Electrical DC Servo Motors:Conclusion

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 employed 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).

In the block diagram Kp is the gain and Kd is the time constant of the lead net- work and T2 is the time constant of the lag network. Velocity of position feedback must be used and it is compared with the demand signal and the error is fed to the Lead-lag network. The output voltage after amplification is fed to the motor. The presence of a derivative term makes this control strategy similar to PID.

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