Probe design for high-precision eddy-current displacement sensors

State-of-the-art industrial Eddy-Current Displacement Sensors (ECDSs) are typically not suitable for use in high-precision applications due to their low resolution and poor stability. By using a smaller, flat sensing coil, a reference coil, a dedicated readout chip and by operating at much higher excitation frequency a higher measurement sensitivity and better mechanical and thermal stability can be achieved. To use the sensor in industrial applications, the chip and the coils must be integrated in a small package. This paper presents a probe design for a high-precision ECDS, aiming at compactness and low thermal sensitivity. In this design, the sensing coil and the reference coil are closely spaced to minimise thermal gradients. The coils can, together with intermediate shielding and capacitive tilt electrodes, be integrated into a single stack only 2 mm thick and 12 mm in diameter, which can be realised on a multilayer PCB. Thermomechanical modelling shows that placing the readout chip on a separate PCB leads to significantly decreased self-heating compared to placement directly on the stack. Experiments show that the inductance behaviour of the realised stack is similar to that of the model.