4 Chapter 16 Cascade Control (multi-loop) Distinguishing features:Two FB controllers but only a single control valve (or other -final control element).2. Output signal of the "master" controller is the set-point for “slave" controller.3. Two FB control loops are "nested" with the "slave" (or "secondary") control loop inside the "master" (or "primary") control loop.Terminologyslave vs. mastersecondary vs. primaryinner vs. outerChapter 16
8 Time Delay Compensation • Model-based feedback controller that improvesclosed-loop performance when time delays are present• Effect of added time delay on PI controller performancefor a second order process (t1 = 3, t2 = 5) shown belowChapter 16
12 Chapter 16 Direct Synthesis Approach (Smith Predictor) Assume time delay between set-point change and controlledvariable (same as process time delay, ).Chapter 16IfthenFrom Block Diagram,Equating...
14 Chapter 16 SELECTIVE CONTROL SYSTEMS (Overrides) For every controlled variable, there must be at least one manipulatedvariable.In some applications# of manipulatedvariables# of controlledvariablesChapter 16Low selector:High selector:
15 Chapter 16 multiple measurements one controller Figure Control of a reactor hot spot temperature by using a high selector.multiple measurementsone controllerone final control element
16 Chapter 16Figure A selective control system to handle a sand/water slurry.Override using PI controllers - “old way” (vs. digital logic)2 measurements, 2 controllers, 1 F.C.E.
17 Chapter 16 Split Range Control 2 Manipulated Vars.: V1 and V2 1 Controlled Var.:Reactor pressureWhile V1 opens, V2 should close
20 Inferential Control Chapter 16 Problem: Controlled variable cannot be measured or has large sampling period.Possible solutions:Control a related variable (e.g., temperature instead of composition).Inferential control: Control is based on an estimate of the controlled variable.The estimate is based on available measurements.Examples: empirical relation, Kalman filterModern term: soft sensorChapter 16
22 Chapter 16 Gain Scheduling Objective: Make the closed-loop system as linear as possible.Basic Idea: Adjust the controller gain based on current measurements of a “scheduling variable”, e.g., u, y, or some other variable.Chapter 16Note: Requires knowledge about how the process gain changes with this measured variable.
23 Examples of Gain Scheduling Example 1. Once through boilerThe open-loop step response are shown in Fig for two different feedwater flow rates.Chapter 16Fig Open-loop responses.Proposed control strategy: Vary controller setting with w, the fraction of full-scale (100%) flow.Example 2. Titration curve for a strong acid-strong base neutralization.
26 Adaptive Control Chapter 16 A general control strategy for control problems where the process or operating conditions can change significantly and unpredictably.Example: Catalyst decay, equipment foulingMany different types of adaptive control strategies have been proposed.Self-Tuning Control (STC):A very well-known strategy and probably the most widely used adaptive control strategy.Basic idea: STC is a model-based approach. As process conditions change, update the model parameters by using least squares estimation and recent u & y data.Note: For predictable or measurable changes, use gain scheduling instead of adaptive controlReason: Gain scheduling is much easier to implement and less trouble prone.Chapter 16