Basic Feedback Control

The concept of an automatic control system is illustrated in Fig. 2.28, based on a temperature-controlled chemical reactor. 

The components of the basic feedback control loop, combining the process and the controller can be best understood using a generalised block diagram (Fig. 2.29). The information on the measured variable, temperature, taken from the system is used to manipulate the flow rate of the cooling water in order to keep the temperature at the desired constant value, or setpoint. This is illustrated by the simulation example TEMPCONT, Sec. 5.7.1. 

---

Unlike the open-loop control, which basically provides a transfer function for the input signals to actuators, the feedback control systems receive feedback signals from sensors then compare the signals with the set point. The controller can then control the plant to the desired set point according to the feedback signal. There are five basic feedback control models (Morriss 1995) ... 

Hypothesis testing can be described in terms of basic feedback control concepts. Using the information in the process model, the controller generates a hypothesis about the controlled process. A test composed of control actions is created to generate feedback useful in evaluating the hypothesis, which in turn is used to update the process model and the hypothesis. 

Next we consider the three basic feedback control modes starting with the simplest moAq, proportional control... 

These are the same basic feedback control elements that we find in the Quality Assurance principles of, for example, the International Standards Organisation... 

Classical Feedback Control. The majority of controllers ia a continuous process plant is of the linear feedback controller type. These controllers utilize one or more of three basic modes of control proportional (P), iategral (I), and derivative (D) action (1,2,6,7). In the days of pneumatic or electrical analogue controllers, these modes were implemented ia the controller by hardware devices. These controllers implemented all or parts of the foUowiag control algorithm ... 

In the basic conventional feedback control strategy the value of the measured variable is compared with that for the desired value of that variable and if a difference exists, a controller output is generated to eliminate the error. 

Figure 8 shows basic elements of a feedback control system as represented by a block diagram. The functional relationships between these elements are easily seen. An important factor to remember is that the block diagram represents flowpaths of control signals, but does not represent flow of energy through the system or process. 

Feedforward control. The basic idea is shown in Fig. 1.8. The disturbance is detected as it enters the process and an appropriate change is made in the manipulated variable such that the controlled variable is held constant. Thus we begin to take corrective action as soon as a disturbance entering the system is detected instead of waiting (as we do with feedback control) for the disturbance to propagate all the way through the process before a correction is made. 

There are three basic types of controllers that are commonly used for continuous feedback control. The details of construction of the analog devices and the programming of the digital devices vary from one manufacturer to the next, but their basic functions are essentially the same. 

However, we can describe the basic structure of several feedforward control systems. Figure 8.7 shows a blending system with one stream which acts as a disturbance both its flow rate and its composition can change. In Fig. 8.7a the conventional feedback controller senses the controlled composition of the total blended stream and changes the flow rate of a manipulated flow. In Fig. %.lb the manipulated flow is simply ratjoed to the wild flow. This provides feedforward control for flow rate changes. Note that the disturbance must be measured to implement feedforward control. 

The basic idea of IMC is to use a model of the process openloop transfer function in such a way that the selection of the specified closedloop response yields a physically realizable feedback controller. 

There is a special type of controller, called a Smith predictor or deadtime compensator, that can be applied in either continuous or discrete form. It is basically a special type of model-based controller, in the same family as IMC. Figure 20.6a gives a sketch of a conventional feedback control system. Let s break up the total openloop process into the portion without any deadtime G j,(s) nd deadtime e... 

The architecture of the self-tuning regulator is shown in Fig. 7.99. It is similar to that of the Model Reference Adaptive Controller in that it also consists basically of two loops. The inner loop contains the process and a normal linear feedback controller. The outer loop is used to adjust the parameters of the feedback controller and comprises a recursive parameter estimator and an adjustment mechanism. 

Figure 8-1 sketches ten major subsystems, represented by the labeled ovals, and their major interaction routes. The solid arrows represent major routes of information flow not all known routes are shown, as this would clutter the diagram. The hatched arrows represent major, known feedback control routes whereby one subsystem has some control over the functioning of another subsystem. The dashed arrows represent information flow routes from the subconscious subsystem to other subsystems, routes that are inferential from the point of view of the ordinary d-SoC. Most of the subsystems are shown feeding information into, or deriving information from, awareness, which is here considered not a subsystem but the basic component of attention/awareness and attention/awareness energy that flows through various systems. 

Isocratic elution is commonly used for the elution of analytes from the column. In isocratic elution, the mobile phase is kept constant throughout the analysis. The mobile phase can be a single solvent or a solution of two or more miscible solvents. The major requirements of isocratic pumps are accuracy and smoothness of flow. Because the pump delivers only one solvent system, simple, inexpensive pulse dampeners and rudimentary flow or pressure feedback control circuits can be used. The basic setup of an isocratic system is illustrated in Figure 3.10. 

Notice that there is a feedback of information from the lower levels back up to the higher levels. The basic idea of feedback control occurs at the unit operation and fundamental control levels. Here, control algorithms are used to adjust manipulated inputs (controller outputs or control variables) to maintain process outputs (process variables) at desired values (known as setpoints). The basic principles and techniques of process control are pre-... 

The digital computer age has brought us sophisticated analytical instruments and computation abilities to delve even deeper into basic sciences. It also has enabled close monitoring and feedback control of processes, even in remote inhospitable atmospheres, to ensure that operations, and materials and products storage, are continuing as intended. 

Basic diagram for a mass spectrometer with two analysers and feedback control carried out by a data system. 

It is well known that important differences do occur with different methods of thermal retorting, and microwave interactive theory indicates even greater differences may be possible. We examined the products of this novel retorting system by using product quality as feedback control to gain some insight into the basic microwave interaction with fuel precursors. 

---