Page 90 - Servo Motors and Industrial Control Theory -

4.7 Properties of Power Unit 83

x :=− (AC) ·B2
1
−
− 3.876 10× − 3
0 (4.35)
x =− 0.029
0
0.12

It can be seen that the errors are very small and when the maximum rated torque of
approximately 10 N m is used the error just becomes ten times greater than the value
given above. It should be noted that units of all rotation is in radian. It should also
be noted that there is a current of 0.12 A passes through the motor to try to correct
the position.
For greater accuracy, the roots must be moved further away from the real axis of
the s-plane. For zero steady state error, an integrator must be added in the forward
path of the system. This increases the order of the system from 5–6 and the system
becomes more complicated. The reader is encouraged to study this as final year or
master project. In this case it may not be possible to measure all state variables. The
use of full order or reduced order observer must be studied.
The purpose above was to show that for servo motor writing the governed dif-
ferential equations in above form all state variables could be measured for feedback.
All roots also can be moved to required position on the s-plane.

4.7 Properties of Power Unit

In the previous section, various forms of power unit for DC servo motors were
discussed. It is important to understand the relationship between the output voltage
and the control signal. In Sect. 4.6, a single gain K was used to transfer the control
signal to a high current and voltage were used. In actual system, a series of rectified
pulses are connected to the motor. The motor then responds to the average output
voltage. The purpose of this section is to study the relationship between the control
signal and the output voltage and to high-light important properties of power unit.
First thyristor controlled power unit will be discussed.
It was shown in Fig. 4.2 that the low level control signal controls the firing angle
of thyristor. A portion of the input signal over one period will pass through the
thyristor. The worst condition when single-phase half rectification is used will be
considered here. Generally the firing angle is proportional to the control signal as

α : K ·V= c c (4.36)

where α is the firing angle, K is the transformation gain, and V is the control signal.
c
c
The average output voltage may be obtained as