Introduction for
Process Control Systems
过程控制系统概论
Shen Guo-jiang
Institute of Industrial Control,
Zhejiang University
Contents
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Origin of Process Control Systems
Important Terms and Objective of Process
Control Systems
Description of Process Control Systems
Types of Control Strategies
Course Tasks
Systems and Control Systems
Outputs
Z
U
Y
Inputs
Systems
D
Control Systems
examples ?
Application of control system
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The military: flight control
Mechanical engineering: centrifugal governor
(Watt)
Electricity and electronics
Process industry (including: petroleum, chemical,
metallurgy, pharmaceutical, environmental
protection, etc.)
Economy and management system: CPI levels of
control
Other…
Origin of Process Control Systems
Purpose: To maintain
the level at a desired
operating value, when
the inlet flow（输入流
量） changes.
Manual Operation:
(1) Measure the level by a water
gauge and the eye (水位表);
(2) Compare the level to its desired
value in the brain;
(3) Based on this comparison,
decide what to do to correct for
any deviation (偏差);
(4) Manipulate the outlet valve (输
出阀) to correct for the
deviation.
Limitations with Manual Control

Require the operator to look frequently at the
level to take corrective action whenever it
deviates from the desired value;

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Different operators would make different
decisions as to how to move the outlet valve,
resulting in inconsistent operation (操作的不一致
性);
Require a large number of operators to maintain
hundreds of variables in most process plants at a
desired value.
Solution ?
Automatic Control System
Solution: Design a control system to accomplish this
control automatically without requiring intervention
from the operator.
Qi
hsp
h
LT
21
Qo
LC
21
Three basic components of all
control systems:
• a sensor / transmitter: the
“eye” of the control system,
• a controller: the “brain” of
the control system,
• final control element: often
a control valve.
Feedback Control Process
1.
Measure the level by a sensor (传感器) and convert the
output from the sensor to an electric signal by a
transmitter (变送器) ;
2. The controller（控制器/调节器） then receives the signal
and compares it with the value desired;
3. Depending on this comparison,
Qi
the controller decides what to
do to correct for any
deviation;
LT
h
21
4. Based on this decision, it sends
a signal to the final control
element（执行单元）, e. g., a
Qo
control valve.
hsp
LC
21
Three basic operations
in every type of control system

Measurement (测量，M)
Measuring the variable to be controlled is usually done by the
combination of sensor and transmitter.

Decision (决策，D)
Based on the Measurement the controller decides what to do
to maintain the variable at its desired value.

Action (动作/执行，A)
As a result of the controller’s decision, the system must then
take an action. This is usually accomplished by the final control
element. The action taken must come back and affect the
measurement.
Important Terms
of Automatic Process Control

Controlled Variable (CV, 被控变量/受控变量)

Setpoint (SP,设定值/给定值)

Manipulated Variable (MV,操纵变量/操作变
量)
The variable that must be maintained or controlled
at some desired value.
The desired value of the controlled variable.
The variable used to maintain the controlled
variable at its setpoint.

Disturbance (DV,扰动/扰动变量)
Any variable that causes the controlled variable to
deviate away from its setpoint.
Objective of Process Control Systems

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 automatic control is important:
1) Safety. Prevent injury to plant personnel and
damage to the process equipment, and protect the
environment by minimizing wastes.
2) Product Quality.
3) Minimum Cost.
Example 1.1
Psp
Pm
PC
51
PT
51
u
F1
P2
f2
P
P1
f1
F2
Variable relations
are as follows:
dP
V
 K1 F1  K 2 F2
dt
F1  KV 1 f1 P1  P
f1 100 u
For the above pressure control
system, please describe its CV,
F2  KV 2 f 2 P  P2
SP, MV, DVs, control diagram as
well as control objective.
Discuss the solution ?
Control Diagram #1
for Example 1.1
f2
P2
Controlled Process
KV 2 f 2 P  P2
P1
Psp
e(t)
+
PC
51
u(t)
Control
Valve
f1
P
F2
KV1 f1 P1  P
_
F1
P(t)
dP
 K 1 F1  K 2 F2
V
dt
P
Pm
PT
51
Point out its MV, DVs ?
Control Diagram #2
for Example 1.1
Psp
+
PC
51
_
u(t)
Control
Valve
P1
P2
f2
Disturbance
Path
(干扰通道)
f1
Control Path
(控制通道)
+
+
Controlled Process
Pm
PT
51
P(t)
Control Diagram #3
for Example 1.1
P1, P2, f2
Disturbance
Path
Psp
+
PC
51
_
u(t)
Control
Valve
F1
Control Path
+
+
Controlled Process
Pm
PT
51
Discuss its correctness
P(t)
Ex. 1.2: Temperature Control System
for a Heat Exchanger
Tsp
u(t)
TC
22
Steam
RV
Problem: point out CV,
SP, MV, DVs, its block
diagram and objective ?
Tm
TT
22
RF , Ti
T
Process Fluid
Condensate
Block Diagram for Heat
Exchanger
Heat Exchanger
RF (t), Ti (t)
Tsp
+
TC
22
_
Tm
u(t)
Steam
Valve
TT
22
RV (t)
Disturbance
Path
Control
Path
+
+
T(t)
General Feedback Control
Systems
Controlled Plant
DVs
ysp
u(t)
+
ym(t)
Controller
Final Control
Element
_
MV
Disturbance
Path
Control
Path
+
+
y(t)
Sensor &
Transmitter
CV: temperature, pressure, flow, level, composition(
成份含量) and property(属性) of process fluid；
General Control Systems
Controlled Plant
DVs
ysp
u(t)
+
ym(t)
Controller
Final Control
Element
_
MV
Disturbance
Path
Control
Path
+
+
y(t)
Sensor &
Transmitter
Controller: Hardware & Software
Control Strategies / Algorithms
(控制策略与算法)
Hardware of Controllers
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Analog control instruments (DDZ-II, III)
Digital control instruments (数字控制器)
Programmable logic controllers (可编程逻辑
控制器，PLC)
Distributed control systems（集散控制系统
或称分布式控制系统，DCS）
Fieldbus control system （现场总线控制系统，
FCS）
Ethernet for Process Automation (过程自动
化以太网，EPA)
Types of Control Systems


Regulatory Control（“定值控制”或“调节控
制”）and Servo Control（”伺服控制”或“跟
踪控制”）
Examples：continuous processes and batch
processes or flight control.
Feedforward Control（前馈控制）and
Feedback Control （反馈控制）
Examples：temperature control system in a
heat exchanger.
Feedforward Control
& Feedback Control
Tsp
Steam
RV
u(t)
Tm
Feedforward
Controller
RFm
FT
31
TT
22
RF
T
TC
22
Steam
RV
Tm
Tim
TT
22
TT
32
RF , Ti
Ti
Process Fluid
Condensate
Tsp
u(t)
Process
Fluid
T
Condensate
Compare the advantages and disadvantages of
both strategies.
Feedforward Feedback Control
Steam
RV
Tsp
Tm
u(t)
Feedforward
Controller
RFm
TC
22
FT
31
TT
22
RF
T
Tim
TT
32
Ti
Process Fluid
Condensate
Types of Control Systems
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Switch Control（开关量控制）and Continuous
Control（连续量控制）
Examples: air conditioner control
Continuous-Time Control（连续时间控制）and
Discrete-Time Control（离散时间控制, 也称
“采样控制”, Digital Control, “数字控制”）
Examples: control systems based on
computers or analog instruments
Types of Control Systems (cont.)
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Multi-input-multi-output Control（MIMO,多
输入多输出控制, or “多变量控制” ), Singleinput-single-output Control （SISO, 单输入
单输出控制）and Multi-loop Control（多回路
控制）
Examples: distillation column control
Linear Control（线性控制）and Nonlinear
Control（非线性控制）
Examples: pH control
Multi-loop Control for Distillation
TC
LC
FC
Processs
Fluid
FC
Distillation
Column
Product
LC
Steam
Residue
Basic Process Control Strategies
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Single-loop PID (单回路PID控制)
Cascade Control (串级控制)
Ratio Control (比值控制)
Override or Selective Control (超驰或选择
控制）etc.
Features: PID based, simple, no model
needed, easy tuning, and for SISO
systems.
Advanced Process Control
(先进控制，APC) Strategies
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Feedforward Control (前馈控制)
Dead-time Compensation (纯滞后补偿)
Decoupling (解耦),
Predictive Control (预测控制）
Adaptive Control (自适应控制), etc.
Features: Model based, complicated,
computer needed, and most for MIMO or
SISO systems with long dead-time.
Primary Task of the Course
Controlled Plant
DVs
ysp
u(t)
+
ym(t)
Controller
Final Control
Element
_
MV
Disturbance
Path
Control
Path
+
+
y(t)
Sensor &
Transmitter
To analyze, design and implement the control
strategies or controller for continuous controlled
processes
Techniques Correlative to
Process Control Engineering
Control
Theory
System
Simulation
Optimization
Process Control
Engineering
Chemical
Engineering
Computer &
Networks
Measuremental &
Control Instruments
Next Lecture
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Defined the types of processes: selfregulating and non-self-regulating processes,
single- and multi-capacitance processes ;
Discussed the modeling from process
dynamics;
Discussed process characteristic parameters
K, T,τ, and their obtaining methods from
process data.