Negative feedback loop

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. 

The female menstrual cycle is divided into four functional phases follicular, ovulatory, luteal, and menstrual.6 The follicular phase starts the cycle, and ovulation generally occurs on day 14. The luteal phase then begins and continues until menstruation occurs.6 The menstrual cycle is regulated by a negative-feedback hormone loop between the hypothalamus, anterior pituitary gland, and ovaries6 (Fig. 45-1). 

Figure M5.3. A sample negative feedback closed-loop generated within Simulink. This servo system has a first order process function and uses a PID controller. The output is sent to a graphing tool for plotting.

A negative phase margin indicates an unstable system. Clearly, first and second order systems are inherently stable as the maximum phase shift of the former is -90° and of the latter -180° (Section 7.8.4). (Note that when such a system is included within a feedback control loop this innate stability may no longer exist—see Section 7.10.3.)... 

What is perhaps not immediately obvious is that in any simple loop, whatever its number of elements and whatever the number and order of the positive and negative interactions, either all the elements of the loop exert a positive control on their own production, or all the elements of the loop exert a negative control on their own production. There are thus two classes of simple feedback loops. In the loops of one class, each element exerts indirectly, via all the other elements, a negative control on its own further production (or activity) let us call these loops negative loops as already suggested in Thomas et al.13-14 In the other class, each element exerts indirectly, via all the other elements, a positive control on its own further production (or activity) let us call these loops positive loops.1314... 

Fig. 13.4 Blue CM/Pf-striatum-GPi-CM/Pf a positive FB loop and red CM/Pf-STN-GPi-CM/Pf a negative FB loop. Somatotopically organized parallel projections form closed loops (vertical, in black) between the motor cortex, the basal ganglia and the motor cortex. These loops signal movement preparation and execution. Concomitantly feedback (horizontal) loops stabilize basal ganglia activity. (From [50] with permission)...

System behavior in system dynamics is modeled by using feedback (causal) loops, stock and flows (levels and rates), and the nonlinearities created by interactions between system components. In this view of the world, behavior over time (the dynamics of the system) can be explained by the interaction of positive and negative feedback loops [185]. The models are constructed from three basic building blocks ... 

Set point control Most process control is done with feedback set point control. This means that the controlled variable is measured as an output of the process and the signal is sent back to be compared with a set point (desired value). The generated error is the difference between the set point and the measured value. The error signal is acted on by the controller to make a corrective action which is sent as a command signal to a final control element. In this kind of control loop, the error is corrected only after it has occurred. The closed-loop negative feedback control works best if the distance velocity lag or dead time (DT) is very short. Fig. 17.4 illustrates atypical closed-loop, negative feedback, control loop. 

The trp repressor controls the operon for the synthesis of L-tryptophan in Escherichia coli by a simple negative feedback loop. In the absence of L-tryptophan, the repressor is inactive, the operon is switched on and the enzymes which synthesize L-tryptophan are produced. As the concentration of L-tryptophan increases, it binds to the repressor and converts it to an active form so that it can bind to the operator region and switch off the gene. 

The fault tree (Figure 7.4-1) has "Pre.ssure Tank Rupture" as the top event (gate G1). This may result from random failure of the tank under load (BEl), OR the gate G2, "Tank ruptures due to overpressure" which is made of BE6 "Relief valve fails to open" AND G3, "Pump motor operates too long." This is made of BE2, "Timer contacts fail to open," AND G4, "Negative feedback loop inactive" which is composed of BE3, "Pressure gauge stuck," OR BE4, "Operator fails to open switch," OR "BE5, "Switch fails to open,"... 

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