Short note: DC servo motor

A DC servo motor is a small DC motor designed for fast, accurate control of position or speed in a closed-loop (feedback) control system. It is the "actuator" that turns the error signal of a servo system into controlled motion.

Construction

• Similar to an ordinary DC motor, but with a long, small-diameter armature to keep inertia ($J$) low.
• Field is either a permanent magnet or a separately excited winding.
• Low-inertia types: slotless (smooth) armature, disc (printed-circuit) armature and shell/cup armature, which give very small electrical and mechanical time constants.

Types and working

1. Armature-controlled: field current is kept constant and the control voltage is applied to the armature. Torque $T = K_t I_a$, so torque and speed vary linearly with armature voltage. This is the most common type (larger power, good damping from back emf).
2. Field-controlled: armature current is kept constant and the control signal is applied to the field winding. Torque $T \propto \phi \propto I_f$. Used for small powers; the small field current is easy to control, but response is slower (large field inductance) and there is no back-emf damping.

 error   +-----------+  Va   +-------+   shaft
 ------->| Amplifier |------>|  DC   |----------> load
   ^     +-----------+       | servo |     |
   |                         +-------+     |
   +------- feedback (pot / encoder) <-----+

Features

• Linear torque–speed and torque–voltage characteristics
• High starting torque, quick reversal, wide speed range
• High torque-to-inertia ratio, fast response

Applications

Robotics, CNC machine tools, X–Y plotters and printers, disk drives, tracking antennas, aircraft control surfaces and position control systems.

Drawback: brushes and commutator need maintenance and produce sparking, so brushless servo drives are replacing them in many uses.