Stepper Motors
DC Servo Motors
Dc servo motors are normally used as prime movers in computers,
numerically controlled machinery, or other applications where starts
and stops are made quickly and accurately. Servo motors have
lightweight, low-inertia armatures that respond quickly to
excitation-voltage changes. In addition, very low armature
inductance in these servo motors results in a low electrical time
constant (typically 0.05 to 1.5 msec) that further sharpens servo
motor response to command signals. Servo motors include
permanent-magnetic, printed-circuit, and moving-coil (or shell) dc
servo motors. The rotor of a shell dc servo motor consists of a
cylindrical shell of copper or aluminum wire coils which rotate in a
magnetic field in the annular space between magnetic pole pieces and
a stationary iron core. The servo motor features a field, which is
provided by cast AlNiCo magnets whose magnetic axis is radial. Servo
motors usually have two, four, or six poles.
Dc servo motor characteristics include inertia, physical shape,
costs, shaft resonance, shaft configuration, speed, and weight.
Although these dc servo motors have similar torque ratings, their
physical and electrical constants vary.
DC Servo Motor Selection: The
first selection approach is to choose a servo motor large enough for
a machine that has already been designed; the second is to select
the best available servo motor with a specific feature and then
build the system around it; and the third is to study servo motor
performance and system requirements and mate the two.
The final servo motor system design is usually the least
sophisticated that meets the performance specifications reliably.
Servo motor requirements may include control of acceleration,
velocity, and position to very close tolerances. This says that the
servo designer must define the system carefully, establish the servo
motor's performance specifications, determine critical areas, and
set up tolerances. Only then will the designer be able to propose an
adequate servosystem and choose a servo motor type. |