Process Characteristics 过程特征 Shen Guo-jiang Institute of Industrial Control, Zhejiang University

Last Lecture Discussed three basic operations in every type of control system; Defined Controlled Variable, Setpoint, Manipulated Variable and Disturbance; Discussed The objective of an automatic process control system ; Defined Regulatory Control and Servo Control; Discussed Feedforward Controland Feedback Control .

Example Variable relations are as follows: For the above pressure control system, please describe its CV, SP, MV, DVs, control diagram as well as control objective. If the is increased suddenly, How the feedback control maintain the P at its set point.

Problem Discussion 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.

Contents Process and Importance of Process Characteristics Introduction of Final Control Elements Types of Processes Obtaining Characteristics from Process Dynamics Obtaining Characteristics from Process Data Summary

Heat Exchanger Temperature Control System The extended controlled process (广义对象) is anything except the controller.

Importance of Process Characteristics Every process has different characteristics Not easy to change the controlled process Very easy to change the controller tuning What we can do is to adapt the controller to the process A good controller is the controller best adapted to the process characteristics

Heat Exchanger Temperature Control System Process description and signal flow diagram?

温度控制系统的信号流程图

Pneumatic Control Valves 0.02 ~ 0.1MPa 功能：根据阀头气压的大小，通过阀杆改变阀体中阀芯的位置，进而调节流经阀体的流体流量。

I/P Converter 功能：将电流信号（4 ~ 20mA）转换成气动模拟量信号0.02 ~ 0.10MPa

Principle of Transducer 应用意义 改变交流电机供电的频率和幅值，因而改变其运动磁场的周期，达到平滑控制电动机转速的目的 异步电动机的变频调速原理 异步电动机定子三相对称绕组空间相隔120角，当通以三相对称电流后，便产生了旋转磁场；其旋转磁场的转速（亦称同步转速）为 n = 60 f 1 / p （r / min） 式中，f 1为定子绕组电源频率；p为磁场对数。 实现方法

变频调速主电路

变频器基本组成

Problem Discussion 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.

Types of Processes Self-regulating processes (or stable processes, 自衡过程/稳定对象) (1) Single-Capacitance Processes (2) Multi-Capacitance Processes Non-self-regulating processes (or unstable processes, 非自衡过程) Ex.: some level processes and some reactors

A Self-regulating Process The controlled process is stable. Why ?

A Non-self-regulating Process The controlled process is unstable. Why ?

A Self-regulating Liquid Level Process The process is self-regulating. Why ?

Approaches to Obtain Process Characteristics Based on Process Dynamics (机理建模) Describe process characteristics with some mathematical equations based on the chemical and/or physical mechanism of a controlled process. Based on Process Data (测试建模) To obtain process characteristics, manually change the input of a controlled process and record the input and output data, then find an appropriate model based on process data.

Modeling Example #1 Material balance equation： Relationship between flow and level： Problem Discussion: How to build the controlled process with SimuLink? (\Simulink\ LevelProcess01.mdl)

Modeling Example #1

Modeling Example #2 For the level controlled process, h2 is selected as its controlled variable, and Qi is the manipulated variable, Qd is the main disturbance variable. The rates of outlet flow are assumed to satisfy the following equations: Please obtain the process characteristics by dynamic equations, and build the corresponding Matlab/SimuLink model.

Modeling Example #2 Material balance equation： Relationship between flow and level： Simulation ex.: \simulink\ LevelProcess02.mdl State equation and linearization ?

Modeling Example #2

Single-Capacitance Processes Ex.1

Single-Capacitance Processes Ex.2

Single-Capacitance Processes Ex.3

Problem Discussion 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.

Terms that Describe the Process Characteristics Process Gain (K) Ratio of the change in output (or responding variable) to the change in input (or forcing function). Process Time Constant (T) Process Dead Time (τ)

Process Gain Calculation Ex.1

Process Gain Calculation Ex.2

Process Gain Calculation Ex.3

Notes to Process Gain Process gain describes the sensitivity of the output variable to a change in input variable. Process gain includes three parts: Sign, Numerical value and Units. Process gain relates only steady-state values, so the gain is a steady-state characteristic of the process.

Process Time Constant (T ) Definition The process time constant for a single-capacitance process is defined as the amount of time counted from the moment the variable starts to respond to reach 63.2% of its total change.

Process Dead Time (τ) Definition the finite amount of time between the change in input variable and when the output variable starts to respond.

Notes to Parameters K, T, τ These numerical values describe the basic characteristics of a real process, which K describes the steady-state characteristic, and T, τ are related to the dynamics of the process. These numerical values depend on the physical parameters of the process as well as its operating conditions. In most cases, they vary with operating conditions, or most processes are nonlinear. The ratio, τ/ T, has significant adverse effects on the controllability of control systems.

Mathematical Description of Single-Capacitance Processes The transfer function for a first-order-plus-dead-time (FOPDT) process is given by

Multi-capacitance Processes Ex.2

Mathematical Description of Multi-Capacitance Processes High-Order Model: Second-order-plus-dead-time Model First-order-plus-dead-time Model

Characteristics of Real Processes Most controlled processes are self-regulating except some liquid level processes; Processes have some amount of dead time; The step responses of controlled processes are often monotonous(单调的) and slow; Most processes are nonlinear, so the numerical values of model parameters vary with operating conditions.

Parameters Describing Process Characteristics Process Gain (K) Ratio of the change in output (or responding variable) to the change in input (or forcing function). Process Time Constant (T) Process Dead Time (τ)

Problem Discussion 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.

Obtaining Process Characteristics from Process Data Obtain the necessary process data by step response testing; (1) Set the controller to manual mode; (2) Make a step change in the controller output; (3) Record the process variable. Obtain parameters K, T, τ from process testing data.

The Step Response Curve for a Heat Exchanger

Obtain the Dynamic Terms from the Step Response Curve

Obtain Process Gain from the Step Response Curve If the span of the temperature transmitter is 100 to 300 ℃, then the change in transmitter output is 4%. Therefore, the total process gain is

Summary 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.

Next Lecture Control valve is divided into Fail-closed valve and Fail-closed valve. what is the physical meaning of them? How to choose them? What is the definition of the feedback controller action? According to the specific object, how to choose the controller action? How to evaluate a performance of control system (qualitative and quantitative)

Next Lecture(Cont.) Describe the input and output relationship of P,PI and PID controller For the common controlled process, why P controller will generate an offset and the PI controller can eliminate the offset? Why the derivative effect of the PID controller dose not used in the most actual process?