Transcript Document 7232165

THE INTRODUCTION
OF AUTOMATIC
PROCESS CONTROL
Xining Ye
GOAL:
Maintaining process variables
(temperatures, pressures, flows,
compositions, levels) at a desired operating
value.
Processes are dynamic in nature, and
changes are always occurring. The
important variables those related to safety,
product quality, and production rates will
not achieve design conditions.
POINTS:
1.1 PROCESS CONTROL SYSTEM
1.2 IMPORTANT TERMS
1.3 TYPES OF CONTROL SYSTEMS
1.4 CONTROL STRATEGIES
1.5 SUMMARY
1.1 PROCESS CONTROL SYSTEM
• Manual process control
• Automatic process control
1.1 PROCESS CONTROL SYSTEM
Steam
Process
fluid
Ti(t)
T(t)
T
Condensate
return
Figure 1-1 Heat exchanger
The purpose of this unit: To heat the process
fluid from some inlet temperature, Ti(t), up to a
desired outlet temperature, T(t).
Steam
Process
fluid
Ti(t)
T(t)
T
Condensate
return
Figure 1-1 Heat exchanger
In this process many variables can change,
causing the outlet temperature to deviate from
its desired value. If this happens, some action
must be taken to correct for this deviation.
• Manual process control
(1) Measure the temperature T(t);
(2) Compare it to its desired value
(3) Based on this comparison, decide what to
do to correct for any deviation. The steam valve
can be manipulated to correct for the deviation.
Steam
Process
fluid
Ti(t)
T(t)
T
Condensate
return
How it works?
Steam
Process
fluid
Ti(t)
T(t)
T
Condensate
return
If the outlet temperature T(t) is above its desired
value, the steam valve can be throttled back to
cut the steam flow(energy) to the heat exchanger;
If the outlet temperature T(t) is below its desired
value, the steam valve could be opened more to
increase the steam flow to the heat exchanger.
Disadvantages of manual process control
(1) The operator should look at the temperature
frequently to take corrective action whenever it
deviates its desired value.
(2) Different operators would make different
decisions as to how to move the steam valve,
resulting in inconsistent operation.
(3) This corrective procedure would require a
large number of operators.
So, We would like to accomplish this control
automatically. Without requiring intervention from
the operator.
• Automatic process control:
SP
(1) Measure the outlet
temperature of the
process stream by a
sensor(thermocouple,
resistance temperature
device, thermisters, etc.)
(2) Transmitter transmits
the signal to the
controller
Controller
TC
Final control
element
Steam
Transmitter
Process
fluid
TT
T(t)
Ti(t)
(3) Controller compare
Sensor
the signal to the desired
T
Condensate
value, and decides what
return
to do to maintain the
temperature at its desired
Fig. 1-2 Heat exchanger control loop
value.
(4) The controller sends a signal to the final control element to
manipulate the steam flow.
Three components of all control systems:
SP
(1)Sensor/transmitter(
检测/变送). The
primary and
secondary elements.
(2) Controller(控制器).
The brain of the
control system.
Controller
TC
Final control
element
Steam
Transmitter
Process
fluid
TT
(3) Final control
T(t)
Ti(t)
Sensor
element(执行器). A
T
Condensate
control valve, but not
return
always. (variablespeed pumps,
Fig. 1-2 Heat exchanger control loop
conveyors and
electric motors)
Three basic operations:
SP
(1) Measurement(M).
Measuring the variable to
be controlled
(2) Decision(D). Based on
the measurement, the
controller decides what to Process
fluid
do to maintain the
Ti(t)
variable at its desired
value.
Controller
TC
Final control
element
Steam
Transmitter
TT
T(t)
Sensor
T
Condensate
return
Fig. 1-2 Heat exchanger control loop
(3) Action(A). As a controller’s decision, the system
must take an action. This is usually accomplished by
the final control element.
1.2 IMPORTANT TERMS
• Controlled variable(被控变量)(process variable,
measurement). The variable that must be controlled at
some desired value.
SP
•Controlled object or
Process(被控对象或过
程). The object that
need to be controlled.
Controller
TC
Final control
element
Steam
Transmitter
TT
Process
fluid
• Set point(设定值). The
desired value of the
controlled variable.
T(t)
Ti(t)
Sensor
T
Condensate
return
Fig. 1-2 Heat exchanger control loop
1.2 IMPORTANT TERMS
• Manipulated variable(操纵变量). The variable used to
maintain the controlled variable at its desired value.
• Disturbance (干扰)(upset).
Any variable that causes the
SP
controlled variable to
TC
deviate away from the set
Controller
Final control
point.
element
Steam
• In the heat exchanger,
possible disturbances.
Transmitter
Inlet process temperature
TT
Process
fluid
Ti(t), the process flow f(t),
T(t)
T (t)
The energy content of the
Sensor
T
steam, ambient condition,
Condensate
process fluid composition
return
and fouling.
Fig. 1-2 Heat exchanger control loop
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1.2 IMPORTANT TERMS
• NOTE. Disturbances are always occurring in processes,
transient conditions are very common. It is because of
these disturbances that automatic process control is
needed. If there were no disturbances, design operating
conditions would prevail, and there would be no
necessity of continuously “monitoring” the process.
With these preceding terms defined, we can say:
The objective of an automatic process control system is
to adjust the manipulated variable to maintain the
controlled variable at its set point in spite of disturbances.
• Why control is important?
(1) Safety: Prevent injury to plant personnel, protect the
environment by preventing emission and minimizing
waste and prevent damage to the process equipment.
(2) Maintain product quality (composition, purity, color,
etc.) on a continuous basis and with minimum cost.
(3) Maintain plant production rate at minimum cost.
So, we can say that the reasons for automation of
process plants are to provide safety and at same time
maintain desired product quality, high plant throughput,
and reduce demand on human labor.
1.3 TYPES OF CONTROL SYSTEM
Two types of control system:
(1) Regulatory control(定值控制): In some processes the
controlled variable deviated from the set point because
of disturbances, regulatory control refers to systems
designed to compensate for these disturbances.
(2) Servo control(随动控制): In some processes, the most
important disturbance is the set point itself. That is, the
set point may be changed as a function of time. Servo
control refers to control systems designed for this
purpose.
In the process industries, regulatory control is far more
common that servo control.
1.4 CONTROL STRATEGIES
Points：
(1) Feedback control (closed-loop control)
反馈控制 (闭环控制)
(2) Feedforward control (open-loop control)
前馈控制 (开环控制)
(3) Choose a proper control system
(1) Feedback control (closed-loop control)
Points:
• How it works?
• The block diagrams of feedback control
• The characteristics of feedback control
1.4 CONTROL STRATEGIES
(1) Feedback control (closed-loop control)
•How it works?
SP
If the inlet process
TC
Controller
temperature decreases,
Final control
thus creating a disturbance,
element
Steam
its effect must propagate
through the heat exchanger
Transmitter
TT
before the outlet
Process
fluid
temperature decreases.
T(t)
Sensor
Once the outlet temperature T (t)
T
changes, the signal from
Condensate
return
the transmitter to the
controller also changes.
Fig. 1-2 Heat exchanger control loop
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1.4 CONTROL STRATEGIES
(1) Feedback control (closed-loop control)
•How it works?
SP
It is then that the controller
TC
Controller
becomes aware that a
Final control
deviation from set point has
element
Steam
occurred and it must
compensate for the
Transmitter
TT
disturbance by
Process
fluid
manipulating the steam
T(t)
T (t)
Sensor
valve. The controller then
T
signals the valve to
Condensate
return
increase its opening and
thus increase the steam
Fig. 1-2 Heat exchanger control loop
flow.
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1.4 CONTROL STRATEGIES
OUTLET
TEMPERATURE
This oscillatory response is typical
of feedback control and shows
that it is essentially a trial and
error operation. That is, when the
controller notices that the outlet
temperature has decreased below
the SET POINT, it signals the valve
to open. But the opening is more
TIME
TIME
CONTROLLER
OUTPUT
At first the outlet temperature
decreases because of the
decrease in inlet temperature, but
then it increases, even above the
set point and continuous to
oscillate until it finally stabilizes.
INLET
TEMPERATURE
The response of feedback control (closed-loop control)
TIME
Fig.1-3 Response
of feedback control
OUTLET
TEMPERATURE
TIME
TIME
CONTROLLER
OUTPUT
Than required. Therefore, the
outlet temperature increases
above the SET POINT. Noticing
this, the controller signals the
valve to close again somewhat to
bring the temperature back down.
This trial and error continued until
the temperature reached and
stayed at SET POINT.
INLET
TEMPERATURE
1.4 CONTROL STRATEGIES
TIME
Fig.1-3 Response
of feedback control
• The block diagrams of feedback control system
Disturbance 1
SP
Controller
Summing
junction
Final control
element
-
Process
or plant
Disturbance 2
Output
Or
Controlled
variable
Sensor/
Transmitter
Fig 1-4 Block diagrams of closed-loop control systems
1.4 CONTROL STRATEGIES
• The characteristics of feedback control
 The advantage of feedback control
Compensate for all disturbances
The result of any disturbance entering the process is to
make the controlled variable deviate from the SET POINT.
Once the controlled variable deviates from the set point,
the controller changes its output to return the controlled
variable to SET POINT(its desired value).
The feedback control loop does not know, nor does it
care, which disturbance enters the process. It only tries
to maintain the controlled variable at set point, and in
this way compensates for all disturbances. The feedback
controller works with minimum knowledge of the
process. Actually, the only information it needs is in
which direction to move, and how much to move is
usually adjusted by trial and error.
1.4 CONTROL STRATEGIES
• The characteristics of feedback control
 The disadvantage of feedback control
SP
Can compensate for a
disturbance only AFTER
the controlled variable
has deviated from the set
point because of the
disturbance.
Can not give the
controlled variable a
timely control (laggard滞
后的)
Controller
TC
Final control
element
Steam
Transmitter
TT
Process
fluid
T(t)
Ti(t)
T
Condensate
return
Sensor
(2) Feedforward control (open-loop control)
Points:
• How it works?
• The block diagram of feedforward control
• The characteristics of feedforward control
• How it works?
The feedforward control is a very common control
strategy in the process industries. It is the simplicity
that accounts for its popularity.
The objective of feedforward control is to measure
the disturbances and compensate for them before
the controlled variable deviates from the set point.
If applied correctly, the controlled variable
deviation would be minimum.
(2) Feedforward control (open-loop control)
Suppose that “major”
disturbance is the inlet
temperature Ti(t). To
implement feedforward
control, this disturbance
must first be measured
and then a decision made
as to manipulate the
steam valve to
compensate for them.
SP
Feedforward
controller
Steam
TT
T(t)
Ti(t)
T
Condensation
retun
Fig 1-5 Feedforward control
(2) Feedforward control (open-loop control)
Fig 1-5 shows this control
strategy.
 measure the inlet
temperature
 Feedforward controller
makes the decision about
how to manipulate the
steam valve to maintain
the controlled variable at
set point.
SP
Feedforward
controller
Steam
TT
T(t)
Ti(t)
T
Condensation
retun
Fig 1-5 Feedforward control
• The Block diagrams of Feedforward control
Sensor/
Tranmitter
SP
Controller
Final control
element
Process flow
Disturbances
Process
or plant
Output
Or
Controlled
variable
Fig 1-6 Block diagrams of feedforward control
• The characteristics of feedforward control
 The disadvantage of feedforward control
Feedforward control cannot compensate
for all disturbances that enter the process
• The characteristics of feedforward control
 The disadvantage of feedforward control
SP
In this example, The
feedforward control
system can compensate
only one of disturbances.
If any of the other
disturbances enter the
process, this strategy
will not compensate for it,
and the result will be a
permanent deviation
from set point of the
controlled variable.
Feedforward
controller
Steam
TT
f(t)
T(t)
Ti(t)
T
Condensation
retun
Fig 1-5 Feedforward control
• The characteristics of feedforward control
 The advantage of feedforward control
SP
It has the characteristic
of forward control
So, if we use this strategy
correctly, the controlled
variable will not deviate
set point.
Feedforward
controller
Steam
TT
T(t)
Ti(t)
T
Condensation
retun
Fig 1-5 Feedforward control
Some examples:
Washing machine
Oven
Feedforward
control system
Microwave oven
Air conditioner
Feedback
control system
(3) Choose a proper control system
Review:
• Feedback control system
• Feedforward control system
?:
How to choose a proper control system?
• Proper Control System
• Can get the output that a process need
• Low cost
• Contrast
Feedforward
Control System
Feedback Control
System
Can not compensate Can compensate all
all disturbances
disturbances
Simple structure
Low cost
Complex structure
High cost
• Choice
An open-loop
system
• Simplicity and low cost
Trade-off:
• Complexity and higher cost
Premise: Ensure the
requirement of
industrial production
A closed-loop
system
• Feedforward control with feedback control
In this example,
Feedforward control
now compensate for
the “major”
disturbance;
feedback control
compensate for all
other disturbances.
TC
SP
Feedforward
controller
+
+
Steam
TT
f(t)
TT
T(t)
Ti(t)
T
Condensation
retun
Fig 1-6 Feedforward control with feedback control
• feedforward control with feedback control
Notice: the three basic
operations, M,D,A are
still present in this
more “advanced”
control strategy.
TC
SP
Feedforward
controller
+
+
Steam
The sensors and
transmitters perform
the measurement.
TT
f(t)
TT
T(t)
Ti(t)
T
Condensation
Both feedforward and
retun
feedback controller
make the decision. Fig 1-6 Feedforward control with feedback control
The steam valve takes
action.
1.5 SUMMARY
• The need for automatic process control
• The principles of a control system, we can
use three letters to describe, M, D and A
• Present the basic components of a process
control system: sensor/transmitter, controller,
and final control element
1.5 SUMMARY
• Present two types of control strategies:
Feedforward control or feedback control, we
also discussed their advantages and
disadvantages,
• Give the principles of choosing the proper
control system