Motion System Design

New sensor design resonates with linear motion

A new type of linear position transducer combines inductive coupling with resonant circuit principles to resolve micron-level movements over a sensing range of several feet. Developed by Balluff Inc., Florence, Ky., the non-contact measuring device consists of two elements: a circuit board and a U-shaped position head that rides along it.

Printed on the circuit board are several conductive paths, or loops, used for sensing, excitation, and reference. The position head, made of ferrite, magnetically couples the loops, making it easier for excitation pulses to induce currents in the reference and sensing circuits. The relative amplitudes of these induced currents, in principle, indicate the position of the ferrite head.

In practice, the inductive position sensor employs a few more wrinkles. The sensing loop on the circuit board, for example, is actually two opposing V-shaped loops. The inverse geometry means that the sum of the voltages on the two loops is theoretically constant. This is used as a reference signal to regulate excitation pulses — in effect compensating for variations such as temperature and vibration. In addition, the sensing loops are printed on both sides of the circuit board, along with excitation loops, for a fourfold increase in output voltage.

The position head is also more sophisticated. It is not merely a ferrite core, but a resonant circuit — consisting of a coil and a capacitor — that charges during the excitation pulse then releases its energy in diminishing sinusoidal cycles. The position signal is constructed with energy collected from the second to the eighth cycle, after the excitation signal and all its parasitics have died down. A control circuit times excitation pulses as well as sampling intervals corresponding to particular phases of the resonant cycles.

Capturing the energy from the resonant circuit provides a clear picture of the position head's location relative to the measuring loops. Accurate readings can be made every 50 µsec or so, achieving an update rate of about 20 kHz. The output signal is not only linear (with 0.02% nonlinearity) but also immune to electromagnetic interference.

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