Automotive Black Box Data Recovery Systems
By Don Gilman
For years, airplane crash investigators have had the benefit of retrieving data from the flight-data recorder, or “black box.” This data has proven invaluable for helping to determine what happened in the seconds before a crash. Now, in order to improve vehicle safety, General Motors is using similar technology in about 40% of its Model Year 1999 vehicles.
Background- Evolving Toward the Current State of the Automotive Art
In the 1970s, electronic sensors gained wide use in production automobiles. This proliferation was largely driven by the industry’s move toward electronically controlled fuel injected engines. The computer, or Engine Control Module (ECM), used sensors to gather information about the current state of engine operation. Though it differed by manufacturer, sensors typically gathered information about Throttle Position, Engine RPM and Airflow. The ECM analyzed the information gathered from the sensors. Then, based on programming in the ECM, instructions were sent to actuators that could vary the length of time a fuel injector pulsed, or specify the amount of spark advance the engine received. These electronic systems were much more efficient than their mechanical predecessors and contributed to significant gains in fuel economy.
Efficiency notwithstanding, many mechanics found these new systems difficult to service. Enter onboard diagnostics. Onboard diagnostics increased the capabilities of the ECM and allowed it to check its well being as well as the condition of its associated sensors. Coupled with this diagnostic capability was a feature that allowed the ECM to store problems that it detected. This ability to store information aided the repair technician, and formed the foundation for the current data
As vehicles became more sophisticated, new electronic systems found their way into the automobile. Each of these systems required new sensors. Acceleration sensors were required for airbag modules. Wheel Speed sensors were required for ABS and Traction Control. Vehicle Yaw Rate sensors were necessary for Stability Control. Along with the new processors and sensors came on board diagnostics, and sometimes a little more. Just as the ECM could store problems, each new module also stored faults that were discovered in that particular system. For example, not only could the Airbag module store a fault, it could (and some do) count the number of times the engine had been started since the fault was generated. This was yet one more step toward storing data andthe idea of data recovery and a “black box.”
Data Recovery Systems (Black Boxes)
In the early 1970s, the National Transportation Safety Board (NTSB) made the recommendation that vehicle manufacturers and the National Highway Traffic Safety Administration work together to gather information on vehicle crashes using on-board collision sensing and recording devices. As a result, General Motors airbag equipped production vehicles have recorded data for impacts that caused a deployment of the airbag since 1974. Many of these systems also recorded data during
impacts that were not severe enough to actually deploy the airbag (“near-deployment” events).
The capability to record pre-crash data originated with some 1999 GM vehicles. While the preceding introduction has been somewhat generic, the remainder of this article will focus specifically on the late model GM vehicles that are equipped with such systems. It is important to note that the extensive use of onboard sensors has made data recording possible. Conceptually,
retaining the data is just a matter of adding memory and the software necessary to sample and capture the existing sensor outputs.
Airbag equipped vehicles use a crash sensing algorithm to decide when to deploy the airbags. The deployment criteria is based on various calibration data stored in the Sensing and Diagnostic Module (SDM). These criteria reflect that particular vehicle model’s response to a wide variety of impact conditions. It is a predictive algorithm, typically making deployment decisions within 15-50 msec after impact. The SDM algorithm determines not only when to fire the airbag, but also helps to determine when to record the pre-crash data.
The amount of data retained for each crash event is limited by available memory. The combination of sampling rate and memory are insufficient for the SDM to record the actual crash deceleration data. However, the crash pulse can be reasonably well represented by the low-frequency velocity change data, (the information of interest to crash reconstructionists typically does not exceed 60 Hz). The SDM calculates the change in velocity by integrating the average of four 312-microsecond
acceleration samples and stores them at 10 msec increments in RAM. Figure 1 shows the delta velocity values for a fairly high severity crash. The data points represent each 10-msec point with a smooth curve drawn through them.
Late model GM vehicles also contain several other sensors which provide information such as driver seat belt status. If there is a deployment or near-deployment event, the last five seconds of data immediately prior to enabling the algorithm are stored in the module’s memory (EEPROM). All of this stored data is available to the accident reconstructionist by using the proper software, interface hardware, and a PC.