Engineers often want to monitor a shaft’s rotational speed but get bogged down trying to get sensors, signal conditioners, cabling, and bulky housings to work together. To solve this dilemma, designers at Electro-Sensors Inc., Minnetonka, Minn. (www.electro-sensors.com), have developed the ST420 shaft tachometer.

It combines sensing, signal processing, and two-wire, loop-powered circuitry in a sealed stainless-steel M18x1 housing. It can sense rotational speeds from a shaft-mounted pulser target (disc or wrap) and outputs a proportional analog current.

Permanent magnets embedded in the target rotate past the stationary sensor. As each magnet passes, the internal sensor puts out a pulse. Pulse frequency is proportional to the shaft rpm and quartz-crystal digital circuitry continuously converts it to a linearly proportional analog signal that ranges from 4 to 20 mA.

There are no sensor bearings to wear out, as with shaft encoders, and the sensor is uncoupled from forces, stresses, and vibrations acting on the shaft. And the sensor and embedded magnets, separated by a gap, do not need to be precisely aligned which simplifies installation. The sensor can also survive harsh environments and is not affected by accumulated dust, grime, dirt, and oil.

The tachometer is factory calibrated and UL tested. Standards and custom rpm ranges and pulser targets are available.

Edited by Stephen J. Mraz