Page 96 - Servo Motors and Industrial Control Theory -

4.9 Conclusion 89

Fig. 4.18 A typical lead-lag Demand Output
network signal Voltage
+ Kp(1 + K s)
d
– 1 + T s 2

Feedback

are fired they continue to conduct until the current becomes zero. This is achieved
by special circuitry design which allow the current becomes zero momentarily and
again they are fired at the right moment.

4.9 Conclusion

First it is studied when only the speed of the motor has to be controlled in open loop.
Hence the torque speed of the motor is established. It is shown that the starting of
torque is very large and therefore a current limiter must be designed in the control
unit. At this stage the inductance of the motor can be ignored. Then the closed
loop of DC motor for a better control of speed and reducing the effect of external
disturbance such the external torque. The proportional control was considered at it
was shown that the speed can be controlled with small steady state error. In closed
loop the inductance cannot be ignored. Quite a fast response can be obtained with
this method. These are proved with deriving the mathematical model for the sys-
tem. Throughout the chapter the classical feedback control theory is used and it is
expected that the reader is familiar with the classical Feedback control theory. The
proportional and integral control strategy was employed and indeed the integral
part causes the error of the system both for the demand signal and external torque
becomes zero. The velocity feedback was used to increase the damping ratio of the
system and hence a lager value of gains could be used.
In position control application the proportional and integral strategy was em-
ployed and it is shown that in closed loop position control the system becomes much
slower than the velocity control case. The characteristic equation becomes fourth
order which is quite complex. With use of Mathcad mathematical software makes
the analysis become much simpler. Throughout this chapter the above mentioned
mathematical software is used. There are other type of mathematical software that
you could use but it is found that the Mathcad mathematical software is very handy
with help of the software build in you can easily handle complex problems.
It should be noted that PID control was mentioned but because there is noise in
the system the derivative term is not considered but instead velocity feedback is
used. Other type of control strategy that may produce a better response is lead-Lag
network. The block diagram below shows the Lead-Lag network that may be used
instead of proportional control strategy. The analysis is left for reader to consider
and do the calculation (Fig. 4.18).