Test Systems Combine Hydraulics, PCs, and Motion Controllers

Hydraulics is the technology of choice for moving or lifting heavy loads or exerting precisely controlled forces. But one of the fastest-growing areas for hydraulics is in materials and structural test equipment.

With proper controls, hydraulic actuators can apply and hold exacting forces on an object. And they can generate varying load cycles that simulate a lifetime of real-world stresses and wear in an accelerated time span. Aerospace and automotive-equipment manufacturers are among the early adopters who are reaping the benefits.

PCs versus motion controllers
Personal computers, with their ability to collect, display, and archive information, have become the de facto standard computing platforms for testing and data acquisition. Besides the fact that PC hardware provides the highest level of computing performance per dollar invested, a wide range of readily available software has made it relatively quick and easy to develop powerful test systems.

Software packages such as LabView from National Instruments and Visual Basic and Visual C++ from Microsoft let test engineers build virtual models of test equipment on screen, collect data from actual tests, display results in tables, graphs, and charts, and save the information locally or transfer it via a network for analysis and storage elsewhere.

PCs, with their flexibility and intuitive human interfaces, are seeing wider use as alternatives to traditional PLC/HMI systems. But PCs by themselves are not ideal for high-performance, closed-loop control. The Windows operating system that is fine for displaying visuals and processing data cannot guarantee predictable responses to high-speed sensor feedback. As a result, hydraulic systems typically require separate closed-loop motion controllers along with a PC.

Closed-loop electrohydraulic motion controllers connect to sensors such as position, force, and pressure transducers, and can drive proportional servovalves. The best controllers close the control loop (that is, receive inputs, make controlling decisions, and generate control outputs) in 1 msec or less — typically an order of magnitude faster than a PC or PLC alone can respond.

Even with slow-moving actuators, it pays to use special-purpose electrohydraulic controllers. That’s because hydraulic motion control typically involves nonlinear relationships between inputs and outputs, so setting up and tuning control loops is much easier using controllers specifically designed for the task.

Advanced controllers can also transition smoothly from accurately positioning a hydraulic actuator to controlling the force it applies — something difficult to do with general-purpose computers without jumps or discontinuities in the motion. Hydraulic motion controllers use special preprogrammed functions to smoothly vary accelerations and decelerations, ultimately permitting faster operating speeds while extending machine life.

The latest electrohydraulic motion controllers also provide straightforward interfaces to standard communications networks. Most support a range of industrial fieldbuses, with the most ubiquitous interface being Ethernet. Using EtherNet/IP, a PC can download motion parameters into the controller and read the results of motion steps. It’s even possible for production-control personnel to monitor processes remotely via an Internet or intranet connection to the machine.

Electrohydraulics in action
One example of electrohydraulics used in R&D involves a hydraulic biaxial wheel tester interfaced to a PC for control and data acquisition. The controls, developed by Innkeeper LLC of Redford, Mich., command hydraulic actuators to exert forces along two axes on a spinning automotive wheel to stress and test its mechanical structure. An image of the PC screen shows how an intuitive visual interface, in this case developed using LabView, lets engineers set motion parameters, initiate action, and display results.

Linear-variable-differential transformers (LVDTs) connected to the cylinders send position information to the motion controller, while load cells provide data on the force applied by the hydraulic actuators. In the motion controller, closed-loop control algorithms process position and force inputs as feedback and generate a variable-voltage control output to drive the proportional valves.

The controller relies on position feedback to properly orient the wheel in the drum in which it spins, and then switches to force feedback to ensure hydraulic axes apply the correct force throughout the test.

Another example involves closed-loop hydraulic control in a test rig developed by Emprise Corp. of Kennesaw, Ga. It measures the strength and integrity of ordnance latches on the U.S. Air Force’s new F-35 Joint Strike Fighter. A six-axis hydraulic system exerts stresses that mimic those the latches would experience during high-speed aircraft maneuvers. A PC handles the HMI and sequences testing operations, collects data, and communicates with the motion controller via Ethernet.

For each load step, application software reads a load-spectrum file that specifies the forces required of each actuator. Calculated setpoints are transmitted from the LabView program to the controller which, in turn, actuates a hydraulic servovalve mounted on each cylinder.

Load cells on the hydraulic actuators measure forces and provide feedback to both the NI hardware and controller. Though the actuators actual displacements are negligible, pressure on the latches ramps up at approximately 3,000 psi/sec. The controller, in this case a Delta RMC150, ensures precise results because its control loops run as fast as once per millisecond.

Each load step takes about a second, and simulating a lifetime of operation requires more than 1 million load points. Actual tests last several life cycles. The durability test generates cyclic fatigue data at all load points, which can be converted into Excel spreadsheets for analysis and archiving.

Selecting EH controllers
Test applications such as these often involve repetitive stress cycles. A controller that supports direct execution of repetitive motion operations makes it quick and easy to set up testing profiles. For example, Delta Computer Systems’ RMC family of controllers can produce precise cyclic motion sequences such as trapezoidal profiles, ramp up/down profiles, and sine waves. The controllers can also generate complex, repetitive profiles using spline functions to connect target points in a motion sequence.

As both the Innkeeper and Emprise examples point out, another aspect of complex tests is the need to simultaneously coordinate several hydraulic axes. It’s often not possible to simulate real-world stresses accurately without submitting a device under test to 3D motion. Therefore, test-system designers should select electrohydraulic controllers that synchronize the motion of multiple axes. This can take many different forms, from simply moving several axes in lock step, to “gearing” or “camming” functions that express the motion of one axis via a mathematical relationship to another axis or an outside stimulus.

These two examples also demonstrate a key factor that contributes to building good test platforms: The ability to work seamlessly with popular data acquisition and control software programs hosted on PCs. The motion controller’s software should give direct access to and from Windows-based PCs running data-acquisition software. Delta Computer Systems’ RMCLink software, for example, lets PC applications read and write registers in the controller and issue motion commands. Direct connection is via serial bus or Ethernet.

It also helps if the motion-control software includes full-function example projects (ideally, embedded in free software downloads) for many programming languages. These ready-to-use examples illustrate concepts such as reading positions and plots and issuing commands.

Tuning wizards in the software are another major time-saver. They simplify and accelerate motion-axis tuning by developing mathematical models of the system. This, in turn, lets test-system designers tune control parameters by merely using a visual “point-and-click” interface.

Whether you’re more familiar with controls or fluid power, there’s no question that combining PCs with hydraulics can result in best-in-class materials testing equipment. As the above examples show, PC software can give “personality” to hydraulics, making it easy to visualize test processes and share and archive results. Likewise, hydraulics, together with an electrohydraulic motion controller that connects easily, controls reliably, and can be tuned to produce precise outputs, gives “muscle” to PC-based test systems.

© 2010 Penton Media, Inc.