Lecture 1.2 _ Design Aspects of a Control System.pptx

Process Dynamics and Control

1. Introduction to Process Dynamics & Control Lecture 1.2: Design Aspects of Process Control System

2. INTENDED LEARNING OUTCOMES By the end of this lecture, you should be able to: Recall the types of variables in a control system (CS) Discuss the steps in designing a control system Differentiate the two types of measurements in a CS Distinguish the types of control configuration problems Discuss the types of control configurations that can be applied in a CS Discuss the advantages and disadvantages of a feedforward and feedback control configurations

3. Controlled Variable (CV) Recall: Types of Variables in a Control System (CS)

4. Designing a Control System

5. Designing a Control System Question: What are the operational objectives that a control system is called upon to achieve? Examples: Ensuring the stability of the process Suppressing the influence of external disturbances Optimizing the economic performance of a plant A combination of the mentioned statements above

6. Designing a Control System Heating Tank System Objective: Safety: Maintain the height of the tank, h to a desired value, Process Specification: Maintain the temperature of the tank, T to a desired value,  

7. Designing a Control System Primary Measurements: Measurements that can be directly measured/monitored Represents the control objectives Example: Temperature using thermocouple (Objective: control outlet T) Flowrate using orifice meter (Objective: control the outlet flowrate)

8. Designing a Control System Secondary Measurements: Measurements that cannot be directly measured/monitored Happens when the control objective are not measurable (unmeasured outputs). Alternative: Measure other variables which can be easily obtain and use it as support (Supporting measurements = secondary measurements) Develop mathematical relationships between the unmeasured outputs and the secondary measurements from empirical, experimental, or theoretical considerations:  

9. Designing a Control System Question: What parameters or output variables can be manipulated to achieve the control objective? Liquid Tank System Objective: Maintain the height of the tank to a desired value, Measured output: Controlled variable (CV) = h  

10. Designing a Control System Question: What parameters or output variables can be manipulated to achieve the control objective? Liquid Tank System Controlled Variable (CV) Manipulated Variable (MV) h & Controlled Variable (CV) Manipulated Variable (MV) h Objective: Maintain the height of the tank to a desired value, Measured output: Controlled variable (CV) = h  

11. Designing a Control System Control configuration is an information structure that connects the available measurements with the available manipulated variables.

12. Designing a Control System Depending upon the control outputs and manipulated inputs, control configurations can be distinguished as: SISO Problems : Single Input, Single Output MIMO Problems : Multiple Input, Multiple Output Heating Tank System SISO : If the control objective (controlled output = CV) is to keep the liquid level (h) at a desired value ( ) by manipulating the effluent flowrate. MIMO : If the control objectives are (more than one) to keep the level and the temperature of the liquid at desired values, by manipulating (more than one) the steam flowrate and the effluent flowrate  

13. Designing a Control System

14. Feedback Control Configuration Diagram: Input: Manipulated Variables (MC) & Disturbance variables Output: Measured and unmeasured outputs Algorithm: The signals from measured outputs will go into the controller action (where targets are already set) The controller will compare the desired/target outputs with the measured outputs and calculates an error value Depending on the calculated error, action will be made to manipulate the input variables (e.g., close/open valves)

15. Feedback Control Configuration Aim: Target the manipulated variables (MV) by controlling the measured outputs . Controlling action only occurs after the disturbance enters the system. Acts in compensatory manner Disturbance is felt then action is taken.

16. Feedback Control Configuration Consider: Simple Distillation Colum Feed enters the column with flowrate F and composition z Overhead vapor that leaves the top tray is condensed in the condenser Condensed liquid is accumulated in the reflux drum (flux accumulator) A part of the condensed liquid is recycled back to the top section of the column, with a flowrate R, and a part is withdrawn as distillate with a flowrate D and a composition The bottom liquid is withdrawn and is subjected into a reboiler, the produced vapor is recycled back into the bottom tray, with a flowrate V , and some amount of the liquid is taken out, with a flowrate B and composition  

17. Feedback Control Configuration Objective: Maintain the top product composition at a desired value   Controlled Variable (CV) Manipulated Variable (MV) Controlled Variable (CV) Manipulated Variable (MV)

18. Feedback Control Configuration Objective: Maintain the top product composition at a desired value   Controlled Variable (CV) Manipulated Variable (MV) Controlled Variable (CV) Manipulated Variable (MV) Control System: Attach a composition analyzer to measure The measured is compared with the desired value and an error is computed The error signal goes to the controller Controlling action is implemented to the reflux valve wherein flowrate R is manipulated.  

19. Feedforward Control Configuration Aim: Target the manipulated variables (MV) by controlling the disturbances (load variable = LV) . Controlling action has already been implemented before the disturbance enters the system. Acts in an anticipatory manner Controlling action/prevention/precaution is taken beforehand

20. Feedforward Control Configuration Control System: A composition analyzer is attached into the feeding pipe which measures the feed composition z The data z goes to the controller which calculates the control action Control action is implemented to the reflux valve wherein flowrate R is manipulated. Controlled Variable (CV) Manipulated Variable (MV) Controlled Variable (CV) Manipulated Variable (MV)

21. Inferential Control Configuration The measured outputs are fed into an estimator (computer algorithm with the help of material & energy balance of the system along with the thermodynamic equations) which calculates unmeasured controlled variables. The unmeasured controlled variables are fed into the controller, compared with the set points, then manipulate the input variables.

22. Inferential Control Configuration It gives a very accurate control, but the drawback is a that we need to apply M&E balance along with Thermodynamic Eq. on every stream to control the process. It becomes a complex and tedious process

23. Inferential Control Configuration Objective: Maintain the top product composition at a desired value   There are instances wherein, is not directly measured due to: most composition analyzers provide large delays of response High investment and maintenance cost We opt for secondary measurements (e.g., temperature) to infer the product composition  

24. Inferential Control Configuration Objective: Maintain the top product composition at a desired value   Control System: Use temperature as secondary variables Measure the temperature using thermocouples The temperature information goes to the estimator (algorithm consisting of equations & correlations) which calculates The calculated is compared with the desired value and an error is computed The error signal goes to the controller Controlling action is implemented to the reflux valve wherein flowrate R is manipulated.   Controlled Variable (CV) Manipulated Variable (MV) Controlled Variable (CV) Manipulated Variable (MV)

25. Reminder: