Anti reset-windup

There will be occasions when the controller saturates. For example a flow controller may encounter a hydraulic limit so that, even with valve fully open, the SP cannot be reached. The integral (or reset) action will respond to this by continually increasing the output but, because the valve is fully open, will have no affect on the flow. This is reset windup. Windup should be avoided because, if the process constraint is removed - for example by starting a booster pump, there will be a delay while the controller removes the windup and can begin actually closing the valve. This is resolved by keeping the permitted output range as narrow as possible, typically —5 to 105 %. 

However the situation becomes more complex with cascaded controllers. The situation can arise where the secondary is controlling at SP but with its output at minimum or maximum. It is important therefore that the primary makes no changes to the secondary s SP which will cause it to saturate. DCS controllers have external anti-reset windup protection, sometimes described just as external reset feedback, to prevent this. 

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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... 

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... 

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. 

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. 

The external reset feedback control structure discussed in Figure 18.3 is inserted in the Aspen Dynamics flowsheet for the flash tank process as shown in Figure 18.8. The feed flow controller has its output signal OPpc sent to a low selector. The other input to the low selector is the output signal OPqrc of the high-level override controller. The override controller is proportional-only, so it does not need anti-reset windup protection. 

Figure 18.17 gives the responses of the process to the 30% disturbances in the setpoint of the feed flow controller for this system with no anti-reset-windup protection. The column pressure drop increases when feed flow rate increases as the temperature controller calls for a higher reboiler duty. When the pressure drop gets to 0.5 bar, the... 

In this configuration each constraint has its own PID controller — a pressure controller on burner inlet and a valve position controller on the fuel valve. In this example both constraint controllers are configured as reverse-acting. The DCS must support certain features. The signal selector must include anti-reset windup to prevent the output of the unselected controller from saturating. Secondly the PID controllers must be of the incremental type, otherwise bumpless initialisation is difficult to achieve. These features are present in most DCS, but should be checked. 

One major advantage of the velocity form Eq. (95) over the positional form Eq. (92) is that, as the summation is lacking, it has inherent anti-reset windup. Also, an initial value of the output bias is not required [7]. 

The anti reset-windup technique discussed above is known as external reset feedback. For most applications either it, or the modification mentioned below, is our preferred scheme. It has the disadvantage that the controller output signal, commonly labeled valve position, is really different from the actual position. It differs by the product of the error signal times the proportional gain. Lag in the reset circuit may cause further error. A modification therefore is introduced by some vendors, particularly in the newer microprocessor controls. This consists of setting the reset time equal to zero when the controller is overridden. This technique is sometimes called integral tracking. It should not be used with auto overrides. 

Manual—automatic switching is not necessary since both controllers have anti reset-windup. 

It should be noted that only a few of the electronic analog controllers on the market have external reset feedback or some other satisfactory anti reset-windup scheme. But newer digital and microprocessor-based controls (as of late 1983) use a variety of techniques, most of which appear to be satisfactory. 

Anti reset-windup for loops with overrides (also simplifying manual-automatic switching)... 

Feedforward and interaction compensation without interfering with either normal reset or anti reset-windup... 

The theory and mathematics involved in combining feedforward con jensation, overrides, controllers (PI and PID), and anti reset-windup have been covered in detail elsewhere and are reviewed briefly below. At this point we wish to point out that (1) the feedforward function, KgTffSl TffS + 1), commonly termed impulse feedforward, is a convenient way of feeding fc>rward without interfering with reset when external reset feedback is used, and (2) making the feedforward time constant, t, equal to the reset time constant, Tji, is usually desirable. This approach to feedforward also provides a convenient way to connect interaction compensators (decouplers) into control loops. 

The use of high or low limits for process variables represents another type of selective control called an override, where a second controller can override or take over from the first controller. This is a less extreme action than an interlock, which is used for emergency shutdown of the process (see Chapter 10). The anti-reset windup feature in feedback controllers (cf. Chapter 8) is a type of override. Another example is a distillation column that has lower and upper limits on the heat input to the column reboiler. The minimum level ensures adequate liquid inventory on the trays, while the upper limit exists to prevent the onset of flooding (Buckley et al., 1985 Shinskey, 1996). Overrides are also often used in forced draft combustion control systems to prevent an imbalance between air flow and fuel flow, which could result in unsafe operating conditions (Singer, 1981). 

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