Page 152 - Servo Motors and Industrial Control Theory -

9.3 Comparison of Results and Design Procedure 147

Fig. 9.2 Velocity varia- Time (sec)
tion for maximum step 0
input of 100 % of external
torque 1
Velocity Steady State 2
Velocity Variation 100
Drop

200
(b)
%

9.3 Comparison of Results and Design Procedure

The choice of a motor for specific application depends on the priority placed on cer-
tain criteria, including the speed of response, stiffness to external torque, size, capi-
tal cost, running cost, reliability, and availability. The main object of this section
is to establish a comparison of speed of response and effect of external torque, so
that a designer is able to choose a specific motor to meet the dynamic performance
requirements. The time response of a servo motor may be split into two parts.
1. The dynamic settling time in the linear mode, where the motor is operating within
its limit condition. This implies that the operating conditions and the change in
the variables involved is small, so they operate in a linear fashion as described by
the mathematical model of the motor in previous chapters. Although it will not
be rigorous, the dynamic settling time due to the inertia of the rotor ( t ) will be
r1
separated from the dynamic settling time due to load inertia referred to the motor
shaft ( t ) for case of comparison.
r2
2. The saturated settling time, where the motor is operating at its limit conditions.
This implies that the motor is drawing its maximum current and, therefore, pro-
viding a constant torque for the initial part of a large change from steady state.
The static settling time is a function of the required velocity variation in addition
to the motor and load inertia.

Figure 9.3 shows the variation of the sizes of motors for various types with respect
to the power rating. It can be seen that the hydraulic motors have the lowest volume
for a given power. This does not include the power pack requirements. Also the
straight line indicates that the smallest hydraulic motor was capable of producing
up to 10 kW power. This may be a disadvantage for hydraulic motors. The rare earth
magnet DC motor produces the highest power for a given size in electrical motors.
Again this does not include the weight of the power unit which should not be too
large. It follows with printed or brushless DC servo motors. Then the induction AC
servo motors have a comparable size to power ratio with brushless DC servo motors