1. Introduction to Process Control
prototype system-stirred tank heater
feedback control
implementation of control
justification of control

3. Control Terminology
Controlled variables - these are the variables which quantify the performance or quality of the final product, which are also called output variables.
Manipulated variables - these input variables are adjusted dynamically to keep the controlled variables at their set-points.
Disturbance variables - these are also called "load" variables and represent input variables that can cause the controlled variables to deviate from their respective set points.

4. Control Terminology(2)
Set-point change - implementing a change in the operating conditions. The set-point signal is changed and the manipulated variable is adjusted appropriately to achieve the new operating conditions. Also called servomechanism (or "servo") control.
Disturbance change - the process transient behavior when a disturbance enters, also called regulatory control or load change. A control system should be able to return each controlled variable back to its set-point.

7. Feedback Control:
Distinguishing feature: measure the controlled variable
Advantages:
Corrective action is taken regardless of the source of the disturbance.
Reduces sensitivity of the controlled variable to disturbances and changes in the process (shown later).
Disadvantages:
No corrective action occurs until after the disturbance has upset the process, that is, until after x differs from xsp.
Very oscillatory responses, or even instability…

8. Manual Feedback Control
Process
Controlled Quantity
Manipulated Flow

9. Feedforward Control:
Distinguishing feature: measure a disturbance variable
Advantage:
Correct for disturbance before it upsets the process.
Disadvantage:
Must be able to measure the disturbance.
No corrective action for unmeasured disturbances.

10. Feedforward Control Is this manual or automatic control?
Process
Controlled Variable
Manipulated Flow
Disturbance
Is this manual or automatic control?
What would automatic control look like?

11. Group discussion : Example 1

12. Group discussion : Example 2

14. Elements in a Process Control Loop
Can be identified from a P&ID (piping & instrumentation diagram)
Sensors
Controllers
Final control elements
Signal lines
Example
- Heated Tank
Thermocouple
Fi Ti
Q (Heater) F T TT TC
Steam in

18. Exercise
Give the control objectives of the control loops
Classify the control configuration of the control loops
Classify the variables of all control loops

21. Why is Process Control Necessary?
Safety
- Protection for well-being of workers and plant
Smooth Operation
- Product specifications can be met despite disturbances
Meet regulations
- Environmental regulations and equipment constraints
Economics
- Profitability can be improved by:
(a) Increased production levels
(b) Reduced raw material costs
(c) Enhanced product quality

22. Limit Operating Variable A2 Average, A1 Time (b) (a) F r e q u n c y
Average of
normal
operation
improved
control
Product
quality
constraint
Product quality distribution curves showing justification for improved control

23. Conceptual View of Control Hierarchy
Computer DDC, SPC, DCS, PLC
Plant Optimization
Unit Optimization
Equipment Optimization
Equipment Control
Single Variable Control
Level 3 - Optimization
Level 2 - Monitoring - Feedback - Cascades - Feedforward - Multivariable - Constraint - Optimization
Instrumentation
Level 1 - Feedback - Cascade

24. Figure 1.6 Major steps in control system development

25. Illustrative Example: Blending system
Notation:
w1, w2 and w are mass flow rates
x1, x2 and x are mass fractions of component A

26. Assumptions:
w1 is constant
x2 = constant = 1 (stream 2 is pure A)
Perfect mixing in the tank
Control Objective:
Keep x at a desired value (or “set point”) xsp, despite variations in x1(t). Flow rate w2 can be adjusted for this purpose.
Terminology:
Controlled variable (or “output variable”): x
Manipulated variable (or “input variable”): w2
Disturbance variable (or “load variable”): x1

27. Design Question. What value of is required to have
Overall balance:
Component A balance:
(The overbars denote nominal steady-state design values.)
At the design conditions, Substitute Eq. 1-2, and , then solve Eq. 1-2 for :

28. Table. 1.1 Control Strategies for the Blending System
1.2 Classification of Control Strategies
Table. 1.1 Control Strategies for the Blending System
Method
Measured Variable
Manipulated Variable
Category 1 x w2
FBa 2 x1 FF 3
x1 and x
FF/FB 4 -
Design change