Tough transducers travel off-road

Jan. 28, 2005

Hydraulic cylinder position-sensing in the rugged environments typically encountered by construction, agriculture, and other off-highway mobile-equipment is critical. There are three reasons to consider sensors, such as Temposonics Model MH sensors:

Richard Schneider

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these sensors are installed inside the cylinder’s protective envelope — embedded and fully sealed

they are designed for ruggedness — with 100-G shock and 25-g vibration ratings plus 200-V/m EMI immunity, and

they incorporate time-based magnetostrictive technology, which is inherently non-contacting, thus eliminating wear and producing exceptional durability and output repeatability.

The compact Model MH stainless-steel position sensors are specifically configured for installation in welded (as illustrated) and tie-rod style hydraulic cylinders with 50-mm or larger bores. They can be positioned from the head end or the rod end to suit the particular application.

The Model MH consists of three main components: a sensor head (a rugged enclosure for the electronics); a pressure-proof sensor tube that houses and protects the internal sensing element (or waveguide); and a donut-shaped permanent magnet — mounted on the piston — which travels along the stationary sensor tube without contacting it when the cylinder strokes.

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Within the sensing element, a sonic strain pulse is induced in the magnetostrictive waveguide by the momentary interaction of two magnetic fields. One field comes from the moving magnet that passes along the outside of the tube. The second field comes from an “interrogation” current pulse applied along the waveguide. The resulting strain pulse travels at sonic speed along the waveguide and is detected at the head of the sensing element. The position of the magnet is determined with high precision and speed by a high-speed counter that measures the elapsed time between the application of the interrogation pulse and the arrival of the resulting strain pulse. Using the elapsed time to determine position of the permanent magnet provides an absolute position reading that never needs recalibration or re-homing after a power loss.

Make contact: MTS Systems Corp., Sensors Div., Cary, N.C.

(919) 677-0100, www.mtssensors.com

About the Author

Richard Schneider | Contributing Editor

Contributing Editor, has been affiliated with Hydraulics & Pneumatics for more than 30 years and served as chief editor from 1987 through 2000. He received a BSME from Cornell University and also completed additional courses at the Milwaukee School of Engineering. His diverse background in industry includes ten years with a fluid power distributor and a variety of other professional positions. He has also been active with the National Fluid Power Association and Fluid Power Society.

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