Processes control time-delay compensation

While the single-loop PID controller is satisfactoiy in many process apphcations, it does not perform well for processes with slow dynamics, time delays, frequent disturbances, or multivariable interactions. We discuss several advanced control methods hereafter that can be implemented via computer control, namely feedforward control, cascade control, time-delay compensation, selective and override control, adaptive control, fuzzy logic control, and statistical process control. 

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. 

The Smith predictor is a model-based control strategy that involves a more complicated block diagram than that for a conventional feedback controller, although a PID controller is still central to the control strategy (see Fig. 8-37). The key concept is based on better coordination of the timing of manipulated variable action. The loop configuration takes into account the facd that the current controlled variable measurement is not a result of the current manipulated variable action, but the value taken 0 time units earlier. Time-delay compensation can yield excellent performance however, if the process model parameters change (especially the time delay), the Smith predictor performance will deteriorate and is not recommended unless other precautions are taken. 

Economic Incentives for Automation Projects Industrial applications of advanced process control strategies such as MPC are motivated by the need for improvements regarding safety, product quality, environmental standards, and economic operation of the process. One view of the economics incentives for advanced automation techniques is illustrated in Fig. 8-41. Distributed control systems (DCS) are widely used for data acquisition and conventional singleloop (PID) control. The addition of advanced regulatory control systems such as selective controls, gain scheduling, and time-delay compensation can provide benefits for a modest incremental cost. But... 

The denominators of YID in (16-24) and Y/Ygp in (16-22) are the same, but the numerator terms are quite different in form. Figure 16.11 shows disturbance responses for Example 16.2 (0 = 2) for PI controllers with and without the Smith predictor. By using the two degree-of-freedom controllers discussed in Chapter 12, it is possible to improve the response for disturbances and avoid this undesirable behavior. In fact, In-gimundarson and Hagglund (2002) have shown that for step disturbances and a FOPTD process, the performance of a properly tuned PID controller is comparable to or better than a PI controller with time-delay compensation. 

The previous discussion of time-delay compensation assumed that measurements of the controlled variable were available. In some control applications, the process variable that is to be controlled cannot be conveniently measured on-line. For example, product composition measurement may require that a sample be sent to the plant analytical laboratory from time to time. In this situation, measurements of the controlled variable may not be available frequently enough or quickly enough to be used for feedback control. 

Digital controllers of the Direct Synthesis type share yet one more characteristic namely, they contain time-delay compensation in the form of a Smith predictor (see Chapter 16). In Eq. 17-61, for Gc to be physically realizable, (Y/Y ) must also contain a term equivalent to which is z, where N = 0/Ar. In other words, if there is a term z in the open-loop discrete transfer function, the closed-loop process cannot respond before NAt or 0 units of time have passed. Using YIYsp)d of this form in Eq. 17-63 yields a Gc containing the mathematical equivalence of time-delay compensation, because the time delay has been eliminated from the characteristic equation. 

II Single-loop PID control with compensation for difficult dynamics (e.g., Smith predictors for time-delays), again with appropriate loop pairing for multivariable processes. Alternatively, the use of explicitly model-based control strategies like direct synthesis control. Internal Model Control (IMC), or Model Predictive Control (MPC) may be appropriate ... 

The presence of significant amounts of dead time in a control loop can cause severe degradation of the control action due to the additional phase lag that it contributes (see Example 7.7). One method for compensating for the effects of dead time in the control loop has been suggested by SMITH<30>. This consists of the insertion of an additional element which is often termed the Smith predictor as it attempts to predict the delayed effect that the manipulated variable will have upon the process output. 

Monomer conversion can be adjusted by manipulating the feed rate of initiator or catalyst. If on-line M WD is available, initiator flow rate or reactor temperature can be used to adjust MW [38]. In emulsion polymerization, initiator feed rate can be used to control monomer conversion, while bypassing part of the water and monomer around the first reactor in a train can be used to control PSD [39,40]. Direct control of surfactant feed rate, based on surface tension measurements also can be used. Polymer quality and end-use property control are hampered, as in batch polymerization, by infrequent, off-line measurements. In addition, on-line measurements may be severely delayed due to the constraints of the process flowsheet. For example, even if on-line viscometry (via melt index) is available every 1 to 5 minutes, the viscometer may be situated at the outlet of an extruder downstream of the polymerization reactor. The transportation delay between the reactor where the MW develops, and the viscometer where the MW is measured (or inferred) may be several hours. Thus, even with frequent sampling, the data is old. There are two approaches possible in this case. One is to do open-loop, steady-state control. In this approach, the measurement is compared to the desired output when the system is believed to be at steady state. A manual correction to the process is then made, based on the error. The corrected inputs are maintained until the process reaches a new steady state, at which time the process is repeated. This approach is especially valid if the dominant dynamics of the process are substantially faster than the sampling interval. Another approach is to connect the output to the appropriate process input(s) in a closed-loop scheme. In this case, the loop must be substantially detuned to compensate for the large measurement delay. The addition of a dead time compensator can... 

Also shown in Figure 3.6 is what the controller response would be without derivative action, i.e. proportional-only. It can be seen that derivative action takes action immediately that the proportional action takes Td minutes to do. In effect it has anticipated the need for corrective action, even though the error was zero at the time. The anticipatory nature of derivative action is beneficial if the process deadtime is large it compensates for the delay between the change in PV and the cause of the disturbance. 

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