Example: Electronic Stability Control (ESC)

The National Highway Traffic Safety Administration (NHTSA) has established FMVSS 126, a new standard that will require all vehicles 4,536 Kg (10,000 pounds) or less to include an Electronic Stability Control (ESC) system as standard equipment starting in 2012 (phase in starting in 2010). Details for FMVSS are online here.

A technical memo describes how the series of tests in FMVSS 126 can be set up in CarSim to enable a one-click run option to obtain a quick idea of how the simulated system would perform in the FMVSS 126 test. Click here to view the tech memo (13-page PDF file).

An example dataset is also available in CarSim 8.

Summary of the FMVSS 126 Test

After completion of a number of control checking and conditioning tests for the brakes and tires, FMVSS 126 requires three sets of tests. First is a series of "Slowly Increasing Steer Tests" to determine the average steering wheel angle associated with a lateral acceleration of the vehicle mass center at 0.3 g. That steering wheel angle is designated A and is used to define and evaluate tests that follow.

Next, two series of "Sine with Dwell" tests are conducted with "a steering pattern of a sine wave at 0.7 Hz frequency with a 500 ms delay beginning at the second peak amplitude" (see Figure 1). The sine amplitude "delta" is increased for each test by an increment of 0.5•A until the final run. The final run in a series is reached when the "delta" is greater than 270 deg. (Also, if the amplitude delta is greater than 300°, then the amplitude for the final run is reduced to 300 deg).

The success or failure of each test in the series is based on the vehicle state at up to three times during the run. The logic is shown in Figure 2 and described below.

1. If the Gain is 5.0 or greater, then a lateral displacement check is made 1.07 sec after the start of the Sine with Dwell. The lateral displacement of the vehicle mass center must be 1.83 m (6 ft) or greater relative to the start of the test for vehicles with GVW of 3,500 kg or less. If the displacement is less, then the vehicle fails the test. For vehicles with GVW greater than 3,500 kg, the required lateral displacement is 1.52 m (5 ft).

2. A peak yaw rate is obtained for the test (see Figure 1). The instant yaw rate 1.0 sec after the steering stops is compared to the peak rate, and must be 35% of the peak rate or less. If the instant yaw rate is higher than 35% of the peak yaw rate, then the vehicle fails the test.

3. The instant yaw rate 1.75 sec after the steering stops is compared to the peak rate, and must be 20% of the peak rate or less (S5.2.2, see Fig. 1). If the instant yaw rate is higher than 20% of the peak yaw rate, then the vehicle fails the test.

If the vehicle is tested to the final run in each direction and does not fail, then the vehicle passes the test.

A Quick Simulation of FMVSS 126 in CarSim

Perhaps the simplest way to define the sequences of test conditions in CarSim is to use events and VehicleSim (VS) commands to set up a single run the covers the entire sequence. This way, a link can be made to an Procedures dataset from the Run screen. Details for the VS commands and sequence of events are provided in the FMVSS 126 tech memo (a PDF file) and in the notes for the datasets used to define the procedure.

The test is demonstrated below using the F3 motorsports vehicle included as an example in CarSim 7, modified to use a less aggressive steering gear ratio. With the high-performance tires it passes the test even without an ESC system. The figure below shows five plots made from the test. The three on the left side show the lateral position of the vehicle CG over time, the values of SWA_Amp used for each of the Sine with Dwell tests, and the absolute peak yaw rate recorded for each test. The two plots on the right show steering wheel angle and yaw rate plotted against the local time (reset to zero at the start of each Sine with Dwell waveform).

Figure 4 shows the steering and yaw rate response for a series of tests of a sedan without ESC, that fails when the yaw rate at 2.93 sec (1.0 sec after the steering input stops) is not less than 35% of the peak.

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