Open-loop control system

An open-loop control system is one in which the control action is independent of the output. An example of an open-loop control system is a chemical addition pump with a variable speed control (Figure 1). The feed rate of chemicals that maintain proper chemistry of a system is determined by an operator, who is not part of the control system. If the chemistry of the system changes, the pump cannot respond by adjusting its feed rate (speed)... 

An open-loop control system is one in which the control action is independent of the output. 

Open hollow fiber membranes, 16 2, 3 Open-loop control systems, 9 56-57 Open-loop dynamics, 20 694... 

A periodically forced system may be considered as an open-loop control system. The intermediate and high amplitude forced responses can be used in model discrimination procedures (Bennett, 1981 Cutlip etal., 1983). Alternate choices of the forcing variable and observations of the relations and lags between various oscillating components of the response will yield information regarding intermediate steps in a reaction mechanism. Even some unstable phase plane components of the unforced system will become apparent through their role in observable effects (such as the codimension two bifurcations described above where they collide and annihilate stable, observable responses). 

A simple example of an open-loop control system would be a steam-jacketed resin kettle very much like that in Fig. 1 except that the steam pressure is regulated automatically by the behavior of the measured jacket pressure but not by the actual temperature of the resin batch in the kettle. In the corresponding closed-loop system the steam pressure is regulated by the temperature of the resin batch as in Figs. 1 and 2. The only way open-loop control can be precise is through a close calibration between steam pressure and batch temperature. Since this close calibration can be maintained inexpensively only in the absence of load changes of any kind, it is obvious that the field of application of open-loop control is limited. In the example of Fig. 1, load changes would result... 

Open-loop control systems are characterized by operation at two levels basal delivery up to and following the absorption of meals, and augmented delivery for short periods associated with the absorption of meals adjusted to the insulin requirement of the respective meal. Augmented flow rates have ranged from 4 X basal (5) to 15 x basal (6,7), and augmented periods have varied from a single bolus injection/meal in humans (8) to 7 h/day in dogs (9). 

Open-loop control systems are used when every input variable of the process should be constant. Open-loop control can effectively be used when a closed control is not needed, when the change in inputs is not strong, or in cases when the feedback control is not good enough. The open-loop control is called feed-forward control when one of the input variables is measured and used for adjusting another input variable. 

Obviously, it is much easier and less expensive to implement an open-loop control system because far fewer components are involved. The compensator and sensors have been ehminated completely. 

Control zone This is the area of connectivity to control systems such as controllers (PLCs), human—machine interfaces (HMIs), and basic I/O devices such as actuators and sensors. Basically, there are three sections shown, namely, a management information system for plant management (which at times shares data with the database zone as discussed earlier), a main process control (namely, closed/open loop control system and data monitoring), and an applicable area control (e.g., choke kill control in offshore drilling, or offsite control like a coal handling plant in a power station). All I/Os are connected to this zone either by hardware directly or by a fieldbus system. This zone has very high priority and firewalls like a control firewall may be deployed. Additional external firewalls may also be used. 

Stepper Motor — Stepper motors can be used in simple open-loop control systems and are generally suited for systems operated at low acceleration and static load. Closed-loop control may be needed for high dynamic loads, particulariy when loads vary. If a stepper motor in an open-loop control system is overtorqued, all knovdedge of rotor position is lost and the system must be reinitialized. To prevent this, the torque capacity of the stepper motor must not exceed the maximum running torque calculated from ... 

FIG. B-11 Open-loop control system. (Source Revolve)... 

Up to + 0.01% in open-loop and 0.001% in closed-loop 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.

Closed-loop control systems are classified according to the number of pure integrations in the open-loop transfer function. If... 

Control systems are classified by the control action, which is the quantity responsible for activating the control system to produce the output. The two general classifications are open-loop and closed-loop control systems. 

Several methods have been investigated to find correlations between physical properties of fuel gas mixtures and the excess air ratio to optimize the combustion procedure. In spite of the varying composition of natural gas it is said to be possible to control a heater system by measurements of the dynamic viscosity of the gas [7]. One explanation could be the correlation between Wobbe number and viscosity With increasing Wobbe numbers the viscosity decreases, and if the Wobbe number of a gas is known, the excess air ratio can be adjusted, resulting in an open loop control. 

Forcing function is a term given to any disturbance which is externally applied to a system. A number of simple functions are of considerable use in both the theoretical and experimental analysis of control systems and their components. Note that the response to a forcing function of a system or component without feedback is called the open-loop response. This should not be confused with the term open-loop control which is frequently used to describe feed-forward control. The response of a system incorporating feedback is referred to as the closed-loop response. Only three of the more useful forcing functions will be described here. 

This heuristic argument forms the basis of the Bode stability criterion(22,24) which states that a control system is unstable if its open-loop frequency response exhibits an AR greater than unity at the frequency for which the phase shift is —180°. This frequency is termed the cross-over frequency (coco) for reasons which become evident when using the Bode diagram (see Example 7.7). Thus if the open-loop AR is unity when i/r = —180°, then the closed-loop control system will oscillate with constant amplitude, i.e. it will be on the verge of instability. The greater the difference between the open-loop AR (< I) at coc and AR = 1, the more stable the closed-loop... 

Since FCC units are usually operated at their middle unstable steady state, extensive efforts are needed to analyze the design and dynamic behavior of open loop and closed loop control systems to stabilize the desirable middle steady state. 

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). 

State feedback control is commonly used in control systems, due to its simple structure and powerful functions. Data-driven methods such as neural networks are useful only for situations with fully measured state variables. For this system in which state variables are not measurable and measurement function is nonlinear, we are dependant on system model for state estimation. On the other hand, as shown in figure 2, in open-loop situations, system has limit cycle behavior and measurements do not give any information of system dynamics. Therefore, we use model-based approach. 

It is interesting to note that there is increased secretion of corticoid after trauma in the face of elevated plasma levels. Some confusion over the mechanisms controlling plasma corticoid has arisen because of this, and two control mechanisms have been proposed, a closed-loop control with negative feedback, in nonstress conditions and an open-loop control with no feedback, in stress. However, Yates and Urquhart (Yl) point out the most likely control system is a closed-loop, negative feedback proportional control, with a variable set point, the set point being raised in stress. Why the set point should be raised (or the type of control changed) in stress when all the evidence indicates a noncausal role for corticoid in the various other responses to stress is a very interesting and as yet unanswered question. 

Again, poles and zeros are important for evaluating stability and controllability properties of the physical system. To find the poles of an open-loop MIMO system one can use the transfer function matrix or the state-space description. They are related by ... 

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