Control systems feedforward-feedback

Figure 24.3. Feedback and feedforward control systems. In feedback control, a measuring instrument obtains information at the output of a process, the signal obtained is compared to a set point, and the difference (or result) is applied to a final actuator. The result is ultimately detected by the measuring instrument and closed-loop control results. In feedforward control a measuring instrument obtains information at the input of a process, the signal obtained is again compared to a set point, but now the result is applied to an actuator that controls another input to the process. The result is not detected by the measuring instrument and open-loop control results. Courtesy of the Foxboro Company.

Other Considerations in Feedforward Control The tuning of feedforward and feedback control systems can be performed independently. In analyzing the block diagram in Fig. 8-32, note that Gy is chosen to cancel out the effects of the disturbance Us) as long as there are no model errors. For the feedback loop, therefore, the effects of L. s) can also be ignored, which for the sei vo case is ... 

Some plants have been using computer control for 20 years. Control systems in industrial use typically consist of individual feedback and feedforward loops. Horst and Enochs [Engineering h- Mining]., 181(6), 69-171 (1980)] reported that installation of single-variable automatic controls improved performance of 20 mineral processing plants by 2 to 10 percent. But interactions among the processes make it difficult for independent controllers to control the circuit optimally. 

Figure 10.4. (a) A feedforward-feedback control system, (b) The diagram after moving G vGp. 

Example 10.2 Consider the temperature control of a gas furnace used in heating a process stream. The probable disturbances are in the process stream temperature and flow rate, and the fuel gas flow rate. Draw the schematic diagram of the furnace temperature control system, and show how feedforward, feedback and cascade controls can all be implemented together to handle load changes. 

When close control is desired, usually the variable that is to be closely controlled is monitored and no changes are made until the measurement differs from what is desired. This is feedback control. It obviously is not an ideal system, since the controller can only react to changes. A better system would be one that anticipates a change and takes corrective action that ensures an unvarying output. This is a feedforward control system. This type of control is very advantageous when the input variables have a wide range of variation. 

Since it is impractical to measure everything that may affect the output variables, even when feedforward control is used feedback control is also included. Figure 7-10 shows how a feedforward system might be used on a waste neutralizer. The purpose of the waste neutralizer is to make certain that the streams leaving the plant are neutralized. First, all the streams are combined together and the feed rate and... 

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. 

Feedforward control systems have gained wide acceptance in chemical engineering in the past two decades. They have demonstrated their ability to improve control, sometimes quite spectacularly. We will illustrate this improvement in this section by comparing the responses of systems with feedforward control and with conventional feedback control when load disturbances occur. 

In practice, many feedforward control systems are implemented by using ratio control systems, as discussed in Chap. 8. Most feedforward control systems are installed as combined feedforward-feedback systems. The feedforward controller takes care of the large and frequent measurable disturbances. The feedback controller takes care of any errors that come through the process because of inaccuracies in the feedforward controller or other unmeasured disturbances. Figure 11.4d shows the block diagram of a simple linear combined fe forward-/ feedback system. The manipulated variable is changed by both the feedforward controller and the feedback controller. 

Figure 11.5h shows a combined feedforward-feedback system where the feedback signal is added to the feedforward signal in a summing device. Figure 11.Sc shows another combined system where the feedback signal is used to change the feedforward controller gain in the ratio device. Figure 11.6 shows a combined feedforward-feedback control system for a distiltetion column where feed-rate disturbances are detected and both steam flow and reflux flow arc changed to hold both overhead and bottoms compositions constant. Two feedforward controllers are required.

A proportional-only controller is used to control the liquid level in a tank by manipulating the outflow. It has been proposed that the sfeadystate offset of the proportional-only controller could be eliminated by using the combined feedforward-feedback system sketched below. 

Now, from its essential notion, we have the feedback interconnection implies that a portion of the information from a given system returns back into the system. In this chapter, two processes are discussed in context of the feedback interconnection. The former is a typical feedback control systems, and consists in a bioreactor for waste water treatment. The bioreactor is controlled by robust asymptotic approach [33], [34]. The first study case in this chapter is focused in the bioreactor temperature. A heat exchanger is interconnected with the bioreactor in order to lead temperature into the digester around a constant value for avoiding stress in bacteria. The latter process is a fluid mechanics one, and has feedforward control structure. The process was constructed to study kinetics and dynamics of the gas-liquid flow in vertical column. In this second system, the interconnection is related to recycling liquid flow. The experiment comprises several superficial gas velocity. Thus, the control acting on the gas-liquid column can be seen as an open-loop system where the control variable is the velocity of the gas entering into the column. There is no measurements of the gas velocity to compute a fluid dynamics... 

A closed-loop system uses the measurement of one or more process variables to move the manipulated variable to achieve control. Closed-loop systems may include feedforward, feedback, or both. 

FIG. 8-34 A block diagram of a feedforward-feedback control system. (Source Sehorg et at., Process Dynamics and Control, 2d ed., Wiley, New York, 2004.)... 

The feedback control in loops 1 and 2 is combined with a feed-forward controller in loop 3 which measures the inlet, temperature T calculates the change in cooling water flow rate Few which is required to bring the reactor temperature Fback to its set point Ts and sends this signal to the feedback controller (the feed-forward controller consists of a model of the process and is therefore not of P-, PI- or PID-type). The feedforward control loop will therefore theoretically eliminate any disturbances in inlet temperature Tv The feedback part of the control system, loop 1, will compensate for any inaccuracies in the feed-forward control model as well as eliminate the effect of other, unmeasured disturbances, e.g. in inlet flow rate Fr... 

The design, optimization, and control of the EMR for the decolorization of Orange II require the implementation of a control system. Two control systems were developed (a) a feedforward system based on the knowledge of kinetics and reactor hydraulics and (b) a feedback system based on the concentration of DO into the reactor, which was observed to be a main variable that provides extensive information about the development of the process. 

Since the controlled variable (the outlet oil temperature) in Figure 34A is being measured, this is a feedback control system. Since the disturbance (the inlet oil temperature) in Figure 34B is being measured, this is a feedforward control system and not a feedback control system. Since both the outlet and inlet oil temperatures in Figure 34C are being measured, this is not a feedback control system. Therefore, the correct answer is Figure 34A. 

The most important fact about piston flow is that disturbances at the inlet are propagated down the tube with no dissipation due to mixing. They arrive at the outlet t seconds later. This pure time delay is known as dead time. Systems with substantial amounts of dead time oscillate when feedback control is attempted. This is caused by the controller responding to an output caused by an input t seconds ago. The current input may be completely different. Feedforward control represents a theoretically sound approach to controlling systems with appreciable dead time. Sensors are installed at the inlet to the reactor to measure fluctuating inputs. The... 

It is clear that our first reaction is to use a composition analyzer to measure the concentration of pentane in the distillate and then using feedback control to manipulate the reflux ratio, so that we can keep the distillate 95% in pentane. This control scheme is shown in Figure 2.2a. An alternative control system is to use a composition analyzer to monitor the concentration of pentane in the feed. Then in a feedforward arrangement we can change the reflux ratio to achieve our objective. This control... 

Describe the steps that you would go through in designing a control system for maintaining the pH of the liquid in a stirred tank (see Figure QI.2) at a desired value What questions must you resolve Develop both feedback and feedforward control configurations for this system. 

In Figure 21.1a we can see the general form of a feedforward control system. It measures the disturbance directly and then it anticipates the effect that it will have on the process output. Subsequently, it changes the manipulated variable by such an amount as to eliminate completely the impact of the disturbance on the process output (controlled variable). Control action starts immediately after a change in the disturbance ) has been detected. In Figure 21.1b we have repeated the schematic of a typical feedback loop so that you can contrast the two control systems directly. It is clear that feedback acts after the fact in a compensatory manner, whereas feedforward acts beforehand in an anticipatory manner. 

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. 

We would expect that a combined feedforward-feedback control system will retain the superior performance of the first and the insensitivity of the second to uncertainties and inaccuracies. Indeed, any deviations caused by the various weaknesses of the feedforward control will be corrected by the feedback controller. This is possible because a feedback control loop directly monitors the behavior of the controlled process (measures process output). Figure 21.7 shows the configuration of a combined feedforward-feedback control system. 

Draw the feedforward and feedback control systems that regulate the flow through a pipe. Do you expect one of them to be significantly better than the other in maintaining the desired flow ... 

Hardware components computer systems, 552-57 control loops, 28-30, 32, 561-66 digital control loops, 557-61 Hardware instructions, 553 Heat exchanger cascade control, 399 control loops, 364 feedback control, 243 feedforward control, 413 modeling, 69-70... 

Part V (Chapters 19 through 22) deals with the description, analysis, and design of more complex control systems, with one controlled output. In particular, Chapter 19 introduces the concept of feedback compensation with Smith s predictor, to cope with systems possessing large dead times or inverse response. Chapter 20 describes and analyzes a variety of multiloop control systems (with one controlled output) often encountered in chemical processes, such as cascade, selective, and split-range. Chapter 21 is devoted exclusively to the analysis and design of feedforward and ratio control systems, while Chapter 22 makes a rather descriptive presentation of adaptive and inferential control schemes why they are needed and how they can be used. 

Returning to the tank heater example, we realize that we can use a different control arrangement to maintain T = Ts when T, changes. Measure the temperature of the inlet stream T, and open or close the steam valve to provide more or less steam. Such a control configuration is called feedforward control and is shown in Figure 1.4. We notice that the feedforward control does not wait until the effect of the disturbances has been felt by the system, but acts appropriately before the external disturbance affects the system, anticipating what its effect will be. The characteristics of the feedback and feedforward control systems will be studied in detail in subsequent chapters. 

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