Rotational Speed

A tachometer is a sensor for measuring rotational speed. The most common tachometers are proximity probes and optical transducers. These transducers generate pulses at a rate proportional to the rotational speed, typically once per revolution.

A proximity probe can detect the presence of a keyway slot. The probe then generates a pulse at a certain fixed amplitude as the keyway slot passes. The following illustration shows a proximity probe working as a tachometer to generate pulses.

Optical transducers observe a piece of reflective tape attached to the shaft. The coincidence of the reflective tape and the optical transducer produces a pulse signal. The following illustration shows an optical transducer working as a tachometer to generate pulses.

Optical transducers are well-suited for machines that cannot tolerate drilled holes or milled slots in the exposed shaft surface. Optical transducers also are appropriate for detecting pulses from high-speed machines.

An encoder is another common tachometer transducer. The following illustration shows an encoder working as a tachometer to generate pulses.

The encoder generates pulses when the slots on the encoding disk interrupt the beam between the photoelectric transducer pair. An encoder usually generates multiple, even several hundred or more, pulses per revolution. Thus, encoders can generate more accurate speed results for low rotational speed measurements that are slower than 100 RPM.