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Inductive position sensor challenges potentiometers

01 April, 2000

Inductive position sensor challenges potentiometers

Novotechnik has developed a non-contact, inductive position sensor as an alternative to potentiometric sensors for providing feedback signals.

The Indres sensor, which can be produced in both rotary and linear formats, is claimed to have an accuracy of up to 0.04% of maximum sensor length and a linearity of better than 0.1%.

The shock- and vibration-resistant sensor uses the inductance from a loop of track on a PCB to act as a transformer coil winding. By placing two PCBs together, one acts as the primary coil, the other as the secondary.

A tuned circuit in the form of a slotted ferrite runs along the edges of both PCBs. This allows a sinusoidal signal in the primary winding to be induced in the secondary. The coupled signal remains at the same amplitude along the length of the winding, but if resistive shunt loops are introduced at regular intervals, there will be a voltage drop at each loop and a corresponding reduction in signal amplitude, from which positioning data can be obtained.

The concept is simple in analytical terms but deriving actual numerical component values entails the evaluation of complicated calculations involving both length and time integrals. The ratiometric principle is similar to that of the voltage divider used in a potentiometer, but uses a tuned circuit ferrite oscillator.

At the recent Drives & Controls 2000 exhibition, Berndt Büttner of Novotechik demonstrated a working model based of the sensor using two lengths of PCB with surface-mount resistors acting as shunt loops. Production versions will use screen-printed resistors and tracks.

Büttner claims that the sensor is not affected by static magnetic fields or by static or dynamic electric fields. It can be used to measure distances up to 1m. Because the electronic circuits are insulated, it is possible to fill the sensor with water or hydraulic fluid.

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