Operation
There are many different patents related to various types of EDR features. Some EDRs continuously record data, overwriting the previous few minutes until a crash stops them, and others are activated by crash-like events (such as sudden changes in velocity) and may continue to record until the accident is over, or until the recording time is expired. EDRs may record a wide range of data elements, potentially including whether the brakes were applied, the speed at the time of impact, the steering angle, and whether seat belt circuits were shown as "Buckled" or "Unbuckled" at the time of the crash. Current EDRs store the information internally on an EEPROM until recovered from the module. Some vehicles have communications systems (such as GM's OnStar system) that may transmit some data, such as an alert that the airbags have been deployed, to a remote location.
Most EDRs in automobiles and light trucks are part of the restraint system control module, which senses impact accelerations and determines what restraints (airbags and/or seatbelt tensioners) to deploy. After the deployment (or non-deployment) decisions are made, and if there is still power available, the data are written to memory. The data downloaded from older EDRs usually contain 6 to 8 pages of information, though many newer systems include many more data elements and require more pages, depending on the make/model/year of the vehicle being evaluated. Depending on the type of EDR, it may contain either a deployment file or a non-deployment file or sometimes both, depending on the circumstances of the collisions and the time interval between them, among other things.
It is also possible that no data can be recovered from a data recorder. One situation where this might occur is a catastrophic loss of electrical power early in a collision event. In this situation, the power reserve in the restraint system control module capacitors may be completely spent by the deployment of the air bags, leaving insufficient power to write data to the EEPROM. There are other circumstances where a module may fail to record a data file as well.
Most EDRs in heavy trucks are part of the engine electronic control module (ECM), which controls fuel injection timing and other functions in modern heavy-duty diesel engines. The EDR functions are different for different engine manufacturers, but most recognize engine events such as sudden stops, low oil pressure, or coolant loss. Detroit Diesel, Caterpillar Inc., Mercedes-Benz, Mack Trucks, and Cummins engines are among those that may contain this function. When a fault-related event occurs, the data is written to memory. When an event triggered by a reduction in wheel speed is sensed, the data that is written to memory can include almost two minutes of data about vehicle speed, brake application, clutch application, and cruise control status. The data can be downloaded later using the computer software and cables for the specific engine involved. These software tools often allow monitoring of the driver hours of service, fuel economy, idle time, average travel speeds, and other information related to the maintenance and operation of the vehicle.
Some EDRs only keep track of the car's speed along its length and not the speed going sideways. Analysts generally look at the momentum, energy, and crush damage, and then compare their speed estimates to the number coming out of the EDR to create a complete view of the accident.
Read more about this topic: Event Data Recorder
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