DC servomotors:
There are many types of dc motors in use in industries. DC motors that are used in servo systems are called dc servomotors. In dc servomotors, the rotor inertias have been made very small, with the result that motors with very high torque-to-inertia ratios are commercially available. Some dc servomotors have extremely small time constants. DC servomotors with relatively small power ratings are used in instruments and computer-related equipment such as disk drives, tape drives, printers, and word processors.
DC servomotors with medium and large power ratings are used in robot systems, numerically controlled milling machines, and so on. In dc servomotors, the field windings may be connected in series with the armature or the field windings may be separate from the armature. (That is, the magnetic field is produced by a separate circuit.) In the latter case, where the field is excited separately, the magnetic flux is independent of the armature current. In some dc servomotors, the magnetic field is produced by a permanent magnet and, therefore, the magnetic flux is constant. Such dc servomotors are called permanent magnet dc servomotors.
DC servomotors with separately excited fields, as well as permanent magnet dc servomotors, can be controlled by the armature current. Such a scheme to control the output of the dc servomotor by the armature current is called armature control of dc servomotors. In the case where the armature current is maintained constant and the speed is controlled by the field voltage, the dc motor is called a field-controlled dc motor. (Some speed control systems use field-controlled dc motors.) The requirement of constant armature current, however, is a serious disadvantage. (Providing a constant current source is much more difficult than providing a constant voltage source.) The time constants of the field-controlled dc motor are generally large compared with the time constants of a comparable armature-controlled dc motor.
A dc servomotor may also be driven by an electronic motion controller, frequently called a servo driver, as a motor-driver combination. The servo driver controls the motion of a dc servomotor and operates in various modes. Some of the features are point to-point positioning, velocity profiling, and programmable acceleration.
The use of an electronic motion controller using a pulse-width-modulated driver to control a dc servomotor is frequently seen in robot control systems, numerical control systems, and other position and/or speed control systems.
