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Sensorless control of brushless DC electromotor

1. University of Belgrade Department of Electrical Engineering Sensorless control of brushless DC electromotor Diploma thesis Candidate Milan Tomi Mentor prof dr Slobodan Vukosavic Belgrade February 2004 Table of contents O 4 Ms A TO 5 Brushless DC electromoftOt editada dada den dba 5 Advantages and drawbacks of BLDC motor control sese 6 Ze POSTION SENSING soi nea ateen aaia aa aa ora e a 7 Types of position SENSING leerla ironia 7 Sensorless position detection ns ideada 8 3 Sensorless position detection TVF metho0d oooccnoccconcccnocnnononononcnonnncconncnon cnn nononnnncnnos 11 E A A kebiehaaovnnbeesets 11 Realization E R e NE dese tt ae dte dhe Ea E dici qu RINT 13 Practical teadlixatiotia IA si ete ox duod asa ULL 16 Method TVF phase delay proDleHisu ro teo a te e pa tes 20 4 IRADK Motor LIV Sf a estes og poset ote D ei D are Feste i UR vast alli 24 BAAR Ta essai ume pt E M Mop OV m tcr tes 24 Power SuDDI Vo aide md avi Cete rude rod teer ipM Ra eaa dates oA UM Re v Qd neca dus ds 24 IRAMSIOUPOO0A power module roce etu ta iba Ai a Aaa 27 Phase current and DC voltage sensing circuits user eeeto fedet etint eese e Fern eva epo red onte 28 PIOUSCHOIS ELE CUI esee esses ee ee pom emot cda atari som d a e rure DRESS HEAT 30 159232 serial illeos eere orbi pe ere eode detti eeu discas adita evel rA Go seid RUE 34 POAT AL Tedesco edem ose candace PEE a telefilm v MER NARNIA CREER ERE 36 High volt
2. ST RCA can also be the SPI Data In SPI mode or data 1 0 I C mode RCS SDO Input output port pin or Synchronous Serial Port data output RC6 TX CK bit6 ST Input output port pin or USART Asynchronous Transmit or Synchronous Clock RC7 RX DT bit ST Input output port pin or USART Asynchronous Receive or Synchronous Data Legend ST Schmitt Trigger input Table 5 1 PORTC functions As we can see all communication to the outside world would go through PORTC since all communication modules are attached to it Modes that can be used are I C SPI and USART Capture Compare PWM functions also use PORTC In addition we can pass Timerl clock input to this port as well as make Timerl oscillator output of it In this project we will use some pins of PORTC as digital inputs from position detecting circuit Pins RC6 and RC7 will be used for asynchronous communication with PC PWM feature will be used also but only as a reference to put it more precisely PWM signal will be copied to desired PORTB pin by software Timers Our microcontroller has three programmable timers Timer timer counter module can be found in all PIC devices Interrupt control bits of this timer are in INTCON register as it is not considered as peripheral of our microcontroller It has the following features 8 bit timer counter 44 Belgrade university Diploma theses Sensorless control of brushless DC motor Readable and writable 8 bit sof
3. micto beotel yu E mail chat mictol978 hotmail com 79
4. output transistor of fourth comparator will turn off ITRIP signal will rise very rapidly time constant value is Rs Ry Es 2 7 T 16045 4 2 approximately ITRIP will rise until 1t reaches 5V because in that moment protection diode of input port pin SHORT CIRCUIT will start to conduct and limit pins voltage to 33 Chapter 4 IRADK motor drive 5V shown on the second figure This will happen in about 50 us from triggering of the fourth comparator ITRIP will stay at 5V until ENABLE signal becomes high or until software reset is applied depending on what was the cause of ITRIP in the first place Diode D18 turns off C25 starts emptying After a time interval of something like 200 us logical level of ITRIP will be 0 since time constant in this case is approximately double than the one in the first case and exponential streaming to the final voltage which s OV is much slower than in the first case when it was practically linear When ITRIP befalls logical 0 IRAMS will be operating again Holdup of 200 microseconds in IRAMS turn off will grant us additional safety Transition process can be observed in software by reading the value of voltage converted on pit AN2 figure RS232 serial link With intention to connect our IRADK to PC standard RS232 serial link is used While IRADK is functioning diverse information will be exchanged between PC and microcontroller on IRADK From PC to microcontroller w
5. t1 Control tStep tStep 5 else tl Control tStep tStep 5 if tl Control lt 637 inc 0 if direction 1 direction 1 else direction 1 tStep 637 TXREG 100 if tlControl gt 40000 OPTION 0X11 N ii TOIE 1 if state amp amp 0x01 test 0 else test 1 if direction testVector testTable 1 state else testVector testTable l state TMRION 0 TMRIIE 0 if tStep lt 10 tStep 10 TMR 1H t1 Control gt gt 8 MATAS TMR1L t1Control WWE 63 Chapter 6 Sensorless mechanism software resources TMRIIF 0 sending PORTC amp 0x0B TXREG sending 5 Let us suppose now that we want to move motor that is standing still Initial value of variable t1 Control is 0 which means that initial speed of our motor will be 0 79 rounds per second as discussed above On every interrupt occurrence state will increase decrease depending on the variable direction 1 Motor will start to move in desired direction When the motor is starting to move interrupts i e state changes are less frequent therefore step should be smaller in order to obtain linear speed augmentation Current step value is contained in variable tStep Now let us take a look at the program presented above supposing that value of variable inc is zero inc 0 and tlControl is less than 40000 As we can see on every interrupt come to pass following events state changes in desired direction tStep decreases by a fixed value previous value of tlContr
6. 1D03 btfss 0x3 0x2 0033 282E goto 0x2E 0034 3400 retlw 0x0 0035 3FFF isr addlw Oxff O7EA 1683 main bsf 0x3 0x5 07EB 1303 bef 0x3 0x6 07EC 0186 clrf 0x6 O7ED 3011 movlw 0x11 O7EE 0081 movwf 0x1 O7EF 168B bsf OxB 0x5 07F0 178B bsf 0xB 0x7 07F1 2FFD goto Ox7FD 07F2 1283 bef 0x3 0x5 07F3 1303 bef 0x3 0x6 07F4 1C06 btfss 0x6 0x0 07F5 2FF7 goto Ox7F7 07F6 2FF8 goto Ox7F8 07F7 2FFA goto Ox7FA 07F8 1006 bcf 0x6 0x0 07F9 2FFD goto Ox7FD O7FA 1283 bef 0x3 0x5 07FB 1303 bef 0x3 0x6 55 Chapter 6 Sensorless mechanism software resources 07FC 1406 bsf 0x6 0x0 07FD 2FF2 goto 0x7F2 O7FE 118A bef 0xA 0x3 O7FF 2827 goto exit 0800 3FFF addlw Oxff 07FC 1406 bsf 0x6 0x0 07FD 2FF2 goto Ox7F2 O7FE 118A bcf 0xA 0x3 Assembler program include p16f873 inc CONFIG CP OFF amp WDT OFF amp PWRTE ON amp XT OSC org 0x00 goto Main org 0x04 goto ISR Cblock 0x0C var w_temp status_temp pclath temp endc bank1 macro bsf STATUS RPO endm bank0 macro bcf STATUS RPO endm Main bank1 movlw 0 movwf TRISB movlw b 00010001 movwf OPTION REG bsf INTCON TOIE bsf INTCON GIE 56 Belgrade university Diploma theses Sensorless control of brushless DC motor bank0 lo btfss PORTB O goto loi bef PORTB 0 goto lo loi bsf PORTB 0 goto lo ISR movwf w_temp swapf STATUS w clrf STATUS movwf status_temp movf PCLATH w movwf pclath temp clrf PCLATH decfsz var goto ilo movlw 0 movwf var goto
7. 25V In addition there is a wire added that can connect via connector auxiliary input source 25V directly to DC bus since we need a source that will supply motor This wire has to be disconnected when connecting IRADK to power grid in order to avoid short circuit that could cause severe damage to IRADK Picture 4 1 Position detecting circuit connected to IRADK 41 Chapter 4 IRADK motor drive Picture 4 2 IRADK board 42 Belgrade university Diploma theses Sensorless control of brushless DC motor 5 PIC16F873 microcontroller Introduction PIC16F873 is a 28 Pin 8 Bit CMOS FLASH general purpose microcontroller It has 4Kb of flash memory In subsequent sections most important features of PIC will be discussed More information can be found on internet in file http www microchip com download lit pline picmicro families 16f87x 30292c pdf Ports PIC has three input output ports PORTA 6 bit PORTB 8b and PORTC 8b Every port has some special features adjoined PORTA is a six bit wide bidirectional port Pins of this port are passed to the input of A D multiplexer Corresponding data direction register is TRISA Setting a TRISA bit 1 will make the corresponding PORTA pin an input i e put the corresponding output driver in a Hi Impedance mode Clearing a TRISA bit 0 will make the corresponding PORTA pin an output i e put the contents of the output latch on the selected pin Reading the PORTA regis
8. More comprehensive control mode change discussion is in next section Variable lt inc gt serves for a direction change lt inc gt is 0 in normal circumstances When we press the button lt inc gt will become 1 motor will start to slow down following the same steps that are followed when it was accelerating i e tStep value will be increasing When tlControl drops below 637 maximal step value direction is changing lt inc gt is back at 0 and motor starts to accelerate again in opposite direction of course Value from tl Control is written to timer registers in two lines since tl Control is a 16 bit variable there is an eight bit shift that allows us to write higher byte of tI Control to TMR1H Last two lines of code presented in this section are for debugging purposes they have no effect on program execution External signals control mode When motor reaches certain speed anterior section BEM forces become significant as a result position can be detected via position detecting circuit As it is discussed in the second chapter in order to provide high quality torque and speed control motor has to be commutated at precise positions Motor position is reflected in BEM forces On subsequent images we can see motor circuit circumstances throughout six commutation states states are defined in PWM section of this chapter A GND Figure 6 3 State 1 electric model External signals condition B g
9. connecting IRADK to European power grid as its connections are not polarized that is we cannot tell the difference between phase and neutral connections IRADK would function either way but with phase voltage connected to neutral connection we would have high voltage on IRADK even when switch is shut down To prevent this I suggest that we connect it randomly and than measure voltage between power grid ground and stabilizer cooler while switch is off If there is voltage we should swap neutral and phase connections I should emphasize again that IRADK has no galvanic isolation hence should not be touched while in function This may be problem during the phase of development of our project But when project is done hardware and software we can place IRADK in a box which will be connected to the ground thus completely harmless Microcontroller slot on IRADK IRADK is supposed to work 8 bit microcontroller On the following figure we can see slot with microcontroller PIC16C72 We will be using PIC16F873 microcontroller in this project which is pin compatible to this one In this section possibilities for connecting other microcontrollers to IRADK will be considered as well If we have needs for higher processing powers we could use DSP that would be connected to IRADK by the use of an adapter Task of adapter task would be to bypass required pins of DSP to matching pins of microcontroller slot 5V R7 1K U1 AN2 gt 4 3 QZ1 20MHz C14 C15
10. have brushes that would have to be changed occasionally Advantages and drawbacks of BLDC motor control Hardware amp software are more complex than in conventional DC motor control but the motor itself is smaller more efficient cheaper lighter more rugged and reliable On the other hand hardware and software are less complex than in regular synchronous control drives The fact that we have to control only one current instead of three and still have we can say vector control makes BLDC a very desirable mode of control We only need one PWM module disadvantage is that the mentioned module has to have access to more pins in our case six Generally the processor which is used has less processing power than the ones used for a regular control of synchronous drives but at this point we cannot do much generalization because required processor power depends on a peripheral hardware current sensors position detector etc Disadvantage of BLDC control are torque peaks which will occur due to commutations These peaks are the consequence of a current decay and can not be eliminated I have to say here that for a majority of applications these peaks are irrelevant We cannot use brushless DC motor for a position control In some cases we can but it will be poor quality control as we do not control position continually our control is discrete At the end of this paragraph I would like to emphasize again that in the BLDC control mode we do not
11. occurs more than once due to fault of undebounced button everything will work normally since there is no big difference if signal is set at logical 1 1 or 2 microseconds Changing of motor direction is accomplished by changing a value of interrupt service routine variable lt inc gt If a protection latch is not set than a user must have been pressed a button because he wants to change a direction of motor rotation Value of lt inc gt variable is complemented Following section will reveal functioning of our program with this variable INTF flag is reset at the end of the procedure for the reason mentioned in previous chapter Due to the fault of undebounced button it may happen that our motor does not change direction if interrupt occurs twice or any even number of times nothing will happen In that case we should press it again This poses little inconvenience Its solution would be far too complicated so I decided simply not to solve it Init of this interrupt is done in main program Interrupt on rising edge of the RBO signal is set and enabled We could work with falling edge there would be no difference 29 Chapter 6 Sensorless mechanism software resources Pulse Width Modulation Beside PWM this section will deal with toggling on off of inverter switches generally PWM is realized on high side switches of our IRAMS inverter for the reasons mentioned in previous chapters There are six states of inverter 1 C
12. state direction if state 0 state 6 else if state gt 6 state 1 OnVector TableOn state OffVector TableOff state if linc tl Control tStep 74 Belgrade university Diploma theses Sensorless control of brushless DC motor tStep 5 else tl Control tStep tStep 5 if tl Control lt 637 inc 0 if direction 1 direction 1 else direction 1 tStep 637 TXREG 100 if tl Control gt 55000 OPTION 0X11 TOIE 1 if state amp amp 0x01 test 0 else test 1 if direction testVector testTablel state else testVector testTable l state TMRION 0 TMRIIE 0 if tStep lt 10 tStep 10 TMRIH t1 Control gt gt 8 TMR 1L t1 Control TMRIIF 0 sending PORTC amp 0x0B TXREG sending 5 if TOIF if waitState gt 0 waitState if RC2 PORTB OnVector else PORTB OffVector if waitState 0 if testVector amp PORTC 0 amp amp test testVector amp PORTC 0 amp amp test state direction if state gt 6 state 1 else if state lt 1 state 6 OnVector TableOn state OffVector TableOff state if state amp 0x01 test 0 else test 1 if direction 1 testVector testTablel state else testVector testTable_1 state waitState 3 j TOIF 0 TXREG state 75 Appendixes j main TRISB 0X03 PORTB setup PORTB 0XFF TRISB 0X01 PORTB 0XFC TRISC 0OXBB C register is used for serial communication T1CON 0X31 TMRI configuring TMRII
13. this section we are dealing with necessary program fraction initialization PORTB and PWM PORTB pins 7 1 have to be outputs and pin 0 input TRISB 0X03 PORTB 0XFF TRISB 0X01 PORTB 0XFC These may look complicated as there is a simpler way to initialize PORTB but it will provide additional security as we must not allow two transistors from the same phase to be turned on First two lines will ensure that all transistors are off and ENABLE signal is an input thus inverter is out of function Last two lines will make ENABLE signal an output on high logical level i e enable PWM init is done through the following lines TRISC 0XBB PR2 0XFF CCPRIL 5 T2CON 0X05 CCP1CON 0X0F First line configures pins of PORTC at this point important is that it configures RC2 to be an output otherwise PWM would not function Lines that follow are not written as their purpose is not related to PWM three dots Lines that follow those lines configure period register at maximum value FF 255 and set initial value of PWM impulse at 5 it is a low level that will not cause high current protection reaction PWM impulse width can be increased decreased by a data received from serial bus this will be discussed later Last two lines configure a timer2 to work adjoined to PWM module as well as a frequency of a clock that drives it Frequency of a clock is set to 1 25Mh F scinaror 16 One quarter of processor clock is prescaled by a
14. to the inner side of stator That is why we will try to detect rotors position implicitly without sensors Sensorless position detection There are various types of position detection All of them are based on extracting Figure 2 2 Diagrams of back electromotor forces Belgrade university Diploma theses Sensorless control of brushless DC motor position information from terminal voltages and sometime currents Position can be detected through detection of saturation of phase inductance The stator is saturated by the magnetic field of the rotor in different axis depending on the rotor position The saturation results in a reduction of the motor winding inductance The variation of the inductance can be detected with a high frequency voltage applied to the motor windings The resulting high frequency currents are modulated by the varying winding inductance If the machine is connected to a pulse width modulated frequency inverter the PWM frequency works as a carrier frequency for the rotor position detection This sensorless method of detecting the rotor position works even in the standstill of the machine which as we will see will not be the case of other sensorless methods Unfortunately this circuit is quite complex and therefore expensive in at least two of the three motor phases the current has to be detected Using this sensorless detecting on a motor with high power efficiency will not work correctly because there is very low sat
15. used in accelerating of Brushless DC motor since position detecting circuit will not work if rotor is not moving or if it moves slowly Timer is an 8 bit timer with a prescaler and a postscaler It can be used as the PWM time base for the PWM mode of the CCP module s The TMR2 register is readable and writable and is cleared on any device RESET T2CON is the register that determines its operation The input clock FOSC 4 has a prescale option of 1 1 1 4 or 1 16 selected by control bits T2CKPSI T2CKPSO T2CON lt 1 0 gt The Timer2 module has an 8 bit period register PR2 Timer2 increments from 00h until it matches PR2 and then resets to 00h on the next increment cycle PR2 is a readable and writable register The PR2 register is initialized to FFh upon RESET The match output of TMR2 goes through a 4 bit postscaler which gives a 1 1 to 1 16 scaling inclusive to generate a TMR2 interrupt latched in flag bit TMR2IF PIR1 lt 1 gt Timer2 can be shut off by clearing control bit TMR2ON T2CON lt 2 gt to minimize power consumption Timer2 scheme is revealed on the following figure FIGURE 7 1 TIMER2 BLOCK DIAGRAM Sets Flag bit TMR2IF TMR2 Output RESET Postscaler 1 1 to 1 16 EQ T2CKPS1 T2CKPSO T20UTPS3 T20UTPSO Figure 5 1 Timer2 block diagram In this project Timer2 will be used in CAPTURE COMPARE PWM module in order to provide reference PWM signal Block scheme of mentioned module when it functions as
16. whether we are using assembler or Hi tech C The difference is only at the beginning of program execution there is a code inserted by linker that has to be executed prior to our program 51 Chapter 6 Sensorless mechanism software resources 6 Sensorless mechanism software resources Introduction In chapter 2 position detecting circuit was presented At the end of the third chapter circuit connection to IRADK was depicted IRADK modification section The theme of this chapter is software integration of our system Software resources located on PC will be described here also this is referring to serial communication program There are several functions that microcontroller software has to implement Our motor needs to be accelerated to some speed in order to produce BEM forces sufficient for position detector functioning Brushless DC acceleration is achieved though open loop control i e timer determines commutation moments When accelerated motor should commutate on regular angle intervals thus signals from position detecting circuit should be read and depending of their values commutation moments should be determined I can say that this is the most important software function although other software functions are indispensable Each one of two previously mentioned functions has a mode of functioning that is adjoined to it On the following block diagram we can see association of those two modes When functioning our system is alw
17. with motors that have sinusoidaly distributed windings because there is no third harmonic in BEM forces There is a way of detecting commutation points by detecting current slope variation When current runs from one phase to another we have V 2RI 21 E EE 2 1 Where Vl is a voltage between active phases as it is shown on the following figure VI L R 4 L R Egb Figure 2 4 Two connected phase model If we assume that terms RI and VI are constant we can determine zero crossing of BEMF on a base of term 2L dI dt which is proportional to slope variation Although this type of position sensing can provide very precise position detection it requires at least two current sensors and a DSP with high processing power and fast A D converters The method that will be presented in this paper will be the simplest of all methods It is based on analog filtration of phase voltages and their comparison It will work with both trapezoidal and sinusoidal BEMF though this time better with sinusoidal It will have some disadvantages also as we will see in the following chapters 10 Belgrade university Diploma theses Sensorless control of brushless DC motor 3 Sensorless position detection TVF method Description TVF stands for Terminal Voltage Filtration We will use sensor here do not let the name sensorless trick you but it will be simple circuit attached to the phases of motor It will have no mechanical parts which would be at
18. 0 enables timerl Second line of code enables interrupt on timerl This initialization is placed in the main procedure like the most of initializations though there is a portion of code in interrupt procedure that will reconfigure this timer when needed Maximum period of timer configured in manner described above is g 2 6 T gt 0 104s 9 53Hz 6 1 oscillator 62 Belgrade university Diploma theses Sensorless control of brushless DC motor Frequency of oscillator is 20Mhz Frequency in parenthesis is the minimal frequency of state shifting Divided by 12 it gives us a number of rotations per second 0 79Hz case of 4 pole motor as we can see it is sufficiently low frequency that is speed Subsequent block of code is the one in charge of motor acceleration 16 bit variable tlControl contains the number that is written to timerl registers at the end of interrupt procedure Increasing of this variable in every cycle i e interrupt occurrence obviously accelerates the motor interrupt void isr static unsigned state 1 Variable definitions static unsigned dutyR 0 static unsigned int tStep 637 static unsigned int t1 Control 0 static signed direction 1 static unsigned inc 0 static unsigned sending 0 gt if TMRIIF state direction changing of state if state 0 state 6 else if state gt 6 state 1 state overflow ctrl OnVector TableOn state OffVector TableOff state if linc
19. 22p 22p L Figure 4 18 Microcontroller slot on IRADK 38 Belgrade university Diploma theses Sensorless control of brushless DC motor On the figure we can see the microcontroller connections to the circuits from previous sections Analog input AN2 is connected to analog signal from ITRIP circuit take a look at protection circuit section Quartz crystal is connected to its corresponding pins in order to provide oscillating frequency of 20MHz If we use DSP or some other microcontroller that is connected to IRADK via adapter it is most probable that it will use its own oscillator so the structure on the picture is only for PIC16C872 pin compatible microcontrollers MCLR Master clear pin is also only for PIC when 12V is on it will be in programming mode when 5V is on it will be in operating mode 5V power is supplied to the slot as its most common power supply for microcontrollers 3V supply is used as A D converter reference when using non PIC microcontroller this voltage can be used only for reference functions as its current capacity is very low All other pins are connected to input or output ports of PIC thus when using non PIC microcontroller should be connected to its ports via adapter IRADK modifications position detecting circuit connection In order to connect position detection circuit we will need three digital inputs Only two are available One of the two available digital inputs is connected to RC1 pin which
20. DK motor drive filled when lower transistor is turned on so they could supply power when high side transistor is on figure simplified scheme DC bus DC bus OFF o ON 15V Figure 4 6 Simplified functioning of bootstrap diodes in IRAMS When we turn off lower transistor diode turns off also leaving the capacitor with floating voltage which can power up a driver circuit This is the cheapest way to realize high side power supply Other types of supply would include transformer for a galvanic separation and that is expensive Disadvantage is that we cannot hold upper transistor turned on all the time We have to turn it off in order to refill capacitors It is recommended that upper side transistors be off at least 10 of time That is the reason why I chose PWM to be on upper transistors high side The PWM side transistors will conduct less time than the ones on the non PWM side One might think that this disadvantage may stop us from holding motors shaft fixed position as we need to have one upper side transistor turned on all the time but it is possible When capacitors gets empty transistor will turn off but current from the motor inductance will turn on lowers IGBT diode capacitor will refill also from the inductance current and upper transistor will start to conduct again Of course it will not be conducting all the time Signals A HI B HI C HL A LO B LO C LO are the ones that control states of transistors in I
21. E 1 INTE 1 Interrupt enable on button press TXSTA 0X23 Serial link configuration RCSTA 0X90 SPBRG 129 RCIE 1 PEIE 1 PR2 0XFF CCPRIL 5 T2CON 0X05 CCP1CON 0X0F GIE 1 Global interrupt enable CREN 0 OERR reset CREN 1 while 1 if RC2 PORTB OnVector else PORTB OffVector 76 Appendixes Appendix B Project making and compilation in Hi tech C for PIC Every program i e file written in Hi tech C for PIC has to adjoined to some project in order to function A simple one file project creation and compilation will be described here On Mp lab menu project we chose the new project We chose some name for project testing for example Then we go to edit project option and set a tool HI TECH we also set desired microcontroller type Then we should click at testing hex when it gets blue after a click we should click edit Following options should be set Language tool PIC C linker Info messages quite desirable Hex format Intel Compile for Mp lab enabled checked All other options should be unchecked Then we should return to previous menu and click add node Testing c file should be added name should match to hex file name with following settings Language tool PIC C compiler Generate debug info desirable Floating point 24 bit Error file desirable Assembler list checked Retain local symbol checked Next step click at Build project project menu Error
22. I respectively RCIF is set when byte is received and is cleared when RCREG is read If we need to send large amount of data we will use transmit interrupt Interrupt flag pair will be TXIE TXIF from registers PIE and PIR1 respectively TXIF is set when transfer is over and can be cleared only by writing new value to TXREG This will 48 Belgrade university Diploma theses Sensorless control of brushless DC motor maximize our transfer rate as our software will enable us that new byte is written as soon as anterior one is sent In this project we will use asynchronous communication via receive interrupt Interrupt system interrupt on button press PIC16F873 has a non vectored interrupt system That way every interrupt that occurs will cause the execution of the same interrupt procedure Determining which interrupt occurred is a task for interrupt procedure Procedure will poll interrupt flags in order to determine interrupt cause Every interrupt has two flags One for enabling it and other which is set whenever interrupt occurs this one is polled in the procedure as we have already seen timer0 section 2 Beside the timer and USART interrupt in this project interrupt from the button is used also Button is attached to RBO bit of PORTB This interrupt flags are INTE and INTF external interrupt Both are the bits of INTCON register Interrupt will occur on rising or falling edge of the signal on RBO pin depending of the stat
23. IC References Contact Abstract In this paper one brushless DC electromotor control method without position sensors on motor is considered In first chapter we will be introduced to widely spread brushless DC motors We will take brief pros and cons analysis of brushless DC drive Later we will be introduced to the types of position sensing in one brushless DC drive explicit and implicit Than we will do thorough analysis of our method Calculation of components and practical realization of position sensing circuit will be given For a practical realization of entire system IRDAK 10 motor drive International rectifier will be used It will be analyzed as well as microcontroller PIC16F873 Microchip which is the brain of our system Afterwards we will take a look at the changes that have to be done on IRDAK in order to connect our position detecting circuit on it Software resources will be described Performance analysis will be done Possible applications and upgrades will be proposed Chapter 1 Introduction 1 Introduction Brushless DC electromotor Brushless DC BLDC electromotor is a name referred not only to a type of a motor but to a type of control also BLDC can be any electromotor with permanent magnets on a rotor Stator windings can be sinusoidaly distributed but it is not necessary a simple linear distribution which produces a trapezoidal back electromagnetic forces BEMF will d
24. PWM can be seen on subsequent figure Registers related to this module are CCPICON and CCP2CON 2 modules In Pulse Width Modulation mode the CCPx pin RC2 produces up to a 10 bit resolution PWM output Since the CCP1 pin is multiplexed with the PORTC data latch the TRISC lt 2 gt bit must be cleared to make the CCP1 pin an output If we want our module to function as PWM bits lt 3 2 gt of CCPICON register have to be set PWM period is defined be PRI register of Timer2 by formula PWM period PR2 1 4 TOSC TMR2 prescale value 46 Belgrade university Diploma theses Sensorless control of brushless DC motor Duty Cycle Registers y COPICONESA gt CCPRiL Bitlis RC2 CCP1 Clear Timer CCP1 pin and latch D C NIE m TRISC lt 2 gt Moca Note 1 The 8 bit timer is concatenated with 2 bit internal Q clock or 2 bits of the prescaler to create 10 bit time base Figure 5 2 PWM mechanism The PWM duty cycle is specified by writing to the CCPRIL register and to the CCP1CON lt S5 4 gt bits Up to 10 bit resolution is available The CCPRIL contains the eight MSbs and the CCP1CON lt 5 4 gt contains the two LSbs This 10 bit value is represented by CPR1L CCP1CON lt S5 4 gt The following equation is used to calculate the PWM duty cycle in time PWM duty cycle CCPR1L CCP1CON lt 5 4 gt TOSC TMR2 prescale value CCPRIL and CCP1CON lt 5 4 gt can be written to at any time but the duty cycle valu
25. RAMS They are TTL compatible 0 5V so they can be connected directly to the pins of microcontroller They work in negative logic When one of them is on low level 0V target transistor will be turned on and vice versa We must take care not to turn on 2 transistors from the same phase at the same time That can damage our inverter although it has protections integrated and in our case outside protection also protection circuit We have to take special care while writing software ITRIP is the signal from the protection circuit which will be discussed in one of the next sections When it is on due to a high current control signals will be ignored and all transistors will be turned off Phase current and DC voltage sensing circuits In order to provide a high quality torque and speed control we need to control current in motor To control it we need to measure it Beside the information about the current we also need the information about the voltage on DC link On the following 28 Belgrade university Diploma theses Sensorless control of brushless DC motor scheme we can see the part of IRADK circuit which is in charge of voltage and current sensing DC_BUS R46 R47 R48 R49 200K 200K 200K 4K7 AN1 K C431 pok 44 22n 22n R58 10 tl 0 68 Ohm 3W Figure 4 7 Phase current and DC bus voltage measuring circuits As we can see it leans on the circuit of rectifier from the first section of t
26. We have two cases of comparison First is when a voltage from disconnected phase is compared to zero phase Wye voltage is half of a high side so we will have 4 3V 3 10 U PWM noise On the other hand when voltage of a disconnected phase is compared to high side phase we have U 8 6V 4 3V 4 3V 3 11 U sw noise U PWM noise highside PWM noise disconnecte phase This noise can cause commutation problems only at low speeds because BEM forces are low but at low speeds PWM duty ratio is significantly lower than 5096 so we can conclude that our filter will successfully eliminate undesired PWM noise It looks like the third harmonic of BEMF could cause errors in commutation as our filter will shift it is phase but if we take a look at chapter one position sensing by detecting of harmonics of induced motor voltages we will see that the third harmonics are equal in all three phases thus they will not affect the comparison In order to realize our system a structure on a figure 3 10 will be used Of course Phase voltage R1 R3 C1 co le comparator H2 Figure 3 10 One phase LP filter model we will have three circuits because we have three phases Let us calculate the values of resistors and capacitors Our circuit will be connected to the phases of motor and will be I7 Chapter 3 Sensorless position detection TVF method supplied from voltage sources of 5V and 15V which we have on IRDAK Ground of these vol
27. a very important feature of IRADK Its purpose is to block the inverter when phase current gets too high Input in this circuit is voltage on shunt resistor that was discussed in previous section 30 Belgrade university Diploma theses Sensorless control of brushless DC motor SHUNT 100p Figure 4 9 Protection circuit 1 3 part 1 of 3 The first part of the circuit is shown on the figure 4 9 In the whole circuit there are four comparators chip LM339 the same that I use for a position detecting circuit only this one is SMD In normal functioning mode output of the first comparator will be logical 1 Output transistor of the open collector comparator will be turned off Rising of current will be followed by falling of voltage on shunt connection which is the same one from the previous section At the current of 10 7A temporary value comparator will change its state Output transistor turns on Second comparator in the chain changes its state figure below second part of the circuit point A is the one from the first part of the circuit 3l Chapter 4 IRADK motor drive RESETSC gt 100 Figure 4 10 Protection circuit 2 3 After the first and second comparator the third and fourth comparator will also change states as we will see and ITRIP signal will become high IRAMS will shut down If the current returns to normal IRAMS will stay shut down because current overload is latched on second comparato
28. age signals strip and power connections 37 Microcontroller slot on IRADK ias 38 IRADK modifications position detecting circuit connection 39 D PICIORS734160r0conttollet 5 ed vecet S ons pel UOS E NU FEET duc die O Ua AR SRL case 43 A A d ae A P ERERINU Spes fdu Hebe o aeta Regg expo CO QU Pee aaa e epos 43 yii E 44 Asynchronous communication module essere 47 Interrupt system interrupt on button press sessssseseeeeeeeenre eene 49 Microcontroller programming osito etie Resta Or bett RR manip uere edu qs Ful rep uo cided 49 MPLAB integrated development environment and HI tech C ssss 50 6 Sensorless mechanism software reSOULCES ceccesceeseeesceeseeeeceeseeeseceeceseeaeeceeeenaeeeees 52 OS 52 Hi tech C Vs regular PIC assembler programming esee 53 Serial link pro ramon PC auao taa see id 58 Serial link on microcontroller SIE ardet vecta ta Cibo rai ed RE 58 Button press HUME OMS sets alcatel iin adaer ot tau EN eral tate Urea Lon abt dle tad tud pu iets 59 Pulse Width Mod l ti oi is peut ted ott dete d 60 Motor acceleration mode dudes uw ra et LO obe A be leu CO Ed 61 External signals control MOR da ed 65 SS A A A O EA A EOS 71 Systemi performance AA O INN 71 Possible Ste mip Stade suite ed cos cata Na 71 Possible ADICIONA a ada ant A LL REA 72 Appendix A SOUPCe COE A ct ha A A A AA 74 Appendix B Project making and compilation in Hi tech C for P
29. anism software resources GND PWM Figure 6 8 State 6 model External signals condition B gt A 0 C gt B 1 A gt C 0 Transition to state 1 should occur when B gt A becomes 1 positive direction Transition to state 5 should occur when A gt C becomes 1 negative direction Position detecting circuit which produces digital signals is discussed in section Practical realization Summary of external signal values and transition conditions through states is presented in the following table 6 1 next page Our program task is to read external signals B gt A C gt B A gt C on distinct time intervals that are much shorter than duration of one state and execute state change when a preferred external signal changes its state Timer0 is configured to generate interrupts on every 200us 4882Hz This configuration is done upon entering in this control mode in interrupt procedure of timerl code from previous section when tlControl variable gets higher than 40000 marked with Timerl oscillator is shut down and timerO interrupt is enabled simultaneously STATE B gt A C gt B A gt C Positive Negative direction direction condition condition 1 1 1 0 C gt B 0 B gt A 0 2 1 0 0 A gt C 1 C gt B 1 3 1 0 1 B gt A 0 A gt C 0 4 0 0 1 C gt B 1 B gt A 1 5 0 1 1 A gt C 0 C gt B 0 6 0 1 0 B gt A 1 A gt C 1 Table 6 1 External signals and state transition conditions throu
30. approximate as we neglected existence of inductivity and transient processes but it is sufficiently precise since we do not practically want to control current here we only want to limit it In addition we can not measure current during the motor startup since PWM impulses are very narrow Acceleration rate has to be limited As we know M J a M is resultant torque is acceleration and J is polar moment of inertia Resultant torque is available electromagnetic torque diminished by the value of load torque If our acceleration is too high term on the right side of the equation can get higher than the one on the left which will cause our motor to block thus value of acceleration should be chosen to be reasonable In order to implement the acceleration of motor Timerl timer counter module is used I chose this module because it has two 8 bit registers tmrlh tmr11 when used with prescaler it can provide longer periods i e lower speeds which are necessary for acceleration In addition resolution is higher and it is possible to accomplish linear speed augment more easily The following portion of software will configure Timer T1CON 0X31 TMRI configuring TMRIIE 1 First line places hexadecimal value 31 in TICON register 0X31 00110001 Bits 7 6 and 2 are irrelevant Bits 5 and 4 11 define prescale value at 1 8 maximal value Bit 3 shuts off oscillator since we are using internal clock Bit 1 selects internal clock Bit
31. ate for this voltage but that would make our circuit 21 Chapter 3 Sensorless position detection TVF method very complicated and expensive On following figures we can see the effect of stator resistance on commutation for sinusoidal and trapezoidal BEM forces commutation delay Figure 3 12 Phase delay in motor with sinusoidal BEM force We can calculate delay angle approximately If 2n E Esin ot E Esna 22 3 18 one commutation point should be at ST aot 3 19 6 3 19 but term RI will delay the commutation for angle 0 e sin E sio 240 RI 3 20 e 2esin JE 0 E 3sin0 3 0 E RI 3 21 jo 3 22 J3 E approximations used here are sin 0 which is all right when is near zero On the following figure we can see commutation delay effect at motor with trapezoidal BEMF 22 Belgrade university Diploma theses Sensorless control of brushless DC motor commutation delay Figure 3 14 Phase delay in motor with trapezoidal BEM force It is easy to calculate that a Ld 3 23 6E which is very similar to the first result That was expected since higher harmonics will not affect significantly the comparison In both cases commutation delay is proportional to a term HT 3 24 This term will be referred to as a phase distortion coefficient It should be as lower as possible That means that we should keep I low and E high I is proportional to load torqu
32. ative direction If we take a look at the Table 6 1 which summarizes signals that should be tested for every state we can notice that it corresponds to a definition of these two tables During the commutation adequate value from these tables is placed in variable testVector code line Variable lt test gt contains anticipated value of external signal that is when external signal which is monitored becomes equal to test value commutation should be executed Code line calculates the value of test If state is even test is 0 if state is odd test is 1 This can also be explained by Table 6 1 Condition check is done by a code line This is the most important code line of this part of program One should note that both logical and bitwise AND operations are 69 Chapter 6 Sensorless mechanism software resources used here Lines that follow are a commutation execution Line and following are the same as for the acceleration mode previous section Lines and calculate test and testVector values respectively Variable waitState is introduced in order to prevent untimely commutation When a commutation is executed it is necessary to wait until a current from the phase that is detached decays through diode as external signals may have incorrect values during this decay This effect is notable only at low speeds since BEMF is small and PWM impulse is narrow Also problem exists only if decay is realized through diode
33. ays in one of these two modes External signals i control mode Mode ACC Read Motor acceleration external control mode signals Yes Speed gt S Mode Ext_signal New STC calculation Figure 6 1 Software resources block diagram 22 Belgrade university Diploma theses Sensorless control of brushless DC motor STC stands for State Transition Condition Blocks on the diagram will be coded in interrupt service routine and their execution will be invoked by timer interrupt Pulse Width Modulation has to be implemented as our desire is to control values of phase voltages One part of software has to be dedicated to the reset of IRADK When IRADK signal ITRIP becomes high due to high current or on startup it need to be reset to low level if we want IRADK to function This function is implemented on the button press interrupt Change of direction of motor rotation is also implemented on button interrupt When motor is not blocked ITRIP 0 button press will cause change of rotation direction otherwise it will unblock the motor One part of program is in charge of communication with PC as voltage command has to be accepted from PC and current position has to be sent in the other direction in order to monitor program execution Microcontroller communicates with PC at the rate of 2400 baud sec When byte is received from PC interrupt will occur All previously defined functions are realized in inte
34. capacitors values is very high 20 at 120Hz 20 C On following figures we can see switching power supply which provides us 15V source R29 D8 STTA106U D13 S00mW STTA106U D14 cat Listas HR ion Ve Undervoltage Lockout Met Undervoltage Lockout 11 Latching Voltage PWM Power Feedback O Ground Input B Error Current Output Amplifier Sense Compensation A Q 1 5 Input A A e a o eo e e a o a a E Gnd 9 Figure 4 3 Internal structure of switching power supply control chip 23 Chapter 4 IRADK motor drive MOSFET Q2 inductance L1 and diode D13 compose buck converter It is a very frequently used structure for lowering the voltage Purpose of UC3842D chip is to provide us control of output voltage through current mode control Internal structure of the chip is shown in the second figure Values of capacitance C33 and resistance R33 will determine the frequency of internal oscillator that is PWM controller frequency Current is measured on shunt resistor R41 and fed back to a controlling chip Resistor capacitor pair R39 C36 will filter the information from shunt resistor Output signal will toggle on off our transistor as it will pass to the output voltages on pins 11 and 8 respectively We will not need high side drivers to toggle transistor because entire circuit is floating that is entire circuit is on the high side Circuit power supply is provided thr
35. controller will be disconnected from their corresponding pins on IRADK microcontroller slot while in functioning mode these pin would connect via jumper 5V voltage source mentioned in previous paragraph is provided through IRADK circuitry 25V voltage source is connected at input of 15V stabilizer The switcher is turned off 25V voltage is supplied from a small isolated transformer We cannot use voltage from switcher i e power grid neither from non isolated transformer auto transformer for example since ICD has its own reference that could be fatal to it MPLAB integrated development environment and HI tech C MPLAB is microchip development environment which is used with all PIC microcontroller types It is a user friendly program It has built in assembler compiler and linker Programmer support is also provided as well as ICD software so we can program the chip and debug directly from MPLAB Program that is written can be simulated in the simulator Simulator executes program line by line while values of registers and other variables are monitored in watches External interrupts can be simulated by using a pin stimulus Serial communication can not be simulated Hi tech C for PIC programming is user friendly tool that can be added to MPLAB development environment so programs can be written and compiled without changing working environment Hi tech C for PIC microcontroller family is completely compatible with regular ANSI C from the pr
36. de voltage but half of its amplitude Edc_bus 2 Voltage of disconnected phase is Vdp Vs Edp where Edp is BEMF of disconnected phase We will assume now that spectral diagram of high side voltage is like the one presented on the following diagram 0 fpwm 2 pwm frequency Figure 3 6 High side voltage spectral diagram It has a DC component as we assume that this voltage is slowly changing term and a high frequency PWM terms Spectral diagram of disconnected phase voltage will look like this 0 t 2 H1 3 f1 fpwm 2 fpwm Figure 3 7 Disconnected phase voltage spectral diagram Voltage on disconnected phase is Vdp Vhs 2 Edf so besides the PWM terms we have low frequency BEMF terms which we can call signal while we will be referring to the PWM terms as a noise Term on frequency 3 fl and higher terms which are not on the figure because their value is insignificant thus will not exist if our motor has sinusoidaly distributed windings Now let us take a look at the voltage that we would like to compare to zero Venn V V RI pe EE dt 3 2 gc If we apply analog low pass filter that would cut PWM noise in Vb and Vc we will eliminate the voltage on inductivity also because it is a high frequency signal that is voltage on the inductivity is equal to zero due to volt second balance We will neglect voltage on the resistor as it is very small in comparison to BEMF Now we have 14 Belgrade university Diploma theses Se
37. dition this testing is done on very light shaft with practically no inertia If we had some weight added to the shaft a rotation would be more stabile and PWM ratio change dynamic would be more liberated Possible system upgrades Speed control is a logical upgrade to this system In order to implement speed control we have to measure speed Speed measurement will be implemented in software as any other type of measurement would be impractical it would demand mounting of sensor on the shaft and all this work is about avoiding that Speed would be measured by measuring time between two consecutive state shifts for lower speeds or by counting state shifts in a determined amount of time for higher speeds If a precision of speed measurement is unsatisfying a software observer can be implemented PI type of speed 71 Chapter 7 Conclusion regulator will be applied Output of this regulator is torque that should be applied to the shaft In order to obtain high quality torque control current has to be controlled Current control can be implemented by using one PI regulator since there is only one current that we are controlling here Special care has to be taken a propos state transition since current peaks in these time intervals are inevitable Current measurement can be problematic as discussed in chapter 4 IRADK Hardware peak detector can be added in order to obtain current value that is valid during the entire PWM period Cur
38. e and E is proportional to speed We conclude that undesired mode of function is at low speeds with high load torque elevator applications 23 Chapter 4 IRADK motor drive 4 IRADK motor drive Introduction IRADK is a three phase variable speed motor drive for appliances and light industrial applications It has 230V input in order to be supplied from standard European power grid It has optically isolated RS232 serial link interface to PC Fault protection for over current and over temperature is provided also There is a slot for standard 8 bit microcontroller Using an adapter we can place a high processing power DSP Auxiliary power supplies 15V and 5V are integrated on IRADK There is AC input EMI filter which eliminates higher harmonics from the input current On off switch is placed on the phase of input voltage IRAMS 10UP60A plug and play power module is used as inverter It is made in 600V NPT IGBT technology It has current rating of 5A at temperature 100 C It has cross conduction prevention logic Reduced Emi is provided by optimized gate drive Drive circuit will be analyzed hare part by part IRADK circuit contains the following sub circuits Power supply IRAMS power module circuit Phase current and DC bus voltage sensing circuits Over current protection Serial link galvanic isolation circuit Additional features LED button additional inputs analog and digital 28 pin microcontroller slot Connection str
39. e can transfer speed torque current or voltage PWM command as well as any other command and from microcontroller to PC we can transfer various bits of information that will allow us to monitor execution of microcontrollers program and debug In addition we can observe all the values that A D converters have captured motor current DC bus voltage etc Baud rate of our serial link can go up to 2400 baud s This is usually sufficient for the communication between microcontroller and PC The problem that arose here is galvanic isolation of serial link As we have seen in previous section our power supply is not isolated from power grid reference since we used buck converter that has no transformer in its structure Voltage on the ground of IRADK the one marked as a ground on all schemes will be hovering between 0 and something like 300V comparatively to the ground voltage of power grid while the ground of serial link cable is the same as grids That is why we will have to apply circuit for signal separation IRADK solution is application of two opto transistors one for each transfer direction On the subsequent figure is a part of IRADK circuit responsible for information transfer between PC and microcontroller 34 Belgrade university Diploma theses Sensorless control of brushless DC motor IDO LL4148 SFH618A 3 Figure 4 13 Serial link galvanic isolation circuit TxD and RxD are transfer and receive pins of microcontroller correspo
40. e is not latched into CCPRIH until after a match between PR2 and TMR2 occurs 1 e the period is complete In PWM mode CCPRIH is a read only register The CCPRIH register and a 2 bit internal latch are used to double buffer the PWM duty cycle This double buffering is essential for glitch free PWM operation Asynchronous communication module PIC has integrated USART Universal synchronous asynchronous receiver transmitter module Here will be discussed aspects of asynchronous communication full 47 Chapter 5 PIC16F873 microcontroller duplex as it is the one used in the project In addition it is the most frequently used form of communication Full duplex means that both communication entities transmit data via one line and receive via the other Data that is being transferred has a ten bit format While idle communication lines are in high state logical 1 Device begins transfer with start bit This is a bit with logical level 0 By changing the state of communication bus it is informing the other device that information is about to be sent Then it sends eight data bits one byte and finishes with stop bit which is 1 of course Parameters of data transmission are defined in register TXSTA Transfer status and control register Relevant bits of this register are TX9 nine bit transmit enable bit enable 1 bit 7 TXEN transmit enable bit enable 1 bit 6 SYNC synchronous asynchronous mod
41. e of INTEDG flag of OPTION register Microcontroller programming Programming of PIC microcontroller can be done by a programmer device In that case we would place our microcontroller in the programmer program it and then get it out of the programmer and place it in IRADK or any other device This technique has great drawbacks It takes a lot of time to move microcontroller from one slot to another and if we do it many times it is very probable that we will damage pins of our microcontroller which would make it unusable Solution of these problems can be found in in circuit programming that programming without getting our microcontroller out of IRADK In circuit programming can be applied by the use of boot loader It is a small program that is programmed in high program locations of the microcontroller by a regular programmer Afterwards that program will receive program via serial bus and place it in microcontroller memory This is known as programming via change of configuration In order to function this programming method low voltage programming has to be enabled when programming boot loader as our microcontroller functions with 5V voltage There is a PIC downloader program on PC side that is responsible for the communication with boot loader that is microcontroller When reset microcontroller will wait some time 0 2s for example to receive program from PIC If this fails to occur it will start with execution of program The disadva
42. e select synchronous 1 bit BRGH high baud rate select bit high baud rate 1 TX9D value of ninth bit in nine bit communication can be parity bit Data reception parameters are located in register RCSTA Relevant ones are SPEN Serial Port Enable bit should be set bit 7 RX9 toggles 9 8 bit transfer modes set for 9 bit communication bit 6 CREN continuous receive enable bit 4 ADDEN Address Detect Enable bit enable 1 bit 3 FERR Framing Error bit if set framing error can be updated by reading RCREG register and receive next valid byte bit 2 OERR Overrun error bit overrun error 1 can be cleared by clearing CREN bit bit 1 RX9D 9th bit of Received Data can be parity bit but must be calculated by user firmware bit 0 Both devices have to be set at the same communication baud rate Baud rate is determined by SPBRG register of Baud rate generator It can be calculated from the equation Baud Rate FOSC 16 X 1 if BRGH 1 or Baud Rate FOSC 64 X 1 if BRGH 0 where BRGH is a bit of transmit status register X is a value of SPBRG register 0 255 Communication registers are RCREG and TXREG RCREG receives incoming data Data that we want to send should be written to TXREG transfer will start as soon as data is written There are two interrupts that can be adjoined to asynchronous communication receive and transmit interrupt Flag pair that serves receive interrupt is RCIE RCIF from registers PIE1 and PIR
43. factor 4 PWM frequency is Fiyy 1 25MHz 256 4882Hz Resolution of PWM is 8 bits two additional PWM bits located in CCPRLI register are not used Motor acceleration mode As it is already mentioned motor has to be accelerated to a speed at which BEM forces will be strong enough that motor position can be extracted from their values In order to accelerate motor commutation moments will be determined by timer To put it 61 Chapter 6 Sensorless mechanism software resources briefly value written in timer register will increase until it reaches maximum When it reaches maximum interrupt will occur Commutation will be executed through interrupt procedure Then new value is written into timer register and so on Values that will be written to timer register should increase That way period of commutation will decrease and frequency i e motor speed will rise In this mode of control there is absolutely no feedback so we will not know the angle between rotor and stator flu The idea is to let a high current through motor windings higher than nominal that will produce high electromagnetic torque which will force rotor to follow stator field i e angle between stator and rotor flux will be small High current and torque guarantee that our motor will move Current in motor windings will be I U 2R 2E where U is phase voltage produced by PWM R is resistance of one phase of motor and E is BEMF of one phase of motor This expression is
44. filter will provide us desired signals Our system is shown on figure 3 3 DCS stands for Digital Control System Realization In order to realize described system we will have to do a brief time and spectral analysis of phase voltages Phase that the current is coming from will be referred to as high side while the other end will be referred to as a low side We will suppose that PWM is on the high side of the drive similar result would be derived for low side There are two cases First one is when voltage that is to be compared with floating voltage disconnected phase is a high side PWM phase and other when it is constant low side OV At this point we will suppose that BEM forces are trapezoidal It will ease calculations but the results are similar for sinusoidal BEM forces We will also suppose that current is constant that is neglect current ripple which is OK while our system is not in the process of commutation BEM forces time diagrams are shown on the diagram Figure 3 4 Emax Figure 3 4 BEM forces diagrams Commutation points will be on crossings of BEM forces as previously defined In the time between two commutations BEM forces are constant on connected phases On figure 3 5 we can see diagram of high side voltage Vhs Edc_bus Figure 3 5 High side voltage diagrams 13 Chapter 3 Sensorless position detection TVF method As PWM is applied voltage can be Edc_bus or zero Wye voltage Vs has the same as high si
45. g motor can be connected to one resistor via 3 phase Graz junction This is much simpler than using three variable resistors When working with a bigger motor FAST K2 maximum load that it can support at speeds lower than 2000 rpm is higher than the maximum that we can apply short circuit At this speed BEMF effective value of the other motor is around 38V When we apply maximum load at this speed speed drops to 1660 rpm One phase current through short circuit of other motor is 3 8A Power conversion is P 3 3Q 3 84 130W 7 1 Three ohms is resistance of one phase 130W corresponds to loading torque M 0 77Nm There is a limitation in PWM duty ratio change dynamic At low speeds PWM duty ratio has to be changed gradually otherwise motor will block This occurs because sudden rise of PWM impulse causes noise in our system that prevents our position detection circuit to work properly As speed gets higher BEM forces rise and this noise becomes insignificant Practical results show that the following sequence of duty ratio command is the most rapid that does not block the rotor 5 12 20 50 of 255 To those who work with this system this may seem a very serious restriction because the only way to control PWM duty ratio is by a serial link and they have to type every single value from the array 5 6 But if we have the software that will smartly increase decrease duty ratio this limitation would be practically insignificant In ad
46. gh states Next block of code will be executing on timer0 interrupt occurrence interrupt void isr static unsigned test 0 68 Belgrade university Diploma theses Sensorless control of brushless DC motor static unsigned test Vector 0 static unsigned waitState 3 static unsigned temp if TOIF if waitState gt 0 waitState if RC2 PORTB OnVector else PORTB OffVector if waitState 0 if testVector amp PORTC 0 amp amp test 11 testVector amp PORTC 0 amp amp test state direction M bi if state gt 6 state 1 else if state lt 1 state 6 OnVector TableOn state OffVector TableOff state if state amp 0x01 test 0 else test 1 Rs if direction 1 testVector testTablel state else testVector testTable 1 state waitState 3 FER TOIF 0 TXREG state j External signals A gt C C gt B and B gt A are connected to PORTC pins 3 2 and 0 respectively Their values are determined by a logical AND test Bits that are to be tested are determined by a two tables one for each direction const unsigned testTable 1 7 0 2 1 8 2 1 8 const unsigned testTablel 7 0 0 1 8 2 1 8 2 Values 8 2 1 correspond to bits RC3 RC1 RCO 8 00001000 RC3 2 00000010 RC1 1 00000001 RCO Values O from the table are never used Table testTablel refers to positive direction of rotation while testTable_1 refers to neg
47. gt A 2 B gt A 3 B gt C 4 A gt C 5 A gt B 6 C gt B Since there are three phases A B and C letter on the left side of the arrow is the phase that the current is running into and a letter on the right side is the phase that current is running from Bits 7 2 are in charge of toggling on off inverter switches so a simple write to PORTB will turn on off desired transistors PORTB lt 7 2 gt A hi B hi C hi A low Blow C low Pin 1 is ENABLE 1 should be written to it Pin 0 of PORTB is input thus values that are written to it are ignored Every state has two assigned vectors one for impulse and one for pause of PWM period They are defined by following tables const unsigned TableOn 7 Oxff Oxce Oxae Oxba 0x7a 0x76 Oxd6 const unsigned TableOff 7 Oxff Oxee Oxee Oxfa Oxfa Oxf6 Oxf6 First hexadecimal number in parenthesis is never used as it has index zero and there is no state zero It is there because an array in C has to start with index zero Value of state zero vectors is FF which guarantee that all switches will be off if this state occurs When written to PORTB value from TableOn will turn on two switches of inverter one on the high side and one on the low side Corresponding TableOff vector will turn off high side switch During state change values of new state vectors are written in variables OnVector and OffVector OnVector TableOn state OffVector TableOfft state These code l
48. have complicated space vector modulation nor multiple current sensing that is we only have to control one current with one PWM module In spite of it is simplicity we still have vector control that is direct control of the torque Belgrade university Diploma theses Sensorless control of brushless DC motor 2 Position sensing Types of position sensing In the machines with permanent magnets on rotor there are various types of determining rotors position We can divide them in explicit and implicit Explicit are the ones that are mounted on the rotor or inner side of the stator They include incremental indicators encoders resolvers and Hall sensors Implicit sensors extract commutation information from phase voltages and can be called sensorless Probably the best ways of a position sensing are optical encoders and synchronous resolvers They can give us a very precise position of the rotor and we can determine very precisely rotors speed by differentiating their position These types of position detectors can give us position of rotor independently of a type of machine Problems with these types of detectors are their price and the fact that they have to be mounted on the shaft which complicates the whole mechanism In brushless DC motor there is no need for continuous position detection We only need to detect position on every 60 degrees in order to commutate properly Low resolution sensors can be used one optical sensor for exam
49. his chapter Voltage measurement is realized on upper part of the image ANI presents analog input of our microcontroller Voltage on 1t will be proportional to the voltage of DC bus For a DC bus voltage of 385V voltage on ANI will be 3V which is maximum input to A D converter as the A D reference voltage is also 3V This result is marked with gray color on the figure It can be calculated easily as the resistors placed between DC bus AN1 and ground are a simple voltage divider Capacitors serve for noise rejection Current sensing problem is more complicated Circuit dedicated to it is on the lower part of figure We are measuring current in DC link Current from electrolytic capacitors supplies motor through inverter as well as other parts of IRADK Assumption is that all current taken from this source is negligible in comparison to the current that goes to motor This is all right because the current that goes to IRADK is seen as current that goes out of DC link multiplied with factor f 15V A o 4 1 f DC bus voltage n due to switcher function So we can say that the current which runs through the shunt resistor R58 is approximately equal to the phase current of the motor Since we are working with brushless DC motor one phase will always be disconnected so voltage on shunt resistor will all the time be proportional to the only current that runs in our motor 29 Chapter 4 IRADK motor drive When working with other types of moto
50. ied ilo movlw 1 movwf var ied movf pclath temp w movwf PCLATH swapf status temp w movwf STATUS swapf w_temp f swapf w_temp w retfie end In the third program assembler original the entire context is saved upon entering in interrupt service routine C program automatically saves the context We can see that program written in C is similar to 1ts assembler equivalent thus in many cases will not produce holdup in execution Context saving as well as some other processes may be unnecessary in that case we may choose to write our program in assembler or add some lines of assembler code in C In high speed real time systems it is recommendable to write interrupts in assembler while specific routines called from interrupt can be easily written in C We can insert assembler code lines into C program by using directives asm and endasm 57 Chapter 6 Sensorless mechanism software resources asm code endasm In code section we can use all variables active in that part of C program We can also use predefined variable RETURN_ to store C function result in it Serial link program on PC Serial communication program on PC side that is used in this project is written in ansi C This program will not be listed here as it is unnecessary This program is adjoined to this project It consists of two ansi C files DOSP C and DOSF C dos is because they work in dos In DOSF C all constants are defined and usartcomm function is
51. implemented DOSP C is a user of DOSF program In it we can change parameters of asynchronous communication parity bits baud rate communication port COMMI COMMA etc Compilation of this program will produce executable program version that can be transferred to every computer It is important to have source code of this program so we can change parameters and recompile it when needed When started this program will automatically write on the screen all values received from the microcontroller Write is accomplished by typing a number we want to send to microcontroller while holding ALT This is DOS program so it may not work in windows NT and later DOS independent operating systems It is the simplest form of PC communication program but its performances are sufficient Serial link on microcontroller side Main program initializes serial communication through the following code lines TXSTA 0X23 Serial link configuration RCSTA 0X90 SPBRG 129 RCIE 1 PEIE 1 er CREN 0 ERE CREN 1 Non parity one start one stop bit asynchronous non addressing communication is the meaning of first two lines Communication baud rate is set at 2400 baud sec Receive interrupt is enabled Lines clear the OERR flag that may be set due to previous overrun error occurrence Next code lines will enable PWM duty ratio modification via serial link if RCIF dutyR RCREG 58 Belgrade university Dip
52. ines are repeated two times throughout the program since there are two modes of function thus two modes of state changing Switch toggling i e PWM is realized by consecutive forwarding of On OffVector values to PORTB PWM module of our microcontroller has its output on RC2 pin of PORTC This module is used here indirectly RC2 output is read by software and only by software and then depending of its value OnVector or OffVector is forwarded to PORTB if RC2 PORTB OnVector else PORTB OffVector These lines should be executed as often as possible in order to have high quality PWM especially if impulses are narrow In our program they are in main and in some parts of interrupt service routine thus will be executed whenever there is no interrupt and in some parts of interrupt procedure Main program without initializations looks like this while 1 if RC2 PORTB OnVector else PORTB OffVector 60 Belgrade university Diploma theses Sensorless control of brushless DC motor AS we can see main program only executes PWM while waiting for interrupts to occur This is not the usual way of PWM realization but it has some important advantages The other way would be to use timer and interrupts but then we would have to calculate pause period or impulse period if pause is defined More important some parts of interrupt service routine of this project can take a lot of processor time halting that way our PWM At the end of
53. ips St Oy Oe ee Power supply High voltage rectifier circuit from IRADK is shown on the following figure 2 ONIOFF DC BUS R45 NEUTRAL EARTH PHASE EMI 3A 2x2mH Figure 4 1 Rectifier circuit 24 Belgrade university Diploma theses Sensorless control of brushless DC motor Variable resistors R45 and R55 serve to limit input current during initialization That is when we plug in our IRADK it has a tendency to drain high current from power grid as electrolytic capacitors C37 C42 are completely empty Capacitors C40 and C41 in conjunction with EMI inductances block higher harmonics of input current EMI filter Element RV1 serves to limit voltage on its connections as it will start to conduct thus will lower the voltage when it gets too high 275V Capacitor C46 is placed between ground and zero connection its purpose is to collect static electricity from our device case when ground is accidentally disconnected Selector S3 is not placed in its position that is it is always in the position on the picture thus this IRADK can only be used in Europe We could place S3 on the board but as it is highly unlikely that we will use this circuit both in Europe and United States and it is very possible that we will destroy IRADK by toggling the switch accidentally for that reason we did not put it Resistors R51 and R53 are used in order to balance the voltages on electrolytic capacitors as tolerance in electrolytic
54. is a highest standard baud rate communication that will function next one is at 9600 We can have a higher receive rate but we cannot have higher transmit rate because of resistor R20 and capacitor C17 which will damage higher frequencies of our data signal Problem could be solved by lowering the values of these components though they are SMD 35 Chapter 4 IRADK motor drive Additional features LED There are three light emitting diodes red yellow and green on IRADK which can be used for various signaling purposes Red could indicate some fault green that everything is all right etc They are in a common anode configuration therefore will light up when logical 0 is applied to corresponding pin 5V LED1 LED2 LED3 RED ds Figure 4 14 LED on IRADK Button One button is sited on IRADK It can be very useful as we will see later Pressing of button will cause low logical level on related pin which is normally on high level This button needs to be debounced in software since pin signal will oscillate after being pressed R21 5V TAST 1K Figure 4 15 Button on IRADK Working with button in interrupt mode can be unreliable nevertheless that is the mode we will use Additional analog input Via this feature we can pass one external signal or a voltage from variable resistor to analog input of microcontroller This is done by putting a drop of tin on a Gl or G2 tin slots We should not put it at both slo
55. is connected to PWM block thus will have to be output pin thus can not be input pin Since LED are not very important we will take out two diodes and put two digital input wires in their place 39 Chapter 4 IRADK motor drive m E D Fy E E B E E E Y i z a r4 L tn tn B z r4 ra T a li Li gU Aj D TELLI ri Scheme 4 1 Position detecting circuit connection to IRADK On the previous figure we can see IRADK modification as well as its connection to female DB9 connector to which position detector will be connected later Green and red LEDs are taken of the board in order to connect inputs from position detector X marks their anterior location We chose them because they are connected to PORTC just like digital input SP2 so we will be able to read states of all digital inputs in one instruction cycle 1 e software will be simplified Auxiliary power inputs are also added to IRADK They are attached between 15V linear regulator input and the ground 25V galvanic isolated power source is connected here The purpose of this is to supply voltage for microcontroller programming I used them also in the first phase of project to test my program as connecting IRADK to power 40 Belgrade university Diploma theses Sensorless control of brushless DC motor grid voltage can be problematic an error in software can damage IRADK while it probably will not damage it if it is connected to
56. is from phase B to phase C Ege will be increasing in the direction marked on the figure In one moment it will become higher than Egb 11 Chapter 3 Sensorless position detection TVF method Figure 3 2 Electrical model of three phase brushless DC motor That is the moment in which the commutation should be done in order to maintain the maximum torque This type of examining is obviously equivalent to the first one figure 2 as it will provide the same moments for a commutation There is no easy way to measure BEM forces because they are buried inside the motor but there is a way to determine their values approximately as they are imaged in phase voltages In our case from figure 3 shifting of current from phase B to phase C should occur when Ege becomes higher than Egb Let us take a look at the phase voltages I V V Ep L T RH V V E 3 1 Vs is wye voltage We can suppose that motor speed is high enough to be Egxm gt gt R I so we can neglect voltage on the resistor If we could eliminate voltage on the inductance we would have Vb Vs Egb gt Vc Vb Egb Egc This means that one simple analog comparator could indicate which BEMF is higher TO EE DES Figure 3 3 Position detector first approximation 12 Belgrade university Diploma theses Sensorless control of brushless DC motor As we will see voltage on the inductance can be eliminated by filtering of terminal voltages One low pass
57. lemented if we want our system to perform simple tasks Variable resistor can be placed on IRADK Voltage from this resistor can be our speed command On button we can implement direction control and or toggle computer manual control This could be useful when validating system functioning Possible applications This system without upgrades has a transfer function of DC motor with fixed excitation permanent magnet It has no mechanical commutator and it is a great advantage because there are no parts that could wear out there is no speed limit and it is more ecological since mechanical commutator usually contains lead that is very hard to dispose Brushless DC motors also have better power to volume ratio It is possible that in near future brushless DC motors will completely substitute regular DC motors A propos our system it can be used whenever a continual speed change is required We can use scalar controlled asynchronous motor as a cheap solution but if we need quicker response this system is superior Disadvantage is its use in systems that require frequent direction change since every direction change implies mode change i e time I suppose here that we have a simple speed control implemented without current sensing With the upgrades from previous section our system can be considered as a high quality drive In this case it is competitive to a drive with regular synchronous motor 72 Belgrade university Diploma the
58. loma theses Sensorless control of brushless DC motor TXREG dutyR CCPR1L dutyR When a value is received from PC it is automatically placed in CCPRIL register echo is returned to PC During the program analysis and debug in some places of program some values are sent to PC All this is accomplished by a simple write to TXREG Button press functions IRADK button is used for two purposes reset of IRADK over current protection latch and changing of motor direction When button is pressed interrupt is generated Button is not debounced so it may happen that one button press causes two interrupts Debouncing is avoided because it would use a timer and would be too complicated and as we will see it will not be necessary Following part of interrupt service routine is meant to work with button if INTF if RC4 temp TRISC TRISC TRISC amp Oxdf RC5 1 TRISC temp eee else if inc inc 0 else inc 1 FERRE INTF 0 When button is pressed INTF flag is set Pin RC5 is connected to reset of protection circuit Information of this circuit state is on the pin RC4 If over current protection latch is set RC4 is on high logical level In order to reset a latch RCS is made an output and logical 1 is placed on it for a short amount of time roughly lus Then a circuit is returned to its previous state RCS is an input thus has no effect on protection circuit functioning If interrupt
59. m will start to read external signals and decide whether or not a commutation should be executed I will mention here that when analyzing a filter function in chapter 3 decay is supposed to be instantaneous From the point of view of filter analysis this approximation is fine considering that current decay time is only a fraction of one state duration time At the beginning of this routine we can see two lines that refresh PWM state These lines are here in order to prevent harming of PWM quality since procedure that follows can take to much time to execute Last line of code will write a current state number on PC screen enabling us to monitor a program execution Naturally it will not affect a program execution 70 Belgrade university Diploma theses Sensorless control of brushless DC motor 7 Conclusion System performance In order to test the quality of this drive two brushless motors connected to the same shaft are used Both motors are of Italian company moog First is FAST K2 nominal speed 3000 rpm with 2 pair of poles Second is FAST T1 with 3 pairs of poles nominal speed is 4500 rpm Our drive can work with both motors though significantly better with the first one as BEMF to winding resistance ratio is much higher for the same load chapter 3 While one of these motors is connected to the drive the other is connected to variable resistors By decreasing the resistance we are increasing the load and vice versa Loadin
60. mmed to microcontroller by a real programmer Each of the PORTB pins has a weak internal pull up A single control bit can turn on all the pull ups This is performed by clearing bit RBPU of OPTION register Four of the PORTB pins RB7 RB4 have an interrupt on change feature This interrupt can wake the device from SLEEP RBO INT is an external interrupt input pin and is configured using the INTEDG bit OPTION _REG lt 6 gt 43 Chapter 5 PIC16F873 microcontroller In this project IRAMS control signals are attached to PORTB 7 2 ENABLE control signal is attached to RB1 Button is attached to RBO INT and it works with interrupt PORTC is an 8 bit wide bidirectional port TRISC register functions in the same manner as TRISB and TRISA PORTC is multiplexed with several peripheral functions PORTC pins have Schmitt Trigger input buffers When enabling peripheral functions care should be taken in defining TRIS bits for each PORTC pin On the following table we can see what can be functions of each and every pin PORTC FUNCTIONS RCO T 10SO T1CKI Input output port pin or Timert oscillator output Timer1 clock input RC1 T1OSI CCP2 bit ST Input output port pin or Timer1 oscillator input or Capture2 input Compare2 output PWM 2 output RC2 CCP1 bit2 ST Input output port pin or Capture1 input Compare1 output PWM1 output RC3 SCK SCL bit3 ST RC3 can also be the synchronous serial clock for both SPI and 12C modes RCA SDI SDA bia
61. mutation Here we will analyze the effect of stator resistance on commutation we will see in some modes of brushless DC functioning it can cause significant commutation delay In order to do an analysis more complete than the previous ones we will take another look at a following figure there is the same figure in this section Figure 3 12 Electrical model of brushless DC motor There are two cases one for each direction of rotation dI 1 Veg Ve V tee E Vg lt Vc we are waiting until V gt Vo and than we commutate se V4 gt Ve we are waiting until V gt V I 2 Vic V V LT RISE E In one case we are disconnecting high side and in the other low side We will analyze only the first case because its situation is very similar to the second As the same filter is applied on both voltages that are about to be compared we can apply low pass filter on the equation 1 we will have Ves anar E Ve high frequency PWM noise V high_frequency_induction_noise 3 16 RI E Ep As analyzed in previous section we can neglect high frequency PWM noise on phase voltages Noise on inductivity will actually decrease total noise because its sign is opposite to a sign of phase noise and its smaller in amplitude That means that we can write V C signal RI Ey Ey 3 17 Term RI will cause commutation delay as there is no way to eliminate it We could add some elements in order to compens
62. n that could occur due to oscillating of comparators output Standard DB9 connector will be used for a connecting of our circuit and IRDAK On our circuit side is the male connector because it does not generate voltages it is completely passive it gets its voltages from IRDAK I have to mention that voltages that come from IRDAK are not isolated that is their potential to earth can be very high so when connected to IRDAK our circuit should not be touched Circuit inputs phases and outputs are marked with colors The same are the colors of the wires that are used to make the circuit The same are the colors of wires that are used on IRDAK female connector All of this is done in order to ease the connection and avoid errors Color of phase C input wire is white I put a brown dash line so we can see it Picture 3 1 Position detecting circuit Signals B gt A C gt B A gt C are led to the pins of microcontroller 16F874 and can be read in any moment by our program In the following chapters IRDAK drive and microcontroller will be described Later we will get into the functioning of our circuit software and the whole drive together Method TVF phase delay problems In previous sections while we were analyzing our circuits function voltage on stator serial resistance was neglected as our assumption was that it is very low thus will 20 Belgrade university Diploma theses Sensorless control of brushless DC motor not affect the com
63. ndication that timerO interrupt occurred and should be cleared at the end of timerO interrupt procedure In this project timer0 will be used a reference time base source of our system Timer1 module is a 16 bit timer counter consisting of two 8 bit registers TMRIH and TMRIL which are readable and writable The TMRI Register pair TMRIH TMRIL increments from 0000h to FFFFh and rolls over to 0000h Flag pair TMRIIF PIR1 lt 0 gt and TMRIIE PIE1 lt 0 gt functions in the same manner as timerO flag pair only here this flags are bits of peripheral interrupt flag enable registers respectively Timerl operation is defined by Timerl control register t1con Timerl can operate in one of two modes as a timer and as a counter The operating mode is determined by the clock select bit TMRICS TICON lt 1 gt When in timer mode Timerl increments on every instruction cycle In counter mode it increments on every rising edge of the external clock input When the Timerl oscillator is enabled TIOSCEN is set the RCI TIOSI CCP2 and RCO TIOSO TICKI pins become inputs That is the TRISC lt 1 0 gt value is ignored and these pins read as 0 45 Chapter 5 PIC16F873 microcontroller Bits TICON lt 3 4 gt are prescale bits By choosing them to be 00 01 10 11 we will get prescale values 1 1 1 2 1 4 and 1 8 correspondingly Timerl can be enabled disabled by setting clearing control bit TMRION T1CON lt 0 gt In this project Timerl will be
64. ndingly Power to left part of this circuit is supplied through pins 4 DTR Data terminal ready and 7 RTS Request to send of RS232 bus For data signals the ones we are transmitting and receiving the on state occurs when the received signal voltage is more negative than 3 volts while the off state occurs for voltages more positive than 3 volts For control signals DTR and RTS in our case the on state occurs when the received signal voltage is more positive than 3 volts while the off state occurs for voltages more negative than 3 volts The voltage between 3 volts and 3 volts is considered to be the transition region and the signal state is undefined DTR signal must be on if we want our communication to function It is desirable that RTS signal be set to off Communication will function with RTS set to on but if a cable that connects IRADK to PC is longer we could have faults due to noise induced in cable Values of control signals are set in PC program responsible for PC IRADK communication I personally used DOS program written in Borland turbo C because that was the simplest way to ensure data transfer which in my case was not a problem considering low data amount that needed to be transferred For more advanced use it is recommendable that we use GUI graphical user interface that is provided with IRADK system Our serial communication can work with speeds up to 2400 baud sec That is 2400 baud sec
65. nsorless control of brushless DC motor Vas Vp V Ey Eye 3 3 cmp Now it is clear that zero crossing of this voltage is the right moment for a commutation In previous paragraph we assumed that filter will cut PWM noise and preserve BEMF signal so one could suppose that one analog first order filter with a pole placed on the frequency of highest possible BEMF frequency let us call it Fh In our system nom sk Soe 3 4 Fy where is nominal speed p is number of pairs of poles not to be mixed with filter poles The problem that is neglected is the phase characteristic of our filter If we place a pole on frequency Fh it will shift phase of our signal and provoke phase delay which would result in delayed commutation that would degrade the performance of our system It is well known that phase characteristic starts to bend one decade behind the pole so placing a pole on frequency 10 Fh would be desirable On the other hand placing a pole on 10 Fh frequency will decrease rejection 0 1 Fh Fh 10 Fh 100 Fh 1000 Fh 40dB dec Figure 3 8 LP filter amplitude characteristic 15 Chapter 3 Sensorless position detection TVF method 0 1 Fh Fh 10 Fh 100 Fh 1000 Fh Pi Z dec Figure 3 9 LP filter phase characteristic My project decision is to place double pole on frequency 10 Fh Amplitude and phase characteristics are shown on the previous figures Bode approximation As we can see from diagrams BEMF compone
66. ntage of this method is use of higher memory locations of microcontroller less resources Besides it is possible to write program over a boot loader by mistake In that case boot loader would have to be programmed again and then we must have the programmer The other form of in circuit programming is to add an adapter to microcontroller slot on IRADK and place a microcontroller in it In programming mode we will connect programmer cable to it and program In program execution mode we will place one jumper on a programmer connection This is the method I used The only difference is that I used ICD In Circuit Debugger instead of programmer It is a device that can do 49 Chapter 5 PIC16F873 microcontroller everything that programmer can do plus it can be connected via serial bus to microcontroller while it is functioning and then it could read the states of microcontroller registers display them on the screen change them etc These features will not be used in the project If we want our ICD to function we have to connect it to pins RB6 RB7 MCLR 5V GND to it Communication will be realized via serial link through pins RB6 and RB7 ICD will get a power supply from 5V and ground pins This voltage will be used for ICD functioning as well as for a conversion to 12V via boost converter that is integrated on ICD 12V supply is sent back to controller via MCLR pin for programming purposes In programming mode RB6 RB7 MCLR pins of micro
67. nts will be completely preserved and their phase will not be shifted In order to analyze PWM noise rejection we will have to adopt PWM frequency and Fh highest frequency in BEMF spectrum Practical realization In order to understand the functioning of this system it is best for the reader to take a look at External signals control mode section at the end of this project figures of six states and transition conditions In this thesis we will use motor FAST K1 of moog It is nominal speed is 3000 rpm it has four poles two pairs which means that our frequency Fh is 100Hz PWM frequency used in this work is 5000Hz Rejection of PWM noise is A 40 log 7 27 96dB 3 5 10 F This is rejection of the first PWM harmonic we will neglect existence of higher harmonics as their rejection is very high gt 40dB We can now calculate ripple of voltage on filter output Ripple will be calculated for a duty ratio of 50 because that is the worst case Amplitude of the first harmonic of PWM on filter input is O wMinput E U Dc bus 2 cos 6 46 U pus 3 6 x C 0 ale Our rectifier is connected to a regular power grid so we have 16 Belgrade university Diploma theses Sensorless control of brushless DC motor U y tas 220V 240V 42 lt 340V 3 7 always Which means PWMinput ur 2 340V 216V 3 8 T Voltage on filter output will be A U yMoutput ia t 10 2 8 6V E 3 9
68. o the job That is an advantage because motors with sinusoidaly distributed stator windings make motor more expensive The idea of BLDC motors is to have one phase disconnected while the current which runs through the other two phases is controlled This current causes torque which pulls rotor When rotor gets into the right position we should do the switch that is we should disconnect one phase and turn on the one that was disconnected The current will then decay through the diode It resembles a step motor with six steps If we have three phases the steps are A gt B A gt C B gt C B gt A C gt A C gt B meaning the current goes from phase A to phase B from phase A to phase C etc take a look at Fig 1 The number of steps will be multiplied by a number of pairs of poles but that will not have influence on a control of our motor and will be neglected at this point that is we will assume that motor has one pair of poles 2 poles HV high voltage DC Figure 1 1 Brusshless DC motor functioning The name BLDC electromotor comes from the similarity of a transfer function of this motor and a simple DC motor If we connect our motor to a voltage it will react in the same manner as a DC motor with independent supplies of armature and stator In that analogy permanent magnet plays the role of a stator in DC motor and stator plays a role of windings Our digital hardware will do the job of the commutation one advantage more we do not
69. ogramming point of view All statements commands and syntax are the same Even include libraries are the same they have the same functions implemented Since interrupt system is not vectored that there is only one interrupt procedure we can define only one function that will be interrupt adjoined interrupt void isr Values of microcontroller registers are accessed by their name for example PORTB TMRIH etc To them we can access from any part of program This method of programming is very simple as there is not much new for an engineer to learn in order to start programming PIC There are also specialized functions that are used to modify and read values of microcontroller registers These functions will enable more organized program development but will not affect program function these functions are not used in this project Mathematical operations of multiply divide and even more complicated like log sin etc from math h include file should be used carefully or avoided Time of their execution can be very long so they are not suitable for real time or any other time critical applications In this project they are avoided Majority of variables that are used in this project are 8 bit variables There is only one 16 bit variable It is natural to use 8 bit variables when working with 8 bit microcontroller 50 Belgrade university Diploma theses Sensorless control of brushless DC motor Regards on simulation are the same
70. ol is increased by tStep value tlControl is written to timer registers TMR1H TMRIL tStep value is decreased by 5 in every cycle its initial value is 637 Motor accelerates until tl Control value reaches 40000 These values 5 637 40000 are more or less empirical that is they are the result of testing not a precise calculation The calculation that is used was only approximate and it only yielded the first results that I changed later through experimenting Though the motor will not be accelerating ideally linear In the following graph we can see the acceleration characteristic Time s 7 46 Figure 6 2 Acceleration characteristic 64 Belgrade university Diploma theses Sensorless control of brushless DC motor Although we do not have linear speed augmentation this type of acceleration will preserve a processor time and resources significantly Any other more linear type would have to use one more timer and or operations like getting a square root reciprocal value multiplication etc and their execution is very slow When t1 Control value reaches 40000 our system shifts to the other mode of control control by digital input signals Value of 40000 written to TMR1H TMRIL register pair is correlated to motor speed of 2 3 rpm for a 4 pole motor 4 6 for 2 pole motor When motor gets to revolve at this speed we are changing a control mode Change of control mode is done by stopping a timerl and turning on timer0
71. ough resistors R30 32 We may wish to supply entire IRADK with lower voltage in that case we will have to short circuit one or two of these resistors in order to provide sufficient current Circuit on the left side of the control chip serves to provide voltage sensing When transistor is off diodes D8 D9 D12 and D13 conduct the current proving us information about output voltage The rest of this circuit filters this information Compensation is done by a local feedback through compensation pin 1 it serves to provide stability of our control loop D10 and D14 are protection diodes Output of our power supply is fed to a 15V linear regulator Stabilized 15V source is used as a supply of certain blocks on IRADK as well as input of a 5V linear regulator following figure C21 C23 100uF U4 100uF 40V 7815 25V 15V swither output C24 15V C29 Figure 4 4 Stabilizing circuits on IRADK 5V source is used as a microcontroller supply it also servers as a supply for some blocks On the figure we can also see a 3V source It is made using potential divider and capacitor which is fine if we bear in mind low current capacity that will be required of this source It will serve us as a reference source for microcontrollers A D converters as well as a reference in protection circuit 26 Belgrade university Diploma theses Sensorless control of brushless DC motor IRAMS10UP60A power module Utilization of this chip will provide ext
72. ple We must bear in mind that dust may cause these sensors not to function So in BLDC drives normally if we want to control only speed not position optical encoders and synchronous resolvers are not used Most frequently used structure for position detection is the set of three Hall sensors on stator Principle of their functioning is based on the Hall effect Current which runs through the hall element in conjunction with magnetic field which we want to measure causes potential difference on Hall sensor due to magnetic force F qv x B Sensors used in brushless DC motor are usually bipolar On fig 2 1 is one typical Hall sensor structure Voc MA a l 1 3 OUTPUT 2 GROUND Figure 2 1 Position sensing via hall element Hall element is marked with X which means that magnetic field is getting into it from upper side of this paper Block reg provides constant current in Hall element Potential difference on ends of sensor is fed to an amplifier and than on hysteretic comparator When magnetic field is stronger than threshold output is 1 and when it is weaker than Chapter 2 Position sensing threshold output is 0 State between thresholds is undefined Output signals from three sensors are used as feedback in our control system Disadvantage of this system is its sensitivity to PWM Noise generated by PWM may cause this system to trigger incorrectly The main problem here is high cost of Hall sensors and their mounting
73. r If we examine circuit from the last figure we will see that it has two stable states due to positive feedback 1 Output is low voltage on pin 5V voltage on pin 3V 2 Output is high output transistor is ON V 0V V 3V RESETSC signal is in high impedance state When everything is fine and our circuit is functioning normally circuit is in state 1 Current overload will change circuit is state to 2 We can return our system to state 1 if apply low voltage at RESETSC pin That is what we will do in software When system gets to state 1 we have to return RESETSC pin to high impedance state otherwise current overload protection will be disabled this must be avoided as consequences can be fatal transistors from active leg can be destroyed due to high dissipation Third comparator inverts the comparison value from second comparator Fourth comparator is one form of logical or gate Its output will set ITRIP signal if occurred current overload or if ENABLE signal is zero ENABLE signal is connected to the pin of microcontroller thus can be modified in software 32 Belgrade university Diploma theses Sensorless control of brushless DC motor 5V SHORT_CIRCUIT ITRIP LL4148 100n Figure 4 11 Protection circuit 3 3 2K2 3K3 microcontroller 5V input port bit SHORT CIRCUIT Figure 4 12 Microcontroller input When current overload occurs or when ENABLE signal changes it state to logical 0
74. remely simple and compact solution C1 22uF 63v PHASE C cs 22uF 63V PHASE B C11 22uF 63V PHASE A gt pc Bus R14 2E2 0 8 A_HIL gt B_HiL_ gt 8C H 7 HALO gt B_LO gt C_LO __ gt ITRIP gt D19 sv c2 100n 100n C12 100n C13 100n 275VAC x1 C19 100n C20 100uF 25V 27 Q C UP VBB C UP OUT NC NC DC BUS NC A LO EMIT B LO EMIT C LO EMIT amp HI IN A iHi IN B HI IN C WLO IN A 8LO IN B 8LO IN C ITRIP VDD 15 GND Figure 4 5 IRAMS inverter connection on IRADK It is one modern inverter It has integrated gate drivers and bootstrap diodes It has temperature monitor as well as temperature and over current shutdown Package is fully isolated It has matched propagation delays for all channels 5V input Schmitt trigger inputs cross conduction prevention logic isolation up to 2500Vac min Inverter power rating is 0 4KW at input voltage 100 253V More information can be found on internet pdf files related to IRADK 10 On the figure we can see IRAMS connection to IRADK Purpose of capacitor C13 and resistor R14 is to delay DC bus voltage appearance as it needs to be delayed to the occurrence of 15V voltage if we want our circuit to function correctly Power supply for hi side drivers is ensured through electrolytic capacitors Cl C5 C11 and their bipolar capacitor pairs For all phases of inverter these capacitors are Chapter 4 IRA
75. rent measured this way would have a PWM ripple This problem could be solved by synchronizing PWM with current measurement Synchronization usually requires both PWM and current measurement to have the same timer reference Hence PWM would have to be on interrupt this implies lower quality of PWM Some hybrid solution would probably be the best Dynamical breaking recuperation can be implemented by a simple software intervention States should be shifted in the manner that stator flux follows rotor flux Practically two tables similar to testTable 1 and testTablel previous chapter should be added Two tables for PWM should be added also While breaking dynamically DC link voltage has to be measured since voltage of the link will rise When DC link voltage gets too high breaking has to be halted otherwise link capacitors can be damaged Dynamical breaking probably would not have any practical value in fact it would only serve us if there is a high inertia on the shaft but we must bear in mind that its implementation demands only some software interventions Serial communication can be improved Graphical user interface can be used in order to communicate more comfort with IRADK microcontroller In addition we can assign an address to our microcontroller so it can serve as a part of a complex system which is controlled using one central PC In that case software has to be changed both on PC and microcontroller side Manual control can be imp
76. rrupt service routines except PWM It is normal to realize PWM via interrupt but this is not the case in this project since there are some efficiency reasons that will be discussed later Beside PWM main program is also in charge of variable initialization just like in all microcontroller systems In this chapter all previously mentioned software functions will be discussed in subsequent sections thus program will be analyzed part by part In appendix A entire program can be found But before we get to the analysis of microcontroller program we will take a brief look at the advantages and drawbacks of C ASM programming and also serial link program on PC side Hi tech C Vs regular PIC assembler programming It is well known that advantages of comfort program writing in C are followed by sluggish execution of program and we know that speed is crucial to real time applications In this section we will see that C program execution is sufficiently fast for this application by comparing one simple program written in C and assembler Following program has interrupt routine that toggles the value of Boolean variable lt var gt In the main program a value of PORTB bit 0 is toggled oscillator on RBO There are three listings C code assembler code produced by program in C and then independently written assembler code that has the same function C program include lt pic1687x h gt interrupt void isr static int var if TOIF if var
77. rs currents have to be calculated from this piece of information only That can be difficult and sometimes impossible DC bus Figure 4 8 Switching process The problem that we have here which will be problem with other motor types also is that we will have current information on shunt resistor only while current runs from one phase to another through upper transistor In PWM interval in which upper transistor is off current will run through the diode and will not close through shunt resistor figure Shunt signal is fed to a filter that will cut higher frequencies Its poles are placed at about 1500Hz That means that PWM component of our signal will be significantly rejected In order to measure the current we will have to multiply a value that A D converter gives us with PWM duty ratio factor in order to obtain the correct value This multiplication has to be done in software and can be problematic We could use a peak detector to hold the value that is measured during the first interval of PWM period when high side transistor is on current runs trough the shunt It would consist of diode and capacitor and perhaps operational amplifier Analog voltage values on ANO pin of microcontroller obtained by measuring the current are marked on figure with gray color They are valid for the case of current that runs through the shunt resistance long enough that we can neglect the existence of capacitors on the scheme Protection circuit This is
78. s on inverters low side If PWM impulses are narrow we can suppose that the situation is like the one in the following image Disconnected phase Figure 6 9 Current decay from disconnected phase V is voltage in the wye It is near to zero volts as BEM forces are low and voltage is zero on both connected phases PWM pause For those reasons voltage on inductivity will be low and current will decay vary slowly If we analyze decay throughout states we can notice that this type of decay will occur in following state transitions 2 gt 3 4 gt 5 6 gt 1 for positive direction and 3 gt 2 5 gt 4 1 gt 6 for negative direction of rotation This will cause states 1 3 and 5 to be skipped at positive direction and states 2 4 and 6 at negative direction In chapter 3 when filter was presented it was supposed that decay is instantaneous This only refers to low speeds and PWM periods there is a correlation between these two entities Variable waitState will solve this problem Value of waitState variable determines how many idle cycles after commutation there should be Evidently this will cause a limit in maximum speed that we can obtain as it will limit the rate of state shifting In our case this maximum speed will be higher than nominal so it is not really a limitation On every commutation i e state change waitState is set to 3 On every next interrupt this value is decreased by 1 Only when this value reaches 0 our progra
79. s that occur will be reported in dos window If there is no errors object file will be built and linker will automatically produce hex file from it It may occur that linker does not start automatically as compiler window One that is finished needs to be closed manually Hex file can be programmed to microcontroller by using programmer There is enable programmer option Pic Start Plus menu I used ICD instead of programmer It can do all that programmer can do and more In order to program microcontroller with ICD development mode has to be changed to ICD options menu Upon mode changing ICD control box will appear On this box options menu microcontroller and crystal types xt in our case should be set All other options should be disabled Program button will program our microcontroller References 1 Speed controlled single spindle drives for textile machines http www s line de homepages bosch sensorless 2 PIC 16F87X user manual http www microchip com download lit pline piemicro families 16f87x 302 92c pdf 3 Position estimator and simplified control strategy for brushless DC motors using DSP technology http www2 ing puc cl power paperspdf dixon 55a pdf 4 IRADK 10 Motor drive reference design kit www irf com technical info refdesigns iradk10 pdf 78 Belgrade university Diploma theses Sensorless control of brushless DC motor Contact Phone 011 367 16 07 Cellular phone 063 587 192 E mail
80. ses Sensorless control of brushless DC motor Brushless DC can not reach the speed of synchronous motor because the gradient of current slope is higher at the same speed and it has a problem of current peaks through commutation but price of this drive will always be significantly lower We also must bear in mind that motor price will be also lower as sinusoidal distributed winding motors that are required for a regular drives with synchronous electromotor and they are more expensive than the ones with linear distribution i e trapezoidal BEMF 73 Appendixes Appendix A Source code include lt pic1687x h gt unsigned OnVector 0xce unsigned OffVector 0xee const unsigned TableOn 7 Oxce Oxce Oxae Oxba 0x7a 0x76 Oxd6 const unsigned TableOff 7 Oxee Oxee Oxee Oxfa Oxfa Oxf6 Oxf6 const unsigned testTable_1 7 0 2 1 8 2 1 8 const unsigned testTablel 7 0 1 8 2 1 8 2 interrupt void isr static unsigned state 1 static unsigned dutyR 0 static unsigned int tStep 637 static unsigned int t1 Control 0 static signed direction 1 static unsigned inc 0 static unsigned test 0 static unsigned testVector 0 static unsigned waitState 3 static unsigned sending 0 static unsigned temp if INTE if RC4 temp TRISC TRISC TRISC amp Oxdf RC5 1 TRISC temp else if inc inc 0 else inc 1 INTF 0 j if RCIF dutyR RCREG TXREG dutyR CCPR1L dutyR j if TMRIIF
81. t A 1 C gt B 1 A gt C 0 Transition to state 2 should occur when C gt B becomes 0 positive direction 65 Chapter 6 Sensorless mechanism software resources Transition to state 6 should occur when B gt A becomes 0 negative direction A GND PWM Figure 6 4 State2 electric model External signals condition B gt A 1 C gt B 0 A gt C 0 Transition to state 3 should occur when A gt C becomes 1 positive direction Transition to state 1 should occur when C gt B becomes 1 negative direction GND Figure 6 5 State 3 model External signals condition B gt A 1 C gt B 0 A gt C 1 Transition to state 4 should occur when B gt A becomes 0 positive direction Transition to state 2 should occur when A gt C becomes 0 negative direction 66 Belgrade university Diploma theses Sensorless control of brushless DC motor Figure 6 6 State 4 model External signals condition B gt A 0 C gt B 0 A gt C 1 Transition to state 5 should occur when A gt C becomes 1 positive direction Transition to state 3 should occur when C gt B becomes 1 negative direction A WM GND Figure 6 7 State 5 model External signals condition B gt A 0 C2 B 1 A gt C 1 Transition to state 6 should occur when A gt C becomes 1 positive direction Transition to state 4 should occur when C gt B becomes 1 negative direction 67 Chapter 6 Sensorless mech
82. tached to the body of motor and that is why we can call it sensorless This sensor can be permanently attached to the drive so it can be considered as a part of the drive A D Figure 3 1 Brushless DC motor commutation positions We want the angle between stator and rotor flux to be closest possible to 90 degrees torque is proportional to sin angle That is the way to obtain maximum possible torque with constant current that is maximal electromechanical conversion maximum efficiency On figure 2 we can see motor with rotor in position for commutation A F are axis of possible stator flux Angle between rotor flux and A axis which is current position of rotor flux is 60 degrees and it is decreasing and the angle between B and rotor is 120 degrees also decreasing getting nearer to ideal 90 degrees It is obvious that we should shift stators flux from axis A to axis B in this position Angle between rotor and stator flux will always be between 60 to 120 degrees and that is the best we can gain with any brushless DC motor The other way to observe things is to take a look at the electrical model of our system Figure 3 2 Current I goes from phase A to phase B In phase C there is no current BEM forces Ega Egb Ege are pulsating with frequency of rotors turning multiplied by a number of pair of poles Gained power of electromechanical conversion is P Ega Egb I Lets suppose that the commutation that is to be done
83. tages is common to ground of DC bus Input voltage can be 380V and voltage range on comparator will be 15V because we will be supplying it with 15V DC That means R 380 R R 15 3 12 We will adopt values R1 27K R2 680K These high resistances will provide low current sink Voltage that capacitor sees is Reisees Ry R R R R 3 13 as R1 has very high value we can neglect it If we choose R3 to much bigger than R2 we could neglect R3 and determine system poles separately due to a principle of poles separation Lets say that R gt gt R we have Reises R Now we can determine value of first capacitor as we know that poles frequency should be 1000Hz 10 Fh Fh 100Hz t 2 m 1000Hz 6283 18rad s a 3 14 TC Ross 1 1 gt C 5 9nF 3 15 e p Re lsees We will adopt value of 5 6nF because that is a closest value commercially available C 5 6nF We will adopt R 270K ten times bigger than R We choose C 680pF More accurate value would be 560pF but since we chose lower value for C we are taking higher value of C in order to balance our system All of this will not affect significantly characteristic of our filter Since comparisons will be executed near ground voltage half of them we need to place one zener diode between ground of our filter and a system ground to avoid errors during the comparison One resistor is placed to provide diode polariza
84. ter reads the status of the pins whereas writing to 1t will write to the port latch All write operations are read modify write operations Therefore a write to a port implies that the port pins are read the value is modified and then written to the port data latch Pin RA4 is multiplexed with the Timer0 module clock input to become the RA4 TOCKI pin The RA4 TOCKI pin is a Schmitt Trigger input and an open drain output All other PORTA pins have TTL input levels and full CMOS output drivers Other PORTA pins are multiplexed with analog inputs and analog VREF input The operation of each pin is selected by clearing setting the control bits in the ADCONI register A D Control Registerl The TRISA register controls the direction of the RA pins even when they are being used as analog inputs The user must ensure the bits in the TRISA register are maintained set when using them as analog inputs PORTB is an 8 bit wide bi directional port The corresponding data direction register is TRISB It works the same way as TRISA Three pins of PORTB are multiplexed with the Low Voltage Programming function RB3 PGM RB6 PGC and RB7 PGD Microcontroller can be programmed with 5V voltage by using this function Furthermore we can program our controller using only boot loader a small program in microcontroller memory that would accept data via serial bus and place it in adequate locations of program memory Of course boot loader would previously have to be progra
85. tion 15V Phase Output 3 3V system ground Figure 3 11 Zener diode circuit Rectangle block is a filter from previous figure Entire circuit is shown on the next page 18 PHASE A Belgrade university Diploma theses Sensorless control of brushless DC motor T5V 15V gt in E T w ES 5 3 o a El 2 O E a O E o G i ES B Ww e a u z ES n z 2 as o a cu o oF Ih uw c i gt in Td Y o S sz BE Lu 8 8 82 E e eo e de te e a m o o gt e ao 9 J e amp la amp amp I E amp de A a s Os p 5 i B M s in na E dm u mm Au A Scheme 3 1 Position detecting circuit Chapter 3 Sensorless position detection TVF method Zener diodes with 15V breakthrough voltage are used for protection Standard decoupling capacitor pair is placed between 15V and ground A capacitor is placed in parallel with zener diode in order to prevent eventual voltage drops due to current variations Standard LM339 chip with four comparator blocks is used for voltage comparison Comparator outputs are open collector transistors which means that we need pull up resistors Pull up resistors are connected to 5V as IRDAK will require TTL compatible levels We use only three comparators output of fourth is always logical 1 state that is output transistor is turned off all the time Besides lowering current sink we are also preventing noise generatio
86. ts because we can cause short circuit and damage the device that we are taking signal from Belgrade university Diploma theses Sensorless control of brushless DC motor R25 20K STRIP1 Figure 4 16 Strip1 Passing a voltage from variable resistor can be very useful in situations when we want our IRADK to function without PC command This could be the way to apply speed command for example Additional digital inputs SP1 2 stands for spare input are shown on the same figure as analog above External digital signal should be TTL compatible Resistors R43 and zener diodes D15 and D16 are placed with the aim of protection Both analog and digital inputs will be passed to microcontroller via strip Using the same strip output PWM signals and ITRIP can be transferred to outside of IRADK This is useful if we want to observe wave diagrams of this signal on oscilloscope High voltage signals strip and power connections STRIP2 Figure 4 17 High voltage signals strip and power connection 37 Chapter 4 IRADK motor drive High voltage signals are present on strip2 of IRADK figure left image They can be passed to various analog circuits that can extract information from them Our information will contain rotor position of brushless DC electromotor Power connections of IRADK are shown on the figure right image Voltage from the power grid should be connected to the first three pins of CON2 Special care has to be taken when
87. tware programmable prescaler nternal or external clock select nterrupt on overflow from FFh to 00h Edge select for external clock Timer0 module functioning is defined through OPTION register Timer mode is selected by clearing bit TOCS OPTION_REG lt 5 gt In Timer mode the Timer0 module will increment every instruction cycle without prescaler If the TMRO register is written the increment is inhibited for the following two instruction cycles The user can work around this by writing an adjusted value to the TMRO register There is only one prescaler available which is mutually exclusive shared between the Timer0 module and the Watchdog Timer A prescaler assignment for the TimerO module means that there is no prescaler for the Watchdog Timer and vice versa Prescaler assignment is defined in PSA OPTION REG 5 If PSA is 0 prescaler is assigned to the timer0 module otherwise it is assigned to the watchdog timer Prescaler value is defined by three least significant bits of OPTION REG lt PS2 PS0 gt using the following table Bit Value TMRO Rate WDT Rate co co e I 2 rg gt ab ad ad ad ad ad ad mb Table 5 2 Prescaler value table Timer0 interrupt control functions via two bits of INTCON register TOIE Timer0 interrupt enable and TOIF TimerO interrupt flag When timer0 wraps over FFh gt 00h TOIF will be set in the same moment if TOIE is set interrupt procedure will be called TOIF is i
88. uration in the iron We can detect position of rotor by detecting harmonics of induced motor voltages The induced motor voltages show harmonics of odd ordinal number i 3 5 7 because the flux in the air gap has a rectangular distribution Figure 2 2 shows BEM forces broken down to harmonics As we can see harmonic voltages with ordinal numbers divisible by three third harmonic on the picture create zero phase sequence systems which are not accessible by the connections of three phase conductors We can also see that commutation moments coincide with maximal amplitude of third harmonic All third harmonics are in the correct phase they cancel each other out between the three phase conductors and therefore cannot be detected from the three phases of the motor in wye connection Figure 2 3 System for position derivation from third harmonic If the wye connection of the motor is accessible the zero phase sequence systems can be detected between this and a virtual wye connection realized with three resistors Fig 2 3 shows the corresponding circuit The voltages are tapped off by potential dividers The reference potential of the sensing circuit may be anywhere between the positive or Chapter 2 Position sensing negative potential of the DC link Problem with this method is that it cannot operate at low speeds because BEM forces are weak this will be problem with other sensorless methods too The other problem is that it will not work
89. var 0 23 Chapter 6 Sensorless mechanism software resources else var 1 TOIF 0 main TRISB 0 PORTB is output OPTION 0X11 TOIE 1 GIE 1 Global interrupt enable while 1 if RB0 RBO 0 else RBO 1 h Compiled C program 0000 0183 poweru clrf 0x3 0001 3000 movlw 0x0 0002 008A movwf OxA 0003 2827 goto exit 0004 OOFF intlev movwf 0x7F 0005 0803 movf 0x3 W 0006 0183 clrf 0x3 0007 00A2 movwf 0x22 0008 080A movf OxA W 0009 00A3 movwf 0x23 000A 018A clrf OxA 000B 1DOB int fu btfss 0xB 0x2 000C 280E goto OxE 000D 280F goto OxF 000E 281D goto OxID 000F 0821 movf 0x21 W 0010 0420 iorwf 0x20 W 0011 1903 btfsc 0x3 0x2 0012 2814 goto 0x14 0013 2815 goto Oxl5 0014 2818 goto 0x18 0015 01A0 clrf 0x20 0016 01AI1 clrf 0x21 0017 281D goto 0x1D 0018 1283 bef 0x3 0x5 0019 1303 bef 0x3 0x6 001A 01A0 clrf 0x20 54 Belgrade university Diploma theses Sensorless control of brushless DC motor 001B OAAO incf 0x20 001C 01A1 clrf 0x21 001D 110B bcf 0xB 0x2 001E 1303 bef 0x3 0x6 001F 1283 bef 0x3 0x5 0020 0823 movf 0x23 W 0021 008A movwf OxA 0022 0822 int_re movf 0x22 W 0023 0083 movwf 0x3 0024 OEFF swapf Ox7F 0025 OE7F swapf Ox7F W 0026 0009 retfie 0027 3020 exit movlw 0x20 0028 0084 movwf 0x4 0029 3022 movlw 0x22 002A 2031 call clear ram 002B 0183 clrf 0x3 002C 118A bef 0xA 0x3 002D 2FEA goto main 002E 0604 xorwf 0x4 W 002F 0180 clrf 0x0 0030 0A84 incf 0x4 0031 0604 clear xorwf 0x4 W 0032

Sensorless control of brushless DC electromotor

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