Motion control for a heavy-duty test stand

NTS Corp., headquartered in Anaheim, California, offers testing, inspection, and certification projects for many industries, including aerospace, defense, automotive, and consumer markets.

“We have well over 150 different tests going on at the same time at our 125-acre Huntsville, Alabama, location alone,” says Jim Birkholz, NTS Huntsville design engineer.

The Huntsville facility uses some large-scale test systems. For example, one test stand developed by NTS, a swing table, weighs about 17,000 lb and has been designed to test naval and aerospace assemblies weighing up to 50,000 lb. The system has been used for testing the integrity of helicopter fuel tanks and for testing jet engines. (See photos at left.)

Orienting the test platform at different angles is necessary for many types of tests. For example, in the case of the helicopter fuel tank test, the manufacturer wants to verify that all the fuel doesn’t flow past baffles in the tank to pool at one end of the tank as the helicopter changes flight attitude. There’s also a vent system in the helicopter fuel tank that must be tested at different flight attitudes. The same swing table has been used to test the oil flow in one of the bearings in a jet engine to verify that it is adequate in all flight angles as the engine runs.

Flexible testing platform

The 16ft x 16ft swing table is mounted approximately 8ft off the floor, suspended between two large gears turned by hydraulic motors; table height is adjustable to change the center of gravity of rotation. During the helicopter fuel tank tests, the table is swung 15° in each direction, 16x per minute in a sinusoidal pattern. NTS’ jet engine customer asked for the ability to turn the whole test assembly upside down.

Photos courtesy of Delta Computer Systems

NTS engineers chose hydraulic motors to drive gears, and since the building already contained a large hydraulic system, use of electric motors would have required installation of an outside transformer. As an added benefit, hydraulic fluids’ compliance delivers a more realistic simulation of real-world conditions when changing directions, and hydraulic actuators move easily to produce smooth, bi-directional motion of heavy loads.

Two separate hydraulic motors move the table – one operating each gear.

“In rotating the two gears, we’re looking to synchronize their positions to within ±0.2°, which is significant given the weight of the platform we’re moving and its speed,” Birkholz says. “But what is most important is to ensure smooth operation of the table, shifting back and forth like a pendulum.”

Selecting a motion controller

Building a flexible test stand such as the NTS swing table requires a motion controller that is easy to program and re-program for different test programs. In this application, the controller must interface easily to rotary encoders mounted on the gears and an inclinometer on the table.

The motion controller must deliver precise multi-axis control to maintain synchronized motion of the two gears as the table moves to programmed motion profiles. NTS used several methods of synchronizing hydraulic cylinders or motors in the past, and frequently found the position sensors (potentiometers) finicky, hard to adjust and keep in adjustment, and the controllers difficult to program.

For the new multi-function swing table test stand, Engineering Manager at Huntsville hydraulics distributor Flow Dynamics and Automation Inc., Jason Woyak, told Birkholz about the RMC motion controller family made by Battle Ground, Washington-based Delta Computer Systems Inc. Birkholz selected the 8-axis RMC150 (see page 23) for this task.

“The Delta controllers proved excellent at synchronizing cylinders,” Birkholz says. “The unexpected bonus was how easy the Delta controllers are to program. I could use the Delta RMC150 motion controller’s built-in sine wave command. I didn’t need to program sinusoidal motion with low-level machine instructions like you need with other motion controllers.”

To avoid the problem of unreliable potentiometer readings, Woyak suggested Birkholz use rotary encoders that interface directly to the motion controller.

Simplified tuning

In addition to facilitating motion programming, Delta’s RMCTools support package also contains components to simplify motion tuning.

“The built-in plotting capability in RMCTools allows you to tune the system very easily and very accurately,” Birkholz says.

The chart on pages 22-23, produced with Delta’s Plot Manager software, shows plots that give a clear view of how actual motion parameter values relate to target values through time.

“You will notice from the plot that the actual positions of the motors lag the target positions, but they track each other extremely well,” Birkholz notes. “We could not tune any tighter due to the backlash in the two gear trains.”

“When we did the first tuning we were getting a little bit of a knocking sound when doing cyclic motion,” Woyak says. “We updated the motion program to put in a delay on one side so there was always a little tension on the gears and the knocking was eliminated.”

“We’ve had excellent experience with Delta controllers and we will use them whenever we do a hydraulic design,” Birkholz concludes.

Delta Computer Systems Inc.
http://deltamotion.com

Reid Bollinger is East Coast Regional Sales Manager for Delta Computer Systems. He graduated from UNC Charlotte with a BS in Business Administration with a concentration in Marketing, and received his hydraulic training through Eaton Corp. and Racine/Bosch. He can be reached at 704.771.6305 or rbollinger@deltamotion.com.