Testing requirements that include control of multiple test stands and real-time data acquisition are integral to manufacturing clutches and torque converters for the automotive and trucking industries at LuK Inc. (Wooster, O.). Clutch materials and designs are evaluated during product research, development, and production, where speed and torque readings are used to calculate friction coefficients.

LuK wanted tighter control of the data acquisition process, but didn't want to devote a PC-based system to each test stand. Using plug-in boards with local microprocessors and memory to offload the PC's processor was the solution. This processor-based strategy yielded the sampling and response rates needed for real-time applications.

LuK's parent company, LuK Group (Buehl, Germany), eventually selected ADwin real-time process control boards from Keithley Instruments (Cleveland, O.). Engineers at LuK's U.S. subsidiary wrote the control software and user-interface software for the test system. Duration and ambient temperatures of these tests are varied to evaluate how hot material can get and still maintain acceptable friction characteristics. The resulting system performs four basic tests on clutch assemblies:

• Slippage At varying motor speeds, torque and pressure are measured to calculate the friction coefficient of the material;
• Engagement The motor is brought up to speed and the clutch is engaged with varying pressures;
• Breakaway This is an inverse engagement test in which a reading is taken of the torque required to slip the clutch, and
• Flush The fluid system is cleaned and the fluid is analyzed to determine how the clutch material degrades over time.

A block diagram of LuK Inc.'s clutch-slippage test system includes analog outputs for controlling motor speed, analog inputs for taking readings, and digital inputs/outputs for test stand power, low oil indication, and fluid pump activation.

Securing slippage data

LuK Inc.'s clutch slippage test system (see block diagram) includes inputs and outputs connected to a process controller in a 200 MHz, Pentium-based PC. The process controller used in the first two test stands is duplicated in two other stands. The controllers' processor modules contain 32-bit RISC processors and 12-bit analog I/O resolution.

These modules were chosen based partially on their ability to accommodate changes easily. Module variations include 60-500 kHz sampling rates and 1-128 MB memory capacity. The module's I/O board has analog inputs (16 single-ended or eight differential-channel), analog outputs (two- to six-channel), and digital I/O channels (6 or 16 each) and counters.

Using these modules, up to 10 processes for fast control, signal processing, and data acquisition can be calculated in parallel. Processes can run concurrently, exchanging global parameters and data independent of each other and the PC. Data transfers between processor and PC occur in the background, so they don't affect real-time operations.

In addition, LuK uses Keithley's ADbasic with the ADwin processor, which provides a development environment for creating the real-time measurement and control processes. All process parameters, delays, and status indications, as well as watch data arrays in a graphical form are displayed in ADshow, a debugging program.

To satisfy real-time requirements of the current application, LuK decided to locate the processing power close to the signal source, and to dedicate resources for processing the data that do not run into PC timing problems. This architecture provides deterministic responses with acceptable latencies.