Page 163 - Servo Motors and Industrial Control Theory -

158 9 The Choice and Comparison of Servo Motors

Because of large dynamic settling time other types of motors are not suitable for
this application.
If the dynamic response is the only criterion, a 5 kW AC or a 1 kW DC with ce-
ramic or rare earth magnet might be selected. The final decision obviously depends
on the capital cost, size, and reliability.

9.5 Conclusion

In this chapter, the dynamic response of a number of types of servo motors com-
monly used in industry have been considered. To obtain the best performance, one
must optimize the forward loop gain, and the integral gain and the feedback gains
and the gains of compensation network in the controller. The dynamic performance
in each case was represented by a step input settling time and dynamic velocity drop
when an external torque is applied. The settling time was divided into a saturated
region where the motor operates in a torque limited condition and a dynamic region
behaves according to the linear mathematical model. A series of simulations were
carried out on different types and sizes of motors and the settling times plotted in
a series of graphs in such a way that the designer can predict the settling time and
dynamic velocity drop to be expected for a particular motor and applications, or
alternatively the motor to choose for a specific dynamic requirements.
As one might expect, no motor excels in all circumstances and the suitability of
a servo motor depends on its application. The following general comments can be
made as follows:
1. Low load inertia and low power motor: For these types of applications hydraulic
motors provide the fastest speed of response. The rare earth magnet and brush-
less DC motors are competitive with hydraulic motors. The effect of external
torque on the velocity of a hydraulic motor is somewhat higher than electrical
motors. The ceramic magnet DC, AC, and stepping motors have the lower speed
of response than the hydraulic and rare earth magnet DC motors.
2. High load inertia, low power motor: Under these conditions, the DC motors pro-
vide the best speed of response, followed by AC, hydraulic, and stepping motors.
The effect of external torque becomes less significant on hydraulic motors than
electrical motors.
3. Low load inertia, high power motor: Here, the hydraulic motors provide the fast-
est speed of response, followed by electrical DC and AC and stepping motors.
AC motors at high power ratings become competitive with conventional DC
motors. The effect of external torque on electrical motors is less than hydraulic
motors.
4. High load inertia, high power rating: For this condition, electrical motors pro-
duce a faster speed of response than the hydraulic motor and the AC motors
became competitive with conventional DC motors. External torque has less
effect on hydraulic motors.