Performance analysis and compensation of M/T-type digital tachometers

New analyses are presented of the widely used M/T-type digital tachometer. This device incorporates accurate time measurement with pulse counting. It is shown that the accuracy of velocity measurement in real implementations is significantly poorer than indicated previously. Encoder nonidealities are found to introduce a bias in the measured velocity output. A significant rms output error can also result. The insensitivity of the error to the form of the encoder noise model is demonstrated, and two analyses of differing complexity are presented. These permit calculation of the tachometer error measures, based on a knowledge of the magnitude of the error characteristics of the encoder. This facilitates the implementation of a compensation function that will reduce the influence of the bias. Experimental results are presented which demonstrate the existence of the bias, verify the validity of the derived formulae, and show the benefit to be gained by compensation.