Manufacturers often need to test their products to verify that they meet customer specifications, and provide the data that verifies specification compliance. This is particularly true when government contracts are involved, public money is being spent, and public safety is an issue. Often, separate electronic subsystems are used to apply the stimulus and measure the results, but if the same module that applies the stress can also record the response, more cost-effective testing systems can be built. An added capability that is often overlooked by material strength and durability test system designers is the power of some motion controllers to also serve as data acquisition support or input devices.Motion controllers make ideal testbed controllers for durable items such as aircraft components, automotive suspension systems, or pressure vessels because they provide programmable stimulus profiles while supporting test data recording. Closed-loop control enables motion controllers to support high testing speeds with precise limit testing, and some controllers have the ability to control force applied as well as position of the axis that is applying stimulus.
Holloway Houston, a manufacturer of lifting chains and slings based in Houston, has a policy of proof-testing every product they make, whether the customer requests it or not. In the case of a production run of nylon rope slings ordered by the U.S. Department of Defense (DoD) for use with military helicopters, the company needed to test its products to a set of tests specified by the DoD to verify a minimum useful life and record the results.
In Holloway’s test system, the rope sling needed to be cycled at four cycles per second. The rope warms up and stretches as it is cycled, and as it stretches, more “pull” is re quired to achieve the equivalent stretching force. The test that the DoD required was to apply tons of force to the sling, cycling the force between a maximum and minimum value, at a rate exceeding four times per second. After cyc ling for five or six minutes, the sling is allowed to cool down. Then the process is repeated until a specified accumulated runtime is reached.
Meeting the Challenge
Hydraulics was selected for this application since hydraulics is more energyefficient than electric motors at exerting large amounts of force. To control the hydraulics, Holloway’s control system integrator, HydrobotWorx of Seabrook, TX, used an RMC75 motion controller manufactured by Delta Computer Systems of Battle Ground, WA (Figure 1). The RMC75 supports synchronized control of two motion axes, with an Ethernet interface to an external PC that is used for programming the RMC75 controller.
The rope sling is tensioned using a double-ended hydraulic cylinder with a 3.25" bore, 2" rod, and 2' stroke, with fluid supplied by a proportional servo valve. The motion control unit controlled the valve and two hydraulic pumps. The pressure sensors were mounted on the cylinders and magnetostrictive displacement transducers (MDTs) with synchronous serial interfaces (SSI) to the motion controller were mounted externally on the double-ended hydraulic pulling cylinder. Figure 3 shows the major components of the 150- foot-long test apparatus (just one side of a double-sided testbed was used for these tests).