Control feedback

In feedback control, after an offset of the controlled variable from a preset value has been generated, the controller acts to eliminate or reduce the offset. Usually there is produced an oscillation in the value of the controlled variable whose amplitude, period, damping and permanent offset depend on the nature of the system and the [Pg.39]

Proportional, in which the corrective action is proportional to the error signal. [Pg.39]

55% weight less than 500 ppm mol. % less than 500 ppm mol. % less than 100 ppm mol. % less than 5 ppm mol. % less than 10 ppm mol. % less than 5 ppm mol. % less than I ppm mol. % less than 15 ppm mol. % less than 5 ppm mol. % less than 1 ppm mol. % less than 5 ppm mol. c/c [Pg.40]

A simple feedback control system is illustrated in Fig. 7.1, the function of which is to control at Y the temperature of the stream leaving the heat exchanger. The temperature 6 of the stream at Y (the controlled variable—Fig. 7.1u—usually denoted by the letter C as in Fig. l. b) is measured by means of a thermocouple, [Pg.560]

Simple feedback control system (a) illustrating components (b) standard nomenclature (c) representation according to British Standard BS 1646 [Pg.561]

The control systems to be considered include feedback, cascade, and feedforward. If the major process loads (feed rate and composition) are reasonably constant and the only corrections required are for variations in heat losses or fouling, feedback control is adequate. If steam flow varies because of demands elsewhere in the plant, a cascade system may be the proper choice. If, however, the major load variables change rapidly and frequently, feed forward or feed forward in conjunction with feedback should be considered. [Pg.299]

Feedback control systems monitor final product quality by manipulating heat input. The internal material balance is maintained by level control on each effect. In feedback control a correction for a disturbance cannot be made until the effect of the disturbance is detected. Some time is also needed to measure the deviation and make the correction. Process time lags require again that time pass before the effect of the correction can be known. Meanwhile, the controlled variable continues to deviate from the desired value for some time. In feedback control, one manipulated variable is controlled from one measured value. [Pg.299]

Feedback control requires that a deviation exist between the measured variable and the set point before control action can take place. The feedback controller changes its output until the deviation (error signal) between the set point and the measured variable is negligible, which is essentially a trial-and-error process characterized by the oscillatory nature that feedback control often exhibits. [Pg.299]

Multielement control occurs when two or more input signals jointly affect the action of the control system. Examples of multielement control are cascade control, ratio control, and override control. [Pg.299]

Cascade Control In cascade control, the output of one controller is the set point for another controller. Each controller has its own measured variable with only the primary controller having an independent set point and only the secondary controller providing an output to the process. The addition of a secondary controller permits variations to be corrected immediately before they can affect the primary variable. [Pg.299]

SHINSKEY Feedback Controllers for the Process Industries SHUGAR, DEAN The Chemist s Ready Reference Handbook SHUGAR, BALLINGER The Chemical Technicians Ready Reference Handbook... [Pg.462]

Figure Bl.27.8. Schematic view of Picker s flow microcalorimeter. A, reference liquid B, liquid under study P, constant flow circulating pump and 2, Zener diodes acting as heaters T and T2, thennistors acting as temperature sensing devices F, feedback control N, null detector R, recorder Q, themiostat. In the above A is the reference liquid and C2is the reference cell. When B circulates in cell C this cell is the working cell. (Reproduced by pemiission from Picker P, Leduc P-A, Philip P R and Desnoyers J E 1971 J. Chem. Thermo. B41.)...

Nearly four biUion L/yr of ethanol are added to gasoline and sold as gasohol (18). The starting or driveabiUty difficulties have been solved, in part, by the advances in vehicle technology employing fuel feedback controls. [Pg.424]

In 1966, the name was proposed (5) for receptors blocked by the at that time known antihistamines. It was also speculated that the other actions of histamine were likely to be mediated by other histamine receptors. The existence of the H2 receptor was accepted in 1972 (6) and the receptor was recognized in rat brain in 1983 (7). receptors in the brain appear to be involved in the feedback control of both histamine synthesis and release, whereas release of various other neurotransmitters, eg, serotinin (5-HT), dopamine, noradrenaline, and acetylcholine, is also modulated (8) (see Neuroregulators). [Pg.135]

Some of the inherent advantages of the feedback control strategy are as follows regardless of the source or nature of the disturbance, the manipulated variable(s) adjusts to correct for the deviation from the setpoint when the deviation is detected the proper values of the manipulated variables are continually sought to balance the system by a trial-and-error approach no mathematical model of the process is required and the most often used feedback control algorithm (some form of proportional—integral—derivative control) is both robust and versatile. [Pg.60]

The main disadvantage of the feedback control strategy is that no corrective action is taken until after a deviation between the measured controlled... [Pg.60]

The feedforward control strategy (Fig. lb) addresses the disadvantages of the feedback control strategy. The feedforward control strategy measures the disturbance before it affects the output of the process. A model of the process determines the adjustment ia the manipulated variables(s) to compensate for the disturbance. The information flow is therefore forward from the disturbances, before the process is affected, to the manipulated variable iaputs. [Pg.61]

The primary advantage of the feedforward over the feedback control strategy is that corrective action is initiated before the controlled variable is upset. Feedforward control, however, has its own drawbacks, ie, variables used to characterize the disturbances must be measurable a model of the response of the controlled variable to the disturbance must be available (when the feedforward strategy is used alone, the control performance depends on the accuracy of the model) and the feedforward control strategy does not compensate for any disturbance not measured or modeled. [Pg.61]

In most process plant situations where feedforward control is appropriate, a combination of the feedforward and feedback control is usually used. The feedforward portion reduces the impact of measured disturbances on the controlled variable while the feedback portion compensates for model inaccuracies and unmeasured disturbances. This control strategy is referred to as feedforward control with feedback trim. [Pg.61]

Classical Feedback Control. The majority of controllers ia a continuous process plant is of the linear feedback controller type. These controllers utilize one or more of three basic modes of control proportional (P), iategral (I), and derivative (D) action (1,2,6,7). In the days of pneumatic or electrical analogue controllers, these modes were implemented ia the controller by hardware devices. These controllers implemented all or parts of the foUowiag control algorithm ... [Pg.68]

Dead-Time Compensation. Dead time within a control loop can greatiy iacrease the difficulty of close control usiag a PID controller. Consider a classical feedback control loop (Fig. 18a) where the process has a dead time of If the setpoiat is suddenly iacreased at time t, the controller immediately senses the deviation and adjusts its output. However, because of the dead time ia the loop, the coatroUer does aot begia to see the impact of that change ia its feedback sigaal, that is, a reductioa ia the deviatioa from setpoiat, uatil the time t +. Because the deviatioa does aot change uatil... [Pg.74]

Technology has been introduced for on-line estimation of the kappa number based on absorption of ultraviolet light (35). This breakthrough ia optical sensor technology permits closed-loop feedback control of digesters from on-line measurement of the kappa number. [Pg.266]

Electrochemical Microsensors. The most successful chemical microsensor in use as of the mid-1990s is the oxygen sensor found in the exhaust system of almost all modem automobiles (see Exhaust control, automotive). It is an electrochemical sensor that uses a soHd electrolyte, often doped Zr02, as an oxygen ion conductor. The sensor exemplifies many of the properties considered desirable for all chemical microsensors. It works in a process-control situation and has very fast (- 100 ms) response time for feedback control. It is relatively inexpensive because it is designed specifically for one task and is mass-produced. It is relatively immune to other chemical species found in exhaust that could act as interferants. It performs in a very hostile environment and is reHable over a long period of time (36). [Pg.392]

Spreadsheet Applications. The types of appHcations handled with spreadsheets are a microcosm of the types of problems and situations handled with fuU-blown appHcation programs that are mn on microcomputers, minis, and mainframes and include engineering computations, process simulation, equipment design and rating, process optimization, reactor kinetics—design, cost estimation, feedback control, data analysis, and unsteady-state simulation (eg, batch distillation optimization). [Pg.84]

Electrotransport technology offers a number of benefits for therapeutic appHcations, including systemic or local adininistration of a wide variety of therapeutic agents with the potential adininistration of peptides and proteins long-term noninvasive administration, improving convenience and compliance controlled release, providing a desired deflvery profile over an extended period with rapid onset of efficacious plasma dmg levels and in some cases reduced side effects and a transport rate relatively independent of skin type or site. Additional benefits include easy inception and discontinuation of treatment, patterned and feedback-controlled deflvery, and avoidance of first-pass hepatic metaboHsm. [Pg.145]

C. D. Ealk andj. J. Mooney, Three-Way Conversion Catalysts—Effect of Closed Eoop Feedback Control and Other Parameters on Catalyst Efficieny, SAE 800462, Society of Automotive Engineers, Warrendale, PA., 1980. [Pg.499]

Feedback Control In a feedback control loop, the controlled variable is compared to the set point R, with the difference, deviation, or error e acted upon by the controller to move m in such a way as to minimize the error. This ac tion is specifically negative feedback, in that an increase in deviation moves m so as to decrease the deviation. (Positive feedback would cause the deviation to expand rather than diminish and therefore does not regulate.) The action of the controller is selectable to allow use on process gains of both signs. [Pg.718]

Feedfoi ward control is usually combined with feedback control to eliminate any offset resulting from inaccurate measurements and calculations and unmeasured load components. The feedback controller can either bias or multiply the feedfoi ward calculation. [Pg.718]

General References SPinskey, Feedback Controllers foi- the Fiocess... [Pg.725]

FIG. 8-23 Both load regulation and setpoint response require high gains for the feedback controller. [Pg.725]

The performance of a controller (and its tuning) must be based on what is achievable for a given process. The concept of best practical IE (lEb) for a step change in load Aq can be estimated (Shinskey, Feedback Controllers foi the Process Industries, McGraw-Hill, New York, 1994) ... [Pg.728]

The effect of the disturbance on the controlled variable These models can be based on steady-state or dynamic analysis. The performance of the feedforward controller depends on the accuracy of both models. If the models are exac t, then feedforward control offers the potential of perfect control (i.e., holding the controlled variable precisely at the set point at all times because of the abihty to predict the appropriate control ac tion). However, since most mathematical models are only approximate and since not all disturbances are measurable, it is standara prac tice to utilize feedforward control in conjunction with feedback control. Table 8-5 lists the relative advantages and disadvantages of feedforward and feedback control. By combining the two control methods, the strengths of both schemes can be utilized. [Pg.730]

FIG. 8-31 (a) Feedback control of a heat exchanger, (h) Feedforward control... [Pg.731]

Other Considerations in Feedforward Control The tuning of feedforward and feedback control systems can be performed independently. In analyzing the block diagram in Fig. 8-32, note that Gy is chosen to cancel out the effects of the disturbance Us) as long as there are no model errors. For the feedback loop, therefore, the effects of L. s) can also be ignored, which for the sei vo case is ... [Pg.732]

The tuning of the controller in the feedback loop can be theoretically performed independent of the feedforward loop (i.e., the feedforward loop does not introduce instability in the closed-loop response). For more information on feedforward/feedback control appications and design of such controllers, refer to the general references. [Pg.732]

One such approach is called cascade control, which is routinely used in most modern computer control systems. Consider a chemical reactor, where reac tor temperature is to be controlled by coolant flow to the jacket of the reac tor (Fig. 8-34). The reac tor temperature can be influenced by changes in disturbance variables such as feed rate or feed temperature a feedback controller could be employed to compensate for such disturbances by adjusting a valve on me coolant flow to the reac tor jacket. However, suppose an increase occurs in the... [Pg.732]

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