Windup, reset

When a controller with integral action (PI or PID) sees an error signal for a long period of time, it integrates the error until it reaches a maximum (usually 100 percent of scale) or a minimum (usually 0 percent). This is called reset windup. 

A sustained error signal can occur for a number of reasons, but the use of override control is one major cause. If the main controller has integral action, it wfll windup when the override controller has control of the valve. And if the override controller is a PI controller, it will windup when the normal controller is setting the valve. So this reset windup problem must be recognized and solved. 

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The use of high or low limits for process variables is another type of selective control, called an override. The feature of anti-reset 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... 

By the very nature of batch processing, it is inevitable that process equipment will have idle time between batches. The idle time also could occur if the controller is not being used at all times during the execution of the batch (e.g., a flow controller may only be used during the feed of one of the reactants). During the idle time, control considerations such as reset windup must be considered to prevent the control signal from going outside of the control limits. 

Seagall, N.L. Taylor, P.A. "Saturation of Single-Input Single-Output Controllers Written in Velocity Form Reset Windup Protection", lEC Process Des. Dev., 1986, 25., 495. 

This is the velocity form algorithm which is considered to be more attractive than the position form. The summation of error is not computed explicitly and thus the velocity form is not as susceptible to reset windup. 

Because of reset windup, this control mode is not well-suited for processes that are frequently shut down and started up. 

Reset windup is an inherent disadvantage of proportional plus reset controllers that are subject to large error signals. 

Proportional plus rate controllers are useful with processes which are frequently started up and shut down because it is not susceptible to reset windup. 

There are several control problems in chemical reactors. One of the most commonly studied is the temperature stabilization in exothermic monomolec-ular irreversible reaction A B in a cooled continuous-stirred tank reactor, CSTR. Main theoretical questions in control of chemical reactors address the design of control functions such that, for instance (i) feedback compensates the nonlinear nature of the chemical process to induce linear stable behavior (ii) stabilization is attained in spite of constrains in input control (e.g., bounded control or anti-reset windup) (iii) temperature is regulated in spite of uncertain kinetic model (parametric or kinetics type) or (iv) stabilization is achieved in presence of recycle streams. In addition, reactor stabilization should be achieved for set of physically realizable initial conditions, (i.e., global... 

I Anti "reset windup (only integrate ettcr if op signal is between 0 and 1... 

In addition to following the load, protective overrides prevent the development of excessively low suction pressures (PIC-02), which could result in drawing oil into the compressor and from overloading the drive motor and thereby tripping the circuit breaker (KWIC). In order to prevent reset windup when the controller output is blocked from affecting the SIC set point, external feedback (EFB) is provided for all three controllers. 

The positional algorithm is preferred when the measurement is noisy, because it works with the error and not the rate of error change when calculating its proportional correction. Velocity algorithms have the advantages of providing bumpless transfer and less reset windup (0 or 100%), and are better suited for controlling servomotor-driven devices. Their main limitations include noise sensitivity, likelihood to oscillate, and lack of an internal reference. 

Selective control configurations also require external feedback to protect them from reset windup. Part (d) has a combination of selective and cascade systems and in such configuration, the external reset (ER) signal (not shown) is taken from the measurement of the slave controller (FRC). 

In order to make sure that the pressure controller (PC-01) set point is changed slowly and in a stable manner, the valve position controller (VPC-02) is provided with integral action only, and its integral time is set to be about 10 times that of PC-01. In order to prevent reset windup when PC-01 is switched to manual or local control from cascade, the valve position controller is also provided with an external feedback signal off the pump speed. 

The load distribution can be computer-optimized by calculating compressor efficiencies (in units of flow per unit power) and loading the compressors in their order of efficiencies. The pressure controller (PC-22) directly sets the set points of SC-21 and SC-23, whereas the balancing controllers (FFC-22 and FFC-24) slowly bias those settings as they follow the total H2 generation of the electrolyzers (FT-4). The flow ratio controllers (FFCs) are also protected from reset windup, as was explained in Section 2.5.4. 

Typical functions of the middle layer are anti-reset windup, variable structure elements, selectors, etc. Although essential for the proper functioning of any practical control system, they have been completely neglected in research circles. We have yet to find an effective multivariable anti-reset windup scheme that works on all our test cases. Can all, or at least most, industrial control problems be solved satisfactorily with some simple loops and minimum-maximum selectors How can the appropriate logic structure be designed How should the loops be tuned to work smoothly in conjunction with the logic How can one detect deteriorating valves and sensors from on-line measurements before these control elements have failed entirely ... 

Unfortunately, this method of control does not solve any of the inherent problems of nonlinear, exothermic temperature control. But it does not prevent the use of any of the traditional remedies, such as adaptive gain, bidirectional gain, or anti-reset windup, since these are all modifications to the PID algorithm. 

When an error e(t) persists for a long time, the value of the integral f e(t) dt increases significantly and may lead the output of a PI controller to its maximum allowable value. We say that the controller has been saturated and in physical terms it means that the valve is fully open or closed before the control action has been completed (i.e., before the error has been driven to zero). This situation is also known as reset windup. How would you handle such a situation You can consult Refs. 7 and 15. 

Tuning typical values are = 0.8/Kp, Tj = 0.75 0 + t) and = 0.0. The proportional band (100/JQ) and the reset time (1/Ti) can be calculated from these. When fine tuning, observe the behavior of both the controlled and manipulated variables. Always use an implementation that includes anti-reset windup protection for when the manipulated variable encounters a constraint. Use an implementation that includes initialization that starts automatic control smoothly from the last manual condition. The digital execution period should be fast with respect to the feedback dynamics, with At 0.05 (0 + r) where possible. 

EXTERNAL RESET FEEDBACK TO PREVENT RESET WINDUP... 

Reset windup problems can occur in override control structures when controllers have integral action. Large swings (bumps) in control valve position can result as control is transferred from one controller to another. One method for achieving bumpless transfer is the use of the Shinskey/Buckley control structure called external reset feedback. Unfortunately, this type of controller is not available in commercial dynamic simulators. 

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