A simple digital tachometer with high precision in a wide speed range

A common method in adjustable speed drives uses an incremental shaft encoder and an electronic circuit for velocity estimation. The usual method of counting pulses coming from the encoder in a fixed period of time produces a high precision velocity estimate in the high speed range. High precision in the low speed range can be achieved by measuring the elapsed time between two successive pulses coming from the encoder. In this paper a mixed method is proposed that combines the best of the two previously mentioned approaches. The proposed method has been implemented using a simple electronic circuit based on one field programmable gate array and one ROM device