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Non-contact position sensor `makes others redundant`

01 September, 2000

Non-contact position sensor `makes others redundant`

Traditional position sensors such as potentiometers and Hall Effect devices could be under threat from a new, low-cost technology developed by a British company. Hampshire-based Gill Instruments claims that its robust induction sensors, which have no moving parts, could make existing sensors redundant in many applications.

The standard induction-based technology for sensing position is the linear variable differential transformer - or LVDT. This consists of a primary coil and two secondary coils arranged so that a continuous signal is produced in the secondaries, depending on a continuous signal generated in the primary coil, and on the position of an electrically conductive element.

The drawback of the LVDT is its cost. An alternative is to use a potentiometer, but this requires physical contact between a wiper and a resistive track and this inevitably leads to wear.

The Gill sensor is designed to address these drawbacks by providing a low-cost form of position sensing that is not subject to wear. It operates by sensing the position of a steel "target" or "activator" which is attached to, or is part of, the sensed item. The target can move in a straight or a curved path and this movement is detected by a pulsed induction sensor.

The sensor consists of a pair of coils connected to a pulse generator. By measuring the voltage (or another electrical variable) across the coil a fixed period after each pulse, a value is obtained for the position of the target. If the two coils are at an angle of about 100 degrees to each other, the output signal is said to vary in an almost linear fashion with the position of the target.

Although it would be possible to use just one coil, Gill`s patent application for the sensor explains that the twin-coil arrangement provides a range of movement with a linear response that is four times longer than is possible with a single coil.

The twin-coil format has other attractions. For example, it:

• results in a compact sensor that is not affected by surrounding materials such as mounting brackets;

• cancels interference effects affecting both coils; and

• allows the unshielded sensors to be mounted directly on steel structures.

By arranging two of the sensors at right angles to each other, it is possible to monitor movements in two dimensions, the patent application adds.

Another possibility is for a coil to move with the target. This could be useful for use with pneumatic or hydraulic cylinders, the document suggests.

Gill originally developed the sensor for automotive applications, such as sensing the position of an accelerator pedal, but says that the technology has far wider applications. The standard target is a 6mm diameter bar, but by varying its shape, different output characteristics can be achieved.

The non-contact sensors are said to be easy to install and to have an unlimited duty cycle ratings. They will operate over a temperature range of -40°C to +85°C, and, being sealed to IP67, can be pressure washed.

Gill claims that the sensors have an accuracy of 3% over the whole operating temperature range and a repeatability of 0.1% of full scale. The typical response time is said to be 10ms.

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