Download Reducing Conducted Transients in Automotive Windsheild Wiper

Reducing Conducted Transients
in Automotive Windshield Wiper
Motors
Robert Langdorf, Shuvra Das, Mohan Krishnan
University of Detroit Mercy
Project Objectives
Study the causes of conducted transients and develop a
low-cost design solution to reduce them
Apply knowledge and skills obtained during other
university coursework
Gain additional understanding of automotive motors
and their electrical/mechanical interrelationships
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SAE 2006-01-0297
Problem Description
When an electric motor is switched off, a large amount of
energy (measured as a negative voltage) can be emitted
to the main power net and can often be damaging to
other devices.
For current design motor, transient emissions of >200V are
possible. Customers desire no more than 100V (even
less for some customers).
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Design Considerations
Cost (there is already a very costly solution
using varistors)
Packaging/Space Constraints
 Use
of standard components
Effects on Other Electrical Requirements
 Other
Conducted Emissions (radio interference)
 Conducted Immunity
 Radiated Emissions
Effect on Motor Performance
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Problem-Solving Approach
1.
2.
3.
4.
5.
Create a working circuit model
Perform some hand calculations on the 2ndorder system
Perform PSPICE simulation
Apply DOE principles to find optimum
solutions using PSPICE, Minitab & Excel
Build and test physical samples to validate
results
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Background Information
The current design:
Inductors
Printed Wiring Board
Capacitors
Terminal Connections to Cover Assembly
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Background Information
The active components during a “switch-off” function are:
– Two 0.47 mF Capacitors
– Two 5 mH Inductor Coils
– Motor (including inherent induction properties)
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Background Information
The circuit used for simulation and analysis:
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Assumptions
The high speed part of the circuit was neglected - there is
no current flowing through it.
Relay was assumed to have a switching time of 0.5ms.
(Ford spec is <1ms)
Motor armature inductance was measured at approximately
970 mH.
Motor resistance, including armature and brushes was
measured at approximately 0.5W, but was assumed
lower due to magnetic effects.
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Assumptions, cont..
The rotational load on the motor (~10Nm) was accounted
for with a 25W resistance from motor ground to source
ground.

There are 2 different grounds in the system
Line resistance was assumed to be 1.25W between each
side of the power source and the motor brush card
terminals.

Note: these two assumptions were derived empirically by
changing values until a solution was found that approximates the
result of a typical experiment.
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Comparison of solution to test result
Production part test result:
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Comparison of solution to test result
PSPICE Result:
200V
0V
-200V
-400V
0s
0.2ms
0.4ms
0.6ms
0.8ms
1.0ms
V(Vout)
Time
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Comparison of solution to test result
The previous voltage responses exhibit:
 Voltage
peaks of similar magnitude
 Similar dampening characteristics
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Ground-to-ground issue
For a production motor, the motor ground to source ground
was captured:
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Ground-to-ground issue
The PSPICE model produces a similar result:
80V
40V
0V
-40V
-80V
0s
0.5ms
1.0ms
1.5ms
2.0ms
- V(Vmg)
Time
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Hand Calculations
Hand calculations were done using the same
model as used in PSPICE.
The following calculation is done to find the
approximate magnitude of the negative
transient spike
Finding the decay takes considerably more
calculation
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Steady State Solution
Current through motor at t=0 is 4.737A
vc1 = 7.588V, vc2 = 5.921V
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Initial conditions
di/dt = 9.184 A/s at t = 0+
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2nd Order Differential Equation
The following equation can be derived:
d 2iR Rm diR
1
 26.25W diL


iR 
2
dt
L dt LC
L
dt
The following parameters can be
calculated:
R

 178.57
2L
0 
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1
 46594.9
LC
SAE 2006-01-0297
2nd Order Differential Equation
The response is underdamped and the natural
frequency can be expressed as:
d  02   2  46954.6
The natural response can be expressed
as:
in (t )  e t  A cos  d t  B sin  d t 
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2nd Order Differential Equation
The forced response, which will be neglected for now, is
expressed as:
 26.25W diL
i f (t ) 
L
dt
 This is neglected because I do not have an expression for
iL related to iR
The parameters A & B in the natural response
equation are calculated by applying the initial
conditions:
A  4.737
B  0.018
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2nd Order Differential Equation
The expression for current with all of the constants
applied becomes:
iR (t )  e 178.57t 4.737 cos 46594.6t  0.018 sin 46594.6t 
The expression for voltage across the capacitor C1
becomes:
t
1
vC1 (t )    iR (t )dt   e178.57t 216.76 sin 46594.6t  1.65 cos 46594.6t 
C0
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2nd Order Differential Equation
Solution
Plot of voltage across C1 versus time:
Voltage across capacitor based on hand calculation
250
200
150
Voltage (V)
100
50
0
-50
-100
-150
-200
-214.2 V @ 32ms
-250
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
Time (s)
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Simulation Result
PSPICE Result:
V = -219.2 V @ t = 23.5 ms
200V
0V
-200V
-400V
0s
0.2ms
0.4ms
0.6ms
0.8ms
1.0ms
V(Vout)
Time
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Experimental Design
Comment on inductors, L1 & L2:
 Changing
the values of the external inductors has very
minimal effect on the transient solution. Inductors in
series simply add and these 5mH coils are negligible
compared to the 970mH motor inductance.
 These coils only will significantly effect the RFI
filtering.
 For the purpose of these experiments, the coils will be
left unchanged.
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Experimental Design
Using PSPICE & Minitab, a DOE was
performed, modifying only the values of
the capacitors, C1 & C2.
Each capacitor was simulated at 5 levels:
 0.047mF,
0.1mF, 0.47mF, 1mF, 4.7mF
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Experimental Design
Using Minitab’s response surface feature,
regression equations were formulated to
help solve for the expected minimum and
maximum peak voltages
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Main Effect Plots
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Main Effect Plots
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Interaction Plots
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Interaction Plots
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Regression Equations
Minimum voltage peak:
V  561.4  601.1C1  106.1C
2
1
Maximum voltage peak:
V  435.1  605.8C1  106.9C12  145.3C2  24.5C22
 Note:
C2 is insignificant in the min. voltage equation
and the interaction C1xC2 is insignificant in both
equations.
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Regression Solution
This yields as an optimum solution:
= 1.18mF, C2 = 2.97 mF
 Vmin = 0V, Vmax = 84.7V
 C1
When tested in PSPICE, the result is:

Vmin = 145.5V, Vmax = 122.7V
?????
This means there must be some other relationship –
try using the log of the capacitance values
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Log Regression Equations
Minimum voltage peak:
V  144.6  174.0 log C1  171.9 log C12  19.1log C22  18.8 log C1 log C2
Maximum voltage peak:
V  83.5  185.0 log C1  51.5 log C2  174.4 log C12  45.3 log C1 log C2
 Note:
C2 is insignificant in the min. voltage equation
and the C22 is insignificant in both equations.
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Log Regression Solution
This yields as an optimum solution (with
minimum peak-to-peak voltage):
= 3.81 mF, C2 = 4.7 mF
 Vmin = -85.6V, Vmax = 51.8V
 C1
When tested in PSPICE, the result is:

Vmin = -86.6V, Vmax = 65.5V
This is a much better model!!!
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Log Regression Solution
Based on feedback from the supplier, it is not
recommended to pursue use of 4.7mF capacitors due to
the high cost of materials. 3.3mF capacitors are
relatively less expensive.
Using 3.3mF as a limit, the log regression model is reoptimized to yield:


C1 = 3.3 mF, C2 = 3.3 mF
Vmin = -90.4V, Vmax = 49.0V
PSPICE yields:

Vmin = -93.8V, Vmax = 64.4V
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Other Possible Solutions
Several other possible solutions exist to fix the transient spike
problem:



Bridge capacitor (Y-type)
Voltage suppressor
Diode
These devices are placed in the circuit in this location:
Since these are much more capable of fixing the problem than only
capacitors, the capacitance used in conjunction with these items
can be reduced (thereby reducing cost).
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Experimental Design #2
Another designed experiment was run to simulate
the effects of the various solutions:
 No
change
 Bridge capacitor (0.47mF)
 Voltage Suppressor (Vishay TPSMA27A)
 Diode (D1N4184 from PSPICE library)
Each option was run at 3 levels of matched C1 &
C2 (matched may be better to suppress RFI):
 0.47mF,
0.047mF, 4.7nF
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Dotplots of PSPICE Results
Dotplots of Min by C3
(group means are indicated by lines)
-1000
39
D1N4148
TPSMA27a
C3
0.47
-2000
None
Min
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Dotplots of PSPICE Results
Dotplots of Max by C3
(group means are indicated by lines)
1000
40
D1N4148
TPSMA27a
C3
0.47
0
None
Max
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Analysis of Dotplots
It is evident from these plots that one of the
recommended solutions may have a major
impact.
Data means for each solution:
 None
 0.47mF Cap  TPSMA27A  D1N4148 -
min
min
min
min
=
=
=
=
41
-974.2, max = 829.8
-188.4, max = 175.6
-30.1, max = 1.0
-3.7, max = 0.5
SAE 2006-01-0297
PSPICE Result for 0.47mF Bridge
Capacitor
200V
100V
0V
-100V
-200V
0s
0.5ms
1.0ms
1.5ms
2.0ms
V(Vout)
Time
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PSPICE Result for Voltage
Suppressor
10V
0V
-10V
-20V
-30V
0s
0.5ms
1.0ms
1.5ms
2.0ms
V(Vout)
Time
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PSPICE Result for Diode
8.0V
4.0V
0V
-4.0V
0s
0.5ms
1.0ms
1.5ms
2.0ms
V(Vout)
Time
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Motor Build and Test Plan
2 sets of parts have been built and tested:
 Motors
DOE)
with the current capacitors (3x3 full factorial
 C1
= 0.47mF, 1mF, 3.3mF
 C2 = 0.47mF, 1mF, 3.3mF
 Motors
with smaller capacitors and 2 of the voltage
reduction solutions previously mentioned (3x2 full
factorial):
 C1
& C2 = 0.47mF, 0.047mF, 4.7nF
 C3 = 0.47mF bridge capacitor, TPSMA30A Voltage
Suppressor
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Comments on Build Plan
Cost is a serious consideration:
 0.047mF
 0.47mF
 1mF
 3.3mF
 TPSMA30A
 Diode
~ $0.025
~ $0.046
~ $0.092
~ $0.13
~ $0.16
~ too expensive to seriously
consider
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Motor Test Plan
All motors were subjected to CE 410
(conducted emissions)
DOE principles are applied to analyze testing
results
They will also be subjected to CE 420 (RFI
emissions). However, timing did not allow
such testing to be completed during the
scope of this project
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Test Results
Following shows how Minitab outputs analysis
results:
General Linear Model: Min versus C1, C2
Factor
Type
Levels
Values
C1
fixed
3
0.47 1.00 3.30
C2
fixed
3
0.47 1.00 3.30
Analysis of Variance for Min, using Adjusted SS for
Source
DF
Seq SS Adj SS
Adj MS
C1
2
1980.0 1980.0 990.0
C2
2
16958.5 16958.5 8479.3
C1*C2
4
3101.1 3101.1 775.3
Error
18
8050.3 8050.3 447.2
Total
26
30090.0
Tests
F
2.21
18.96
1.73
P
0.138
0.000
0.187
P = 0 translates to virtually 100% confidence that the factor is significant.
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Test Results – Experiment #1
Assumptions of normality, independence of the testing
order, constant variance and independence from
other variables are deemed adequate based on
analysis of residuals.
For the minimum peak voltage:



The value of C1 is ~86% significant
The value of C2 is 100% significant
The interaction is ~81% significant
The effect plots (next slide) show that the optimum
condition is when both capacitors are 3.3 mF, similar
to the simulation results.
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Test Results – Experiment #1
Optimum Settings at C1, C2 = 3.30
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Test Results – Experiment #1
For the maximum peak voltage:
 The
value of C1 is 100% significant
 The value of C2 is ~80% significant
 The interaction is ~99.4% significant
The effect plots (next slide) show that the optimum
condition is when both capacitors are 3.3 mF, similar
to the simulation results.
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Test Results – Experiment #1
Optimum Settings at C1, C2 = 3.30
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Test Results – Experiment #1
Using the optimum settings from this
experiment (C1 & C2 are 3.3 mF):
 The
negative peak is about -110V
 The maximum peak is about +50V
Compared to the current capacitor design:
 Negative
peak ≈ -200V
 Positive peak ≈ +85V
 This is a significant improvement
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Test Results – Experiment #2
Assumptions of normality, independence of the testing
order, constant variance and independence from
other variables are deemed adequate based on
analysis of residuals.
For the minimum peak voltage:



The effect of the matched capacitors is not statistically
significant.
The effect of the suppression device is 100%
The interaction effect is ~93% significant.
Based on the effect plots (next slide), the ideal solution
is the combination of 4.7 nF capacitors and the
voltage suppressor.
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Test Results – Experiment #2
Optimum Settings at C3 = TPSMA30A & C1C2 = 0.0047
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Test Results – Experiment #2
For the maximum peak voltage:
 The
effect of the matched capacitors is ~92%
significant
 The effect of the voltage suppression devices is
100% significant
 The interaction is ~97% significant
The effect plots (next slide), show that the optimum
solution is the combination of 4.7 nF capacitors and
the voltage suppressor, similar to the simulation
results.
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Test Results – Experiment #2
Optimum Settings at C1C2 = 0.0047 & C3 = TPSMA30A
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Test Results – Experiment #2
Based on analysis, the overall optimal solution includes:


Matched Smaller Capacitors ~4.7nF
Vishay Voltage Suppressor (TPSMA30A or similar)
Using these optimum settings:


The negative peak is ~ -90V
The maximum peak is ~ +15V.
Compared to the current capacitor design:



Negative Peak ≈ -200V
Positive peak ≈ +85V
This is a significant improvement, even better than the
optimized design as determined in Experiment #1.
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Conclusion
Cost comparison:
 Optimal
3
x $0.025 + $0.16  $0.235 per motor.
 Current
3
cost
x $0.046  $0.138 per motor
 Varistor

solution
solution
 $0.38 per motor
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Conclusion
Questions
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