Antiwindup

Fig. 4. Block diagram of the robust approach with antiwindup action (10)...

If the preload setting is too high, response is similar to that without the antiwindup switch (Fig. 4.5). With too little preload, throttling begins prematurely and the controller must bring the measurement to the... 

Some integration must take place, because a sizable error always exists at the time h when the antiwindup switch activates the reset circuit. The load q (output at time <2) will differ from the preload b by the amount of this integration ... 

A more severe form of interaetion exists in pneumatie eontrollers whose reset and derivative circuits are in parallel feedbaek about the amplifier. Pneumatic controllers equipped with an antiwindup switeh are connected in this way. The only difference between these and the eonven-tional interacting controller is in the effective proportional band ... 

Measure the period of oscillation t and set derivative and reset time both to t /2t. (The optimum value of D and R in Table 4.2 is 0.64Td, which is 2Td/x or To/2it.) For a two-mode controller, set R at t<,/2.4. When adjusting a three-mode pneumatic controller with antiwindup, always keep R>2D to retain proportional stability. 

The linear controller must, above all, be preloaded to the anticipated process conditions at the new set point. This was found to be advantageous whenever an antiwindup switch is used in the reset circuit. And because reset is normally required with a dual-mode arrangement, the control system must include an antiwindup switch into which the preload setting is introduced. The arrangement of the system for increasing set-point changes is shown in Fig. 5.17. 

Occasionally a situation n-ill arise when none of the controllers in a selective loop assumes control. Some processes are prone to this, although in most it cannot happen. In the process shown in Fig. 6.15, neither controller would take over if suction pressure were to fall low enough. In this case, discharge pressure and flow would both be below their set points and the motor would go to maximum speed. Both controllers would then saturate. When suction pressure was restored, discharge pressure or flow or both could overshoot their set points. If this were a common occurrence, an antiwindup switch should be inserted in the common feedback line at the output of the selector. 

In order to avoid overshoot, the primary controller must be equipped with an antiwindup switch with preload applied to the reset circuit. It... 

Answers to problems, 349-353 Antiwindup switch, 97, 98 Auctioneering, 168 Averaging control, 147, 148 Azeotrope, 340 binary, 340, 341 heterogeneous, 340 homogeneous, 341, 342 ternary, 341... 

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