Feedforward dynamic compensator

The lowest layer of process control applications is described as regulatory control. This includes aU the basic controllers for flow, temperature, pressure and level. But it also includes control of product quality. Regulatory is not synonymous with basic. Regulatory controls are those which maintain the process at a desired condition, or SP, but that does not mean they are simple. They can involve complex instrumentation such as on-stream analysers. They can employ advanced techniques such as signal conditioning, feedforward, dynamic compensation, overrides, inferential properties etc. Such techniques are often described as advanced regulatory control (ARC). Generally they are implemented... 

In general, the feedforward dynamic elements will not be physically realizable. In other words, they cannot be implemented exactly. For instance, if the process disturbance measurement contains dead time, or lag, then the feedforward dynamic compensation would have to be a predictor, which of course is impossible unless an exact... 

Feedforward control can also be applied by multiplying the liquid flow measurement—after dynamic compensation—by the output of the temperature controller, the result used to set steam flow in cascade. Feedforward is capable of a reduction in integrated error as much as a hundredfold but requires the use of a steam-flow loop and dynamic compensator to approach this. 

Feedforward control system that provides constant separation by manipulating the distillate flow (top). At the bottom, a variety of dynamic compensators are shown, which can be used to match the "dynamic personality" of the process. 

For noninteracting control loops with zero dead time, the integral setting (minutes per repeat) is about 50% and the derivative, about 18% of the period of oscillation (P). As dead time rises, these percentages drop. If the dead time reaches 50% of the time constant, I = 40%, D = 16%, and if dead time equals the time constant, I = 33% and D = 13%. When tuning the feedforward control loops, one has to separately consider the steady-state portion of the heat transfer process (flow times temperature difference) and its dynamic compensation. The dynamic compensation of the steady-state model by a lead/lag element is necessary, because the response is not instantaneous but affected by both the dead time and the time constant of the process. 

Block Diagram Analysis One shortcoming of this feedforward design procedure is that it is based on the steady-state characteristics of the process and, as such, neglects process (Ramies (i.e., how fast the controlled variable responds to changes in the load and manipulated variables). Thus, it is often necessary to include dynamic compensation in the feedforward controller. The most direct method of designing the FF dynamic compensator is to use a block dir rram of a general process, as shown in Fig. 8-34, where G, represents the disturbance transmitter, (iis the feedforward controller, Cj relates the disturbance to the controlled variable, G is the valve, Gp is the process, G is the output transmitter, and G is the feedback controller. All blocks correspond to transfer fimetions (via Laplace transforms). 

From the earlier discussion and from Table 57.1 [11], we would expect that a combined feedforward-feedback control system would retain the superior performance of the feedforward controller and the insensitivity of the feedback controller to uncertainties and inaccuracies. Figure 57.4 shows the main components of a feedforward-feedback control system they include the feedforward controller incorporated with the process model, the feedback controller, and the dynamic compensator. The function of feedback element is to correct the action of the feedforward controller in the case of measurement and modeling inaccuracy. Courtois et al. [12] and Bruce and McFarlane [13] implemented the combined feedforward-feedback control system for mixed-flow grain dryers. 

The third Ingredient of a feedforward system Is dynamic compensation. A change In one of the major loads to the process also modifies the operating level of the manipulated variable. If these two Inputs to the process enter at different locations, these usually exists an unbalance or inequality between the effect of the load variable and the effect of the manipulated variable on the controlled variable. This imbalance manifests itself as a transient excursion of the controlled variable from set point. If the forward calculation is accurate, the controlled variable returns to set point once the new steady-state operating level is reached. 

The feedforward controller therefore made the correct change it just did so too soon. The dynamics of the PV with respect to the DV are now much slower that its dynamics with respect to the MV. We therefore need to include some dynamic compensation that, in this case, delays the feedforward correction. Failure to properly include such compensation can result in the addition of feedforward causing the scheme to perform less well than the standalone feedback controller. 

We apply dynamic compensation in the form of a deadtime/lead-lag algorithm. This is tuned in exactly the same way as described in Chapter 6 covering bias feedforward. By performing open loop steps on the MV we obtain the dynamics of both the inferential and... 

Alternatively, we could attempt to obtain the feedforward gains (AO empirically by plant testing, providing that we can introduce a disturbance into feed enthalpy. We may be able to determine K from analysis of historical data but if these were collected while tray temperature (or some other composition) control was in service then it will only be possible to model steady state behaviour. Similarly we could identify K from steady state simulation. Dynamic compensation would then have to be tuned by trial and error. 

Of the two, load response is the more important, because set-point changes are ordinarily less frequent. Ideally, the load signal should be delayed by t, before it is multiplied, and then advanced by Tm- It is impossible to create a time advance, however. So dynamic compensation is best introduced in this apphcation by delaying the feedforward signal by an amount Tq — Tm. If t > compensation is impossible. 

Because forward loops exhibit absolutely no oscillatory tendencies, to talk of gain and phase is rather inconsequential. Step responses will be used throughout, since they constitute the most severe test of system performance. The response of systems under feedforward control, both with and without dynamic compensation, differs markedly from that experienced with feedback control. For this reason, it is not surprising that dynamic elements in the forward loop bear little resemblance to the conventional modes of feedback controllers. 

A qualitative appraisal of the requirement for dynamic compensation may be obtained from a comparison of open-loop response curves. Because an increase in the manipulated variable acts in opposition to the load, their individual stq>reqx)nse curves will diverge. One or the other response will have to be inverted so that the two curves may be supaimposed, as is done in Fig. 8.10. The response of such a process under uncompensated feedforward control appears as the difference between these two curves. 

It might be possible to construct an extremely complete dynamic model of a process, but any compensator with more than three terms to adjust would be unreasonably difficult to cope with. Furthermore, the purpose of dynamic compensation is to minimize an error which is already transient, so perfection is not really warranted. In most cases, a simple lead-lag function will be perfectly adequate and will be able to reduce the absolute area of the response curve by tenfold or more, distributed uniformly. Figure 8.14 compares the load response of the heat exchanger under dynamically compensated feedforward control with that encountered under feedback control. 

Estimate the peak loeation of the uncompensated response curve which would result from feedforward control of the process whose dynamic characteristics are given in Fig. 8.10. What settings of lead and lag will be necessary for dynamic compensation ... 

As is typical of feedforward control loops, dynamic compensation is necessary to ensure that the effect of a distillate-rate change be manifest at the same time as the feed-rate change which promoted it. Because feed enters the tower at a location considerably removed from where distillate is withdrawn, their dynamic effects upon composition diff er by a corresponding amount. The response of a tower due to a change in feed rate appears as the sum of an incident and a reflected wave, just as is the case with distillate rate, but the incident path is longer and the reflected path is shorter. Figure 11.21 illustrates the difference in the length of the paths. 

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