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Improving the Makerbot 3D Printer

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1. nV MXN 7 x A al O o signal opto interrupter Es SN BK Figure C 2 End stop circuit schematic 80 Appendix D G Code Reference The subset of G code interpreted by the system is described in the following sections D 1 Language The G code machine is implemented as in 2 1 5 The following subsections specify the syntax and registers of the language D 1 1 BNF lt instruction gt lt reg write gt lt new line gt lt reg write gt lt reg name gt number lt white space gt lt reg write gt comment lt reg name gt A Z lt comment gt lt line comment gt lt block comment gt lt line comment gt lt line comment start gt lt non newline gt lt new line gt lt line comment start gt lt block comment gt lt non close bracket gt D 1 2 Register Types amp Behaviour Table D 1 shows the type and reset behaviour of each of the G code registers D 2 Actions Each instruction should write a value to the G or M register Depending on the value written the printer will carry out a different action These actions are specified below 8l Register Type Reset at instruction start A Integer Float Float Float Float Float Integer Float Float Float Float Float Integer Float No Float No Integer No Float Float Float Integer Float Float Float Float Float Float Oo O OO O KAZZZAZLZKAZAZLZLZL
2. Low Power HH mo NEM High Power Area Area Passive External Connections Reserved For Future Use Figure 4 4 Top level layout of main board reset button and power connections not shown 4 1 1 Layout amp Board A prototyping board designed for working with DIP Dual In line Package components such as the Mbed was selected Figure 4 3 B Conventional strip board 4 3 A is not ideal for these components as it would require many connections to be cut between the columns of pins The main board contains electronics for both low power systems such as the Mbed and high power systems such as the heater and motors and electrical interference between these parts must be minimised The high and lower power parts have been kept physically separate on the board figure 4 4 each with their own power supply connections The board also has a continuous track covering the whole board which can be connected to ground known as a ground plane helping reduce noise Sil06 A full circuit schematic and pin out for the board is given in Appendix C 1 26 Drain Gate Source Gate Drain Source A Physical Appearence B Symbol Figure 4 5 Metal Oxide Semiconductor Field Effect Transistor MOSFET Drain to Source Voltage 15V Drain to Source Current A Gate to Source Voltage V Figure 4 6 IRLU8729PbF Typical Tran
3. figure A 7 The fill pattern is not visible 68 Figure A 7 Fill pattern being printed Figure A 8 Finished cuboid being ejected when the object is completed It is used as it significantly reduces the time and plastic required to fill objects while still retaining strength The cube takes about 8 minutes to print A 5 4 Cool down Eject and Self Clean Once the print completes the platform is cooled down to allow the object to solidify completely This takes around 2 minutes When the object has cooled the platform conveyor is activated and the object is peeled off the platform and deposited in front of the printer figure A 8 As plastic oozes out of the extruder after the print completes it must once again be cleaned using the same process as the start of the print Finally the printer returns to the home position and the heaters and power supply turn off If another print job is started at this point the extruder will still be hot but the platform having cooled slightly will need to be partly reheated before the print can begin Many prints can be run successively in this way with no human intervention unless a print fails 69 Figure A 9 Strings of plastic left during printing requiring manual removal A 6 Final Print Once the printer ejects the object it will still be hot and may still be slightly soft in some places Care should be taken when handling the object until it has fully cooled The border prin
4. Marius Kintel Clifford Wolf Openscad http www openscad org 2012 Accessed 19 April 2012 Adam Dunkels The uIP 1 0 Reference Manual Swedish Institute of Computer Science Mon Jun 12 11 56 02 2006 edition June 2006 Adam Dunkels Iwip http www sics se adam lwip 2012 Accessed 17 April 2012 FreeRTOS http www freertos org 2012 Accessed 24 April 2012 Git http git scm com 2012 Accessed 24 April 2012 AMS Inc Stepper motor system basics http www stepcontrol com steppingi01 html 2012 Accessed 24 April 2012 Makerbot Industries Cupcake cnc Makerbot Wiki http wiki makerbot com cupcake 2012 Used under GNU Free Documentation License 1 3 Ac cessed 24 April 2012 Makerbot Industries Faq frequently asked questions Makerbot Wiki http wiki makerbot com faq frequently asked questions 2012 Used under GNU Free Documentation License 1 3 Accessed 13 April 2012 Makerbot Industries Pid controller tuning Makerbot Wiki http wiki makerbot com pid controller tuning 2012 Used under GNU Free Docu mentation License 1 3 Accessed 11 April 2012 61 Ind12d Int04 Int09 JE12 Jet12 Lom12 Mai Mar12 Max01 NXP10 Pro12 rep12a Rep12b Rep12c Rep12d Makerbot Industries Thingiverse http www thingiverse com 2012 Ac cessed 14 April 2012 Intel ATX Specification version 2 2 edition 2004 International Rectifier JRLU8729PbF Datashee
5. On power on the PSU is not turned on and so the heaters and motors do not receive power The only way to start the heaters is to open a new connection to the printer and send the required PSU and heater instructions This design means that if the printer is powered on unintentionally it will not do anything unsafe It also means that resetting the printer has the effect of putting it into a safe state where an explicit action is required to restart it 4 4 3 Stop Button The electronics connect the Mbed s reset pin to a larger and easier to press red button figure 4 20 Because of the safe power on behaviour and indifference to software failures this button can always put the system into a safe state 4 4 4 Watchdog Timer The most safety critical software process is that of the heater controller If this routine fails for any reason the heaters may become stuck powered on As over heating is not as obvious to the operator as for example a jammed axis this is a particularly dangerous fault To catch such faults the Mbed includes a hardware watchdog timer WDT The WDT contains timer which upon timing out resets the system The WDT must be fed 41 Figure 4 21 Breadboard with a prototype end stop circuit reset periodically to prevent it resetting the system To feed the WDT specific values must be loaded in the correct order into a control register This mechanism can catch bugs which cause the routine to jump to a rando
6. mand completing and another starting is minimised This minimises the error introduced during very short bursts of extremely detailed movements Some G code implementations have error checking and retransmission mechanisms built in but these are poorly specified and designed for use with serial connections 21 3 1 3 G code Interpreter Though existing G code interpreters are available they are generally tightly integrated with the control logic they were designed with and not easily used standalone G code implementations also vary widely with 3D printers but generally only a require a very small subset of the language features As a result a small G code interpreter will be implemented for the required subset of G code 3 1 4 Drivers In order to interface with the electronics and facilitate accurate timing various peripherals on the Mbed will be used requiring supporting driver code In particular the following features will be needed General Purpose Input Output GPIO Allows digital TTL Transistor Transfer Level signals to be produced and read from the pins on the microcontroller For example stepper control and end stop signals Analog Input Read analog signals from the electronics for example readings from temperature sensors Timer Used to produce interrupts at precisely timed intervals to allow stepper control signals to be generated Watch dog Timer To ensure fail safe behaviour a watch dog timer can be used
7. transmission 06 cag a imm quid in eee EOS AIVE s 95 E Tob ETOS e dc hice ah ache Sp EA n dr AA AAA bit a 95 F 2 Status Monitoring Protocol 4 8 hu obs ur cR LEY ea IO d 95 List of Figures 1 1 1 2 1 3 1 4 2 1 ZA 2 3 3 1 3 2 3 3 4 4 2 4 3 4 4 4 5 4 6 4 7 4 8 4 9 4 10 4 11 4 12 4 13 4 14 4 15 4 16 4 17 4 18 4 19 4 20 4 21 5 1 3D printed Gear Cube produced by the printer upgraded in this project Unmodified Makerbot Cupcake CNC photo by Rayshobby Pro12 Makerbot key components a Slicing a 3D model into layers to be printed stepper motor operation o 2l EL ERR a Extr der components e Los re dit sacer pact ee e ed Sth sak J Mbed microcontroller a o eee Rome RM EGER Soto High level diagram of overall system architecture G code processing pipeline as uerb e OEE dE Rem d Stepper controller board with connections labelled photo Rep12g Components of the main board 0 0 00 eae Electronics installed with key components labelled Types of prototyping board x urs ex Deo dd SS ERAS Top level layout of main board reset button and power connections not SOW sca dove o Eder line Peta a tese dote e o ee LM So S Metal Oxide Semiconductor Field Effect Transistor MOSFET IRLU8729PbF Typical Transfer Characteristics reproduced from Fig 3 O ss ue cart eoe acetate Une gees Oo ea ere wea G
8. The G code was initially implemented using TCP but a bug in pIP s flow control implementation meant an alternative implementation was required and built on UDP The two protocols are outlined below along with an additional TCP interface used for reporting the system s status 37 TCP G code Interface The TCP G code interface consists of an open port listening for connections Once connected a client simply sends the G code to the printer and disconnects when finished Telnet or net cat nc can be used to send G code files to the printer for example nc 192 168 3 100 1818 q O lt model gcode Where model gcode is the file to send and 193 168 3 100 is the IP of the printer As the G code subset supported does not support return values nothing is returned by the printer UDP G code Interface Unfortunately as discussed in 85 3 the TCP implementation in uIP is incorrect Due to time constraints this was not fixed during the project Instead a UDP based protocol was implemented UDP provides a facility for sending datagrams containing a small amount of data to a remote host These datagrams are not guaranteed to arrive or to arrive in the order they are sent They do include a checksum and so if a datagram arrives it can be safely be assumed to be intact Finally no flow control mechanism is provided excess packets are silently dropped They are however lightweight and a good base for building custom protocols UDP alone cannot be
9. and latency is not important as changes occur very slowly This system is very simple to implement with negligible overhead Heater Control Loop A naive controller could simply turn on the heaters whenever the temperature drops below some target temperature or set point and then off when it was met or exceeded This type of controller can cause the temperature to overshoot and fall below the set point as the heating and cooling of the system is not immediate Instead a proportional integral derivative PID controller is used which can control heaters which takes into 32 Comparator Set Point Temperature Figure 4 13 PID heater controller schematic account such behaviours PID controllers are widely used where a process which is not completely understood must be controlled Ben93 Figure 4 13 shows a schematic of the PID controller used in the system At time t the comparator calculates the error e t between the actual temperature and the set point A value is calculated from this error using a factor proportional to the error the error accumulated over time integral and the error s rate of change derivative These factors are weighted by the constants Kp K and Kg respectively and the result used to control the heater The three weights must be chosen manually to produce sensible behaviour 5 5 discusses how these values were selected The value calculated could be used to control an analog output or PWM or a t
10. dig Methodology Y cose bus deo toca nib eoa Etre eti eec ted 42 Avice MVHS MAB A CE es ak BG BA soe eh Ses 43 Acro e Version COntr l ds doe a ds ius adele he ante he wa P HF wae 43 4 5 4 Debugging Uh RS ve itu eii ee ol e Oe a c P 43 Testing amp Evaluation 44 DoD VESHBGDEONMIGO E o M hie Soto ES oer ett Me steh Ae 7 Moe E d 44 add AW SUES 2 todo ts E e dr oae Nes m gg 44 DEZ Ernd st ps cote pum eti oer tane Aus ur redi e do qu des 45 Dub eL DE soe serta dep hace tes debi ao M e aa Rs POR uta an du NE o 45 5 2 FreeR TOS c EXPECT 45 5 9 UIP E NODWOEEOIBE avide das Ge beat e Ah er a m Y 45 5 4 Temperature Readings 2 224k ob a 47 or PID Onbrali se sores 20 eek Ve e a a Ge TS Ka GEE es 48 5 0 Stepper Control ld ars atos ork ats RE E We v MI SCIES 49 BOs AAA ES en but Jess ES SR NI aetna adea b tied 49 5 6 25 Steppin EE c gs A eS we ee Bre 49 Bot UEROESTODS e ity a ee egeo ede doas nae DE ag wale he ean di PE US 50 5 8 Buffer Utilisation um ion etus etu aa ke orae TE VO B Re eut 50 5 9 System Testing us aeneae clem tm Elke gene pr b pu E arg 51 59 1 Synthetic Tests s kesa 24 SER eii ER BIRGONCAS SRO ERS 51 DUO Test ODJECtS oh ds oh ae Ge ea S e ee Venus 53 Conclusions amp Future Work GT SPO o E Goals NEN EE A tech A ie nh ok gs Bea 6 1 1 Electronics Replacement s asm poc Rem ea a EGO Rez 3 6 1 2 Microcontroller Improvements o Lum End Stops aina vs bd Aid a ARE eek Ow eee Be ee a A 0s Future W
11. 1 is used to power the main board Signal Colour Notes Ground Black 3 3V Orange 5 Tolerance Unused 5V Red 5 Tolerance 12V Yellow 5 Tolerance 12V Blue 10 Tolerance Unused Power Good Gray Signal asserted when all voltages are correct and stable 5V Standby Purple Power available at all times Max 2A 3 3V Sense Brown Unused Power On Green Active Low signal pulled up to 5V Table 4 1 20 pin ATX Connector Signals Int04 The Mbed is connected to the 5V standby supply allowing it to remain connected to the network and power on the system on demand The maximum power consumption of the Mbed is 200mA well within the ratings of the ATX specification Sem12b The Mbed s on board regulator provides a regulated 3 3V supply used by the Mbed and the low power electronics attached to it The 3 3V supply from the PSU is not used because the regulator in the Mbed offers a cleaner supply which is always available 30 A Without Magnetics B With Magnetics Figure 4 11 X ray of RJ45 sockets with and without integrated magnetics Lom12 To allow the PSU to be turned on by the Mbed the power on signal is attached to a GPIO pin Because the Mbed is a 3 3V logic device a MOSFET is used to connect the 5V Power On signal to ground thus turning on the Power Supply using the 3 3V signal from the Mbed 4 1 7 Ethernet Ethernet requires relatively complex circuitry to drive it With the exception of the Ethernet magnetics
12. Deu go AR afi Ros ases oko Ios ce e P a ders ed ui 4 Implementation Act SEICODPODIGS acido id amar cierre fedet n tothe Sere ees Ier Tet S 4 1 1 Layout amp Board eade 3 ohn oe E acti E i ee e ee 10 10 11 11 13 14 14 14 16 16 16 17 17 17 18 18 18 19 19 20 20 21 21 22 22 22 22 22 23 23 4 1 2 Heaters amp DC Motors n 27 41 9 DhermisbobS sds Reo OOS vo m RETE de e 28 ZI Stepper Motots de pet eet pe Ane mega pu eu HA eins 29 Zo BNOSSLODS sara E rate so REORIAESS mE Xe gum su 29 AO POWEL a2 vik dt ferta de aie bate de RR ORO S dA S ded 30 dq HG GHC feu dm oP ave odo patet Hd ae teen v dre eaa 3l ZO DETIDOS sd n uu duce cas d e dae A ai cer eb b Aeon 31 4 2 1 FreeRTOS on the Mbed o o 31 4 2 2 Temperature Control Loca pe A BORG 32 42 3 Stepper Control 24 une E ARA ASS 33 4 2 4 G Code Interpreter xs e Sah as ya SOS 36 4 2 5 Printer Controller ly ote Di ea Gok ke Be ee EAS 37 4 2 6 Network Interface lisis i en See ee ee ug 37 4 amp 3 Utilities ASA A a Ap Tae ae Ma he cad gen ena ate GR My had ge Vee tie eae 39 AAN ut e tetos eit p o ASQ WI Ie nadie e pl a Va aspe ot nant QU M PEU d 41 4 4 1 Heater Indicator LEDS 4 e B SRS EE 41 4 4 2 Power on Behaviour cxi E Sm Sere ee 41 AAS Stop Button o uos oe a oso Reg Re REY 41 T Watchdog Titer qu arya v E apie we Ps 4e TS Pee eS de 41 4 5 Methodology amp Tools wd rer or RG eu T NEAR EE x S 42
13. Mbed As well as high performance processors designed for phones ARM also design the Cortex M series of microcontrollers For this project a Cortex M3 based Mbed microcontroller was chosen to replace the pair of existing 8 bit microcontrollers Sem12b The reasons for the suitability of this choice are justified in this section 17 Figure 2 3 Mbed microcontroller 2 2 1 Mbed The Mbed is a small microcontroller prototyping board centred around the NXP LPC1768 ARM Cortex M3 based microcontroller figure 2 3 It has has four debugging LEDs a USB port for loading programs various input output facilities including facilities for attaching an Ethernet port The pins on the device expose these input and output capa bilities and fit into standard 0 1 spaced circuit boards and prototyping breadboards The Mbed provides a USB flash drive like interface This interface is used to program the device by simply copying a binary file onto it This mechanism is used to support the device s unusual choice of purely web based official development tools Web based development was not ideal for this project and an alternative solution is discussed in 4 2 1 allowing conventional development tools to be used 2 2 2 NXP LPC1768 The LPC1768 microcontroller behind the Mbed provides the ARM Cortex M3 processor with various useful peripherals It runs at up to 100MHz and has 32KB of ram NXP10 While still a seemingly tiny amount compared to e
14. NXP Semiconductors Mbed NXP LPC1768 Technical Reference http mbed org handbook mbed NXP LPC1768 2012 Accessed 29 March 2012 John S Steinhart and Stanley R Hart Calibration curves for thermistors Deep Sea Research and Oceanographic Abstracts 15 4 497 503 1968 C E Shannon Communication in the presence of noise Proc Institute of Radio Engineers 37 1 10 21 January 1949 Lili Manolis Sherman 3d printers lead growth of rapid pro totyping Plastics Technology http www ptonline com articles 3d printers lead growth of rapid prototyping 2012 Accessed 14 April 2012 Silicon Labs C8051F xxx Printed Circuit Board Design Notes preliminary rev 0 1 11 06 edition 2006 Skeinforge wiki http fabmetheus crsndoo com wiki index php Skeinforge 2012 Accessed 24 April 2012 Tracy Staedter Phantom ray drone completes first flight Discovery News http news discovery com tech phantom ray drone completes first flight 110504 html 2012 Ac cessed 13 April 2012 J G Ziegler and N B Nichols Optimum settings for automatic controllers Transactions of the ASME 64 pages 159 768 1942 63 Appendix A Example Printing Workflow The following appendix contains a walk through of the complete process of designing and printing an object from scratch The process begins with the production of a 3D model of the design The next step is to convert the model into G code instructions and send them to the printer A de
15. The interface must also be reliable because a missed corrupted or out of order G code instruc tion would cause potentially dangerous results TCP offers both reliable communication and flow control mechanisms and is implemented in uIP making it an appropriate choice for this task The status querying interface should be kept minimal and human readable A telnet compatible interface should be created 3 1 2 G code Processing Pipeline The main task of the firmware is to process incoming G code from the network and to control the printer appropriately without stalling A pipeline architecture figure 3 2 was selected where G code from the network is buffered before being interpreted and converted into low level commands The low level commands are placed in another buffer and then executed in sequence to drive the printer In order to reduce stalls due to data processing between commands space is exchanged for computation time by keeping all command arguments in formats directly used by the printer in the low level commands For example distances should be represented as an integral number of steps and not floating point millimetre values At the start of the pipeline the effect of network latency should be minimised by allocating a large G code buffer giving the network longer to respond to changes in the rate of command execution before the buffer drains Finally by assigning a high priority to the printer controller the delay between a com
16. a list of categories for which status should be fetched and returns a tab delimited data file containing those values and exits The p option causes the printer to be repeatedly polled for status information until the utility is terminated The output of this utility is compatible with GNU Plot E 3 4 makebed live sh To provide a live view of the printer s status during a print a shell script has been provided which plots live data from the printer s status interface It is a simple wrapper for the makebed sh get command A screen shot is shown in figure 4 19 on page 40 93 Appendix F Protocol Specifications The printer provides two network interfaces for which the protocols are described in the following sections The port numbers used by these services are shown in table F 1 F 1 UDP G code Transmission Protocol This interface is required to reliably send data to the printer and provide flow control such that the printer doesn t receive data it can t handle A discussion of the protocol s general design and requirements can be found in 84 2 6 F 1 1 Sender Packets Packets sent to the printer consist of an unsigned 32 bit integer sequence number followed by bytes of payload data up to the window size advertised by the receiver Initially the sequence number is 0 and the window size is assumed to be zero This means that the first datagram consists of the sequence number alone F 1 2 Acknowledges The receiver is req
17. again the test was initially conducted using a test program running on the Mbed and then by G code sent over the network The number of segments the circle was divided into was increased from 30 to 3 000 and the speed set to 330mm minute and 3300mm minute to test slow and fast movements 49 The performance of the system was measured by visual inspection of the movement and circles plotted by attaching a pen to the extruder The time taken the plot the circles was also measured using a timer on the Mbed to see if the processing overhead between each segment caused significant drift Above around 100 segments the circles drawn appeared smooth and at low and high speeds the overhead after 20 circles had been plotted at each speed and resolution was less than the 1ms resolution of the timer Finally to ensure that steps were not missed the system was moved to a known point between tests and this did not drift after all tests had completed 5 7 End stops The endstops were tested under various lighting conditions to observe the effect of external lighting on the opto interrupters The following lightings conditions were tested e Ambient strip lighting e Ambient halogen lighting Ambient incandescent lighting Ambient natural light e Direct halogen lighting e Direct incandescent lighting e Darkened room Testing consisted of blocking each opto interrupter by moving the printer axes so that the end stop is triggered and observing the dig
18. extremely easy to integrate into the project FreeRTOS itself provided the right balance of features and performance for the project The operating system did not place restrictions on the use of low level system registers and simply provided preemptive multitasking and some atomic operations as required The operating system was initially tested for timing accuracy using a frequency probe attached to an I O pin toggled by a simple demonstration program to ensure the system was behaving as expected This test yielded a mismatch from the expected frequency which was found to be an incorrect definition of the system clock speed in a FreeRTOS header file Once fixed the system ran as expected running the included demo and test programs as defined No further issues were found during the course of the project 5 3 uIP amp Networking Various tests were conducted on the uIP stack during the project concentrating on performance and correctness of the features used This section discusses these tests and concludes with an evaluation of uIP s suitability for the project Wireshark was used to monitor the packets sent between the Mbed and computer where it became apparent that every packet from the Mbed was being duplicated After ruling out network problems as the cause the bug was traced down to the Ethernet driver provided by the demo The driver duplicated every packet sent to the network including IMCP Ping Requests figure 5 1 As well as wasting b
19. heater control have performed well with the silent operation notable over the previous system They also allow the possibility of variable power controls for the motors and heaters with only software changes The only notable issue remaining is its compatibility with certain ATX PSUs Despite the simplicity of the fix time was not available to implement and test the change Future work should aim to address this issue 6 1 2 Microcontroller Improvements The choice of using the Mbed microcontroller proved overall to be a good decision It provided all the hardware required in a small easily integrated package The device was also fast enough to host the printer software providing a major improvement over the old system Unfortunately the lack of exposed debugging facilities significantly hindered de velopment in some areas For example with JTAG debugging facilities available adding a new networking stack may have been possible within the project time scale The firmware written for the Mbed proved to be an improvement over the previous system making more detailed prints possible due to improved system performance and 57 communications It is also far more modular in design making expansion and experi mentation feasible The print quality achieved by the printer is now largely consistent with the constraints of the hardware itself rather than inadequacies of the firmware or microcontroller FreeRTOS proved a good choice of operating syst
20. http reprap org wiki G code 2012 Used under GNU Free Documentation License 1 2 Accessed 24 April 2012 RepRap Generation 3 electronics RepRap Wiki http reprap org wiki Generation 3 Electronics 2012 Used under GNU Free Documentation License 1 2 Accessed 24 April 2012 RepRap Optoendstop 2 1 RepRap Wiki http reprap org wiki OptoEndstop 2 1 2012 Used under GNU Free Documentation License 1 2 Accessed 20 April 2012 62 Rep12e Rep12f Rep12g Rep12h Sch12 Sem1 2a Sem1 2b SH68 Sha49 She12 Sil06 ske12 Stal2 ZN42 RepRap PC power supply RepRap Wiki http reprap org wiki PCPowerSupply 2012 Used under GNU Free Documentation License 1 2 Accessed 9 April 2012 RepRap Reprap wiki http reprap org wiki 2012 Used under GNU Free Documentation License 1 2 Accessed 14 April 2012 RepRap Stepper controller board photo RepRap Wiki http reprap org wiki File Cache 3218206144_6461b3e2c0 jpg 2012 Used under GNU Free Documentation License 1 2 Accessed 29 March 2012 RepRap Stepper motor driver 2 3 RepRap Wiki http reprap org wiki Stepper_Motor_Driver_2 3 2012 Used under GNU Free Documentation License 1 2 Accessed 21 April 2012 OSI School Duplicate ACK http www osischool com protocol tcp duplicate ack 2012 Accessed 11 April 2012 NXP Semiconductors Mbed compiler http mbed org handbook mbed Compiler 2012 Accessed 4 April 2012
21. made PCBs resulted in an other circuit board on the printer reducing some of the simplicity gained from simplifying the main electronics The CAT 5 cables used to connect the board to the rest of the elec tronics are also bulky especially considering that only a single signal wire is used within each 8 core cable Due to time limitations the end stops were only used for positioning Though not used to their full capability the end stops provide a valuable improvement to the printer s operation Other uses for the sensors and a fix for the ambient lighting issues are described in 6 2 2 6 2 Future Work There are many possible avenues of future work which would either strongly complement the project or build directly on the system The most interesting of these possibilities are presented below 6 2 1 Network Interface The interface used to interact with the printer is an important part of the system as it is both user facing and performance critical Unfortunately while adequately performant 58 the interface built falls short in other areas and these issues are deserving of further work G code Interface No job control security or multiple user support is provided by the G code interface of the printer Each of these shortcomings restricts the printer to networks of trusted users who are able to collaborate to manually schedule print jobs Future work might build upon conventional printer interfaces for example integrating w
22. make the overhead of the scheduler unacceptable and so the FreeRTOS timer cannot be used Instead a hardware timer which interrupts the microcontroller causing a light weight interrupt service routine ISR to run will be needed Because the Mbed only provides a limited number of hardware timers just one is used to control all three steppers As each stepper may not be in phase the time between signals being produced for each stepper can become very small even when the step period is large This is another example of a sampling artefact where the resulting errors are at worst 8 for a timer running at fHz Because the timer is already sufficiently fast to make such errors insignificant no extra precision is required when sharing a timer The timers provided on board the Mbed consist of a register comparator and counter which is incremented by the system clock after being passed through a clock divider figure 4 17 The timer was configured such that when the counter matches the value in the register the counter is reset and the CPU is interrupted The timer was configured to run at IMHz which exceeds the 264kHz requirement calculated above with some additional margin The ISR for the timer interrupt determines for each stepper whether its step signal should be toggled and calculates how long until another interrupt is required The timer compare register is updated the ISR returns and normal program execution resumes GCC generates approximatel
23. platform M109 SO Heater set target to O c M107 Conveyor Turn off stepper motors M18 Power off PSU MO D 3 3 Skeinforge Print Prefix Prefix added to all print jobs to heat up and prepare the printer before a print job power on M 1 G21 set units to mm G90 set positioning to absolute 86 Start in parking position G92 X 60 Y 45 Z10 Raise up to avoid the loop G1 212 F100 Move to squirt position Gi X 55 Y 10 F1000 G1 Z7 F100 Heat up M104 8225 M109 8120 M6 Extrude a bit and stop M101 G4 P5000 M103 G4 P6000 Wipe G1 Y10 F2000 Go to origin M101 G1 XO YO ZO F2400 0 D 3 4 Skeinforge Print Postfix Postfix added to all print jobs to cool down and eject the object after printing G1 XO Y40 F3300 0 move platform to ejection position cool down platform M104 8225 M109 S80 M103 Extruder off GO4 P100000 wait t 1000 seconds M106 conveyor on GO4 P10000 wait t 1000 seconds M107 conveyor off start wipe Move to squirt position G1 X 55 Y 10 F1000 G1 Z7 F100 87 Heat up extruder M104 8225 M6 Extrude a bit and stop M101 G4 P5000 M103 G4 P6000 Wipe Gi Y10 F2000 Go to starting position G1 Z12 F100 G1 X 60 Y 45 F3300 G1 Z10 F100 Turn off heaters M104 SO set extruder temperature M109 SO set heated build platform temperature power off MO D 3 5 Home X amp Y Axes Home the X amp Y axes using the end stop
24. prototype were subsequently modified and used as part of the off printer client software 42 4 5 2 Languages The firmware was written entirely in C As well as a mature compiler and manufacturer provided header files C strikes a good balance between the availability of sensible abstrac tions and providing low level access to the underlying hardware Thanks to a policy of manual static memory management no garbage collection or dynamic memory allocation was required which helps make system performance more deterministic The off printer utilities are written primarily in Python and include a wrapper shell script written in Bash These languages provide very high levels of abstraction simplifying and accelerating development The performance costs incurred by using such languages are not relevant on a modern PC for the simple tasks required and so represent a good fit for the project 4 5 3 Version Control To track changes and keep snapshots of the project s code Git was used for version control git12 Git provides facilities for quickly comparing versions of code It also allows experimental copies or branches of the code base to be created and independently modified Useful changes can be made in their own isolated branches and easily be merged back into the main version of the system Changes can also be quickly reverted These features allowed new ideas to be created and tested without fear of irreversibly altering the system 4 5
25. this is provided on board the Mbed A jack containing the magnetics figure 4 11 was used to complete the system 4 2 Firmware In this section the firmware for the Mbed is described starting with the infrastructure required for development on FreeRTOS on the Mbed and then moving on to the ma jor components of the system Finally the safety precautions taken and the tools and development practices used are outlined 4 2 1 FreeRTOS on the Mbed The Mbed is designed for use with a web based IDE and compiler Sem12a This system is not appropriate for use in the project as the process of uploading code to be compiled is laborious and the compilation options restricted The CodeSourcery G None EABI toolchain includes a GCC ARM cross compiler linker and LibC compiled for various ARM based microcontrollers It was selected over closed source alternatives because it has a large community of users and produces good quality code An unofficial port of FreeRTOS for the Mbed is available designed for the CodeS ourcery toolchain It provides a base FreeRTOS configuration with a demonstration HIP based web server as well as other simple operating system demos Also included are headers for the ARM Cortex Microcontroller Software Interface Standard CMSIS which defines a common interface for ARM Cortex microcontrollers ARM12 Finally headers defining macros and pointers for all control registers in the Mbed are included 3l Analog In i Comp
26. to reset and power down the system in the event of software malfunction 3 1 5 Safety The system must behave safely in the event of a software failure and should be inter ruptible by a user at any time The system must also start up in a safe state so that unintentionally powering on the machine cannot result in dangerous behaviour Read ings from the system must also be sufficiently accurate that they do not mislead the user about the system s safety 3 2 Electronics amp Hardware New electronics are required to replace the motherboard relay and extruder boards As well as this electronics and hardware must be added for the proposed end stop sensors The requirements for these parts are described below 3 2 1 Stepper Control The printer s three primary axes are controlled by stepper motors which require complex circuitry to drive them Off the shelf RepRap stepper motor drivers figure 3 3 will be used as in the existing electronics Rep12h The boards connect to the stepper motor and power supply and provide a TTL control interface They also provide a connection 22 Stepper Motor End stops TTL Interface Figure 3 3 Stepper controller board with connections labelled photo Rep12g for two end stop sensors to be attached the output from which they passively forward back through the TTL interface The enable direction and step signals from each stepper board must be driven by the microcontroller s GPIO p
27. used to send data to the printer and simple protocol has been built on top which provides a communications channel which is e Reliable e Unidirectional e Order guaranteed e Flow controlled Figure 4 18 shows the format of datagrams sent between the printer and sender Datagrams are sent to the printer which contain a sequence number and a payload whose length can be calculated based on the datagram s size contained in the UDP header The sender then waits for a response from the printer containing the same sequence number and a window size The printer will only respond if a datagram with an appropriate sequence number is received discarding out of order datagrams If a response with a matching sequence number is not received by the sender within a short timeout the datagram is retransmitted This mechanism facilitates reliable in order transmission The window size returned by the printer is used for flow control and is the amount of space in the G code buffer or the maximum datagram size whichever is smallest The sender may send up to this amount of data to the printer in its next datagram ensuring the printer is always sent as much data is it can deal with If the G code buffer becomes full then the window size will become zero the sender must poll the printer until the window size becomes non zero 85 3 discusses the performance and practical implications of this protocol and the full specification is given in Appendix F 1 38 T
28. 4 Debugging Microcontrollers are typically debugged using a JTAG Joint Test Action Group interface which allows the microcontroller to be paused stepped and examined during program execution Unfortunately the Mbed does not expose this interface and so all debugging must be carried out through the input output facilities provided The port of FreeRTOS used included a demonstration web server which was modified during the first iteration of firmware development to allow program variables to be ex posed through this interface Eventually the demonstration code was removed and the status interface described in 84 2 6 was put in its place providing equivalent facilities Both interfaces allowed internal variables to be polled examined and graphed on a PC As well as the firmware itself its interactions with the network also required debug ging To do this Wireshark a network analyser was used This allows the individual packets sent and received by a computer to be monitored filtered and examined It also includes protocol specific features such as protocol checking and protocol specific statistics 43 Chapter 5 Testing amp Evaluation As the system was built the individual parts were unit tested extensively before integra tion into the rest of the system Integration tests were then performed followed finally by a series of full system tests This strategy meant testing was a continuous process throughout the implementation and identi
29. 5 Z 0 3 In this example the action is move to as determined by the G register The X Y and Z registers determine where to move and F determines the speed If this is followed by M104 225 then the following registers will be set F 3000 M 104 S 225 X 10 Y 15 Z 0 3 Note that the G register has been undefined but the others have remained In this example the action is determined based on the M register to be set extruder temperature and the S register determines the temperature X Y Z and F are ignored Finally if G1 Z0 6 is encountered the following registers are set F 3000 G 1 S 225 X 10 Y 15 Z 0 6 Once again the machine determines the action to be move to and moves to the position defined by X Y and Z at the speed in F ignoring the S register Because the values of X and Y have not been changed the machine will move only the Z axis 2 1 6 Support Software The G code used by the printer is generated from 3D models using an open source tool called Skeinforge ske12 Skeinforge is typically used as part of ReplicatorG a graphical user interface for preparing and printing 3D models rep12a ReplicatorG can also handle the translation of G code into the compressed format used by the RepRap firmware Other less mature tools such as Slic3r are available but less frequently used 2 2 ARM amp
30. Current unit D 2 2 M Actions M 2 Home axes Custom extension to the standard G code actions Slowly move the specified axes until hitting the endstop The current position in the X Y and Z registers is then set to the locations of the end stops essentially calibrating the printer s position Argument Description Unit A Axis Selection Bit mask Bit 1 X Bit 2 Y Bit 3 Z M 1 amp MO Turn the PSU on off Turns the PSU on and off respectively Note that if the PSU is not powered on many actions may block indefinitely This action blocks until mains power is available if the microcontroller is being powered via USB M 1 is a custom extension to the standard G code actions Argument Description Unit No arguments M6 Wait for heaters Block until both heaters have reached their target temperature Contrary to the stan dard this command will not block waiting for the heaters to cool down to a new target temperature Argument Description Unit No arguments M17 amp M18 Power stepper motors on off Enable or disable power to all stepper motor drivers The steppers are automatically powered on when moved 84 Argument Description Unit No arguments M101 M102 M103 Extruder motor forward backward off Set the extruder motor moving forward backward or not at all respectively M102 is not supported by the system and will raise an error and stop the motor The extruder should not be tur
31. Linux will allow several packets to arrive before acknowledging them all at once saving bandwidth reducing the effect of network latency This delays the transmission of the next packet by uIP as it waits for the ACK By duplicating each packet the receiving computer is forced to immediately send an ACK as from receiver s perspective a duplicate packet may indicate that the original packet was delayed in the network and the sender did not receive an ACK in time in this case because the receiver had never sent it As a result the receiver sends the ACK immediately allowing LIP to send the next packet Though this wastes bandwidth it reduces the wait between each packet being sent and in practice dramatically increases the bandwidth available when sending data from the Mbed to the computer Disabling this work around means sending multiple packet bursts of data to a computer is more time consuming but removes a barrier to flow control being used successfully The only data sent from the Mbed are status responses which are small enough around 30 bytes to fit in a single packet Disabling packet duplication in favour of removing a barrier to proper flow control is a good trade off Unfortunately problems with flow control persisted with no improvement after dis abling packet duplication With the duplicate packets removed the traffic became clearer To enable flow control TCP sends a window size with each packet representing the amount of data t
32. PIN_EXTRUDER Extruder Motor 30 30 PIN PLATFORM Platform Motor 31 D USB Unused 32 D USB Unused 33 TD MagJack Ethernet 34 TD MagJack Ethernet 35 RD MagJack Ethernet 36 RD MagJack Ethernet 37 IF Reserved Unused 38 IF Reserved Unused 39 VU USB 45V Unused 40 VOUT 3 3V Out Table C 1 Mbed pin connections TT Lg r d latform Thermistor pat Thermistor 5 E nT 84 4 4K i 4 4K E y Eo EE 21 TIE 3 3 E 5 mbed es le s es Si a SL a Si a Sl a E 3 E E 3 1 GND vour 49 E E 3 S E 5 El 2 viN vu 39 amp E a ri 3 3 38 e tos reset ae IE LITE 1 2 4 nR IF4 37 o a 4 Teto tn SL n 5 p E E F 4r Power OK RD XH XH A A 8 X Endstop Min RD 35 s s s s gnd _7 X Endstop Max TD 34 8 2K 8 2K 8 2K 8 2K 8 X Stepper Step TD 33 H YH M Y 2 X_Stepper_Dir p 32 10 x_Stepper_NEN D 31 11 Y Endstop Min Platform Motor 30 12 y Endstop Max Extruder Motor 29 13 v Stepper Step Platform Heater 28 14 Y Stepper Dir Extruder Heater 27 15 Y Stepper NEN 26 16 Z Endstop_Min 25 17 Z Endsto
33. UNIVERSITY OF MANCHESTER School of Computer Science Project Report 2012 Improving the Makerbot 3D Printer Author Supervisor Jonathan Heathcote Alasdair Rawsthorne Abstract Improving the Makerbot 3D Printer Author Jonathan Heathcote Personal 3D printers allow anyone to manufacture complex physical objects on demand with minimal skill and effort The Makerbot Cupcake CNC is a low cost easy to build 3D printer with a wide community of users Though relatively capable the machine is held back by its primitive control electronics and microcontroller New electronics based around a more powerful ARM microcontroller were produced along with new firmware to control the printer The system was built on the widely used FreeRTOS and uses the wIP TCP IP stack allowing a clean easily extended design to be implemented The new system improved the print quality achievable by the printer thanks to improved timing accuracy Additional sensors also allowed the printer to act with increased inde pendence Supervisor Alasdair Rawsthorne Acknowledgements Thanks are due to my supervisor Alasdair Rawsthorne who has been both supportive and helpful throughout every stage of the project Also many thanks to Dave Bowden who constantly supplied me with all manner of tools and components at short notice throughout Finally thanks too to my parents and girlfriend who ve been kind enough to put up with my ranting and assist with proof reading
34. ZzZ e ooo O uN NK MSS CHMDOVOAZZ CACAO MIAO ZZZZZZZZZZ ooo O OO O0 6 Table D 1 G code register types and behaviours 82 D 2 1 G Actions GO amp G1 Move extruder to coordinate G1 moves the extruder through a straight line in 3D space from the current position to the position given in the arguments GO is included for compatibility reasons and does the same thing as G1 Argument Description Unit X X coordinate Current unit Y Y coordinate Current unit Z Z coordinate Current unit F Feed rate speed of movement Current unit per minute G4 Sleep Pause the printer for a specified period Argument Description Unit P Period Milliseconds G20 amp G21 Set unit Set the unit used to specify movements to inches or millimetres respectively Argument Description Unit No arguments G90 amp G91 Set absolute relative positioning Sets whether positions are specified absolutely or relative to the current position Relative positioning is not supported by the system Argument Description Unit No arguments G92 Set origin Set the current position without moving the extruder Used to set the initial position of the extruder to the origin or some known homing location If this instruction is not used the current position is undefined and moving the extruder may have unexpected effects 83 Argument Description Unit X X coordinate Current unit Y Y coordinate Current unit Z Z coordinate
35. and is a characteristic constant for the device available in the data sheet Using the Analog to Digital converter in the Mbed voltages but not resistances can be read directly As a result a potential divider figure 4 8 is used to produce a measurable voltage which is proportional to the resistance to measure it indirectly A reference voltage Vref is placed across two resistors R and Ra and the voltage between 28 Viet Figure 4 8 Potential divider Infra Red LED Photo transistor Figure 4 9 Photo interrupter with a photo transistor in a Darlington pair them at V is measured The relationship between these variables is Ro V Var 4 2 ref R Ry e Thus if the thermistor is placed as Rs the resistance can be calculated using f R Vet 377 4 3 deles Vref The value of R was chosen to evenly spread the voltages from the expected range of thermistor resistances over the full range of 0 to Vef volts This maximises the utilisation of the analog to digital converter available over the temperature ranges used Using 4 1 and 4 3 with a potential divider circuit will allow the temperature of the thermistor to be measured 4 1 4 Stepper Motors The stepper controllers chosen accept TTL signals and connect via a ten pin insulation displacement connector IDC An IDC socket was placed on the board and the pins connected directly to the Mbed and ground plane as required 4 1 5 End stops Optical end stop
36. andwidth if a TCP packet ac knowledgement ACK from the Mbed was to be delayed in the network retransmission 45 ping 192 168 3 100 PING 192 168 3 100 192 168 3 100 56 84 bytes of data 64 bytes from 192 168 3 100 icmp_req 1 ttl 64 time 0 364 ms 64 bytes from 192 168 3 100 icmp req 1 tt1 64 time 0 385 ms DUP 64 bytes from 192 168 3 100 icmp req 2 ttl 64 time 0 175 ms 64 bytes from 192 168 3 100 icmp req 2 tt1 64 time 0 195 ms DUP 64 bytes from 192 168 3 100 icmp req 3 ttl 64 time 0 195 ms 64 bytes from 192 168 3 100 icmp req 3 tt1 64 time 0 212 ms DUP 64 bytes from 192 168 3 100 icmp req 4 ttl 64 time 0 179 ms 64 bytes from 192 168 3 100 icmp req 4 tt1 64 time 0 195 ms DUP C 192 168 3 100 ping statistics 4 packets transmitted 4 received 4 duplicates 0 packet loss time 3000ms rtt min avg max mdev 0 175 0 237 0 385 0 081 ms 00000000000 WwW Ww Figure 5 1 Ping responses being duplicated by the uIP driver will result in four duplicate ACK packets This causes the sending computer to retrans mit and incorrectly adjust its expectations of the network connection when the packets are eventually received Sch12 This behaviour is one factor that can prevent TCP flow control from functioning correctly The packet duplication behaviour is often added to uIP implementations to work around problems caused by IP only allowing one packet to be sent at a time Mai Modern systems such as Windows and
37. arator Digital to Analog Register Control Logic Figure 4 12 Successive approximation analog to digital converter Appendix E 2 2 contains specific compilation instructions for the final system with the CodeSourcery toolchain 4 2 2 Temperature Control To drive the two heaters an active feedback loop is used where input from a thermistor is used to drive a heater via a MOSFET The following subsections describe how analog values are read by the Mbed and the theory and operation of the feedback loop that controls the heaters Analog Input The Mbed includes a 12 bit successive approximation analog to digital converter ADC for reading analog values NXP10 The ADC uses a digital to analog converter and a comparator to binary search for an approximation to the analog value while the input is held constant figure 4 12 Once an appropriate number of iterations of the binary search have been carried out the value in the register is returned Max01 The ADC sampling process takes at least 5us or approximately 500 CPU cycles there fore the CPU can carry out another task while the ADC process takes place NXP10 Though an interrupt is provided when the ADC completes as well as a direct memory access DMA facility this was not used Instead a slow poll is used where the task reading from the ADC is suspended for a time typically adequate for ADC operation The temperature sensors are sampled at an extremely low rate around 2Hz
38. ation and Goals The Makerbot while a very capable machine has many limitations in its hardware electronics and firmware In this project the control electronics and microcontroller are the primary area for improvement The existing system has trouble with complex designs 11 Filament Extruder Print Platform Figure 1 3 Makerbot key components e e A B C Figure 1 4 Slicing a 3D model into layers to be printed 12 where dense sequences of instructions exceed the microcontroller s limited resources and slow serial interface The electronics are also a complicated configuration of several circuit boards using crude mechanical relays Finally the printer does not have sensors to indicate the positions of the platform and extruder requiring the platform to be carefully positioned before prints This is a time consuming and error prone task which also means that the printer can t detect mechanical errors during printing In this project each of these three complaints are addressed by the following primary project goals Simplify control electronics Produce a single board which contains all required com ponents using only reliable solid state parts Improve performance Upgrade to a more powerful microcontroller and develop new firmware to exploit the resulting improvements in speed and communications capa bilities Add sensors for platform and extruder movements End stop sensors at the end of each axis of movement wi
39. code into an appropriate sequence of commands to drive the electronics The third component is a driver interface for the various Mbed peripherals In this section each of these components is examined followed by a brief discussion of the safety requirements of the system 3D Modeling G Code Generation G Code Interpreter Printer Control Printer Controller Electronics Hardware Network Interface Status Status Monitoring Monitor Computer Mbed Firmware Hardware Figure 3 1 High level diagram of overall system architecture 20 Netas G Code G Code Command Printer Dios Buffer Interpreter Buffer Controller Figure 3 2 G code processing pipeline 3 1 1 Network Interface The firmware will provide an interface for streaming G code to the printer and an interface for querying the printer s status This will be done via a network interface primarily to increase the bandwidth of the connection with the computer The G code interface should be a simple open port which accepts G code streams and feeds them into the pipeline without the need for specialised software on the sending computer It will need to support flow control as the rate at which the printer is able to accept G code instructions will vary depending on what instructions are being exe cuted For example while waiting for heaters to warm up no instructions are executed but during complex movements many instructions are executed in rapid secession
40. d After an extended period of being powered on the MOSFETs became hot running at around 50 C The data sheet for the IRLU8729PbF MOSFETs states that the operating temperature range is from 55 C to 175 C and so this temperature is safely within operational limits 44 5 1 2 End stops Printed plastic paddles were originally planned as the triggers for use with the end stops Unfortunately acrylonitrile butadiene styrene ABS plastic used by the Makerbot is transparent to the infra red wavelengths used by the opto interrupters and so this material is unsuitable The design was changed to instead use wooden craft lollipop sticks which fit into pre cut slots in the Makerbot and easily trigger the opto interrupters 5 1 3 ATX PSU Some ATX PSUs require a certain load on all provided voltages in order to power up properly Rep12e While a large load is drawn on the 12V line by the heaters and motors the 5V line only powers the Mbed which draws little power The result of this is that the 12V line attached to the heaters only provided 9V and so could not warm up to the required temperature A resistor can be added to the 5V line to draw extra current and fully power up the PSU Rep12e Due to time constraints an alternative PSU was used which did not feature this behaviour rather than modifying the circuit With the new PSU all voltages met their specified requirements 5 2 FreeRTOS The availability of the FreeRTOS port made it
41. d model A print from Skeinforge G code before raft removal B and after raft removal Oia vec RE du 53 5 8 3D printed vase with detailed corners lll 54 5 9 3D printed Z axis handle comparison te a ets eem oem og 54 5 10 3D printed herringbone gear comparison 55 5 11 Failed large print showing warping A and a collision with the extruder B 55 5 12 Folding butterfly comb printed without a raft 56 A 1 OpenSCAD showing a cube model compiled and rendered 65 A 2 ReplicatorG GUI showing a cube model loaded 66 A 3 Skeinforge profile selection dialogue llis 66 A 4 Extruder placed in the homing bracket 67 A 5 Extruder extruding plastic during a self clean 68 A 6 First layer being printed 2 9 E A OR RS 68 A Fill pattern being printed 20 us uer ace EI IMEY iM 69 A 8 Finished cuboid being ejected llle 69 A 9 Strings of plastic left during printing requiring manual removal 70 B 1 Flexible snake to test printing many thin fins 71 B 2 Starfish to test layering appearance o 71 B 3 Doorstop to test large smooth gradients 72 B 4 Butterfly to test intricate islands of print 72 B 5 Rabbit outline to test very thin structures 72 B 6 Letter A to test simple large shapes 73 B 7 Phone sta
42. de to the printer makebed py send cube gcode While print jobs are running the status information can be requested or polled For example makebed py get temperature Additionally a simple wrapper makebed_live sh is provided written in Bash using GNU Plot to plot various pieces of printer status information in real time figure 4 19 Usage information for the utilities can be found in Appendix E 3 39 Extruder Temperature Temperature C T T Current Target 300 Time Seconds Platform Position Temperature C Gcode Buffer Bytes Platform Temperature T Current Target 300 Time Seconds Buffer Usage T T Gcode Commands 200 300 Time Seconds Command Buffer Commands x 946 151 y 323 246 y2 3 87551 Figure 4 19 makebed live sh screen shot 40 Figure 4 20 Emergency stop reset button 4 4 Safety Because the system contains heaters and moving parts safety was a major consideration when implementing the system This section outlines the key features used to ensure operation is as safe as possible including human interaction features and fail safe design 4 4 1 Heater Indicator LEDs The two heaters each use an indicator LED on the microcontroller to indicate when the heaters are powered on This enables an operator to easily and safely check the state of the heaters during operation 4 4 2 Power on Behaviour
43. des a more user friendly front end to the Skeinforge G code generator rep12a Skeinforge relies on a calibrated printer profile to guide its decisions A profile 64 OpenSCAD New Document File Edit Design View Help cube size 20 20 10 Compiling design CSG Products 4 generation Compiling design CSG Products normalization CSG generation finished Total rendering time 0 hours 0 minutes 0 seconds Viewport translate 8 50 2 09 9 97 rotate 62 70 0 00 337 80 distance 295 25 Figure A 1 OpenSCAD showing a cube model compiled and rendered was produced compatible with ReplicatorG 27 and Skeinforge 35 as part of the project which must be copied into replicatorg 0027 skein engines skeinforge 35 skeinforge application prefs Once ReplicatorG has been started the STL file from the previous step is loaded using Open from the File menu figure A 2 Once loaded the model should be positioned centrally on the platform using the tools on the right of the preview Clicking Move provides features to Center the model and Put on Platform to ensure the model is not printed in mid air Once the model is correctly positioned the G code is generated using the Generate GCode button A dialogue will prompt for a profile to be selected SF35 cupcake A BP raftless should be used figure A 3 This process can take some time for complex models A gcode file i
44. ds inefficiencies in MOSFETS when only partially driven 33 Enable Direction MN Forward 1 Step Fx 1 1 i itsh tsl ts th tt Figure 4 14 Stepper control signal wave diagram Period Meaning Timing Constraint tsh Step high gt lys ANOS ta Step low gt lus ANOS ts Setup time gt 200ns ANOS tn Hold time gt 200ns A1109 ty Step time gt 757us Experimentally determined Table 4 2 Stepper timing constraints Timing Constraints The stepper controller boards are based on an Allegro A3982 stepper controller which defines additional timing requirements Table 4 2 gives the timing constraints for these signals Figure 4 14 shows an example waveform with three forward steps followed by four backward steps On the positive edge of the step signal the direction is sampled and the motor is stepped The motors in the printer add an additional timing constraint t due the limit on how often they can step defined by the mechanical properties of the printer This is important because the faster the stepper is driven the less torque is available and so the stepper may miss steps and fail to move Timer Requirements If a stepper is set to run at its maximum speed steps will be 757s apart meaning that the signal must be toggled every 379s Therefore the output signals may change at up to 1 32kHz FreeRTOS provides timing guarantees for scheduling arbitrary delays within tasks Unfortunately this uses the s
45. e is no tooling cost associated with changing a design custom manu facturing is also possible This has applications both for personalised goods and more recently in the manufacture of bespoke medical implants Mar12 The cost of entry level 3D printers has recently become much lower with DIY devices such as the RepRap costing between 300 and 620 to build Shel2 Rep12f This has helped grow communities such as Thingiverse where people share their designs for printable objects with the goal of making physical things as easily accessible as any other digital media Ind12d 1 2 Makerbot The Makerbot figure 1 2 is an open source DIY 3D printer which can produce plastic objects up to 10cm x 10cm x 13cm in size It consists of a moving platform onto which an extruder melts plastic filament and deposits a thin strand of plastic figure 1 3 Objects are produced by moving the platform underneath the extruder and to form layers of plastic which are stacked one on top of each other until the complete shape is formed Figure 1 4 shows how a cone A might be sliced into layers B and how each layer might be printed C Software running on a computer handles the process of slicing a 3D model and gen erating the list of movements required to print it out A simple microcontroller on the Makerbot receives this list of instructions and generates the carefully timed electronic signals needed to drive the printer s components 1 3 Project Motiv
46. e n e a ls e Oe Ae tr da en e RA D 1 2 Register Types amp Behaviour 20 4 D2 tos REI a OK a a E OR SS TODD Ma VOI acd Ge arre de ti 13 2 2 INACIO E pede 3 4 se utra I RYS daw Sash RU xa wie goa x 13 37 SESCUIDIGS S 2d St sat acted ido obest ago Ee Ilo Ee Tei ID Ee oie he te a St P31 Power On Heat Up di De ra Aa cS 57 57 57 57 58 58 58 59 59 59 60 60 61 D 3 2 POWeOE dOWIE gt uae de a ede Pa a bone We GY 86 D 3 3 Skeinforge Print Prefix ae Shab EE gas Sees 86 D 3 4 Skeinforge Print Postfix cds ita Goa bd do Gos o my 87 D 3 5 Home X amp Y Axes o ai ETE ak E SA RO bar ekg ed 88 Te OEE C 5o oie vos Seay E oM E eM eg eke ee SSE oie So 89 E Code Documentation 90 EAD File Citne od clon iol x wr ode Ere ba ee erat at eve fa edo irata tandi ae 90 E2 EIFE ta Ec tees LAR f aho dr papae arise e ed 90 E 2 1 Dependencies het x RO we etum Be eee a A es 90 B2 2 Compilations c a SP Gs be Ee dp eps Sed cic bea erts 90 ES Configuration 12 o9 da GA oa BCE EC pio RI SCR el be ORC E 92 E3 Utilities MAs cde tec ter n POR oret erar sn ad ear ad UR rie dio Cds 92 E 3 1 Dependencies ooa aaa eR e 92 pi O 92 NA Geen Rus a O E E RN wah Se ees 93 E34 cmakebed liye sh pessoa as er pery eA eek ee A eG 93 F Protocol Specifications 94 F 1 UDP G code Transmission Protocol 4 ond wes REESE SESS 94 F 1 1 Sender Packets St A A UE TAS ee EMEN 94 F 1 2 Acknowledges ta X xd ir Se ed ty end E scu 94 Bsp R
47. e print to fail Figure 5 5 shows a print where the spiral was printed too loosely and it did not adhere to itself These prints were repeated varying the platform temperature Z axis position height and the tightness of the spiral until the tests performed as described above These tests ensure that the printer is capable of operating all its major components coherently in order to produce printed object To test the system with G code generated by Skeinforge a simple 3D model of a cuboid Figure 5 7 A was printed This print yields a cuboid of known dimensions and used to check calibration settings for Skeinforge and assess the printer s performance 52 inch mm mT mt MMA Nn ve M 90 100 MO 120 130 440 mm D URATOOL OFF ON a Figure 5 6 Digital callipers being used to measure test cuboids Figure 5 7 Test cuboid model A print from Skeinforge G code before raft removal B and after raft removal C The cuboid was initially printed on top of a thick raft of plastic used to ensure an even printing surface and then separated using a chisel The dimensions of the cuboids were checked using a pair of digital callipers figure 5 6 to ensure that the printed object is of the correct size The Makerbot wiki claims that 0 1mm resolution is possible on a correctly tuned machine and this requirement was met by most of the printed cuboids Ind12b In a small number of cases the Z axis of the printer did not
48. eading that the device is unsafe to touch The error at high temperatures may be a result of not being able to use the thermometer to measure the internal parts of the extruder where it is hottest 5 5 PID Control The PID controller has three constants Kp K and Ka which must be manually tuned to yield sensible system performance An optimal system has oscillations in temperature that are as small as possible PID controller tuning is a non trivial problem for which automated solutions are either highly specialised or unavailable Heuristics exist such as The Ziegler Nichols method for selecting good values which work in many cases and require human interpretation ZN42 48 Constant Extruder Value Platform Value K 5 143 7 0 K 0 0612 0 342 Ka 108 0 36 0 Table 5 1 Generic PID controller constants for a Makerbot Ind12c The Makerbot wiki suggests values given in table 5 1 which yield adequate perfor mance on many printers Ind12c After selecting these values the printer s performance was monitored both while idle and during printing and the size of the oscillations were measured Performance was similar in both states with the exception of a temperature jump in platform temperature at the start of printing caused by molten plastic being ex truded on top of the thermistor Oscillations were 2 C for the extruder and platform Due to time constraints further improvements in tuning could not be achieved Though this is gr
49. eater than the 1 C recommended print quality was not adversely affected 5 6 Stepper Control The amount of plastic deposited at a given point during a print is dependent on the rate at which the plastic is extruded and also the rate at which the platform moves The accuracy of the timing combined with the mechanical properties of the machine determines the platform s rate of movement and thus the print quality The stepper control system consists of code for producing accurately timed steps and code for coherently moving the stepper motors These two parts were tested separately as described in the following subsections 5 6 1 Timing To ensure timing accuracy the three stepper signals were driven at a combination of frequencies with a frequency probe attached to the step pin These tests were generated initially using a program on the microcontroller so that the system was not under any load and then using G code sent over the network interface while other requests were being made to place the system under reasonable load The frequencies measured were exact to within the accuracy of the probe four sig nificant figures for all tests 5 6 2 Stepping To test that steps happened coherently with sequences of steps correctly spaced apart the system was connected to the printer and circles were plotted The circles are made up of short continuous line segments where the relationship between the movements on each axis varies Once
50. em as it provided a reliable multi process environment It also didn t place any restrictions on development allowing straightforward code for interacting with the low level features of the Mbed LIP though the best option within the constraints of the project due to its simplicity wasn t especially well suited to the task The flow control problem greatly diminished the utility of the stack by forcing the development of a custom protocol As well as this the API it enforces requiring protothreads and protosockets is likely to make future expansion of the network interface difficult due to its heavy restrictions in the name of unneeded performance savings Future changes to overcome these problems are discussed in 86 2 1 The stepper and heater control systems developed though relatively naive proved performant and improved on the old system Possible future improvements are described in 86 2 4 Overall the improvements made to the microcontroller and firmware were successful and have had a positive effect on the performance and future expandability of the printer 6 1 3 End stops Though susceptible to glitches under direct lighting the end stops developed functioned correctly and allowed automatic calibration of the axis positions as required They also obey the standard interface used by other Makerbot end stops meaning future modifica tions may make use of standard designs The need to build a control circuit rather than using pre
51. er 0 and a zero window is returned indicating a terminal error F 2 Status Monitoring Protocol To monitor the printer s status newline terminated commands are sent to the printer over a TCP connection Responses consist of a series of integers separated by spaces and terminated with a newline Table F 2 lists the commands and their responses The Integer x100 format is used for decimal values as printing floating point values are not supported by the standard libraries used The floating point value is multiplied by 100 and truncated to an integer 95 Command Response Unit Format tmp Extruder temperature C Integer x100 Extruder set point C Integer x100 Extruder heater on Boolean Platform temperature C Integer x100 Platform set point C Integer x100 Platform heater on Boolean gcd G code instructions interpreted Integer Last interpreter error number Integer buf G code buffer utilisation Bytes Integer G code buffer size Bytes Integer Command buffer utilisation Commands Integer Command buffer size Commands Integer Command buffer overrun count this print Integer pow PSU Power On Boolean pos X axis Position Millimetres Integer x100 Y axis Position Millimetres Integer x100 Z axis Position Millimetres Integer x100 dbg Debug Register Undefined Undefined Table F 2 Status interface commands and responses 96
52. es with the motherboard via a custom RS485 interface Relay Board This circuit board contains a mechanical relay for turning each of the two heaters on and off The relay board is driven by the extruder controller using simple digital signals Stepper Motor Driver x3 Circuit boards which produce the high power signals re quired to drive the stepper motors These boards are connected to the motherboard via a simple digital interface that abstracts away many of the electrical and timing difficulties driving a stepper motor 2 1 3 Microcontrollers The electronics use a pair of Arduino compatible microcontrollers to drive the printer These devices have a fairly minimal 8 bit instruction set limited amounts of memory 4KB of RAM and a very limited set of options for high speed communications A faster microcontroller capable of faster communication and command processing is needed to solve the performance problems described in 1 3 2 1 4 Firmware The firmware on the microcontrollers is responsible for two main tasks receiving print data from a computer and producing the signals required for printing The signals pro duced have strict real time timing requirements and so to meet these specialised timing hardware within the microcontroller must be used 3D printers typically receive print data in the form of G code files Rep12b G code is the de facto standard for controlling computer numerical control CNC machines such as 3D pri
53. fied bugs early In this chapter the tests carried out during development are described along with an evaluation of the results Each section describes the tests a major component of the sys tem was subjected to concluding with an evaluation of the system s overall performance 5 1 Electronics The major components of the electronics were prototyped and tested on a breadboard with the use of a multimeter The higher power components such as the heaters and motors were initially disconnected or tested with low power test loads until the circuit was deemed to be correct Once connected these components were then tested under operational loads with careful supervision to ensure that they functioned correctly and that the current flowing through each part of the circuit was as expected The tested circuit designs were then soldered together on circuit boards where the connections were first tested for continuity and checked for short circuits before performing integration tests on the system Overall the electronics performed well and no problems caused by electrical noise generated when switching high power loads were observed The parts of the system which were found to exhibit unexpected behaviour are outlined in the following subsections 5 1 1 MOSFETs The MOSFETs were able to switch on the heaters and bring the system from room temperature to operating temperature within 10 minutes matching the performance of the previous system as expecte
54. h tight resource constraints and without the need for extra hardware such as a Memory management unit MMU 2 3 2 FreeRTOS FreeRTOS is a widely used open source RTOS designed for use with a range of microcon trollers including many Cortex M3 based devices fre12 The two key features provided by FreeRTOS are tasks and queues which are described below Tasks A system built on FreeRTOS can be structured as several tasks executing in parallel Tasks are similar to processes or threads on a conventional operating system with each task having its own set of registers and a stack Because the microcontroller can only run one task at once the FreeRTOS uses pre emptive scheduling to approximate this behaviour where the current task is periodically interrupted by a timer preempted and a different task put in its place If tasks are switched fast enough they appear to run simultaneously Tasks may be given different priorities and can be suspended until events such as a timer expiring or a hardware resource becoming available occur The timing of the operating system s actions can be guaranteed allowing real time systems to be developed Queues amp Mutexes To provide synchronisation and communication between tasks FreeRTOS provides a queue structure Queues are defined which allow data to be inserted or removed with a first in first out FIFO access scheme These queues can be used safely by multiple tasks simultaneously w
55. he receiver is able to receive uIP reports a constant window size until the method uip_stop is called when the window size is set to zero and any further packets received are discarded The zero window message is resent until uip restart is called and the old window size is restored A packet is then sent to the computer to 46 20000 T gt E go E 10000 E d fan 0 120 140 160 180 200 220 Time s Figure 5 2 Exponential back off by computer when uIP flow control used request data transfer to continue To test this mechanism a program was written which produced a long sequence of small movement G code instructions This causes the buffer to initially fill up and then be constantly kept topped up by the computer while being restrained by flow control The actual behaviour of the connection shown in figure 5 2 shows the buffer is initially filled the first spike but then the sender waits an exponentially growing period until it starts filling the buffer again regardless of when the window size is made non zero By inspecting the packets sent and received the window size is always obeyed by the computer but the packet announcing the window size becoming non zero appears to be ignored as transmission is not immediately resumed Wireshark s protocol checker did not report any errors and comparison with the known working implementation of TCP flow control in Linux did not reveal any obvious differences Unfortunately furt
56. her study did not yield a diagnosis for the problem Due to the time constraints imposed by the project TCP had to be dropped for G code transmission in the project 5 4 Temperature Readings The temperature readings depended on correct values being read from the analog inputs and on correct calculation of the temperature based on these readings Consistency of readings is important reading temperatures close to the actual value however is relatively unimportant This is because the temperature is fairly uneven within the heated components of the printer and so it is difficult to define a correct reading The actual temperature values used during printing are calibrated manually and so the absolute temperature in C is not significant To test analog input a selection of resistors with known values within the range of values the thermistor could exhibit were tested As well as breadboard testing the test was repeated on the final circuit board as the screw terminal used to connect the thermistors could also be used to connect a test resistor directly When converted to a resistance using 4 3 the values read were found to be within 2 of the resistor value as read by a multimeter a difference which is accounted for by the fact that a second resistor with a 5 tolerance is used in the potential divider To test that temperatures were being correctly calculated an infra red thermometer figure 5 3 was used to take reference temperatu
57. his assumption does not hold if the motor is unable to provide enough torque turning power to move its load As a result the motors used by the printer are designed to be powerful enough to reliably provide the torque required so that sensors are not needed to judge the position of the system Extruder The extruder uses a simple DC motor and gearbox to force a filament of acrylonitrile butadiene styrene ABS plastic the material from which Lego is made into a heater and out of a fine nozzle figure 2 2 A temperature sensor is installed in the heater allowing the temperature to be carefully controlled to ensure an even flow Platform The platform contains a heated build surface which helps prevent prints warping and this also improves the adhesion of the print to the build surface 15 The build surface is also a conveyor belt powered by a small DC motor and gearbox It is used to eject printed objects after printing completes allowing continuous printing operations 2 1 2 Electronics The Makerbot is controlled by the RepRap s generation 3 electronics Rep12c These consist of Motherboard This circuit board hosts an 8 bit microcontroller which communicates with a computer via a custom serial interface and controls the printer s operation Extruder Controller This circuit board hosts a second small microcontroller along with electronics for the extruder s motor and temperature sensor The extruder controller communicat
58. hresh old value used to decide whether a heater is on or off bang bang control Bang bang control was used as the previous electronics proved this method to be adequate The PID control loop is executed in its own task twice a second when each temperature is read and the heaters switched on and off as appropriate Executing the loop more frequently would not be useful because of the slow rate of change in the system Doing so would also be costly due to floating point calculations being carried out in software 4 2 3 Stepper Control To drive the stepper controllers accurately timed pulses must be produced to cause the stepper motors to move at the correct speed These pulses also need to be coherent between motors so that the three axes can move simultaneously to plot straight paths Principle of Operation Before each movement begins a periodic timer is set based on the frequency at which steps must occur and is used to toggle the step signal This is used instead of Bresenham s line algorithm to simplify implementation Bre65 Since expensive floating point calculations are required for value conversion into machine units the additional calculation introduced is not significant During each timer tick only cheap integer operations are required Pulse width modulation PWM is a method of approximating analog outputs by rapidly switching a signal on and off with a varying duty cycle This is cheap to implement in hardware and avoi
59. ic homing instructions The makebed py utility is used to stream a G code file to the printer makebed py send cube gcode This command will block until all print data is sent to the printer Complete documentation and safety advice for using makebed py to send files to the printer is provided in appendix E 3 2 A 5 Printing Process The G code generated by Skeinforge profile created in this project goes through several phases during a print Each of these is described in the following subsections A 5 1 Warm Up amp Self Clean The printer first enables the platform and extruder heaters setting the temperatures to 120 C and 225 C respectively Next it moves the extruder to the heating position to the left of the platform and in front of the rubber cleaning peg The printer then stays in this position until both heaters are up to temperature taking around 10 minutes Some plastic may ooze from the extruder during heating leaving an unknown quantity of plastic in the extruder s heating chamber Before printing the extruder is refilled with plastic and any excess wiped off against the cleaning peg figure A 5 A 5 2 Printing The First Layer Once everything has heated up and the extruder is free of excess plastic a rectangle is drawn on the platform which surrounds the area the object will be printed in This allows the operator to check that the print will fit on the platform and to allow manual See appendix D 3 5 for an examp
60. ical components This work is the focus of many hobbyists and organisations with improvements in further generations of the Makerbot design building on these ideas Porting promising ideas from other printers could provide valuable improvements in performance complementing the improved control of the printer achieved in this project 6 3 Final Conclusions The limitations of the network interface are probably the most significant but with some modest restrictions are not fatal Future work in this area could address these limitations and explore possibilities for network printing with 3D printers Overall the project has been a success with the performance and extensibility of the printer being improved As shown in appendix A the printer is usable and follows essentially the same usage pattern as other 3D printers 60 References A1109 ARMI2 Ben93 Bre65 Can12 CW12 Dun06 Dun12 fre12 git12 Inc12 Ind12a Ind12b Ind12c Allegro MicroSystems Inc A3982 Datasheet 11 23 09 edition 2009 ARM Ltd Cortex Microcontroller Software Interface Standard CMSIS 3 01 edition March 2012 Stuart Bennett A history of control engineering 1930 1955 IET 1993 p p 48 ISBN 978 0 86341 299 8 Jack E Bresenham Algorithm for computer control of a digital plotter IBM Systems Journal 4 1 2530 January 1965 CandyFab Candyfab wiki http wiki candyfab org 2012 Accessed 14 April 2012
61. iles E 2 Firmware This section describes the compilation and configuration process for the Mbed firmware E 2 1 Dependencies The tools libraries required to build the firmware are listed in table E 2 E 2 2 Compilation The following steps can be used to build the Mbed firmware 1 Make sure the dependencies mentioned above are installed 2 Ensure that arm none eabi gcc is in your path 3 In the Makefile set RTOS SOURCE DIR to the Source directory of your FreeRTOS distribution 4 In the Makefile set the path of the Mbed mount point on your computer in the install all make rule 5 Run make to build the firmware 6 make install will build as above and also copy the firmware to the Mbed Reset the Mbed to start the newly installed firmware 90 Filename Description core_cm3 h cr_startup_lpc17 c FreeRTOSConfig h freertos_hooks c h LPC17xx h main c MakebedConfig h makedefs Makefile mbed_boot c h rtosdemo_rdb1768_Debug ld system_LPC17xx h analog analog_in c h float_parsing strtod c h GPIO gpio c h makerbot makerbot c h network emac h network EthDev h network network c h network network uip state h network network debug c h network network_gcode c h pid pid c h stepper stepper c h thermistor thermistor c h util makebed py util makebed_live sh util gsend py util debug py util gcode gcode watchdog watchdog c h ARM CMSIS header for Cortex M3 Defines interru
62. ins 3 2 2 Heater amp DC Motor Control The heaters and motors both require large amounts of current at 12 volts to run This far exceeds the output capabilities of the GPIO pins on the Mbed so a circuit will be needed to switch the power for these devices Previously mechanical relays were used to control the heaters but instead a solid state solution should be sought to allow the possibility of varying heater power 3 2 3 End stops By adding end stop sensors on each axis the axes can be accurately and consistently positioned at the start of a print job Electronics compatible with the interface exposed by the stepper controller board will be required Optical end stops have been selected as the Makerbot has pre drilled mounting holes at the end of each axis for mounting them Optical end stops are also non contact and so do not disrupt the movement of the axes 23 Chapter 4 Implementation This chapter describes the implementation of the system in detail First the electronics produced are described and followed by an explanation of the firmware which drives them Finally safety considerations and a brief discussion of the development methodology used is presented 4 1 Electronics Two circuits were produced one which hosts the Mbed and the electronics needed to drive the heaters motors and temperature sensors and another which provides an interface between the end stops and stepper controller boards The main board hosts
63. into relative movements measured in stepper motor steps and speeds converted into step periods measured in timer ticks These low level commands are then added to the command buffer to be executed by the printer controller 4 2 5 Printer Controller The printer controller uses a FreeRTOS queue in a high priority task to buffer low level commands from the G code interpreter The controller consists of a simple loop which executes each command requiring minimal processing As a result this task spends most of its time blocked waiting for actions to complete but responds quickly when required 4 2 6 Network Interface The network interface consists of two services built on the uIP stack a G code transmis sion interface and a status monitoring interface uIP provides a simple API for sending and receiving data over TCP and UDP within applications built within its protosocket and protothread frameworks Dun06 Protothreads are extremely lightweight threads using cooperative multitasking imple mented in pure C These threads do not preserve registers or variables during blocking phases of execution and use minimal system resources Protosockets are a simplified UNIX style socket interface built on protothreads These libraries are designed with ex tremely low power microcontrollers in mind As a result only a minimal application is implemented within the uIP framework which passes data from the network immediately to the other parts of the system
64. ital value read by the Mbed and the state of the debugging LED Under all but the direct lighting conditions the correct value was read and the debugging LEDs were either completely off or completely on Under direct lighting especially incandescent lighting the state read by the Mbed for some end stops became stuck due to external light shining into the opto interrupters In these cases the debugging LEDs did not become completely off indicating that the opto interrupter was being only partially triggered Though these results suggest that the end stops cannot be used under direct lighting from halogen or incandescent bulbs the system was found to perform well outside these conditions Although not tested due to unfavourable weather conditions direct natural light may also have caused similar problems While these restrictions are unfortunate they are not unreasonable and still allow the system to be used in practice 5 8 Buffer Utilisation To ensure that the printer pipeline did not stall during print jobs the G code and low level command buffers were monitored during the execution of various test jobs If buffer 50 underruns occurred or poor buffer utilisation was observed it could indicate a perfor mance issue in the G code interpreter network interface or their interaction with the operating system The utilisation of each buffer along with a counter for the number of underruns ex perienced by the command buffer were logged
65. ith the central Unix printing system CUPS to provide a more polished user experience Network Stack Due to the limitations and bugs in uIP an alternative stack such as lwIP could be used Dun12 This would provide a higher level FreeRTOS integrated network interface and with its better tested flow control features would enable TCP to be used for G code transmission This improved interface could also allow changes to the network interface to be significantly cleaner to implement Web Interface The Mbed is powerful enough to generate and host dynamic web pages as demonstrated by the FreeRTOS web server demo A web application for control and monitoring of the printer via a web browser could make the printer significantly easier to use by not requiring specialist software 6 2 2 Endstop Support To eliminate problems caused by lighting a more advanced system could be implemented The infra red LEDs in the opto interrupters could be rewired such that they are pulsed the photo transistor s signal is then checked by the microcontroller for the presence of these pulses External light sources are unlikely to contain matching pulses and so the system can be sure of the origin of the light passing into the photo transistor As well as this the end stops could be used for safety to detect when the axes have moved outside their operating area unexpectedly This could help prevent damage to the printer and ensure safer operation when G code w
66. ith unreachable coordinates are used or mechanical failures occur 6 2 3 G Code Support The subset of G code supported by the printer is limited to that produced by Skeinforge To allow other tools to be used for G code generation and also to allow more features of the printer to be exposed the G code support could be extended 6 2 4 Firmware Improvements Two major improvements could be made to the firmware to better account for the me chanical properties of the printer These are outlined below 59 Complex stepper control systems take into account the properties of stepper motors and do not step at a constant rate Instead the speed is increased and decreased grad ually making use of the increased torque available at low speeds during acceleration and deceleration Such behaviours unfortunately also require more precise control of the ex truder as the amount of plastic required throughout each line segment would vary with the speed of movement The heaters could potentially respond more appropriately if variable amounts of power could be supplied rather than just on and off This requires PWM support to be added and changes made to the PID controller PID controllers for variable power systems often require complex additions to function correctly and require careful set up 6 2 5 Mechanical Improvements As well as the aspects focused on in this project many improvements could be made by modifying the printer s mechan
67. ithout race con ditions and so are ideal for inter task communication Because of this safety they also form the basis for standard parallel programming constructions such as semaphores and mutexes When accessing a FreeRTOS queue a task may become blocked for example when adding an item to a queue that is already full FreeRTOS can provide timing guarantees on timeouts waiting for these functions to complete It also allows a task s priority to be temporarily raised when resumed after a blocking call ensuring it is scheduled as soon as possible reducing the delay before any new data is processed 19 Chapter 3 Design The 3D Printing system can be divided up into three main parts as shown in figure 3 1 These major components and their requirements and design are described in the following sections This project is principally focused on the development of the firmware and electronics used by the printer In particular the process of generating 3D models and G code as well as mechanical operation is out of the scope of the project The printer hardware and off the shelf open source tools provided by the Makerbot project will be used for these purposes 3 1 Firmware The microcontroller firmware will consist of three main components running on top of the FreeRTOS operating system The first will be the uIP network stack for communication with the computer software The second is a G code processing pipeline which will translate G
68. le of such an automatic homing G code file 67 Figure A 5 Extruder extruding plastic during a self clean Figure A 6 First layer being printed adjustments to the height of the extruder to ensure that it is at the correct distance from the bed It is critical that the distance between the extruder and platform is correct during the printing of the first layer If it is too far the plastic will not adhere to the platform and will not accurately produce the shape required If it is too close the plastic printed will not fit under the extruder causing the printed object to get caught on the extruder as it moves around during the print Adjustments should be made by turning the adjustment handle on the Z axis during the printing of the border The border will be discarded after printing and so mistakes can be corrected while it is produced Once the border is drawn the extruder prints out the first layer of the object figure A 6 This is done at a slower speed than other layers allowing more plastic to be deposited producing thicker lines helping the print adhere better to the platform A 5 3 Main Printing Phase During this phase the model is printed layer by layer Each layer is usually printed first by drawing three solid outer shells and then filling in the centre Layers near the top and bottom of the model are filled completely to produce a solid finish Layers inside the model are filled with a hexagonal fill pattern
69. lities are provided for interacting with the printer over the network Their dependen cies and use are described below They should be executed from the util directory The utilities require that the printer s IP address is specified in makebedrc E 3 1 Dependencies The utilities depend on the programs mentioned in table E 3 E 3 2 Send Files can be sent to the printer using the command makebed py send which accepts G code on its standard input or from a file specified as an argument The program will block until the entire file has been received by the printer If the printer is suffering buffer underruns the rate at which the utility polls the printer can be increased with the p option which takes a number of seconds defaulting to 0 1 to wait between poll requests This should not need changing for most prints and can result in a large amount of network traffic being generated if set very low Safety If the program is terminated while sending a G code file the printer will keep running after executing the last intact G code instruction received During a typical print this could leave the printer stationary and extruding plastic It can also potentially result in damage or unsafe behaviour As a precaution it is recommended that the printer is reset using the reset button if the utility is stopped prematurely 92 E 3 3 Get The command makebed py get allows printer status information to be fetched The command expects
70. ll be added to allow the system to position itself 1 4 Report Outline This report first discusses the background of the project covering 3D printing and the technologies selected for the project In chapter 3 the design of system is proposed fol lowed by details of the implementation in chapter 4 Chapter 5 describes how the system was tested and evaluates the new system s performance Finally chapter 6 concludes the report and describes opportunities for future work following on from the project 13 Chapter 2 Background In this chapter the background of the project is described The principle components of the Makerbot are discussed in more detail followed by a description of the tasks car ried out by the firmware required to drive them Afterwards the ARM based Mbed microcontroller used in this project is introduced along with the FreeRTOS operating system 2 1 Makerbot The Makerbot Cupcake CNC used in this project is the first generation of a series of printers based on the RepRap 3D printer Ind12a The RepRap and consequently the Makerbot are open designs which are freely available for use and modification In this section the primary components of the printer are described followed by the electronics firmware and microcontroller that drives them 2 1 1 Printer Components The printer can be broken down into three major components the axes along which the machine s components can move the extruder which
71. m location block for an excessive period or otherwise become corrupted The heater controller loop executes at 2Hz and the WDT is fed at the end of each iteration of the control loop The WDT is set with a timeout of two seconds meaning that it should never come close to a timeout except in the event of a control loop has failure or the system resources have somehow become saturated After the WDT has reset the system it enters a safe state where the heaters do not have power and are allowed to cool safely 4 5 Methodology amp Tools In this section the methodologies used for the implementation of the system are discussed followed by the choice of languages and development tools and why they were selected Finally the methods used to debug the firmware running on the Mbed are described 4 5 1 Methodology Development followed an iterative bottom up process where components were built tested and then integrated into the system as a whole Prototyping was used heavily in the development of the electronics The major parts of the circuit were initially prototyped on a solderless breadboard and powered by a bench power supply figure 4 21 In these prototypes different components could easily be swapped in and out of the design and measurements easily taken with the system running at different voltages The UDP protocol was also initially prototyped using a high level language Python allowing quick development and testing Some parts of the
72. melts the plastic and the platform itself on which the design forms Each of these are described below Axes of Movement There are three axes of movement in the Makerbot Two horizontal axes along which the platform can travel the X and Y axes and one vertical axis along which the extruder is moved the Z axis The X and Y axes move along rails and are belt driven by a stepper motor The Z axis moves up and down four threaded rods which are connected together via a belt and driven by a single stepper motor When the threaded rods turn the extruder is moved by the screwing effect of the rods Stepper motors allow precise movements to be made In contrast with simple DC mo tors which turn electrical energy into continuous movement stepper motors turn energy into discrete steps of movement 14 Figure 2 1 Stepper motor operation Filament DC Motor Heater Nozzle Extrusion Figure 2 2 Extruder components A simple stepper motor consists of four toothed electromagnets and a toothed mag netic central rotor By turning on each electromagnet in sequence the teeth of the rotor are moved to align with the energised electromagnet causing a single step of movement to be executed an example is given in figure 2 1 Inc12 By controlling these steps the motor s rotation and speed can be exactly controlled Stepper motors lack active feedback mechanisms and rely solely on the motor successfully completing every step T
73. move the required amount due to the mechanism jamming a known problem with the Makerbot design Jet12 As a result these prints were of the incorrect height but were otherwise correct and the problems not attributed to the firmware 5 9 2 Test Objects To test the printer s ability to produce useful objects various objects were printed in cluding objects with moving parts or near the print size limits of the printer These tests check the system s ability to deal with large and complex loads A selection of printed test objects is provided in Appendix B 53 Figure 5 8 3D printed vase with detailed corners A Figure 5 9 3D printed Z axis handle comparison of old A and new B electronics Detailed Prints Prints with detailed areas were used to test that the system could process the larger density of G code at the required rate and also to ensure that steps were not missed during printing For example figure 5 8 shows a vase which yields very short line segments while printing the corners of the shape The previous electronics would not be able to process this design fast enough and would skip instructions causing steps to be missed With the new electronics no buffer underruns or printing problems occurred during a run featuring a large version shown and a smaller version containing finer detail Figure 5 9 shows an object which has a small cylindrical handle printed with the old and new electronics With the old sy
74. n data to error checking As well as various language features the actions available and their precise behaviours differ To keep the system as simple and fast as possible only features and actions required to support the output of Skeinforge s G code generator were implemented The resulting language simply supports comments and writing to registers The syntax supported is given in Backus Naur Form BNF in appendix D 1 1 Actions and their treatment of registers were also selected based on the output of Skeinforge and include e Unit selection e Calibration e Movement 36 e Motor control e Power control e Temperature control e Delays A complete description of the actions supported is given in appendix D 2 Parsing The G code subset supported is very simple and so a small parser was implemented by hand rather than using a standard parser generator such as GNU Bison Such tools also generate code that is not optimised for running on a microcontroller in a real time environment and would have been time consuming to learn A simple state machine was built which parses and executes the G code setting and reading registers as described in 2 1 5 When unexpected characters or register values are encountered a flag is set and the parser continues from the next character or instruction Once each instruction is parsed and the register values set the values are converted into machine units For example movements are converted
75. nd to test steep gradients ooo a a a eae 73 B 8 Tooth to test large models with large overhanging areas 73 B 9 Multiple metabricks to test building batches of objects 2 74 B 10 Twistable heart to test simple mechanisms and multi part objects 74 B 11 Whistle with pea printed inside to test precise air tight objects with simultaneously printed sub components o 74 B 12 Tweezers to test flexible designs frequently used to remove excess extru C 1 C 2 sion produced during self cleaning o o o 75 Main board circuit diagram direct connections to stepper controllers omit O 78 End stop circuit schematic ita ddr Dal a a SS 80 List of Tables 4 1 4 2 9 1 C 1 G2 C 3 DI E 1 E 2 E 3 F 1 F 2 20 pin ATX Connector Signals Int04 a scu ex aie a RR xg 30 Stepper timing constraints 2 4 3 8 3x 09s 9 Se wR E ROS eA 34 Generic PID controller constants for a Makerbot Indl2c 49 Mbed pin connections Sds eu x SER Ge ES al eee IHR 77 End stop interface cat 5 wire allocations Repl2d 80 Opto interrupter connection colour codes 04 80 G code register types and behaviours lll len 82 Key source file listing dox qe bati Ew RA Perd e PR OG 91 Firmware software dependencies oo o a a a 500 ae 91 Utility dependencies ele ase Quis dr po d RI er re e ean reb eun 92 Network interface ports ar excea z
76. ndix contains the documentation required for the circuits produced within the project C 1 Control Electronics The pinout for the Mbed table C 1 and schematic C 1 for the main board produced in the project are given below The connections to stepper motors have been omitted for clarity but the associated Mbed pins labelled 76 Pin Name Definition Connection 1 GND PSU Ground 2 VIN PSU 5V Standby 3 VB Real time Clock Battery Unused 4 nR Reset switch 5 5 PIN POWER OK PSU Power OK 6 6 PIN ENDSTOP O MIN X axis Min End stop 7 7 PIN_ENDSTOP_O_MAX X axis Max End stop 8 8 PIN_STEPPER_O_STEP X axis Stepper Step 9 9 PIN_STEPPER_O_DIR X axis Stepper Direction 10 10 PIN STEPPER O NEN X axis Stepper nEnable 11 11 PIN ENDSTOP 1 MIN Y axis Min End stop 12 12 PIN ENDSTOP 1 MAX Y axis Max End stop 13 13 PIN_STEPPER_1_STEP Y axis Stepper Step 14 14 PIN_STEPPER_1_DIR Y axis Stepper Direction 15 15 PIN_STEPPER_1_NEN Y axis Stepper nEnable 16 16 PIN_ENDSTOP_2_MIN Z axis Min End stop 17 17 PIN_ENDSTOP_2_MAX Z axis Max End stop 18 18 Unused 19 19 PIN_THERMISTOR_PLATFORM Platform Thermistor 20 20 PIN_THERMISTOR_EXTRUDER Extruder Thermistor 21 21 PIN_STEPPER_2_STEP Z axis Stepper Step 22 22 PIN STEPPER 2 DIR Z axis Stepper Direction 23 23 PIN_STEPPER_2_NEN Z axis Stepper nEnable 24 24 PIN_POWER_EN PSU Power On 25 25 Unused 26 26 Unused 27 27 PIN_HEATER_EXTRUDER Extruder Heater 28 28 PIN_HEATER_PLATFORM Platform Heater 29 29
77. ned on unless it has heated up enough to melt the incoming filament Argument Description Unit No arguments M104 Set extruder temperature Set the target temperature of the extruder This action does not block To wait for heating to complete use M6 Argument Description Unit S Target temperature C M106 amp M107 Platform conveyor on off Turn the platform conveyor belt on or off respectively Argument Description Unit No arguments M108 Set extruder speed Set the speed at which the extruder motor turns Not supported by the system Argument Description Unit No arguments M109 Set platform temperature Set the target temperature of the platform This action does not block To wait for heating to complete use M6 Argument Description Unit S Target temperature C 85 D 3 Examples The following examples of G code files show how it can be used for various useful tasks D 3 1 Power On Heat Up Heats the system up to a temperature suitable for printing Can be used to prepare the printer before starting a print Turn on the PSU M 1 Set the target temperature for the extruder to 225 c M104 8225 Set the target temperature for the platform to 120 c M109 8120 D 3 2 Power down Powers down all components and then the PSU When the PSU is turned back on the heaters and motors will still remain off Turn off extruder M104 SO Heater set target to O c M103 Motor Turn off
78. ng synthetic benchmarks as well as printing real world objects These tests also aided in calibration of the G code generator Skeinforge The synthetic tests allow the printer s performance to be objectively measured while real objects show the real world performance of the printer in its intended application 5 9 1 Synthetic Tests As a simple test of using all of the printer s components coherently the circle plotting test was modified to produce spirals and the heaters and extruder enabled Figure 5 4 shows samples of the output of these tests A The extruder was moved at a safe distance from the platform to ensure that all components move coherently but without the risk of the extruder colliding with the platform or blocking the nozzle of the extruder 5l B C D Figure 5 4 Synthetic 3D printer tests for basic calibration Figure 5 5 Bad print of the test in figure 5 4 D B As in A but the extruder is moved closer to the platform to test that the system can safely operate close to the platform and that the plastic adheres properly and then is properly detached when ejected C A larger spiral was printed to test that the plastic adheres closer to the colder edges of the platform and that warping due to temperature changes during the print does not cause problems D The winding of the spiral was tightened to test that the plastic adheres to itself and the platform and that warping does not cause th
79. nters laser cutters and lathes The language is human readable and defines step by step instructions for machine actions such as move to X Y Z or enable heater The RepRap generation 3 firmware on the motherboard uses a custom serial protocol to communicate with the host computer This protocol is designed to be simple for the microcontroller to use and as well as various diagnostic features contains a compressed version of G code 16 2 1 5 G Code G code is assumed to execute on an abstract G code machine The machine consists of 26 numeric registers named A to Z A G code instruction consists of a set of register assignments after which the machine executes a specific action based on the contents of its registers Some registers may be reset to an undefined state before each instruction allowing the machine to identify when a register is written For example the G and M registers are used to specify what type of action should occur and if not cleared it would be impossible to determine which register contains the action required Each register accepts either a floating point or integer value for example the G and M registers only accept integer action numbers while X Y and Z are floating point and accept coordinates For example when the instruction G1 X10 Y 15 Z0 3 F3000 is encountered the fol lowing register values are set F 3000 G 1 X 10 Y 1
80. o Printer UDP Header Sequence No Payload G code Figure 4 18 UDP G code Sender Datagram Format Status Interface A simple Telnet compatible TCP interface listening on port 2777 was implemented that can be used to request the contents of various status and debugging variables As flow control was not necessary the TCP implementation in uIP is adequate for this purpose The interface listens for keywords separated by white space and responds with tab separated data compatible with GNU Plot For example tmp 22523 22500 0 11800 12000 1 The command tmp requests current temperature information The response contains the current temperature set point target and whether the heater is on or off for the extruder and the platform Temperature readings are given in degrees Celsius multiplied by 100 This is due to the version of LibC provided with the CodeSourcery tool chain not supporting printing of floating point values A utilities for using the status monitoring facility are described in the next section and full documentation for the protocol is given in Appendix F 2 4 3 Utilities Two utilities for interacting with the printer were written in Python The first a client implementing the UDP G code transmission protocol and the second a utility for con veniently monitoring the printer status These utilities are part of the makebed py com mand For example the following command will stream the G code contained in cube gco
81. oe nk ee Ex RO Kk P 94 Status interface commands and responses ls 96 Chapter 1 Introduction 3D printing is a manufacturing process where computer models of objects are automat ically reproduced in a physical form JE12 Figure 1 1 shows an example of a complex 3D printed object Various 3D printing technologies exist some of which have gained a number of hobbyist friendly implementations In this project a number of refinements to the popular Makerbot Cupcake CNC 3D printer were made to improve its performance In this chapter the applications of 3D printing are examined followed by an introduc tion to the Makerbot and the improvements made by this project 1 1 Applications 3D printing technologies allow complex objects including complete mechanisms to be easily manufactured based on digitized designs in one go with a single piece of equip ment Various materials are used including metal various plastics resins and even sugar Can12 Rapid prototyping is an obvious application where the flexibility to quickly manu facture a wide range of objects extremely valuable For example Boeing are using 3D printing to reduce the tooling cost and speed up prototyping its aeroplanes Sta12 Figure 1 1 3D printed Gear Cube produced by the printer upgraded in this project 10 amp wae E ss z m 50 Figure 1 2 Unmodified Makerbot Cupcake CNC photo by Rayshobby Pro12 Because ther
82. oject Raftless Printing Though the first prints were completed on top of a raft it was later disabled This reduced print time improved print quality and allowed intricate designs such as combs to be printed where removal of a raft would damage the design figure 5 12 Many objects were reprinted using raftless printing and performance was generally 95 Figure 5 12 Folding butterfly comb printed without a raft similar with the exception of larger designs These experienced greater warping without the support of the raft and thus were more prone to failure due to the extruder hitting the object 56 Chapter 6 Conclusions amp Future Work In this chapter the outcomes of the project are compared against the initial goals and future work is suggested based on lessons learnt and the limitations the project 6 1 Project Goals The three project goals have each been addressed within the project and in this section the degree to which they have been met is discussed 6 1 1 Electronics Replacement The new electronics functionally replace all the features of the old electronics and have proved reliable in testing With a single board the system is also a lot simpler than the previous three boards removing the need for a custom communications protocol and extra microcontroller Though not quite as polished as a printed circuit board PCB it offers the possibility of future expansion The solid state MOSFETs used for
83. ork E oca m idee puke Bile nike alt Rie diel Ree EE wey 6 2 1 Network Interface IA RR DR A AG eee ES 6 2 2 Endstop Support quor e exi m E 6 2 3 G Code Support 4 3cs E eres TOP Sx ro a Se Mex RE ders 6 2 4 Firmware Improvements V leida e A 6 2 5 Mechanical Improvements co ase ed uo ER ee TE 6 3 Final Conclusions C ec AA A c a ale hia RR dra xd References Example Printing Workflow AJ 3D Modelling OpenSCAD uz iuh RR UR RR RAT A 2 G Code Generation ReplicatorG amp Skeinforge A 3 Status Monitoring c AC S sce Sere SES a EXE AIRES IAS A 4 G Code Streaming 1o 4058 oh Se oo dee A RAE E d Avo Printing Process x ase ao SERS na qiie e ar EET aee die egi A 5 1 Warm Up amp Self Clean sa ma ss do exeo ee ew A 5 2 Printing The First Layer aa sede aa dn qx eK Yo A 5 3 Main Printing Phase aoe ap each Ea RS a nad qd A 5 4 Cool down Eject and Self Clean A0 Pinal JP EHE eee oy ecu Sates qr e Ye e atu Duet E gre ure x ue en Example Prints Belt Intricate Prints kon SR e M REP ei wegen mdr 1 2 Latge Prime aoid uon A te ote B 3 Functional Prints s Circuit Diagrams Cl Control Electronics 9 ua ko RR UR EE AE E RA a C 2 End stop Electronics E s tre Eden Weeks am epu oir gee Bie say esa CZ Witing Color Codes a cente ms vete aD OA G 2 2 Schematic aeree X o mete Pk wk E D duc DO ott A boe onn 2S G Code Reference W Languages EDIDI cc c XM MENU T
84. p Max Power On 24 CH m 18 Z Stepper NEN 23 EE E 8 19 Platform Thermistor Z Stepper Dir 22 RP lx s s O 20 Extruder Thermistor Z Stepper Step 21 29 9 q 3 EH gnd W l a 2 jo lo lu lo la ja jo w 3 gt 3 x v a las ker laike da The kese S ee e S io la l le la jz la is Iz o S PS ON 2 3 gnd 1 1 Figure C 1 Main board circuit diagram direct connections to stepper controllers omitted for clarity C 2 End stop Electronics The schematics and wiring colour codes for the end stop electronics are given in this section C 2 1 Wiring Colour Codes The colour codes for the connections from the end stop board are given in table C 2 and the connections to the opto interrupters are given in table C 3 C 2 2 Schematic The endstops are interfaced via the circuit in figure C 2 This circuit is duplicated once for each end stop on the end stop board 79 Colour Connection Green Solid Signal Green Striped Unused Blue Solid 5V Blue Striped 5V Orange Solid Unused Orange Striped Unused Brown Solid Ground Brown Striped Ground Table C 2 End stop interface cat 5 wire allocations Rep12d End stop LED LED Photo transistor Collector Photo transistor Emitter X axis Red Brown Orange Yellow Y axis min Red Brown White Black Y axis max Green Blue Orange Yellow Z axis Green Blue Purple Gray Table C 3 Opto interrupter connection colour codes
85. pt handler functions System amp networking parameters FreeRTOS callback handlers NXP CMSIS header for LPC17xx series Initialises system starts all tasks Configuration for 3D printing firmware Definitions used by the Makefile Makefile which builds the project Functions to boot up chip peripherals Linker script ARM CMSIS header for Cortex M3 Analog input driver Implementation of strtod General purpose input output driver Print manager scheduler FreeRTOS EMAC driver Ethernet driver Network interface uIP application State struct for uIP application Debugging Status interface G code transmission interface PID controller Stepper motor driver Thermistor library Communication utility Live printer status monitor G code UDP library Status interface library Example G code files Watchdog timer driver Table E 1 Key source file listing Package Version CodeSourcery G Lite 2011 03 42 FreeRTOS GNU Make 7 0 2 3 81 Table E 2 Firmware software dependencies 91 Package Version Python 2 6 Bash 4 1 GNUPlot 4 4 patchlevel 0 Table E 3 Utility dependencies E 2 3 Configuration All printer related parameters can be found fully documented in MakebedConfig h To set the system s IP net mask and MAC addresses the definitions configIP_ADDRO 3 configIP ADDRO 3 and configMAC_ADDRO 5 can be set as the contents of the dot file FreeRTOSConfig h DHCP is not supported E 3 Utilities Uti
86. re SEN Sexe ER oe Example MOSFET circuit with pull down resistor Potential divider 2 rro 41e tu tee eb Shea Se ed Bae Ye eS Photo interrupter with a photo transistor in a Darlington pair Photo interrupter endstop being interrupted by the X axis X ray of RJ45 sockets with and without integrated magnetics Lom12 Successive approximation analog to digital converter PID heater controller schematic o o Stepper control signal wave diagram a a e e a e a Nyquist Shannon sampling theorem example with high frequency signal Example of sampling artefacts 2 2 vue xwv doe 4 x RECO S Ra Mbed timer architecture i e 2495 4 ood bu a meo REY XE UDP G code Sender Datagram Format makebed live sh screen shot sa pags See eed E eee ES OE Emergency stop reset button 0 e 000502 eee Breadboard with a prototype end stop circuit Ping responses being duplicated by the uIP driver T 10 11 12 12 15 15 18 20 21 23 24 25 26 26 2 5 2 Exponential back off by computer when uIP flow control used 47 5 3 Checking platform temperatures using an infra red thermometer 48 5 4 Synthetic 3D printer tests for basic calibration 52 5 5 Bad print of the test in figure 54 D o o 52 5 6 Digital callipers being used to measure test cuboids 53 5 7 Test cuboi
87. re readings from the extruder and plat 47 Figure 5 3 Checking platform temperatures using an infra red thermometer form The heaters were turned on and readings were taken every minute as the extruder and platform heated up and then every five minutes for half an hour as it cooled down The tests were repeated multiple times alongside other experiments each time with sim ilar results to ensure consistency These values were then compared against the value calculated using 4 1 The radius of the area measured by the thermometer becomes larger as it is moved further away from the target Because the temperature across the platform and extruder vary greatly depending on location the thermometer was placed close to the centre of the extruder nozzle and the centre of the build platform where the thermistors reside The temperatures recorded for the extruder were within 1 C below 100 C but rose to around 8 1 C around 220 C normal operating temperature The platform temperatures were initially incorrect by 10 C or more This was caused by a simple connection error Once the problem was corrected readings followed a similar pattern to the extruder up to the 125 C the platform is designed to operate at The results above represent adequate performance for the task of maintaining a desired temperature and also show that the temperatures read are close to their real values such that an operator can safely tell from a temperature r
88. s Assumes that the Z axis is initially placed at the correct height to fit in the homing bracket Power on M 1 Use mm G21 Set the Z axis as we re not homing that G92 XO YO Z10 Lift the head out of its hole G1 Z15 F100 Home x 1 and y 2 at the same time 1 2 3 M 2 A3 88 Move to the calibration ring G1 X 56 Y 44 Z15 F3300 G1 Z10 F100 Power off MO D 3 6 Circle Plots a circle segmented into lines using the X and Y axes Assumes the extruder is hovering safely above the centre of the platform before moving Power on M 1 Use mm G21 Assume we re starting in the middle G92 XO YO ZO Move to the edge of the circle G1 X40 000000 YO 000000 F330 000000 Plot the circle Gi X32 360680 Y23 511410 F330 000000 Gi X12 360680 Y38 042261 F330 000000 Gi X 12 360680 Y38 042261 F330 000000 Gi X 32 360680 Y23 511410 F330 000000 Gi X 40 000000 YO 000000 F330 000000 G1 X 32 360680 Y 23 511410 F330 000000 G1 X 12 360680 Y 38 042261 F330 000000 G1 X12 360680 Y 38 042261 F330 000000 Gi X32 360680 Y 23 511410 F330 000000 Move to the center of the circle G1 XO YO F330 000000 Power off MO 89 Appendix E Code Documentation This appendix contains the documentation required to configure and build the microcon troller firmware It also contains the required documentation for using the communication utilities E 1 File Listing Table E 1 describes the purpose of each of the key project f
89. s consist of a photo interrupter containing an infra red LED and a photo transistor arranged across a gap Figure 4 9 Photons from the LED activate the photo transistor allowing current to flow but when the gap is blocked the transistor is switched off and no current flows Due to problems sourcing the interface boards for the end stops a circuit was built which is compatible with the stepper controller interface 5V and ground are provided and a TTL logic signal is expected by the interface To ease debugging an indicator LED was also added which is lit when the end stop is unobstructed The LED in the photo interrupter is driven via a current limiting resistor and the signal output and indicator LED are connected through the photo transistor A pull down resistor is used to pull the signal to ground when the photo transistor is powered off 29 Figure 4 10 Photo interrupter endstop being interrupted by the X axis The circuitry is placed on a board with cables running to each end stop shown mounted in figure 4 10 and to each of the CAT 5 sockets on the stepper controller boards Strain relief is included so that the connections are not damaged if the cables are caught in the machine A circuit diagram is provided in appendix C 2 2 4 1 6 Power The printer uses an ATX power supply unit PSU commonly found in desktop computers A 20 pin connector containing both power and various control signals for the power supply see table 4
90. s produced containing the printer data in the same directory and with the same name as the STL file A 3 Status Monitoring To monitor the printer during a print a utility called makebed_live sh is provided which shows heater temperatures extruder position and buffer usage It is helpful to have this open during a print to monitor the progress of the heating and cooling stages as well as to check for buffer underruns Figure 4 19 page 40 shows the utility in use during a print A 4 G Code Streaming Before any G code is sent to the printer the printer should me moved to its home position figure A 4 This can be done manually by hand or automatically by sending a 65 A Not Connected ReplicatorG 0027 Seg File Edit GCode Machine Thingiverse Help Move Object Center Put on platform Xx 10 xt Y 10 Y zs 10 Z Lock height Left drag to move object Right drag to rotate view p View Move Rotate Mirror Scale Generate GCode Figure A 2 ReplicatorG GUI showing a cube model loaded 4 Generating gcode o le xl Base Profile SF35 cupcake ABP raftless LS Y Use raft Use support material None v C Use Print O Matic stepper extruders only i Cancel GenerateGcode il Figure A 3 Skeinforge profile selection dialogue 66 e lo N Figure A 4 Extruder placed in the homing bracket G code file to the printer containing automat
91. scription is then given of the key phases of the printing process Once the print is complete the printed object is ready after a small amount of cleaning if required A simple cuboid is used as an example throughout as the process of designing complex 3D models is not the focus of this report The cube will measure 20mm wide 20mm deep and 10mm tall A 1 3D Modelling OpenSCAD There are many programs available for 3D modelling but in this example OpenSCAD is used Other tools may be used provided that they can export models in the widely used Standard Tessellation Language STL file format which is accepted by the G Code generation software shown in this example Alternatively ready made models in STL format can be downloaded from websites such as Thingiverse Ind12d OpenSCAD is a constructive solid geometry modelling tool that takes descriptions of 3D models as text and outputs them as STL files CW12 In order to define the cube model we specified the following code is used cube size 20 20 10 Typing this into the OpenSCAD GUI allows you to see a rendered version of the object by choosing Compile and Render from the Design menu figure A 1 The object can be exported to an STL file ready for G code generation by choosing Export as STL from the same menu A 2 G Code Generation ReplicatorG amp Skeinforge Once an STL file has been produced G code for printing the model must be generated ReplicatorG provi
92. sfer Characteristics reproduced from Fig 3 Int09 4 1 2 Heaters amp DC Motors The heaters and DC motors in the extruder and platform operate at a higher voltage and are considerably higher current than the microcontroller can provide on its output pins To control these a transistor can be used Transistors act like a switch which allows high power components to be switched on and off using only a small current from the Mbed An IRLU8729PbF MOSFET Metal Oxide Field Effect Transistor was selected as it can switch large loads up to 58A with very little on resistance reducing energy wastage through heat Int09 A MOSFET has three connections called the gate drain and source Figure 4 5 When the voltage between the gate and source is OV no current flows from the drain to the source As the voltage between the gate and drain are increased the current allowed to flow increases rapidly when it passes a certain threshold Figure 4 6 By connecting the gate to a pin on the Mbed and the source to ground a large current from a device such as a heater or motor attached to the drain can be switched The behaviour of a MOSFET when the gate is left floating disconnected is generally undefined and can damage the component When the Mbed powers on its output pins 27 12V Load e g Heater Mbed Pin Pull Down Resistor Ground Figure 4 7 Example MOSFET circuit with pull down resistor default to a floating state which co
93. stem the G code was not processed fast enough resulting in the printer stalling and depositing extra plastic The new system was able to process the same amount of G code without stalling Figure 5 10 shows a pair of herringbone gears printed with the old and new electronics A and B show how the new system is able to produce rounder circles due to better timing accuracy C and D show the effect of improved timing accuracy on the teeth of the gears 54 A B Figure 5 10 3D printed herringbone gear comparison of old A amp C and new B amp D electronics Figure 5 11 Failed large print showing warping A and a collision with the extruder B Large Prints Large prints stress the printer and electronics for long periods and can reveal missed motor steps or instructions The previous system had frequent issues printing large test objects due to skipped instructions or steps and is a particular area for improvement Of the large objects printed only one failed to print figure 5 11 This was due to the object warping A and then becoming detached from the platform during the print causing the tip of the extruder to rip the object off the platform B Unevenness in the temperature of the object during printing is the cause of this distortion This is partially caused by unevenness in the temperature of the build platform Unfortunately this is a problem with the printer s design which is not addressed in this pr
94. t 11 23 09 edition 2009 Stuart Nathan Jon Excell The rise of additive manufacturing The Engineer http www theengineer co uk in depth the big story the rise of additive manufacturing 1002560 article 2012 Accessed 13 April 2012 Jetguy Re Z axis skipping jamming Posted to the Makerbot Forums http wiki makerbot com forum t 349029 2012 Accessed 21 April 2012 Pete Lomas Rj45 magnetics x rays Raspberry Pi Blog http www raspberrypi org archives 781 2012 Accessed 21 April 2012 Richard Barry FreeRTOS Maintainer re uip users tcp duplicate packets Posted to the uip users mailing list on sics se Wed 09 Jun 2010 05 40 49 0100 Paul Marks 3d printer provides woman with a brand new jaw New Scientist http www newscientist com blogs onepercent 2012 02 3d printer provides woman with html 2012 Accessed 14 April 2012 Maxim Understanding SAR ADCs Their Architecture and Comparison with Other ADCs http www maxim ic com app notes index mvp id 1080 2001 Accessed 4 April 2012 NXP Semiconductors LPC17xx User manual UM10360 rev 2 19 august 2010 edition 2010 Ray s Hobby Projects First time experience with the makerbot http rayshobby blogspot co uk 2011 01 first time experience with makerbot html 2012 Used under At tribution ShareAlike Creative Commons 3 0 license Accessed 17 April 2012 ReplicatorG http replicat org 2012 Accessed 19 April 2012 RepRap G code RepRap Wiki
95. ted at the start can be snapped off and the process is complete For more complex models strings of plastic left during printing figure A 9 may also need to be removed using a knife 70 Appendix B Example Prints This appendix shows a representative selection of the objects that were printed as test cases during system testing All designs printed were downloaded from Thingiverse or included as part of ReplicatorG B 1 Intricate Prints Figure B 1 Flexible snake to test printing many thin fins Figure B 2 Starfish to test layering appearance 71 Figure B 3 Doorstop to test large smooth gradients Figure B 4 Butterfly to test intricate islands of print Figure B 5 Rabbit outline to test very thin structures 72 B 2 Large Prints Figure B 6 Letter A to test simple large shapes Figure B 7 Phone stand to test steep gradients Figure B 8 Tooth to test large models with large overhanging areas T3 Figure B 9 Multiple metabricks to test building batches of objects B 3 Functional Prints Figure B 10 Twistable heart to test simple mechanisms and multi part objects Figure B 11 Whistle with pea printed inside to test precise air tight objects with simultaneously printed sub components 74 Figure B 12 Tweezers to test flexible designs frequently used to remove excess extrusion produced during self cleaning 75 Appendix C Circuit Diagrams This appe
96. the Mbed electronics for controlling heaters and motors read ing from temperature sensors and connections for the stepper controller boards figure 4 1 A second board is used for the end stop electronics as these parts may be replaced separately from the main electronics and connect via the existing stepper controller in terface The completed system as installed in the printer is shown in figure 4 2 with the major components labelled To keep the system easy to build requiring readily available tools 0 1 spaced electron ics were used throughout These are easy to work with using only a standard soldering iron and basic tools Components of this size can also be used for prototyping with a solderless breadboard Heater amp Motor gt Heaters amp Motors Control Temp ag vure Thermistors Reading Stepper Interface Power Control Ethernet Figure 4 1 Components of the main board Power Supply Network eni Stepper Boards 24 End stop Interface End stop Board HN i To End stops Stepper Controller Z To Heaters Motors Main Board Stepper Controller Y Ethernet Interface Programmer Interface Stepper Controller X Stop Reset Button To Thermistors To PSU ATX Power Supply Figure 4 2 Electronics installed with key components labelled 25 0 1 0 1 ECECEDEDECECEUETEDEDECED 0 1 0 1 A Strip Board B DIP Board Figure 4 3 Types of prototyping board
97. uired to acknowledge each datagram received and accepted and ad vertise its current window size Packets consist of a copy of the 32 bit sequence number followed by an unsigned 32 bit integer window size Datagrams are only accepted when e Their sequence numbers are sequential e An empty first packet with sequence number zero has been received e The data contained fits into the system buffer i e obeys the window size Port Service 1818 G code interface UDP amp TCP 2777 Status interface TCP Table F 1 Network interface ports 94 F 1 3 Retransmission If the sender does not receive an acknowledgement with a matching sequence number within a certain period it will repeatedly resend the request If a zero window size is reported the sender must poll the printer until the window re opens by issuing new empty packets with increasing sequence numbers Exponential back off is not used in an effort to reduce the latency between the window opening and sending the next piece of data Shorter timeouts result in more traffic and less delay in restarting transmission after a zero window Longer timeouts result in less wasted traffic but if too long can result in buffer underruns before more data is sent after a zero window F 1 4 Errors If the printer receives some unexpected data for example if the printer is reset and receives a datagram from the middle of a transmission before it was reset an acknowl edgement with sequence numb
98. uld cause a MOSFET to unexpectedly switch on or become damaged To prevent this happening the gate is connected to ground via a resistor When the output pin is floating the gate is pulled to 0V by the resistor When the output of the pin is not floating the gate is pulled to that voltage overriding the pull down resistor A high resistance value is used so that the Mbed can easily override the pull down resistor Figure 4 7 shows the circuit used to control the two heaters and two motors using a MOSFET and pull down resistor When driving motors a flyback diode is usually used to prevent a voltage spike occurring when the power is removed from the motor This voltage spike is caused by the magnetic field in the motor s coils collapsing This is not included in the circuit as the MOSFETs used already contain an appropriate diode It should be noted that this circuit does not allow the motors to be driven in both directions as this is not needed by the printer If this was required a more complex circuit such as an H bridge would be needed 4 1 3 Thermistors To measure the temperature of the heaters thermistors are used The resistance of a thermistor changes non linearly with temperature and can be modelled using an equation derived from the Steinhart Hart Equation SH68 rms 4 1 Where T and R are the current temperature and resistance of the thermistor Ty and Ro are the temperature and resistance at a reference temperature
99. ven a modest smart phone this is a large amount for a microcontroller without the overhead of running a fully fledged general purpose operating system and associated software The chip contains various peripherals such as hardware timers analog interfaces and importantly fast Ethernet support These timers will as with the previous microcon trollers be vital for driving the electronics properly Analog inputs are also needed to interface with the electronics Ethernet support will allow the microcontroller to quickly receive detailed print data over the network As well as these features ARM devices are widely used and boast mature open source development tools making it an ideal choice for expanding the open source Makerbot design 2 3 Real Time Operating Systems The firmware consists of various complementary parts such as communication and con trol which are easily managed with the use of an operating system Due to the absolute 18 timing and performance requirements FreeRTOS a real time operating system RTOS was selected 2 3 1 Differences With Non Real Time Systems Real time operating systems like other operating systems provide a scheduler which allows multiple processes to run as if simultaneously It also provides facilities for com municating between these processes Unlike regular operating systems an RTOS can provide exact timing guarantees They are also generally targeted at microcontroller de velopment wit
100. while various G code files were sent to the printer Most testing was carried out during the system testing phase but the following synthetic tests were also used Circle plotting with low detail This test ensured that given a sequence of slow G code instructions the printer keeps all buffers reasonably full and that the network interface can cope with small infrequent bursts of data Circle plotting with high detail This test ensures that given a constant sequence of fast G code instructions the printer keeps all buffers reasonably full and that no underruns occur during busy periods Circle plotting with high detail and pauses This test ensures that given a sequence of fast G code instructions separated by pauses where the buffers filled and the network interface paused the transmission can quickly restart after the pause In all tests the buffer levels were generally above half full in the worst case In a small number of instances the underrun counter was triggered but no effect on the printer was observed These underruns may have been the result of FreeRTOS not allocating enough resources to the G code interpreter until the low level command buffer emptied but while the G code buffer was still full Once the G code interpreter was allowed to execute printing would have continued immediately resulting in the unobservable pauses observed 5 9 System Testing The system was tested as a whole to assess its performance in both printi
101. while working on this report Contents 1 Introduction 1 D Appications eaei S S deus k eek eu Gye re d SOE Sue 2 Makerbot zcv Sect Ad un mau Nato eem vue oH RH 1 3 Project Motivation and Goals u tn ca DEL era ed set dr ticos TA Repot Outline v stata thus ead he meten Dus cech Jake ette a 2 Background BM Makerbot luo ut SUR Cv E aen Dar ees Betty BUR S 2 11 Printer Components e 2 125 EleGtrODI A hve nee ite ey Ts E af o en d 2 1 3 Microcontrollers CS deti dr E been A A a Sue SE 6 a ek NAE eA YG DO Hp S 2401 90 Code 2 Gil Soa aee Bk SU ee EC es fees oS 2 1 6 Support Software v ud zoo med we warum eR Eo dx es 2 2 ARMS MBCA a co e oar o acean Bg estan E RR ry PINE TOT DLL 2220 NXP LOCAS due E ar A at te A O be BS i 2 3 Real Time Operating Systems ke sa ek ee ee oP Ka uw 2 3 1 Differences With Non Real Time Systems 2 0 2 ALEGADOS ui are ol deo ate ESA via Br Clo EIS OR 3 Design Seb VEIBRIWSEO 3x0 ape deze aeree de eque qe EE RN 3 1 1 Network Interface ss reter pr roa rgo a eR OVES 3 1 2 G code Processing Pipeline 004 3 1 3 G code Interpreter 2 Sob E Ee een GSE Se ay Dolo Drivers a ce An a A ee A te TVs deis Gigs esas ea gc ts Dalai SAEY eis go a a eS ensi ox betae Baek AA PS 3 2 Electronics amp Hardware sica lastra mah bw DPI bu eed Ge a d DA Stepper Control e uk a peas ueque Ew PRAE 3 2 2 Heater amp DC Motor Control llle n 3 29 End Stops a
102. y 100 instructions for the ISR taking an estimated 250 cycles to execute Assuming the CPU executes one instruction per cycle at 100MHz the 2Estimate based on informal analysis of the generated assembly code 35 Counter Clock Interrupt Figure 4 17 Mbed timer architecture amount of CPU time used by the ISR in the worst case can be calculated as follows Interrupts Per Seccond x ISR Cycles _ 1 32 x 10 x 250 Cycles Per Seccond 100 x 106 Overhead 0 33 4 4 This design results in very little overhead in the system but provides very accurate signals to the stepper motors Usage An API is provided which allows a number of steps in a given direction with a specified period to be sent to a stepper motor To allow sequences of coherent movements to be made a method which blocks until all steps have been completed is also provided This method uses a semaphore which is released by the ISR when all steps have been completed This is also used to temporarily give a higher priority to the controlling task during which time the next sequence of steps can be started immediately 4 2 4 G Code Interpreter In this subsection there is a discussion of the selection of G code features implemented by the system Following this the implementation of the parser and interpreter is described Feature Subset Selection G code interpreters support a large variety of features ranging from comments and in structions which retur
103. ystem timer which ticks at 1kHz which is far too low for the frequencies such as those discussed above According to the Nyquist Shannon sampling theorem a frequency of fHz can only be generated by a timer running at gt 2 Hz This means the timer resolution must be above 2 x 1 32kHz 2 64kHz Sha49 Figure 4 15 shows aliasing caused by trying to reproduce a signal at 2 gt 5 the frequency of the timer where whole cycles are missing in the output 34 Timer Clock Desired Signal Generated Signal Figure 4 15 Nyquist Shannon sampling theorem example with high frequency signal Sample Clock Desired Signal Generated Signal Figure 4 16 Example of sampling artefacts The effect of sampling artefacts can cause further inaccuracies Figure 4 16 shows a signal below I of the sampling frequency displaying such artefacts In the worst case when sampling at f Hz a signal change may be IS late Therefore increasing the sampling frequency decreases the error introduced by such artefacts High print quality depends on smooth even steps as a result the timer resolution must be not only above 2 64kHz due to the Nyquist Shannon sampling theorem but also be high enough to reduce sampling artefacts to an acceptable level If errors due to artefacts are to be reduced in the worst case to for example one hundredth of the step period then a frequency of 264kHz is needed Increasing the speed of the system timer to such a frequency would

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