Transcript Introduction to Process Control

Chapter One
Introduction to Process Control
controller
control
element
process
sensor
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.
Disturbances - These are also called the “load” variables
and represent inputs that can cause the controlled variables
to deviate from their respective set points.
Control Terminology
Servo control - The set-point signal is changed and the
manipulated variable is adjusted appropriately to achieve
the new operating conditions.
Regulatory control – The set-point is fixed at a constant
value. When any disturbance enters the system, the
manipulated variable is adjusted to drive the controlled
variable back to its fixed set-point.
Chapter 1
Why do we need automatic
process control?
• Industrial processes are not static but rather
very dynamic; they are continuously
changing as a result of many types of
disturbances.
• It is principally because of this dynamic
nature that control systems are needed to
continuously and automatically watch over
the variables that must be controlled.
Illustrative Example: Blending System
Notation:
• w1, w2 and w are mass flow rates
• x1, x2 and x are mass fractions of component A
Assumptions:
1. w1 is constant
2. x2 = constant = 1 (stream 2 is pure A)
3. Perfect mixing in the tank
Control Objective:
Keep x at a desired value (or “set point”) xsp, by adjusting flow
rate w2, despite variations in x1..
Terminology:
• Controlled variable (or “output variable”): x
• Manipulated variable (or “input variable”): w2
• Disturbance variable (or “load variable”): x1
Design Question: What value of w2 is required to have
x  xSP ?
Overall balance:
0  w1  w2  w
(1)
Component balance:
w1 x1  w2 x2  wx  0
(2)
(The overbars denote nominal steady-state design values.)
• At the design conditions, x  xSP. Substitute Eq. (1), x  xSP and
x2  1 , then solve Eq. (2) for w2:
xSP  x1
w2  w1
1  xSP
(3)
Control Question
Suppose that the inlet concentration x1 changes with
time. How can we ensure that x remains at or near
the set point xSP ?
As a specific example, if x1  x1 and w2  w2 , then
x  xSP .
Possible Control Strategies
Method 1. Measure x and adjust w2.
Intuitively, if x is too high, we should reduce w2
and vice versa.
1. Manual control
2. Automatic control with proportional feedback
control law.
w2  t   w2  Kc  xSP  x  t 
(4)
Method 2. Measure x1 and adjust w2.
Thus, if x1 is greater than x1, we would decrease w2 so
that
w2  w2 ;
One approach: Consider Eq. (3) and replace x1 and w2
with x1(t) and w2(t) to get a control law:
w2  t   w1
xSP  x1  t 
1  xSP
(5)
Because Eq. (3) applies only at steady state, it is not
clear how effective the control law in (5) will be for
transient conditions.
Method 3. Measure x1 and x, adjust w2.
• This approach is a combination of Methods 1 and 2.
Method 4. Use a larger tank.
• If a larger tank is used, fluctuations in x1 will tend to be damped
out due to the larger capacitance of the tank contents.
• However, a larger tank means an increased capital cost.
Classification of Control Strategies
•
Method
Measured
Variable
Manipulated
Variable
Category
1
x
w2
FBa
2
x1
w2
FF
3
x1 and x
w2
FF/FB
4
-
-
Design change
Feedback Control
• Distinguishing feature:
– measure the controlled variable.
– It is important to make a distinction between negative feedback
and positive feedback.
• 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
Feed Forward Control
• Distinguishing Feature: Measure a
disturbance.
• Advantage: Correct for disturbance before
it upsets the process.
• Disadvantage:
– Must be able to measure the disturbance
– No corrective action for unmeasured
disturbances.
Justification of Process Control
•
•
•
•
•
•
•
•
•
Increased product throughput
Increased yield of higher valued products
Decreased energy consumption
Decreased pollution
Decreased off-spec product
Increased Safety
Extended life of equipment
Improved Operability
Decreased production labor
Economic Incentives - Advanced Control
(days-months )
5. Planning and
Scheduling
(hours-days )
4. Real-Time
O ptimization
(minutes-hours )
(seconds-minutes )
3b. Multivar iable
and Constraint
Control
3a. Regulatory
Control
(< 1 second )
2. Safety, E nvironment
and Equipment
Protection
(< 1 second )
1. Measurement
and Actuation
Process
Figure 1.7 Hierarchy of
process control activities.
Figure 1.9 Major
steps in control
system development