Presentation on theme: "Tuning PID Controller Institute of Industrial Control,"— Presentation transcript:
1 Tuning PID Controller Institute of Industrial Control, Zhejiang University, Hangzhou, P. R. China2013/03/27
2 Last Lecture Selection of Valve Action Action of Feedback Controllers Performance Criterion of Process Control SystemsUnderstand Typical Controllers (P, PI and PID)
3 Single-loop PID Control System Problem: For an unknown extended controlled process, how to design and tune our PID controller ?
4 Proportional-Integral-Derivative (PID) Controller Ideal PID ControllerTd is the derivative time.Industrial PID Controller (design and realization ?)The derivative gain Ad = 10.
5 Problem DiscussionExplain the function of PID controller for a stable controlled process.Analyze the effect of PID parameter changes on control performancesHow can we realize the industrial PID controller in Simulink ?PID tuning example (See ../PIDControl /PIDLoop.mdl )
6 ( discuss the initial condition setting) Problem 2-1For the level controlled process, h2 is selected as its controlled variable, and Qin is the main input of the process. Suppose the sectional area of two tanks are A1 and A2. The rates of outlet flow are assumed to satisfy the following equations:Please obtain the process characteristics by dynamic equations, and build the corresponding SimuLink model.( discuss the initial condition setting)
7 ( load the file temp.mat ) Problem 2-2For a heater with electricity, the step response data of outlet temperature can be shown as follows. Please determine its characteristic parameters K, T,τ if the span of temperature transmitter is 0 to 50 ℃.t, min369121518212427U, %6040T, ℃38.738.338.839.038.437.035.333.63033363942454851545732.231.030.229.529.028.628.328.027.927.8( load the file temp.mat )
8 Contents 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
9 Type Selection of PID Controllers *1: For some slow processes with long time constants, the derivative action is suggested to use. However, if there exists strong measurement noises, a first-order or average filter should be added.Controlled VariableController TypeTemperature / CompositionPID*1Flow / Pressure /Liquid-LevelPILiquid-LevelPPlease analyze the rule of type selection ?
11 Offline Tuning Based on Process Parameters: K, T,τ Step 1: switch the controller to manual mode, change the output of controller in step form, and record input/output data of controller.Step 2: obtain process characteristics: K, T,τ, from the step response data.Step 3: set the PID parameters Kc, Ti , Td, and switch the controller to automatic mode.Step 4: increase or decrease the gain Kc until obtaining the satisfactory response.
12 Simulation of Offline Tuning step 1: Step Testing See ../PIDControl/PIDLoop.mdl
14 Step 3: Obtain Initial PID Para. (Ziegler-Nichols Method) ControllerKcTiTdPPIPIDNote: the above method was developed for
15 Step 3: Obtain Initial PID Para. (Lambda Tuning Method) ControllerKcTiTdPPITPIDτ/2Initial ValueNote: the above method is not limited by the value of
16 Simulation Example #1 K = 1.75 T = 6.5,τ= 3.3 min For PI Controller, Z-N tuning: Kc = 1.0, Ti = 11 minLambda tuning: Kc = 0.56, Ti = 6.5 min
17 Simulation Example #2 K = 1.75 T = 6.5,τ= 6.3 min For PI Controller, Z-N tuning: Kc = 0.53, Ti = 20.8 minLambda tuning: Kc = 0.30, Ti = 6.5 min
18 Procedure of Online Tuning: Ziegler-Nichols Technique Step 1: with the controller online (in automatic mode), remove all the reset (Ti = maximum) and derivative (Td = 0) modes. Start with a small Kc value.Step 2: make a small set point or load change and observe the response of CV.Step 3: if the response is not continuously oscillatory, increase Kc, or decrease PB, repeat step 2.Step 4: Repeat step 3 until a continuous oscillatory response is obtained.
19 Example of Online Tuning See ../PIDControl/PIDLoop.mdl
20 Online Tuning: Ziegler-Nichols Technique The gain that gives these continuous oscillations is the ultimate gain (临界增益), Kcu. The period of the oscillations is called the ultimate period (临界周期), Tu. the ultimate gain and the ultimate period are the characteristics of the process being tuned. The following formulas are then applied:ControllerKcTiTdP0.5KcuPI0.45KcuTu /1.2PID0.65KcuTu /2Tu /8
23 Auto-tuning Based on Relay Feedback (基于继电反馈的参数自整定) Here we suppose the process gain > 0
24 Relay Feedback Example The controlled process can be described asThe amplitude of relay controller is d = ±2.0
25 Response of Relay Feedback Oscillation period TU & amplitude AY（振荡周期与幅度）?See the detailed results:../ PIDLoopAutoTuning.mdl
26 The Ultimate Gain (临界增益) Kcu Calculation 经FT变换可知，控制输出的一次谐波幅度为而对应的控制器临界增益为
27 Online Z-N Tuning Parameters ControllerKcTiTdP0.5KcuPI0.45KcuTu /1.2PID0.65KcuTu /2Tu /8If we use a PID controller, then we select the following parameters ……
28 Closed-loop Response of PID Feedback System Above auto-tuning method can be applied to other controlled processes ?
29 Characteristics of Flow Loops Fast dynamic responseZero dead time, which results in an infinite controller gain in every tuning equationLarge measurement noiseTo decrease the change of control valve, a PI controller is common used with very small proportional action and a large integral action to approximate an integral controller. (Why?)
30 Tuning Example of Flow Loops See ../PIDControl/FlowLoop.mdlPlease compare the proportional gain with the integral gain
32 Characteristics of Level Loops Very often levels are integrating processesThere are two types of possible control objectives when the input flow varies:(1) Tight Level Control;(2) Average Level Control(“液位均匀控制”)
33 Tight Level ControlThe objective is to control the level tightly at set point, and the output flow can be allowed to vary without limitationIf a level process happens to be self-regulated, and it is possible to obtain K, T andτ, the above tuning techniques can be used directlyIf a level process is integrating, a PI controller is common used with large proportional action and a very small integral action
34 Average Level ControlThe objective is to smooth the output flow from the tank, which feeds the downstream unit, the level in the tank must be allowed to “float” between a high and a low levelA P controller is common used in Average Level Control with a small proportional gainTuning: the gain should be set to be as small as possible, as long as the level changes between a high and a low level for the expected flow deviation from the average flow.
35 Example of Level Control See ../PIDControl/ LevelLoop.mdl
36 Analysis of Average Level Control Systems Dynamic equation of the controlled process:where A is the area of the tank.Suppose
37 Analysis of Average Level Control Systems (cont.) For a proportional controller, Gc = －Kc,Please analyze the above models.
38 Examples of Average Level Control Systems Please see ../PIDControl/ AverageLevelLoop.mdl
39 Simulation Results of P-type Average Level Control
40 Please see the following simulation example Reset Windup ProblemPlease see the following simulation example…/PIDControl/PidLoopwithLimit.mdl
41 Simulation Result with Reset Windup in a Single-Loop System Discussion:Which difference exists between reset windup and the open or closed status of the control valve completely ?
42 The Principle of Preventing Reset Windup Principle: remove the reset or integral action if the control output is beyond the normal operation range.
45 Summary Selection of PID Controller Types Tuning of PID Controller ParametersTuning of PID Controller for Flow LoopsTight / Average Level ControlReset Windup and Its Prevention
46 Problem DiscussionFor an unknown stable temperature control system, can you determine PID parameters in Offline and Online tuning methods ?Please realize the industrial PID controller in Simulink ?For the fast flow control loop, show me your tuning principle and explain why.For the AVERAGE level control loop, show me your tuning principle and explain why.Explain the existing reason of reset windup and show me your prevention schemes
47 Exercise 3.1A controlled process is shown in the Problem 2-1 (p.34) in Automated Continuous Process Control.calculate its characteristics parameters K, T and τ;decide on the action of the valve and the controller;tune your PID controller.