A frameless eddy current sensor for cryogenic displacement measurement

This paper presents a frameless eddy current sensor for cryogenic displacement measurement, for instance, the monitoring of axial displacement in liquid propellant engine. The frameless sensor probe, which consists of flat spiral coils and long flat cables, is manufactured on a polyimide substrate directly using the flexible printed circuit board (FPCB) process. Manganese cooper, instead of general cooper, is used as the material for coil because of its thermal stability of conductivity in the low temperature. The developed frequency-modulated (FM) oscillator is used to improve sensitivity and stability. A cryogenic calibration system is designed and fabricated to verify the performance of the sensor. Experimental results show that the flat manganese cooper coil with frameless construction can be specific about the effect of low temperature. The sensor has good thermal stability, sensitivity, temporal stability and repeatability in the temperature range of 20–273 K. Moreover, the flat sensor is flexible to easily adhere inside of the axial bush to monitor the axial displacement in practice.