Chapter1 Process Control System 1

发布于:2021-06-11 03:09:44

Process Control System 过程控制系统
北方工业大学自动化2008 李小坚 2011年9月5日

Chapter 1 Introduction
? 1.1 What is Process Control ? There are many of the basic quantities: we may wish to keep them constant in a process, to regulate them as we like. ? These quantities are: ? Flow ? Liquide - level or pressure ? Temperature ? Composition ? Pressure

the Questions of Process Control: The process engineers routinely have to answer about process control include the following:
? I have this process. What should be controlled? ? Where on the process do I put my control loops? ? As I proceed, How to design a process, ? What aspects of control should I consider at stages? Two Basic Tasks: ? how to design controllers ? how to design complete control systems.

Why Control? Chemical plants are intended to be operated under known and specified conditions. There are several reasons why this is so:
Safety: Operability: Economic:

1 . Control for Safety
? Formal safety and environmental constraints must not be violated. There should be emergency control and alert system

? ? ? ?

Intrinsically safe The ultimate safety system Self-diagnosis Recoverable and Reliability

2. Control for Operability
The operator of a process quite simply has to ? know what it is doing ? be able to make it do what he or she wants, rather than to follow its natural inclinations. ? The issue of making a plant behave in this way is called operability.

3. Control for Profitability
? ? ? ? ? The Cost and Efficiency is very important. In the Design and Operation Production rate, Product quality, and Energy economy.

The Technical Requirements of Process Control : -? 稳:Stable, Steady, Smooth,Robust , ? 准:Precisely, Accurately, correct, exact and detailed ? 快:Fast, quickly , in time

The Deficulties of Process Control
? ? ? ? 1、被控对象的多样性,The diversity 、被控对象的多样性, 2、对象存在纯滞后,Large delay 、对象存在纯滞后, 3、对象具有非线性,nonlinearty 、对象具有非线性, 4、控制系统比较复杂 complexity 、控制系统比较复杂,

Different process control systems
? ? ? ? 1 simple process control systems 简单过空 2 complex process control systems复杂过控 3 advanced process control systems先进过控 4 total integrated process control systems集成 过控

The development of Process control system过程控制系统的 过程控制系统的 发展概况
? ? ? ? ? 1,初期阶段:40’s年代 2,仪表阶段:50’s 3,综合自动化:60-80s,集散控制 4,集成化阶段:90s--,多级网络的整体 5,过程控制走向 更深 更广 更大 更智能

1 Basic Concepts of Feedback Control

The Setpoint Control -Server Control and Programs
? The system has been disturbed. This is the common situation for a chemical plant subject to all sorts of external upsets. However, the control system does not need to know what the source of the disturbance was. ? The setpoint has been changed. In the absence of external disturbance, a change in setpoint will introduce an error. The control system will act until the measured quantity reaches its new setpoint.

1.2 The Process Control System
Functions of a Control System: ? Measurement This is essentially an estimate or appraisal of the process being controlled by the system. In this example, this is achieved by the right hand of the operator. ? Comparison This is an examination of the likeness of the measured values and the desired values. This is carried out in the brain of the operator.

. Comperation and Computation
In order to keep the temperature at a desired value. Certain calculation was carried out.

. Correction
by the controller and actuator This is ultimately the materialisation of the order for the adjustment.

. Control action
It is just like the left hand of the operator takes the necessary actions following the order from brain

? Automatic Control System ? Based on the above process, we can easily set up an automatic control system as shown in the next figure.

Hardware of a Control System
Examining the automatic control system, it is found that it contains the following hardware. – Sensor - a piece of equipment to measure system variables. It serves as the signal source in automatic control. These will be discussed at length in a later module. – Controller - a piece of equipment to perform the functions of comparison and computation. The actions that a controller can take will be discussed at length in a later module. – Control Element - a piece of equipment to perform the control action or to exert direct influence on the process. This element receives signals from the controller and performs some type of operation on the process. Generally the control element is simply a control valve.

Software of a Control System
? Associated with a control system are a number of different types of variables. First we have the Controlled Variable. This is the basic process value being regulated by the system. It is the one variable that we are specially interested in - the outlet water temperature in the example above. In feedback control the controlled variable is usually the measured variable. ? An important concept related to the controlled variable is the Setpoint. This is the predetermined desired value for the controlled variable. The objective of the control system is to regulate the controlled variable at its setpoint.

To achieve the control objective there must be one or more variables we can alter or adjust. These are called the Manipulated Variables. In the above example this was the input hotwater flow rate. Conclusively:in the control system we adjust the manipulated variable to maintain the controlled variable at its setpoint. This meets the requirement of keeping the stability of the process and suppressing the influence of disturbances.