robotics//sensor//tachometer

A tachometer measures how fast a shaft turns, and in a control loop it is the sensor that closes a speed loop: a conveyor held at constant speed, a spindle at its rpm, a motor whose speed must not drop when the load rises. The classic form is the **tachogenerator**, a small DC generator on the motor shaft whose output voltage is proportional to speed, typically 6 to 12 volts per thousand rpm, which an analogue drive could compare directly with a setpoint voltage.

A tachometer measures how fast a shaft turns, and in a control loop it is the sensor that closes a speed loop: a conveyor held at constant speed, a spindle at its rpm, a motor whose speed must not drop when the load rises. The classic form is the tachogenerator, a small DC generator on the motor shaft whose output voltage is proportional to speed, typically 6 to 12 volts per thousand rpm, which an analogue drive could compare directly with a setpoint voltage.

Today most speed loops measure with an encoder instead, a disc that emits pulses as the shaft turns, and compute the speed by counting pulses per sample period or by timing the interval between them. The digital measurement does not drift and gives the position as well; its weakness is low speed, where few pulses arrive per sample and the speed estimate becomes coarse.

The speed loop is the textbook control loop: a tachometer as the sensor, a drive as the actuator, the motor with its load as the plant. In a servo axis it usually sits inside a position loop, which gives it its setpoint (cascade control), and inside it there is often a current loop, faster still (motion control).

Its ripple goes straight into the error. A tachogenerator's voltage carries a ripple from its commutator, and an encoder's speed estimate is quantised at low speed; a fast speed loop passes either to the drive unless the measurement is filtered, which is one reason speed loops rarely use derivative action.

Many drives estimate speed without any sensor, from the motor's own currents and voltages (sensorless control). It is cheaper and good enough for pumps and fans, and weaker at very low speed, where a shaft sensor is still fitted (VFD).