TAILIEUCHUNG - Process Control Systems Episode 3

Tham khảo tài liệu 'process control systems episode 3', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Characteristics of Real Processes F1G . An uncompensated pH loop tends to exhibit a distorted oscillation. 1 53 load change of any magnitude can then drive the pH far enough up the curve to reduce the loop gain to the point where recovery is extremely slow. Compensation for such a severe nonlinearity is essential if a satisfactory degree of performance is to be obtained. The nonlinearity is so severe that even poor compensation is noticeably effective. Reduction in loop-gain variation from 100 1 to 5 1 for example is bound to improve the situation. The shape of the neutralization curve is a function of the reacting If the ingredients of the are subject to change the slope of the curve at a given may also change. This amounts to a second gain variation superimposed on the first. This is not a predictable characteristic. The response of a typical uncompensated loop to a load change is pictured in Fig. . Oscillations are flattened by the change in gain. Recovery is considerably retarded by reduced loop gain away from the vicinity of the set point. space is devoted to control in Chap. 10. Variable Dynamic Gain Figure is a record of temperature and flow of a product stream leaving a heat exchanger. Temperature was being controlled by manipulating the flow of to the exchanger. that the temperature record at 80 percent flow is overdamped whereas at 40 percent flow F1G . Both magnitude and damping of the transient are functions of flow. 0 Time 54 I Understanding Feedback Control damping is slightly heavier than 14-amplitude. It appears that the loop will be only marginally stable at 20 percent flow. It is as if the proportional band had been changed. But the more lightly damped curve departs farther from the set point-contrary to the effects of changing proportional band as in Fig. . Therefore it is the process gain which has changed the process is more lightly damped and more sensitive to disturbance at low rates of flow. The problem has been .

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