Page 155 - Servo Motors and Industrial Control Theory -

150 9 The Choice and Comparison of Servo Motors

Fig. 9.5 Saturated settling 0.5 × 0.001 2
time for a unit change of
velocity without any load
inertia 0.4 1
Ts1 (sec/(rad/sec)) 0.3 3

0.2

0.1
5 6
4
0
0 2 4 6 8 10
Power (kw)

A simulation using the mathematical model presented in previous chapters is
used to find the dynamic settling time of different types of servo motors. The dy-
namic settling time of servo motors without and load inertia is greatly influenced
by the maximum torque availability of the motor and its rotor inertia. The saturated
settling time of servo motors without any load inertia due to a unit change in angular
velocity is approximated by

I
t: = r (9.2)
1 s
T
m
where T is the maximum torque of the motor and I is the inertia of the rotor. The
m
r
above equation is derived by writing the Newton’s second law of motion and solv-
ing the resulting equation for a unit velocity change.
Figure 9.5 shows the saturated settling time of servo motors and Fig. 9.6 shows
the corresponding dynamic settling time. It can be seen that the hydraulic motors
give the lowest dynamic and saturated settling time compared with electric motors.
At low power range, the electric motors are a good replacement for hydraulic mo-
tors.
The rare earth magnet and printed DC motors having low inertia have the fastest
response time and then it follows by ceramic magnet DC motors, stepping motors,
and induction AC servo motors.
Figure 9.6 shows that the hydraulic motors have the fastest dynamic response.
The rare earth magnet and printed DC servo motors have a slightly larger settling
time but they are competing well with hydraulic motors. The dynamic settling time
also depends on the power unit used. It can be seen that PWM drive for DC motors
produces a competitive response with hydraulic and brushless DC servo motors.
The thyristor controlled DC servo motors have larger response time. The AC induc-
tion servo motors having a lower inertia even though have lower maximum torque
compared with DC motors, have become good competitively for DC servo motors.