Introduction forProcess Control Systems

过程控制系统概论

Shen Guo-jiang

Institute of Industrial Control,

Zhejiang University

Contents 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

Systems

U

D

Z

Y

Control Systems

examples ?

Inputs

Outputs

Application of control system 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

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.

Purpose: To maintain the level at a desired operating value, when the inlet flow （输入流量） changes.

Limitations with Manual Control

Require the operator to look frequently at the level to take corrective action whenever it deviates from the desired value;

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 SystemSolution: Design a control system to accomplish this control automatically without requiring intervention from the operator.

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.

h

hsp

Qi

Qo

LT21

LC21

Feedback Control Process1. 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, the controller decides what to do to correct for any deviation;

4. Based on this decision, it sends a signal to the final control element （执行单元） , e. g., a control valve.

h

hsp

Qi

Qo

LT21

LC21

Three basic operationsin 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 )The variable that must be maintained or controlled at some desired value.

Setpoint (SP, 设定值 / 给定值 )The desired value of the controlled variable.

Manipulated Variable (MV, 操纵变量 / 操作变量 )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

For the above pressure control system, please describe its CV, SP, MV, DVs, control diagram as well as control objective.

2211 FKFKdt

dPV

PPfKF V 1111

2222 PPfKF V

Variable relations are as follows:

Discuss the solution ?

P

P1

P2Psp Pm

u

f1

f2

F1

F2PC51

PT51

uf 1001

Control Diagram #1 for Example 1.1

Point out its MV, DVs ?

PC51

Psp e(t)

+_

Pm

PT51

Control Valve

P(t)u(t)

222 PPfKV

PPfKV 111

f1

P1

F1

2211 FKFKdt

dPV

P

P2

F2

f2

P

Controlled Process

Control Diagram #2 for Example 1.1

PC51

Psp

+_

Pm

PT51

Control Valve

P(t)u(t) f1

P2

Controlled Process

++

Disturbance Path

(干扰通道)

Control Path(控制通道)

P1

f2

Control Diagram #3 for Example 1.1

PC51

Psp

+_

Pm

PT51

Control Valve

P(t)u(t) F1

Controlled Process

++

Disturbance Path

Control Path

P1, P2, f2

Discuss its correctness

Ex. 1.2: Temperature Control System for a Heat Exchanger

Problem: point out CV, SP, MV, DVs, its block diagram and objective ?

Steam

Tsp

Tm

T

RV

RF , Ti

u(t)

Condensate

Process Fluid

TC22

TT22

Block Diagram for Heat Exchanger

Tsp

+_

Tm

++ T(t)RV (t)

RF (t), Ti (t)Disturbance

Path

TT22

Steam Valve

Control Path

TC22

Heat Exchanger

u(t)

General Feedback Control Systems

CV: temperature, pressure, flow, level, composition(成份含量 ) and property(属性 ) of process fluid ；

ysp u(t)

+_

ym(t)

++ y(t)MV

DVs Disturbance

Path

Sensor & Transmitter

Final Control Element

Control Path

Controller

Controlled Plant

General Control Systems

Controller: Hardware & Software

Control Strategies / Algorithms

(控制策略与算法 )

ysp u(t)

+_

ym(t)

++ y(t)MV

DVs Disturbance

Path

Sensor & Transmitter

Final Control Element

Control Path

Controller

Controlled Plant

Hardware of Controllers 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

T

RV

RF

Tsp

Tm

u(t)

Ti

Feedforward Controller

Condensate

Process Fluid

Steam

TT22

FT31

TT32

RFm Tim

Compare the advantages and disadvantages of both strategies.

Steam

Tsp

Tm

T

RV

RF , Ti

u(t)

Condensate

Process Fluid

TC22

TT22

Feedforward Feedback Control

T

RV

Tm

u(t)

Steam

Condensate

Process Fluid

Feedforward ControllerTsp

TC22

TT22

RF Ti

FT31

TT32

RFm Tim

Types of Control Systems 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.)

Multi-input-multi-output Control （ MIMO,多输入多输出控制 , or “ 多变量控制” ), Single-input-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

LC

TC

LC

FC

FC

DistillationColumn

SteamResidue

ProcesssFluid

Product

Basic Process Control Strategies

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

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

To analyze, design and implement the control strategies or controller for continuous controlled processes

ysp u(t)

+_

ym(t)

++ y(t)MV

DVs Disturbance

Path

Sensor & Transmitter

Final Control Element

Control Path

Controller

Controlled Plant

Techniques Correlative to Process Control Engineering

Process Control Engineering

Control Theory

Optimization

Chemical Engineering

Measuremental & Control Instruments

Computer & Networks

System Simulation

Next Lecture

Defined the types of processes: self-regulating 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.