International Conference On Information Communication And Embedded System (ICICES2016)
Implementation of Real Time Low Voltage System for Enhancement of Safety in an Electric vehicle Manoj Rohit Vemparala, Santosh Ram Somasundaram, Saket M Anandkrishnan, Ashok. B*, Kavitha. C VIT University, Vellore, India. *Email:
[email protected] Abstract - Energy conservation, environmental conservation and sustainable development are the need of the hour and in this regard the development of an electric vehicle has become topmost priority. There is a growing concern about the safety of EVs, which have transformed the future of the automobile industry. With this rapid growth there is a increasing requirement for monitoring and development of safety circuits for battery pack, drivetrain and during emergencies. In this paper we have focused on developing a safety system for a formula electric car keeping design simplicity in mind. All the systems and circuits proposed in this paper are in accordance with the FSAE rules. As part of this project an analysis methodology for analysis of safety systems has been developed and the potential impacts of these circuits have been studied. Keywords— EV (electric vehicle), battery pack, FSAE, safety systems, monitoring
I. INTRODUCTION International Automobile and Motorsport arena are gradually taking a paradigm shift towards EVs. With the increased awareness and attention towards pollution backed up by gasoline price hike due to worldwide growing scarcity of fossil fuels, there is a huge emphasis on alternate vehicle development. The more diminishing the world fossil fuel leads to an idea to make a national electric car, and nowadays it has become material for automotive discussion. Due to this an alternate source of vehicle propelling system is needed for the futuristic purpose. One among that is the propulsion of automobile by an electric source[1-4]. The electric voltage system is an integration of vehicle body, electric propulsion, energy storage battery, and energy management [5-8]. The design approach will be more accepting when referencing to the people's perception and expectation. Developing an Electric car has its own challenges associated with it, perhaps what convolutes the case is the proper safety measures that needs to be in place to ensure the well-being of the driver [9].The objective is to increase the aspects of usability and liability of the product manufactured. So, it can meet the user's tangible- and intangible quality expected to be better. Every car, no matter what class it comes under, has safety as its top priority [10-12]. This Paper emphasizes more about the Low Voltage System of Electric Car. To achieve a safe car, firstly failure mode effect analysis (FMEA) was performed that jotted down the possible failure modes of the car. Based on the analysis, several circuits were designed that was ultimately integrated into different printed circuit boards. To complement the board, the ECU of the car was developed to work alongside. All the Low Voltage Systems are powered up by the DC/DC converter which converts High Voltage into 12V DC.
II. SYSTEM OVERVIEW The role of the entire low voltage system in the electric car is to monitor, manage the functioning of the High Voltage components and also improve driver ergonomics. Consider a system consisting of two motors in series electrically, coupled through a mechanical shaft. The intermediate shaft (via chain drive) connects the motors that are connected to the differential by a chain drive. The temperature and voltage of the battery pack are monitored by the Battery Management System. The accumulator pack altogether consists of a well-constructed battery with Li-Po cells, two Accumulator Isolation Relays (AIRS), DC-DC converter, Battery Management System (BMS), High Voltage Disconnect (HVD). In addition to accumulator pack, motor is controlled by the motor controller, which in turn protected by a Pre-Charge circuit. The overall system’s insulation is measured by an Insulation Monitoring Device. If any kind of fault is observed by the above systems then the AIRs will open thus disabling the tractive system shown in Fig. 1.There are also different sensors around the car which determine the extent to which the high voltage systems could be used. Sensors like Potentiometers (Used as Torque encoders), Current Sensors, Voltage Sensor and Proximity Sensor (Speed Determination). There are two ways of disabling the tractive systems. 1) Opening the AIRs-: This is achieved by disabling the current to the relay coils. 2) Disabling the power to the motor-: This is achieved by cutting off the connection between the torque encoders and the motor controller. The shutdown circuit disables the tractive system by opening the Accumulator Isolation Relay. In case of emergency three shutdown buttons are provided, one in the cockpit, two on both the sides that completely de-energize the tractive system. The function of Electronic Control Unit (ECU) is to monitor the critical sensors in the vehicle and control the dashboard. It monitors torque encoders, brake encoder, voltage sensor, AIRs, wheel speed sensor and after ensuring the proper functioning of them, it generates Ready to Drive Sound (RTDS) which indicates that the vehicle is ready to drive. In case of occurrence of an implausibility, it generates a fault signal which disconnects the torque encoders with the motor controller without completely de-energizing the AIRs thus, assisting the shutdown system in ensuring the high electrical safety in the vehicle. Apart from these critical applications, it displays the speed and error signals on the dashboard.
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International Conference On Information Communication And Embedded System (ICICES2016)
Figure 1. Tractive system
III. SHUTDOWN CIRCUIT AND ECU A. Shutdown Circuit
The BMS Used for our setup is Elithion lithuminate Pro. It has 3 main fault lines (LLIM for reports of voltage less than 2.9V, HLIM for over Voltage faults and FAULT for Generic Faults such as communication errors or abnormal temperatures.) In the case of the BMS as well, the signals are sent to ECU for further processing. When the signal received is confirmed is a fault, the signal to activate the interlocks is immediately sent. All the latches are reset in the same fashion, by means of a reset button for each fault. The thyristor are reset by momentarily applying a ground potential to the SCR. This restores the latch to its initial (inactive) state. Shutdown buttons and IMD/BMS faults operate simultaneously to open AIRs. If BMS/IMD fault is triggered then AIR's will open and it can only be reset by the provided reset button for IMD/BMS latch, or by power cycling GLVMS. It is not possible to reset IMD/BMS fault by shutdown buttons. Latching circuit is powered up via DC-DC converter which is connected across the battery. The shutdown circuit is made up of a series connection of relays. And the latches are used to control relays. These relays are configured to be normally open. So in the de-energized state shutdown circuit is opened i.e, each circuit will remove the current controlling the AIRs
B. Electronic Control Unit (ECU)
The shutdown circuit is installed to ensure the safety of the vehicle as well as the driver. It consists of various monitoring devices, systems and switches, which disconnect the batteries and motor controller in case of any fault thus, completely shutting off the vehicle. The circuit has been strategically designed keeping in mind all possible conditions which may occur. The LV system is powered by the DC/DC converter which takes supply from the HV batteries. The Grounded Low Voltage Master Switch (GLVMS) controls the LV power supply. The tractive system cannot be energized until all the faults checked by the IMD, BMS and BSPD are cleared; shutdown buttons which are mounted on the sides and cockpit, and Tractive System Master Switch (TSMS) are not closed .Fig. 2 shows the three main subsystems used in the shutdown circuit. Faults are stored even when GLVMS is powered off; thereby tractive system is de-energized when GLVMS is switched back on.
Figure 3. Block diagram of electronic control unit (ECU)
Only one ECU is used in the vehicle. PSoC 5LP is used as ECU for the car. As shown in Fig. 3 signals from Pedal Assembly, Proximity Sensor are the main input signals for the ECU. ECU is responsible for Enabling/Disabling the Motor Controller. Updating the Dash Board, Controlling the Brake Light and Data Logging important signals used in case of maintenance.
Figure 2. Block diagram of shutdown subsytems
A Bender A-ISOMETER® ISO-F1 IR155-3203/-3204 approved and certified IMD for automotive use is used in the vehicle. It monitors the insulation resistance between the high voltage tractive system and the low voltage system. The faults that may occur in the vehicle may be momentary and hence must be latched. This Signal is then sent to ECU for further processing. When the signal received is confirmed is a fault, the signal to activate the interlocks is immediately sent. For this purpose, an efficient latching system using Thyristors (SCR’s-2P4M) has been designed.
Figure 4. Pedal Assembly Algorithm
978-1-5090-2552-7/16/$31.00 ©2016 IEEE
International Conference On Information Communication And Embedded System (ICICES2016) There are three Potentiometers in Pedal Assembly. Curtis PB-8 Rotary type POTs is used in the vehicle. Two of the sensors are connected to the Throttle Pedal and one Sensor is connected to the brake. If the difference between the two sensors is more than 10% or TE1 is more than 50% and brake is pressed or any of the sensors gives Low or High Signal (0V or 5V) then the Motor Controller is disabled. If there is plausibility, then accordingly the LCD on the Dash Board gets updated. This process is shown in Fig. 4. The Speed of the Vehicle is calculated by two proximity sensors located near the Hubs of the front wheels. 10 Holes are drilled on the front wheel Hub. If the Proximity Sensor detects 10 Holes then ECU reads it as 1 Rotation. Average number of Rotations from two proximity sensors is taken and accordingly updated in the Dashboard’s LCD. If there is no change in Holes for a long time, then the Speed is updated to 0 RPM. This case might occur when the vehicle is just stopped. 10 Holes = 1 Rotation. X holes are detected in 500msec. Number of Holes in 1sec= 2*X. Number of Rotations= 2*X/10= X/5 RPS= 12*X RPM.
IV. PRECHARGE AND DISCHARGE
Figure 6. Precharge circuit graph
Figure 6 indicates the graph plot between voltage across Motor Controller and time. Discharge time is observed to be 7.50sec (90% of the Voltage).
B. Discharge Circuit
A. Precharge Circuit Pre-Charge circuit limits the high inrush current when motor controller is connected to the batteries. If it is not provided, then the high inrush current can severely damage the capacitors in the motor controller. The pre-charge circuit consists of a resistor, a relay and a delay circuit which drives the main contactor of the battery pack and the pre-charge relay indicated in the schematic. Whenever the Tractive System Master Switch (TSMS) is switched ON, the value of the value is compared with a reference voltage through a comparator which triggers the main contactor after a specific delay during which the motor controller is charged up to 90%. A similar procedure is used to trigger the SPST pre-charge. As soon as the car is switched ON, the pre-charge relay closes and creates a path for charging the motor controller through a pre-charge resistor. After the delay is over, the pre-charge relay opens and the main contactor closes completing the HV system. Figure 5 indicates R1 as Discharge Resistance which is 10KΩ which has a continuous Power Rating of 50W and overload Power Rating for 1sec of 500W.
Figure 5. Precharge circuit schematic
Even if the AIRs are disconnected it is not necessary that the tractive system voltage is reduced because the motor controller is still charged. Also for pre-charge circuitry to work the next time car is switched ON, we need to ensure that the motor controller is discharged. To overcome these problems, the discharge circuit is designed. It will simply, discharge the motor controller with a suitable resistor. A discharge circuit consists of a discharge resistor and SPST-NC (Normally Close) relay. It is electrically connected between motor controller terminals. When the car is switched ON, the Tractive system Master switch gives a High Signal. This powers up the coil of NC relay. It is pulled open and discharge path is disconnected and the pre-charge path is activated. Figure 7 indicates R1 as Discharge Resistance which is 10KΩ which has a continuous Power Rating of 50W and overload Power Rating for 1sec of 500W.
Figure 7. Discharge circuit schematic
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International Conference On Information Communication And Embedded System (ICICES2016)
Figure 8 . Discharge rate of motor controller
Figure 8 indicates the graph plot between voltage across Motor Controller and time. Discharge time is observed to be 7.50sec (90% of the Voltage).
Figure 9. Latching subsystems for BMS, IMD, BSPD
A. Latch circuit
V. LATCH CHARACTERISTICS AND PERFORMANCE As mentioned there are three major systems, BMS, IMD and BSPD present, which have latched subsystems present in the shutdown board. The main purpose of latching is to keep the tractive system disconnected until the fault is cleared and manually reset. The signals for the three systems vary slightly, but all of them have in common a main latch circuit which is discussed in this section. The outputs of all the latch circuits are fed to relay the board, which control the operation of the AIR’s. Since the BMS has 3 fault signals 3 latching circuits are used. The block diagram of the three latching systems is shown in Fig.9. The BMS and IMD fault lines coming from the BMS and IMD respectively are given as inputs to the latch control and monitor unit (LCU). The BMS signals are active HIGH while the IMD signal is active LOW. The signals coming from the Latch control and monitor unit are given to the main latch circuit and the signals are active HIGH. It is seen that the IMD takes settling time 2 seconds as a result of which, if the LCU is not used, the IMD latch gets triggered on starting the vehicle. The LCU prevents this from happening by maintaining no fault state for the IMD till the settling time is completed. In order to reset the vehicle easily after fault is corrected it is recommended to keep the reset buttons in an easily accessible location (out of drivers reach) such as on the back panel. The BSPD latching system takes two inputs, one of the brake and one of the current sensor. The LCU monitors both inputs of the BSPD. When both hard braking and more than 5KW of power is passed to the motors for more than 0.5 seconds, the fault line of the latch circuit is logic HIGH. The LCU is present in order to reduce the effects of noise leading to impulses creeping in and triggering the latches when no fault is present. The LCU also keeps a log as to when and how many times each of the latches has been triggered and it also controls the indicators for the three systems which are present in the dashboard. Pull-up resistors are present, which trigger the latch circuit in case the LCU fails.
Figure 10. Latch Circuits
The latch circuit block from Fig 9. is shown in its expanded form is shown in Fig 10. The latch circuit uses a 2p4m SCR (U1) for the latch element. The SCR property is such that if a voltage is given at its gate then it conducts. It remains conductive even after the removal of the gate signal, provided the current flowing through it from anode to cathode exceeds the holding current. In order to bring it back to nonconductive state we drop the current flowing through it below holding current of the SCR (U1). This is done with the help of a push button which diverts current through it. In fault state when a gate input is HIGH, SCR (U1) will be in conductive state and will remain in a conductive state even when the fault is removed. In no fault state SCR (U1) will be non-conductive and it is done by pressing the first push button (provided the fault is not present). Thus the SCR (U1) behaves as a latching element. The SCR anode voltage is given as an input to the AND gate IC 4081 (U2: A).
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International Conference On Information Communication And Embedded System (ICICES2016) If only SCR is used to control the relay board and when main supply power is reset when the SCR (U1) is in a fault state (conductive) then the SCR (U1) loses its fault state and shows no fault state (becomes nonconductive). To avoid this we use a memory circuit along with the SCR (U1). The memory circuitry basically consists of a diode and a capacitor. The capacitor (C1) charges when there is a fault present and it stores the charge when the main power supply is off. This charge can be stored for a potentially long time, even though there may be a small leakage current. Using the formula, (VI-VF) / time = leakage current / capacitance Where Vi= initial voltage, Vf= final voltage. The capacitor can store the fault state efficiently for a minimum of 44.44 min considering, (VI-VF) = 8, capacitance= 1000uf and max leakage current 3uA. Practically, we have observed the charge to hold for a day. The diode (D1) ensures the capacitor doesn’t discharge when no fault is present. The MOSFET (Q1) controls the charging of the capacitor and charges it during a fault state. The MOSFET (Q2) serves as an inverter which inverts the logic of the capacitor. This inverted capacitor logic is given as a second input to the AND gate 4081 IC. This basically the AND gate 4081 IC uses Anode voltage of the SCR (U1) and inverted capacitor (C1) voltage as inputs. If any one of these inputs is low the output of AND gate will be low as a result of which AIR will be in an open state (Tractive system disconnected). Caution should be taken care when resetting. First SCR (U1) should be reset following which capacitor should be reset. If the capacitor push button is pressed first before the SCR reset switch the capacitor will charge until the SCR is reset which may cause improper reset leading to system continuing to be in a fault state. In proper vehicle running condition when no faults are present, Both the SCR anode is HIGH as well as capacitor charge is LOW, which after logic inversion is fed to the AND gate 4081 IC, resulting in HIGH output from the AND gate IC. The HIGH output is fed to the relay board which then allows current flow to the AIR’s. Table 1. Summarizes the conditions affecting the operation of the vehicle. The specifications of the latching circuit are further mentioned in Table. 2 in the following subsection.
Sr No.
Specification
Value
1
Supply Voltage
Min 9V Typical 12V Max 15V
2
Operation temperature
All components work efficiently between -55C to 105C
3
Current Drawn by Latch circuit block (Vcc=12V)
a. No Fault 12.22mA (typical) b. Fault (SCR nonconductive) 20.14 mA (typical) c. Fault (SCR conductive) 24.21 mA (typical)
4
Current Drawn by Relay Board block (Vcc=12V)
a. No Fault 139.46 mA(typical) b. Fault 12.002 mA(typical)
5
Fault time
1us (typical)
6
Resetting time (Vcc=12V)
12.5 ms (typical) 20 ms (Max)
7
Acceptable noise at input
0.2 V (Max)
activation
response
Table 1. Different cases of latch working
Sr No
SCR anode voltage logic
Capacitor voltage logic
AND gate output
Vehicle status
1
HIGH
LOW
HIGH
NO FAULT (RUNNING)
2
LOW
HIGH
LOW
FAULT
3
HIGH
HIGH
LOW
FAULT Fig 11. Fault getting activated
B. Latch characteristics Table 2. Main specifications relating to latch schematic
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International Conference On Information Communication And Embedded System (ICICES2016) VI. CONCLUSION
Fig 12. Latch system reset by pushing both reset buttons
The system consisting of various subsystems was designed and installed in an electric car made by Team Ojas for the formula season 2015. All subsystems were designed keeping in mind FSAE guidelines and the FSAE rulebook. The low voltage system forms an integral part of the vehicle and is essential for the safety of the driver and the environment as well as monitoring performance like speed, dashboard functions etc. Our system mainly focuses on driver safety and making sure the vehicle operates within optimum standards. Much future advancement is possible depending on the design parameters of the vehicle. Additional interlocks need to be added in case of electronic differential and gearbox systems. Data acquisition of all latch circuit status and sensor readings can be logged by the ECU for performance analysis and displayed in a GUI. Through this paper, we wish to establish a standard, robust, economical, low voltage system design and implementation that helps in further development.
ACKNOWLEDGMENT We wish to acknowledge Team Ojas, VIT University for providing us support and encouragement as well as the opportunity to implement and test the systems. REFERENCES
Fig13. Latch system reset after pushing capacitor reset button
C. Relay Board The relay board, we use contains the ULN2803 Darlington driver. Each channel of the chip contains a back-emf snubbing diode, so it is perfect for driving relays and motors. One of the coil terminals of the relay is connected always to positive 12 volts, DCDC positive. The output of the latch is given to the ULN2083 IC which inverts the input logic. The corresponding output from the ULN2083 IC is given in the other coil terminal. A 100 ohm resistor and LED is connected across the coil terminals to identify when the relay is switched from NC to NO. LED On indicates the switch is connected in NO condition. When there is no fault in latch, the output of the latch is logic HIGH. This is given to the ULN2083 IC which gives LOW at its output. As a result a 12 volt drop appears across the coil causing it to trigger. The voltage drop also causes LED to turn ON. The relay control circuit is shown in Fig 14.
[1] Shukla, A. K., A. S. Arico, and V. Antonucci. "An appraisal of electric automobile power sources." Renewable and Sustainable Energy Reviews (2001): pp.137-155. [2] Emadi, Ali, et al. "Topological overview of hybrid electric and fuel cell vehicular power system architectures and configurations." , IEEE Transactions on Vehicular Technology (2005): pp.763-770. [3] MacLean, Heather L., and Lester B. Lave. "Evaluating automobile fuel/propulsion system technologies." Progress in energy and combustion science (2003): pp.1-69. [4] Lave, Lester, et al. "Life-cycle analysis of alternative automobile fuel/propulsion technologies." Environmental Science & Technology (2000): pp.3598-3605. [5] Chan,C.C, “An Overview of Electric Vehicle Technology”; IEEE Vol.81:9, 1993. [6] Chan, C. C., and Y. S. Wong. "Electric vehicles charge forward." , IEEE Power and Energy Magazine (2004): pp.24-33. [7] Amjad, Shaik, S. Neelakrishnan, and R. Rudramoorthy. "Review of design considerations and technological challenges for successful development and deployment of plug-in hybrid electric vehicles." Renewable and Sustainable Energy Reviews (2010): pp.1104-1110. [8] Gao, David Wenzhong, Chris Mi, and Ali Emadi. "Modeling and simulation of electric and hybrid vehicles." Proceedings of the IEEE (2007): pp.729-745. [9] Michon, John A. "A critical view of driver behavior models: what do we know, what should we do?." Human behavior and traffic safety. Springer (1985). pp.485-524. [10] Sri Naga Sruthi, Rishikesh Vibhute,Udayan Karmarkar, Shray Chandra, Sudharshun Mukundan Iyengar, “Safety System of an Electric Vehicle for Formula Racing” 2014 IEEE International Conference on Vehicular Electronics and Safety (ICVES) December, (2014). [11] Galea, F, Msida, Malta, Casha, O. Grech, I, ” Control Unit for a Continuous Variable Transmission for use in an Electric Car,” 17th IEEE International Conference on Electronics, Circuits, and Systems (ICECS)”, (2010). [12] Seyed Mohammad Rezvanizaniani; Zongchang Liu , Yan Chen , Jay Lee, ”Review and recent advances in battery health monitoring and prognostics technologies for electric vehicle (EV) safety and mobility,” Journal of Power Sources (2014), pp.110-124.
Fig 14. Single relay circuit on relay board
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