What device is used to monitor rotational speed?

• A. Encoder
• B. Tachometer
• C. Controller
• D. Sensor

Answer: (B)

Measuring rotational speed is the job of a tachometer. A tachometer directly reports how fast a shaft is turning, typically in RPM, and can be analog or digital. In motor control, tachometer feedback helps keep the speed on target by feeding into the controller. An encoder, while it can be used to infer speed by counting how many pulses occur over time, is mainly a device for measuring position or angle, not a dedicated speed readout. A sensor is a general device that detects a physical quantity, and a controller is the unit that processes signals and makes decisions, not a speed measurement itself.