Which device is commonly used to monitor rotational position of a motor shaft?

• A. Limit switch
• B. Optical encoder
• C. Reed switch
• D. Proximity sensor

Answer: (B)

Measuring rotational position requires a sensor that can translate shaft angle into a precise electrical signal for feedback. An optical encoder fits this need because it attaches to the motor shaft and, as the shaft spins, a light source and a patterned disk generate pulses. The relative timing (quadrature) of those pulses encodes both how far the shaft has turned and the rotation direction, so a controller can count pulses to determine position, speed, and angle with high resolution. This kind of feedback is essential for accurate motor control and closed-loop systems.

Limit switches are binary indicators used to detect endpoints or limits in linear or limited-stroke motion, not continuous rotation. Reed switches sense magnetic fields but are typically used for simple on/off sensing with magnets and don’t provide the high-resolution position information needed for rotational tracking. Proximity sensors detect the presence of nearby objects (often metal) but do not offer the angular position data needed to know where a shaft is in its rotation.