Control system feed-forward

Two main control schemes exist feedback control and feed-forward control (Fig. 6). In feedback control (by far the most common), system performance is monitored, deviations from desired conditions are quantified, and controlled variables are modified to return the system to the desired state. In feed-forward control, process inputs are monitored. As they deviate from desired values, their effect on the system is predicted, and controlled variables are modified to minimize their effect. Feedback control is safer, since it guarantees performance by controlling it directly, but it is also slower corrective action is taken only after the perturbation has affected process performance. Feed-forward control is faster it acts on input deviations as soon as they are detected. However, it is riskier if the detected... 

The feedback control in loops 1 and 2 is combined with a feed-forward controller in loop 3 which measures the inlet, temperature T calculates the change in cooling water flow rate Few which is required to bring the reactor temperature Fback to its set point Ts and sends this signal to the feedback controller (the feed-forward controller consists of a model of the process and is therefore not of P-, PI- or PID-type). The feedforward control loop will therefore theoretically eliminate any disturbances in inlet temperature Tv The feedback part of the control system, loop 1, will compensate for any inaccuracies in the feed-forward control model as well as eliminate the effect of other, unmeasured disturbances, e.g. in inlet flow rate Fr... 

An extraction plant should operate at steady state in accordance with the flow-sheet design for the process. However, fluctuation in feed streams can cause changes in product quaUty unless a sophisticated system of feed-forward control is used (103). Upsets of operation caused by flooding in the column always force shutdowns. Therefore, interface control could be of utmost importance. The plant design should be based on (/) process control (qv) decisions made by trained technical personnel, (2) off-line analysis or limited on-line automatic analysis, and (J) control panels equipped with manual and automatic control for motor speed, flow, interface level, pressure, temperature, etc. 

Appropriate design features may include feed-forward temperature control, high temperature alarms, high-temperature cutouts to stop feed flow and open a vent to atmospheric or closed system, adequate temperature monitoring through catalyst beds, etc. 

In this short initial communication we wish to describe a general purpose continuous-flow stirred-tank reactor (CSTR) system which incorporates a digital computer for supervisory control purposes and which has been constructed for use with radical and other polymerization processes. The performance of the system has been tested by attempting to control the MWD of the product from free-radically initiated solution polymerizations of methyl methacrylate (MMA) using oscillatory feed-forward control strategies for the reagent feeds. This reaction has been selected for study because of the ease of experimentation which it affords and because the theoretical aspects of the control of MWD in radical polymerizations has attracted much attention in the scientific literature. 

Feedback control can never be perfect as it only reacts to the disturbances which are already measured in the system output. The feed-forward method tries to eliminate this drawback by an alternative approach. Instead of using the process output, the measured variable is taken as the measured inlet disturbances and its effect on the process is anticipated via the use of a model. The action is taken on the manipulated variable using the model to relate the measured variable at the inlet, the manipulated variable and the process output. The success of this control strategy depends largely on the accuracy of the model prediction, which is often imperfect as models can rarely predict the... 

Figure 6 Schematic of a control system, using a stirred tank as an example. A stream enters the tank at temperature T-. The system is designed to maintain the exit temperature at Tout- In a feedback mode, the exiting temperature is measured and the stream feed is opened or closed as necessary. Tout is known, but the effects of variability are corrected only after they have entered the system. In a feed-forward mode, the incoming temperature is measured and the stream feed is modified to prevent its variability from entering the system. However, OUt is unknown.

Forcing function is a term given to any disturbance which is externally applied to a system. A number of simple functions are of considerable use in both the theoretical and experimental analysis of control systems and their components. Note that the response to a forcing function of a system or component without feedback is called the open-loop response. This should not be confused with the term open-loop control which is frequently used to describe feed-forward control. The response of a system incorporating feedback is referred to as the closed-loop response. Only three of the more useful forcing functions will be described here. 

Blending also plays a role in the chemical industry. The use of Raman spectroscopy to control gasoline blending, particularly oxygenated species such as methanol, ethanol, propanols, and butanols, is discussed in US patent 5,596,196.64 The system can be operated in a feed-forward and/or feedback control scheme. In feed-forward, the composition of each of six remote feed tanks is measured sequentially. A control computer then... 

The steps in setting up the Aspen Plus simulation are outlined below. The rigorous RCSTR model is used, which requires specifying reactions and kinetic parameters. An alternative, which is useful in some systems with reversible reactions, is the RGIBBS reactor module. Kinetic parameters are not required. Chemical equilibrium compositions are calculated for given feed and reactor temperature and pressure. If the forward and reverse reactions are known to be fast, so that the reactor effluent is at equilibrium conditions, the RGIBBS reactor provides a simple way to model a reactor. In Chapter 3 we will illustrate how this type of reactor can incorporate some approximate dynamics for developing control systems. 

Propose a control system based on feed-forward - feedback control, cascade control and inferential control to achieve these control objectives. 

A simple way to circumvent the problem and improve the control is to use predictive control, often called feed forward. The idea is to use a control signal, CCff, which in some way reflects the expected time-course of CC that is needed to keep CO close to CL In technical systems this control is frequently called model predictive control (MPC [12]), because CCff is typically derived from a mathematical model. In biological systems, the feed forward is mostly delivered by nerve signals, more rarely by hormones. 

Another familiar example is the control of muscle movements [7, 14]. On top of complex sensory feedback systems in muscles and spinal cord, the cerebellum and basal ganglia deliver a feed forward signal that takes the person s reaction time into consideration and presets the muscles to a situation some 50-100 ms ahead. The PID control can then make small corrections. The precision of the movements can... 

The development of gradient systems, available with both feed-forward and feedback control systems, is an important capability of the pumping system. In order to troubleshoot problems with the gradient system, it is valuable to have a feed-back control gradient system. 

The advantage of such a control system is that the feeding of fuel can be adapted to the time curve of the thermal output almost instantaneously. This ensures stable controlling which is necessary for a continuous operation without on/off-operation. On the other hand, a fuel control reacting exclusively to the boiler temperature or the flow temperature, only allows a very slow adjustment of the fuel quantity to the load curve, as a change in load becomes effective only after a certain time. Due to this dead time, there is in general no stable control without disturbance value feed-forward... 

The major limitations of the feed forward control strategy presented here are that (i) it is only as good as the fundamental data which are used in the models and (ii) it can only be used for systems which conform to the conventionally accepted mode of behaviour of free radical chain polymerisation in solution. However, the same approach can be used with the appropriate models for any copolymerisation process. The range of application can be increased by making an arbitary assessment of the parameters necessary for the control models and/or by introducing a feedback loop which incorporates some state measurement device, e.g., an in-line gas chromatograph for measurement of residual monomers concentrations. Such a scheme is shown in Figure 21. 

Figure 8. Trends of glutathione, reducing sugar, dry cell weight (DCW) and ethanol concentration in the broth during the glutathione fermentation in 120-kl fermenter using the feed-forward/feedback control system.

The three processes are coordinated and controlled by the overall controller, which predicts the activity that will keep the overall energy expenditure of the respiratory system at a minimum. For this prediction the controller uses information fed back during previous breaths. In this sense, the model possesses predictive elements—i.e.y feed forward control with associated memory. As yet, extensive testing of the model with experimental data has not been reported. 

An advanced feed-fiirward control system was installed. E adi time a powa-outlet was used in the control room, the feed-forward system was affected just like running an electric appliance interferes with TV recqitian. This caused erratic reflux and rdxnl behavior. 

Pyruvate kinase is also activated by fructose-1,6-bisphosphate. Why fructose-1,6-bisphosphate It is an example of feed-forward activation. This glycolytic intermediate is controlled by its own enzyme system. If glycolysis is activated, then the activity of pyruvate kinase must also be increased in order to allow overall carbon flow through the pathway. Feed-forward activation ensures that the enzymes act in concert to the overall goal of energy production. 

The dynamic model was validated both in steady and unsteady-state conditions, which is quite interesting in case that a control based on feed-forward strategy is applied. The prediction of the final concentration of Orange II from the initial data would allow the system control to modify the flow rate of MnP, Orange II or H2O2, in order to adapt the conditions to the desired final value. 

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