Tachometer

A tachometer (revolution-counter, tach, rev-counter, RPM gauge) is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine.

Prolonged use at high speeds may cause inadequate lubrication, overheating (exceeding capability of the cooling system), exceeding speed capability of sub-parts of the engine (for example spring retracted valves) thus causing excessive wear or permanent damage or failure of engines.

On analogue tachometers, speeds above maximum safe operating speed are typically indicated by an area of the gauge marked in red, giving rise to the expression of "redlining" an engine — revving the engine up to the maximum safe limit.

Aircraft tachometers have a green arc showing the engine's designed cruising speed range.

In older vehicles, the tachometer is driven by the RMS voltage waves from the low tension (LT contact breaker) side of the ignition coil,[4] while on others (and nearly all diesel engines, which have no ignition system) engine speed is determined by the frequency from the alternator tachometer output.

Tachometers driven by a rotating cable from a drive unit fitted to the engine (usually on the camshaft) exist - usually on simple diesel-engined machinery with basic or no electrical systems.

Hall effect sensors typically use a rotating target attached to a wheel, gearbox or motor.

The probe is mounted with its head a precise distance from the target wheel and detects the teeth or magnets passing its face.

The slotted disk is typically sandwiched between two circuit boards containing a photo-diode, photo-transistor, amplifier, and filtering circuits which produce a square wave pulse train output customized to the customer's voltage and pulses per revolution requirements.

The sensors mounted around the circumference of the disk provide quadrature encoded outputs and thus allow the vehicle's computer to determine the direction of rotation of the wheel.

It is difficult to prove conclusively that the vehicle is stationary, other than by waiting a certain time to ensure that no further pulses occur.

This calibration must be done while coasting at a fixed speed to eliminate the possibility of wheel slip/slide introducing errors into the calculation.

Automatic calibration of this type is used to generate more accurate traction and braking signals, and to improve wheel slip detection.

To compensate for this, secondary odometry inputs employ Doppler radar units beneath the train to measure speed independently.

In analogue audio recording, a tachometer is a device that measures the speed of audiotape as it passes across the head.