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Process Control Engineering

Process Control Engineering. Institute of Industrial Control, Zhejiang University 20 13/06/26. 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 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）

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Process Control Engineering

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

  2. 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 • 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

  3. 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）

  4. Feedback Control Process（反馈控制过程） • Measure the level by asensor(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.

  5. 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.

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

  7. P2 Controlled by a single loop?

  8. 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

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

  10. 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

  11. 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)

  12. 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.

  13. Obtain the Dynamic Terms from the Step Response Curve

  14. 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

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

  16. Action of Controllers • Direct Action (正作用) 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.

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

  18. 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 (+) (+) (+)

  19. 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.

  20. 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.

  21. 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.

  22. 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

  23. Obtain Initial PID Para.(Ziegler-Nichols Method) Note: the above method was developed for

  24. Obtain Initial PID Para.(Lambda Tuning Method) Note: the above method is not limited by the value of

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

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

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

  28. 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）

  29. 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）

  30. 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 ?

  31. 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.

  32. General Cascade Control Diagram Note: D1denotes 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 inremote set point or cascade mode.

  33. 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.

  34. Cascade Control Examplesof Heat Exchanger Scheme #2 Scheme #1 Scheme #3

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

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

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

  38. Feedforward / Feedback Control

  39. Comparison of Feedforward and Feedback Control

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

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

  42. 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）

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

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

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

  46. 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.

  47. Selective Control Example

  48. 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

  49. Combination of Parallel Valves

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

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