Hydraulics needs a better mousetrap

Alan Hitchcox editor [email protected]

When I first became a staff member of Hydraulics & Pneumatics, I was quickly indoctrinated on what the hot topics were at the time. One of the hottest topics then, 20 years ago, was the integration of electronics into hydraulics — which I came to know as electrohydraulics.

Since then, we have published hundreds of articles about electrohydraulics in this magazine. Of course, back then, electrohydraulics meant analog electronics. More recently, we’ve written about digital electrohydraulics. And even more recently, it’s been digital electrohydraulics in fieldbus systems — both in stationary and mobile equipment.

One thing hasn’t changed, though: the need for a universal linear displacement transducer (LDT) that can easily and inexpensively be integrated into most hydraulic cylinders.

Don’t get me wrong, several different designs of acceptable LDTs already exist, each with characteristics that suit them to specific types of applications. At the low end are resistive sensors, at the high end are magnetostrictive transducers, and in between are various other designs that have gained limited acceptance. The problem is, resistive sensors have physical limitations that will likely prevent them from ever gaining widespread use in hydraulic cylinders. Magnetostrictive transducers have become the favored technology for high-performance applications. Magnetostrictive transducers have been a real success story for electrohydraulics. They have allowed incorporating electrohydraulic cylinders into countless applications that would not have been possible otherwise. However, their relatively high cost prevents them from being used in most applications. Furthermore, the extensive machining required to accommodate in-cylinder installation only adds to their initial cost.

Therefore, a key to more widespread use of electrohydraulic cylinders is development of an LDT with the performance on par with magnetostrictive transducers but with a lower cost and that does not require extensive machining of the cylinder. What’s encouraging is that some new designs of LDTs are emerging that hold potential to provide low-cost position sensing in pneumatic cylinders. For example, Motion Controls, LLC, Hartford, Wis, is developing a sensor that fits in the cap end of an air cylinder and uses (non-laser) light and a light sensor to determine piston position. Another, from Astyx, Chicago, uses microwave technology to measure piston position.

The question is, can these or other technologies migrate over to hydraulic cylinders? Probably not — at least not without substantial redevelopment. The inside of a hydraulic cylinder is a vastly different environment from that of an air cylinder.

Eventually, though, someone will develop a low cost, high-performance LDT for use in hydraulic cylinders. When they do, you can bet that the hydraulics world will beat a path to their door.