Brake Intensity Advisory
System Design
Assessment
Jace Hall
Michael Purvis
Caleb Trotter
Edward Yri
ECE 4007-L01
11/02/2011
Project Details
• What
- BIAS (Brake Intensity Advisory System) operates by
illuminating an auxiliary set of LEDS located around
the perimeter of the standard brake lights when “hard
braking” occurs.
• Cost
- The cost associated with mass production of the BIAS
is projected to be $75.84.
2
Project Motivation
• Why
- The percentage of rear-end collisions (17% of all
claims) ranks second among all automobile accidents.
• Who
- BIAS is intended for drivers and car manufacturers
who are concerned with operating and manufacturing
safer vehicles.
3
BIAS
4
Design Goals
- Simple integration with current braking systems
- Non-intrusive design
- Real-time response
- Software definability based on vehicle brake force
parameters
5
Technical Objectives
- Receive a voltage sensor output between 0-3.3V
corresponding to a defined force.
- Categorize the inputs into three defined states using the
Mbed microcontroller.
- Illuminate the auxiliary LEDS when a
voltage of 1.9V or higher is received by the
microcontroller.
- Achieve a visual indication of braking force in
real time.
6
High Level BIAS Schematic
7
Force Sensing Resistor (FSR) Operation
- The FSR is a polymer
thick film device.
- The output resistance
decreases as the force
applied increases.
- FSRs provide a cost
benefit of two orders
of magnitude over
load cells (another
commonly used force
sensor.)
8
Interpreting FSR Output as Voltage
- The FSR must be connected to a voltage divider circuit to
interpret the applied force as a voltage.
- A voltage regulator must be inserted to convert the input
voltage to 3.3V due to Mbed voltage input constraints.
9
Determining the Fixed Resistor Value
- The fixed resistor value
in the voltage divider
helps determine the
sensitivity of the output.
RF
VOUT =
´VIN
RF + FSR
- Decreasing RF allows
for a more dynamic
range of force values to
be converted to a
corresponding voltage.
300
VOUT =
´ 3.3 = 2.71V
300 + 65
- Test: Dynamic Braking
Resistor Value:
63Ω-2000Ω
10
300
VOUT =
´ 3.3 = .43V
300 + 2000
Confirming the Theoretical Threshold
- The FSR was attached
to a brake pedal and
braking pattern
voltages were
recorded.
- The “hard braking”
threshold was
determined to be any
voltage above 1.9V.
- A custom bracket was
fabricated to ensure
the applied braking
force was absorbed
solely by the FSR.
11
Problems Encountered with FSR
- Loose connections
creating transients.
- Determining the
proper fixed
resistance value.
12
High Level BIAS Schematic - Microcontroller
13
Mbed I/O
14
Why Mbed?
• Extensive libraries for rapid prototyping
• Meets technical requirements of proposal
• Integrated 6 PWM channels
15
Program Overview
State: Normal
No braking force detected.
State:
Hard Braking 2
PWM1 = 80% Dc.
PWM3 = 80% Dc.
LED Arrays 1,2 & 3
on.
Braking
>0
Yes
Yes
Braking
=0
State:
Light Braking
PWM1 = 40% Dc.
LED Array 1 is on.
Delay
0.025 s.
State:
Hard Braking 1
PWM2 = 80% Dc.
LED Arrays 1 & 2 on.
16
Yes
Braking
> 1.9V
High Level BIAS Schematic – LED Array
17
LED Array and Driver Specifications
• LEDs
- Outdoor and automotive rated
- Max luminous intensity: 5500 mcd
- Max junction temp: 130°C
• LED Drivers
- PWM control for dimmable LED output
- Max current : 1 A
- Max input voltage: 30 V
18
LED Array Design
Vin 12V
C1
Vin
R1
GND
LED
SET
Driver
CTL
PWM
Signal
19
SW
Z1
L1
Current Limiting Resistors in LED Array
400
Current Limiting
Resistor Values
350
Resistance (Ω)
300
250
4 LEDs in Series
3 LEDs in Series
2 LEDs in Series
200
150
100
50
0
1
2
3
4
5
# of Parallel LED Strings
20
6
7
8
Brake Light Configuration
21
Brake Light Operation
• Voltage threshold < 1.9 V.
1
22
Brake Light Operation
• Voltage threshold ≥ 1.9 V.
2
1
23
Brake Light Operation
• Voltage threshold ≥ 1.9 V after a 25 ms delay.
3
2
1
24
Budget/Cost Analysis
• Prototype Budget Status
- Of the requested $405, $98.99 has been spent. All
parts have been procured at this time.
• Projected Implementation Cost
- The BIAS prototype includes a more expensive
microcontroller and added voltage source. The cost of
the production model will reflect these deducted costs.
25
Production Cost Break Down
Parts
Mass Production Cost
Prototype Cost
FSR
$16.00
$7.95
Wire/Connectors
$5.50
Donated
Resistors/Voltage
Regulator/Capacitors
$4.95
Donated
Perforated Boards
$5.75
Donated
$10.00
Donated
LEDs
$19.25
$19.25
LED Drivers
$4.50
$4.50
Parts Total
$65.95
$31.70
Contingency (15%)
$9.89
$4.76
$75.84
$36.46
Microcontroller
TOTAL
26
Future Work
27
Task
Deadline
Assemble LED Array
11/4
Program microcontroller
11/16
Solder LED array circuit to perforated board
11/19
Test LED array and microcontroller
11/21
Integrate LED array, microcontroller, and FSR
11/28
Finalize prototype
12/2