DC servo motor and applications

A d.c. servo motor is a d.c. motor (separately excited or permanent-magnet) designed for fast, accurate speed and position control, with a low-inertia armature and high torque-to-inertia ratio.

Construction

• Armature: long and thin laminated core, light, low inertia. Often a coreless (hollow-cup or disc) armature in small motors.
• Field: permanent magnet, or separately excited field winding.
• A tachogenerator or an encoder or potentiometer is mounted on the shaft for feedback.

Operating principle

The torque of a d.c. motor is T = k φ I_a and the speed is N ∝ (V − I_a R_a)/φ.

Two methods of control:

1. Armature control: the field is kept constant (or a permanent magnet) and the armature voltage is varied. The torque and speed are proportional to the armature voltage. This gives a linear torque-speed curve and is the common method.
2. Field control: armature current is kept constant and field voltage is changed. This has a slow response because of field inductance, so it is used rarely.

 Ref -> (+) -> Amp -> [Armature] M -> Load
        ^ (-)                |
        +---- Tachogenerator / encoder

The error between the reference and the feedback signal is amplified and applied to the armature. Torque is produced in proportion to the error, so the motor turns the load until the error becomes zero. Reversal of the error signal polarity reverses the direction of rotation.

Characteristics

• Torque proportional to armature current, speed proportional to voltage, a linear torque-speed curve.
• High starting torque and fast response (small electrical and mechanical time constants).
• Speed range is wide, from a few rpm to thousands.

Applications

Robotics, CNC machine tools, position control of antennas and radar, tape drives, disc drives, automatic cameras, printers, process control valves, aircraft control surfaces and toys.