Antireset windup

Process-variable feedback for the controller is achieved by one of two methods. The process variable can (I) be measured and transmitted to the controller by using a separate measurement transmitter with a 0.2-I.0-bar (3-15-psi pneumatic output, or (2) be sensed directly by the controller, which contains the measurement sensor within its enclosure. Controllers with integral sensing elements are available that sense pressure, differential pressure, temperature, and level. Some controller designs have the set point adjustment knob in the controller, making set point adjustment a local and manual operation. Other types receive a set point from a remotely located pneumatic source, such as a manual air set regulator or another controller, to achieve set point adjustment. There are versions of the pneumatic controller that support the useful one-, two-, and three-mode combinations of proportional, integral, and derivative actions. Other options include auto/manual transfer stations, antireset windup circuitry, on/off control, and process-variable and set point indicators. 

The use of high or low limits for process variables is another type of selective control, called an override. The feature of antireset windup in feedback controllers is a type of override. Another example is a distillation column with lower and upper limits on the heat input to the column reboiler. The minimum level ensures that liquid will remain on the trays, while the upper limit is determined by the onset of flooding. Overrides are also used in forced-draft combustion control systems to prevent an imbalance between airflow and fuel flow, which could result in unsafe operating conditions. 

Antireset windup can be implemented by simply not allowing the integral to accumulate when the manipulated variable is saturated. The manipulated variable is saturated when the control valve on the line supplying the manipulated variable is either closed or fully open. A saturated control valve can be identified when there is sustained offset between the maiupulated variable level... 

FIGURE 15.66 Response of a feedback system to a saturated manipulated variable, (a) Conventional PI controller and (b) PI controller with antireset windup. 

Tuning. Typical values are K.. = O. IK, Tj = 0.75 (0 + x), and = 0.0. The proportional band (lOO/A" ) and the reset time (1/T,) can be calculated from these. When fine tuning, observe the behavior of both the controlled and manipulated variables. Always use an implementation that includes antireset windup protection for use when the manipulated variable encounters a constraint. Use an implementation that includes initialization that starts automatic control bumplessly from the last manual condition. The digital execution period should be fast with respect to the feedback dynamics, with At < 0.05 (0 -i- x) where possible. 

A convenient method for providing feedforward compensation that does not interfere with either norm reset or antireset windup involves the use of an impulse relay and a summer as shown in Figure 9.8. In pneumatics these fimctions are sometimes combined into a single device. The impulse function... 

Cascading temperature or other variables to flow-ratio control should be avoided unless ratio turndown and flow turndown are less than 2 1. This applies whether one uses the divider (closed-loop) or multiplier (open-loop) flow-ratio scheme. Tlie problem is that the gain of the ratio loop as seen by the primary loop is zero at low flow and high at high flows. We have not foimd a convenient way to compensate for this with analog equipment without interfering with antireset windup. 

At this point it may be appropriate to note that a viable alternative to PI level control is PL level control. It has transfer functions very similar to those of the PI level control for a damping ratio of unity, the transfer functions are identical if one reduces the PL level controller gain, by a factor of 2. For Kd, < 1, it requires auto overrides just as the PI controller does. It has the feature, useful in some circvimstances, of not needing antireset windup. It is not a standard commercial item but usually can be assembled with various standard devices. 

In Chapter 12 we proposed a particular control-loop stmeture that incorporates overrides and antireset windup. It also accommodates feedforward compensation and advanced control techniques without interfering with either normal reset or antireset windup. We also suggested that decouplers could be designed to compensate for interactions in e same way that feedforward compensators are designed. The technique here leads to stable, noninteracting control, but not necessarily to optimum control. Modem control theory, with its more sophisticated approaches to multivariable control, sometimes reqtiires some interaction for optimality. 

Experimental Evaluation of Digital Algorithms for Antireset Windup, lEC Proc. Des. Dev., 15 (Apr. 1976). 

It inherently contains antireset windup, because the summation of errors is not explicitly calculated. 

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