High frequency demodulation technique for instantaneous angular speed estimation

Abstract

This paper adapts the super-heterodyne method from telecommunication field to speed estimation from an optical encoder signal. After a presentation of our gear box test bench and the associated electronic devices, the paper explains the classical methods for Instantaneous Angular Speed (IAS) estimation from an optical encoder. Firstly, elapsed time method that measures the interval between consecutive (rising) edges of an optical encoder is investigated: aliasing problem and quantification effect are explained. Secondly, frequency demodulation technique based on temporally sampled optical encoder signal is investigated. An analysis of this signal shows its imperfections (crosstalk, presence of a second order harmonic, …). The necessity of a high-sampling frequency, the aliasing problem and the noise impact are also illustrated. Next, this method is compared with elapsed time. In the second section, a new method is proposed: high frequency demodulation. This method uses an analog frequency shifting before sampling the signal. Therefore, the required sampling frequency is reduced and linked with the speed fluctuation frequency. Spectrum of optical encoder signal is analysed and used to explain how to set up the frequency shift and limitations. Finally, a comparison is made with elapsed time.