Process Control Engineering Institute of Industrial Control, Zhejiang University 2013/06/26

考核说明 考核：平时成绩 50%，包括课堂提问、平时练习、综合练习；期末考试（闭卷）50% 课堂提问 平时练习 综合练习 20分 满分20分。缺课1次扣5分。 平时练习 满分10分。平时作业的平均分 综合练习 20分

概论 Origin of Process Control Systems（过程控制系统的由来） Important Terms and Objective of Process Control Systems（过程控制的术语与目标） Description of Process Control Systems（过程控制系统的描述） Types of Control Strategies （控制策略分类） Course Tasks（课程任务）

Feedback Control Process （反馈控制过程） 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) The controller（控制器/调节器） then receives the signal and compares it with the value desired; Depending on this comparison, the controller decides what to do to correct for any deviation; Based on this decision, it sends a signal to the final control element（执行单元）, e. g., a control valve and change the controller process.

Important Terms （重要术语） 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. Controller Output (OP, 控制器输出) 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.

Control Diagram Variable relations are as follows: For the above pressure control system, please describe its CV, SP, OP, MV, DVs, control diagram as well as control objective.

P2 Controlled by a single loop?

Process Characteristics Importance of Process Characteristics (过程对象特性的重要性) Introduction of Final Control Elements (执行机构介绍) Types of Processes (过程特性分类) Obtaining Characteristics from Process Dynamics (过程特性机理建模法) Obtaining Characteristics from Process Data (过程特性测试建模法) Summary

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

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

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)

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.

Obtain the Dynamic Terms from the Step Response Curve

PID Controller Selection of Valve Action (调节阀作用选择) Action of Feedback Controllers (反馈控制器的正反作用) Performance Criterion of Process Control Systems (过程控制系统的性能指标) Understand P, PI and PID Controllers Problem Discussion

Types of Control Valves Fail-opened Valve (气关阀) Fail-closed Valve (气开阀)

Action of Controllers Direct Action (正作用) Reverse Action (反作用) when the signal from the transmitter increases, the controller output also increases. Reverse Action (反作用) when the signal from the transmitter increases, the controller output decreases on the contrary. Note: The set point is not part of decision.

Selection of Controller Action Principle: to construct a negative feedback loop ?

Controller action selection based on loop analysis Ex. 1 Step 1: plot block diagram Step 2: indicate the action direction for each block except the controller. Step 3: determine the action of the controller to construct a negative feedback loop (+) TC 22 must be reverse (+) (+) (+)

PID Controller: Effect of P on Control Performances P controllers have only one tuning parameter, Kc. However, they suffer a major disadvantage – there exists an Offset of the controlled variable from the set point. (Why ?) For a given step disturbance, the magnitude of the offset depends on the value of the gain. The larger the gain, the smaller the offset. Above a certain Kc, most processes go unstable.

Effect of Integral Action on Control Performances PI controllers have two tuning parameter: the gain or proportional band, and the integral time or the integral rate (1/Ti ). The advantage is that the integration removes the offset. (Why ?) The disadvantage of PI controllers is that the addition of integration adds some amount of instability to the system. The smaller the integral time, the stronger the integral action, the faster the system removes the offset, but the weaker the stability of the system.

Effect of Derivative Action on Control Performances PID controllers have three tuning parameter: the gain, the integral time and the derivative time. The derivative action gives the controller the capability to anticipate. PID controllers are recommended for use in slow processes with long time constants, such as temperature loops, which are usually free of noises. For fast processes with noises, such as flow loops and pressure loops, the use of derivative action will amplify the noise and therefore should not be used.

PID Tuning Selection of PID Controller Types （PID控制器类型选择） Tuning of PID Controller Parameters （控制器参数整定） Flow Control （流量控制） Level Control （液位控制） Reset Windup and Its Prevention （积分饱和与防止） Summary

Obtain Initial PID Para. (Ziegler-Nichols Method) Controller Kc Ti Td P PI PID Note: the above method was developed for

Obtain Initial PID Para. (Lambda Tuning Method) Controller Kc Ti Td P PI T PID τ/2 Initial Value Note: the above method is not limited by the value of

Digital PID Concept of Digital Control Systems Selection of Digital Filters Digital PID Controllers and Its Improved Version Concept of Distributed Control Systems Summary

Digital PID Positional Algorithm Ideal analog PID algorithm Digital PID positional algorithm （位置算法）

Digital PID Incremental Algorithm Digital PID positional algorithm Digital PID incremental algorithm（增量算法）

PID incremental algorithm with derivative action first Digital PID incremental algorithm Digital PID incremental algorithm with derivative action first （微分先行PID增量算法）

Cascade Control Concept of Cascade Control（串级控制概念） Characteristics of Cascade Control（串级控制系统的特性分析） Design Principle of Cascade Control （串级控制的设计原理） Implementation and Tuning of Controllers （串级控制器的实现与参数整定） Simulation Examples（仿真举例）

Process Example: A Cascade Control Scheme (1) This scheme consists of two sensors, two transmitters, two controllers, and one valve. (2) This scheme results in two control loops, one loop controlling T and the other loop controlling Fgas. Note: The flow of fuel gas is used only as an intermediate variable to improve control performance. Please plot block diagram of the system ?

Cascade Control Diagram for Outlet Temp. of Process Fluid where TC 23 is called “primary/master controller (主控制器)”, and FC 13 is called “secondary/slave controller (副控制器)”; D1 denotes disturbances entering the outer loop, D2 denotes disturbances entering the inner loop.

General Cascade Control Diagram Note: D1 denotes the effect of primary disturbances on primary CV, D2 denotes the effect of secondary disturbances. “Primary Loop” presents the outer loop where the inner loop is closed and set in remote set point or cascade mode.

Design Principles of Cascade Control Systems The secondary variable must respond faster to changes in some disturbances than the primary variable does — the faster, the better Secondary loop or inner loop must include some obvious disturbances to primary variable— the more the better If possible, secondary loop should include some nonlinear plant Typical cascaded loops: temp. to flow, concentration to flow, pressure to flow, level to flow, temp. to pressure, temp. to temp.

Cascade Control Examples of Heat Exchanger Scheme #2 Scheme #1 Scheme #3

Feedforward Control Feedforward Concept Design of Linear Feedforward Controllers Design Examples of Feedforward Control Feedforward-Feedback Control Simulation Results Summary

Design of Linear Feedforward Controllers (cont.) Design formula for the feedforward controller: Design Objective:

Design of Linear Feedforward Controllers (cont.) Design formula for the feedforward controller: ( Why ? )

Feedforward / Feedback Control

Comparison of Feedforward and Feedback Control Feedforward Control Feedback Control Disturbances are measurable CV is measurable Control MV based on disturbances Control MV based on control ERROR Open-loop, No Stability Problem Closed-loop, Stability is the most important Only some disturbances are detected All disturbances are detected Accurate model needed for both of Control and Disturbance Paths No accurate model needed Not adaptable to nonlinear or time-varied systems Adaptable to nonlinear or time-varied systems

Ratio Control Concept of Ratio Control Design of Ratio Control Schemes Cross-limiting Control of Air/Fuel Ratio in a Boiler or Furnace Summary

Ratio Control Suppose both of the flow transmitters are linear. Sometimes, they are nonlinear Steady-state condition:

Air/Fuel Ratio in a Boiler Control Scheme Cross-limiting control with O2 trim （带有O2调节的双交叉控制）

Override and Selective Override/Constraint Control Problem Design of Constrain Control Systems Reset Windup and Its Prevention in Constrain Control Selective Control Schemes

Override Control Scheme LS: Low Selector（低选器） u(t) = min(u1, u2) LS (－) (＋) Smooth Switch problem between two loops ?

Override Control Scheme LS: Low Selector（低选器） u(t) = min(u1, u2) RFB: (external) reset feedback（外部积分反馈）

Reset Windup Prevention in Constraint Control Discuss: ONLY the controller in closed-loop condition has integral action, and the output of inactive controller will follow the output of active controller.

Selective Control Example

Parallel Positioning Control Concept of Parallel Positioning Control (分程控制) Application of Parallel Valve-Positioning Control --- Batch Reactor Control Concept & Application of Valve Position Control

Combination of Parallel Valves

Parallel Valve Positioning Control Scheme #1 Problem: Choose the FO or FC type of control valves; Plot the diagram; Determine the controller action Analyze the whole control process

Methods for Compensating Process Nonlinearity Nonlinear Valves Cascade Control Variable Ratio Control Nonlinear Gain Compensation Nonlinear Transformation Identification + Adaptive Control

Dead Time - Smith Predictor Prediction Error Filter：

Coupling Analysis of Multivariable Systems (多变量系统的关联分析)

Computation of Relative Gain for n×n Systems Note: “●” means the multiplication of matrix elements K non-invertible?

Decoupling Control Schemes of Multivariable Systems (多变量系统的解耦控制)

Block Diagram for a General 2*2 System with Decoupler #1 Decoupling Conditions ?

Decoupler #1 for a General 2*2 System If

About Decoupler #1 Problem: (1) initial MVs’ value; (2) “Man/Auto” mode switch; (3) limit of MVs.

Block Diagram for a 2*2 System with Feedforward Decoupler

Boiler Process Diagram and Control Problems of Boiler (锅炉设备的生产流程与控制问题) Characteristic Analysis & Three-element Control for Drum Level (汽包水位特性分析与三冲量控制) Cross-limiting Air/Fuel Ratio Combustion Control (双交叉空燃比燃烧控制)

Two-element (双冲量) Control Problem discussion: (1) Point out the kind of control methods ? (2) Obtain control diagram of the scheme. (3) Select the controller action, the symbol and the value of C2, if the valve is a fail-open valve and C1=1.

Distillation Distillation Principle & Control Problems Column Pressure Control Material Balance (物料平衡) Control Product Purity Control (1) Distillate Purity Control (2) Bottom Purity Control (3) Both Distillate & Bottom Purity Control Other Control Schemes