September 17, 2009 by markarndt
A paper recently published at the 2009 SAE World Congressdemonstrates the accuracy and utility of speed data collected by the Powertrain Control Module (PCM) of late model Ford vehicles. Testing described in the paper was completed in conjunction with an evaluation of Electronic Stability Control (ESC) systems supported by Tab Turner of the law firm Turner & Associates.
An instrumented 2005 Ford Explorer was used to evaluate speed data provided from its PCM at high slip angles and other dynamic maneuvers. The slip angle is the angle between the heading of a vehicle and its velocity direction –- a vehicle that is side-slipping or spinning out has a high slip angle.
PCM speed was compared to speed and slip angle collected from a calibrated velocity sensor. In addition to speed, slip angle and other standard handling test measurements the vehicle brake switch and throttle were recorded so PCM data could be synchronized. After each test run the vehicle ignition was turned off and the PCM was downloaded using commercially available Bosch hardware and software. The principal maneuver was the National Highway Traffic Safety Administration (NHTSA) sine-with-dwell test consisting of a 0.7 HZ sinusoidal steer with a 0.5 second dwell at the steer reversal peak.
Runs were conducted with the vehicle’s Electronic Stability Control (ESC) disengaged so that the test vehicle would achieve large slip angles. Other dynamic maneuvers included: NHTSA’s sine-with-dwell with ESC engaged; 100% accelerator to 80 mph with 0.5G braking to stop; and acceleration to 50 mph with maximum ABS braking to stop.
Results demonstrate agreement between the speed recorded by the calibrated instrumentation and speed recorded by the vehicle’s PCM for conditions when the vehicle slip angle and rear wheel slip were near zero. PCM speed was lower than instrumented speed in high slip angle maneuvers. PCM on average underreported during maximum ABS braking and at medium to high speed in 0.5G braking. In acceleration the PCM speed had no detectable under-reporting error except at the highest speeds with 100% accelerator application.