Design of IIR filter in capacitive rotary position sensor based on FPGA

In the capacitive rotary position sensor, noise jamming generated by the motor is mixed in the response signal, so it is difficult to measure precisely. This paper presents an IIR filter solution with cascade structure. An IIR band-pass filter of 8-order is designed by Matlab, and then be disintegrated into 4 sub-filters. The band-pass frequency of the filter ranges from 18 kHz to 22 kHz with date sample rate of 400 kHz. All coefficients of the sub-filters are quantized by 2¹⁴ to satisfy the multiplication requirement in FPGA. The filter is described modularly by Verilog HDL and implemented in FPGA of Cyclone III series. The filter has been tested well with square wave. Signal attenuation outside the band-pass frequency is more than 50 dB. Modular structure makes it very convenient to design filters with different characteristics. At present, the solution is applied in a capacitive rotary position sensor.