Regenerative Braking System
for Illini Formula Electric
Design Review
ECE 445
Team 8
Josh Seibert & Nick Meinhart
TA: Zitao Liao
September 30, 2015
1
Table of Contents
1. Introduction ..................................................................................................................................... 3
1.2 Objectives .................................................................................................................................... 3
1.3 Fun ............................................................................................................................................... 3
1.4 Benefits ........................................................................................................................................ 3
1.5 Features........................................................................................................................................ 3
2. Design..................................................................................................................................... 4
2.1 Block Diagram ................................................................................................................... 4
2.2 Block Descriptions ............................................................................................................. 5
2.3 Controller .......................................................................................................................... 5
2.4 Battery ............................................................................................................................... 7
2.5 Inverter/Drive .................................................................................................................... 7
2.6 Motor/Generator ................................................................................................................ 7
2.7 Schematics & Simulations ................................................................................................. 8
2.8 Calculations ..................................................................................................................... 11
3. Requirements and Verification ........................................................................................... 13
3.2 Tolerance Analysis .......................................................................................................... 13
4. Cost Analysis and Schedule ................................................................................................ 14
4.1 Cost Analysis ............................................................................................................................ 14
4.2 Schedule .......................................................................................................................... 15
5. Ethics & Safety .................................................................................................................... 15
5.1 Ethics ......................................................................................................................................... 15
5.2 Safety .......................................................................................................................................... 16
6. References ............................................................................................................................ 16
2
1. Introduction
1.1 Purpose
Regenerative braking systems are used in today’s electric and hybrid vehicles in order to
capture and recover the kinetic energy that is lost as heat during conventional braking with rotors
and brake pads. These systems help improve the efficiency and driving range of the vehicle per
full charge of the battery.
This project is to develop a regenerative braking system for the Illini Formula Electric
vehicle. The current vehicle is fully electric with two batteries, a single drive system, and single
motor. However, the vehicle’s range is dependent on the combined charge capacity of the two
batteries. With the addition of a regenerative braking system, the range and efficiency of the
current vehicle would increase. The system we have planned in developing would be a system
that recaptures energy while the vehicle is coasting. Here the driver would allow the electric car
to slow by depressing the accelerator and regenerative braking would take place if certain
conditions are met. Conventional braking would still be present and used when the driver uses
the brake pedal.
1.2 Objectives



Increase the driving range of the Formula electric car given normal driving conditions
Add a minimally invasive system without major modifications to the existing car
Add the least amount of weight possibly while still achieving significant gains
1.3 Functions

Stores kinetic energy of braking that can be used later as electrical energy
1.4 Benefits


Extends driving range of vehicle
Decreases wear on brakes
1.5 Features


Controller to provide non-human interaction
Choice between no regen, low regen, or high regen
3
2 Design
2.1 Block Diagrams
Location of design
Figure 1: Design Location
Figure 2: Overall Block Diagram
4
Figure 3: Controller Block Diagram
2.2 Block Descriptions
2.3 Controller
The controller is the main component of our design. This component will take in inputs from the
battery sensor, speed sensor, and brake signal that will be used to determine whether or not
regenerative braking should take place. These signals will be taken in and processed by their
respective circuits. Then the signals will be sent to the drive to identify when regenerative
braking can or cannot take place.
2.3.1 Battery Charge Circuitry
The battery charge circuitry will take in as an input the signal that gives the current charge level
of the battery. This circuit can be seen in figure 4 with the pin numbers of the comparator. The
sensor is already part of the current vehicle’s signals. This sensor provides a voltage level that
indicates the charge of the battery. The maximum voltage is 5 volts at a full charge and drops off
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linearly as the charge of the battery drops. This circuit will use this signal and compare to a set
voltage level of 4.5 volts. This means that regenerative braking will only take place once the
charge of the battery drops down to 90%. This will ensure that the battery will not be
overcharged at any point. In figure 5 it can be seen that with a charge level voltage signal set low
to 4 volts, which is below the threshold voltage for charge, will give a high output voltage signal.
In figure 6, it can be seen that when the charge level voltage signal is above the set voltage
comparison level, the output voltage signal will be low. It can be seen with these two figures that
the regenerative braking could be control with this circuit based on battery charge.
2.3.2 Speed Circuitry
The speed circuitry is to determine if the vehicle is going at speed that would allow for
regenerative braking. This circuit will use two comparators with one determining if the vehicle is
going fast enough and the other determining if the vehicle is going slow enough. This is because
of the back emf generated from the motor, which is dependent on speed. The back emf cannot be
too high as well as too low. Therefore the speed needs to be monitored. The speed signal is being
generated by a hall sensor and the circuit for this component can be seen in figure 7.
2.3.3 Brake Detection
The brake detection is use to determine if the driver has applied the brakes. If the brakes are
applied, then the regenerative braking system will not be used. This is to help ensure that the
driver as a more natural braking force applied to the car. Although kinetic energy will be lost as
heat, we did not want the driver to unexpectedly have two braking systems applied at the same
time. This could cause abnormal vehicle control. This is simply taking in a signal that is applied
by the driver when the brake pedal is pressed and sends a signal to the brake lights on the car.
We will be pulling of this signal in order to determine when the driver is using the brake.
2.3.4 And Gate/ Driver Switch
The AND gate and driver switch is the last part of the controller. This consist of a single AND
gate that takes in the outputs of the battery charge circuitry, speed circuit, and brake signal. This
AND gate will then send an output to a MOSFET. This MOSFET will turn on and then complete
a looped circuit. In this looped circuit, there will be a 3 position switch which will give three
different paths. Each of these paths will consist of a different resistance in which will give a
different voltage level that can be sent to the drive. The drive will then inhibit the regen current
depending on the voltage level input.
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2.3.5 12 to 5 Volt DC Converter
The 12 to 5 volt DC converter is to be used in the controller in order to provide the correct
voltage for the logic. The comparators being used call for an input 5 V DC. This will ensure that
the comparators will operate correctly, as well as other logic.
2.4 Battery
The battery component is already part of the current electrical system of the car. This consists of
a 50 amp discharge current, and 10 Amp charge current. Each battery bank consist of smaller
cells that have a voltage level of 4.1 V at maximum charge and a 3.7 nominal voltage level.
There already exists a battery level sensor that we plan on using in order to determine if the
battery can accept any charge. This is the data being collected from the battery in figure 1.
2.5 Inverter/Drive
The inverter/drive is a variable frequency drive by Sevcon. The model is Gen 4 Size 8.The drive
is already being used in the current electrical system of car with it primary purpose being to
convert the DC voltage from the battery packs to AC for the motor. This component will be used
in the regenerative braking design as a rectifier as it also has the ability to rectify the AC voltage
from the motor/generator to DC. This component will also be used in limiting the current under
10 amps, or more specifically the current will be limited to a maximum of 9 amps. This
restriction will inhibit the regenerative capabilities, but is required in order to not damage the
batteries. This drive will also be programed to read certain voltage values from the input in order
to determine how much regen to allow. There will be three modes controlled by the driver which
are off, low, and high. These all control the amount of current that will flow with 9 amps being
the maximum.
2.6 Motor/Generator
The motor/generator is an existing component on the vehicle by YASA Motors. The motor is a 4
pole, permanent magnet, synchronous motor. This motor has a maximum rpm of 7500 at 340
Newton meters. This motor is ideal for regenerative braking as the rotor requires no excitation
current in order to create current in the rotors. This is because of the permanent magnet which
will provide the flux. The motor has a back emf of 0.062 Vrms/rpm.
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2.7 Schematics and Simulations
Figure 4: Battery Charge Circuit
Figure 5: Battery Charge Circuit simulation with low charge level
8
Figure 6: Battery Charge Circuit simulation with high charge level
Figure 7: Speed Circuit
9
Figure 8: Speed Circuit simulation
Figure 9: AND gate and Switch circuit
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2.8 Calculations
2.8.1 Battery Charge Circuit
In order to set our design to only allow regen at or below a 90% battery charge, we had to size
the resistors correctly in the circuit for the comparator to take in the correct voltage value. The
system on the car outputs 5V for a full charge. At 90% charge the output voltage from the system
on the car would be 4.5V. So the 5 volt system would need to be dropped down to this level
which is most simply down with resistors. The total resistance and the 5 volt input is 12.5 kΩ.
This would give a current of 0.4 mA. Then to select the correct resistance, 0.5 volts can be
divided by the current to give 1.25 kΩ as can be seen in equation 1. The power consumed by this
circuit was minimized by our selection of total resistance seen. The power consuming
components include the resistor and the comparator. The total power consumed by the resistors
can be seen in equation 2. For the power consumed by the comparator, we used the standard
values given by the data sheet. The values given were an input of 5 volts with a flow of 30 µA.
This calculation can be seen in equation 3. The sum of equations 2 and 3 gives the total power
consumption to be 2.15 mW.
0.5 𝑉
= 1.25 𝑘Ω
1
= 2 𝑚𝑊
2
5𝑉 ∗ 30µ𝐴 = 150 µ𝑊
3
0.4 𝑚𝐴
5𝑉
12.5 𝑘Ω
2.8.2 Speed Sensor Calculations
The speed circuit calculations are similar to the battery circuit calculations. First, a hall sensor had to be
selected that could read up to the fastest speed of the car. The car is capably of 7500 rpms max.
Therefore a hall sensor would see a magnetic at a frequency of 125 Hz. The hall sensor that we have
selected is capable of 10 kHz. The variable resistors are in place to allow the team to adjust the voltage
levels which will adjust the speed range. We have decided to put in 500 kΩ max variable resistor to
allow the team a wide variety in speed. The 100 kΩ resister labeled as R3 was selected high to cap the
maximum power consumption in the event that both variable resistors were set to zero. The maximum
power by this is 1.44 mW. As the variable resistors are adjusted, the power consumption will decrease
as the resistance will increase. As the range between the variable resistors is lowered, the window for
regen will widen. The power consumption for the comparators is the same as equation 3 as both
comparators are on the same chip. To power the hall sensor, the 12 volt system is being used which will
provide at maximum of 10 mA when the magnet is present. In the worst case, the current would be
constant which would bring the power consumption of the hall sensor to 0.12 W. This can be seen in
equation 4. This brings the maximum power consumption of this circuit to 0.121 W.
10𝑚𝐴 ∗ 12𝑉 = 0.12 𝑊
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4
2.8.3 AND gate and switch circuit calculations
This circuit is to provide three levels of regen depending on the position of the switch and which
loop is selected. For no regen, the voltage signal sent to the drive should be zero. This can be
seen by looking at figure 9 in which all of the voltage is dropped across the 100 kΩ resistor. For
the switch in the low regen mode, the loop selected would be the 70 kΩ resistor. In series with
the 100 kΩ resistor, this would give a total resistance of 170 kΩ. Then by equation 5, it can be
seen that the current is 70.5 µA. Then by equation 6, it can be seen that the voltage sent to the
drive would be 4.94 V which is nearly 5 V. For the last loop, the 500 kΩ resistor would be in
series with the 100 kΩ resistor. Then by following similar methods as in equations 5 and 6, the
voltage signal sent to the drive would be 10 V. The time that the circuit would be consuming the
most power would be when the switch is set to the path with no extra resistance. The power
consumed by this loop can be seen in equation 7 which is 1.44 mW. Then by the same methods,
the power consumption of the other two loops can be seen to be 847 µW and 240 µW
respectively. The AND gate will also consume some power. The input voltage is set at 5 volts
and by the data sheet; the input current is 2.4 mA. This would lead to 12 mW of power
consumption.
12 𝑉
170 𝑘Ω
= 70.5 µ𝐴
70.5 µ𝐴 ∗ 70 𝑘Ω = 4.94 𝑉
12𝑉 2
100𝑘Ω
= 1.44 𝑚𝑊
5
6
7
2.8.4 Total Power Consumption
The total power consumption of our design varies based on the set levels of the variable resistors
in the speed circuit and which mode is selected by the switch. However, the worst case power
consumption can be computed by taking the worst case power consumptions of each circuit. This
comes out to be 0.136 W.
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3 Requirements and Verification
3.1 Requirements and Verification Points Allocation
Requirements
1. Current Flow
Current flow must be reversed
to travel from the motor to the
drive, and charge the battery.
Also, must not exceed 10
amps while charging the
battery.
2. Charging Control
Turn on regen when battery
charge is below 90%, Turn off
regen if battery charges above
90% total charge. Car
currently has a battery
monitoring system linearly
scaled at 5V. 5V means 100%
charge, 0V means 0% charge.
4.5V = 90%
3. Speed Control
Only allow regen in effective
speed range. (TBD as car is
not fully operational at this
time) Back EMF must be
greater than battery voltage
4. Switch Circuit
Allow three ranges of regen,
off, low & high
Verification
Measure current when the car is
accelerating, when regen is applied, the
meter should read a negative current with a
magnitude of 10 amps or less.
Points
50
Apply above 90% voltage to the controller
signal and verify that regen is not allowed.
Apply below 90 % voltage to the controller
signal and verify regen is working
15
At 89% charge, apply excessive regen
charging and verify the controller will shut
regen off at 90% charge.
At speeds with a back EMF lower than 300V
or speeds with back EMF greater than 340V,
disengage regen, verify by driving
characteristics.
With no regen, output of circuit will read 0V
Low regen, output = 5V
High regen, output = 10V
25
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3.2 Tolerance Analysis
The component that has the most ability to affect this design is the hall sensor. The tolerance
analysis will need to be able to identify the range of distance between the hall sensor and the
magnet on the wheel. Since the hall sensor output depends on the proximity of the magnet, we
will have to test several differences in distance in order to establish a position. The data collected
from these test will be compared to the speed read via radar gun.
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We will also have to test the strength of the magnet. The strength of the magnet will need to be
strong enough to affect the hall sensor as it passes by at a high speed, but not high enough to
affect the sensor too much as it is on the other side of the wheel. We will need to relate the
strength to the output of the hall sensor.
4 Cost Analysis and Schedule
4.1 Cost Analysis
4.1.1 Labor
Total = Hourly rate x
2.5 x Total Hours
$14,656.25
$14,656.25
$29,312.50
Engineer
Hourly Rate
Total Hours
Josh Seibert
Nick Meinhart
Total
$33.50
$33.50
$67.00
175
175
350
Part Number
74LS21
S14100400AM1314
55100-3M-03-A
LT1017CN8
STP16NF06
POT3106W-1-504K
ECE Shop
N/A
LVC-12V5-3A
N/A
Quantity
1
1
1
2
1
1
1
18in x 24in x .093in
1
1
4.1.2 Parts
Parts List
Item
AND Gate
3 Toggle Switch
Hall Sensors
Comparator chip
MOSFET
Variable 500k Resistor
Resistor Pack
Plexiglas
12/5 Volt Converter
MISC
Total
Unit Price
$0.88
$5.66
$10.77
$2.42
$0.88
$0.79
N/A
$10.98
$9.95
$30.00
4.1.3 Total
Total Cost
Labor
Parts
Total
$29,312.50
$74.75
$29,387.25
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Total Price
$0.88
$5.66
$10.77
$4.84
$0.88
$0.79
N/A
$10.98
$9.95
$30.00
$74.75
4.2 Schedule
Week
9/14
9/21
9/28
10/5
10/12
10/19
10/26
11/2
11/9
11/16
11/23
11/30
12/7
Task
Write up proposal
Review proposal
Design Controller/Eagle Assignment
Design Capacitor Bank/Eagle Assignment
Design Review/Lab Safety
Schedule time with TA/Design Review/Lab Safety
Review Design /Solder assignment
Review Design/Solder Assignment
Assemble/Test Controller
Safety Manual & Assemble/Test Controller
Test Controller & Review Safety Manual
Test Controller & Construct Protective Case
Test components on IFE vehicle/Progress Report
Test Components on IFE vehicle/Progress Report
Fine Tune system/Prepare Demo
Fine Tune system/Schedule demo with TA
Prepare for Demo
Prepare for Demo
Review Demo Results
Review Demo Results
Write Final Paper
Write Final Paper
Prepare Presentation/Review Final Paper
Prepare Presentation/Review Final Paper
Turn in lab notebook/Lab Checkout
Turn in lab notebook/Lab Checkout
Responsibility
All
All
Josh
Nick
Josh
Nick
Josh
Nick
Josh
Nick
Josh
Nick
Josh
Nick
Josh
Nick
Josh
Nick
Josh
Nick
Josh
Nick
Josh
Nick
Josh
Nick
5 Ethics & Safety
5.1 Ethics
This project plans to abide by IEEE ethics. Most notable is the approach that our project is
avoiding injury to the driver, anyone on the Formula Electric team and their property.
Furthermore, the project will improve our understanding of technology and technical
competency. Lastly, our design is assisting colleagues to build a better car and expanding their
professional development.
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5.2 Safety Statement
Implementing a regenerative braking system can be very complex and dangerous if not
constructed properly. It is our duty to design and fabricate the regenerative braking system is a
safe manner for the driver and anyone else that may be working on the car. Many precautions
must be made when dealing with a three phase motor, 300 volt battery packs and reversed
current flow.
Our design will reverse the current to travel from the motor through the drive before reaching
and charging the battery. This will mean educating everyone on the Illini Formula Electric team
about our added design and the possible dangers associated with it. Educating will be done with
safety meetings and a physical safety manual that will be provided to the team.
The components implemented in our design will need to be reliable and protected from any
conditions that may arise during a race. This will require the components be protected from
moisture, rocks and other debris that could cause damage. Therefore, appropriate components
will be fitted with an all-weather protective casing.
6 References
[1] Sevcon Gen 4 size 8 manual
[2] LT1017 Comparator Datasheet
http://cds.linear.com/docs/en/datasheet/10178ff.pdf
[3] 12/5 Volt Converter Part Listing
https://www.superbrightleds.com/moreinfo/bar-strip-accessories/12vdc-to-5vdc-voltageconverter-/1549/3652/?utm_source=googlebase&utm_medium=base&utm_content=LVC-12V53A&utm_campaign=GoogleBaseChild&gclid=CjwKEAjw7aiwBRCPgdu70arX70wSJADK6iD
DhPaNokafmc-LRGitIiy6Kvk4lIc59Ra7hdhW0A4N6xoC7F3w_wcB
[4] AND gate Datasheet
http://www.skot9000.com/ttl/datasheets/21.pdf
[5] MOSFET Datasheet
http://www.st.com/web/en/resource/technical/document/datasheet/CD00002501.pdf
6. Hall Sensor Datasheet
http://www.mouser.com/ds/2/240/55100%20IssueAG-275263.pdf
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