Part 4: Data Accuracy, Limitations, and Validation
Event information consists of discrete and variable data. Discrete data includes: brake switch status, manual passenger airbag cutoff switch position, and the driver seat belt switch status. Variable data includes: the analog acceleration information from which ΔV is computed, vehicle speed, engine RPM, and throttle position. Table 2 shows the accuracy and resolution for the variable-type parameters recorded for the 1999 SDM.
|
Parameter |
Full Scale |
Resolution |
Accuracy |
How Measured |
When Updated |
|
ΔV |
+ 55.9 mph |
0.4 mph |
~ + 10% |
integrated acceleration |
recorded every 10 msec, calculated every 1.25 msec. |
|
Vehicle speed |
158.4 mph |
0.6 mph |
+ 4 % |
Magnetic pickup |
vehicle speed changes by > 0.1 mph |
|
Engine Speed |
16383 RPM |
1/4 RPM |
+ 1 RPM |
Magnetic pickup |
RPM changes by > 32 RPM. |
|
Throttle Position |
100% Wide open throttle |
0.4 % |
+ 5% |
Rotary potentiometer |
Throttle position changes by > 5%. |
Table 2: Accuracy and Resolution of Data Recorded
There are three main sources of error in estimating ΔV. One error comes from the tolerance of the components in the SDM and the microcontroller. The hardware elements include the accelerometer, the analog-to-digital converter (ADC), low pass filter, and signal conditioning. The accelerometer and ADC contribute the largest portion of the total system error. Accelerometer accuracy is about 8% of full scale which equates to a ΔV error of + 4.5 mph. ADC error is about 0.25 gs, not including quantization noise. Over a 150 msec recording period, the ADC contributes a maximum error of + 0.8 mph.
The second ΔV error is due to integer-based arithmetic and representing ΔV using single data bytes. For a 56 mph full-scale value, 7 bits (plus a sign bit) equates to a precision of 0.438 mph.
The third error source, which applies only to 1999 model vehicles, results from the crash-sensing algorithm continuously applying a 1g bias acceleration in the opposite direction to that seen in frontal impacts. This bias prevents inadvertent airbag deployments resulting from ΔV accumulation when driving on rough roads and contributes an underestimation error of 3.3 mph at the end of 150 msec. GM is in the process of updating its software to eliminate this error source. In the meantime, the downloading tool will utilize software to compensate for the bias.
In the worst case, the total error in ΔV is 5.7 mph (4.5 + 0.8 + 0.4) for a full-scale reading of 56 mph. The RMS error, assuming independent error sources, is approximately 1.53 mph.
Another less predictable error comes from the potential for losing electrical power during the crash. While the SDM maintains the defacto industry standard energy reserve for airbag deployment, the reserve is insufficient to guarantee that all event data will be recorded in every crash. However, if it is not recorded, the SDM indicates this condition in the data record. A power loss during a crash generally will not affect data that was previously recorded.
Retrieving Event Data from GM Vehicles
Initially, General Motors used a proprietary Event Data Retrieval Unit (EDRU) that interfaced with a standard Tech 1 scan tool to download data through the vehicle diagnostic connector. Data could be viewed on the Tech 1 or printed from the EDRU's printer, and all data is displayed in a hexadecimal format. For vehicles that have sustained electrical system damage, interface cables are provided for powering the system and connecting the SDM directly to the EDRU (see Figure 3).
Figure 3: GM Event Data Retrieval Unit
In 1998, General Motors licensed manufacturing rights to the Vetronix Corporation to build a data retrieval tool for the Sensing and Diagnostic Module (SDM) - based EDR. In early 2000 Vetronix Corporation began selling its Crash Data Retrieval (CDR) system. The system allows the user to connect directly between a notebook computer and many General Motors vehicles equipped with an SDM. The connection can be made between the vehicle's diagnostic connector, typically located below the steering wheel or directly to the SDM in those situations when the vehicle's electrical system has been damaged in the crash (See Figure 4). The Vetronix Crash Data Retrieval (CDR) system consists of hardware and software that downloads pre- and post-crash data from the vehicle's airbag module (SDM) to a laptop computer. The Windows® based CDR software presents this data in easy-to-read graphs and tables.
The CDR system can be purchased directly from Vetronix by calling (800) 321-4889. The current cost of the system is $2,495.00. The CDR system contains: a 6' extension cable; AC/DC 12V power supply; airbag module; interface cables (2); cigarette lighter power cable; crash data retrieval module; PC interface cable; vehicle interface cable; Windows® 95/98 based software CD including help files / manual and a storage case for the entire kit.
All SDM recorded data is measured, calculated, and stored internally, except for the following: vehicle speed, engine speed, and percent throttle data is transmitted, once a second by the powertrain control module (PCM) via the Class 2 data link, to the SDM. Brake switch circuit status data is transmitted, once a second by either the ABS module or the PCM via the Class 2 data link, to the SDM. Depending on vehicle option content, the brake switch circuit status data may not be available. In most cases, the driver's belt switch circuit is wired directly to the SDM. In some vehicles, the driver's belt switch circuit status data is transmitted from the Body Control Module (BCM), via the Class 2 data link, to the SDM. The passenger front airbag suppression switch circuit is wired directly to the SDM.
