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
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
i 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
ii 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
iii 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
1 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
2 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.
2.
The energy management system should be able to charge a group of batteries.
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.
3 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 be simple
for placement within the
vehicle
System monitoring should be
as minimalistic as possible
The system should supply
the onboard devices with the
correct power
The system should be as
small as possible
The system and its parts
should be robust
One charge should allow the
vehicle to run for at least an
hour
The weight should be as
small as possible
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
4 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.
5 2
Design Requirements Specifications
Marketing
Requirements
Engineering Requirements
Justification
1,2
The 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
1,2
The 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.
4,5
The HV and LV systems must be
kept separate as to not interfere
The battery packs and the control
with each other, and must be
unit will be housed in separate anticovered to protect from shorting
conductive containers
due to dropped or misplaced
metal objects
1,7
There is a need for constant
Each cell will have a slave PIC16
monitoring of the battery pack’s
monitoring temperature and voltage, temperature, in case it overheats.
communicating to a master PIC32,
Also, if there is a highly overthat can shut down the entire system charged cell the PIC needs to shut
if conditions become unsafe
the system down to prevent fire
and explosion
7
The master PIC32 will relay
In order to communicate with the
pertinent information using a CAN
drive controller, a CAN bus
bus system to the drive control team,
system is needed to bridge the
so data can be displayed to the
two groups
driver
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
Weather may be a factor, and
Design must provide a safe driving
must be considered when housing
experience, whether it be wet or dry
equipment
8
The EMS system, mainly the control
unit, will be shielded from EM by Interference from EM forces can
being enclosed in polycarbonate and throw off previously calculated
use shielded wires with minimum
expectancies
lengths
1,2,9
Each individual cell will have its own
The cells must be charged evenly
balancing circuit that will bypass all
to prevent over-charging any
current with a Zener Shunt Regulator
individual cells
once the battery reaches 3.6V
Figure 2: Engineering Requirements
6 3
Accepted Technical Design
3.1 Level 0 Block Diagrams:
Input Power(Charging)
Accumulator
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
Designer
Inputs
Outputs
Accumulator
Scott E. Perry
Input of charginging current < 30A (0.3CA)
Total e nergy from battery pack < 5500Wh
Charging unit takes an i nput of 120V AC, and depending on the monitoring i nput i t Description
enables the charging unit to start, continue or stop charging e ach cell.
Table 1: Level 0 Accumulator Theory of Operation
7 Current draw signal
PIC data signal (x28)
Software
Monitoring
Module
Current draw
State of charge
Temperature warning
Figure 4: Software Monitoring Block Diagram
Module
Designer
Software Monitoring Module
Luis A. Cabrera
Current draw signal
Inputs
PIC data signal ( x28) ( temperature and voltage)
Current draw
Outputs
State of charge
Temperature/Voltage warning
Monitors the whole system by reading e ach battery's temperature and voltage and sending a warning signal to the drive control, i t Description also monitors the total current draw on the battery pack, calculates the state of charge and passes i t along with the current draw value.
Table 2: Software Monitoring Theory of Operation
8 3.2 Level 1 Block Diagrams:
Power, 120VAC
Power Output
to Load/Motors
Ground
Low Voltage
Instrumentation
Accumulator/HV
Charger
Power
1. Voltage (for N cells)
2. Temperature (for N cells)
3. Current
Isometer
Main Control
Module
Low
Voltage
Supply
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
Designer
Accumulator/HV
Scott E. Perry
Inputs
Current <18A charging
Outputs
Power Output: 72V and up to 400A
Charger operaters off 120VAC, and sends power to Accumulator for charging purposes. The Main Control Module will monitor voltage, current, and Description
temperature data and decide when to stop charging. The Low V oltage supply will supply all l ow voltage i nstrumentation.
Table 3: Level 1 Accumulator Theory of Operation
9 CAN message (current draw, temperature/voltage warning, state of charge)
CAN module
Refresh buffer
Data
Initialize
main()
Current monitor
Initialize
PIC monitor
adcRead()
PIC data (temperature, voltage)
Current draw
Figure 5: Level 1 Software Block Diagram
10 Module
Designer
Input
Outputs
main()
Luis A. Cabrera
Current monitor data
PIC monitor data
CAN clear buffer
Data
adcRead() i nitialize
CAN module i nitialize
Current monitor i nitialize, charge/discharge continue
PIC monitor i nitialize
After i nitialization of all functions and modules it calls on the current monitor and PIC monitor to retrieve data. Once data i s retrieved i t Description
determines i f the system i s operating normally and sends data to the CAN module. If opertion i s not safe a shutdown sequence i nterrupt.
Module
Designer
PIC monitor
Luis A. Cabrera
main () i nitialize
Inputs
adcRead() response
adcRead() query
Outputs
Status data to main()
This process will run e very 10ms and query the adcRead() temperature and voltage channels. Description
The data will be sent back to main() to determine i f e verything i s running ok.
Module
Designer
Inputs
Outputs
Current monitor
Luis A. Cabrera
main () i nitialize
adcRead() response
adcRead() query
Data to CAN module
This process will run after the PIC monitor process finishes. It will query the adcRead() Description
current channel, calculate and send the state of charge as well as the current drawn to main().
11 Module
Designer
adcRead()
Luis A. Cabrera
main() i nitialize
Current draw
Inputs
PIC data ( temperature, voltage)
Current monitor query
PIC monitor query
Current sensor channel ( 0)
Outputs
Temperature/Voltage channel(1, 2)
Whenever the monitors need data, i t will Description respond with sampled data from the queried channels ( 0, 1, 2). Module
Designer
Inputs
Output
CAN module
Luis A. Cabrera
main() i nitialize
Data
Refresh buffer
CAN message
Once i nitializedby main(), i t will periodically refresh the message buffer and send a new Description one using the data from main() that contains current draw, average temperature, average voltage, and state of charge.
Table 4: Software Level 1 Theory of Operation
12 3.3 Level 2 Block Diagrams:
Input
120Vac
Accumulator
Balancing Circuit
Feedback
Battery
Pack
Power
Analog
Circuit
State
Power
Output
Current
Shunt
Power
Output
Charger
Light Signal
Charging
Rate
PIC
Optoisolator
Voltage Signal
(2.5V-3.65V)
Ground
Isometer
LV Signal
Microcontroller
Temperature
LV Isolated
System
Figure 6: Level 2 EMS Block Diagram
13 CAN
Transceiver
CAN Output to Drive
Control
Module
Designer
Inputs
Outputs
Balancing Circuit
Scott E. Perry
<18A current, voltage l evel f rom battery
Current to Charge battery
The Balancing Circuits which distribute the power to the battery cells. When these cells are f ull or uneven the Description
balancing circuit slave PICs read this and alerts the master PIC, which sends this i nformation to the Controller Unit.
Module
Designer
Inputs
Battery Pack
Scott E. Perry
Input charging current f rom charger of <18A
Power Output to Motors, 72V and up to 400A through a current shunt
The battery pack becomes charged by the charger, being balanced by the analog circuit or PICs and outputs power to Description
the motors. The current shunt measures the current output and sends the i nformation to the master PIC
Outputs
Module
Designer
Inputs
Outputs
Microcontroller
Scott E. Perry
Power: 120V AC at 60Hz
Power Output to Motors, 72V and up to 400A
Input power goes through Charging Unit and i s turned i nto correct charging power. This power i s sent to the Balancing Circuits which distribute the power to the battery cells. Description When these cells are f ull or uneven the balancing circuit slave PICs read this and alerts the master PIC, which sends this i nformation to the Controller Unit. The batteries are also being monitered by the Software Unit.
Module
Designer
Inputs
Outputs
Isometer
Joshua D. Shepard
Input f rom HV and LV system
40kOhm to ground
Connects the HV and LV system to ground with a 40kohm Description
resistance i nbetween f or competition measruing purposes.
Table 5: EMS Theory of Operation
14 Main flowchart
main()
Initialize variables,
defaults, modules
Set up Shutdown
sequence interrupt
while(1)
While PIC monitor
running
Check Operation
status from data
Safe
Is safe
NOT SAFE
Continue
Charging/
Discharging
Shutdown
sequence interrupt
While PIC monitor
running
Clear CAN buffer
Send stored data
to CAN module
Figure 7: Main Software Flowchart
15 Temperature-Voltage Monitor
Current Monitor
Start
Start
Receive current
draw reading from
adcRead(0)
Check PICs (x28)
Calculate state of
charge
Receive
temperature/
voltage from
adcRead(1, 2)
Save state
of charge
and current
draw
Ok
Save
temperature
and voltage
Return control to
main()
Figure 8: Software Flowcharts
16 Return control to
main()
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!"# !"##
Thunder sky:
𝑁!!!"#$%&'( =
5,500 𝑊ℎ
= 28 𝑐𝑒𝑙𝑙𝑠
192 𝑊ℎ
A123:
17 (2)
𝑁!!"# =
5,500 𝑊ℎ
= 666 𝑐𝑒𝑙𝑙𝑠 8.25 𝑊ℎ
The formula electric race car will be judged on speed; therefore the total weight of the
battery pack is an important factor to consider. Computing weight with (3) gives the total
weight of the battery pack.
𝑊!"#$% = (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!"#$%&'(" = (𝐿)(𝑊)(𝐻)
18 (4)
𝑉!"#$%&'( = 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 !
19 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.
20 F1
FUSE
R3
1.2k
R1
5k
4N33
6
5
1
4
2
R4
TIP 147
Q1
330
R7
1k
V2
3.65Vdc
U1
LM431
R6
1k
U3
R8
1k
R2
10k
0
R5
4
D2
1N6266/TO
D1
1N6266/TO
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.
21 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;
22 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
23 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.
24 Figure 10: Matlab Track Simulation Results
25 4
Parts List
Qty.
28
28
28
28
28
28
Part Num.
LM431BCZ/NOPB
TIP147
SSL-LX5093GD
SSL-LX5093SOC
1.5KE9.1A
PIC16F505-E/SL
PIC32MX795F512L1 80I/PT
28 4N33
28 LM61CIZ
Description
Adjustable Precision Zener Shunt Regulator
PNP Epitaxial Silicon Darlington Transistor
Green LED through-hole mount
Orange LED through-hole mount
Transiet Voltage Suppressor
PIC16 F505
PIC32
Optocoupler
Temperature Sensor
26 Cost
Total Cost
$0.64
$17.92
1.44
40.32
1.03
28.84
0.48
13.44
0.43
12.04
0.90
$0.90
11.76
0.27
0.85
Total
$11.76
7.56
23.80
$156.58
5
Project Schedule
Table 6 Gantt Chart for EMS
27 28 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+chargi
ng&hl=en&sa=X&ei=nnRfT6OXDMnz0gHbpaHYBw&ved=0CDIQ6AEwAA#v=onepa
ge&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 29 9
Appendices
30 31 32 33 34 35 36