Persistent feedback control

Control by negative feedback is a regulating mechanism that produces corrective action. The system output is monitored continuously or sampled at discrete points in time and the results are compared to pre-established norms or goals. The measured difference between results and norm is used as input to control actions. It affects the system in a direction that gives a result closer to the norm. 

In the feedback control of accidents, we use different types of standards. In workplace inspections, for example, we are concerned with deviations from technical requirements defined in safety regulations, etc. Another example is feedback control of SHE performance, as measured by one or more SHE performance indicators. We will present such indicators in Part IV. 

An associated term is feed forward. Anticipation is the heart of the feedforward mechanism. Here, the information used as input to control the system is not obtained directly by measuring system performance but indirectly through anticipation. One typical example is when we introduce control measures on the basis of results of risk analyses. 

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Persistent feedback control, on the other hand, is an ongoing process where the SHE performance is monitored periodically and compared to pre-established SHE goals, compare Figure 3.2. Basic principles of feedback control are reviewed in this chapter. We will revert to applications in Part IV on SHE performance measurement. 

It is not always clear in SHE management practice which principle applies. The diagnostic process and persistent feedback control are partially overlapping. In both cases, it is important to close the loop, i.e. to ensure that the necessary actions are identified following the diagnosis or measurement and that these are implemented and the effects monitored. 

We find application of persistent-feedback-control principles at this level. In workplace inspections, the conditions at the workplace are monitored and compared to pre-established norms (the regulations, internal procedures, accepted practice). Decisions are made during the inspections to correct deviations (e.g. replace faulty guards, improve housekeeping). 

In this Part, we will present SHE performance measures or indicators for use in the types of SHE management activities that were presented in Chapter 10 and named persistent feedback control. These activities involve the establishment of SHE performance goals and the followup of such goals through measurement of actual performance. The principles for establishing goals may differ ... 

When processes are subject only to slow and small perturbations, conventional feedback PID controllers usually are adequate with set points and instrument characteristics fine-tuned in the field. As an example, two modes of control of a heat exchange process are shown in Figure 3.8 where the objective is to maintain constant outlet temperature by exchanging process heat with a heat transfer medium. Part (a) has a feedback controller which goes into action when a deviation from the preset temperature occurs and attempts to restore the set point. Inevitably some oscillation of the outlet temperature will be generated that will persist for some time and may never die down if perturbations of the inlet condition occur often enough. In the operation of the feedforward control of part (b), the flow rate and temperature of the process input are continually signalled to a computer which then finds the flow rate of heat transfer medium required to maintain constant process outlet temperature and adjusts the flow control valve appropriately. Temperature oscillation amplitude and duration will be much less in this mode. 

Feed-forward control takes quick action on any disturbance while feedback ensures that any persisting or increasing offset in the level is taken care of. Feed-forward control is often used in case of slow changing processes where the state variable reacts slowly on a distiubance. This happens, for example, in distillation columns with many trays, which are subject to large disturbances (for example feed changes from furnaces). Most process units, however, can be well controlled by using feedback control only. 

Feedback control only takes action after an error has developed. The error must be large or persistent before feedback control makes a large correction for an upset. Feedforward control has the noble idea of calculating a corrective action to compensate for an upset before an error appears. The most common load upsets (disturbances) are influent feed and composition, flow feedforward is widely used and is often referred to as ratio control since the reagent flow is ratioed to the influent flow. The... 

To analyze the obtained drift velocity field it is suitable to choose the distance between the two measuring points dp = 2a as control parameter [53]. For dp/X 1, the drift velocity field looks very similar to that induced by one-point feedback (compare section 9.3.1, [47]). It includes a set of circular-shaped attracting manifolds called resonance attractors [21], as shown in Fig. 9.2. This attractor structure still persists for distances dp/X < 0.5. For example, the drift velocity field obtained for dp/X = 0.45 is shown in Fig. 9.10(a). The thick solid line represents the drift... 

If a chemical string is not feasible, batch treatments using persistent film inhibitors may be used. The inhibitor is designed to form a tough film that is not too soluble in the production stream so it will last for a sufficient time between treatments. The batch may be displaced with liquids, gas, or nitrogen. Squeeze inhibitors must be designed to be stable in the formation, and not cause severe emulsion problems. The adsorption characteristics should be controlled for proper feedback of the inhibitor. Pumped wells, can be treated by continuous addition or batching down the annulus. 

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