No contact is the best contact

Mark Fletcher reviews a non-contact measurement device that will suit the most arduous hydraulic applications

A position sensor that fits inside hydraulic cylinders combines non-contact operation with a novel method of detection.

The concept, developed by Variohm in conjunction with Novotechnik, uses conventional resistance, collector tracks and a capacitive pick up. Unlike a conventional potentiometer, where the resistance track is connected to a DC supply, the TLI units are driven from an AC source.

The main application will be in the measurement of hydraulic cylinder positioning. But its method of operation does not prevent it from being used in other industries where linear measurement is required.

The capacitive pick up can detect the AC signal and couple it to the collector track, where it is available for further conditioning. The amplitude of the AC signal varies along its length from zero to a maximum. The detector picks up this amplitude and equates it to a relative position. The amplitude of the AC wave is proportional to the position on the track.

The capacitive pick-up slides, in its carrier, along the extrusion very smoothly thanks to a bearing plastic rail. The pick-up/extrusion fit is very precise but the shroud around the pick-up has a reasonable amount of play in it which helps to absorb any non-linearity in the piston travel. Any non-axial movement in the shroud does not translate to the pick up so the accuracy of the unit is not compromised.

Developing resistance to hydraulic oils was part of the design process. Another major problem was maintaining mechanical accuracy to give reliable and repeatable results. This was overcome by the use of a special extrusion which carries the capacitive pick up. The precise extrusion maintains the proper track-to-pick up relationship as well as offering an attractive mechanical package. Three varieties are available to suit the majority of hydraulic cylinders.

The final element of the design was the provision of an integral electronics system. This had to be supplied from a DC source and provide a stable AC signal to the resistance track. It also had to convert the AC from the collector track back into a 0 to 10V DC level which a customer could use. With this conversion hardware any potential customer can treat the unit as they would any other kind of potentiometer measuring device by putting a DC signal in and getting a DC signal out.

The development was performed in conjunction with Mulag of Bad Pertersal, Germany which manufactures specialist vehicles including bin lorries. Typically lorries of this type will carry loads of up to 3.9 tonnes of compressed waste. The high compression forces are generated by hydraulic pistons which have the TLI sensors fitted to them. This application was deemed to be a suitable test bed because of the environmental conditions faced by the units. They must operate at temperatures as low as -40C, humidities of up to 95 per cent and in the presence of corrosive salts.

The TLI operates from -40 to 90C at operational speeds of up to 10m/s in oil pressures of 350bar (600 bar peak). In all these conditions the unit has to maintain sealing to IP67.

Design Pointers
Non-contact measurement means wear-free operation

Designed to suit hydraulic applications, the sensor can cope with adverse environmental conditions

The measurement head position is constant and precise but the shroud can move to combat deflection in the cylinder

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