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Formula SAE Safety Interlock Circuit & Indicator

1. 32 33 WID ans 33 3 2 1 3 Physical pun e aate nda 33 3 22 Tractive System Master Switch ttr ht portent 33 BZ Za 33 WINNS 34 3 2 2 3 Physical MOURLDDE estar ne ueste 34 3 2 3 Shutdown B ttonS tsp fe tol oa eei qn tdt 34 9055 ments bn ans s 34 9 Uc WINE 35 3 2 3 3 Physical erre etna 35 3 2 4 Brake Over 36 REGUINCMENIS Agassi aita ben E e pe pA 36 En dienen esas Rr REPE 37 3 2 4 3 Physical MOULE 37 3 2 5 Inertia SeSi 37 3 3 3 4 2201 Requirements 37 38 3 2 5 3 Physical copia p shui ats 38 Circuit A URN 38 3 3 1 Design PEOCESS ya amata 39 3 3 2 Component Selection 39 3 3 3 41 3 3 3 1 Header 1 H1 External Safety Elements 41 2529 2 BMS TritetlocE tp o reda dii d 42 3335 44 3 334 BPS Interl ek RO 45 3 3 3 5 Motor Lo
2. 86 TABLE 4 2 DASHBOARD PHYSICAL WIRING DETAILS 89 Nomenclature 5 5 5 5 5 RtD LV HV HSS NO NC LED SPI EUSART 12 SAE REV PCB IO GND BMS EVMS uC ICSP DC Safety Circuit Box Control System Master Switch Tractive System Master Switch Ready to Drive Low Voltage High Voltage High Speed Steel Normally Open Normally Closed Light Emitting Diode Serial Peripheral Interface Enhanced Universal Synchronous Asynchronous Receiver Transmitter Inter Integrated Circuit Society of Automotive Engineers Renewable Energy Vehicle Project Printed Circuit Board Input Output Ground Battery Management System Electric Vehicle Management System Microcontroller In Circuit Serial Programming Direct Current 1 Introduction 1 1 Electric Vehicle Motivation The human civilization has heavily relied on the internal combustion engine since its inception some 200 years ago The pivotal role that this remarkable technology has played in the advancement and industrialization of the human race cannot be overstated This 19 century invention has persisted to this day and remains central to keeping the economies around the world ticking as well as maintaining the standard of living as we know it However most prominently in the last decade there has been significant research and commercializing of new technologies with the electric vehicle bein
3. at 91 5 1 1 No resistor on transistor 2 1 9 5 1 2 not necessary on charge interlock 93 5 1 3 BPS Reset Button Issue 93 5 1 4 IMD Interlock Trigger Issue reipsa 95 5 Cornclusioris sg op ded eee 97 Gels Future la i 97 ZEN Slice M 1 List of Figures FIGURE 2 1 SEPARATION OF LV amp SYSTEMS 7 FIGURE 2 2 LV ELECTRIC SYSTEM BREAKDOWN 8 FIGURE 2 3 EVMS DEVICE AND TERMINALS 11 FIGURE 2 4 CHOSEN AUXILIARY BATTERY SPECIFICATIONS 7 14 FIGURE 2 5 1 BOTTOM HALF LV ELECTRIC SYSTEM 2 044 44 17 FIGURE 2 5 2 BOTTOM HALF LV ELECTRIC SYSTEM 18 FIGURE 3 1 I V CURVE FOR LED OF VARIOUS COLOUR 12 25 FIGURE 3 2 SIMPLE LED CIRCUIT 13 26 FIGURE 33 LED MULTIPLE XING CIRCUIT 27 FIGURE 3 4 BASIC INDUCTOR CIRC UII siccis odiis PAM O AE 29 FIGURE 3 5 FREEWHEELING DIODE USED IN
4. 9 ground 5_ _ Connect to main contactor terminal the battery side EVMS PRECH Connect to main contactor Prech B terminal on the motor controller side Drive EVMS DRIVE Connections to the relay which Enable EVMS DRIVE ENABLE closes when allowed to drive Charge EVMS CHARGE Connections to the relay which Enable EVMS CHARGE ENABLE closes when allowed to charge EVMS MAIN Positive terminal of main Main Ctr contactor EVMS GROUND MAIN Negative terminals of main Ground contactor EVMS AUX CTR Positive terminal of auxiliary Aux Ctr contactor EVMS GROUND AUX Negative terminal of auxiliary Ground contactor Table 2 1 EVMS terminals and description Note The Terminal Name column lists the name by which the EVMS terminals are referred to in the thesis 10 12VDC Ground Key In Chg Sense BMS In Status R Status G Status B Buzzer Figure 2 3 EVMS device and terminals Scenario 1 Drive Input KEY IN HIGH Preconditions 1 BMS IN LOW 2 CHG SENSE HIGH Effect 1 DRIVE amp ENABLE pins closed 2 AUX CTR HIGH ON 3 Pre charge occurs over main contactor 4 MAIN CTR HIGH ON Scenario 2 Charge 11 Input CHG SENSE LOW Preconditions 1 BMS IN LOW 2 KEY IN LOW FLOAT Effect 1 CHARGE amp ENABLE pins closed 2 MAIN CTR LOW OFF 3 AUX CTR HIGH ON Scenario 3 Battery Fau
5. Table 3 7 Wiring details of Brake Over Travel Switch Note For more detail on Connector Termination please see Section 3 4 2 3 243 Physical Mounting The BOTS is mounted on the pedal box assembly in line with the movement of the brake pedal In this position the driver will not be physically able to reset the BOTS if it is actuated as per rule T7 3 2 The pedal assembly is currently in progress by another student and hence no further details are available at this time 3 2 5 Inertia Sensor An inertia sensor triggers when it experiences a predefined deceleration They are widely used in conventional combustion engine vehicles where they stop the fuel pump in the event of an accident With the advent of electric cars inertia sensors remain a core requirement of the safety system In an accident just as you don t want fuel spraying around over a damaged car with potential sparks and hot engine parts it is similarly dangerous to have live electric tractive wiring short circuiting over the chassis or in contact with the driver In the Formula SAE car the inertia sensor will open the main safety circuit path causing the contactors to isolate the voltage to within the battery boxes only 3 25 1 Requirements 5 7 2 The inertia switch must be part of the Shutdown Circuit and must be wired in series with the shutdown buttons such that an impact will result in the 37 Shutdown Circuit being opened The inertia switch
6. Main contactor to Main MAIN_CTR contactor Drives the Aux Main relay N A EVMS_MAIN_CTR which in turn EVMS_MAIN_CTR powers the Main relay Drives the Aux Aux relay which N A EVMS_AUX_CTR EVMS_AUX_CTR in turn powers the Aux relays IMD Signal High Low status IMD_SIG 8 signal from IMD IMD Reset Btn MD Reet N A button mounted terminal 1 on enclosure IMD Reset IMD Reset Btn N A button mounted BMS O IMD I terminal 2 on enclosure Brake Panic Switch N A 906 anie es IMD O BPS I terminal 1 N A Brake Panic Switch BPS CMN terminal 2 JMP N A 4 4 BPS_RST_T1 Brake Panic Switch BPS_BTN_T1 reset button terminal 1 5 1 H5 P2 H5 P3 5 4 High Low status N A N A N A N A N A N A HP5 P1 S5 P4 HP5 P2 N A HP5 P3 N A HP5 P4 N A N A N A N A N A N A N A N A N A N A N A N A N A C5 P4 N A N A N A N A N A N A N A EVMS_GROUN T D EVMS GROUN D EVMS_KEY_IN C6 P15 EVMS_RED C6 P16 EVMS_GREEN C6 P17 EVMS_BLUE C6 P18 N A EVMS_BMS_IN signal from BMS BMS_SIG modules BMS Reset BMS Reset Btn N A Nd button mounted GND terminal 1 on enclosure BMS Reset BMS Reset Btn button mounted BMS_BTN_T2 terminal 2 on enclosure Supply power to N A JMP HP11 P5 BMS Interlock BMS_1
7. 044 44 42 30 FIGURE 3 6 HEADER 1 SCHEMATIC 42 FIGURE 3 7 BMS INTERLOCK CIRCUIT 5 43 FIGURE 3 8 IMD INTERLOCK CIRCUIT SCHEMATIC eeeeeeee entes 44 FIGURE 3 9 BPS INTERLOCK CIRCUIT SCHEMATIC ipea t edt vr ud adeundi 46 FIGURE 3 10 MOTOR LOOP MODULE CIRCUIT SCHEMATIC 48 FIGURE 3 11 AUX amp MAIN RELAY MODULE SCHEMATIC 49 FIGURE 3 12 READY TO DRIVE INTERLOCK CIRCUIT SCHEMATIC 50 FIGURE 3 13 READY TO DRIVE SOUND CIRCUIT SCHEMATIC 2 52 FIGURE 3 14 CHARGE INTERLOCK CIRCUIT SCHEMATIC entente 53 FIGURE 3 15 VOLTAGE DIVIDER INPUTS TO MICROCONTROLLER 55 FIGURE 3 16 LED DRIVER CIRCUIT a 27 FIGURE 3 17 5V POWER SUPPLY CIRCUIT 58 FIGURE 3 18 SAFETY CIRCUIT PCB ROUTE LAYOUT 62 FIGURE 3 19 SAFETY CIRCUIT PCB MAIN SAFETY CIRCUIT PATH TRACE THICKENING FIGURE 3 20 CNTR M IP67 P18 AND CNTR_M_IP67_P18 CONNECTOR SPECIFICATION Zaa aa an Ea ORE qaa 67 FIGURE 3 21 CNTR M IP67 P7 ANDCNTR M IP67 P7 CONNECTOR SPECIFICATION l ua a u aa Sha sau DUM a ER ONERE 67 FIGU FIGU FIGU FIGU
8. P 9767 P 9768 IP66 rated waterproof 4 and 7 pin connectors for installa tions outdoors or in areas prone to moisture build up Compatible with Amphenol C16 series connectors LTW PWF series 4 pin series 15A rated 7 pin series 10A rated Price Each Cable Entry RRP 4 10 P 9767 7 pin chassis female 19 50 17 55 15 60 P 9768 7 line male 5 5 10 8mm 19 50 17 55 1560 Figure 3 21 67 7 67 7 connector specification 7 3 4 2 2 Connector Placement and Wiring There are a number of connectors used in the safety system in particular for IO of the Safety Circuit Box The following list describes all connectors used in the 67 Safety Control System associated abbreviated names used throughout the text Connector Socket Location Name Connector Plug Name CNTR_F_IP67_P7 SCB S1 CNTR 67 P7 Cl CNTR_F_IP67_P7 SCB 52 CNTR 67 P7 C2 CNTR_F_IP67_P7 5 53 CNTR M IP67 P7 C3 CNTR F IP67 P18 SCB 54 CNTR M IP67 18 C4 CNTR F IP67 P18 SCB S5 CNIR M 67 P18 C5 CNTR F IP67 P18 Dashboard 56 CNTR M IP67 18 C6 HDR 10 SCB H1 1 5 HDR_PLUG_P10 HP1 1 HDR_PCB_P10 SCB 1 2 PLUG 10 HP1 2 HDR 10 SCB H2 1 i HDR_PLUG_P10 HP2 1 HDR_PCB_P10 SCB H2 2 HDR_PLUG_P10 HP2 2 HDR_PCB_P6 SCB H3 HDR_PLUG_P6 HP3 HD
9. THE UNIVERSITY WESTERN AUSTRALIA School of Electrical Electronic and Computer Engineering Final Year Project Thesis Formula SAE Safety Interlock Circuit amp Indicator Instrumentation Matthew Michalek Supervisor Professor Thomas Br unl Date of Submission 274 November 2012 Abstract With ever increasing fuel prices and environmental impact becoming increasingly concerning the REV Project continues to push the development of electric vehicles The safety of such vehicles is paramount and must be addressed to ensure the viability of electric vehicles The aim of this project is to develop a comprehensive safety system for the 2012 Formula SAE race car The primary objective is that it must help to protect the driver and crew from dangerous situations In addition it must comply with the rules stipulated by the Society of Automotive Engineers Australia A highly integrated safety indicator system is also developed Its purpose is to pinpoint faults in the safety system and effectively communicate this to the driver A modular component of this system is the dashboard that forms the vital interface between the driver and the state of the car From the logical design of the electric system to the safety circuit implemented on a printed circuit board this is a fully custom design to meet the exact requirements of the Formula SAE car Due to the importance of safety development of a robust and reliable safety system
10. Its primary purpose is to ascertain whether the current state of the car or more precisely whether the state of various buttons sensors and interlock modules implies a safe condition If the current state of the car is safe the Safety Control System will activate the tractive system by powering a series of contactors that in turn close the tractive circuit which physically allows current to flow between the accumulators and motors So it is clear that the HV Tractive and LV Electric Systems are both logically and physically separate systems with the contactors forming an interface between the two as illustrated in Figure 2 1 LV Electric System HV Tractive System Tractive Circuit Tractive Circuit Contactors Interface between HV and LV Systems Safety Control C System Figure 2 1 Separation of LV amp Systems detailed operation of the high voltage tractive system is beyond the scope of this thesis and no further discussion on this topic will follow As the car currently stands the Safety Control System differs little from the larger overall LV Electric System containing only a few extra components such as the auxiliary battery Electronics and componentry used for a variety of purposes other than safety may be added in the future These would offcourse be part of the LV System but not relevant to the Safety Control System and for this reason we will keep this
11. Obstruction i Figure 4 2 Left side of dashboard Shutdown Button 1 940 00 5 LED Left Shutdown LED Shutdown Button LED Button 1 Z 145 00 m di Mount hole T pon 42 00 TSMS Right Shutdown LED LED Button LED Steering Wheel Obstruction Figure 4 3 Right side of dashboard 81 4 2 1 Shape Dimensions only rule that effects the design of the dash is 14 2 1 A vertical cross section which allows the template shown in Figure 9 to be passed horizontally through the cockpit to a point 100 mm 4 inches rearwards of the face of the rearmost pedal when m the inoperative position must be maintained over its entire length If the pedals adjustable they will be put in their most forward position 3 That is the template seen in Figure 4 1 must be able to pass between the dashboard and the seat Other than this there is no mention about requirements particular to the dashboard or mention of one at all The requirements of rule T4 2 1 coupled with the tight and compact nature of the Formula SAE car exacerbates the challenge of designing a full featured user friendly and unobstructed dashboard Given this my approach was to maximise the size of the dashboard while remaining compliant with rule Tt4 2 1 This would give the largest surface area to work with and consequently provide m
12. right position the key switch input is directed to the key switch drive output which in turn pulls the EVMS_KEY_IN terminal HIGH Provided the EVMS_BMS_IN input is LOW and EVMS_CHG_SENSE is HIGH the EVMS will output power on EVMS_AUX_CTR which turns ON closes the auxiliary contactors Following immediately the pre charge occurs over the currently inactive main contactor When the pre charge is complete power is output on EVMS_MAIN_CTR which turns ON the final main contactor 19 At this stage all contactors ON CLOSED which implies that the tractive circuit is physically closed and that the DC DC converter is now ON However the car will still not respond to actuation of the torque encoder accelerator pedal When EVMS_KEY_IN was pulled HIGH the EVMS also internally closed the EVMS_DRIVE and EVMS_DRIVE_ENABLE terminals This allows current to flow through the HV Relay coil and onto the Ready to Drive RtD Interlock Circuit When the RtD button is pressed the RtD circuit latches closed completing the circuit just described which therefore turns the HV relay ON This in turn connects the HV terminal to the motor controller enable terminals pulling them HIGH The car will now respond to the torque encoder This completes the process of putting the car in the DRIVE state 2 4 2 Charge the Car Again let s assume we start in the OFF SAFETY_FAULT state Provided all the safety elements are ON CLOSED again we have a closed path
13. 107 Appendix Connector Table 108 Header 1 1 1 1 1 2 H1 1 P3 H1 1 P4 H1 1 P5 H1 1 P6 H1 1 P7 H1 1 P8 H1 1 P9 H1 1 P10 H1 2 P1 H1 2 P2 H1 2 P3 UHM S93elN PCB Socket Header Cable connecti Plug amp amp 1 1 1 51 1 1 1 2 51 2 1 1 S1 P3 1 1 4 51 4 1 1 5 1 1 6 55 1 1 1 7 1 1 8 1 1 9 51 5 1 1 10 51 6 1 2 1 52 1 1 2 2 52 2 HP1 2 P3 S2 P3 UHM S93elN Plug Plug Connector Cable Connector amp amp pin pin C1 P1 N A C1 P2 N A C1 P3 N A C1 P4 N A N A N A 5 1 1 5 1 6 2 1 2 2 C2 P3 N A UHM S93elN Socket Device connectio Description Name 1 Connection n amp pin Negative terminal N A Aux Battery DC DC GND converter Positive terminal Aux N A Battery DC DC 12V converter Control System N A Master Switch CSMS_t1 terminal 1 Control System N A Master Switch CSMS_t2 terminal 2 Supply power to N A JMP JP5 4 BMS Interlock CSMS t2 Jumper Wire Supply power to N A EVMS_ 12VDC EVMS CSMS t2 IMP Supply power to N
14. 2 6V Hence we have 12 2 6 gt 3 9 R1 705 4700 3 9 Therefore 4700 resistors were used in the LED Driver Circuit 3 1 3 LED multiplexing Ordinarily each LED used would require its own dedicated pin on the microcontroller However in many applications large numbers of LEDs may be necessary Either a microcontroller with the required IO is utilized with the likely undesirable side effects of a larger physical footprint and higher cost or a more efficient method to drive LEDs is employed 26 technique of LED multiplexing is such method that be used to significantly reduce the number of IO pins required to drive larger number of LEDs The Safety Circuit PCB implements 11 LEDs as part of the LED Indicator Subsystem so we ll develop the theory based on this The idea of multiplexing is that a logical array of LEDs is developed as shown in Figure 3 3 below SENI AW 2 Vl EV lt lt 3 I Bt Bt Figure 3 3 LED Multiplexing Circuit Each row of LEDs has the anode connected to a uC pin Similarly each column has the cathode connected to a C pin To turn ON an LED the corresponding anode and cathode pins on the pC are pulled HIGH In this schematic the pC pins actually drive transistors which in turn drive the LEDs This reduces current the microcontroller would otherwise have to supply
15. 24 Safety Circuit mounted SCB enclosure 3 4 4 External Safety Element Mounting Right Shutdown Button Inertia Sensor Figure 3 25 Left Mount Plate Figure 3 26 Right Mount Plate 72 3 4 5 Cables Wiring 3 4 5 1 Complete Safety System Cabling A complete and comprehensive Safety Control System wiring table is available in Appendix C 3 45 2 Theory The selection of a cable for a particular application should be a considered measured and calculated process The primary specification of any cable is the cross sectional area of the conductor The cross sectional area relates to a quantity known as the ampacity of a cable otherwise known as current carrying capacity which specifies the maximum electrical RMS current that can safely flow while remaining in the safe temperature range of the cable Exceeding this value means the conductor will begin to sustain progressive or immediate deterioration leading to potentially hazardous situations It is important to note however that the determination of the ampacity is not a direct conversion from the cross sectional area as one might expect The ampacity of the cable really depends on other factors including Insulation temperature rating Electrical resistance of the conductor material Frequency of the current AC only ox par Ability to dissipate heat which depends on conductor geometry and surroundings 5 Ambient temperature So the que
16. Author Matthew Michalek Created on 18 September 2012 12 34 PM include lt stdio h gt include lt stdlib h gt include lt xc h gt define _XTAL_FREQ 4000000 define R1 RB3 define R2 RB4 define R3 RBS define C1 RB2 define C2 RB1 define C3 RD7 define C4 RD6 __CONFIG MCLRE_ON amp CP OFF amp BOREN OFF amp WDTE OFF amp PWRTE_OFF amp FOSC NOCLKOUT amp OFF amp LVP OFF Function Prototypes void CheckSafetyElementFault void LightSafetyElementFault void PreRtD int safetyElementStatus1 0611111111 int safetyElementStatus2 0011111111 void main INITIALISE TRISB 0 TRISD 0 ANSEL 0x00 ANSELH 0x00 for CheckSafetyElementFault if safetyElementStatus1 amp amp safetyElementStatus2 0 PreRtD else LightSafetyElementFault return EXIT_FAULT void CheckSafetyElementFault if RA7 0 TSMS FAULT safetyElementStatus1 255 else if RE2 0 SD LEFT FAULT safetyElementStatus1 127 if RE1 0 SD RIGHT FAULT safetyElementStatus1 63 if REO 0 SD DASH FAULT safetyElementStatus1 31 if 5 0 BOTS FAULT safetyElementStatus1 15 if RA4 0 INERTIA FAULT safetyElementStatus1 7 if 0 BMS FAULT safetyElementStatus1 3 105 else if RA0 0 IMD FAU
17. Essentially we want to source power for the IMD interlock from an area which is not controlled or influenced by the BMS interlock or any other safety element We do not want the tripping of another safety element to also trip the IMD interlock as is currently occurring due to the BMS interlock The solution to this problem involves a cut to a small PCB trace This makes the disconnection as seen in Figure 5 4 Figure 5 4 IMD Fix A nearby source of power as described above needs to be identified so it can be fed into the cut trace This is done as shown in Figure 5 4 by feeding from power after the CSMS This new point will provide power so long as the CSMS is ON The point is not influenced by any safety circuit element or interlock Now if the BMS interlock trips power to the IMD interlock will not be interrupted and hence remain CLOSED as required 96 6 Conclusions aim of this thesis was to design build implement reliable robust safety system that would adhere to all relevant rules stipulated by the SAEA but first and foremost protect the driver and crew from potentially dangerous situations The heart of the Safety Control System is the centralized Safety Circuit Box It contains the custom designed Safety Circuit PCB that was tested manufactured tested again and revisions made to rectify small faults It is also heavy in regard to IO since it requires myriad connections to the wider Safety System The
18. FIGU FIGU FIGU FIGU FIGU FIGU FIGU FIGU FIGU FIGU FIGU FIGU FIGU RE 3 22 RE 3 23 RE 3 24 RE 3 25 RE 3 26 RE 3 27 RE 3 28 RE 3 29 RE 3 30 4 1 4 2 RE 4 3 RE 4 4 5 1 RE 5 2 RE 5 3 RE 5 4 PLACEMENT OF CONNECTORS 51 52 S3 54 55 ON THE SC B LID 70 PLACEMENT OF CONNECTOR 56 MOUNTED BEHIND THE DASHBOARD 70 SAFETY CIRCUIT PCB MOUNTED IN SCB ENCLOSURE 72 LEFT MOUNT PLATE Qu u ludis u na 71 RIGHT MOUNT 72 SAFETY CONTROL SYSTEM STATE DIAGRAM eee 76 INDICATE FAULT ra EUREN 77 STANDBY FLOWCHATRT A 78 READY TO DRIVE FLOWCHART 78 INITIAL DASHBOARD CONCEPT DESIGN 80 LEFT SIDE OF DASHBOARD RO An 81 RIGHT SIDE OPDASEBOARBD 81 MAIN CABLE RUN BETWEEN FRONT AND REAR OF THE CAR 87 TRANSISTOR DAMAGED BY EXCESSIVE BASE CURRENT 92 REVISED SCHEMATIC FOR BPS ISSUE 94 ADDITION OF REQUIRED DIODE a 95 IMD EDG 222 sauna qa 96 List of Tables TABLE 2 1 EVMS TERMIN
19. SPI but can also be used as a simple digital IO It cannot be used for digital IO RC5 if SPI is required and should be left free for this purpose If digital IO is required for instance it would be sensible to first use up P21 and P22 because they can only perform digital IO and are not part of any other peripheral that may be used in the future 3 3 4 Board Layout When designing a circuit and drawing up the schematic diagram the primary objective is the correct application of electronics theory and meeting application requirements so that the final product works as intended Once this is complete the important step of PCB trace routing must be performed This is the transformation from a circuit design schematic to a trace layout which will be the basis on which the PCB is manufactured PCB trace routing is an artistic process Those who have used the auto router in various CAD packages often exclaim how poor the results are This is because the auto router function is an inherently algorithmic process it cannot think artistically as humans can The trace layout for the Safety Circuit PCB is presented in Figure 3 18 61 0 000090000009000006000090 83 i IEN C 22 171411 lo 444444 9 8 Re A o o 9999 B ER 90000000096000000000 0000 9096040600000090042 Es Figure 3 18 Safety Circuit route lay
20. Using C5 in this way means the buzzer cannot then be mounted to the SCB because that would hinder removal of C5 if required and hence hangs freely with the plug connector The buzzer is extremely light and does not require securing Ideally 52 the buzzer would mounted the chassis with direct connection to inside the SCB but this would require another hole compromising the waterproof integrity of the 5 enclosure On balance it did not make sense to do this when free pins were available on the C5 connector In this configuration the buzzer can also be quickly replaced if it becomes faulty 3 3 3 9 Charge Interlock The Charge Interlock Figure 3 14 controls the input to the EVMS_CHG_SENSE terminal and hence whether the car is in a charge mode or not Figure 3 14 Charge Interlock circuit schematic CHG_INTLK MICROC_12V BUZ supplies the HIGH signal for EVMS_CHG_SENSE which puts the EVMS in a non charge state When the key switch is flicked to charge KEYSW_CHRG_EN is HIGH powering the relay coil This in turn creates a path from EVMS_CHG_SENSE to GND which puts the EVMS in the charge mode 53 3 3 3 10 LED Indicator Subsystem Although the end user experiences the LED Indicator subsystem via the dashboard the heart of the LED Indicator Subsystem is implemented on the same Safety Circuit PCB as the rest of the modules covered in this section Not only does this centralise the circuitry and hence alleviate the need
21. as wasted power dissipated as heat For efficiency conscious applications it may be desirable to minimise the wasted power Ohms Law 3 10 demonstrates the voltage drop across a conductor Varop IR 3 10 As current I through the cable increases or the length of the cable run increases thereby raising the resistance of the cable the voltage drop across the cable increases Firstly the current flowing through the cable can be determined by the application it is being used for If the current is not always constant using the maximum possible value in the subsequent calculations will give a worst case voltage drop which is what we want to know Secondly we must determine the resistance of our cable run Often the manufacturer will specify the resistance per kilometre or similar parameter However in case it is omitted or spare cable is found and utilized the resistance of the conductor in a cable can be calculated using formula 3 11 below 74 pst 3 11 Where p resistivity L length A cross sectional area The resistivity is a physical property of the metal used As can be seen in formula 3 11 the length of cable is directly proportional to resistance whereas cross sectional area is inversely proportional to resistance Normally we don t have much choice in the material used as the conductor so resistivity is not a variable we can play with greatly 3 5 Programming the Microcontroller 3 5 1 Purpose
22. more compact safety circuit and sourcing specific insulating material is difficult 3 3 3 8 Ready to Drive Sound Rule EV4 14 stipulates the requirement of a Ready to Drive sound The READY TO DRIVE RtD state is defined in the rule below EV4 142 The car is ready to drive as soon as the motor s will respond to the input of the torque encoder accelerator pedal 3 This function will be performed by the microcontroller utilizing a piezo electric buzzer The circuit shown in Figure 3 13 relates to the RtD Sound portion of the Safety Circuit PCB 51 4 Figure 3 13 Ready to Drive Sound circuit schematic An NPN transistor is simply driven by a digital output of the microcontroller When the signal is HIGH the transistor is ON current supplied from CHG_INTLK MICROC_12V BUZ of H 22 flows through the buzzer via the header through the transistor to ground The sound should be easily recognisable for the benefit of car identification In addition the rule below must be complied with 4 141 The has to make a characteristic sound once not continuous for at least 1 second and a maximum of 3 seconds when it is ready to drive 8 Some thought has been given to the desired sound and the final result will be continuous momentary bursts of noise gradually speeding up until almost continuous The program code implementation can be seen in Appendix B The buzzer itself is terminated to C5 utilizing P12 and P13
23. must latch until manually reset 8 3 25 2 Wiring Termination of the cable at the Inertia sensor is done via the accompanying crimp connector This connector simply plugs into the inertia sensor socket Table 3 7 below summaries the cabling involved for the inertia sensor Inertia Sensor I O Connector Wire length Wire Type Termination Common C C3 P3 1m Normally Closed NC C3 P4 1m GEN 1 terminal Normally Open NO Unused N A N A terminal Table 3 8 Wiring details of Inertia Sensor Note For more detail on Connector Termination please see Section 3 4 2 3 2 5 3 Physical Mounting The Inertia Sensor is located on the left side of the car It is mounted on the Left Mount Plate see Figure 3 25 3 3 Circuit Design The Safety Circuit PCB comprises of the internal safety elements summarised in Table 3 1 and indicator circuitry working together in harmony Although the dashboard and mounted componentry are a physically separate component from the 5 it is important to remember that the electronics of the LED Indicator Subsystem are actually integrated on the same Safety Circuit PCB That is the physical components on the dashboard are driven and controlled by the SCB This section will thoroughly cover the design detail operation and purpose of the internal safety circuit elements and indicator circuitry 38 3 3 1 Design Process The development of electronics is an iterative pro
24. of connecting a 12V battery in parallel using jump leads than can supply the current to activate the contactors momentarily at which point the DC DC converter will kick in and take over supplying the power The last consideration was the capacity required for the auxiliary battery The trade off is between capacity and size and weight with the latter being desirable to keep as low as possible for an electric race car As we will see in Section 2 4 3 for car demonstration purposes the car will be in the STANDBY state where by definition the contactors are OFF implying the DC DC is OFF well Therefore power to the entire LV Electric System including the EVMS SCB and dashboard indictors is supplied by the auxiliary battery A battery with sufficient capacity to drive these low power devices for a considerable time is desirable Item Current SCB inc indicator LEDs 50mA EVMS lt 50mA TOTAL 100mA Table 2 2 Current consumption by components of the Safety Control System Assuming we want to be able to leave the car in this state for at least 48hours and taking 100mA from Table 2 2 Battery Capacity Ah 1 x 48 4 8Ah 2 1 The auxiliary battery chosen has a capacity of 7 2Ah This is well beyond the capacity of Equation 2 1 and should in fact last up to 72 hours on a full charge It is highly recommended that when the car is not in use the CSMS switch be 16 turned OFF This cuts power supplied
25. on at once meaning the current is kept within those limits it is best to minimise microcontroller loading 3 3 3 10 3 LED Driver Circuit As described in Section 3 3 3 10 2 above the microcontroller does not power or drive the LEDs directly Instead it does so via bipolar transistors see Figure 3 16 56 AM AMAA Figure 3 16 LED Driver circuit schematic Further a technique of LED multiplexing discussing Section 3 1 3 is implemented to significantly reduce the number of microcontroller output pins required Table 3 9 summaries the connection details for the LED multiplexing implemented on the Safety Circuit PCB Logical LED Connector Pin C4 uC pin LED Indicator Row Column Anode Cathode 1 1 14 RB3 RB2 TSMS 1 2 1 5 RB3 RB1 SD Left L3 1 6 RB3 RD7 SD Right 14 1 7 RB3 RD6 SD Dash 2 1 2 4 RB4 RB2 BMS 2 2 2 5 RB4 RB1 Inertia 2 3 2 6 RB4 RD7 BOT 2 4 277 RB4 RD6 IMD 3 1 3 4 RB5 RB2 Motors 3 2 3 5 RB5 1 BPS 57 3 3 3 6 RB5 RD7 RTD 3 4 3 7 RB5 RD6 unused Table 3 10 LED Multiplexing implementation summary 3 3 3 11 5V Power Supply The Safety Control System is mostly a 12V system However the microcontroller is a 5V device In section 3 3 3 10 we saw the use of voltage dividers to ensure a maximum 5V input However the microcontroller itself must be powere
26. this manner the resulting explanation would be hard to follow and likely incoherent 3 3 3 1 Header 1 H1 External Safety Elements The Safety Control System consists of simple elements such as the Shutdown Dash button and more involved interlocks that require implementation on the Safety Circuit PCB The purpose of Header 1 is to provide an interface to the SCB from all the external elements which are essentially two terminal devices The current flows through the header starting at 12V deviating through the external safety elements and exiting at INTERTIA T2 as seen in Figure 2 6 41 5 6 P8 III III 1 OS Figure 3 6 Header 1 schematic snippet In addition points P5 P6 P7 P8 are jumper wires that supply power to various areas of the PCB and external EVMS unit The resistor and LED are wired such that it will be lit when power is live at the box This is a little diagnostic tool I like to incorporate in my circuits When problems occur one wants to eliminate as many variables as possible This circuit aids in this endeavour by confirming availability of power at a glance 3 3 3 2 BMS Interlock After the current has passed through Header 1 and the associated safety elements there are some on board interlocks it must then pass through before powering the contactors The first of these is the BMS interlock The following rules concerning battery safety must be satisfied EV5 1 4 If the Shu
27. to the key switch When the key switch is turned to the Charge position current continues on to the EVMS_CHG_SENSE terminal pulling it HIGH In this case provided the EVMS_BMS_IN input is LOW and EVMS_CHG _SENSE is HIGH the EVMS will output power on EVMS_AUX_CTR which turns ON closes the auxiliary contactors This closes the circuit between the charger and the batteries It is worth mentioning that the DC DC converter is now ON The main contactor which connects the motor controllers to the batteries is specifically left OFF in the charge state to ensure the car cannot possibly move The batteries will continue to charge until the charger s automatic end of charge detection mechanism activates 2 4 3 Demonstrate the Car It is inevitable that he Formula SAE car will be demonstrated at university events and various functions pushing the cause of electric vehicles The LV safety control system has been designed with this in mind resulting in the ability to safely leave the car unattended In this state there is no chance that that the tractive system can be accidently activated even while the electronic 20 instrumentation such the indicator system is still functional be played with To enable the car for demonstration simply turn the key switch to the Standby position and then remove the key for safe keeping In this state users can activate various safety elements for example the Dash Shutdown Butt
28. to the physical implementation of the safety system are considered 3 41 PCB Manufacture The PCB manufacturing process was conducted using the photo resist method This method can achieve excellent results having both a better success rate and producing a better quality product than using the Laser Transfer Method 26 This is because it is based on photolithography the extremely accurate process used in microfabrication usually on a silicon substrate 27 Photos from the fabrication process can be seen in Appendix A 2 3 4 2 I O Connections 66 3 4 2 1 Connector Specification The connectors in Figures 3 20 amp 3 21 have been sourced from Altronics and used extensively for the SCB IO They are IP67 rated ensuring longevity of the connectors and minimising the probability of water ingress into the SCB itself 67 Rated 5 18 Pin Lock Connectors 67 18 CNTR_M_IP67_P18 9506 95 67 rated waterproof 18 connectors for installations outdoors or in areas prone to moisture build up Screw cap available to Suit socket and line plug when not connected 5A rated LTW D series Price Each Cable Entry RRP 4 10 P 9506 18 line male 55 10 8mm 22 229 2090 P 9526 18 pin chassis female 2695 2490 2790 Figure 3 20 IP67 P18 and CNTR 67 18 connector specification 7 IP67 Rated 10 15A Screw Type Connectors CNTR_M_IP67_P7 CNTR_F_IP67_P7
29. was critical and no shortcuts were made to obtain the best possible product Acknowledgments I would like to extend my sincere thanks to Professor Thomas Br unl for the opportunity to work with and be part of the REV Project It has been a gratifying experience that will surely help ease my transition into the workforce next year I wish you and all your future endeavours the best of luck The EAGLE community for maintaining and adding to the library of components Finally I would like to thank the REV team for all your help throughout the year I m sure you will all attest to the fact that we had a lot of fun Contents 1 UCR OMe sacs castes 1 1 1 EElectric Vehicle Motivation 1 12 TheREV Projecte 2 1 2 1 Wa aapniciacarciovcatictearneaoutantadianiccnunipnarantoniesganneaadiamenisans 2 1 2 2 REV Formula SAE ite tip Rb Res dd 2 1 2 3 Personal 3 129 ObIJecHVeS 3 1 3 1 3 1 3 2 LED Fault Indicator System 4 1 3 3 4 2 Low Voltage Electric System
30. 1 and pins 2 amp 5 to light LED 6 If done fast enough meaning 100Hz or more 15 the human eye sees both LEDs constantly lit For small numbers of LEDs lit at once it is not a problem to cycle each LED at 100Hz However to keep the 100Hz rate for each LED the clock of the uC goes up as a greater number of LEDs are lit at once That is for each cycle which is 10ms at 100hz the processor must step through each LED required to be lit So for example to give the effect of lighting 2 LEDs simultaneously a 2 x 100 200Hz processor is required and for 100 simultaneous LEDs a 10 x 100 1kHz processor is required This off course is the bare minimum clock and does not take into account other tasks undertaken by the uC There is no doubt multiplexing should always be considered to drive LEDs However large numbers of LEDs lit at once require more processor power in terms of clock speed to give that illusion 3 1 4 Freewheeling diodes Freewheeling diodes also known as fly back diodes serve the purpose of protecting sensitive semiconductor devices or switch contacts when dealing 28 with inductive loads First let s review the basic operation of inductor considering the circuit of Figure 3 4 Figure 3 4 Basic inductor circuit Keep in mind the fundamental characteristic of an inductor which is that it will always oppose or resist a change in current whether it is increasing or decreasing When the step voltage is initi
31. 2V Jumper Wire Brake Panic Switch S6 P14 reset button terminal 2 56 12 BMS LED Cathode S6 P15 Key Switch P 2 EVMS LED Red 30016 Anode P R EVMS LED Green SEDE Anode P G S6 P18 EVMS LED Blue Anode P B 7 Bibliography 1 6 UWA REV Project Online Available http www therevproject com Accessed 15 Oct 2012 The REV Project Formula SAE 2010 Online Available http www therevproject com vehicles sae2010 php Accessed 15 Oct 2012 Society of Automotive Engineers Australia Formula SAE A Competition 2012 Online Available http www saea com au formula sae a Accessed 15 Oct 2012 ZEVA Zero Emission Vehicles Australia Online Available http zeva com au Products Accessed 15 Oct 2012 ZEVA Electric Vehicle Management System v1 1 User Manual Online Available http zeva com au Products Manuals EVMS 620manual pdf product ZEVA EVMSv1 1 Accessed 15 Oct 2012 Kennedy Alternative Energy LiFePO4 Online Available http www kennedyalternativeenergy com lifepo4 html Accessed 15 Oct 2012 Altronics Altronics Catalogue 2011 12 Online Available http download2 altronics com au Altronics Catalogue 2011 pdf Accessed 2012 Oct 15 SAE International 2013 Formula SAE Rules 2012 1 10 11 12 13 14 15 16 17 18 Electrotechnik Freewheeling Fly bac
32. 6 E WI uu C 6 22 Electric Vehicle Management System cuisse aiii td iecit 9 2 2 1 F ctionality 9 23 Jualary Components uq rive iiie toris iO eed dice da 13 2 3 1 CONT ACTORS 13 2 3 2 Battery BOX CS 13 2 3 3 DC DC Conve I 14 2 3 4 M 14 14 2342 15 24 Safety System Operation 17 2 4 1 Drive the Ca Tm 19 2 42 20 3 Safety Circuit 3 1 3 2 2 4 3 Demonstrate che Caro se ee epe Motu eee 20 3 _3 ____ ___ _ ___ 22 CCU INCOR 22 3 1 1 22 3 1 1 1 Transistor as 22 3 1 1 2 Selecting base 23 3 1 2 Current limiting resistor for LED sess 25 3 1 3 LED multiplexing acis oia 26 3 1 4 Freewheeling 910968 as 28 Elements of the Safety Circuit sida us assets a 31 3 2 1 Control System Master Switch
33. 6 P3 18 RC3 SCK SCL SPI H6 P4 17 RC2 P1A CCP1 Digital IO H7 P1 40 RB7 ICSPDAT Programming H7 P2 39 RB6 ICSPCLK Programming H7 P3 33 RBO AN12 INT Digital IO 7 4 1 RE3 MCLR VPP Programming H8 P1 26 RC7 RX DT EUSART H8 P2 25 RC6 TX CK EUSART H8 P3 21 RD2 Digital IO H8 P4 22 RD3 Digital IO H8 P5 23 RC4 SDI SDA SPI H8 P6 24 RC5 SDO SPI Table 3 11 PCB Spare Header Pins to Microcontroller Functions The Pin Functions column describes the functionality or uses for each pin Most pins have multiple functions referred to as multiplexing functionality on a pin The Suggested Use Column groups pins used for important communication peripherals Overall these headers make it extraordinarily simple to interface with the microcontroller whether that be additional safety elements sensors or even communication with a whole new subsystem See Section 6 1 for ideas on future work It is important to note such breaking out of spare pins is not an afterthought In particular the communication peripherals of the microcontroller including Serial Peripheral Interface SPI and EUSART 60 will useful even necessary to communicate with external systems Care was taken not to use pins related to these peripherals for the purpose of the Safety Circuit PCB In the end it is up to the future designer to choose what each pin will ultimately be used for For example P24 of the pC is one of three pins required for
34. A Charge Interlock CSMS t2 HP2 2P16 Wire IMP Supply power to N A LED module CSMS t2 HP2 1 P1 Jumper Wire Tractive System N A Master Switch TSMS_t1 terminal 1 Tractive System N A Master Switch TSMS_t2 terminal 2 Left Shutdown N A button terminal 1 Left Shutdown button terminal 2 N A Right Shutdown button terminal 1 H1 2 P4 H1 2 P5 H1 2 P6 H1 2 P7 Right Shutdown H1 2 P8 H1 2 P9 H1 2 P10 H21 P1 H21 P2 H21 P3 21 4 21 5 21 6 21 7 21 8 21 9 21 10 1 2 4 52 4 1 2 5 52 5 1 2 6 52 6 1 2 7 53 1 1 2 8 53 2 1 2 9 S3 P3 HP1 2 P10 S3 P4 HP21 P1 N A HP21 P2 SA P1 HP21 P3 54 2 21 4 S4 P3 HP21 P5 S4 P4 HP21 P6 54 5 21 7 S4 P6 21 8 54 7 21 9 54 11 21 10 55 2 2 4 C2 P5 N A C2 P6 N A C3 P1 N A C3 P2 N A C3 P3 N A C3 P4 N A N A N A C4 P1 C6 P1 C4 P2 C6 P2 C4 P3 C6 P3 C4 P4 C6 P4 C4 P5 C6 P5 C4 P6 C6 P6 C4 P7 C6 P7 C4 P11 C6 P11 C5 P2 N A NA button terminal 2 Dash Shutdown N A button terminal 1 Dash Shutdown N A button terminal 2 Brake Over Travel N A Switch terminal 1 B
35. ALS AND DESCRIPTION 10 TABLE2 2 CURRENT CONSUMPTION BY COMPONENTS OF THE SAFETY CONTROL T o 16 TABLE3 1 INTERNAL amp EXTERNAL SAFETY ELEMENTS 32 TABLE 3 2 EXTERNAL SAFETY ELEMENT CABLE TYPE 32 TABLE 3 3 WIRING DETAILS OF CSMS 33 TABLE 3 4 WIRING DETAILS OF TSMS 34 TABLE 3 5 WIRING DETAILS OF SHUTDOWN BUTTONS 35 TABLE 3 6 SHUTDOWN BUTTON MOUNTING DETAILS 36 TABLE 3 7 WIRING DETAILS OF BRAKE OVER TRAVEL SWITCH 37 TABLE 3 8 WIRING DETAILS OF INERTIA SENSOR 38 TABLE 3 9 COMPONENTS USED IN THE SAFETY CIRCUIT PC B 41 TABLE 3 10 LED MULTIPLEXING IMPLEMENTATION SUMMARY 58 TABLE 3 11 SPARE HEADER PINS TO MICROCONTROLLER FUNCTIONS 60 TABLE 3 12 CUSTOM DESIGNED FOOTPRINTS AND CIRCUIT SYMBOLS 66 TABLE 3 13 CONNECTORS USED IN SCB AND DASHBOARD 68 TABLE 4 1 DETAILS OF DASHBOARD HOLES
36. AULTS state and continues to cycle Each state is broken down into flow charts First we start with the INDICATE FAULT state of Figure 3 28 76 State Indicate Faults Start m State transisition Standby NO Light MOTOR LOOP OTOR LOOP Light TSMS NO NO Light BPS Light SD LEFT Fault LED kaya Fault LED NO NO Light IMD Light SD RIGHT Fault LED Fault LED NO NO Light BMS Light SD DASH 2 _ SD DASH OFF YES EGER NO NO Light INERTIA Light BOTS lt gt BOTS OFF YES PX CUN NO Figure 3 28 Indicate Fault Flowchart 77 State Standby Safety Element Flash RtD LED Triggered OFF RtD Button Pressed YES YES State transition State transisition Ready to Drive Indicate Faults Figure 3 29 Standby Flowchart State Ready to Drive Any Safety Element Triggered OFF YES State transition Indicate Faults Figure 3 30 Ready to Drive Flowchart 3 5 4 Code See Appendix B for a listing of the programming code running on the microcontroller of the LED Indicator Subsystem 78 4 Dashboard dashboard is a module of the Safety Control System particularly relating to the LED Indicator subsystem It forms the important visual interface between the driver and the current safety s
37. LED Indicator Subsystem and dashboard module work to alert the driver of faults in the Safety Control System and form the visual interface between the state of the car and driver Importantly the Safety Control System and wider LV Electric System have been implemented in the Formula SAE car and are proving to be working well The final working product is arguably the most important step of the engineering product development cycle which this thesis traverses from start to finish 6 1 Future Work The Formula SAE car is not a matured project that has been refined over many years It therefore leaves exciting future potential for enhancements additions and expansions particularly in the electronics department 97 One addition that I have kept at the back of my mind is logging system that would log a myriad number of car sensors that have not even been implemented as yet such as 1 Speed 2 Accelerometer 3 Gyroscope 4 GPS location 5 Various heat sensors Not only would such a project include sourcing various sensors and hardware it would require circuit design to interface with hardware and store the data in an efficient way As we saw in Section 3 3 3 13 I have left provisions on my custom designed Safety Circuit PCB for future expansions In particular the communication peripherals of the microcontroller such as SPI EUSART and have been left free and broken out to a PCB header By adding new code on the microcon
38. LT safetyElementStatus1 1 m if RA1 0 BPS FAULT safetyElementStatus1 255 im if RA2 0 MOTOR LOOP FAULT safetyElementStatus1 127 safetyElementStatus1 0600000000 safetyElementStatus2 0600000000 void LightSafetyElementFault while safetyElementStatus1 amp amp safetyElementStatus2 0 if safetyElementStatus1 255 TSMS FAULT 1 2 1 else if safetyElementStatus1 127 SD LEFT FAULT 1 1 1 else if safetyElementStatus1 63 SD RIGHT FAULT RB3 1 7 1 else if safetyElementStatus1 31 SD DASH FAULT RB3 1 RD6 1 else if safetyElementStatus1 15 5 FAULT 4 1 RD7 1 else if safetyElementStatus1 7 INERTIA FAULT 4 1 RB1 1 else if safetyElementStatus1 3 BMS FAULT 106 RB4 1 RB2 1 else if safetyElementStatus1 1 IMD FAULT RB4 1 RD6 1 else if safetyElementStatus1 255 5 FAULT 5 1 RB1 1 else if safetyElementStatus2 127 MOTOR LOOP FAULT RB5 1 2 1 CheckSafetyElementFault return void PreRtD CheckSafetyElementFault while safetyElementStatus1 amp amp safetyElementStatus2 0 flash Rtd LED RB5 1 RD7 1 delay ms 1000 RB5 0 RD7 0 if RD5 1 RtD pressed turn off Rtd LED RB5 0 RD7 0 return
39. Motor Controller MTR_CONTROL_E Enable terminals N Ready to Drive SETS button terminal 2 RTD BTN 12 N A EVMS CHG SENSE div d Supply power to CHG INTLK MICRO N A JMP HP11 P7 Charge Interlock C 12V BUZ Jumper Wire un Ready to Drive CHG_INTLK MICRO Buzzer ve Y C_12V BUZ Buzzer ve Connection to Ready to Dri Ready to Drive RTD BUZZER Buzzer ve Buzzer ve Connects to one end of the motor loop N A Motor L MTR LOOP Other end T grounded to chassis Current path to keyswitch Positi ill 56 10 Key switch P1 amp P3 RE 12V SDC OUT determine car state Idle Drive Charge Battery Box 1 AUX Supplies power contactors 1 amp 2 contactors the AUX_CTR Battery 2 AUX contactors 3 amp 4 battery boxes Jump connector pins 5 amp 6 H3 P4 H3 P5 H3 P6 Supplies power H4 P1 H4 P2 H4 P3 H4 P4 H4 P5 H4 P6 HP33 P4 S5 P10 HP3 P5 S5 P8 HP3 P6 S5 P9 H4 P1 S5 P5 H4 P2 N A H4 P3 N A H4 P4 S4 P15 S4 P14 4 5 Not a direct connection See Modified circuit Not a direct connection See Modified 34013 circuit C5 P10 N A C5 P8 N A C5 P9 N A C5 P5 N A N A N A N A N A C4 P15 BPS_T1 BPS T2 N A N A 13
40. ON after the button is released When the button is momentarily pressed energizing the relay at point E relay contacts C and NO are now closed creating a new path to ground thereby latching the relay permanently Of course at D contacts C and NO are also closed thereby closing IN A and OUT B The circuit is in the ON state 43 What happens if a fault occurs at the batteries thereby pulling BMS_SIG HIGH The transistor will turn OFF cutting power to the relay and so E and D revert to their original positions and IN A and OUT B open The circuit is now in the OFF state If BMS_SIG happens to go LOW again the circuit will not turn ON until the reset button is again pressed This is part of the interlock operation In addition to comply with Rule EV5 1 4 the reset button is placed on the SCB which is out of the reach of the driver 3333 IMD Interlock After the BMS interlock is CLOSED the next safety element the current must pass is the IMD interlock circuit Similarly to the BMS interlock the current enters the IMD interlock see Figure 3 8 at IN A and exits at OUT B provided the circuit is in a safe state IN A and OUT B are normally open complying with the fail safe principle OUT B Pole 2 Ma ND IN A Figure 3 8 IMD Interlock circuit schematic The operation of the IMD interlock is similar to the BMS interlock but will be covered now for
41. P socket This component is N A soldered in place of PIC16F887_uC REG_7805 7805Voltage Regulator NPN337 NPN BC337 bipolar transistor T1 T3 T4 T5 T6 17 T8 T9 PNP338 PNP BC338 bipolar transistor T2 CAP 100U 100 Capacitor C1 C2 DIODE 1N4004 1N4004 diode D1 D2 40 R10k Resistor 10kQ R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R13 R14 R25 R26 R27 R28 R29 R30 R31 R32 R33 R15k Resistor 15kQ R15 R16 R18 R19 R20 R21 R22 R23 R24 R34 R35 470 Resistor 4700 11 12 17 102 LED1 LED Green low intensity 80mcd LED1 SPST 67 SPST IP67 Dome momentary push IMD Reset Button button BMS Reset Button see Figure 3 23 Table 3 9 Components used in the Safety Circuit PCB 3 3 3 Circuit Schematic The method being taken to explain the Safety Control System is a top down approach Firstly a high level perspective on the operation of the LV Electric System was presented in Section 2 4 This gave us the ability to understand how the system works and how the various states of the car are achieved without getting bogged down low level circuit detail It is now appropriate to delve into the Safety Circuit PCB details Appendix A 3 contains the entire circuit schematic of the Safety System PCB The following explanation will be broken up into individual logical sections of the circuit and analysed one at a time Without breaking it down in
42. P6 1 5m Table 3 4 Wiring details of TSMS Note For more detail on Connector Termination please see Section 3 4 2 3223 Physical Mounting As per the rules the TSMS is located on the right side of the car next to the CSMS It is mounted on the Right Mount Plate see Figure 3 26 3 2 3 Shutdown Buttons The safety circuit consists of three shutdown buttons 1 Shutdown Button Dash Located on the dashboard 2 Shutdown Button Left Located on Left Mount Plate 3 Shutdown button Right Located on Right Mount Plate They can be regarded as panic or emergency buttons where if any fault occurs they can be quickly pressed As with all the safety elements the safety buttons are wired in series with the main safety circuit path These buttons are normally closed allowing current to flow however when the button is pressed the safety circuit is opened resulting in disconnection of the tractive system 3 23 1 Requirements EV5 3 1 A system of three shut down buttons must be installed on the vehicle 8 EV5 3 2 one of the shut down buttons must separate the tractive system from the accumulator block by opening the Shutdown Circuit see also 5 1 8 34 EV5 3 3 Each shut down button must be push pull push rotate emergency switch where pushing the button opens the Shutdown Circuit The shut down buttons must not act through logic e g a micro controller 8 EV5 3 4 One button must
43. R_PCB_P6 SCB H4 HDR_PLUG_P6 4 HDR P4 SCB H5 HDR PLUG P4 HP5 HDR P4 SCB H6 HDR PLUG P4 N A HDR P4 SCB H7 HDR PLUG P4 N A HDR PCB P6 SCB H8 HDR PLUG P4 N A Table 3 13 Connectors used in SCB and Dashboard Note 1 This thesis will refer to connectors given by the Name in Table 3 12 2 Individual pins of connectors are referenced by appending to the connector name where X is the pin number 3 Use of N A indicates spare PCB headers that currently are not utilised The internal wiring of SCB is very dense All wiring lengths between the PCB and lid connectors is such that it does not hinder the removal of the lid It can be taken off it can be easily placed to the side of the SCB However the wiring is also kept as short as possible to ease congestion in the enclosure once the lid is 68 following figures show the placement of external connectors listed Table 3 12 69 k 8 Figure 3 22 Placement of connectors 51 52 53 84 55 on the 5 lid Figure3 23 Placement of connector 56 mounted behind the dashboard 70 3423 Connector Pin Allocation Connector Type Pin Allocation CNTR_F_IP67_P7 CNTR_F_IP67_P18 3 4 3 Safety Circuit Box Final Product amp Mounting Figure 3 24 shows the Safety Circuit PCB mounted in the SCB enclosure The relays and PCB header plugs have been omitted for clarity 71 Figure 3
44. The microcontroller is a major component of the LED Indicator Subsystem Its use has been thoroughly covered in Section 3 3 3 10 3 52 In Circuit Serial Programming ICSP The microcontroller is programmed on board using the In Circuit Serial Programming protocol via the programming header described in Section 3 3 3 12 3 5 3 Logic Flow diagram Before programming commenced it is always beneficial to draw up some basic state or flow diagrams This helps to translate the logic of the application into code The Safety Control System has a concept of state but is also procedural in nature Therefore it is best represented by a combination of state and flow charts to describe the full picture The state diagram is given in Figure 3 27 75 All Safety Elements ON po i Standby Safety Element Triggered OFF b Ready to Drive Press RtD Button Figure 3 27 Safety Control System State Diagram Only states relevant to the embedded program are considered here For example the STANDBY state could be split into a CHARGE state but its distinction from other states is irrelevant to the program and hence remains merged The system starts in the INDICATE FAULTS state If all safety elements are ON CLOSED the system transitions to the STANDBY state Once the RtD button is pressed the READY TO DRIVE state is engage and the car is driveable If a safety element is trigged OFF then the system goes back to the INDICATE F
45. We ll begin this chapter by covering the external safety elements Safety Element Location Control System Master Switch CSMS External Tractive System Master Switch TSMS External Dash Shutdown Button External Left Shutdown Button External Right Shutdown Button External Brake Over Travel Switch External Inertia Sensor External BMS Interlock Circuit Internal IMD Interlock Circuit Internal Brake Panic Switch Interlock Circuit Internal Motor Loop Circuit Internal Table 3 1 Internal amp External Safety Elements The cable used for all external safety element wiring is summarised in Table Name Manufacturer Conductor Conductor Max Current Cores Diameter GEN_1 Generic 2 imm 10A Table 3 2 External safety element cable type 3 2 1 Control System Master Switch The Control System Master Switch CSMS is the first element of the safety circuit Its purpose is to control the power supply to the safety system In Figure 2 5 we can see that when CSMS is OFF power to the entire LV Electric System is cut When the car is unused the CSMS should be kept OFF to prevent any power usage 32 3 2 1 1 Requirements EV522 CSMS must completely disable power to the Control System and must be direct acting i e it cannot act through a relay or logic 8 EV5 2 3 CSMS must be located on the right side of the vehicle in proximity to the Main Hoop at shoul
46. acity of the SCB any number of contactors can potentially be driven 3 3 8 7 Ready to Drive Interlock amp HV relay Once the main safety circuit path is closed allowing current to close the contactors the car should not yet respond to the accelerator pedal The rule is 49 EV4 11 3 Only closing the Shutdown Circuit must not set the car to ready to drive mode The car is ready to drive as soon as the motor s will respond to the input of the torque encoder acceleration pedal Therefore additional actions are required by the driver to set the car to ready to drive mode e g pressing a dedicated start button after the tractive system has been activated 3 The ready to drive interlock controls the activation of the motor controller enable line via an external HV relay The additional action required by the rules is performed by the driver pressing the Ready to Drive RtD button This will enable the motor controllers and hence the car will respond to the torque encoder The functionality of this circuit see Figure 3 12 depends on the EVMS and also the external HV Relay box Firstly when the RtD button is pressed RTD_BTN_T2 goes HIGH powering the relay Contacts C and NO are now connected meaning the relay has a new power source RTD_12V that does not flow through the button So when the button is released it is a momentary button the relay remains latched in the ON position Figure 3 12 Ready to Drive Interlock circuit sc
47. ally applied switch closed the current essentially sees and open circuit As the current begins to flow through the wire that is the inductor a circular magnetic field is established and begins to build As this occurs the building magnetic field of each winding sweeps across every other winding inducing a voltage in those windings that opposes the voltage applied to the inductor 16 This is confirmed by Faradays Law which states that a voltage will be induced in a conductor placed in a varying magnetic field The reason the voltage is opposite comes from Lenz s Law which states an induced EMF gives rise to a current whose magnetic field opposes the original change in flux 17 The only way a current can oppose the change is flux is by flowing in the opposite direction and so in summary an induced current will flow in the opposite direction to the current that induced it The effective result of this expanding magnetic field is to hinder the rise of the current When the magnetic field reaches its maximum size steady state the inductor no longer impedes current flow as there is no long a varying magnetic field to 29 induce opposing EMF and hence current 18 Therefore the inductor acts like a low resistive wire and current flows according to ohms Law Now assume the switch is abruptly opened Since there is no longer a power supply the built up magnetic field begins to collapse As it does so it again sweeps the win
48. arch labs vc theory photolith html Accessed 15 Oct 2012 Georgia Tech College of Engineering Photolithography Online Available http www ece gatech edu research labs vc theory photolith html Accessed 15 Oct 2012 28 Interface Bus AWG Cable Description Online Available http www interfacebus com Copper Wire AWG SlIze html Accessed 15 Oct 2012
49. ashboard in the first place The LEDs are placed in groups and positioned in a neat fashion 83 such that room for labelling is available and that there will be no confusion as to which label refers to which element This can be seen in Figure 4 2 and Figure 4 3 4223 Ready to Drive Brake Panic Switch Buttons and Key switch These buttons are simply placed where they can be seen and easily accessed The Brake Panic Switch fit nicely to the left of the column of LEDs filling in the gap there but remaining outside the non visible area The key switch off course has a protruding key so it was best to keep it right away from the movement of the steering wheel hands and arms Thus it was placed to the far right along with the Ready to Start button It makes logical sense to keep these close as the sequence to start the car involves turning the key succeeded by pressing the Ready to Start button 4 2 3 Weather Resistance In a project such as the Formula SAE car that will be invariably exposed to the elements providing at least some degree of weather resistance shows prudent design that will last for the long haul a quality that eludes so many commercial products in this day and age From selecting components with appropriate waterproof ratings to sealing metal exposure the longevity of the product can be dramatically increased All wiring terminals have been protected with heat shrink where practicable and silicon sealed otherwise Th
50. avel Switch BOTS is physically actuated by the brake pedal in the event a mechanical fault occurs allowing the brake pedal to travel beyond its typical range As with all the other safety elements the BOTS is located in series with the main safety circuit path so when it is actuated it opens this circuit The chosen BOTS is a SPST push pull type button meaning once it has been actuated by the brake pedal further contact cannot reset it to the CLOSED ON position as per rule T7 3 2 A manual reset by hand is required 3 2 4 1 Requirements 17 3 1 A brake pedal over travel switch must be installed on the car as part of the shutdown system and wired series with the shutdown buttons This switch must be installed so that in the event of brake system failure such that the brake pedal over travels it will result in the shutdown system being activated and controlling the systems as defined in C4 IC vehicles or EV5 4 electric vehicles 8 17 3 2 Repeated actuation of the switch must not restore power to these components and it must be designed so that the driver cannot reset it 8 36 3 2 42 Wiring Termination of the cable at the 5 is done inserting the stripped end of the wire into the terminal and tightening the screw Table 3 6 below summarises the cabling involved for the Shutdown Buttons BOTS I O Connector Wire length Wire Type Termination Terminal 1 C3 P1 1 5m GEN_1 Terminal 2 C3 P2 1 5m
51. aximum flexibility of element placement discussed next and leave room for possible future addition of elements Finally the dashboard design is in two pieces Figure 4 2 left side and 4 3 right side This was necessary because the steering wheel mount member would obstruct the placement of the dashboard flush against the tabs Following these ideas resulted in the final outline of the dashboard 4 2 2 Placement of elements Element placements should be made judiciously to ensure a practical safe and user friendly design Such considerations include e Direct line of sight The element should not be obstructed by the steering wheel lacking a direct line of sight The driver should be able to clearly see the element clearly at all times e Logical grouping of elements Easy actuation of elements that involve driver interaction Buttons and switches are free from obstructions and easy for the driver to see and 82 access This is not only for convenience but have safety implications as well As seen in Figure 4 2 and Figure 4 3 the dashboard is quite limited in area and the driver s view of the dashboard is moderately obstructed by the steering wheel In conjunction with mechanical students a decision was made to swap with the smaller more compact steering wheel of the old 2010 Formula SAE car The red dotted line labelled Steering Wheel Obstruction in Figure 4 2 and Figure 4 3 outlines the position of the new steer
52. be located on each side of the vehicle behind the driver s compartment at approximately the level of the driver s head The minimum allowed diameter of the shutdown buttons on both sides of the car is 40 mm 8 3 23 2 Wiring Termination of the cable at the Shutdown Buttons is done by inserting the stripped end of the wire into the terminal and tightening the screw Table 3 4 below summarises the cabling involved for the Shutdown Buttons Shutdown Button I O Connector Wire length Wire Type Termination Left Shutdown Button Terminal 1 C2 P1 1 5m GEN 1 Left Shutdown Button 2 2 15m Terminal 2 Right Shutdown Button Terminal 1 SES Hom GEN 1 Right Shutdown Button Terminal 2 ot Dash Shutdown Button Terminal 1 on GEN_1 Dash Shutdown Button C2 P6 3m Terminal 2 Table 3 5 Wiring details of Shutdown Buttons Note For more detail on Connector Termination please see Section 3 4 2 3 2 3 3 Physical Mounting As per the rules there are three shutdown buttons mounted in the required locations on the car They are summarised in Table 3 5 below 35 Button Mounting Further Details Left Shutdown Left Mount Plate Figure 3 25 Button Right Shutdown Right Mount Plate Figure 3 26 Button Dash Shutdown Dashboard Section 4 2 2 1 Button Figure 4 3 Table 3 6 Shutdown Button Mounting Details 3 2 4 Brake Over Travel Switch The Brake Over Tr
53. by the auxiliary battery or DC DC converter to the entire LV Electric System The dimensions and weight of this battery Figure 2 4 were also considered easily manageable for the Formula SAE car 2 4 Safety System Operation There are a number of states the safety system can be in The easiest way to gain 5 a full insight on the operation of the safety system is to follow the steps required to attain both the DRIVE and CHARGE B states The LV Electric System largely E comprising of the Safety Control System is _ wami presented Figure 2 5 numbers 5 5 4 the figures indicate where lines should 55 u 5 connect B Figure 2 5 1 Bottom half LV Electric il System 20pe00 X V AH AH JEUILWI9 6VIILII91 1016 22V 11023407 1030 z 17 Figure 2 5 2 half LV Electric System IMD SIG BMS SIG In Lu a EN i i Insulation ems Brake Panic Tractive System Monitoring Device Interlock Circuit Interlock Circuit Master Switch Cabi Location SD Enclosure Location SD Enclosure 4 i L 4 3 18 following the current from the voltage sour
54. car such as the Formula SAE where currents can peak 800A contactors are not a choice but a necessity 2 3 2 Battery Boxes The battery boxes house the accumulator batteries contactors and BMS modules The battery chemistry used is LiFEPO having a nominal cell voltage of 3 3V 6 There are two battery boxes each comprising of 8 sets of LiFEPO4 cells in series resulting in an accumulator of 3 3 x 8 26 4V nominal The two accumulators are wired in series to produce a total 52 8V nominal 13 There is BMS module for each set of parallel cells the battery box They provide over and under voltage protection Essentially the signal line is daisy chained between each BMS module to ground If the voltage is at a safe level the MOSFETs inside each module ON and conduct the signal path The result is that the end of the daisy chain is LOW or grounded If a fault occurs at any BMS module the MOSFET is turned OFF opening the signal path The end of the line is now floating This signal is used by the BMS interlock of the Safety Circuit PCB discussed in Section 3 3 3 2 2 3 3 DC DC Converter The DC DC converter takes the 52 8VDC nominal of the combined accumulators in series and reduces it to 13 8VDC for the LV Electric System The choice of 13 8V for what is the 12V LV Electric System is explored next 2 3 4 Auxiliary Battery 2 3 4 1 Specification Sealed Lead Acid Batteries Premium commercial grade long life and
55. ce all the way through the system and considering the outcomes of the signals on various components we will see exactly how the Safety Control System operates In the process of covering the important DRIVE and Charge states the other states will be covered as a matter of course The proceeding section is heavily procedural with many cause and effect actions To gain better insight it would be of great assistance to follow Figure 2 5 along with the explanatory text 241 Drive the Car We start off in the OFF SAFETY_FAULT state where all contactors are open implying that the DC DC converter is off Therefore the only source of power possible to the safety system at this stage is the auxiliary battery The CSMS is then closed which powers on the EVMS In addition we can see a potential path through a multitude of various safety circuit interlocks sensors buttons For simplicity each of these elements can be thought of as a switch OPEN if it is in the FAULT state and CLOSED if the element is in the SAFE state The detail and inner working of each element is thoroughly covered in Chapter 3 Assuming each of these elements is CLOSED we have a closed path to the key switch If the key switch is in the Standby middle position then we are currently in the STANDBY state To follow the remaining process you may want to refresh your memory on the detailed functionality of the EVMS see Section 2 2 By turning the key switch to Drive
56. cess for all but the most basic of circuits Once the design is complete the next step is to fabricate the PCB and install componentry The first fabricated board is normally referred to as prototype prototype enables the design to be put to use and thoroughly tested Any problems or design flaws encountered can be rectified However it is important to realise that the fabrication process can be costly and time consuming Ideally the prototype will have no flaws and can be used as the final product This is off course not realistic and usually at least one prototype will be required aim is to tweak the design and sort out all the issues in the first prototype Too many of these cycles will lead to cost and time blow outs It just goes to show that thorough analysis and a systematic review of the circuit design and PCB layout will save time and money in the long run When comparing this with software development it is evident that a very different approach and mindset is required in electronics design In software development errors in the code can be corrected and the software recompiled with a few presses of a button This cycle can occur continuously with virtually no time or monetary cost Contrasting to electronics design the analogy to compiling software is like manufacturing a PCB and we have seen this is something we do not want to repeat too many times 3 3 2 Component Selection This section will cover the specificatio
57. completeness Firstly let s look at the NPN transistor Its base terminal is driven by IMD_SIG which is the signal from the IMD device If it is HIGH the IMD has determined that the insulation condition of the car is in a safe state and hence the transistor is ON However if IMD_SIG is LOW or 44 FLOATING there is an insulation fault in the car and the transistor is OFF pull down resistor ensures the transistor is OFF when IMD_SIG is not HIGH Now assuming the transistor is ON the last obstacle to allowing the current to flow through the relay is the BMS Reset button When it is pressed current flows from IN A through the BMS Reset button via header H5 P2 and H5 P3 through the relay coil through the transistor and to ground The relay is turned ON This offcourse connects the contacts C and NO of both poles A new path to IN A is created which actually provides a new voltage source via pole 2 This latches the relay in the powered state That is when the BMS Reset button is released the newly created path keeps the relay powered latched Pole 1 also allows current to flow through it and onto the next safety element that is the BPS interlock The IMD interlock is in the ON CLOSED state If IMD_SIG now goes LOW or FLOATING in which case the pull down resistor will pull it LOW anyway the transistor will turn OFF This opens the current path for powering the relay and therefore it turns OFF reverting the poles to the original pos
58. cured by a nut from behind elements were chosen that could be mounted in this way The LEDs were secured by first inserting an LED bezel into the hole Then the LED is inserted from behind which applies pressure to the bezel and holds it in place To increase the strength super glue was applied around the base of the LED bezels The connector mount is secured behind the dashboard using a bolt washer and nut The connector itself is inserted and tightened using the included plastic nut In addition two sets of a bolt washer and nut are used to provide extra strength in diagonal corners of the connector see Figure 3 23 Finally labels were printed and applied as per the design 43 1 4 Connector amp Wiring The functionality of the dashboard is driven by the Safety Circuit PCB Therefore cabling will be required to interface these two systems located at either end of the car The main cable running between the Safety Circuit PCB rear of car and dashboard front of car is illustrated in Figure 4 4 86 Car Front Car Rear P1 11 13 mm m EE P12 EVMS_LED_GROUND 12 14 GROUND P14 5 2 P15 EVMS_KEY_IN P15 BPS 1 P16 EVMS LED RED P16 N A P17 EVMS LED GREEN P17 N A P18 EVMS LED BLUE 18 Figure 4 4 Main cable run between front rear of the Connector C4 and C6 terminate each end of a cable that runs from
59. d from a 5V source The safety circuit utilises a 5V linear regulator to attain the required voltage input see Figure 3 17 Figure 3 17 5V Power Supply circuit schematic Bypass ceramic capacitors are used at the input and output of the regulator as well as close to the microcontroller input This minimises the noise and provides a clean and stable 5V power supply to the microcontroller 3 3 3 12 Programming Header The inclusion of a programming header adds immeasurable convenience to the programming process of an embedded system As discussed in Section 3 3 1 58 programmin g is iterative cyclic process where the program be updated and uploaded to the microcontroller numerous times over the lifetime of a project Thousands of times is likely for a complex project By using an integrated programming header 7 the vast number of potential updates that will occur during the testing and debugging phase will not be a worry because of the simplicity of the process The programmer is simply connected straight to the programming header and the new programs uploaded The programming is done completely on board using the In Circuit Serial Programming ICSP Microchip protocol The alternative is to use an external programmer prevalent in the past The microcontroller would have to be physically removed from the circuit and placed in a standalone programmer where the programming function was performed Then the micr
60. der height and be easily actuated from outside the car 8 32 12 Wiring Termination of the cable at the CSMS is done by wrapping the stripped end of the wire around the terminal and tightening the nut Table 3 2 below summaries the cabling involved for the CSMS TSMS I O Connector Wire length Wire Type Termination Terminal 1 C1 P3 1 5m GEN_1 Terminal 2 1 4 1 5m Table 3 3 Wiring details of CSMS Note For more detail on Connector Termination please see Section 3 4 2 3 2 1 3 Physical Mounting As per the rules the CSMS is located on the right side of the car It is mounted on the Right Mount Plate see Figure 3 26 3 2 2 Tractive System Master Switch The Tractive System Master Switch TSMS is one element of the safety circuit Its purpose is to simply open or close the main safety circuit path thereby directly controlling the contactors and state of the tractive circuit 3 2 2 1 Requirements EV5 24 TSMS must be located next to the CSMS and must open the Shutdown Circuit The TSMS must be direct acting it cannot act through a relay or logic 8 33 3 2 22 Wiring Termination of the cable at the TSMS is done wrapping the stripped end of the wire around the terminal tightening the nut Table 3 3 below summaries the cabling involved for the TSMS TSMS I O Connector Wire length Wire Type Termination Terminal 1 C1 P5 1 5m GEN_1 Terminal 2 C1
61. dings of the inductor and induces a voltage which forces current to continue flowing in its current direction This is similar to the previous case when the magnetic field was instead building In effect the inductor has used the stored energy in its magnetic field to keep the current flowing after the disconnection of the power supply 12VDC GND GND Figure 3 5 Freewheeling diode used in circuit Consider the circuit in Figure 3 5 19 When the transistor is ON the inductor reacts and behaves as previously described However when the transistor turns OFF there is no longer a closed path allowing current to flow through the inductor The current is forced to 0 meaning almost instantaneously meaning di dt is very large Looking at the mathematical definition of an inductor di V L 3 10 dt we can conclude that a VERY large negative voltage will be induced across the inductor This is likely to be detrimental to other components in the circuit 30 especially semiconductor devices such as the transistor the circuit of Figure 3 5 A relatively simple solution to this problem is the use of a freewheeling or fly back diode placed across inductive components The diode is connected in reverse bias to the power supply as in Figure 3 5 so it is effectively invisible during normal operation of the circuit When the transistor turns OFF the diode clamps the voltage across the inductor to about 0 7V protecting th
62. distinction LV Electric System Safety Control System Auxiliary Battery p LED Indicator Sub system P A gt DC DC Converter Safety Circuit Box PCB gt Battery Boxes EVMS I 7 Future Electronic 2 Systems Dashboard Figure 2 2 LV Electric System Breakdown The LED Indicator system is a subsystem of the larger Safety Control System and is responsible for pinpointing the safety element at fault and notifying the driver or crew Figure 2 2 The Safety Control System which includes the LED Indicator sub system is physically implemented via the Safety Circuit Box SCB EVMS and dashboard Figure 2 2 At the heart of the Safety Control System is the custom designed and built Safety Circuit PCB contained within the Safety Circuit Box SCB which will be thoroughly examined Chapter 3 For now we begin our analysis by covering the basic operation of the other components that make up the LV Electric System namely the EVMS and auxiliary components Once this foundation is laid we move onto a high level overview of the entire LV Electric System with an emphasis on the Safety Control System This will aid in explaining its overall operation and how all the elements work together to put the car in the all important DRIVE and states Even at this relatively high level view there plenty of interc
63. e transistor or other potential componentry 3 2 Elements of the Safety Circuit All safety circuit elements are part of the main safety circuit path which is the electrical artery of the Safety Control System The main safety circuit path is the one which travels from the power supply through all the safety elements The current which flows via this path directly powers the contactors and is in line with rule EV5 1 1 The Shutdown Circuit directly carries the current driving the accumulator isolation relays AIRs 8 The safety elements are placed in series with this path An element is in the SAFE state when it has closed its portion of the main safety circuit path This is referred to as the element being ON or CLOSED If an element is in the FAIL state then it has opened its portion of the main safety circuit path This is referred to as the element being OFF or OPEN It follows that in order to have a closed main safety circuit path and hence ability to power the contactors all elements must be in the safe state If just one element switches to the FAIL state then the main safety circuit path is electrically opened and the contactors cannot be powered The elements range from simple buttons to more complicated interlock circuits and are either external to or implemented inside the Safety Circuit Box A list of 31 internal and external safety elements part of the Safety Control System 15 presented in Table 3 1
64. e dashboard itself has been sprayed with a clear lacquer to hinder corrosion and upkeep the aesthetic appeal 4 2 4 Future Expansion and Maintenance In all my design work I always consider ease of accessibility for maintenance or future expansion purposes I believe this completes the package in terms of a robust design Using a modular technique by incorporating a connector exemplifies this approach The dashboard is now nothing more than a separate component or module of the larger Safety Control System It is attached to the Safety Control PCB via one simple to use connector and disconnected in the 84 same way If a fault develops with element the dashboard be quickly disengaged and taken to a workbench for repair In mission critical situations it would be possible to plug and play spare dashboards implying minimal downtime The advantages are stark and it is clear just how difficult a simple maintenance task would be if the cabling was all hard wired 4 3 Physical Implementation 4 3 1 Dashboard Construction The manufacturing of a personal design is an immensely satisfying step of the overall product production I did not use any workshop time for either the dashboard or Safety Circuit PCB This meant more complicated fabrication for other aspects of the car could be prioritised instead The REV lab is our pseudo workshop and it can only be best described as modest While the end results may not be of commercial qual
65. e ground which it is terminated at This keeps the relay energized so long as the motor loop cable is not severed meaning IN A and OUT B are closed allowing the current to continue to the next module 3 3 3 6 AUX amp Main Relay Module As covered in the Section 2 2 the EVMS controls powering the auxiliary and main contactors The EVMS was not a custom design for the Formula SAE car unlike the Safety Circuit Box this thesis covers Consequently it does not fit in with all the requirements of the Safety Control System In particular the inability of the EVMS to drive more than one contactor as required by the Safety Control System was found out the hard way The EVMS documentation is fairly poor especially regarding to specifying electrical ratings After contacting the manufacturer it was made clear that driving four contactors 48 would be acceptable However an experiment to power two contactors in parallel damaged the EVMS which then had to be repaired The solution to this problem is the AUX MAIN Relay module as shown in Figure 3 11 Figure 3 11 Aux amp Relay Schematic Instead of the EVMS directly driving the contactors it instead drives relays internal to the SCB Specifically the EVMS MAIN output drives the SCB MAIN relay and the EVMS AUX CTR output drives the SCB_AUX relay I am no longer subject to the limitations of the EVMS Subject to limitations of the chosen relays and current cap
66. e microcontroller would certainly damage it The nature of the voltage divider implies there will be a continuous loss of power to ground This can be minimised by a judicious selection of resistors We first need to find the ratio of the resistors to achieve 5V Assume R 10k Then x 14k closest resistance to this from the 12 standard without going lower is 15k This ensures the voltage to the microcontroller is slightly under 5V or more precisely 10k Eo 15k 4 8V is well above the minimum input voltage of 2V of the PIC16f887 12 4 8V microcontroller data sheet p251 20 to register as a HIGH input The amount of current leaking to ground with 10k and 15k resistors is 12 _ 25k OSH Using resistors of 1k and 1 5 k would achieve the same result in terms of voltage division but if we check the current flow now we have Pc d GA yup edm This is a 10 times increase of wasted current per voltage divider used of which there are 10 This is essentially completely wasted power which can be avoided by just simply making correct design decisions 3 3 3 10 2 Microcontroller Outputs The microcontroller outputs are used to drive transistors which in turn power the LED indicators and also the buzzer covered in Section 3 3 8 The PIC16F887 has a maximum source or sink current of 25mA per pin and a maximum of 75mA for all ports combined datasheet p245 20 Even though only one fault LED is ever
67. eader plug HP5 P1 to S5 P4 T2 and HP4 P1 to S5 P5 T3 The last transistor without a base current limiting resistor is T10 which is part of the Ready to Drive Sound circuit The base pin is directly driven by a microcontroller pin and not by an external device such as in the case of the BMS and IMD interlock transistors This means there is no wire to insert the resistor in line with Instead the trace on the PCB is cut and the resistor soldered over the broken track This is a neat and simple fix 92 5 1 2 not necessary charge interlock The resistor R1 as seen in the charge interlock Figure 3 3 3 9 does not serve any useful purpose When the car is not in charge state EVMS CHRG SENSE is pulled high INTLK MICROC 12V BUZ which is HIGH so long as the CSMS is switched on The EVMS has no requirement to limit current into EVMS CHRG 5 via a resistor During design I was probably stuck in the mindset of using a resistor for all pull up functions which is often required but not in the case of this circuit The next important step is to perform is a quick analysis as to whether the resistor will affect the operation of the circuit and as it turns out it does not Technically the resistance coupled with any input capacitance which is negligible in this case will increase the rise time according to the time constant RC The pull up of EVMS CHRG SENSE is not a time critical operation and hence inconsequentia
68. ey Switch P1 The input signal Key Switch P3 into the key switch is ON when all elements of the safety circuit are closed Otherwise the input signal is OFF 11 Key switch pin 4 56 11 2 9 gt Key Switch 4 This signal line pulled HIGH when the key switch is in the charge position This powers Charge 88 Interlock Relay pulling EVMS_CHG SENSE LOW 12 BMS LED Cathode S6 P12 EVMS_GND gt BMS LED Cathode terminal 13 BPS Reset Button S6 P13 C4 P13 terminal 1 gt BPS Reset Button terminal 1 14 BPS Reset Button S6 P14 GROUND terminal 2 gt BPS Reset Button 2 15 Key switch pin 2 S6 P15 EVMS KEY IN Key Switch P2 Signal line goes HIGH when the key switch is in the drive position 16 BMS LED Red S6 P16 EVMS STATUS R BMS LED Red terminal 17 BMS LED Green S6 P17 EVMS STATUS G BMS LED Green terminal 18 BMS LED Blue S6 P18 EVMS STATUS B BMS LED Blue terminal Table 4 2 Dashboard physical wiring details Note Column 1 is a list of pins on the connector socket would otherwise occur at the connector 89 Column 2 states which elements the pin is electrically connected to Column 3 shows the sequence that wiring physically occurs In cases where multiple elements connect to one pin the elements and connector pin are daisy chained in series instead of a star configuration from the pin This minimises wire used and al
69. for further PCBs enclosures mounting and space requirements the Safety Circuit PCB has all the required signals from each safety element in one convenient location circuitry of the LED Indicator Subsystem consists of the microcontroller and LED drive transistors which will be covered next 3 3 3 10 1 Microcontroller Inputs All microcontroller inputs utilized are part of the LED Indicator Subsystem In Section 3 2 the idea of the main safety circuit path was introduced The LED Indicator Subsystem works by connecting the output of each safety element to the input of the microcontroller When all elements are ON the signal on each input pin will be HIGH If any element turns OFF then that element and all proceeding element input pins will read LOW By knowing the order of the safety elements it is possible to identify which safety element was triggered See Appendix B for code implementation 54 safety element signals safety element signals gue input pins A Figure 3 15 Voltage Divider inputs to Microcontroller In actual fact the inputs to the microcontroller IO pins cannot be directly connected rather they must be fed through a voltage divider This is because the microcontroller is a 5V device whereas the safety circuit including the main safety circuit path runs off 12V Inputting 12V into th
70. fused with the Formula SAE 2010 car 2 which was an electric conversion using an old UWA Motorsports chassis The proactive and ambitious nature of REV is further demonstrated by a new and exciting project to begin in 2013 It will attempt the conversion of a jet ski the first electrically powered marine vehicle to enter the REV line up 1 2 2 REV Formula SAE The REV Formula SAE 2012 vehicle is the latest and current project being pursued by REV team 2012 The marked difference with past endeavours is that this is not a conversion instead a combination of electrical and mechanical student engineers will collaboratively build this car from scratch This off course brings new and exciting challenges but also gives students the chance to innovate and design every aspect of their respective components without the restriction of having to adapt to previous designs The ultimate goal besides having a fully functioning car is to participate in the Formula SAE A 2012 competition 3 held at Victoria University Industrial Skills Campus This will really enable us to put our designs through the strain of the myriad vigorous tests involved which will give us the best real world practical evaluation of our final product 1 2 3 Personal Motivation When choosing my thesis topic I desired a project that would draw upon the skills and experience gained over my past 5 years at UWA Setting solid foundation prior to my entrance into the workf
71. g at the forefront As the world s insatiable demand for fossil fuels continues to grow year by year issues of finite resources and environmental consequences are suddenly becoming significant factors that warrant thoughtful consideration if we are to continue on a sustainable growth path into the future Electric motors are by no means a new concept however their use as the propulsion system in vehicles is The general idea is that the internal combustion engine and the fuel which it relies on is replaced by an electric motor s and batteries respectively The fundamental point to note is that by sourcing electricity generated using renewable methods results in a vehicle with significantly reduced emissions As this technology is developed and prices come down we just find ourselves at the dawn of new era of electric dominated vehicles 1 2 The REV Project 1 2 1 Brief Overview The UWA Renewable Energy Vehicle Project REV headed by Professor Thomas Braunl 15 long term initiative to develop solutions to secure the vision of a clean and sustainable energy future 1 Some of the recent successfully completed REV projects have included the conversion of a Hyundai Getz in 2008 called the REV Eco and a Lotus Elise in 2009 called the REV Racer into electrically propelled vehicles Currently the REV Formula SAE 2012 race car is under construction to be complete by end 2012 and is the focus of this thesis This is not to be con
72. hat will it work Sure for a home one off project This is precisely the situation I find myself constructing one of project on a limited budget and limited selection of components from Altronics from whom we are encouraged to buy due to a sponsorship For the rest of the traces calculating the required trace width for 1A gives 0 300mm width Using a value of 1A far exceeds the current that any other trace in the circuit will conduct The actual trace chosen is again compromised this time up to 0 508mm Off course using a large width is always acceptable in terms of current capacity However the reason a larger trace was chosen was because I planned to manufacture this board myself Clearly my capacity to make a PCB and tolerances involved in my process will be markedly worse than a professional fab house However a width of 0 508mm small enough to effectively route with but at the same time ensuring they are wide enough to result in solid traces in my PCB manufacturing process 63 3 3 4 1 Custom Component Footprints The library of components available in EAGLE 25 is extensive but lacks consistent nomenclature and organisation Due to the countless number of electronics componentry available today CADSOFT chose to use an open community type paradigm for the building of the library This off course allows the electronics community at large to add their own footprints and symbols of various components While this leads to an extens
73. hematic 50 Since the contactors are the EVMS DRIVE and EVMS DRIVE ENABLE terminals are already closed Hence from the Safety System Figure 2 5 we can see the current can now flow through these EVMS terminals to the HV relay through the closed contacts of the interlock circuit relay and then to ground This turns on the HV relay The current from the HV tractive supply can now flow through the HV relay and to the motor controller enable terminals turning them ON At first it may seem that the external HV relay is redundant Why not directly control the motor controller enable line with the RtD interlock circuit relay There is a good reason why it has been designed and implemented this way The HV relay is named so because it passes the tractive HV through its contacts If there was no external HV relay the relay from the RtD interlock would have to pass this HV This implies that HV cabling would enter the SCB The problem with this approach is that the rules are very strict when dealing with HV Specially rated insulating barriers must be used between HV and LV areas in an enclosure and the specified spacing has to be maintained on the PCB The safety circuit design purposely ignored these spacing requirements so that the circuit could be better compacted This off course meant no HV would be allowed inside SCB Overall the chosen implementation is better because it avoids mixing HV and LV which minimises risk of contact a
74. hich is shown in Figure 3 1 is non linear unlike that of a resistive component such as a resistor that complies with Ohms Law 9 Forward x gt Current s 95 295 ex lt gt Conventional Current Flow 50 Anode 4 Cathode 40 notch Short Lead LED Type amp Colour LED and its I V Characteristics Figure 3 1 I V for LED of various colour 12 Without going into too much detail on the operation of a diode one characteristic of a diode is the ON forward voltage When a diode is forward biased in this way it is ON an able to conduct a current However as the forward voltage is slowly exceeded the resistance of the diode exponentially drops off This will cause a larger current to flow that can very quickly burn out a diode To prevent this a resistor is used to limit the current to an appropriate level Consider the simple circuit given in Figure 3 2 25 Figure 3 2 Simple LED circuit 13 Where Vs Supply voltage i LED forward current in Amps Vf LED forward voltage drop in Volts The power supply Vs is known by the user and Vf is obtained from the LED datasheet The value of i depends on the type of LED being driven but for a basic indicator LED a safe value is 5mA 12 The resistor therefore needs a value of _ Vs Vf R1 r 3 8 The values of the LED Driver Circuit Figure 3 14 are Vs 12V i 20mA and Vf
75. ing wheel in front of the dashboard when seated The portion of the dashboard below the line will be obstructed by the new steering wheel As the design demonstrates placement of elements in this area has been avoided 4221 Shutdown Dash Button This is arguably one of the more important elements as it relates directly to the safety of the driver It is critical for safety reasons that the driver can freely and easily actuate the Dash Shutdown button in an emergency situation Therefore it is particularly vital that it is free from all obstructions particularly the steering wheel in any rotational position It has hence been placed to the left area of the dashboard free from clutter of other elements and well clear of the steering wheel 4222 LED Indicators These LEDs indicate faults within the Safety Control System When some safety elements are triggered OPEN OFF such as the Shutdown Dash button and the Brake Panic Switch the driver can reset the elements to turn them back ON and hence CLOSE the main safety circuit path On the other hand reset buttons for other elements such as the IMD and BMS interlocks are purposely out of reach of the driver rule requirements Either way the purpose of the dashboard is to interface the state of the car with the driver and so all elements should be unobstructed and clearly visible I argue that if a particular element is not really required to be viewed by the driver then it should not be on the d
76. ition The IMD interlock is now in the OFF OPEN state If IMD_SIG goes back to HIGH the circuit will not automatically turn on again The IMD reset button will need to be pressed again This is part of the interlock operation of the circuit It is also in line with rule EV 5 1 4 that states a manual action is required if the safety circuit has been triggered by the IMD A freewheeling diode has been incorporated in this circuit as we have the relay of the coil in series with a semiconductor device namely the transistor As covered in Section 3 1 4 the inductive voltage spike of the relay coil when it is abruptly turned off by the transistor will likely damage the transistor The diode clamps the voltage across the relay coil to its turn on voltage and allows current to dissipate in the diode relay loop 3 3 3 4 BPS Interlock The Brake Panic Switch interlock circuit ensures the main safety circuit path is opened when the brake panic switch is actuated The details of the switch and 45 its placement being managed the student designing the pedal box related rule is 17 4 1 All vehicles must be equipped with a brake panic switch as part of the shutdown system such that in the event of a panic braking incident the brake panic switch will be activated This switch must kill the engine and cut the power to any electrical fuel pumps and for electric vehicles it must open the Accumulator Isolation Relays 3 The assumpti
77. ity take great pride in my work as we will see of the final product see Appendix 4311 Cut Out The dashboard is made from 2mm sheet aluminium It was cut using a jigsaw and appropriate cutting blade The outline of the dashboard is pencilled on aluminium sheet secured and using a jigsaw cut free hand A bracket was also required for mounting the connector to An aluminium angle 90 bracket was used and cut to size using a manual hacksaw 4 3 1 2 Drilling Holes Following the design all holes are pencilled in It is good practise to centre punch all holes before drilling so prevent the drill from slipping For the larger holes spade drill bits were used When drilling metal it is important to keep the speed low preferably resting every 10 seconds to allow the drill bit to cool Drilling too fast will cause the drill bit to become very hot which will cause the bit to blunt very quickly Table 4 1 summarises the details of each hole 85 Drill Bit Drill Bit Type Diameter Shutdown Dash Button Spade 25mm 2 Ready to Drive Button Spade 13mm Brake Panic Switch 3 Key Switch Spade 22mm 4 LEDs Twist HSS 6 4mm 5 Connector Mount hole for connector Spade 25mm 6 Connector Mount for securing Twist HSS 3mm 7 Mounting Dash Holes Twist HSS 8mm Table 4 1 Details of Dashboard Holes 43 1 3 Component Assembly components except the LEDs are simply placed in their respective holes and se
78. ive database of components unfortunately the quality of the designs cannot be guaranteed Therefore my principle is that you either check the footprint dimensions of each component you use from the library or implement them yourself Each library component is known as a package and consists of a symbol used in the schematic and a footprint used in the PCB layout In the case of the safety shutdown circuit the standard components such as the resistors BJT transistors and diodes have matching packages Packages for others such as the relay bases PCB headers and PIC16F887 microcontroller don t exist The implementation of these components is displayed in Table 3 11 below Schematic Symbol Layout Footprint RLY_SPDT_12V 64 RLY_DPDT_12V_1 j 79 0 9 PIC16F887_uC 65 HDR_PCB_P6 id EA io B io p B io io B gt Table 3 12 Custom designed Footprints circuit symbols 3 4 Physical Design amp Implementation At this stage we have covered a great deal on the Safety Circuit PCB on a design level The external safety elements have been covered in depth and a thorough analysis of the design and operation of the Safety Circuit PCB has followed However this is really just the first step of the product development cycle Next the designs and decisions related
79. k diodes and their function Online Available http www electrotechnik net 2009 09 freewheeling or fly back diodes and html Accessed 15 Oct 2012 Battery Tender Float Maintenance Charging Is it really necessary Online Available http batterytender com includes languages english resources FLO AT MAINTENANCE pdf Accessed 15 Oct 2012 MIT Electric Vehicle Team A Guide to Understanding Battery Specifications Online Available http mit edu evt summary battery specifications pdf Accessed 15 Oct 2012 7 Electronics Tutorials Electronics Tutorial about Light Emitting Diodes Online Available http www electronics tutorials ws diode diode_8 html Accessed 15 Oct 2012 Sparkfun Electronics LED Current Limiting Resistors Online Available http www sparkfun com tutorials 219 Accessed 15 Oct 2012 Tristan Tech Led Display Multiplexing Online Available http www tristantech net articles led display multiplexing Accessed 15 Oct 2012 fpga4fun LED Multiplexing Online Available http www fpga4fun com Opto4 html Accessed 15 Oct 2012 Answers How does and inductor work Online Available http wiki answers com OQ How does an inductor work Britannica Encyclopedia Lenz s Law Online Available http www britannica com EBchecked topic 336163 Lenzs law Accessed 15 Oct 2012 My Electronics Basic Inducto
80. l interrupt the current to the coil of relay R1 closing contacts C and NC which puts the BPS interlock back ON 3 3 3 5 Motor Loop Module The purpose Motor Loop Module is to open the main safety circuit path in the event of an accident where the mechanical integrity of a wheel hub motor is compromised The rule for this is stated below EV4 23 Outboard wheel motors are allowed where the motor is outside of the frame but only if an interlock is added such that the Shutdown Circuit V5 1 is opened if the wheel assembly is damaged or knocked off the car 3 A long length of cable will run the perimeter of the car with particular attention paid to how it is attached to the wishbones and motor of each wheel Crimped spade connectors are strategically placed in series with this cable on both sides of the wishbone The idea is that in an accident the connectors will rip apart causing the circuit and hence the main safety circuit path to open The main safety circuit path passing through the Motor Loop Module see Figure 3 10 between IN A and OUT B is OPEN fulfilling failsafe requirements 47 Figure 3 10 Motor Loop Module circuit schematic The current can only reach the Motor Loop Module if the previous Brake Panic IMD and BMS interlocks are all in the ON state Once it reaches the Motor Loop Module it will flow through the relay coil and then through the long motor loop cable If the cable is in order it will conduct the current to th
81. l to the correct operation of the charge interlock Due to this the circuit will not require any modification Nevertheless in a final product design for mass production the resistor would be eliminated from the circuit 5 1 3 BPS Reset Button Issue The Brake Panic Interlock requires a normally closed NC reset button to operate correctly Please refer to Section 3 3 3 4 for more detail The assumption was made that such a part would be easily available Unfortunately a NC variant of the BTN SPST IP67 Table 3 8 did not exist In fact no NC IP67 momentary buttons were available at local suppliers Instead of going to the expense and waiting for delivery times the NO SPST IP67 button would be used However the circuit would need modification to work correctly If a major logical or design fault was identified then there would be little choice but to redesign the circuit board and remanufacture However as we will see the solution to this fault only requires the addition of two components Off course doing this directly on the board would be very difficult as the PCB routing was done in an efficient and compact manner The revised design can 93 nevertheless easily implemented placing the components line with the wiring running between the PCB header plugs and socket connectors on the lid The revised schematic for PCB header H4 is shown in Figure 5 2 In section 3 3 3 4 we saw that once the BPS is actuated curre
82. ldering it underneath the board as shown in Figure 5 3 Figure 5 3 Addition of required diode It is within the clearance of the PCB standoffs in the SCB enclosure and so does not interfere in any way 5 1 4 IMD Interlock Trigger Issue As we have seen the Safety Circuit PCB consists of a series of interlock circuits that either OPEN or CLOSE the main safety circuit path When viewing the IMD interlock circuit in its own right there are no issues However it is only until a higher level view it taken that an issue surfaces We know from Section 3 3 3 3 that the IMD interlock draws from the main safety circuit path to power its relay and the interlock itself is situated after the BMS interlock Now if an IMD fault occurs and IMD_SIG is pulled low the IMD interlock opens and the IMD fault light will activate If the IMD fault is rectified implying IMD_SIG is pulled HIGH then the IMD reset button can be pressed to close the interlock Everything is so far ok 95 If however the BMS interlock is triggered opening the main safety circuit path which powers the IMD interlock the IMD interlock will lose power and open also The outcome is that both interlocks have triggered when only the BMS interlock should have The effect on operation will be that both the BMS and IMD reset buttons will need pressing to close both interlocks again While the circuit can still operate in this way it is neither as intended nor intuitive to the user
83. leviates the cable congestion that Column 4 provides quick reference to where the signal line is ultimately connected to at the SCB The Safety Circuit PCB header and pin is given and a small description if necessary 4 3 2 Final Product Summary I am pleased with the final manufactured dashboard see Appendix A 1 amp A 2 Keeping in line with my design principles it forms a modular component of the safety circuit and car overall Precautions have been taken to ensure weather resistance as much as practicable The layout of the elements has been given consideration for easy driver viewing labelling has made it intuitive to use and space remains for small additions of various instrumentation in the future Last of all it is compliant will all rules effecting the design of the dashboard 90 5 Testing Debugging For all but the most basic of circuits thorough testing of the hardware debugging of software is a fundamental part of the development process We saw in Section 3 3 1 that at the prototyping stage the design is thoroughly put to the test and all problems identified and fixed No matter how well a circuit is reviewed often there will be oversights and some problems are difficult to foresee and only become evident once the device is operated in practise The Safety Circuit PCB design is no different and therefore required some minor revisions commercial setting the circuit would be remanufactu
84. lt Input BMS IN HIGH FLOAT Preconditions 1 N A Effect 1 AUX CTR LOW OFF 2 MAIN CTR LOW OFF 3 DRIVE amp ENABLE pins open 4 CHARGE amp ENABLE pins open Scenario 4 Standby Input 1 KEY IN LOW FLOAT 2 CHG SENSE HIGH Preconditions 1 N A Effect 1 AUX CTR LOW OFF 2 MAIN CTR LOW OFF 3 DRIVE amp ENABLE pins open 4 CHARGE amp ENABLE pins open 12 Notes 1 interlock acts between the charge and drive states Its purpose is to ensure the car cannot transition from DRIVE to CHARGE or vice versa without first passing through the standby state The idea is that the car contains a charge door which pulls CHG SENSE LOW when opened for charging When the car is connected for charging having opened the charge door the interlock will prevent the car from accidently being set into drive mode even if KEY IN is pulled HIGH Similarly if driving and the charge door is accidently opened thereby pulling CHG SENSE LOW the interlock will prevent the car from entering the charge state which would open the main contactor and hence break the tractive circuit 2 See 5 for full EVMS Manual 2 3 Auxiliary Components 2 3 1 Contactors The contactors form the interface between the LV Electric System and the HV Tractive System Contactors are electrically controlled switches very much like a relay The significant difference is that contactors are heavy duty and rated for much higher currents In a
85. maintenance free sealed lead acid batteries Cells are fully sealed to prevent leakage of electrolyte Applications include security systems camcorder packs medical equipment computers and UPS Beware of cheaper consumer grade batteries that often have a shorter life span 550908 12V 7 2 2895 26 5 2350 12V Models H L Connections Weight 50908 101 151 65 4 8mm spade 2 55kg Figure 2 4 Chosen Auxiliary Battery Specifications 7 14 2 3 4 2 Justification The team was advised by Prof Br unl to try and avoid using an auxiliary battery In past REV projects such as the Hyundai Getz Eco Vehicle the auxiliary would go flat and hence starting the car would be impossible This caused great frustration and angst and therefore a robust solution was required for the new Formula SAE car The DC DC converter Section 2 3 3 takes the 52 8VDC nominal of the tractive battery packs and steps down the voltage to 13 8VDC to power the control system However connecting this idea with the following rule EV3 52 accumulator isolation relays must cut both poles of the accumulator If these relays are open no HV may be present outside of the accumulator container 8 results in an unfortunate and unavoidable circular condition The contactors must be powered to close the poles of the battery boxes which in turn supply power to the DC DC converter However the contactors can t be ON in the first place because the DC DC co
86. mam ans 73 Programming the Microcontroller ete 75 3 5 1 75 2532 Circuit Serial Programming ICSP 75 35 53 OPI 75 3 5 4 CoOde PM E 78 discs sut 79 Initial Concept etude 79 ISIN ra mE 80 4 2 1 Shape and Dimensions 82 4 2 2 Placement of elements 82 42 44 Shutdown Dash Button 83 4 2 2 2 LED 83 4 22 3 Ready to Drive Brake Panic Switch Buttons and aii Utama dt 84 4 2 3 Weather Resistance eua dg etie I 84 4 2 4 Future Expansion and Maintenance 84 Physical Implementation tar chiede ie E A HERR 85 4 3 1 Dashboard Consteacllonc o aae ierit dit a dc ners 85 LILT ua pusan erani eet acetic eria abd utin 85 Z 91 2 Db emet eoa diten 85 4 3 1 3 Component Assembly 86 4 3 1 4 Connector amp Wiring ecran ees o eei 86 4 3 2 Final Product Summaty i ct ether estrena ture 90 5 Testing and Debugging 91
87. ns of all the components used in the Safety Circuit Box The following table enumerates all components used in the SCB and their associated component ID as used in the circuit schematic see Section 3 3 3 and Board Layout Figure 3 19 It particularly aids in the placement of components on a blank PCB 39 Component ID Component Description SCB Component ID CNTR_M_IP67_P18 Connector plug male rated IP67 CA C5 C6 18 pin CNTR IP67 P7 Connector plug male rated IP67 7 C1 C2 C3 pin CNTR F IP67 P18 Connector socket female rated S4 S5 S6 IP67 18 pin CNTR F IP67 P7 Connector socket female rated S1 S2 S3 IP67 7 pin RLY SPDT 12V Relay 12V coil SPDT K4 K5 K7 K8 K9 16A 250VAC 110VDC rated K10 contacts RLY DPDT 12V 1 Relay 12V coil DPDT K2 K3 K11 8A 250VAC 30VDC rated contacts RLY SCKT 1 Relay socket base for N A RLY SPDT 12V This component is soldered in place of all RLY SPDT 12V elements RLY SCKT 2 Relay socket base for N A DPDT 12V RLY DPDT 12V 2 This component is soldered in place of these two elements HDR PCB P4 PCB Header 4 pin 7A rated H5 H6 H7 HDR PLUG P4 Plug for HDR P4 N A HDR PCB P6 PCB Header 6 pin 7A rated H3 H4 H8 HDR PLUG P6 Plug for HDR P6 N A HDR PCB P10 PCB Header 10 pin 7A rated H1 1 H12 2 1 2 2 HDR_PLUG_P10 Plug for HDR_P10 N A PIC16F887_uC PIC16F887 microcontroller U6 SCKT_PDIP40 40 PDI
88. nt flows through the C NO contact through the normally closed BPS reset button and then through the relay coil to ground This keeps the relay R1 latched in this position and so the main safety circuit path is OPEN However we now know we are using a NO BPS reset button A NPN transistor is connected between the H4 P6 and H4 P5 where the BPS Reset button would normally connect to The transistor is pulled high via R1 turning ON the transistor and imitating a normally closed BPS button Then terminal 1 of the BPS reset button is connected to the base via a resistor with terminal 2 connecting to ground Now when the BPS reset button is actuated it will pull the base to ground turning the transistor OFF Just as in Section 3 3 3 4 this cuts power to the relay R1 resetting it to its normal contact position of C NC The circuit still operates as described in Section 3 3 3 4 just the implementation of the actual BPS button has some added complexity as just described Figure 5 2 Revised schematic for BPS issue A transistor is now in series with the relay coil The topic of inductor spikes covered in Section 3 1 4 now applies to this modified circuit Freewheeling diodes as utilised in the BMS and IMD interlocks in a similar scenario must be used to ensure the voltage spikes do not damage the transistor However it 94 must added to the physical board ideal way to do this was so
89. nverter is OFF The only solution to this problem is to utilize an auxiliary battery which will initially power the LV Electric System and hence contactors Once the contactors close the DC DC converter will now have input power and hence can sustain the power requirements of the LV Electric System A significant flaw in the Getz auxiliary battery implementation is that there was no mechanism to keep it in a charged state This was rectified in the Formula SAE design where in Figure 2 5 we can see that once the DC DC converter is ON it will charge the auxiliary battery in parallel 9 states that for a sealed lead acid battery the ideal float charge is 2 25V 2 27V per cell A float charge is defined as a charger voltage continuously applied to the terminals of the battery in order to keep it in an optimally charged state when ready to use 10 Since a 12V lead acid battery is made of 6 cells and using the 2 27V value the ideal float voltage for the lead acid auxiliary battery is 6 x 2 27 13 62V From Section 2 3 3 the DC DC converter outputs 13 8V which aligns closely with the correct float charge voltage for our lead acid battery It is conceivable the 15 auxiliary battery may be floated for long periods of time however there is chance of overcharging as the very definition of floating as given by 11 is the voltage that it is safe to apply indefinitely If for whatever reason the auxiliary goes flat it will be a simple matter
90. ocontroller had to be placed back into the main circuit At first this may not seem like a heavy burden but the nature of programming alluded to above would quickly convince anyone that in circuit programmin g is the way to go There are pins on the microcontroller related to the ICSP protocol but they multiplexed with other functionality Microchip 21 recommends the clock PGC and data PGD pins should be kept free from other passive circuits to ensure trouble free programming highlighting the vulnerability of these critical signals to capacitive loading 22 Since the chosen microcontroller the PIC16F887 is a 40 device with plenty of IO to meet the requirements of the Safety Circuit PCB there is no need to utilise the programming pins for anything other but that purpose This will ensure another variable that could cause a problem is eliminated resulting in one less issue to think about during troubleshooting 3 3 3 13 Spare Headers As part of a robust design provisions for possible future additions or expansions should be provided This practise is demonstrated in the Safety Circuit The microcontroller is large and not fully utilised number of 59 spare pins have been broken out to header pins summary is listed Table 3 10 Header amp Microcontroller Pin Functions Suggested Use Pin Pin H6 P1 20 RD1 Digital IO H6 P2 19 RDO Digital IO H
91. on and watch the corresponding LEDs light up All LV Electric System components will work because power is available By removing the key we are preventing the key switch from being placed in the drive position which would send a HIGH signal to EVMS_KEY_IN and turn ON the contactors Contactors OFF implies tractive system OPEN and hence no way for current from accumulators to reach the motors or motor controllers 21 3 Safety Circuit 3 1 Circuit Theory Here we will cover some circuit theory and design concepts that have been utilized in the Safety Circuit PCB design covered in Section 3 3 3 1 1 Transistors Transistors are a ubiquitous semiconductor device number of transistors are used in the safety circuit where they provide a convenient interface to control a relay as in the case of the BMS and IMD interlocks They are a necessity to enable the limited current drive capacity of a microcontroller to drive the LEDs and buzzer 3 1 11 Transistor as a switch Transistor can be used as an amplifier device or an ON OFF switch Since its function as a switch is relevant to the safety circuit we will cover it now The key to using a transistor as a switch is to operate it in the SATURATION region This ensures the transistor is in the fully ON state where the voltage across the transistor is almost zero On the other hand to turn the transistor fully OFF it is put in the CUTOFF region The power developed across a s
92. on in this circuit design is that the brake panic switch will be of normally closed type To comply with fail safe practise due to R1 configured as NC R2 had to be added to ensure the BPS interlock was open when not powered see Figure 3 9 R2 R1 2 i OUT B 4 6 H4 P5 H4 P4 IN A Figure 3 9 BPS Interlock circuit schematic As soon as the two previous BMS and IMD interlocks turn ON it is clear current can flow through the coil of R2 and the BPS interlock becomes ON As soon as the BPS is actuated its corresponding pins of H4 P4 and H4 P5 are closed allowing current to flow and power relay coil 1 However the BPS is only a momentary switch because we don t want to have to manually reset the BPS every time it is actuated In addition the BPS will be in a difficult to access 46 position near the pedal box This means the relay would normally de energize once the BPS was released However what actually happens is that once the BPS is pressed contacts NO and C of R1 become closed allowing current to bypass the BPS flow through the normally closed BPS reset button via H4 P5 and H4 6 and then through the relay coil and ground When the BPS 15 released R1 is now powered via an alternate path via BPS reset button and remains ON meaning the BPS interlock stays OFF To reset the BPS interlock back to the ON state is simply a matter of pressing the BPS reset button This wil
93. onnections events occurring it would be unwise to be distracted by low level details of the Safety Circuit PCB at this stage 2 2 Electric Vehicle Management System The electric vehicle management system EVMS device manufactured by Zero Emission Vehicles Australia ZEVA 4 was purchased for the Formula SAE car The primary requirement of this device was that of a Battery Management System BMS however it also featured staged pre charging contactor control and a status lieht 2 2 1 Functionality The EVMS forms an important component of the Safety Control System There is a heavy cause and effect relationships between the various terminals of this device Therefore the operation of the EVMS is best described by examining various scenarios as a whole instead of individual terminals Nevertheless a brief overview of the EVMS terminals is given in Table 2 1 and shown in Figure 2 3 EVMS Terminal Terminal Name Terminal Description 12VDC EVMS_ 12VDC Positive power supply Ground EVMS_GROUND Negative power supply EVMS Signal to drive car HIGH to drive LOW otherwise BMSIn EVMS_BMS_IN cae System Status R EVMS STATUS Red EVMS status signal Status G EVMS STATUS G Green EVMS status signal Status B EVMS STATUS B Blue EVMS status signal Buzzer EVMS BUZZER Audible EVMS status Ground EVMS GROUND STATUS Buzzer and status indicator
94. op Module 47 3 3 3 6 AUX amp Main Relay Module 48 3 3 3 7 Ready to Drive Interlock amp HV relay 49 3 3 3 8 Ready to Drive 51 C 53 3 3 3 10 LED Indicator Subsystem 54 353 1 SV Power Supplyuuusaqa ahua aii 58 3 3 3 12 Programming 58 292135 5 59 3 3 4 Board Layouts anna nf 61 3 3 4 1 Custom Component Footprints 64 Physical Design amp Implementation sse 66 3 4 1 Manufacture ooa ete e a e dra RA ERU ERE TAN RAS 66 3 4 2 Connections rere _ 66 3 4 2 1 Connector Specification 67 3 4 2 2 Connector Placement and Wiring 67 3 5 4 Dashboard 4 1 4 2 4 3 3 4 2 3 Connector Pin Allocation 71 3 4 3 Safety Circuit Box Final Product amp Mounting 71 3 4 4 External Safety Element Mounting sss 72 3 4 5 Cables and WIODE C 73 3 45 1 Complete Safety System Cabling 73 8 5 2s Ghheoryz a
95. orce was a top priority Preferably it would involve circuit design and embedded systems a deep interest of mine that would allow me to explore and gain proficiency in this field Most importantly I didn t want to stop at the design phase instead proceeding to physically implement and build my designs for a real practical purpose This would immensely drive my motivation and ensure I remained enthusiastic in important consideration for a project being performed over two semesters I would also relish the opportunity to work a team requiring collaboration with other student engineers to strengthen by communicational skills In other words I wanted a project that epitomised what engineering is all about These are the attributes that attracted me to the REV Project 1 3 Objectives The following sections outline conceptually the intended purpose and functionality of a safety system and fault indicator system 1 3 1 Safety System The Formula SAE car will operate entirely on electrical power Electricity can be a lethal force if not respected In fact under law you must hold an electricians license in order to work on voltages greater than 50VDC Due to the fact that mainly students will actually work to build and maintain the car it is clear that for the sake of cost and convenience the tractive voltage of the SAE Formula car will need to be a maximum of 50V The rules of the Formula SAE A competition stipulate the requirement of a com
96. out Other than achieving a nice clean and consistent route layout the other consideration is the width of the actual traces The safety circuit deals with current in the range of 6 8 which flows in the main safety circuit path but also with small currents of lt 100mA Clearly a trace suitable for the latter will not be so for the former 23 presents formulas to calculate PCB trace widths based on the IPC 2221 24 current vs conductor cross section graph The formulas have been generated by curve fitting to empirical data The main safety circuit path must be rated for at least 8 A Using 23 to calculate the required trace width for 8 gives 5 29mm This is quite a large trace for a PCB making it very difficult to route and may require a larger PCB I decided to make a compromise by choosing a trace width of 2mm The additional required cross section for the trace is obtained by applying a liberal amount of solder along the main safety circuit path as shown in Figure 3 19 62 11995 II Figure 3 19 Safety Circuit Main safety circuit path trace thickening Mcgyvr 9 exclaims that adding solder to a trace is not a good reliable way to increase the current carrying ability Further they suggest to use a wider trace or use thicker copper cladding I agree this is the professional method to use for a commercial mass produced product They then go onto say Having said t
97. prehensive safety shutdown system A major proportion of this thesis will explore in detail the development of a 3 comprehensive safety system This includes the custom circuit design through to physical construction and implementation all while overviewing the theory calculations and good practise taken into consideration along the way Ultimately the safety shutdown system must both ensure the safety of the driver and compliance with all the required rules 1 3 2 LED Fault Indicator System There will be an assortment of safety elements that can trigger the Safety System It would be of great convenience to have a fault indicator system that pinpoints the location of the problem It will comprise not only of circuitry to sense the faults and drive the LED indicators but also the important physical dashboard The dashboard is an integral component of any car with instrumentation and forms the visual interface with the driver It will be my responsibility to design and physically construct the dashboard that adheres to the rules and provides the necessary functionality to start or charge the car indicate safety shutdown circuit faults and shutdown the car in an emergency I will also liaise with the relevant team members to determine if additional instrumentation such as an energy meter and speedometer can be mounted 1 3 3 Summary The preceding text presents ambitious objectives that will underpin the rest of this thesi
98. r Operation Online Available http electronicsways blogspot com au 2011 10 basic inductor 19 20 21 22 23 24 25 26 27 operation html Accessed 15 Oct 2012 University of Nevada Las Vegas Relay Basics Online Available http www physics unlv edu bill PHYS483 relay pdf Accessed 15 Oct 2012 Microchip PIC16F887 Data Sheet Online Available http ww1 microchip com downloads en DeviceDoc 41291D pdf Accessed 15 Oct 2012 Microchip PICkit 3 Microcontroller Programmer User s Guide Online Available http wwl1 microchip com downloads en DeviceDoc PICkit 3 User Guide 51795A pdf Accessed 15 Oct 2012 microEngineering Labs In Circuit Serial Programming ICSP Online Available http melabs com support icsp htm Accessed 15 Oct 2012 The Circuit Calculator Blog PCB Trace Width Calculator Online Available http www circuitcalculator com wordpress 2006 01 31 pcb trace width calculator Accessed 15 Oct 2012 IPC 2221 Generic Standard on Print Board Design February 1998 Online Available http www the bao de divers ipc2221 pdf Accessed 15 Oct 2012 CadSoft EAGLE PCB Design Software Online Available http www cadsoftusa com eagle pcb design software language en Accessed 15 Oct 2012 RepRap MakePCBlInstructions Online Available http www ece gatech edu rese
99. rake Over Travel Switch terminal 2 Inertia switch N A terminal C Inertia switch N A terminal NC Supply power to N A JMP HP11 P8 LED module LED_12V Jumper Wire S6 P1 LED Anodes Row 1 LED_ROW1 S6 P2 LED Anodes Row 2 LED_ROW2 S6 P3 LED Anodes Row 3 LED_ROW3 S6 P4 LED Cathodes Col 1 LED_COL1 S6 P5 LED Cathodes Col 2 LED_COL2 S6 P6 LED Cathodes Col 3 ED COLS 6 7 LED Cathodes 4 LED_COL4 Powers relay when key is in S6 P11 Key switch P4 charge position which pulls EVMS_CHRG_SE NSE LOW Supplies power to Ready to N A EVMS_KEY_IN Drive Interlock RTD_12V that has passed entire Safety H22 P1 H22 P2 H22 P3 Circuit amp Key 22 4 22 5 22 6 22 7 22 8 H3 P1 H3 P2 H3 P3 HP22 P1 S4 P8 HP22 P2 S5 P6 HP22 P3 S5 P7 HP22 P4 S4 P9 HP22 P5 S5 P3 HP22 P6 N A HP22 P7 S5 P12 HP22 P8 S5 P13 HP3 P1 S5 P11 HP3 P2 S4 P10 53 5 HP3 P3 S3 P6 C4 P8 6 8 5 6 5 7 4 9 6 9 C5 P3 N A N A N A Ready to Drive C5 P12 Buzzer N A terminal ve Ready to Drive C5 P13 Buzzer terminal ve C5 P11 N A C4 P10 C6 P10 C3 P5 N A C3 P6 N A switch Ready to Drive ners button terminal 1 EVMS_DE N A
100. red however since the faults only required small design adjustments they could be easily integrated on the current board This solution 15 not ideal but is a trade of I am willing to make for the sake of time and budget constraints In this chapter I will outline the small design faults discovered during the test procedure Full solutions to these faults will be thoroughly detailed including modified designs and the approach to physically implement the alterations 5 1 Circuit Faults 5 1 1 No resistor on transistor base A number of transistors on the PCB did not have current limiting transistors on the base pin These include the transistors T2 and T3 from the BMS and IMD interlock circuits respectively While reviewing the circuit initially the resistors on the transistor bases were mistaken for current limiting resistors when they 91 were really pull up and pull down resistors During circuit testing this was discovered after the transistors successively emitted a noise and burnt out internally removed damaged transistor is seen Figure 5 1 where a crack can be seen Figure 5 1 Transistor damaged by excessive base current Now a solution is required The transistors are easily de soldered removed and replaced However base resistors need to be added The base of each transistor is routed to a PCB pin header The easiest and cleanest way to add a resistor is to simply insert it inline from the wire going from the PCB h
101. s The overarching aim of this thesis is to thoroughly document all my work such that is serves as a useful and precise reference in the future As we will see there is a lot of detail in the designs implementation and cabling that makes up the entire Safety System By including a comprehensive examination of not only the theory but all implementation details if any faults should occur in any part of my project in the future successive REV teams will not be left in the dark In particular any changes modifications or errors and corrections made while physically implementing this project in the car will be documented to ensure no inconsistencies exist between the designs and physical implementation This process is in line with my principles of delivering a complete engineering package functioning robust product precise consistent and detailed documentation 2 Low Voltage Electric System 2 1 Overview The overall electrical system of the Formula SAE car can be separated into two distinctly separate areas of operation the High Voltage HV Tractive System and Low Voltage Electric LV System The HV Tractive System includes the accumulators motor controllers electric motors and all high current wiring Broadly speaking the LV Electric System includes all components running on the 12V power supply The Safety Control System is contained within the LV Electric System and is currently the most substantial system contained within
102. stion remains how can one determine the cross sectional area of cable required for a given amperage that ensures safe and reliable operation Of course you would not want to use a cable with a rated ampacity equal or close to the actual current it will conduct Instead a generous margin or tolerance must be incorporated in the calculation Essentially we want a conversion value of conductor cross sectional area per amp which can be easily used to determine 73 the cross sectional area of the cable for required amperage Such value must be experimentally derived conservatively rated and remain valid for all but extreme cases of the 5 points listed above 28 states a conversion value of 300cmil amp as a safe robust and reliable value to use in practice Of course in extreme conditions where for example cabling is laid in extremely hot factory conditions or where numerous cables are bundled and run in enclosed conduit the heat dissipation may be drastically reduced and additional tolerances required of the conversion value i e value increased Another consideration is the voltage drop over the cable run While the ampacity requirements may be met the voltage drops incurred especially in long cable runs or high current applications may become significant This may lead to the potential problem of insufficient voltage to run circuitry devices at the other end of the cable run In addition this voltage drop across the cable run manifests
103. tatus of the car 4 1 Initial Concept Design The first step was to draw up a preliminary concept design including all intended components Figure 4 1 The concept was based on dimensions obtained from the car chassis CAD drawings Unfortunately the CAD drawings did not reflect the reality of the physical chassis accurately enough Then the problem was exacerbated by the addition of two steel members near the dashboard This relinquished the prospect of having a speedometer and TBS energy meter on the dashboard 79 Shutdown Button 1 Shutdown Brake Panic Button 1 Inertia BMS IMD Switch amp LED LED LED Car Start Button amp LED Brake Panic Inertia BMS IMD SD Left BOT SD Right TSMS CSMS Motors BMS Status Left Shutdown Button LED Right Shutdown Button LED Travel TSMS CSMS LED LED Charge Drive Cockpit Internal Cross Section Template l I I I I I I I I I 1 Initial Dashboard concept design Figure 4 4 2 Final Design The final design that was manufactured is displayed in two patts in Figure 4 2 80 and Figure 4 3 Brake Panic Rear bracket Switch amp mount hole LED adios LED Mount hole N 145 00 7 Motors Mount hole 42 00 status Ready to Drive Button amp LED Steering Wheel 1 Motors LED
104. tdown Circuit is opened by the AMS or the IMD the tractive system must remain disabled until being manually reset by a person directly at the car which is not the driver 3 Note The AMS Accumulator Management System is what we refer to as the BMS Battery Management System 42 The current enters the BMS interlock Figure 3 7 at IN A and exits at OUT B provided the circuit is in a safe state IN A and OUT B are normally open complying with the fail safe principle OUT B EE IN A Figure 3 7 BMS Interlock circuit schematic So the question is how does the circuit close IN A and OUT B Let s look at the PNP transistor T2 The base is driven by BMS_SIG from H5 P1 Remember that BMS SIG is the signal from the BMS modules in the battery boxes that is LOW when there is no fault and floating otherwise A floating signal is problematic and so a pull up resistor R3 is used to keep the base of T2 HIGH and hence OFF Now a BMS Reset Button is connected to H5 P2 and H5 P3 GND Providing BMS_SIG is LOW and hence T2 is ON if the button is pressed the circuit will be complete from BMS_12V to GND and hence current will flow through the relay coil turning it ON When the button is released the circuit is opened i e there is no longer a ground point However the circuit is configured in such a way that the relay actually latches in the ON position So how does the relay remain
105. the back of the car Safety Circuit PCB to the front of the car dashboard via conduit under the seat Pins 1 11 and 13 wired directly between C4 and C6 Pins 12 and 14 18 do not terminate at the other connector instead terminating directly to other devices as illustrated in Figure 4 4 C4 plugs into S4 on the SCB and C6 plugs into 56 on the dash The physical wiring internal to the dashboard is summarised in Table 4 2 S6 Connection Internal SCB Connection amp Connector Dashboard Wiring Description Pin Detail 1 Anodes of LEDs S6 P1 H2 P2 TSMS gt TSMS anode SD Left gt SD Left anode SD Right gt SD Right anode SD Dash gt SD Dash anode 2 LED Row 2 Anodes S6 P2 H2 P3 gt BMS anode 87 gt Inertia anode gt BOT anode gt IMD anode 3 LED Row 3 Anodes S6 P3 H2 P4 gt Motors anode gt BPS anode gt RtD anode 4 LED Column 1 56 4 2 5 Cathodes gt TSMS cathode gt BMS cathode gt Motors cathode 5 LED Column 2 S6 P5 H2 P6 Cathodes gt SD Left cathode gt Inertia cathode gt BPS cathode 6 LED Column 3 S6 P6 H2 P7 Cathodes gt SD Right cathode gt BOT cathode gt RtD cathode 7 LED Column 4 S6 P7 H2 P8 Cathodes gt SD Dash cathode gt IMD cathode 8 Ready to Drive S6 P8 H2 P11 Button terminall gt RtD Button terminal 1 9 Ready to Drive S6 P9 H2 P14 Button terminal 2 gt RtD Button T2 10 Key switch pin 1 amp 3 S6 P10 H3 P2 K
106. to directly power the LEDs Also it simplifies the logic of turning on a LED Without transistors the anode pins would act as a voltage source but the cathode pins would be a ground Hence to turn ON an LED the corresponding anode pin would be pulled HIGH but the cathode pin pulled LOW to ground So we can see that 7 pins are required to drive 12 LEDs which would have otherwise required 12 pins This is a 4276 saving The pin savings become more 27 pronounced larger numbers of LEDs used For example if 100 LEDs required 20 uC pins are needed This is a whopping 80 saving There is one potential downside to LED multiplexing Although it does not apply to the Safety Circuit PCB it should be mentioned for completeness LED multiplexing is often used in displays to light multiple LEDs that form numbers or figures etc 14 When multiplexing it is evident that only individual or square groups of LED can be lit at once For example referring to Figure 3 3 lighting LED 1 is a matter of driving inputs 1 and 4 HIGH Now what if we want to light both LED 1 and 6 the type of activity we would want to perform to display a character consisting of multiple LEDs Naively on first glance one may say send inputs 1 2 4 and 5 HIGH However this would actually light not only LEDs 1 and 4 but also LEDs 2 and 5 which is undesirable This problem can be gotten around by quickly alternating between output HIGH on pins 1 amp 4 to light LED
107. troller and wiring this interface to the new logging system all sorts of information relating to the state of the Safety Control System could be determined For example the time when the car was driven charged or when a particular safety element was triggered could be logged Further a mechanism to upload the data to a PC would be necessary be it RS232 USB or even wirelessly for a nice challenge This data could be analysed by custom written software an example of which could be to average the speed of trips plotting values such as instantaneous speed or plotting the GPS locations on a map to show the travelled route The complexity of this analysis is limited only by your imagination 98 Appendix Figures A 1 Dashboard Figure A 1 1 99 Figure 1 2 Ts H 2 MD pus SD Left IC SD Rioht Remo e DI Figure A 1 3 100 Manufacturing 2 Figure 2 2 Figure 2 1 Figure A 2 4 Figure A 2 3 101 Figure 2 5 102 A 3 Safety Circuit Schematic 103 Appendix Program code listing File main c
108. urrent our calculations to get the highest current value This value is the minimum base current required to saturate the transistor 23 10 suggests the minimum base current is increased 30 to guarantee the transistor is in SATURATION This is incorporated in the follow equation _ Supply Voltage 3 3 B 1 3 Let s now follow these steps to find the appropriate base resistor for the transistors in the Safety Circuit PCB LEDS 0 02 Isa s 0 0003333 3 4 B min HFE min 60 Supply Voltage 5 11 5460 3 5 X13 0 0003333 x 13 Based on this 10 resistors have been used to drive LED transistors For BMS IMD relay The maximum current through the relay coil has been experimentally derived to be at most 100mA Ic max 0 1 Ip min 0 001666A 3 6 _ Supply Voltage _ 5 mam P X13 0 001666 x 1 3 3 7 Based this 1kQ resistors have been used to drive BMS IMD and buzzer transistors It is important to note that these calculations provide a good guideline to base resistor selection They do not need to be adhered to exactly and it is safe to choose a lower value as this just further saturates the transistor 24 3 12 Current limiting resistor for LED Light Emitting diodes LED behave in accordance with the characteristics exhibited by a diode The I V curve an example of w
109. witching transistor V is 13 P I lt x Ver 3 1 22 When CUTOFF fully OFF is 0 so the power is 0 When SATURATED fully ON Veces 0 so the power is very small When the transistor is in SATURATION the voltage drop across it Vee is as close to zero as it can get This ensures the power consumed by the transistor is very small ensuring heat generation is kept to a minimum Selecting the correct base resistor will ensure the transistor remains in SATURATION when ON If however the resistance is too large limiting the required base current the transistor will operate in the LINEAR region where the voltage drop across the transistor VC is no longer zero The full voltage is now split between the load and the transistor The voltage drop across the transistor leads to increased generated heat and the load itself may no longer have the required voltage to work properly Both of these are highly undesirable for switching transistor operation 3 1 1 2 Selecting a base resistor By selecting an appropriate base resistor we can ensure the transistor remains in SATURATION The important ratings of a switching transistor used in the calculation of a base resistor are 13 HFE min Minimum current Ip min Minimum base current Ic max maximum collector current To find the appropriate base resistor we first calculate Ic max Ip min 3 2 We use the minimum maximum collector c

Formula SAE Safety Interlock Circuit & Indicator

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Formula SAE Safety Interlock Circuit &amp; Indicator

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Formula SAE Safety Interlock Circuit & Indicator