Session 09 - Agenda Activity Estimated Time 1. Agenda – Class Roster 10 Chapter 21- Learning Objectives 2. Historical Background 3. DCS Operation 20 4. MUX/DEMUX 5. DCS Advantages 6. Sample Questions Chapter 17- Learning Objectives 7. Control Schemes - Control Modes 8. Control Schemes for Various Processes 9. Sample Questions 5 BREAK 15 9. Grades – Progress Reports ------- Instrumentation 1 – Session 09 Instrumentation I - Session 09

Session 09 - Objectives Explain the purpose of a DCS. In general terms, describe the operation of a DCS. Explain the function of a multiplexer/demultiplexer. Explain advantages of a DCS over an analog control system: Instrumentation 1 – Session 09

History of Digital Control Instrumentation 1 – Session 09

DCSs? Distributed Computer Systems, DCSs, are computer – based systems that are used to control processes In 1970s the use of computers in process control became more prevalent than before It was called “direct” because a single computer was directly connected to a particular part of process. Disadvantages? The entire unit was shut down if a computer failed Instrumentation 1 – Session 09

Early Computer Control Instrumentation 1 – Session 09

Distributed Control Systems Instrumentation 1 – Session 09

DCS Operation DCS is a series of distributed computer processors or nodes that are all interconnected via a computer network They are able to manage multiple process loops at one time If one node (a single computer in a data highway or computer network) goes down or fails signal can be rerouted to one of the other nodes and the process can continue Instrumentation 1 – Session 09

Individual Node Instrumentation 1 – Session 09

DCS Operation A DCS divides production control in a plant into several subparts or nodes Each node acts independently A node may include a considerable control loop A local computer in each node communicates with the host computer which is a central computer Instrumentation 1 – Session 09

DCS Operation The function of a typical DCS node can be explained as a series of steps as follows: The DCS node processes signals received from the control loop such as temperature, level, flow, pressure, and on/off signals The signals processed by the DCS are then converted from analog to digital through an A/D convertor The digital signals produced by the A/D converter are then forwarded to a local computer by a multiplexer (MUX) Instrumentation 1 – Session 09

MUX/DEMUX Functioning Instrumentation 1 – Session 09

DCS Operation The local computer then processes the incoming information and sends out control commands based on internally programmed logic and instructions The control commands generated by the local computer are forwarded by the demultiplexer (DEMUX) to a digital – to analog D/A convertor The analog signals produced by the D/A converter are then forwarded to the appropriate valve or control element and the host computer Instrumentation 1 – Session 09

Analog and Digital Instruments Instrumentation 1 – Session 09

Analog and Digital Signals Instrumentation 1 – Session 09

DCS Advantages Precision and accuracy Speed Cost Data management Space efficiency Safety and Reliability Instrumentation 1 – Session 09

Match From the List Greater precision in process management Less interaction needed between instrumentation techs and operator Greater ability to handle data management Less expense for installation and maintenance A DCS is preferred over earlier analog based systems for all the following reasons except: Instrumentation 1 – Session 09

Match From the List Greater precision in process management Less interaction needed between instrumentation techs and operator Greater ability to handle data management Less expense for installation and maintenance A DCS is preferred over earlier analog based systems for all the following reasons except: Instrumentation 1 – Session 09

Match From the List demultiplexer plc smart transmitter multiplexer   Multiple signals from field instruments would be forwarded to a controller by a Instrumentation 1 – Session 09

Match From the List demultiplexer plc smart transmitter multiplexer   Multiple signals from field instruments would be forwarded to a controller by a Instrumentation 1 – Session 09

Chapter 17 - Objectives Identify and describe types of control schemes On/off Lead/lag Feedback Feedforward Describe a control scheme that allows the following modes Local manual control Remote manual control Local automatic control Remote automatic control Cascading control Match appropriate control schemes to a process Given a process scenario, design an appropriate process control scheme Instrumentation 1 – Session 09

Control Schemes Two categories of control types: 1. Control schemes (on/off, lead/log, feedback, feedforward) 2. Control modes local manual local automatic control remote manual control remote automatic control cascading of the remote automatic control Instrumentation 1 – Session 09

Control Scheme What is the impact of variability in the control part of this process? Should be the primary question asked when selecting a control scheme, sensors, and final control elements for each area of the process. Instrumentation 1 – Session 09

On/off control Used when a controlled variable cycling above and below the setpoint is considered acceptable Common type of controls used in homes Ovens, central heat and air, and water heaters When a large control zone is not a problem in industry an on/off control is used High level point/low level point Instrumentation 1 – Session 09

Lead/Lag control How a process reacts to a disturbance e.g., a temperature change leads to a pressure change Or, a measurement change lags a controller output change In a boiler system, lead/lag control specifically means to utilize both high and low selector relays to force the fuel to follow the airflow on a rising steam demand, and forcing the air flow to follow the fuel on a falling steam demand Instrumentation 1 – Session 09

Feedback control Is a closed loop control strategy when error drives the controller output to change the position of the final control element to eliminate or minimize the error Instrumentation 1 – Session 09

Feedforward control Is a control strategy where the controller output is based on knowledge of the relationship between the output of the controller and the input received from the point in the process where the disturbance is measured. Feedforward control is designed to predict and prevent an error from occurring in the first place by changing the position of the final control element before the measured variable senses the change Instrumentation 1 – Session 09

Feedback and Feedforward controls The Three types of Control System (a) Open Loop (b) Feed-forward (c) Feedback (Closed Loop) Based on Hopgood (2002) Instrumentation 1 – Session 09

Local Controller Instrumentation 1 – Session 09

Remote Controller Instrumentation 1 – Session 09

Local and Remote Settings Instrumentation 1 – Session 09

Cascading Controller Instrumentation 1 – Session 09

Match Appropriate control schemes to a process 1. A reactor’s catalyst feed is manipulated due to a predicted load change. A. On-Off Control B. Feedback Control C. Feedforward Control D. Lead-Lag Control Instrumentation 1 – Session 09

Match Appropriate control schemes to a process 1. A reactor’s catalyst feed is manipulated due to a predicted load change. A. On-Off Control B. Feedback Control C. Feedforward Control D. Lead-Lag Control Instrumentation 1 – Session 09

Match Appropriate control schemes to a process 2.The liquid in this tank is emptied once it reaches a specific level. A. On-Off Control B. Feedback Control C. Feedforward Control D. Lead-Lag Control Instrumentation 1 – Session 09

Match Appropriate control schemes to a process 2.The liquid in this tank is emptied once it reaches a specific level. A. On-Off Control B. Feedback Control C. Feedforward Control D. Lead-Lag Control Instrumentation 1 – Session 09

Match Appropriate control schemes to a process 3. A parallel piece of equipment begins to operate due to an increase in process load. A. On-Off Control B. Feedback Control C. Feedforward Control D. Lead-Lag Control Instrumentation 1 – Session 09

Match Appropriate control schemes to a process 3. A parallel piece of equipment begins to operate due to an increase in process load. A. On-Off Control B. Feedback Control C. Feedforward Control D. Lead-Lag Control Instrumentation 1 – Session 09

Match Appropriate control schemes to a process 4. A tank level is manipulated because an error signal has developed. A. On-Off Control B. Feedback Control C. Feedforward Control D. Lead-Lag Control Instrumentation 1 – Session 09

Match Appropriate control schemes to a process 4. A tank level is manipulated because an error signal has developed. A. On-Off Control B. Feedback Control C. Feedforward Control D. Lead-Lag Control Instrumentation 1 – Session 09

Identify the Control Scheme Feedback Control Loop Feedforward Control Loop Instrumentation 1 – Session 09

Identify the Control Scheme Feedback Control Loop Feedforward Control Loop X Instrumentation 1 – Session 09

Identify the Control Scheme Feedback Control Loop Feedforward Control Loop Instrumentation 1 – Session 09

Identify the Control Scheme Feedback Control Loop Feedforward Control Loop X Instrumentation 1 – Session 09

Homework Assignments - HWAs Design 10 True/False questions From chapter “13” using the following format: Times New Roman, 12 pt. font size, according to the form distributed by Instructor. Handwriting will not be accepted Instrumentation 1 – Session 09

Homework Assignments - HWAs The answer key for this question and the address in the Book.(the edition, Page and line) Due date: Please email me these questions before coming Friday (October 6th ) at 8 AM. The following week you will ask your questions from the students and explain the answer to us. Your grade for this HWA will be for your extracting and presenting these questions and answers to the class. This HWA will have 5% of your total grade. Please send your questions before the due date otherwise you will lose the whole point. Instrumentation 1 – Session 09

Homework Assignments - HWAs Example: A curve upper surface of a column of a liquid material is called convex 2. A distance from a zero reference point to the surface is called Height False True Instrumentation 1 – Session 09