Process control 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... 

Example 21.4 illustrates how feedback control, feed-forward control, or a combination of both can be used to control a process unit. 

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. 

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... 

Determine the transfer functions of the feed-forward control scheme assuming linear operation and negligible distance-velocity lag throughout the process. Comment on the stability of the feed-forward controllers you design. 

Finally, if these sensors are necessary under a process control point of view, and by taking into account the fact that in order to install two sensors, one in the input and the other at the output, is not financially possible, the limited knowledge of the bioprocess and their dubious character encourage control engineers to consider feedback control rather than feed-forward control. A sensor, if it is available, will preferably be installed at the output of the process rather than at the input. 

Quality attributes trending (SPC or MSPC) Process control (feedback or feed forward)... 

Wright of Advanced Micro Devices discusses the use of Raman microspectroscopy to measure the integrity of a film on semiconductor wafers during manufacture in US patent 6,509,201 and combined the results with other data for feed-forward process control [181]. Yield is improved by providing a tailored repair for each part. Hitachi has filed a Japanese patent application disclosing the use of Raman spectroscopy to determine the strain in silicon semiconductor substrates to aid manufacturing [182]. Raman spectroscopy has a well established place in the semiconductor industry for this and other applications [183]. 

What will the measurement results be used for Will they be used for process control (closed loop, open loop, feed-forward, feedback) Will they be used as a safety interlock (This puts very strict requirements on the analyzer reliability, and may even require duplicate analyzers.) Will the results be used to accept raw materials or to release product Will they be used to sort or segregate materials How frequently does the measurement need to be made How rapid does one individual measurement need to be How accurate does it have to be How precise How much analyzer downtime is acceptable ... 

Feed-Forward - Feedback Control. In practical applications, feedforward control is normally used in combination with feedback control. The feedforward part is used to reduce the effects of measurable disturbances, while the feedback part compensates for inaccuracies in the process model, measurement errors and unmeasured disturbances. The feedforward and feedback controllers can be combined in several different ways, as discussed in most standard control text books. 

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... 

Furnace temperature is measured with thermocouples. It is controlled to 1300 K by adjusting fuel combustion rate and air preheat temperature. Feed forward control based on scheduled changes in spent acid composition and feed rate is employed to optimize the process (Rohm and Haas, 2003). 

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. 

A control plan is the sum of the procedures and equipment used to ensure that the internal requirements are met. It includes control charts, sampling plans, 100% inspection, feed-forward/feedback mechanisms, and mistake-proofing tech-niques/devices. Some items may be performed by equipment like automatic controllers, and some are performed by operators and checkers. There may not be a single document called a control plan. Instead, these controls might be spread across a number of documents including a statistical process control plan, an inspection plan, an operator manual, and various other standard operating procedure and specifications. 

An elaboration of open-loop control which sometimes can be competitive with closed-loop control is feed-forward control, in which the controller is apprised of factors which affect the key process variable to be controlled, but is not directly apprised of the behavior of this variable. Thus for the kettle of Fig. 1, it would be possible to measure the flow rate and temperature of the feed and have a simple computer analyze the information and on the basis of this information set the steam pressure to be maintained by the controller. So long as the computer considered all pertinent factors, the control would be satisfactory, but an unaccounted factor, such as a change in agitator speed, might make the control ineffectual. 

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 additional potential application of particulate counting is process control and monitoring. With the improvement in aqueous particle counters, on-line measurement of number concentrations and size distributions for particulates larger than 1-2 pm is now feasible. Both feedforward and feedback process control applications can be envisioned. Feed-forward control could be used to estimate the coagulant chemical requirements needed for particle destabilization based on measurement of particle count and estimation of particulate surface area (32). Feedback control possibilities include control of the particle size distribution entering a filter, control of chemical dosing prior to granular-media filtration, and control of filter operation. 

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