EG&G Structural Kinematics, a company providing simulation testing services to automobile and truck manufacturers and their suppliers, needed a PC-based data acquisition (DAQ) system to test automotive electronic control modules (AECMs). The system had to simulate engine signals for different driving situations into a control module, and monitor the responses.
Understanding the complexities of the testing operations requires an appreciation of the tasks assigned to AECMs. AECMs receive signals from various sources, such as the transmission, the crank and cam shafts, the throttle, the manifold and the O2 sensors. This information is processed by the module and signals—such as fuel-injector timing, spark timing and tachometer, and engine warning light—are generated to control the engine and powertrain components.
System Requirements
To test AECMs, EG&G needed to accurately stimulate and monitor various driving situations. For instance, the signals generated by the cam and crank shaft must maintain proper phase relations, and the throttle position sensor must set levels that correspond to the cam and crank signals. Six analog output waveforms must be generated at a rate of 10 kS/s each.
The AECM tester must create many driving scenarios, called modes, and play them for long periods of time. Modes such as park idle, 50-mph uphill and wide-open throttle are among those used to create a profile. Profiles are then played, repeated or chained together to perform the complete driving simulation for the module.
Concurrent with simulation, the test system must monitor the AECM output for correct responses. Forty-eight analog voltage channels must be sampled at 20 Hz per channel and 12 analog voltage channels at 2 kHz/channel. Output codes are monitored at the RS-232 port.
Each analog channel requires independent and programmable alarm limits. When alarm limits are reached, all data must be streamed to disk 2 seconds prior to the alarm and 4 seconds after. Total system throughput, both input and output, is 85 kHz.
The last requirement was the user interface. Parameters and system status need continual display. The current position within a profile and alarm limit information and status were needed.
System Implementation
EG&G designed the test system using DAQ hardware from National Instruments, including:
AT-MIO-16F-5 and AT-MIO-16 multifunction DAQ boards.
An AT-AO-10 analog output board.
SCXI-1001 and SCXI-1000 12- and four-slot signal conditioning chassis with eight SCXI-1140 simultaneous sample-and-hold modules (Figure 1).
LabVIEW software from National Instruments was used to develop the graphical user interface.
Signal Generation
A significant section of the AECM tester is waveform generation. Crank, cam, wheel speed and mass airflow sensor signals are generated maintaining accurate amplitude and phase relationships. The CPU creates the waveforms to be output in real time.
Direct memory access (DMA) circuitry on the AT-AO-10 output board and the computer’s motherboard allow CPU-free transfers of data from PC RAM to the output channels. By regenerating or automatically recycling the data, the tester outputs the constant frequency portion of the simulated drive modes.
Since some of the modes are very long, regeneration of a small periodic waveform eliminates creating huge waveforms that will not fit in the computer’s RAM. When the waveform frequencies ramp from one constant frequency to the next, the software driver allows the CPU to simply write the frequency sweep waveforms into an intermediate output buffer. The driver then seamlessly merges the waveforms into the output on the next regeneration.
The sweep waveform must be immediately followed by a constant frequency waveform to prevent regeneration (Figure 2). Phase and amplitude are accurately maintained because each point in each waveform is calculated and synchronously fed to the module. Additional non-DMA analog output channels generate varying DC voltages to simulate manifold air pressure and throttle position sensors.
AECM Monitoring
During the simulated drive modes, the AECM responds with outputs in the form of analog and digital discrete voltages, analog waveforms and RS-232 codes. The AECM contains more than 50 outputs, all of which must be monitored.
A DAQ board (AT-MIO-16H9) with a simultaneous sample-and-hold multiplexer system (SCXI-1001 with SCXI-1140 modules) acquires data from 48 low-speed channels. A second board (AT-MIO-16F-5) and multiplexer system (SCXI-1000 and SCXI-1140s) acquire the 12 high-speed channels at 2 kHz per channel.
Two DAQ boards separate the high- and low-speed channels. Once again, DMA technology collects the data without requiring the CPU to perform the transfers.
Every 2 seconds, the CPU completes its analog output check and reads the backlog of the analog input channels. Data acquired in the analog input buffers is checked against the input alarm limits. If any channel falls outside the limits, the CPU stores the data to disk. The RS-232 buffer is read for proper module codes.
The user interface is updated after each iteration of the test. The display indicates the current position in the profile, the alarm status, the manual alarm override and voltage levels.
Benefits of a PC-Based Simulator/Monitor
The AECM simulator/monitor provided significant advantages over current methods of automotive electronic module test. First, the entire application was written by a single engineer over a period of six months. He was also responsible for the hardware and electrical development. Typically, test systems of this complexity had a team of people programming for that long.
Next, the test system provided repeatable automated testing. Finally, the PC-based system allowed cost-effective, yet complex, testing for the development and verification phase of the modules.
About the Authors
Chuck Karam is the Program Manager in the Truck Group at EG&G Structural Kinematics. He has been with the company for eight years, performing electrical, environmental, fatigue and simulation test and test development. EG&G Structural Kinematics, 950 Maplelawn, Troy, MI 48084, (810) 643-4622.
Dave Wilson is the Data Acquisition Marketing Manager at National Instruments. Before joining the company in 1991, he was a Regional Product Specialist for Sales at Keithley Asyst. Mr. Wilson earned a B.S. degree in applied physics from the State University of New York. National Instruments, 6504 Bridge Point Parkway, Austin, TX 78730, (512) 794-0100.
Copyright 1996 Nelson Publishing Inc.
March 1996
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