Improved digital tachometer with reduced sensitivity to sensor nonideality

A constant sample-time digital tachometer, similar to Ohmae's 'M/T' method, is described. This system is capable of accurate determination of shaft velocity, based on the outputs of an incremental encoder. Previous error analyses of such devices which consider only the asynchronous nature of counter operation are shown to be specious. It is highlighted that sensor nonidealities, including variations in the transition locations from their nominal values and phasing errors between encoder channels, can produce much larger tachometer errors. An estimate is made of the actual rms output error, based on a simple encoder model and simplified analysis. It is shown that the error can be significantly reduced by operating four sub-circuits in parallel, each dealing with either a low-to-high or high-to-low transition of a single incremental encoder signal. A field-programmable-gate-array/digital-signal-processor-based practical implementation is described. Both simulation and experimental results are presented which confirm the utility of the enhanced digital tachometer.