Feedback undesirable

Strong process interacHons can cause serious problems if a conventional multiloop feedback control scheme (e g., PI or PID controllers) is employed. The process interacHons canproduce undesirable control loop interac tions where the controllers fight each other. Also, it may be difficult to determine the best pairing of controlled and manipulated variables. For example, in the in-hne blending process in Fig. 8-40(<7), should w be controlled with and x with tt>g, or vice versa ... 

In any imaging and spectroscopic mode of the STM, a bias is required between the sample and the tip. In an electrochemical solvent, faradaic current between the tip and sample can interfere with, and sometimes completely obscure, the tunneling current. This undesirable situation makes it very difficult to control the feedback and to maintain a constant tunneling gap between the tip and the sample. For example, in our laboratory, we have found that feedback control is lost on our present microscope if the faradaic current, ip, assumes a value greater than one-half that of the tunneling current, it. Use of partially insulated tips alleviates this condition, but unfortunately, does not completely eliminate the problem (57). 

A very useful application of the circuit of Figure 8.3b is produced if we replace the feedback resistor Ru with a resistive transducer such as a coated semiconductor thermistor. The commercial availability of small, rapid-response, chemically inert thermistors that have conveniently measurable resistances at temperatures of 200-600 K makes them an excellent choice as the transducer in chemical applications that require the rapid and accurate measurement of temperature. Unfortunately, a continuous current in Ru may produce undesirable... 

Managers often view performance appraisals as an undesirable task, but careful planning and implementation should make them less onerous. Performance appraisals are closely finked to employee motivation, performance, commitment, and turnover. Numerous systems are available, each ofwhich has its strengths and drawbacks. The formal appraisal must be accompanied by frequent and substantive informal feedback. The appraisal interview is key to the success of the appraisal system. The allocation of organizational rewards must be finked closely to the results of the appraisal process. 

There are a number of additional targets that may lead to effective antithrombotic therapy in ACS. In terms of anticoagulants, the concepts of agents that have dual inhibitor sites such as the one we find in heparin but that lack in some of its undesirable qualities could be very useful. The same concept may apply to drugs that have both anticoagulant and antiplatelet properties, It is quite probable that inhibitors of tissue factor as well as of the platelet ADP receptor when combined with aspirin might be very effective. An ability to block the feedback action of the polyphosphates released from platelets upon activation is also an attractive aim (Fig. 3),... 

Electro chemists are aware of the annoying residual uncompensated solution resistance Ru between the Luggin probe and the working electrode, see for example [74]. Although it is possible in principle to compensate fully for the iR error thus introduced [131,132], this is rarely done, as it introduces, in practice, undesirable instrumental oscillations or, in the case of damped feedback [132], sluggish potentiostat response. 

DPL has been proved to be an efficient tool to modify and structure polymer surfaces, permitting one to circumvent the numerous drawbacks of contact mode lithography. The antagonism between writing process and feedback can be eliminated by carving surfaces through overlapping holes (pulse-train). On the other hand, DPL leads to the formation of undesir-... 

If enzyme E, were subject to feedback inhibition at high levels of metabolite A, then high levels of A6 would lead to decreased production of metabolite A2. This could have the undesirable effect of decreasing the production of metabolite A3, via the action of enzyme E2. This problem is overcome if enzyme E3, which catalyzes the first committed step in the production of A6, is subject to metabolic control,... 

Accidents are very rare relative to the number of near accidents and human errors. Fortunate as it may seem, this poses a real problem for complex systems with a high catastrophy potential (nuclear power plants, chemical plants, commercial aviation) few accidents means few cases to analyse and hardly any feedback to learn from. This leads to the undesirable situation of ad-hoc corrective measures after each single accident, because the database is far too small to generate statistically sensible preventi ve measures. 

It now looks as if we have achieved the best of all worlds a thermally efficient process with an easy-to-control reactor Can this be true Not quite. What we forget are the undesirable effects on the reactor that thermal feedback introduces. In Chap. 4 we explained in detail how7 process feedback is responsible for the same issues we tried to avoid in the first place by selecting an adiabatic plug-flow reactor. It is necessary that we take a close look at the steady-state and dynamic characteristics of FEHE systems. 

Figure 15.54b is a schematic of a feedforward controller applied for steam drum level control. If the flow rate of the makeup feedwater is equal to the steam usage, the drum level remains constant. One is tempted to conclude that the feedforward controller is aU that is needed for this application. Unfortunately, the measurements of the steam usage and the feedwater flow rate are not perfectly accurate. Even small errors in measured flow rates add up over time, leading to one of two undesirable extremes. The drum can till with water and put water into the steam system, or the liquid level can drop, exposing the boiler tubes, which can damage them. As a result, neither feedback nor feedforward are effective by themselves for this case. In general, feedforward-only controllers are susceptible to measurement errors and umneasured disturbances, and, as a result, some type of feedback correction is typically required. 

In order to eliminate the undesired effects, we would like to have an open-loop feedback signal that carries current and not delayed information, such as... 

The advantage of feedback control is that corrective action is taken regardless of the source of the disturbance. Its chief drawback is that no corrective action is taken until after the controlled variable deviates from the set point. Feedback control may also result in undesirable oscillations in the controlled variable if the controller is not tuned properly that is, if the adjustable controller parameters are not set at appropriate values. Although trial-and-error tuning can achieve satisfactory performance in some cases, the tuning of the controller can be aided by using a mathematical model of the dynamic process. 

Undesirable Effects Women - virilism (hirsutism), menstrual irregularities. Men - prostatic hyperplasia or cancer, gynecomastia (high dose or with liver disease), pattern baldness, reduced sperm count (negative feedback). Both sexes -hypercalcemia, coagulopathies, sodium and water retention, hyperlipidemia, atherosclerosis, cholestatic hepatitis, liver cancer. 

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