Energy Management System For SAE-EV - University … Management System For SAE-EV !!! Midterm Design...

Energy Management System For SAE-EV

Midterm Design Report

Cabrera, Luis Perry, Scott

Shepard, Joshua

Design Team 06

Faculty Advisor: Dr. Tom Hartley Senior Design Coordinator: Gregory A. Lewis

Date Submitted: November 15, 2012

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Table of Contents List of Figures ................................................................................................................................ ii

List of Tables ................................................................................................................................ iii

1 Problem Statement ................................................................................................................ 2

1.1 Need ...................................................................................................................................... 2

1.2 Objective ............................................................................................................................... 2

1.3 Background ........................................................................................................................... 2

1.4 Marketing Requirements ....................................................................................................... 3

1.5 Objective Tree ....................................................................................................................... 4

2 Design Requirements Specifications .................................................................................... 6

3 Accepted Technical Design ................................................................................................... 7

3.1 Level 0 Block Diagrams: ....................................................................................................... 7

3.2 Level 1 Block Diagrams: ....................................................................................................... 9

3.3 Level 2 Block Diagrams: ..................................................................................................... 13

3.4 Design Calculations: ........................................................................................................... 17

3.4.1 Battery Management System Theory of Operation ................................................ 20

3.4.2 Battery Management System, Software Theory of Operation ................................ 21

3.5 Matlab Simulation ............................................................................................................... 22

5 Project Schedule .................................................................................................................. 27

6 Design Team Information ................................................................................................... 29

Luis Cabrera, Team Leader, Software Manager, CpE .............................................................. 29

Scott Perry, Hardware Manager, EE ......................................................................................... 29

Joshua Shepard, Archivist, EE .................................................................................................. 29

7 Conclusions and Recommendations .................................................................................. 29

8 References ............................................................................................................................. 29

9 Appendices ........................................................................................................................... 30

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List of Figures Figure 1: Objective Tree ..................................................................................................... 4 Figure 2: Engineering Requirements ................................................................................. 6 Figure 3: Accumulator Block Diagram .............................................................................. 7 Figure 4: Software Monitoring Block Diagram ................................................................. 8 Figure 5: Level 1 Software Block Diagram ..................................................................... 10 Figure 6: Level 2 EMS Block Diagram ........................................................................... 13 Figure 7: Main Software Flowchart ................................................................................. 15 Figure 8: Software Flowcharts ........................................................................................ 16 Figure 9: Analog Current Bypass Circuit ........................................................................ 21 Figure 10: Matlab Track Simulation Results ................................................................... 25

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List of Tables Table 1: Level 0 Accumulator Theory of Operation ......................................................... 7 Table 2: Software Monitoring Theory of Operation .......................................................... 8 Table 3: Level 1 Accumulator Theory of Operation ......................................................... 9 Table 4: Software Level 1 Theory of Operation .............................................................. 12 Table 5: EMS Theory of Operation ................................................................................. 14 Table 7 Gant Chart for EMS ............................................................................................. 27

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Abstract

Electric vehicles are leading the way in alternative methods to move away from the combustion engine. With growing battery technology, electric vehicles are becoming a financial risk worth taking on the large manufacturing scale. This has triggered a Formula SAE event specifically tailored for an Electric Vehicle event. Yet batteries are still not fail proof, and the correct set up is still needed to operate them at the optimum level without damage to the cells. So an entire Energy Management System (EMS) must be put in place to control the charging and discharging of the battery pack. This is just as important as selecting the appropriate batteries themselves. So as participants of the SAE competition a balancing circuit must be implemented to make the EMS as efficient and safe as possible

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1 Problem Statement

1.1 Need Formula SAE will host the first electric vehicle competition in 2013. Since the University of Akron has been participating in the original Formula SAE competitions, the team will need a new way to manage how the electric system will function. The Energy Management System (EMS) must be within the rules of the competition. The EMS will have to consider the charging of the batteries, and how energy will be supplied to all the components of the Formula SAE vehicle. During competition and testing, providing safety is the number one priority, along with designing the best energy management system possible to give the best chance to win the competition.

1.2 Objective To design a proper energy management system that will fulfill the power demand of the vehicle, this system must supply enough energy to get the electric vehicle through all road tests. There should also be a method of feedback on the storage system. These should gauge a rate of usage, remaining power, efficiency, and temperature of each cell, all as possible examples of feedback information that would be useful. The EMS will provide emergency shutdown switches that are easily accessible for the driver to use in case of an emergency. The vehicle will be inspected before the competition and must pass all inspections in order to be eligible. The system must keep cost and parts to a minimum because a cost report and bill of materials will be submitted. The high-voltage and low-voltage systems must be separate and follow all codes and regulations set by the competition. Figure 1, the objective tree, is a layout of how to build a correct energy management system that uses good system design and is easy to use. A Gantt chart has been created to help with the process of completing the design of our system, table 1 shows out design process in detail.

1.3 Background SAE is the society of automotive engineers, and they sponsor an automotive competition for formula racing among other competitions. This competition is available for university undergraduates and graduates pursuing a degree in engineering. Students are required to construct a working formula car assuming a manufacturing firm has hired them to do so. The vehicle will be graded on both static and dynamic events. The static events are design, cost analysis, and presentation. The dynamic events are acceleration, skid-pad, autocross, fuel economy, and endurance. The design is graded by the students explaining

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and defending their design to a panel of judges. The acceleration portion will judge the cars ability to accelerate from standing still to 75 meters. The skid-pad section will grade how well the car can take lateral forces. The car will continually move around two concentric circles in the shape of an “8”. Autocross will be measured by performing a run on a tight course showing maneuverability, handling, accelerating, braking, and cornering all into one event. Endurance is the test of a 1.1 mile course that will show acceleration speed, handling and reliability all in one. The endurance test will be a large part of what our design team has to accomplish with the energy storage and discharge system. The static events are worth 325 points, while the dynamic events are worth 675 points for a total of 1000 points. For the 2013 SAE Electric Car challenge, only first year cars are allowed to participate in the North American challenges. There are two competitions in North America, one in Michigan, and one in Nebraska. The competition in Michigan accepts 120 teams, while the competition in Nebraska accepts 80 teams. The 2013 the formula one vehicle will use only electric power to complete all of the goals. This puts a great need for high performance and high endurance electric system to control the vehicle. Our design team will be responsible for the charging, discharging, protection of the energy system, and control of the energy system. Figure 2, the charging unit, is responsible for all charging of the battery pack. The battery pack must be charged and discharged at the same rate, Figure 3; the control unit takes care of this. Feedback is provided from the monitoring unit, shown in Figure 4.

1.4 Marketing Requirements 1. The energy management system should be able to charge a group of batteries. 2. The system should supply the motors, sensors, and other electrical devices within the

vehicle with the correct amount of energy. 3. One complete charge should be sufficient for at least the duration of the event. 4. The system should be simple for quick servicing and placement on the vehicle. 5. The size and weight of the system should be as small as possible. 6. The system should be able to operate in a dry and wet environment. 7. The monitoring of the system should be as minimalistic as possible to save energy. 8. The system should be shielded from EM noise from the other components. 9. The system should have two modes, charging/servicing mode, and a supplying mode. 10. The system along with its parts should be robust due to the environment it operates in.

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1.5 Objective Tree

Energy Management System

Energy Management System Design Easy to Use

The system will balance all cells for an even charge and

discharge.

The system should supply the onboard devices with the

correct power

One charge should allow the vehicle to run for at least an

hour

The system should be simple for placement within the

vehicle

The system should be as small as possible

The weight should be as small as possible

System monitoring should be as minimalistic as possible

The system and its parts should be robust

Figure 1: Objective Tree

Figure 1 is the objective tree which is a layout of how the EMS will succeed in the SAE Electric vehicle challenge. In order to be competitive, the EMS will need to be as small as possible in terms of size and weight. Safety and performance are the two biggest goals

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with the EMS. In terms of safety the battery pack must safely balance the charging and discharging of the battery pack cells. For performance the system must be able to operate for an hour, which is the limit for the endurance race, and the system must also be able to output the necessary power that the motors require for peak performance.

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2 Design Requirements SpecificationsMarketing

Requirements Engineering Requirements Justification

1,2The EMS will charge each battery

cell in the pack to a maximum voltage of 3.6V

This allows a fast charging rate without harming the batteries

1,2The EMS will include a battery pack

that supplies low voltage power at ±12V

There is low voltage instrumentation that needs to be

powered

2,3

The EMS battery packs will supply 72V and hold at most 5500wh of energy, which holds the ability to complete an endurance track of

22km within a 60 minute time limit

The power limit must abide the SAE rulebook, while still being able to complete the necessary

tasks.

4,5The battery packs and the control

unit will be housed in separate anti-conductive containers

The HV and LV systems must be kept separate as to not interfere

with each other, and must be covered to protect from shorting

due to dropped or misplaced metal objects

1,7

Each cell will have a slave PIC16 monitoring temperature and voltage, communicating to a master PIC32,

that can shut down the entire system if conditions become unsafe

There is a need for constant monitoring of the battery pack’s temperature, in case it overheats.

Also, if there is a highly over-charged cell the PIC needs to shut

the system down to prevent fire and explosion

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The master PIC32 will relay pertinent information using a CAN

bus system to the drive control team, so data can be displayed to the

driver

In order to communicate with the drive controller, a CAN bus

system is needed to bridge the two groups

6,10

The EMS will be protected from elements using a non-conductive mesh screen that allows for air

cooling

Any debris or microscopic particles can interfere with the

electronics

6 Design must provide a safe driving experience, whether it be wet or dry

Weather may be a factor, and must be considered when housing

equipment

8

The EMS system, mainly the control unit, will be shielded from EM by

being enclosed in polycarbonate and use shielded wires with minimum

lengths

Interference from EM forces can throw off previously calculated

expectancies

1,2,9

Each individual cell will have its own balancing circuit that will bypass all

current with a Zener Shunt Regulator once the battery reaches 3.6V

The cells must be charged evenly to prevent over-charging any

individual cells

Figure 2: Engineering Requirements

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3 Accepted Technical Design

3.1 Level 0 Block Diagrams:

AccumulatorInput Power(Charging) Output

Power(Discharging)

Figure 3: Accumulator Block Diagram

The accumulator module in Figure 4 is a low level representation of how the total system will work. The Accumulator is the total energy system, including the battery pack and all of the monitoring hardware.

Module AccumulatorDesigner Scott E. PerryInputs Input of charginging current < 30A (0.3CA)Outputs Total energy from battery pack < 5500Wh

DescriptionCharging unit takes an input of 120V AC, and depending on the monitoring input it enables the charging unit to start, continue or stop charging each cell.

Table 1: Level 0 Accumulator Theory of Operation

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Software Monitoring

Module

Current draw signal

PIC data signal (x28)

Current draw

Temperature warning

State of charge

Figure 4: Software Monitoring Block Diagram

Module Software Monitoring ModuleDesigner Luis A. Cabrera

Current draw signalPIC data signal (x28) (temperature and voltage)Current drawState of chargeTemperature/Voltage warning

Description

Monitors the whole system by reading each battery's temperature and voltage and sending a warning signal to the drive control, it also monitors the total current draw on the battery pack, calculates the state of charge and passes it along with the current draw value.

Inputs

Outputs

Table 2: Software Monitoring Theory of Operation

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Accumulator/HVCharger

Main Control Module

Power

Power, 120VAC

Power Outputto Load/Motors

Low Voltage Supply

Isometer

Low Voltage Instrumentation

1. Voltage (for N cells)2. Temperature (for N cells)

3. CurrentGround

The charger accepts 120Vac at 60Hz power and charges the Accumulator which also has the monitoring system to keep the system operating in safe conditions. The main control module receives the data about the battery pack such as voltage, temperature and current to determine what the system should do.

Module Accumulator/HVDesigner Scott E. Perry

Inputs Current <18A charging

Outputs Power Output: 72V and up to 400A

Description

Charger operaters off 120VAC, and sends power to Accumulator for charging purposes. The Main Control Module will monitor voltage, current, and temperature data and decide when to stop charging. The Low Voltage supply will supply all low voltage instrumentation.

Table 3: Level 1 Accumulator Theory of Operation

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main()

PIC monitorCurrent monitor

adcRead()

Current draw

PIC data (temperature, voltage)

CAN module

CAN message (current draw, temperature/voltage warning, state of charge)

Refresh buffer

Initialize

DataInitialize

Figure 5: Level 1 Software Block Diagram

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Module main()Designer Luis A. Cabrera

Current monitor dataPIC monitor dataCAN clear bufferDataadcRead() initializeCAN module initializeCurrent monitor initialize, charge/discharge continuePIC monitor initialize

Description

After initialization of all functions and modules it calls on the current monitor and PIC monitor to retrieve data. Once data is retrieved it determines if the system is operating normally and sends data to the CAN module. If opertion is not safe a shutdown sequence interrupt.

Module PIC monitorDesigner Luis A. Cabrera

main () initializeadcRead() responseadcRead() queryStatus data to main()

Description

This process will run every 10ms and query the adcRead() temperature and voltage channels. The data will be sent back to main() to determine if everything is running ok.

Module Current monitorDesigner Luis A. Cabrera

main () initializeadcRead() responseadcRead() queryData to CAN module

Description

This process will run after the PIC monitor process finishes. It will query the adcRead() current channel, calculate and send the state of charge as well as the current drawn to main().

Input

Outputs

Inputs

Outputs

Inputs

Outputs

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Module adcRead()Designer Luis A. Cabrera

main() initializeCurrent drawPIC data (temperature, voltage)Current monitor queryPIC monitor queryCurrent sensor channel (0)Temperature/Voltage channel(1, 2)

DescriptionWhenever the monitors need data, it will respond with sampled data from the queried channels (0, 1, 2).

Module CAN moduleDesigner Luis A. Cabrera

main() initializeDataRefresh buffer

Output CAN message

Description

Once initializedby main(), it will periodically refresh the message buffer and send a new one using the data from main() that contains current draw, average temperature, average voltage, and state of charge.

Inputs

Outputs

Inputs

Table 4: Software Level 1 Theory of Operation

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Battery Pack

Analog Circuit

PIC

Power

Power Output

Charging Rate

Temperature

Input 120Vac

Power Output

LV Signal

Charger

Opto-isolator

Light Signal

Balancing Circuit

Voltage Signal (2.5V-3.65V)

State

Feedback

LV Isolated System

IsometerGround

Accumulator

CAN TransceiverMicrocontroller

Current Shunt

CAN Output to Drive Control

Figure 6: Level 2 EMS Block Diagram

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Module Balancing CircuitDesigner Scott E. PerryInputs <18A current, voltage level from batteryOutputs Current to Charge battery

Description

The Balancing Circuits which distribute the power to the battery cells. When these cells are full or uneven the balancing circuit slave PICs read this and alerts the master PIC, which sends this information to the Controller Unit.

Module Battery PackDesigner Scott E. PerryInputs Input charging current from charger of <18A

Outputs Power Output to Motors, 72V and up to 400A through a current shunt

Description

The battery pack becomes charged by the charger, being balanced by the analog circuit or PICs and outputs power to the motors. The current shunt measures the current output and sends the information to the master PIC

Module MicrocontrollerDesigner Scott E. PerryInputs Power: 120V AC at 60HzOutputs Power Output to Motors, 72V and up to 400A

Description

Input power goes through Charging Unit and is turned into correct charging power. This power is sent to the Balancing Circuits which distribute the power to the battery cells. When these cells are full or uneven the balancing circuit slave PICs read this and alerts the master PIC, which sends this information to the Controller Unit. The batteries are also being monitered by the Software Unit.

Module IsometerDesigner Joshua D. ShepardInputs Input from HV and LV systemOutputs 40kOhm to ground

DescriptionConnects the HV and LV system to ground with a 40kohm resistance inbetween for competition measruing purposes.

Table 5: EMS Theory of Operation

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main()

Initialize variables, defaults, modules

while(1)

Set up Shutdown sequence interrupt

Main flowchart

While PIC monitor running

Check Operation status from data

Is safe

Continue Charging/

Discharging

Safe

While PIC monitor running

Clear CAN buffer

Send stored data to CAN module

Shutdown sequence interrupt

NOT SAFE

Figure 7: Main Software Flowchart

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Temperature-Voltage Monitor

Check PICs (x28)

Ok

Current Monitor

Receive current draw reading from

adcRead(0)

Calculate state of charge

Return control to main()

Receive temperature/voltage from

adcRead(1, 2)

Return control to main()

Start Start

Save state of charge

and current draw

Save temperature and voltage

Figure 8: Software Flowcharts

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3.4 Design Calculations:

Two types of batteries are being considered for use in the battery pack, Thundersky and A123. The Thundersky batteries are Lithium Iron Phosphate (LiFePO4) with a nominal voltage of 3.2V and a nominal capacity of 60Ah at C. The A123 cells are also LiFePO4, with a nominal voltage 3.3V and a nominal capacity of 2.5Ah at C. Using (1) will give the amount of energy that each cell gives.

𝐸!"# !"## = (𝑉!"#$!%&)( 𝐴ℎ!) (1)

Thunder sky:

𝐸 = (3.2𝑉)(60𝐴ℎ) = 192 𝑊ℎ − 𝑝𝑒𝑟 𝑐𝑒𝑙𝑙

A123:

𝐸 = 3.3𝑉 (2.5𝐴ℎ) = 8.25 𝑊ℎ − 𝑝𝑒𝑟 𝑐𝑒𝑙𝑙

The total battery pack energy is limited to 5,500 Wh by the SAE Formula Electric rules. In order

Use (2) to determine the maximum number of cells the battery pack can contain.

𝑁!"##$ = 𝐸!"#/𝐸!"# !"## (2)

Thunder sky:

𝑁!!!"#$%&'( =5,500 𝑊ℎ192 𝑊ℎ = 28 𝑐𝑒𝑙𝑙𝑠

A123:

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𝑁!!"# =5,500 𝑊ℎ8.25 𝑊ℎ = 666 𝑐𝑒𝑙𝑙𝑠

The formula electric race car will be judged on speed; therefore the total weight of the battery pack is an important factor to consider. Computing weight with (3) gives the total weight of the battery pack.

𝑊!"#$% = (𝑁!"##$)(𝑊!"# !"##) (3)

Thundersky:

𝑊!!!"#$%&'(!!"# = 28 𝑐𝑒𝑙𝑙𝑠 2.4 𝑘𝑔 = 67.2 𝑘𝑔

𝑊!!!"#$%&'(!!"#! = 28 𝑐𝑒𝑙𝑙𝑠 2.6 𝑘𝑔 = 72.8 𝑘𝑔

𝑊!!!"#$%&'(!!"# = 28 𝑐𝑒𝑙𝑙𝑠 5.29 𝑙𝑏 = 148.15 𝑙𝑏

𝑊!!!"#$%&'(!!"#! = 28 𝑐𝑒𝑙𝑙𝑠 5.73 𝑙𝑏 = 160.5 𝑙𝑏

A123:

𝑊!!"# = 666 𝑐𝑒𝑙𝑙𝑠 0.076 𝑘𝑔 = 50.616 𝑘𝑔

𝑊!!"# = 666 𝑐𝑒𝑙𝑙𝑠 0.1675 = 111.589 𝑙𝑏

Another important factor in vehicle design is space, while weight is very important the space in the vehicle is limited. Equation (4) is the volume for a rectangular prism, which can be used to find the volume of a Thundersky cell. Using (5) will give the volume of a cylinder which is used to find the volume of an A123 cell.

𝑉!"#$%&'(" = (𝐿)(𝑊)(𝐻) (4)

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𝑉!"#$%&'( = 𝜋 𝑟 ! ∗ 𝐻 (5)

Thundersky:

𝑉!!!"#$%&'( = 115 𝑚𝑚 61 𝑚𝑚 215 𝑚𝑚 = 0.015082 𝑚!

𝑉!!!"#$%&'( = 4.528 𝑖𝑛 2.4 𝑖𝑛 8.46 𝑖𝑛 = 0.053 𝑓𝑡!

A123:

𝑉!!"# = 3.14 (13.075 𝑚𝑚!) 65.5 𝑚𝑚 = 0.000352 𝑚!

𝑉!!"# = 3.14 (0.515 𝑖𝑛!) 2.57 𝑖𝑛 = 0.001242 𝑓𝑡!

The Thundersky cells are rectangular and can be set up in 4 columns of 7 cells each. In order to determine the total volume the battery pack will take up use (6) for a pack of Thundersky cells, and (7) for a pack of A123 cells.

𝑉!!!"#$%&'(!!"#$ = 7𝑥𝑊 4𝑥𝐿 (𝐻) (6)

𝑉!!"#!!"#$ = (𝑉!!"#)( 𝑁!"##$ !!"#) (7)

Thundersky:

𝑉𝑻𝒉𝒖𝒏𝒅𝒆𝒓𝒔𝒌𝒚 = 0.16.81102 𝑖𝑛 (18.11 𝑖𝑛) 8.46 𝑖𝑛 = 1.49 𝑓𝑡!

A123:

𝑉!!"# = 2.145622 𝑖𝑛! 666 𝑐𝑒𝑙𝑙𝑠 = 0.827 𝑓𝑡!

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After considering all of these calculations the Thundersky were chosen to be used because they would require less cell management because each cell would need to be monitored. Comparing 28 managing circuits to 666 managing circuits will save both money and space. While A123 cells would save weight, the monitoring circuits would be exponentially more complicated and expensive than using the 28 Thundersky cells.

3.4.1 Battery Management System Theory of Operation

The battery management system (BMS) must provide protection against unsafe areas of operation such as overvoltage, overcharging, undercharging and high temperatures [5]. Multiple safety instrumentation is used to prevent these dangerous operations.

An analog current bypass circuit shown in Figure 9 is used to protect against overcharging of each cell in the BMS. For safety reasons, a fuse will be used to disconnect the circuit from too much current flowing through so the circuitry will not fail. To stop the cell from charging past 3.6V, the LM431 zener shunt regulator, which is reference designator U1 in Figure 9, is used to bypass the current once the voltage reaches 3.6V. The LM431 compares the voltage from the voltage divider created by R1 and R2 to 2.5V. The resistors were chosen to cutoff the charging of the battery at 3.6V. Once 3.6V is reached, the current is switched by a TIP 147 darlington PNP transistor, which is reference designator Q1 in Figure 9, which bypasses the current into the 4Ω power resistor stopping the battery from being charged. This analog circuit is a form of redundancy if the PIC were to fail controlling the voltage of the battery pack.

There will be 28 monitoring circuits, one for every cell of the battery pack. These 28 slave PICs will all need to report to a master PIC. The slave PIC will all provide voltage, temperature, and current with I2C protocol to the Master PIC. In order to provide the correct voltage an opto-isolator is used because the grounds would be different between each slave PIC and the Master PIC without the opto-isolator. The opto-isolator which is reference designation U3 in Figure 9 is provided with a light signal from a diode, which signals the darlington transistor to activate and provide the voltage. Two LEDs provide information about the charging of the battery cell by inspection. There is a green LED and an orange LED, the Green LED is active when the battery is charging, and the orange LED becomes active once the current is being shunted to signify that the battery is charged to 3.6V.

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Figure 9: Analog Current Bypass Circuit

3.4.2 Battery Management System, Software Theory of Operation

The software control that the Energy Management System uses must keep track of the current drawn from the Accumulator, the temperature and voltage of each individual battery in the Accumulator and determine if the system is running safely. This software control also sends operating data to the Drive Control System using CAN communication.

The entire software control is shown in Figure 6 as a pictorial representation of the separate software functions and modules. The main() function is where the control starts and is shown as a flow chart in Figure 7. It initializes functions, modules and an interrupt, then runs a continuous loop where data is collected from the Current and PIC modules using the adcRead() function. The adcRead() function has separate channels that represent unique hardware signals (total current draw, temperature, and voltage). This function call takes a channel as its parameter and returns a sampled data value.

The first module that uses the adcRead() function is the PIC monitor. As shown in Figure 8, the flow chart must run a separate loop to retrieve temperature (channel 1) and voltage (channel 2) from each battery, this is done using an explicit call to adcRead(). Once all the battery data is recorded control is given back to main(). The data that was saved is checked to see if the temperature and the voltage are within safe operating range, if either is not, a Shutdown sequence interrupt kicks in, where all operations are stopped. If the data shows that the operation is safe, a signal is sent back to the individual PIC to allow the battery to charge or discharge.

R15k

R210k

R31.2k

R4

330

R54

V23.65Vdc

R61k

R71k

D21N6266/TO

D11N6266/TO

F1

FUSE

U1LM431

U3

4N33

16

2

5

4

R81k

Q1TIP 147

0

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The following process that runs is the Current module. It also uses the adcRead() function with channel 0 as the parameter. Once the current draw value is received, the Accumulator State of charge is calculated with (8).

SOC(t) = 𝑄! − 𝑖 𝜏 𝑑𝜏!!!

(8)

The state of charge and the current draw value are sent back to main() before control is given back to it.

The final step of this process is to send a CAN message to the Drive Control System. First the CAN bus message buffer needs to be cleared, then all the data that has been saved must be compiled. Current draw and state of charge can be sent as is, but the temperature and voltage values must be averaged and sent as two separate values.

3.5 Matlab Simulation

In order to really understand the total energy consumption the vehicle would use, a Matlab simulation was created. With a joint effort of Design team 6, Drive control team accurate results were achieved that are important to pick parts for the vehicle. The Matlab program calculates speed limits, energy used, power used, current used, acceleration at any point, and velocity at any point. The maximum potential of the autocross vehicle is the goal of this simulation, in order to accomplish that a similar autocross track was used. The autocross competition scores on the fastest time per lap, and its track has both straights, slaloms, and half circle segments. Dividing the previous completions track into segments, the forces the vehicle experiences can be calculated. The main loop used for this Matlab follows.

while (true) %look ahead for braking tempPosition = Position + (Velocity * TimeDelta); tempVelocity = Velocity; isBraking = 0; targetVelocity = inf;

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while (tempVelocity > 0) if (round(tempPosition) + 1 > length(SpeedLimits)) isBraking = 0; break; else if (SpeedLimits(round(tempPosition) + 1) < tempVelocity) targetVelocity = Velocity - (tempVelocity - SpeedLimits(round(tempPosition) + 1)); isBraking = 1; break; end end if (targetVelocity > Velocity - (tempVelocity - SpeedLimits(round(tempPosition) + 1))) targetVelocity = Velocity - (tempVelocity - SpeedLimits(round(tempPosition) + 1)); end tempPosition = tempPosition + (tempVelocity * TimeDelta); tempVelocity = tempVelocity - (BrakingDecel * TimeDelta); end DesiredAcceleration = ((targetVelocity - Velocity) * TimeDelta); ForceDrag = (0.5)*AirDensity*DragCoeff*FrontArea*(Velocity.^2); DesiredForceAtWheels = (targetVelocity - Velocity) * Mass / TimeDelta + ForceDrag; %ForceDrag = (0.5)*AirDensity*DragCoeff*FrontArea*(Velocity.^2); ForceAtWheels = min(2 * PeakTorque/WheelRadius * GR * MechanicalEfficiency, DesiredForceAtWheels); if (isBraking == 1) ForceAtWheels = 0; BrakingForce = Mass * BrakingDecel; BrakingForce = min(abs(DesiredForceAtWheels), BrakingForce); Acceleration = - (BrakingForce + ForceDrag)/Mass; else Acceleration = (ForceAtWheels - ForceDrag)/Mass; end Velocity = Velocity + Acceleration * TimeDelta; Position = Position + Velocity * TimeDelta; TimeStep = TimeStep + 1; if (Position > length(SpeedLimits)) break; end

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PositionLog(TimeStep) = Position; VelocityLog(TimeStep) = Velocity; AccelerationLog(TimeStep) = Acceleration; PowerLog(TimeStep) = Velocity * max(ForceAtWheels, 0); CurrentLog(TimeStep) = PowerLog(TimeStep) / 72; EnergyLog(TimeStep) = EnergyLog(max(TimeStep - 1,1)) + PowerLog(TimeStep) / 1000 / 3600 * TimeDelta ; end

The simulation results are shown in figure 10. The first plot is the velocity throughout the track lap. Our vehicle simulation peaks at about 30 m/s which is roughly at about 67 mph which is below The SAE electric formula teams max speed of 70 mph. The next plot is the power consumption; we peak at about 55 kW, which is at the SAE limit, as well as ours, for the total Accumulator energy stored. The power consumption plots peak for short periods of time, so it is safe to assume that our system can handle the power consumption. The Energy used is an important plot since it can tell us the total energy we use in a race. This simulation was done for only one lap, and for the autocross, it requires 3, so in total the autocross race uses only 900 kWh. The battery current plot shows the current drawn at anytime during the lap of the track. Each motor requires 400 A during full acceleration, so since we use two motors, we would peak at around 800 A total.

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Figure 10: Matlab Track Simulation Results

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4 Parts List

Qty. Part Num. Description Cost Total Cost28 LM431BCZ/NOPB Adjustable Precision Zener Shunt Regulator $0.64 $17.9228 TIP147 PNP Epitaxial Silicon Darlington Transistor 1.44 40.3228 SSL-LX5093GD Green LED through-hole mount 1.03 28.8428 SSL-LX5093SOC Orange LED through-hole mount 0.48 13.4428 1.5KE9.1A Transiet Voltage Suppressor 0.43 12.0428 PIC16F505-E/SL PIC16 F505 0.90 $0.90

1PIC32MX795F512L-80I/PT PIC32 11.76 $11.76

28 4N33 Optocoupler 0.27 7.5628 LM61CIZ Temperature Sensor 0.85 23.80

Total $156.58

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5 Project Schedule

Table 6 Gantt Chart for EMS

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6 Design Team Information

Luis Cabrera, Team Leader, Software Manager, CpE

Scott Perry, Hardware Manager, EE

Joshua Shepard, Archivist, EE 7 Conclusions and Recommendations The SAE Formula Electric competition is a complicated task in the aspect that it requires many subsets to work in conjunction to complete. For the energy management system to be complete, safety and performance characteristics are of the upmost importance. A rulebook is issued at the beginning of the competition which explains the safety guidelines that must be followed in order to compete.

8 References [1] Hyundai Motor Company. (2007, 5 22). Retrieved 3 19, 2012, from http://www.google.com/patents?id=AykIAAAAEBAJ&pg=PA4&dq=car+battery+charging&hl=en&sa=X&ei=nnRfT6OXDMnz0gHbpaHYBw&ved=0CDIQ6AEwAA#v=onepage&q=car%20battery%20charging&f=false

[2] SAE. (2011, 9 14). SAE Students. Retrieved 3 22, 2012, from http://students.sae.org/competitions/formulaseries/fsae/handbook.pdf

[3] Tesla Motors. (2006, 8 16). Retrieved 3 14, 2012, from http://webarchive.teslamotors.com/display_data/TeslaRoadsterBatterySystem.pdf

[4] http://www.nrel.gov/vehiclesandfuels/energystorage/pdfs/aabc_lv.pdf

[5] http://batteryuniversity.com/learn/article/batteries_for_transportation_aerospace

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9 Appendices

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