Page 157 - Servo Motors and Industrial Control Theory -

152 9 The Choice and Comparison of Servo Motors

Fig. 9.7 Saturated settling 0.1
time for a unit change of
velocity and for a unit load
inertia
0.08 3 2
ts2 (sec / kg-m 2 / (rad/sec)) 0.06

0.04

0.02 6 4
1
5
0
0 2 4 6 8 10
Power (kw)

inertias, and the results were averaged and scaled for a unit load inertia. Figure 9.8
shows the results, and it can be seen that the hydraulic motor has the largest dy-
namic settling time. This is mainly due to the fact that because the maximum torque
of the hydraulic motor is much smaller than electric motors and also the inertia of
the hydraulic motor is much smaller that electric motors. It is also more sensitive
to load inertia. The dynamic settling time of the stepping motor is larger than other
electrical motors due to its sensitivity to load inertia. DC motors have the smallest
dynamic and saturated settling time due to their capability of producing large torque
for a short period of time.
The dynamic and saturated settling time of electrical motors due to load inertia
reduces as the power rating of the motor increases, because higher power motor are
capable of higher torque. As stated previously, the single hydraulic motor covered
the range up to 10 kW power range and, therefore, the result is a straight line. To
give a smaller dynamic and saturated settling time the reader is encouraged to study
the behavior of larger power rating of hydraulic servo motors. It should be noted
that the property of a servo valve also influences the result.
From Figs. 9.5, 9.6, 9.7, and 9.8 a designer, by knowing the rotor and load inertia
and maximum velocity variations, is then able to predict the total settling time for a
particular motor using eq. (9.1).
In many applications, the variation in angular velocity due to an external torque
is important. Figure 9.9 shows the dynamic velocity drop for a unit step input of
torque and without any load inertia. It can be seen that the electrohydraulic motor
has the highest velocity drop, followed by AC and DC motors. The speed of recov-
ery is similar to the dynamic settling time, which can be obtained from Fig. 9.6. It
appears that the velocity drop for a step input largely depends on rotor inertia. The