1. Session 09 - Agenda Activity Estimated Time 1. Agenda – Class Roster10
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

2. 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

3. History of Digital Control
Instrumentation 1 – Session 09

4. 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

5. Early Computer Control
Instrumentation 1 – Session 09

6. Distributed Control Systems
Instrumentation 1 – Session 09

7. 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

8. Individual Node
Instrumentation 1 – Session 09

9. 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

10. 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

11. MUX/DEMUX Functioning
Instrumentation 1 – Session 09

12. 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

13. Analog and Digital Instruments
Instrumentation 1 – Session 09

14. Analog and Digital Signals
Instrumentation 1 – Session 09

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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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

28. Local Controller
Instrumentation 1 – Session 09

29. Remote Controller
Instrumentation 1 – Session 09

30. Local and Remote Settings
Instrumentation 1 – Session 09

31. Cascading Controller
Instrumentation 1 – Session 09

32. 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

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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

39. 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

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

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

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

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

44. 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

45. Homework Assignments - HWAs
The answer key for this question and the address in the
Book.(the edition, Page and line)
Due date:
Please 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

46. 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