Almost every vehicle on the road today is packed with electronics that offer an array of special features. But the capabilities provided by the electronics do not come without challenges—particularly for test engineers.
Our latest request was to develop a PC-based system that could control and monitor anti-lock braking system (ABS) modules during long-term durability testing.
We found the solution using LabVIEWTM, SCXI, and I/O boards from National Instruments.
The testing consisted of applying specific preprogrammed profiles to the ABS modules. These profiles were actual recorded ABS stops, PC-generated standard stops, and ignition cycles.
Four channels of waveform generation at a rate of 5,000 S/s were used to output these profiles. The simulation cycled through the profiles until the required amount of stops had been applied to the ABS modules. Sixteen channels of data from the modules were collected at 2,000 S/s throughout the test.
This data included wheel speeds, brake pressure, battery voltage, and other system parameters. To record fault codes, serial communications with the modules were maintained throughout the test. The ABS modules were placed in an environmental chamber mounted on a shaker table. With the use of these tools, different climatic variations and road conditions could be simulated.
To perform an accurate test, the system had to be robust enough to make the ABS module believe that it actually was receiving its signals from a vehicle. If the signals were not as the ABS module believed they should be, the module would generate a fault.
LabVIEW software and NI data acquisition hardware were chosen as the platform for the simulation. This design reduced the system costs and development time and provided the tools needed to create an accurate simulation. The nature of the simulation allowed results between different modules to be compared because the testing conditions were identical for each module.
Waveform generation was used to simulate wheel-speed signals and brake-pedal pressure. Sine waves of varying frequencies were used for the wheel speeds, and a DC ramp signal was used for the brake-pedal pressure. These waveforms were critical for providing an accurate simulation.
To portray an accurate representation of a vehicle stop, amplitude and phase modulation had to be maintained between the different stops contained within the profile as well as between the different wheel signals. The relationship between the wheel signals was accomplished by calculating each point in each waveform and then synchronously saving the waveform to the hard drive.
During playback, a bridge between stops was generated. The simulation tracked the current position of playback within the profile. This allowed it to know what waveform would be executed next.
The simulation used this information to determine the starting frequency and phase of the next waveform. Then the bridge was generated in real time, starting with the frequency and phase of the previous waveform and ending with the starting frequency and phase of the next waveform.
The length of this bridge was set in the test configuration. A 5-s bridge accommodated most ranges of speeds. This method provided a seamless transition between stops.
The operator interface needed the capability to generate the profiles, monitor the test, and report any faults. A data editor was necessary to create the different stops used within the profiles. It imported wheel-speed traces in ASCII or a spreadsheet format and converted them into equivalent wheel-speed sine/square waves (Figure 1).
The first step in this process converted the actual wheel speed (mph) into a frequency using a known correlation between wheel speed and frequency. Then a sinusoidal waveform was created for each frequency point. To maintain the resolution of the waveform, 25 points per cycle were used for each frequency point.
Waveforms were generated for each of the front-wheel speeds and then one waveform for the rear-wheel speed. By creating these waveforms simultaneously, the phase between each of them could be maintained. This was a critical issue with the simulation because the ABS module compared each of these inputs. A phase discrepancy between the left front-wheel speed and right front-wheel speed could signify a difference in speed between each of these wheels, which would cause the ABS module to generate a fault.
The generated waveforms had to accurately portray the acquired wheel-speed signals. To maintain the seamless playback and accommodate the time necessary to develop the bridge between waveforms, a cycle was added to each waveform. The starting and ending of frequency and phase of this cycle would be the same. This simulated a constant speed while the simulation prepared for the next stopping sequence. Then these waveforms were saved to the hard drive to be used during the actual simulation.
The operator generated stops with the data editor by entering a starting speed, an ending speed, and a deceleration rate. The editor created the appropriate wheel speed sine/square waves using this information. The profile generator allowed for nesting and repeating of created stops (Figure 2). This optimized both time and hard-drive space for lengthy million-cycle tests.
Monitoring the test consisted of graphical and digital representations of acquired data from the ABS module. Elapsed test time and the current position within the test were displayed and tracked. This was done to accommodate pausing of a test or for a power failure.
Tests could resume from where they were stopped or interrupted, a critical aspect of long-term durability testing. The ABS module fault codes were logged when they occurred. Alarm conditions were set for each acquired data signal, and the data was stored to the hard drive when these alarm conditions were met.
This PC-based simulation allowed engineers and technicians to test ABS modules with millions of stops without leaving their office or putting a mile on a vehicle.
About the Author
Craig Mira is the senior project engineer for the Automated Test Equipment Division of Dateppli. He has been working at the company for the past five years. Mr. Mira earned a B.S. degree in electrical engineering from Michigan Technological University. Dateppli, 3333 E. Patrick Rd., Midland, MI 48642, (517) 839-1040, www.dateppli.com.
Copyright 1998 Nelson Publishing Inc.
May 1998