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).
The tests started with the motion controller in closed-loop mode running sine wave motion profiles using a single axis of control. Control was switched to open-loop mode, and a second valve controlled by the RMC75 was used to increase the hydraulic fluid flow as the temperature of the rope sling rose, the rope increased in length, and the speed of operation was increased to a minimum of four cycles per second. After the maximum stress was applied, the motion controller was switched back into closed-loop mode to slowly exercise the rope while it cooled down for about 25 minutes.
During each cycle of the closed-loop control phase, the motion controller was first instructed to perform position control to produce the desired amount of stretch in the rope, and then the controller was shifted to perform pressure control to ensure that the correct amount of tension was applied to the rope before changing direction. “Our biggest challenge was providing enough flow,” said Mike Zilai of HydrobotWorx. “The RMC’s ability to control pressure/force as well as position, and to switch smoothly between control modes, is one of the things that makes the controller well suited for strength testing applications. Our innovation was using the second RMC75 axis to add flow using a second valve and hydraulic pumping system.”
As the test sequence proceeded, the controller allowed the test engineers to monitor the flow into the cylinders in real time by reading values inside the controller using an attached personal computer running Delta Computer Systems’ RMCTools motion software package. The real-time data monitoring enabled the Holloway test engineers to make changes to the test sequence on the fly. “Early on, we could see that we needed quicker valves and more fluid flow when the system wasn’t responding quickly enough,” said Mike Zilai.
To provide real-time analysis and documentation of the test sequence as the motion controller operated, Holloway test engineers used the Plot Manager that is provided with the RMCTools software. “Being able to see the velocities and pressure and differential force made life a whole lot easier for us,” said Zilai.
This is the first time that Holloway had built a test platform incorporating a motion controller. Previously, the company’s test systems were manually operated. “Thanks to the controller, the new machine can apply a lifetime of wear to the rope very quickly under a controlled environment to verify that it will meet its useful life specification, and it can help document the results,” said Zilai. “This is only one of two machines in the world that can do this.”
The Holloway test system is an example of how motion controllers can excel in data acquisition roles in addition to control. Because they can detect subtle changes in how materials react to stresses placed on them, motion controllers can also be used for non-destructive test (NDT) applications that apply loads to test an item’s breaking point without actually breaking it.
This article was written by Bill Savela of Delta Computer Systems, Battle Ground, WA. For more information, Click Here