Long-loop negative feedback

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,"... 

Synthesis of lutropin and follitropin is controlled by gonadotropin releasing hormone from the hypothalamus, which in turn is influenced by the sexual center in the brain. The sexual center, hypothalamus, anterior lobe of the pituitary and gonads or ptecenta form a feedback loop in which the sex hormones from the gonads exert negative feedback on the brain. Females display a cycUc pattern of variation in the hormone levels (menstrual cycle). In males, the variations are acyclic. In addition to these long-term variations, there are diurnal fluctuations in the hormone levels. 

The closed-loop system will be stable if this root is negative. Because time constants are always positive (t > 0), the feedback control system will be stable if KcK Kp > -1. This means that as long as the controller has the correct control action (i.e., reverse- or direct-acting, as per Section 8.3), then the system will be stable. For example, if > 0 and Ky > 0, then the controller must be made reverseacting so that Kc > 0. By contrast, if Kp < 0, then a direct-acting controller Kc < 0) is required. 

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