30079_28b - Then the system is switched into automatic...

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Then the system is switched into automatic mode. Digital computers are often used to replace the manual adjustment process because they can be readily coded to produce complicated functions for the start-up signals. Care must also be taken when switching from manual to automatic. For example, the integrators in electronic controllers must be provided with the proper initial conditions. 28.7.5 Reset Windup In practice, all actuators and final control elements have a limited operating range. For example, a motor-amplifier combination can produce a torque proportional to the input voltage over only a limited range. No amplifier can supply an infinite current; there is a maximum current and thus a maximum torque that the system can produce. The final control elements are said to be overdriven when they are commanded by the controller to do something they cannot do. Since the limitations of the final control elements are ultimately due to the limited rate at which they can supply energy, it is important that all system performance specifications and controller designs be consistent with the energy-delivery capabilities of the elements to be used. Controllers using integral action can exhibit the phenomenon called reset windup or integrator buildup when overdriven, if they are not properly designed. For a step change in set point, the proportional term responds instantly and saturates immediately if the set-point change is large enough. On the other hand, the integral term does not respond as fast, It integrates the error signal and saturates some time later if the error remains large for a long enough time. As the error decreases, the pro- portional term no longer causes saturation. However, the integral term continues to increase as long as the error has not changed sign, and thus the manipulated variable remains saturated. Even though the output is very near its desired value, the manipulated variable remains saturated until after the error has reversed sign. The result can be an undesirable overshoot in the response of the controlled variable. Limits on the controller prevent the voltages from exceeding the value required to saturate the actuator, and thus protect the actuator, but they do not prevent the integral build-up that causes the overshoot. One way to prevent integrator build-up is to select the gains so that saturation will never occur. This requires knowledge of the maximum input magnitude that the system will encounter. General algorithms for doing this are not available; some methods for low-order systems are presented in Ref. 1, Chap. 7, and Ref. 2, Chap. 7. Integrator build-up is easier to prevent when using digital control; this is discussed in Section 28.10. 28.8 COMPENSATION AND ALTERNATIVE CONTROL STRUCTURES A common design technique is to insert a compensator into the system when the PID control algo- rithm can be made to satisfy most but not all of the design specifications. A compensator is a device that alters the response of the controller so that the overall system will have satisfactory performance.
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This note was uploaded on 05/02/2010 for the course ME 100 taught by Professor Any during the Spring '10 term at Purdue University-West Lafayette.

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30079_28b - Then the system is switched into automatic...

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