Article: "The accuracy of speed recorded by an SDM...," Reust, T., et al

Collision: The International Compendium for Crash Research

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Volume 1 Issue 1

Accuracy of Speed Recorded by an SDM & the Effects of Brake & Yaw Events Analysis of the GM Sensing & Diagnostic Module in 360 Degree Linear Momentum Collisions: Real Case Analysis CDR Data Presentation & Validation in Legal Proceedings

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Contents june 2006

of interest 2

From the Editor

4

Calendar of Events

6

Organization News

8

Letter From NAPARS

On The Cover Photo By: Scott Baker Taken at the Bremerton Race Track in Washington at the 2002 ARCCSI Crash Conference. This was approximately a 52 MPH collision.

Trivia: This photo was used as the framework for the ARC-CSI logo.

features 9

The Accuracy of Speed Recorded by an SDM and the Effects of Brake, YAW and other Factors

17

Analysis of the GM Sensing and Diagnostic Module in 360 Degree Linear Momentum Collisions

43

CDR Presentation and Validation in Legal Proceedings

53

A Review of Various ACM Module Types and Data Recorded

56

Rational Legislative and Organizational Policy for Automotive Event Data Recorders

69

Practical Ascpects of CDR Using the Vetronix System

82

Motorcycle Collinear Collisions Involving Motor Vehicles Equipped with EDRs

case study 28

Crash Data Case Study

97

Case Study Solution

28

The Accuracy of Speed Recorded by an SDM and the Effects of Brake, Yaw and Other Factors -Timothy J. Reust, James M. Morgan Accident Science, Inc.

to evaluate the event.To date, only a limited study of the SDM-reported Many 1999 and newer speed accuracy has been reported by General Motor vehicles Lawrence. have an event data recorder (EDR) that can record data which NTRODUCTION includes pre-event speed. The EDR is incorporated into the air bag Late model General Motors system sensing and diagnostic module vehicles incorporate an (SDM). In this study, the accuracy of event data recorder with the vehicle-reported speed was tested the sensing and diagnostic module. during acceleration, coasting, braking The SDM can record pre-event and yaw at speeds between 1 to 90 speeds at one-second intervals for miles per hour. The accuracy was approximately five seconds preceding also tested during heavy acceleration, the event. The speed is in units of when an undersized spare tire was miles per hour and is recorded in 1 mounted on the drive axle and with mile per hour increments. Other tires having new and worn tire tread information recorded by the SDM depths. The vehicle-reported speed includes, engine RPM, throttle was compared to the speed measured percent and brake application. The by a calibrated GPS speed measuring recorded event can be triggered by system. The results showed that the air bag deployment or a deceleration. vehicle-reported speed matched well The vehicles used for this study were during acceleration and coasting, manufactured by General Motors and underestimated the speed during included both front wheel drive and braking and underestimated the speed rear wheel drive. This information during a yaw event. Information can be downloaded using the Vetronix downloaded from a SDM after Crash Data Retrieval (CDR) kit. an accident event can be used in conjunction with conventional The current trend in accident methods of accident reconstruction reconstruction is to make use of the

A

BSTRACT

I

available electronic data. The data can make a significant contribution to the engineering reconstruction of a vehicle accident. In some cases, the SDM data can provide information that traditional methods cannot.

This paper addresses the accuracy of the SDM-reported speed and the affects of brake, acceleration and yaw events.

M

ETHODS

A series of well-documented tests were conducted with various types of vehicles.

The testing included 12 different vehicles which included both front and rear wheel drives. The vehicle included passenger type, SUV, pickup and minivan. Some of the vehicles were equipped with the StabiliTrak electronic stability control system. All the testing was conducted on dry and level roadway surfaces. The tires of the vehicles were the original equipment size and were inflated to their recommended pressures. More than 80 instrumented tests were conducted for this research paper.

Some of the testing was done by slowly accelerating the vehicle up to speed, others with maximum acceleration and Collision

some also included coasting and was done for the purpose of checking the accuracy of the vehicle-reported speed versus an instrumented speed.

The brake testing speeds ranged from 15 to 90 miles per hour. The majority of the brake tests were conducted with the ABS brake system active, however some were with the system deactivated. Most of the brake tests were conducted with maximum brake application and some tests were done within the normal range of brake application. The yaw testing was conducted with and without brake application. A few tests were conducted with heavy acceleration induced wheel slip. Testing was also conducted with an undersized spare tire which was obtained from a vehicle and mounted to its drive axle.

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ehicles

The following vehicles were used during the testing:

A. 2001 Buick Regal B. 2002 Chevrolet Venture.

C. 2002 Chevrolet Camaro SS (manual transmission) D. 2004 Pontiac Bonneville E. 2005 GMC Yukon XL (with StabiliTrak) F. 2005 Chevrolet Suburban (with StabiliTrak) G. 2005 GMC Sierra Z71 H. 2005 Pontiac Bonneville I. 2005 Pontiac Bonneville J. 2005 Chevrolet Trailblazer K. 2005 Buick Le Sabre L. 2005 Cadillac Deville

I

NSTRUMENTATION

Vehicle speed was measured using a Racelogic VBox III GPS system .The VBox system has a speed accuracy of 0.063 mph averaged over four samples.

Collision 10

The Racelogic (IMU) inertial measurement unit was used during the yaw tests to measure yaw rate and lateral acceleration. The VBox also recorded brake and throttle data. The initiation of brake pedal application was recorded by placing a tape switch on the surface of the brake pedal.

Throttle sensor position information was measured by placing wire taps onto the leads of the vehicle’s throttle position sensor

and provided a relatively smooth data line. The difference between the VBox speeds at both the beginning and end of the step was used as the vehicle-reported speed error. (Figure 1) For the brake tests and yaw tests a percent error was calculated between the VBox speed (actual speed) and the Tech 2 (vehicle-reported speed).

The tests of the tires with worn and new tread depth were compared and used to show the variation in speed accuracy.

The instrumentation signals were ESULTS recorded by the VBox system. The vehicle-reported speed and additional The low acceleration tests engine and running gear data were provided data at speeds recorded as a snapshot through the between 1 to 76 miles per vehicles’ data link connector using hour and were combined together in a the Vetronix Tech 2. common graph. Based on this testing,

R

Each test produced two data sets: the instrumentation data (VBox) and the snapshot data (Tech 2). The data acquisition system of the VBox records 100 samples per second. The Tech 2 recorded a snapshot with approximately 3 to 10 samples per second

D

ATA REDUCTION

The throttle sensor data from both the VBox data acquisition and the Tech 2 snapshot were used as the common trace to synchronize the two data sets together.

The VBox speed was used to be the actual speed of the vehicle. After the recorded data was synchronized, the VBox speed and the snapshot speed from each test were evaluated.

For the low acceleration, coasting and tire tread depth tests, the vehicle-reported speed provided several data points at one speed and then would “step” to the next mile per hour value. During that time the VBox speed was constantly changing

the speed accuracy is ±1.5 mile per hour during low acceleration. (Figure 2)

The coasting tests were conducted at speeds between 24 to 70 miles per hour and were combined in a common graph.

Based on this testing, the speed accuracy is +0.5 to -2.0 miles per hour during coasting. (Figure 3) The maximum acceleration tests were conducted at full throttle (100%) using the 2002 Chevrolet Camaro SS (rear drive with 5.7 liter LS1 engine) and the 2005 Pontiac Bonneville (front drive with 3.8 liter engine). The results show that speed errors are generally within the ±1.5 mile per hour except during initial takeoff and gear shifts. The graphs show the maximum error at each speed.

(Figures 4 and 5) The maximum effort brake with ABS tests showed the vehicle-reported speed is underreported by approximately 8 to 18 percent at vehicle-reported speeds of 30 miles per hour and higher. The exception to this upper range was the pickup which had an error of 25 % which would occur briefly at approximately 0.28 seconds after brake application. A common trend

A chart was prepared to show the low level deceleration rate versus the percent that the vehicle-reported speed is underreported. These results are for speeds of 30 miles per hour and higher.

(Figure 12) During maximum and rapid brake application with a non-ABS system, the vehicle-reported speed will drop to zero approximately 0.72 to 1.07 seconds after the beginning of brake application.

(Figure 13 and 14)

The yaw tests when only steering was used showed the vehicle-reported speed underreported by approximately 2 to 7 percent at speeds of 30 miles per hour and higher. (Figures 15 and 16)

of the 2002 Venture (front wheel drive) van resulted in an over reported vehiclereported speed. The standard size tire was a P215/70R15 with a measured circumference of 83.75 inches. The spare tire was a T125/70R15 with a measured circumference of 69.125 inches. The following graph shows the relationship to the speed values. (Figure 20)

Yaw tests involving both steering and braking showed the vehicle-reported speed underreported by approximately 3 to 18 percent at speeds of 30 miles per hour and higher.

(Figures 17 and 18)

Speed Error Coasting

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The test with heavy acceleration induced wheel slip showed a much higher vehiclereported speed than the VBox measured speed. (Figure 19)

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was seen at vehicle-reported speed below 20 miles per hour where the percent error would increase to a much higher value. (Figures 6, 7, 8, and 9) The lower level brake tests did not activate the ABS system but again showed an underreporting of the vehicle-reported speed. (Figures 10 and 11)

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Figure 5 Collision 11

The following formula was developed to correct speed downloaded from an SDM when an undersized tire has been found on the vehicle. (Formula 1) C=circumference Download speed x ((Cspare + Cstock)/2)/ Cstock = correct speed

(Formula 1) Testing with tires having new and worn tire tread depths resulted in only a slight difference in the speed error. Worn tires will cause a slightly higher speed to be reported. The results show that the vehicle-reported speeds are still within the ±1.5 mile per hour error range. The new and worn tires were both Yokohama AVS ES100 P275/40ZR17 98W. The new tire had a tread depth of 10/32 inch and the worn tire had a tread depth of 2/32 inch. The results are shown near the speed of 45 miles per hour because tires are calibrated at that speed (see SAE J966 Recommended Practice). (Figure 21)

D

ISCUSSION

The vehicle-reported speed is calculated by the vehicle system using the input from the vehicle speed sensor (VSS) and programmed settings for pulses per distance and time. The VSS is located on the output shaft of the transmission.

The vehicle speed that is read by the VSS is a function of the wheel speed and may differ from the ground speed.

An example of differing speed is during a hard brake application and when the ABS brake system is activated causing the wheel speed will be less than the ground speed due to wheel slip.

The speed sensor used for the Tech 2 speed is the VSS. The speed sensor used for the SDM downloaded speed is also the VSS. The synchronized data from the VBox and the Tech 2 show good timing agreement with each other in that they increase and decrease in unison.

Throughout this paper, the Tech 2 speed has been used interchangeably as the vehicle-reported speed.

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Graphs were prepared by combining the data from the VBox, Tech 2 and CDR download. The graphs show that there is a good agreement between the Tech 2 and CDR in terms of speed when a slight adjustment to the one second time intervals of the CDR data is made. Based on these tests, the Tech 2 speed appears to be a good representative of the SDM download speed. (Figures 22 and 23)

C

ONCLUSION

Based on the tests conducted for this paper, the accuracy of the vehicle-reported speed during low acceleration is approximately ±1.5 mile per hour at speeds ranging from 1 to 76 miles per hour. During maximum acceleration at full throttle, the accuracy is approximately ±1.5 mile per hour except during initial takeoff and gear shifts at speeds ranging from 1 to 89 miles per hour. During coasting, the accuracy is approximately +0.5 to -2.0 miles per hour.

A significant finding of this paper is showing how the ABS brake system operation will cause the vehicle-reported speed to be underreported and how that value is further affected as it reaches lower speeds.

During maximum brake application and when the ABS system is activated, the vehicle-reported speed is underreported by approximately 8 to 18 percent at speeds of 30 miles per hour and higher.

The exception was the pickup with an error of 25% which would occur briefly at approximately 0.28 seconds after brake application.

vehicle-reported speed will drop to zero approximately 0.72 to 1.07 seconds after brake application.

When a vehicle is in a yaw the vehiclereported speed will be underreported by approximately 2 to 7 % when only steering has been used and will be underreport by approximately 3 to 18 % when both steering and braking have been used at speeds of 30 miles per hour and higher.

Heavy acceleration induced wheel slip conditions will cause the vehicle-reported speed to be over reported.

An undersized spare tire mounted on a drive axle of a front wheel drive vehicle will cause the vehicle-reported speed to be over reported. A method to correct a downloaded speed is given in the discussion section of this paper.

Tread wear on a tire results in only a slight difference in the vehicle-reported speed error. The results show that with worn tires the vehicle-reported speeds are still within the ±1.5 mile per hour error range.

CKNOWLEDGMENTS

A

The authors would like to thank Brad Muir for his help and encouragement and Loretta Reust

for her support.

R

eferences

Lawrence JM, Wilkinson CC, Heinrichs BE and Siegmund GP. “The accuracy of preDuring normal levels of brake application crash speed captured by event data where the ABS is not activated the recorders” SAE 2003-01-0889, 2003. vehicle-reported speed is underreported.

This trend is shown in (Figure 12).

During maximum and rapid brake application with a non-ABS system, the

Reust TJ. “The Accuracy of Speed Captured by Commercial Vehicle Event Data Recorders” SAE 2004-01-1199, 2004.

Reust TJ, Morgan JM. “Commercial Vehicle Event Data Recorders and the Effect of ABS Brakes During Maximum Brake Application” SAE 2006-01-1129, 2006.

2005 Pontiac Bonneville bon16 ABS Brake, 0.828g Brake Application at 24.73

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Collision 13

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Figure 12 AZ4 2004 Pontiac Bonneville Non ABS, 0.774 g Brake Application at 9.85 sec

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Figure 18 Collision 14

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Figure 23 Collision 15