Testing, testing, 1-2-3

June 1, 2012
Learn how fluid power with closed-loop motion control enables speedier, simplified nondestructive product testing.

One of the fastest growing uses for hydraulic and pneumatic actuators and closed-loop motion control is in product testing systems. Why? Fluid power offers distinct advantages over electric motors in many testing applications because hydraulics can meter small fluid volumes in and out of an actuator for precise control of pressure and force. This uses less energy than a motor that needs significant amounts of current to maintain force or torque. What's more, in applications requiring predictable force or movement of heavy loads, load cells may not be as practical as the pressure sensors of fluid-power actuation for determining net force.

Simulated reality

Motion actuators for testing are typically configured to simulate real-world conditions. For example, some apply force to aircraft landing gear to evaluate new runway materials and aircraft designs that affect wear and tear. Hydraulic actuators in building-truss testers apply loads to simulate harsh environmental conditions such as wind and snow loading. In these applications and others, including the testing of products as diverse as vehicle leaf springs, flexible pipe sections, and medical prostheses, precisely controlled motion actuators apply varying stresses over hours or days to simulate conditions that such items encounter over a lifetime of use.

Closed-loop control

In addition to precise position and speed control, coaxing the best performance out of the motion sources for nondestructive testing applications requires a motion controller capable of implementing closed-loop control of the pressure or force applied to the device under test. Simply controlling actuator position is insufficient, because it isn't possible to detect subtle physical changes in the test subject unless the net force required to flex the device is closely monitored. In a fluid power system, the net force being applied by an actuator can be discerned by monitoring the pressure difference between pressures on each side of the piston.

For example, consider a hydraulic tester designed to repetitively flex leaf springs while measuring the force to do so. The tester's motion controller determines force by connecting to pressure sensors, and precisely measures actuator position by connecting to a magnetostrictive linear displacement transducer (MLDT). In a typical test operation, position control is used to closely position the actuator before a controlled force is applied by the motion controller. Engineers should look for an electronic motion controller that is both easy to program and can smoothly transition between controlling position and regulating pressure or force.

In this scenario, the motion controller drives the hydraulic cylinder by sending analog signals to a proportional servo valve capable of making precise cylinder pressure adjustments (via sinusoidal or other waveforms) to control actuator force or position. An accumulator stores hydraulic pressure, ensuring that consistent supply pressure is available to operate the servo valve during spring compression cycles.

Motion programming

Nondestructive testing programs often apply repetitive stress cycles to devices being tested. A motion controller that supports directly executed cyclic motion operations makes it quick and easy to set up testing profiles. However, cyclic loading isn't necessarily simple. Proper motion controller operations are required to simulate complex real-world motion scenarios for one or more motion axes.

On the other hand, in non-cyclic testing applications, the motion controller should easily generate other motion patterns for stressing the device under test. For example, spline functions can produce smooth motion curves that can replicate seemingly random stimulus on the device under test — as in a racecar suspension tester that simulates the stresses of track driving.

Spline functions emulate flexible strips that can be bent between fixed points and used to draw smooth curves. With them, implementing smoothly curving motion profiles is as easy as providing the motion controller with position coordinates (as a function of time or the position of another axis) and instructing it to connect the dots. The motion controller is simpler to program because it doesn't require the system designer to calculate velocities and acceleration rates. Finally, system performance improves because a single spline function replaces many point-to-point steps.

Using splines, complex motion profiles can be specified graphically using either a visual curve tool or an Excel spreadsheet: The machine designer defines the positions and lets the spline algorithm compute the acceleration and velocity necessary to move smoothly from one point to another. As the spline function executes, the motion controller calculates the acceleration and velocity needed to move from the present point to the next point in the motion profile, such that the accelerations are continually varying at a linear rate for smooth motion curves.

Leveraging PC-based application software

A wealth of relevant testing and data acquisition software is available for PCs, so it is important for the motion controller to easily interface with a PC running .NET assembly and ActiveX control and software packages, such as LabVIEW from National Instruments or Visual Basic from Microsoft.

Consider a flexible pipe testing application for the oil and gas industry. Here, a test designer must setup different test stimuli to be applied to the pipe, view plotted test results, and log the data. One solution is to enter test parameters (such as positive and negative deflection angles, maximum and minimum tensions to apply, and number of repetitions) in an Excel spreadsheet, and then display the graphs using an interface developed with LabVIEW. In one such implementation, built for a flexible-pipe supplier to the energy industry, system integrator Jason Woyak of Flow Dynamics and Automation Inc., Birmingham, built an interface within Excel using Visual Basic for Applications (VBA). The interface outputs motion coordinates to the motion controller obtained directly from a spreadsheet.

VBA is an implementation of Visual Basic, an event-driven programming language and associated integrated development environment (IDE) that is built into most Microsoft Office applications. By embedding the VBA IDE into applications, developers can build custom control interfaces using Visual Basic. Note: The communications interface with the motion controller itself relies on software provided by the motion-controller vendor. For example, a .NET assembly and ActiveX software package called RMCLink from Delta Computer Systems uses the motion controller's built-in Ethernet interface to connect to the PC. To use LabVIEW to display test results, a virtual instrument extracts the appropriate information from the motion controller's registers. For example, the RMCLink software contains virtual instruments for use by LabVIEW users.

Motion controller enables non-destructive testing

Interrogating the motion controller to monitor motion parameters during testing is an important capability. Reconsider the leaf spring tester, used by spring manufacturer Rockwell American, Seagoville, Tex. Here, the application's motion controller accurately follows operator selectable, internally generated target force profiles using an HMI. Each spring movement is controlled by continually adjusting drive output to the hydraulic valve 1,000 times per sec.

Minimum and maximum spring deflections are monitored in real-time during the force cycling and compared against allowable limits, to determine any change in spring properties. For each spring tested, these limits are found when the motion controller is commanded to enter force control, as initiated by the operator via the HMI touchscreen. At the beginning of the testing cycle, the system compresses the spring to minimum and maximum force setpoints while recording and storing the corresponding minimum and maximum spring deflections.

If the hydraulic actuator's position exceeds operator-specified tolerances during the continuous force control cycle testing, the spring properties may be changing, the spring may be ready to break, or one leaf in the spring may have already broken. If this happens, the motion controller's continuous monitoring of position tolerances during force cycling leads to an automatic system shutdown. Because of these controls, the machine can run continuously with minimal supervision.

Motion controller facilitates certification

A motion system able to store test data can also document compliance with regulatory requirements. For example, consider a prosthetics manufacturer with an ISO-imposed requirement for testing elastic ankle joints to ensure they can flex under realistic conditions through at least two million cycles. Key to proving that they are tested realistically is making sure that during each cycle, the joint displacement is within a certain limit when a particular force is applied.

Orion Test Systems and Engineering Inc., Lake Orion, Mich., manufactures such a tester, which uses two pneumatic cylinders controlled by a two-axis motion controller. One cylinder is positioned to press on the heel, and one to push on the toe, of an artificial foot. In this system, rather than measuring differential pressure in the cylinder, a load cell affixed to each cylinder measures the force being applied, while a MLDT affixed to each piston measures each actuator's position. Cylinders alternate their motion to flex the joint; during each cycle, the motion controller increases the force being applied on each cylinder until it reaches a predetermined setpoint and then measures the joint deflection to ensure that it doesn't exceed maximum allowable force.

Because data on the amount of deflection is collected every cycle by a PLC, the tester can measure the onset of fatigue before catastrophic failure occurs. The motion controller is able to cycle the tester between two and three times per sec — a rate twice as fast as the previous controller used in this application — doubling the testing facility's throughput.

Pneumatics was selected over hydraulics for this tester to keep the test system's weight as low as possible. The compressibility of air complicates tuning, but the Delta RMC75E motion controller's real-time plotting and tuning tools make tuning easier.

Motion systems in unlikely places

The ability of motion systems to acquire data, make decisions, and produce outputs enables engineers to use them in unusual applications. For example, Catalina Cylinder Corp., Garden Grove, Calif., makes aluminum cylinders used to transport gases such as oxygen and carbon dioxide. Catalina's engineer, John Kishel, had the idea that perhaps a hydraulic actuator — under closed-loop control by a motion controller — could be a source of pressurization for the gas cylinders under test.

Here's how it works: The system uses a hydraulic cylinder acting as a pump, with the cylinder's rod end entering a tube that squeezes water into the gas cylinder. The gas cylinder is tested according to a motion profile dictated by the program running on the motion controller. Very high pressures are achieved quickly, even though the cylinder moves relatively slowly. Further, because the hydraulic cylinder moves slowly, the pressurization system has a long life. Kishel confirms that by using a smart motion controller, the system can achieve its target pressure to within 1% at 3,000 psi, enabling Catalina's testing process to achieve more precise control and a higher degree of pressure-related repeatability than other techniques.

For more information, contact the author at [email protected] or visit

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Single and double-disc style couplings from Ruland Mfg. deliver accuracy in test and measurement industries. Comprised of two black anodized aluminum hubs and multiple flat stainless steel disc springs, disc couplings have low inertia and are torsionally stiff, suitable for precise zero-backlash systems with speeds to 10,000 rpm. Single disc styles are suitable where compact installation is required; double disc styles add a center spacer to increase the misalignment capabilities. Thin disc springs allow for substantial misalignment between shafts while remaining rigid under torque loads. Couplings are available in both clamp and setscrew styles with bores from 3 to 30 mm. For more information, visit

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