PIDS

The integral action of a PID algorithm is designed to remove this offset. The integral action term is given by... 

PID controller is a flexible, effective, and rehable controller for the process industries. A considerable range of controller actions is possible by selecting tuning parameters to provide different weights to the present (proportional), the past (integral), and the projected future (derivative). References related to the tuning of PID controllers are available (20—24). 

Dead-Time Compensation. Dead time within a control loop can greatiy iacrease the difficulty of close control usiag a PID controller. Consider a classical feedback control loop (Fig. 18a) where the process has a dead time of If the setpoiat is suddenly iacreased at time t, the controller immediately senses the deviation and adjusts its output. However, because of the dead time ia the loop, the coatroUer does aot begia to see the impact of that change ia its feedback sigaal, that is, a reductioa ia the deviatioa from setpoiat, uatil the time t +. Because the deviatioa does aot change uatil... 

K. J. Astrom and T. Automatic Tuning of PID Controllers, Instmment Society of America, Research Triangle Park, N.C., 1987. 

For example, pelvic inflammatory disease (PID) is usually caused by a concurrent infection with both N. gonorrhoeae and C. trachomatis (94). For these reasons many sexually transmitted diseases, including PID, are treated with mixtures of antibiotics to provide broad coverage for mixed, aerobic, and... 

A final area of research focuses on how the piU. can best be used by women. Physicians used to recommend pid-free hoHdays, but it is now known that there is no vaUd reason for this practice. Similarly, further research indicated that the U.S. EDA s restrictive guidelines for prescribing the piU. to women over the age of 35 are not justified, and those guidelines have been changed. EinaHy it is clear that cigarette smoking increases the risk of cardiovascular side effects, especially in women over 35 who utilize combination oral contraceptives. 

Proportional-plus-Integral-plus-Derivative (PID) Control. 8-12... 

Proportional-plus-Integral-plus-Derivative (PID) Control The derivative mode moves the controller output as a function of the rate-of-change of the controlled variable, which adds phase lead to the controller, increasing its speed of response. It is normally combined... 

In some analog PID controllers, the integral and derivative terms are combined serially rather than in parallel as done in the last equation. This results in interaction between these modes, such that the effective values of the controller parameters differ from their set values as follows ... 

The performance of the interacting controller is almost as good as the noninterac ting controller on most processes, but the tuning rules differ because of the above relationships. With digital PID controllers, the noninteracting version is commonly used. 

For a PI or PID controller, the integrated deviation—better known as the integrated error IE—is related to the controller settings ... 

FIG. 8 26 Resp onse for a step change in disturbance with tuned P, PI, and PID controllers and with no control. 

A recent addition to the model-based tuning correlations is Internal Model Control (Rivera, Morari, and Skogestad, Internal Model Control 4 PID Controller Design, lEC Proc. Des. Dev., 25, 252, 1986), which offers some advantages over the other methods described here. However, the correlations are similar to the ones discussed above. Other plant testing and controller design approaches such as frequency response can be used for more complicated models. 

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. 

The above FF controller can be implemented using analog elements or more commonly by a digital computer. Figure 8-33 compares typical responses for PID FB control, steady-state FF control (.s = 0), dynamic FF control, and combined FF/FB control. In practice, the engineer can tune K, and Tl in the field to improve the performance oTthe FF controller. The feedforward controller can also be simplified to provide steady-state feedforward control. This is done by setting. s = 0 in Gj. s). This might be appropriate if there is uncertainty in the dynamic models for Gl and Gp. 

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