THE INTRODUCTION OF AUTOMATIC PROCESS CONTROL

1
THE INTRODUCTION OF AUTOMATIC PROCESS CONTROL

• Xining Ye

2
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.
3
POINTS
1.1 PROCESS CONTROL SYSTEM
1.2 IMPORTANT TERMS
1.3 TYPES OF CONTROL SYSTEMS
1.4 CONTROL STRATEGIES
1.5 SUMMARY
4
1.1 PROCESS CONTROL SYSTEM

• Manual process control

• Automatic process control

5
1.1 PROCESS CONTROL SYSTEM
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).
6
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.
7

• 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.
8

• How it works?

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.
9

• 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.
10

• Automatic process control

(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
(3) Controller compare the signal to the desired
value, and decides what to do to maintain the
temperature at its desired value.
Fig. 1-2 Heat exchanger control loop
(4) The controller sends a signal to the final
control element to manipulate the steam flow.
11

• Three components of all control systems

(1)Sensor/transmitter(??/??). The primary and
secondary elements.
(2) Controller(???). The brain of the control
system.
(3) Final control element(???). A control valve,
but not always. (variable-speed pumps, conveyors
and electric motors)
Fig. 1-2 Heat exchanger control loop
12

• Three basic operations

(1) Measurement(M). Measuring the variable to be
controlled
(2) Decision(D). Based on the measurement, the
controller decides what to do to maintain the
variable at its desired value.
Fig. 1-2 Heat exchanger control loop
(3) Action(A). As a controllers decision, the
system must take an action. This is usually
accomplished by the final control element.
13
1.2 IMPORTANT TERMS

• Controlled variable(????)(process variable,
  measurement). The variable that must be
  controlled at some desired value.

• Controlled object or Process(???????). The object
  that need to be controlled.

• Set point(???). The desired value of the
  controlled variable.

Fig. 1-2 Heat exchanger control loop
14
1.2 IMPORTANT TERMS

• Manipulated variable(????). The variable used to
  maintain the controlled variable at its desired
  value.

• Disturbance (??)(upset). Any variable that
  causes the controlled variable to deviate away
  from the set point.

• In the heat exchanger, possible disturbances.
  Inlet process temperature Ti(t), the process flow
  f(t), The energy content of the steam, ambient
  condition, process fluid composition and fouling.

Fig. 1-2 Heat exchanger control loop
15
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.
16

• 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.
17
1.3 TYPES OF CONTROL SYSTEM
Two types of control system
(1) Regulatory control(????) 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(????) 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.
18
1.4 CONTROL STRATEGIES
Points
(1) Feedback control (closed-loop control)
???? (????)
(2) Feedforward control (open-loop control)
???? (????)
(3) Choose a proper control system
19
(1) Feedback control (closed-loop control)
Points

• How it works?

• The block diagrams of feedback control

• The characteristics of feedback control

20
1.4 CONTROL STRATEGIES
(1) Feedback control (closed-loop control)

• How it works?

If the inlet process temperature decreases, thus
creating a disturbance, its effect must propagate
through the heat exchanger before the outlet
temperature decreases. Once the outlet
temperature changes, the signal from the
transmitter to the controller also changes.
Fig. 1-2 Heat exchanger control loop
21
1.4 CONTROL STRATEGIES
(1) Feedback control (closed-loop control)

• How it works?

It is then that the controller becomes aware that
a deviation from set point has occurred and it
must compensate for the disturbance by
manipulating the steam valve. The controller then
signals the valve to increase its opening and
thus increase the steam flow.
Fig. 1-2 Heat exchanger control loop
22
1.4 CONTROL STRATEGIES

• The response of feedback control (closed-loop
  control)

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
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
CONTROLLER OUTPUT
Fig.1-3 Response of feedback control
23
1.4 CONTROL STRATEGIES
INLET TEMPERATURE
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.
OUTLET TEMPERATURE
CONTROLLER OUTPUT
Fig.1-3 Response of feedback control
24

• The block diagrams of feedback control system

Fig 1-4 Block diagrams of closed-loop control
systems
25
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.
26
1.4 CONTROL STRATEGIES

• The characteristics of feedback control

• The disadvantage of feedback control

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???)
27
(2) Feedforward control (open-loop control)
Points

• How it works?

• The block diagram of feedforward control

• The characteristics of feedforward control

28

• 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.
29
(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.
Fig 1-5 Feedforward control
30
(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.

Fig 1-5 Feedforward control
31

• The Block diagrams of Feedforward control

Fig 1-6 Block diagrams of feedforward control
32

• The characteristics of feedforward control

• The disadvantage of feedforward control

Feedforward control cannot compensate for all
disturbances that enter the process
33

• The characteristics of feedforward control

• The disadvantage of feedforward control

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.
Fig 1-5 Feedforward control
34

• The characteristics of feedforward control

• The advantage of feedforward control

It has the characteristic of forward control
So, if we use this strategy correctly, the
controlled variable will not deviate set point.
Fig 1-5 Feedforward control
35
Some examples
Washing machine
Feedforward control system
Oven
Microwave oven
Air conditioner
36
(3) Choose a proper control system
Review

• Feedback control system

• Feedforward control system

?
How to choose a proper control system?
37

• Proper Control System

• Can get the output that a process need

• Low cost

38

• Contrast

39

• Choice

• Simplicity and low cost

Trade-off

• Complexity and higher cost

Premise Ensure the requirement of industrial
production
40

• Feedforward control with feedback control

In this example, Feedforward control now
compensate for the major disturbance feedback
control compensate for all other disturbances.
Fig 1-6 Feedforward control with feedback control
41

• feedforward control with feedback control

Notice the three basic operations, M,D,A are
still present in this more advanced control
strategy. The sensors and transmitters perform
the measurement. Both feedforward and feedback
controller make the decision. The steam valve
takes action.
Fig 1-6 Feedforward control with feedback control
42
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

43
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

44
THANK YOU!