June 2019

INTEGRATED SYSTEMS 40 HYDRAULICS & PNEUMATICS June 2019 www.hpmag.co.uk fluid compressibility in the lines. Where possible, using metal tubing instead of flexible hose will minimize controllability problems due to expansion in the lines. Which transducers are best Hydraulic systems commonly employ linear displacement transducers to measure cylinder position, and pressure transducers or load cells to measure force. It is critical that devices be chosen that have high response – significantly faster than the control loop time of the motion controller – and fine granularity of measurement. A common mistake is to choose a contacts to wear out and cause maintenance problems, and repeatability of measurements is high. Figure 1 shows how the components of an electrohydraulically-controlled system are connected. In this system, force exerted by the cylinder can be measured by calculating the difference in reading between the outputs of pressure transducers mounted on either side of the piston. System designers will benefit in terms of best system repeatability, highest productivity and longest machine useful life by selecting the highest quality components according to the guidelines above. This comes at a cost, however, and some designers are tempted to use less-than-optimal components to reduce system costs. Note that even the best motion controller may not be able to compensate for a poor system design or implementation. Programming the motion controller This being said, some electrohydraulic motion controllers, such as the RMC200 from Delta Computer Systems, Inc. (Figure 2), can compensate for some design problems. For example, Delta Computer Systems engineers recently analysed a customer system that was exhibiting the effects of static friction, sometimes called a stick-slip problem. The application, a gang saw ‘wiggle box’ that needs to move in following the curve of a cut log, was vibrating when it should have been moving smoothly. The problem was diagnosed as being caused by the bores of two of the cylinders being too small, meaning that the system had a low natural frequency and was not as stiff as it should be. Hence, it wasn’t responding quickly enough to control signals from the motion controller. The ‘D’ velocity term in the PID loop equation wasn’t producing enough drive to each valve. The Delta engineers Figure 3. The motion control algorithm should include the potential for feedforwards (FFs in diagram above) and second derivative (D Diff.) term in addition to standard P, I and D terms. “It is critical that devices be chosen that have high response – significantly faster than the control loop time of the motion controller – and fine granularity of measurement.” transducer that provides the same resolution as the required motion tolerance. Instead, the transducer resolution should preferably be ten times greater or more. For systems with poor controllability, even higher resolution may be required to take advantage of compensating control algorithms. A big advantage to using linear magnetostrictive displacement transducers (LMDTs) is that they always know the cylinder’s position; no homing step is required at system initiation or reset. LMDTs have the additional advantage of providing non- contacting measurements, which means that there are no