Technique for capacitance-type blade tip clearance measurement based on PLL carrier frequency tracking

A capacitance FM type blade tip clearance measuring scheme based on PLL carrier frequency tracking was designed to cope with the interference of stray capacitance and meet demand of on-line monitoring. The PLL non-difference carrier frequency tracking loop effectively inhibited the slow carrier frequency drift caused by stray capacitance. The signal band was set at 200 kHz by choosing 1 MHz as IF frequency and signal peak acquisition control timing was developed to improve the system's working efficiency. Also the signal's nonlinear model was derived theoretically and verified. Simulative results show that the system has a high level of sensitivity and an ability of high speed on-line detection.