Control systems feedforward

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. 

There are many advanced strategies in classical control systems. Only a limited selection of examples is presented in this chapter. We start with cascade control, which is a simple introduction to a multiloop, but essentially SISO, system. We continue with feedforward and ratio control. The idea behind ratio control is simple, and it applies quite well to the furnace problem that we use as an illustration. Finally, we address a multiple-input multiple-output system using a simple blending problem as illustration, and use the problem to look into issues of interaction and decoupling. These techniques build on what we have learned in classical control theories. 

Figure 10.3. A feedforward control system on a major load variable with measurement function GMl and feedforward controller GFF.

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. 

A host of gadgets and software are available to perform a variety of computations and logical operations with control signals. For example, adders, multipliers, dividers, low selectors, high selectors, high limiters, low limiters, and square-root extractors can all be implemented in both analog and computer systems. They are widely used in ratio control, in computed variable control, in feedforward control, and in override control. These will be discussed in the next chapter. 

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. 

Keep the control system as simple as possible. Everyone involved in the process, from the operators up to the plant manager, should be able to understand the system. Use as few pieces of control hardware as possible. Every additional gadget that is included in the system is one more item that can fail or drift. The instrument salesperson will never tell you this, of course. 1 Use feedforward control to compensate for large, frequent, and measurable disturbances. 

In the last chapter we used Laplace-domain techniques to study the dynamics and stability of simple closedloop control systems. In this chapter we want to apply these same methods to more complex systems cascade control, feedforward control, openloop unstable processes, and processes with inverse response. Finally we will discuss an alternative way to look at controller design that is called model-based 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. 

Figure 11.4c shows the f forward control system. The load disturbance still enters the process through the G/, ) process transfer function. The load disturbance is also fed into a feedforward control device that has a transfer function. The feedforward controller detects changes in the load and makes changes in the manipulated variable. ... 

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.

There are no inherent linear limitations in feedforward control. Nonlinear feedforward controllers can be designed for nonlinear systems. The concepts are illustrated in Example 11.3. 

Let us choose a feedforward control system that holds both reactor temperature T and reactor concentration Cj conslant at their steadystate values, f and. The feed flow rate F and the jacket temperature Tj are the manipulated variables. Disturbances are feed concentration C o and feed temperature 7. ... 

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... 

Control System Development Model-based design space development offers an ideal segue between process and control development. Quite literally, a model-based design space would provide the template for development of feedforward process control models. Moreover, development of a process design space using a model-based framework would facilitate control system validation and identification of science-based, in-process, and release specifications. 

Feedforward Control A feedforward system uses measurements of disturbance variables to position the manipulated variable in such a way as to minimize any resulting deviation. The disturbance variables could be either measured loads or the set point, the former being more common. The feedforward gain must be set precisely to reduce the deviation of the controlled variable from the set point. 

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.)... 

Flow as a secondary cannot only overcome the effects of valve hysteresis, but also insures that line pressure variations or badly selected valve characteristics will not affect the primary loop. For these reasons, in composition control systems, flow is usually set in cascade. Cascade flow loops are also useful in feedforward systems. Flow controllers invariably have both proportional and integral modes. If their proportional band exceeds 100%, they must have an integral mode. 

Feedforward control system that provides constant separation by manipulating the distillate flow (top). At the bottom, a variety of dynamic compensators are shown, which can be used to match the "dynamic personality" of the process. 

Whenever the feed composition is unpredictable, one must directly control the compositions of both products. The main benefit of dual composition control is minimized energy consumption. The main limitation is caused by the interactions between the two composition loops. On the left of Figure 2.93 an example of a feedforward dual composition control system is shown. In this configuration, the distillate flow is manipulated to control the distillate composition by maintaining the relationship ... 

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. 

We showed through nonlinear dynamic simulations how the process reacts to various disturbances and changes in operating conditions. We have not shown any attempts to optimize process performance, to improve the process design, or to apply any advanced control techniques (model-based, nonlinear, feedforward, valve-position, etc.). These would be the natural next steps after the base-level regulatory control system had been developed to keep the process at a stable desired operating point. 

Feedforward Control A feedforward system uses measurements of disturbance variables to position the manipulated variable in such a way as to minimize any resulting deviation. The disturbance... 

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