Page 149 - Servo Motors and Industrial Control Theory -

144 9 The Choice and Comparison of Servo Motors

As discussed in previous chapters, there are also a number of power units available
for driving the electrical DC motors and these are thyristors controlled with up to
a frequency of 300 Hz and Pulse Width Modulated control unit of up and above
2 kHz pulsations.
It should be noted that only the classical feedback control strategy has been used
to compare different types of servo motors. State variable feedback control strategy
as presented in Chap. 4 is still the subject of further research even though the theory
is well established.
The smallest hydraulic motor available is with power of 10 kW with maximum
speed of 5,000 rpm. But electrical motors are available in a wide range of power.
Therefore, for small power ratings there are servo motors to meet the required pow-
er requirements.
The results of the analysis are given in terms of a number of parameters which
have been separated out in a way that is not strictly rigorous, but which allows pre-
sentation in a series of graphs which are easy to design purpose. It should be noted
that in order to compare servo motors with rated velocity of 1,500 rpm has been
used. For each type of motor, the lead-lag network together with an integrator with
their linearized mathematical model for small variation of inputs have been opti-
mized to get the dynamic behavior for both small input signal and external torque.
When referring to diagrams, the following key has been used:
1. Ceramic magnet DC motors
a. Controlled by 50 Hz thyristor bridge
b. Controlled by 150 Hz three phase thyristor bridge
c. Controlled by pulse width modulated (PWM) with frequency of 2 kHz
2. AC induction motors
3. Stepping servo motors
4. Electrohydraulic servo motors
5. Rare earth magnet DC motors
6. Printed or brushless DC motors

9.2 Theory and Performance Criteria

A servo feed-drive system consists of four subunits:
1. The controller which has low level signals and electronics
2. The power unit which deals with high current signals
3. The load mechanism which may contain gear box to convert the high speed to
low rotational speed or may have a lead screw type speed reduction mechanism
to convert rotational motion to linear motion.

As was discussed in previous chapters, a mathematical model is developed for each
of the subunits by employing linear or linearized relations. The load mechanism is