Development and experimental investigation of a high-precision capacitive displacement transducer of the inertial sensor for TianQin

Capacitive sensing electronics are critical to space gravitational wave detection missions, providing the test mass motion information for the inertial sensor's non-sensitive axis control and the drag-free control of the satellite. For the TianQin mission, the capacitive sensor requirement is 6.9x10-7pFHz-1/2 at 6 mHz, corresponding to a displacement noise of 1.7nmHz-1/2 . This paper presents the system design scheme, noise analysis, and performance testing of the capacitive sensor for the TianQin mission. Based on the previous work of our group, a capacitive sensing scheme based on a transformer bridge and modulation-demodulation is adopted. A transformer with a better high LQ product is fabricated, and the preamplifier current noise is reduced to further improve the performance above mHz. The amplitude stability of the AC carrier and the performance of the lock-in amplifier circuit are further improved to reduce the low-frequency 1/f noise at mHz and below. Finally, a capacitive sensor prototype is developed and tested, the experimental results show that the capacitance measurement noise is better than 3x10-7pFHz-1/2 above 1 mHz and better than 6x10-7pFHz-1/2 at about 0.1mHz , which meets the overall performance requirement of the TianQin mission.