Auto-alignment of a high-precision eddy-current displacement sensor using a thermal slider actuator

Abstract

Eddy-current displacement sensors can reach nanometre resolution by operating at high excitation frequency (∼150 MHz) and low standoff (∼50 μm). Alignment is more critical at lower standoff, requiring auto-alignment functionality. Auto-alignment of sensors can be performed using the Thermal Slider Actuator (TSA), which was until now only used in combination with capacitive probes. In this paper a new miniaturised design of the eddy-current probe is proposed that can be integrated in the TSA. The eddy-current coils are embedded in a PCB disk of 12-mm diameter, which is clamped between the fingers of the TSA. Although the PCB is relatively rough, the TSA was shown to produce net motion in both directions, attaining speeds of 13 μm/hour. Furthermore, in contrast to a capacitive probe, the eddy-current probe contains electronics, which generates heat, leading to a thermal gradient in the TSA. A thermomechanical model of the eddy-current probe and the TSA shows that the resulting deformation of the TSA is acceptable.