Closed loop control system response

Time-Delay Compensation Time delays are a common occurrence in the process industries because of the presence of recycle loops, fluid-flow distance lags, and dead time in composition measurements resulting from use of chromatographic analysis. The presence of a time delay in a process severely hmits the performance of a conventional PID control system, reducing the stability margin of the closed-loop control system. Consequently, the controller gain must be reduced below that which could be used for a process without delay. Thus, the response of the closed-loop system will be sluggish compared to that of the system with no time delay. 

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. 

Feedback is information in a closed-loop control system about the condition of a process variable. This variable is compared with a desired condition to produce the proper control action on the process. Information is continually "fed back" to the control circuit in response to control action. In the previous example, the actual storage tank water level, sensed by the level transmitter, is feedback to the level controller. This feedback is compared with a desired level to produce the required control action that will position the level control as needed to maintain the desired level. Figure 3 shows this relationship. 

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... 

There are adaptive PCs. They are control system that changes the settings in response to changes in machine performance to bring the product back into its preset requirements or specification. The shift is maintained so that the control has adapted to changing conditions. It is a technique typically used to modify a closed loop control system. The process control comparator is the portion of the control elements that determines the feedback error on which a controller acts. 

Computer-controlled automatic profile dies with electrical controlled sensors in closed-loop control systems have developed greater efficiency and accuracy to extrusion coating, cast film, and sheet lines. A scanner measures the web thickness and signals the computer, which then converts the readings to act on thermally actuated die bolts. The individual adjusting bolts expand or contract as ordered by the computer to control the profile. The more sophisticated systems measure adjusting bolt temperature and provide faster response time with less scrap and quicker startups. The scanner is typically an infrared, nuclear, or caliper-type gauge. 

The ultimate goal of insulin replacement therapy is a closed-loop controlled system or device in which the insulin release rates are adjusted by the system in response to glucose levels in the blood. Several different approaches for glucose-responsive insulin delivery have been investigated. 

Fig. 33.3. Step responses of a closed loop control system.

Control algorithms We have discussed that in closed-loop control systems a corrective action is taken by the controller in response to feedback from a transducer. The exact corrective action depends on the algorithm which has been developed. The simplest control approach is a two position control which turns the control element on and off based on the monitored value of the output. With an on/off strategy, the process value will typically oscillate above and below the set point. The most common controller is the PID (proportional, integral, and derivative) loop controller which is able to detect an early trend, adjust quickly, and prevent an over-correction. A PID controller can maintain temperatures within 1°F. The controller provides the means to define the control algorithm by assigning a constant for each of the three control modes. Typically, most of the adjustment is accomplished with the proportional control element, with the control action, u be-... 

Liquid fuel was seldom utilized in the previous ACC studies because it was not only difficult to actuate liquid-fuel injection at high frequencies, but the combustion delays associated with liquid-fuel atomization, droplet heating, vaporization, and burning processes made such a control extremely slow for fast-response in situ type controllers. As a result, the use of liquid fuel was confined to either steady injection process ]13] or upstream addition of prevaporized fuel ]4, 7] which limited the ACC flexibility associated with temporal responsiveness. The goal of this project is to make ACC more practical for propulsion systems by studying direct liquid-fueled ACC in a closed-loop controller setting. 

Figure 20. Simulated conversion response of continuous polymerization system to a load disturbance under closed-loop control with IAE optimum controller tuning constants and manipulation of initiator flow rate at 0.06 mol/L H20 surfactant and 50°C catalyst feed concentration—STD feedback (-) vs.

Process design modifications usually have a bigger impact on operability (dynamic resilience). Dynamic resilience depends on controller structure, choice of measurements, and manipulated variables. Multivariable frequency-response techniques have been used to determine resilience properties. A primary result is that closed-loop control quality is limited by system invertability (nonmin-imum phase elements). Additionally, it has been shown that steady-state optimal designs are not necessarily optimal in dynamic operation. 

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. 

For purposes of this report, the process control system includes the closed-loop control of plant processes having control response times ranging from minutes to fractious of seconds. Higher-level information management and control is covered under Information Management Systems in Chapter 4. 

In contrast, input multiplicity [46] can occur when different sets of input variables produce the same set of output variables. This input multiplicity will depend on the choice of output or measured variables. It is associated with the so-called zero dynamics of the system, which can be observed by an unexpected inverse response of the outputs after a step change has been applied to the inputs. Therefore, it can have severe implications for closed loop control [46]. We will come back to this point in Section 10.8. 

From tliese results it can be concluded, that the combinations of controlled variables 3,22, 3,10 and 3,11 yield the best closed-loop performance. These combinations allow very fast rise times, and approximate decoupling of the closed-loop system can be achieved. All other structures give slower responses and stronger couplings and are therefore rejected. The structure 3,22 shows a good dynamic behavior, but this is due to the fact that the vapor stream is determined by the heat duty in a nearly proportional manner. Controlling the vapor stream by the heat input means to replace one degree of freedom by another does not provide closed-loop control of the process itself. This structure is therefore discarded. 

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