Sensor Sense: Inductive Position-Measurement Sensors

May 8, 2008
Analog inductive sensors, as discussed in the Jul y 2006 Sensor Sense (, return the relative distance of the target from the sensor face.

But their limited range and linearity limit many of their applications to detecting odd or irregularly shaped targets rather than actual position.

On the other hand, inductive linear-position sensors use analog sensing to return the position of the target over a distance of up to 360 mm with a linearity error of ±0.4 mm and resolution of 360 μm. They can also be used for rotational angle measurements for 360° applications.

Linear-position sensors perform this feat using multiple sensing coils in an array that spans the sensing distance. A special target called an attenuator slides along the sensing range, moving from one coil to the next buried in the main body of the sensor. A microprocessor integrates the output from each coil, translating the information into a true target position that is sent as a 0-to-10-V or 4-to-20-mA analog signal.

The microprocessor adds several advantages: Not only does it evaluate position using the multiple coils along the length of the sensor, it compensates for temperature variations and applies linearization correction to boost measurement accuracy. Future connectivity and programmability options might include scaling, range setting, limit detection, and value evaluation as well as communication interfaces such as CANopen and RS-232.

Each linear-position sensor comes with an attenuator. But any ferrous material will also work as an attenuator provided it maintains the specified width across the entire active region of the sensor. For example, a specified 8-mm-wide attenuator centered directly over one sensing coil partially overlaps the coils ahead and behind. The microprocessor reads the value from all three coils, calculating the center line of the attenuator based on the ratio of the coil outputs and the attenuator’s 8-mm width. A wider or thinner attenuator would reduce accuracy and resolution.

The actual distance of the attenuator above the sensor is immaterial as long as it stays within the specified range, typically 1 to 6 mm, with a corresponding reduction in distance if the attenuator is nonferrous metal.

This transparent view of an inductive linear-position sensor shows the multiple sensing coils buried within the main body of the sensor. A microprocessor within the main body calculates the position of the attenuator as it moves from coil to coil.

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