An Absolute Capacitive Encoder Using Sample-and-Hold Demodulator to Reduce Interference Between Sine and Cosine Channels

This paper presents an absolute capacitive rotary encoder using a sample-and-hold demodulator (SHD) to reduce interference between sine and cosine channels. The capacitive encoder measures the rotation angle of a shaft and is widely used in industrial fields, such as servo motors, robot joint and gimbal positioning. The encoder converts the capacitive changes of sine and cosine channel electrodes, which vary with the rotation of the sinusoidal-patterned rotor, into voltage signals through a charge amplifier. Since the charge amplifier receives signals from each channel simultaneously, interference can occur between the sine and cosine channels. To reduce the interference, this paper proposes SHD. The SHD is a simple circuit composed of an operational amplifier and switches, which effectively separates multi-channel signals by adjusting the sampling timing. The signals from the sine and cosine channels separated by the SHD are smoothed by low pass filters to become analog signals. These analog signals are converted into digital signals and processed by a field programmable gate array to calculate the rotation angle. A MATLAB simulation was conducted to demonstrate that SHD can ideally reduce channel interference by 94.9%. The proposed encoder was designed and tested. It achieved a signal-to-noise ratio of 68.7 dB, accuracy of ±0.022°, resolution of 0.0055°, and nonlinearity of 0.27%.