Glossary term
Tachometer
An instrument or sensor system used to measure the rotational speed of shafts, rotors, wheels, motors, or machinery.
Definition
deviceA tachometer is an instrument or sensor system that measures rotational speed of a shaft, rotor, wheel, or rotating machine.
Tachometers convert periodic mechanical rotation into an electrical, optical, magnetic, or digital signal from which speed is estimated. They are used in engines, turbines, motors, machine tools, pumps, vehicles, test rigs, and control systems where rotational speed affects performance, safety, vibration, or synchronization.
A tachometer measures rotational speed by detecting repeated events tied to shaft angle. The signal may come from a magnetic pickup, optical mark, Hall sensor, encoder channel, alternator frequency, toothed wheel, proximity probe, or contact wheel. Speed is then computed from pulse frequency or from the time interval between pulses.
For a sensor that produces P pulses per revolution, rotational speed can be estimated from frequency f_p as:
where N is in revolutions per minute. At high speed, frequency measurement is often convenient. At low speed, period measurement may give better resolution, but it becomes more sensitive to pulse timing jitter and eccentricity.
Engineering use
Tachometers support speed control, overspeed protection, order tracking, torsional vibration testing, rotor balancing, engine calibration, conveyor synchronization, and condition monitoring. In rotating machinery analysis, a once-per-revolution phase reference is often as important as speed magnitude because it allows vibration data to be related to shaft angle.
The measurement chain includes the target geometry, sensor air gap, signal conditioning, trigger threshold, sampling rate, filtering, cable shielding, counter resolution, and update algorithm. Dynamic measurements must also state latency and bandwidth; a tachometer that is adequate for steady dashboard display may be too slow for control or transient run-up analysis.
Common mistakes
A common mistake is reading a noisy pulse train as true speed variation when the cause is runout, missing teeth, weak signal-to-noise ratio, trigger bounce, or timing jitter. Another is comparing tachometer values from instruments with different averaging windows. A strong tachometer review states pulses per revolution, sensing principle, speed range, resolution, update rate, phase reference, filtering, mounting method, and how missed or false pulses are handled.