Preload, reset

A logical solution to the reset-windup problem is to add enough intelligence to the controller to make it aware of a shutdown condition. This is done by placing in the controller s reset circuit a switch energized by the output. Whenever the output exceeds 100 percent, the switch disables the reset circuit, leaving a proportional (or proportional-plus-deriva-tive) controller. In the absence of automatic reset action, a bias must take its place. Because this bias equals the output of the controller at zero deviation, it is ordinarily adjusted in relation to the expected process load. For this reason it is sometimes called the preload setting. 

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 ... 

When placing one of these controllers into operation, it is first necessary to adjust proportional, derivative, and reset for maximum performance in the steady state, just as would be done for a continuous process. Once these are set, response during startup depends entirely on the value of preload. 

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. 

While e > ei, the on-off operator is energized, sending 100 percent output to the process. The preload setting q is sent to the proportional-plus-reset-plus-derivative controller. 

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

For a problem such as this, the dual-mode eontrol system deseribed in Fig. 5.17 is extremely effective. The preload is estimated as before, but no correction is required for integration, because reset aetion is not initiated until the error is nearly zero. Full heating ean be applied to within 1 or 2 percent of the set point, far beyond the eapabilities of a 25 pereent proportional band. Yet full cooling need only be applied for a time delay of perhaps a minute to dissipate the energy stored in the jaeket. As pointed out in Fig. 5.18, the switching parameters are easy to adjust and tolerant of maladjustment. 

Alternatively, Shinskey (1996) recommends using a bias term (or preload) uq in the control law to prevent reset windup ... 

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