Part 12 Cascade, Ratio, and Feedforward Control
Slide 2Overall Course Objectives Develop the aptitudes important to work as a mechanical procedure control build. Abilities Tuning circles Control circle configuration Control circle investigating Command of the wording Fundamental comprehension Process elements Feedback control
Slide 3Cascade, Ratio, and Feedforward Control Each of these systems offers preferences concerning unsettling influence dismissal: Cascade diminishes the impact of particular sorts of aggravations. Proportion lessens the impact of nourish stream rates changes Feedforward control is a general strategy for making up for measured unsettling influences.
Slide 4Compensating for Disturbances Reduces Deviations from Setpoint and Settling Time
Slide 5Level Controller on a Tank With and Without Cascade Control
Slide 6Analysis of Cascade Example Without a course level controller, changes in downstream weight will irritate the tank level. With course level controller, changes in downstream weight will be consumed by the stream controller before they can essentially influence tank level on the grounds that the stream controller reacts quicker to this aggravation than the tank level process.
Slide 7Key Features for Cascade Control to be Successful Secondary circle ought to lessen the impact of at least one aggravations. Auxiliary circle must be no less than 3 times speedier than ace circle. The CV for the auxiliary circle ought to directly affect the CV for the essential circle. The optional circle ought to be tuned firmly.
Slide 8Cascade Reactor Temperature Control
Slide 9Analysis of Example Without course, changes in the cooling water temperature will make a noteworthy surprise for the reactor temperature. With course, changes in the cooling water temperature will be consumed by the slave circle before they can fundamentally influence the reactor temperature.
Slide 10Multiple Cascade Example This approach works on the grounds that the stream control circle is considerably quicker than the temperature control circle which is substantially speedier than the structure control circle.
Slide 11Example Draw schematic: A temperature controller on the outlet stream is fell to a weight controller on the steam which is fell to a control valve on the condensate.
Slide 12Solution
Slide 13Ratio Control Useful when the controlled variable scales specifically with the nourish rate to the procedure. Dynamic pay is required when the controlled variable reacts powerfully extraordinary to nourish rate changes than it does to an adjustments in the controlled variable.
Slide 14Typical Performance Improvements utilizing Ratio Control
Slide 15Ratio Control for Wastewater Neutralization
Slide 16Analysis of Ratio Control Example The stream rate of base scales specifically with the stream rate of the acidic wastewater. The yield of the pH controller is the proportion of NaOH stream rate to corrosive wastewater stream rate; in this manner, the result of the controller yield and the deliberate corrosive wastewater stream rate turn into the setpoint for the stream controller on the NaOH expansion.
Slide 17Ratio Control Applied for Vent Composition Control
Slide 18Ratio Control Requiring Dynamic Compensation
Slide 19Example Draw schematic: For a control framework that alters the proportion of fuel stream to the stream rate of the procedure liquid to control the outlet temperature of the procedure liquid. Utilize a stream controller on the fuel.
Slide 20Solution
Slide 21Feedforward and Feedback Level Control
Slide 22Analysis of Feedforward and Feedback Level Control Feedback-just should retain the varieties in steam use by criticism activity as it were. Feedforward-just handle variety in steam utilization however little mistakes in metering will in the end discharge or fill the tank. Consolidated feedforward and criticism has best components of both controllers.
Slide 23Derivation of FF Controller
Slide 24Lead/Lag Element for Implementing FF Control
Slide 25Effect of Lead/Lag Ratio
Slide 26Static Feedforward Controller A static feedforward controller make a rectification that is straightforwardly corresponding to the unsettling influence change. A static feedforward controller is utilized when the procedure reacts in a comparative mold to an adjustment in the aggravation and the controlled variable.
Slide 27Feedforward When t p « t d
Slide 28Example of Feedforward Control for t d < t p
Slide 29Static Feedforward Results When the bay temperature drops by 20ºC, Q is instantly expanded by 20 kW. Deviations from setpoint result from element jumble
Slide 30Perfect Feedforward Control FF amendment is identical representation of unsettling influence impact. Net impact is no change in controlled variable.
Slide 31Required Dynamic Compensation Since the Q influences the procedure slower than T i , at first overcompensation in Q is required trailed by decreasing Q to 20 kW.
Slide 32Results with Dynamic Compensation
Slide 33Feedforward Control Action
Slide 34Effect of Lead/Lag Ratio
Slide 35Tuning a FF Controller Make introductory evaluations of lead/slack parameters in view of process information. Under open circle conditions, change K ff until unfaltering state deviation from setpoint is limited.
Slide 36Tuning a FF Controller Analyzing the dynamic crisscross, modify q ff .
Slide 37Tuning a FF Controller Finally, modify ( t ld - t lg ) until around equivalent territories above and beneath the setpoint result.
Slide 38Demonstration: Visual Basic Simulator Tuning a FF Controller
Slide 39Feedback Control Can viably dispense with aggravations for quick reacting forms. In any case, it holds up until the aggravation irritates the procedure before making restorative move. Can get to be distinctly shaky because of nonlinearity and unsettling influence upsets.
Slide 40Feedforward Control Compensates for d's before procedure is influenced Most viable for moderate procedures and for procedures with noteworthy deadtime. Can enhance dependability of the input controller by diminishing the deviation from setpoint. Since it is a direct controller, its execution will disintegrate with nonlinearity.
Slide 41Combined FF and FB Control
Slide 42Combined FF and FB for the CSTR
Slide 43Results for CSTR
Slide 44Analysis of Results for CSTR FB-just comes back to setpoint rapidly however has substantial deviation from setpoint. FF-just lessens the deviation from setpoint yet is ease back to come back to setpoint. FF+FB decreases deviation from setpoint and gives quick come back to setpoint.
Slide 45Example Draw schematic: For a consolidated feedforward and criticism controller in which the bay encourage temperature is the feedforward variable and the outlet temperature is the input variable. The joined controller yield is the setpoint for a steam weight controller.
Slide 46Solution
Slide 47Overview Cascade can viably evacuate certain unsettling influences if the slave circle is no less than 3 times speedier than the ace circle. Proportion control is compelling for procedures that scale with the sustain rate. Feedforward can be viable for measured unsettling influences for moderate reacting forms the length of the procedure nonlinearity is not very extraordinary.
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