第1章-FUNDAMENTALS-OF-AUTOMATIC-CONTROL.pptx

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,2014-8-25,#,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,2014-8-25,#,单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,2014-8-25,#,自动化专业英语,目录,单击此处添加文字内容,1,FUNDAMENTALS OF AUTOMATIC CONTROL,单击此处添加文字内容,2,MEASUREMENTS AND ACTUATORS,单击此处添加文字内容,3,ADVANCED CONTROL SYSTEMS,单击此处添加文字内容,4,COMPUTER CONTROL SYSTEMS,单击此处添加文字内容,5,AUTOMATIC CONTROL SYSTEMS,单击此处添加文字内容,6,ARTIFICIAL INTELLIGENCE TECHNIQUES AND APPLICATIONS,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.1,Introduction to Process Control,1.1.1 Illustrative Example,Continuous stirred-tank heater.,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.1,Introduction to Process Control,The control objective for the stirred-tank heater is to keep the exit temperature T at a constant reference value TR.The reference value is referred to as a set point in control terminology.Next we consider two questions.,1.1.1 Illustrative Example,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.1,Introduction to Process Control,Question 1.,How much heat must be supplied to the stirred-tank-heater to heat the liquid from an inlet temperature T,i,to an exit temperature T,R,?,A steady-state energy balance for the tank indicates that the heat added is equal to the change in enthalpy between the inlet and exit streams:,1.1.1 Illustrative Example,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.1,Introduction to Process Control,Question 1.,How much heat must be supplied to the stirred-tank-heater to heat the liquid from an inlet temperature T,i,to an exit temperature T,R,?,Making this substitution in Eq.(1.1.1)gives an expression for the nominal heat input:,1.1.1 Illustrative Example,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.1,Introduction to Process Control,Question 2.,Suppose that inlet temperature T,i,changes with time.How can we ensure that T remains at or near the set point T,R,?,Method 1.,Measure T and adjust Q.One way of controlling T despite disturbances in Ti is to adjust Q based on measurements of T.Intuitively,if T is too high,we should reduce Q;if T is too low,we should increase Q.This control strategy will tend to move T toward the set point TR and could be implemented in a number of different ways.For example,a plant operator could observe the measured temperature and compare the measured value to TR.The operator would then change Q in an appropriate manner.This would be an application of manual control.However,it would probably be more convenient and economical to have this simple control task performed automatically by an electronic device rather than a person,that is,to utilize automatic control.,1.1.1 Illustrative Example,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.1,Introduction to Process Control,Question 2.,Suppose that inlet temperature T,i,changes with time.How can we ensure that T remains at or near the set point T,R,?,Method 2.,Measure,T,i,adjust,Q,.As an alternative to Method 1,we could measure disturbance variable,T,i,and adjust,Q,accordingly.Thus,if,T,i,is greater than,we would decrease,Q,;for,T,i,we would set,Q,.,Method 3.,Measure,T,adjust,w,.Instead of adjusting,Q,we could choose to manipulate mass flow rate,w,.Thus,if,T,is too high we would increase,w,to reduce the energy input rate in the stirred tank relative to the mass flow rate and thereby reduce the exit temperature.,1.1.1 Illustrative Example,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.1,Introduction to Process Control,Question 2.,Suppose that inlet temperature T,i,changes with time.How can we ensure that T remains at or near the set point T,R,?,Method 4.,Measure,T,i,adjust,w,.In analogy with Method 3,if,T,i,is too high,w,should be increased.,Method 5.,Measure,T,i,and,T,adjust,Q,.This approach is a combination of Methods 1 and 2.,Method 6.,Measure,T,i,and,T,adjust,w,.This approach is a combination of Methods 3 and 4.,Method 7.,Place a heat exchanger on the inlet stream.The heat exchanger is intended to reduce the disturbances in,T,i,and consequently reduce the variations in,T,.This approach is sometimes called“hog-tieing”an input.,1.1.1 Illustrative Example,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.1,Introduction to Process Control,Question 2.,Suppose that inlet temperature T,i,changes with time.How can we ensure that T remains at or near the set point T,R,?,Method 8.,Use a larger tank.If a larger tank is used,fluctuations in,T,i,will tend to be damped out due to the larger thermal capacitance of the tank contents.However,increased volume of tank age would be an expensive solution for an industrial plant due to the increased capital costs of the larger tank.Note that this approach is analogous to the use of water baths in chemistry laboratories where the large thermal capacitance of the bath serves as a heat sink and thus provides an isothermal environment for a small-scale research apparatus.,1.1.1 Illustrative Example,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.1,Introduction to Process Control,The control strategies for the stirred-tank heater are summarized in Table 1.1.1.,1.1.2 Classification of Control Strategies,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.1,Introduction to Process Control,It is informative to examine the effects of these various types of disturbances on the feed forward and feedback control strategies discussed above.,First,consider the feed forward control strategy of Method 2 where the disturbances in Ti are measured and the measurements are used to adjust the manipulated variable Q.,Next,we will consider how the feedback control strategy of Method 1 would perform in the presence of disturbances in Ti or w.,1.1.2 Classification of Control Strategies,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,Reading Material:,A schematic diagram of a general control system.,Overview of Control Engineering,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,Reading Material:,Many control systems include both types of signals:the real-valued signals that we will consider,and Boolean signals,such as fault or limit alarms and manual override switches,that we will not consider.,Overview of Control Engineering,System Design and Control Configuration,1,Control System Testing,Validation and Tuning,5,Modeling,2,Controller Design,3,Controller Implementation,4,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.2,What is Feedback and What are Its Effects?,As seen Eq.(1.2.1),feedback affects the gain G of a nonfeedback system by a factor 1+GH.,1.2.1 Effect of Feedback on Overall Gain,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.2,What is Feedback and What are Its Effects?,To investigate the effect of feedback on stability,we can again refer to the expression in Eq.(1.2.1).,1.2.2 Effect of Feedback on Stability,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.2,What is Feedback and What are Its Effects?,The input-output relation of the overall system is,1.2.2 Effect of Feedback on Stability,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.2,What is Feedback and What are Its Effects?,The sensitivity of the gain of the overall system,M,to the variation in G is defined as,1.2.3 Effect of Feedback on Sensitivity,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.2,What is Feedback and What are Its Effects?,By using Eq.(1.2.1),the sensitivity function is written,1.2.3 Effect of Feedback on Sensitivity,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.2,What is Feedback and What are Its Effects?,Let us refer to the system shown in Figure 1.2.3,in which r denotes the command signal and n is the noise signal.,1.2.3 Effect of Feedback on Sensitivity,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.2,What is Feedback and What are Its Effects?,In the absence of feedback,H=0,the output y due to n acting alone is,1.2.4 Effect of Feedback on External Disturbance or Noise,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.2,What is Feedback and What are Its Effects?,With the presence of feedback,the system output due to n acting alone is,1.2.4 Effect of Feedback on External Disturbance or Noise,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.3,Stability of Closed-Loop Control Systems,Consider the feedback control system shown in Figure 1.3.1 with the following transfer functions:,EXAMPLE,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.3,Stability of Closed-Loop Control Systems,Figure 1.3.1 Standard block diagram of a feedback control system.,EXAMPLE,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.3,Stability of Closed-Loop Control Systems,To determine the effect of Kc on the closed-loop response c(t),we consider a unit step change in set point,R(s)=1/s.We have derived the closed-loop transfer function for set-point changes:,Solution,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.3,Stability of Closed-Loop Control Systems,Substituting(1.3.1)and(1.3.2)into(1.3.3)and rearranging gives,Solution,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.3,Stability of Closed-Loop Control Systems,Definition of stability.,An unconstrained linear system is said to be stable if the output response is bounded for all bounded inputs.Otherwise it is said to be unstable.,1.3.1 General Stability Criterion,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.3,Stability of Closed-Loop Control Systems,Characteristic Equation,As a starting point for the stability analysis,consider the block diagram in Figure 1.3.1.Using block diagram algebra,we obtain,1.3.1 General Stability Criterion,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.3,Stability of Closed-Loop Control Systems,Characteristic Equation,For the moment consider set-point changes only,in which case Eq.(1.3.5)reduces to the closed-loop transfer function,1.3.1 General Stability Criterion,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.3,Stability of Closed-Loop Control Systems,Characteristic Equation,After a rearrangement it can be factored into poles(pi)and zeroes(zi)as,1.3.1 General Stability Criterion,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.3,Stability of Closed-Loop Control Systems,Characteristic Equation,we obtain,：,1.3.1 General Stability Criterion,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.3,Stability of Closed-Loop Control Systems,Characteristic Equation,General Stability Criterion.,The feedback control system in Figure is stable if and only if all roots of the characteristic equation are negative or have negative real parts.Otherwise,the system is unstable.,1.3.1 General Stability Criterion,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.3,Stability of Closed-Loop Control Systems,The Routh Stability Criterion is based on a characteristic equation that has the form,1.3.2 Routh Stability Criterion,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.3,Stability of Closed-Loop Control Systems,If all of the coefficients are positive,we next construct the following Routh array:,1.3.2 Routh Stability Criterion,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.3,Stability of Closed-Loop Control Systems,The elements in the remaining rows are calculated from the formulas:,1.3.2 Routh Stability Criterion,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.3,Stability of Closed-Loop Control Systems,Routh Stability Criterion.,A necessary and sufficient condition for all roots of the characteristic equation in Eq.,(1.3.12),to have negative real parts is that all of the elements in left column of the Routh array are positive.,1.3.2 Routh Stability Criterion,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.4,The Design Process of Control System,In principle,the flight control problem could be formulated as a general optimization problem:to minimize a mathematically specified performance index subject to mathematical constraints.In practice,however,this approach is all but doomed to failure,for many reasons,including the following:,Multiple objectives and hidden constraints:,It is rare that everything you want a system to do can be expressed in a single performance criterion to be optimized.,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.4,The Design Process of Control System,Number of state variables:,The number of state variables in a physical system is often larger than you want or need to deal with.,Disparate time scales:,Many processes entail phenomena that occur at widely disparate time scales.,Uncertain dynamics:,The dynamics of the process are generally not as well known as you would like.,Failure to provide insight:,Even with confidence in the dynamics and a mathemati-cally justifiable performance criterion,you may be unwilling to entrust the solution of the problem to a computer-generated optimization,because you may not be able to get an intuitive appreciation of the solution.,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.5,Controller Tuning,Similar guidelines are available for selecting the initial controller settings for the startup of a new plant.,1.5.1 Guidelines for Common Control Loops,Flow Control,1,Composition,5,Liquid Level,2,Gas Pressure,3,Temperature,4,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.5,Controller Tuning,Controller field tuning is often performed using trial and error procedures suggested by controller manufacturers.A typical approach for PID controllers can be summarized as follows:,Step 1.,Eliminate integral and derivative action by setting tD at its minimum value and tI at its maximum valueStep 2.,Set Kc at a low value(e.g.,0.5)and put the controller on automatic.,1.5.2 Trial and Error Tuning,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.5,Controller Tuning,Step 3.,Increase the controller gain Kc by small-increments until continuous cycling occurs after a small set-point or load change.The term“continuous cycling”refers to a sustained oscillation with constant amplitude.,Step 4.,Reduce Kc by a factor of two.,Step 5.,Decrease tI in small increments until continuous cycling occurs again.Set tI equal to three times this value.,1.5.2 Trial and Error Tuning,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.5,Controller Tuning,Step 3.,Increase the controller gain Kc by small-increments until continuous cycling occurs after a small set-point or load change.The term“continuous cycling”refers to a sustained oscillation with constant amplitude.,Step 4.,Reduce Kc by a factor of two.,Step 5.,Decrease tI in small increments until continuous cycling occurs again.Set tI equal to three times this value.,Step 6.,Increase tD until continuous cycling occurs.Set tD equal to one-third of this value.,1.5.2 Trial and Error Tuning,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.5,Controller Tuning,Table 1.5.1 Ziegler-Nichols Controller Settings Based on the Continuous Cycling Method,1.5.3 Continuous Cycling Method,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.5,Controller Tuning,More conservative settings are often preferable,such as the modified Z-N settings in Table 1.5.2.,1.5.3 Continuous Cycling Method,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.5,Controller Tuning,Typical process reaction curves:,1.5.4 Process Reaction Curve Method,CHAPTER 1,FUNDAMENTALS OF AUTOMATIC CONTROL,1.5,Controller Tuning,The Ziegler-Nichols tuning relations for the process reaction curve method are s