Closed-Loop Feedback Systems

Much of human behavior is hard-wired. But, unlike the heart, liver, or even our genes, the brain can respond in a dynamic way not only to internal physiological cues but also to unpredictable external ones, and it can embody that response in future behavior. 

FIGURE 4.4.4 Generalized feedback control loop. A portion of the output signal is fed back to the controller input, where it is subtracted from the reference level input. The controller then acts upon this difference, feeding it to the process to he controlled, known as the plant. The feedback signal may be conditioned before being compared with the reference input. 

Feedback usually gives very good control. The input level is sensed and compared to a set-point level. The difference between these two is then used to form a correction at the output, and this changes the input level. This is called proportional control. 

There are times when this feedback can lead to oscillations, especially if there are delays, as discussed earlier. Oscillations in feedback control systems represent loss of control because they are difficult to stop once started. To improve stability, we could add rate of change information somewhere in the control system. A system that responds not only to the input level but also to the rate of change of the input anticipates where the input level will be and corrects ahead of time. Perhaps that is why so many biological receptors are sensitive to the rate of change in addition to the actual level of input. 

We have just described proportional (P) control and proportional plus derivative (PD) control. Integration can be added to a controller, which not only gives it reset action, but also can exacerbate instability. There are proportional plus integral (PI) and proportional-integral-derivative (PID) controllers. These classical types are used where the system dynamics (the Plant) are well defined. 

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The function of the oxygen sensor and the closed loop fuel metering system is to maintain the air and fuel mixture at the stoichiometric condition as it passes into the engine for combustion ie, there should be no excess air or excess fuel. The main purpose is to permit the TWC catalyst to operate effectively to control HC, CO, and NO emissions. The oxygen sensor is located in the exhaust system ahead of the catalyst so that it is exposed to the exhaust of aU cylinders (see Fig. 4). The sensor analyzes the combustion event after it happens. Therefore, the system is sometimes caUed a closed loop feedback system. There is an inherent time delay in such a system and thus the system is constandy correcting the air/fuel mixture cycles around the stoichiometric control point rather than maintaining a desired air/fuel mixture. 

AFM, invented in 1986 [25], has been optimized as a surface imaging tool for a variety of materials from metals to insulators by scanning the surface with a thin metallic cantilever, with a sharp tip on the end, coming to a point with a radius on the order of 10 nm. As the tip is scanned across a surface, the tip-stage position is controlled in three dimensions by a set of oriented piezo crystals (see Figure 4.2). An optical lever comprised of a laser in conjunction with a photodiode is used to measure cantilever deflection in a closed loop feedback system (based on amount of deflection of the cantilever in the simplest mode of operation). As the tip rasters across the surface, a three-dimensional topography map of the surface is created. 

The principal regulator of cortisol secretion is ACTH (see Chapter 31), the release of which is regulated by CRH and by circulating unbound cortisol. The balance of the effects of CRH and cortisol on the anterior pituitary maintains fairly constant ACTH stimulation of the adrenals and circulating levels of cortisol. This closed-loop feedback system is superseded by neural signals from higher regions of the brain in nonsteady-state conditions (e.g., circadian rhythm, stress). 

Establish closed loop feedback system with field service personnel and customers to ensure quick and proper disposition of newly discovered EHS issues. 

With this technology it is now possible to achieve extremely accurate speed control of the order of 0.01 % to 0.001 %. To achieve such high accuracy in speed control, closed-loop feedback control systems and microprocessor-based control logistics can be introduced into the inverter control scheme to sense, monitor and control the variable parameters of the motor to very precise limits. 

Very accurate controls through microprocessor-based closed-loop feedback control systems... 

Fig. 4.1 Block diagram of a closed-loop control system. R s) = Laplace transform of reference input r(t) C(s) = Laplace transform of controlled output c(t) B s) = Primary feedback signal, of value H(s)C(s) E s) = Actuating or error signal, of value R s) - B s), G s) = Product of all transfer functions along the forward path H s) = Product of all transfer functions along the feedback path G s)H s) = Open-loop transfer function = summing point symbol, used to denote algebraic summation = Signal take-off point Direction of information flow.

Fig. 8.10 Closed-loop control system with full-order observer state feedback.

Figure 3 Feedback in a Closed-Loop Control System. 3...

Feedback is information in a closed-loop control system about the condition of a process variable. This variable is compared with a desired condition to produce the proper control action on the process. Information is continually "fed back" to the control circuit in response to control action. In the previous example, the actual storage tank water level, sensed by the level transmitter, is feedback to the level controller. This feedback is compared with a desired level to produce the required control action that will position the level control as needed to maintain the desired level. Figure 3 shows this relationship. 

Feedback is information in a closed-loop control system about the condition of a process variable. 

We have chosen the steady state with Yfa = 0.872 and FCD = 1.0 giving a dense phase reactor temperature of Yrd = 1.5627 (Figure 7.14(b) and (c)) and a dense-phase gasoline yield of x-id = 0.387 (Figure 7.14(a)). This is the steady state around which we will concentrate most of our dynamic analysis for both the open-loop and closed-loop control system. We first discuss the effect of numerical sensitivity on the results. Then we address the problem of stabilizing the middle (desirable, but unstable) steady state using a switching policy, as well as a simple proportional feedback control. 

Positioning systems can use either an open-loop or a closed-loop control system. In closed-loop motion control, such as the optimized positioning of solar collectors based on measuring their shadows, the positions of both the collector and the shadow are continuously detected. Based on this feedback, the position and velocity of the collector can both be controlled. The reported position is continuously compared to the desired one, and the collector is moved to reduce the error between the two. This is called servo control (Figure 3.154). 

There are adaptive PCs. They are control system that changes the settings in response to changes in machine performance to bring the product back into its preset requirements or specification. The shift is maintained so that the control has adapted to changing conditions. It is a technique typically used to modify a closed loop control system. The process control comparator is the portion of the control elements that determines the feedback error on which a controller acts. 

Suppose the design of the engine conld be snbstantially improved snch that k conld be reduced to say 0.1 (approaching a valne for a typical closed-loop feedback controlled system). Table 2.5 shows comparable results to those given in Table 2.4. 

Hence the closed-loop control system for steady state conditions may be described by the forward transfer function of (2.52), using the positive root, and the feedback transfer function of... 

The torque produced by the motor is therefore reduced in proportion to the square of the rms value of the applied voltage. Circuits are available for both star and delta connected motors. Closed loop feedback control may be used to adjust the firing of the thyristors, thereby making accurate speed regulation possible. These systems are only used for small machines, e.g. up to 20 kW because they tend to produce many harmonic currents and voltages in the supply. 

The management system model can also be characterized as a feedback or closed-loop control system. In this version, the management team is the controller (who), the process is the system being controlled (what), and the instrumentation (how) monitors the system states and feeds these back to the controller so that deviations between the actual and the desired states can be nulled. The interfaces between each of the elements also represent the management process. Between the what and the how elements is the measurement-to-data interface. Between the how and who elements is the information portrayal/information perception interface. And between the who and the what elements is the decision-to-action interface. Viewed from the perspective of this model, the management of a function would entail ... 

Like any closed-loop system, the behavior of the respiratory control system is defined by the continual interaction of the controller and the peripheral processes being controlled. The latter include the respiratory mechanical system and the pulmonary gas exchange process. These peripheral processes have been extensively studied, and their quantitative relationships have been described in detail in previous reviews. Less well understood is the behavior of the respiratory controller and the way in which it processes afferent inputs. A confounding factor is that the controller may manifest itself in many different ways, depending on the modeling and experimental approaches being taken. Traditionally, the respiratory control system has been modeled as a closed-loop feedback/feedforward regulator whereby homeostasis of arterial blood gas and pH is maintained. Alternatively, the respiratory controller may be viewed as a... 

The PDCA management model is applied to coal mine hazard information closed-loop management system. It will implement the closed-loop hazard information management according to the science program of plan-do—check-action, in the meanwhile, it increases the work of information audit and feedback about the hazard information input, the hazard rectification, assessment and other information. It makes various processes closely linked and hazard information timely and effective... 

Servo-controlled robots are run in a "closed loop . A feedback control system is used to drive the robot to certain sequenced desired points. Normally the robots are run with a limited accuracy therefore the final position is determined by an error signal. The error signal is created by the difierence between the manipulator s momentary position and the position the manipulator is to be directed. The closed-loop servo system continuously minimizes the error signal and drives the manipulator to keep the positioning error as small as possible. 

Developments in web tension control systems are providing increased capability and function to eliminate problems (Fig. 3-34). They include ultrasonic roll diameter sensors, pneumatic pressure gauge tension monitors, capston-Mt. Hope tension systems, and so on. As an example, replacement of a web-tensioning system s conventional electromechanical drive with an ordinary ac motor enables processors to lower system cost and improve web consistency, as has been done for many years. A vector control system uses a belt and pulley arrangement to remotely couple an encoder to the shaft of the ac induction motor. This approach provides closed-loop feedback, without requiring that one modify the relatively inexpensive motor by installing a special feedback device on it. 

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