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Automated Continuous Process Control Part 4

Tham khảo tài liệu 'automated continuous process control part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | c03.qxd 7 3 2003 8 23 PM Page 50 50 FEEDBACK CONTROLLERS not be use in processes with noise. An advantage of the derivative mode is that it provides anticipation. Another advantage is related to the stability of the system. Theory predicts and practice confirms that the ultimate gain with a PID controller is larger than that of a PI controller. That is KCU IpiD KCU I PI The derivative terms add some amount of stability to the system this is presented in more detail in Chapter 5. Therefore the controller can be tuned more aggressively now. The formulas we ll use to tune controllers will take care of this. 3-2.4 Proportional-Derivative Controller The proportional-derivative PD controller is used in processes where a proportional controller can be used where steady-state offset is acceptable but some amount of anticipation is desired and no noise is present. The describing equation is m t m KCe t KC T D de t dt and the transfer function is GC s KC 1 TDs E s 3-2.14 3-2.15 Based on our previous presentation on the effect of each tuning parameter on the stability of systems the reader can complete the following KCU PD KCU P 3-3 RESET WINDUP The problem of reset windup is an important and realistic one in process control. It may occur whenever a controller contains integration. The heat exchanger control loop shown in Fig. 3-1.1 is again used at this time to explain the reset windup problem. Suppose that the process inlet temperature drops by an unusually large amount this disturbance drops the outlet temperature. The controller PI or PID in turn asks the steam valve to open. Because the valve is fail-closed the signal from the controller increases until because of the reset action the outlet temperature equals the desired set point. But suppose that in the effort of restoring the controlled variable to the set point the controller integrates up to 100 because the drop in inlet temperature is too large. At this point the steam valve is wide open and therefore the control