Application Briefs

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.”

Figure 3. The Holloway rope sling tester with major components indicated.
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, visit

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