Primary feedback signal

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.

Saturable reactors, which are adjustable by a small dc signal, have also been used for both primary (stator) and secondary (rotor) control. In the primary they control motor voltage and therefore torque. In combination with fixed secondary resistors and feedback from a tachometer, this system can be used for precise speed and torque control of cranes, hoists, etc. Even reversing can be accomplished by using two saturable reactors in each of two (of three) phases. Other combinations of fixed or saturable reac tors in the primaiy and/or secondaiy, all combined with secondary resistors, provide a wide range of capabiUties and flexibihty for the wound-rotor motor. 

A DC motor is feedback-controlled by a current sub-control loop and a primary speed control loop. In order to close the control loop, the actual current value is fed back to the current control loop and a speed signal to the speed control loop. While current is measured in the power converter, a shaft encoder on the motor is required for speed signal feedback. Either a tacho-generator or a digital encoder is used as a speed transmitter. If speed measurement accuracy is not very important, the speed feedback can be measured via armature voltage. In this case, this measurement can also be done within the power converter. Static control accuracy reaches... 

Control systems may be classified from their signal flow diagrams as either open-loop systems or closed-loop systems depending on whether the output of the primary control circuit is fed back to the controlling component. As Fig. 2 suggests, the typical control circuit consists of sequential arrays of components deployed about the process under control. If the controller is not apprised of the behavior of the controlled variable, the control system is an open-loop one. Conversely, if the measuring means on the controlled variable sends its signals back to the controller so that the behavior of the controlled variable is always under the scrutiny of the controller, the system is a closed-loop or feedback control system. 

From Figure 21.5b we notice that the feedforward loop retains all the external characteristics of a feedback loop. Thus it has a primary measurement which is compared to a set point signal and the result of the comparison is the actuating signal for the main controller. In substance, though, the two control systems differ significantly, as pointed out in Section 21.1. 

Fig. 9.5. Scheme of the model considered for signal-induced, intracellular Ca" oscillations based on Ca Mnduced release (CICR). The stimulus (S) acting on a cell surface receptor (R) triggers the synthesis of IP3 the latter intracellular messenger elicits the release of Cif from an IPj-sensitive store (X, concentration X) at a rate proportional to the saturation function (B) of the IP3 receptor. Fast replenishment of X could involve activation of Ca" uptake from the extracellular medium into the IPj-sensitive store. Cytosolic Ca (Z, concentration Z) is pumped into an IPj-insensitive intracellular store Ca in the latter store (Y, concentration Y) is released into the cytosol in a process activated by cytosolic Ca ". This feedback, known as CICR, plays a primary role in the origin of Ca " oscillations. In its minimal version, the model contains only two variables, i.e. Y and Z, as X is treated as constant owing to fast replenishment of the IPj-sensitive Ca pool (Dupont et ai, 1991). 

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