PC-based control reinvigorates lab workhorse

Joseph R. Britton -- Control Engineering, 5/1/2000

Beyond improving capability, upgrading test stands also requires enhancing their versatility, efficiency, ease of operation, and maintenance. During one recent upgrade, BFGoodrich Aerospace (BFG, Troy, O.) overcame these challenges with help from VMIC (Huntsville, Ala.).

BFG designs and manufactures aircraft wheels and brakes. The facility features a test lab, which operates special-purpose test stands, such as the Young static test stand or Young Press. Commissioned in the mid-1950s, this dual-axis press applies independently controlled vertical and horizontal loads to the test specimen. Testing requirements imposed by aircraft manufacturers and the FAA recently prompted BFG to upgrade the Young Press so the horizontal axis could statically apply torque to a wheel, brake, or tire assembly.

The torque upgrade meant BFG would also upgrade the machine's controls. The existing manual open-loop controls forced operators to juggle control of both load axes while following test procedures. In fact, a second operator was often required to control parameters, such as tire pressure. The test stand needed automated closed-loop control.

Since long-term objectives required more versatility and faster execution speeds than BFG felt a PLC could deliver, its engineers decided to implement PC-based control. System requirements included execution of automatic control programs, calibration required by frequent transducer substitution, deterministic data sampling, and a graphical user interface (GUI).

IOWorks, VMIC's PC-based control package, met all of BFG's requirements for precise control. To collect, file, save, and communicate dynamometer and other test data, BFG and VMIC also installed a VMEbus system, using an Intel Pentium single-board computer running Microsoft Windows NT, as the new system's architecture. IOWorks, as the control engine, allows programming in either conventional ladder diagrams or C++ code.

VMIC's VMEbus analog-to-digital converter boards send critical test parameters, such as vertical and horizontal loads, torques, and tire pressures to the control system. These boards are accessed with function calls that, when coded into a ladder-logic function block, are nearly transparent to the programmer.

Digital and analog I/O devices are controlled via a Profibus network. The Profibus master is daisy-chained to slave stations. Sensors and actuators are wired to I/O modules on the nearest slave. This configuration reduces wiring and provides flexibility for future upgrades.

The risk of BFG and VMIC's approach has been rewarded by performance beyond initial expectations. Control of critical parameters is very tight with minimal operator intervention. Test requirements are being met more accurately, more quickly, and with less time and labor than previously possible.

"It's a night and day difference. The new setup makes my job much easier. The reliability and repeatability of tests is 10 times better than previous manual controls," says John Yost, BFG's machine operator. "The old system was a kind of balancing act adjusting the control knobs. The new system is much more precise."

For more information, visit www.vmic.com visit www.citect.com; or visit www.controleng.com/freeinfo.