1、4.1 Introduction4.2 Connection of parts of a control system4.3 Objective of tuning the controller4.4 Tuning ways Controller design involves the selection of the control modes to be used and the determination of the values of parameters of each mode.For continuous systems,the proportional mode is usu
2、ally combined with the integral and/or derivative modes to form PI,PD and PID controller.The parameters for three modes are the proportional gain(),the integral time(),and the derivative time constant().PKITDT4.1 Introduction When the appropriate control mode has been chosen for the given controller
3、 in a control system,the two-thirds is only completed in the task of the control system design.The final part of the take is to connect the parts of the constitute parts of a control system together for to form a completely control system and chose the value of the parameters,the proportional gain()
4、,the integral time(),and the derivative time constant(),of the control modes chosen in the controller.PKITDTThe simplest design method is to specify a PID controller and then determine the control mode parameters in the field during start-up.This field adjustment is called“setting the controller”,an
5、d a procedure called the ultimate cycle method is used to determine the mode parameters.4.2 Connection of parts of a control system A closed-loop feedback control system must be a negative feedback control system.For obtaining a negative feedback control system,we ought to elect correctly every elem
6、ent constructing a control system.4.1.1 The symbol rule of every element constructing a control system When connecting every element and forming a control system,we must note the action way of every element constructing the control system.The action way of every element has usually two basic types,s
7、uch as the air-to-open or air-to-close of a valve,the direct-action or reverse-action of a controller and a positive or negative object and so on.The action way of every element is described with a symbol rule.1.Controller There are two action ways in controllers usually,the direct action way contro
8、ller and the reverse action way.When the algebra value of the input signal,the error signal,enlarging and the algebra value of the output signal,the control signal,enlarges immediately in a controller,the controller is the direct action controller.The direct way of controller is described with the s
9、ymbol of positive,“+”.When the algebra value of the input signal enlarging and the algebra value of the output signal decreases immediately in a controller,the controller is reverse action controller.the reverse action way of controller is described with the symbol of negative,“”.2.Control valve or
10、manipulating elementThere are two action ways in manipulating elements usually,the direct action way and the reverse action way.When the algebra value of the input signal,the control signal,enlarging,and the algebra value of the output signal,the manipulating variable,enlarges immediately in a manip
11、ulating element,the manipulating element is the direct action manipulating element.The direct way of manipulating element is described with the symbol of positive,“+”.When the algebra value of the input signal decreasing and the algebra value of the output signal decreases immediately in manipulatin
12、g element,the manipulating element is reverse action manipulating element.The reverse action way of manipulating element is described with the symbol of negative,“”In control valves,there are two types of the air-to-open and air-to-close valve.The action way of air-to-open is described with the symb
13、ol of positive,“+”,and the action way of air-to-close is described with the symbol of negative,“”.3.Process or controlled element There are two types in processes or elements usually,the positive process or element and the negative process or element.When the algebra value of the controlled variable
14、 in a process or controlled element increases following it to increase immediately for the quantity of energy or quality flowing into a process or controlled element,the process or controlled element is the positive and described with the symbol of positive,“+”.Contrarily the process or controlled e
15、lement is negative and described with the symbol of positive,“”.4.Measuring element There are two action ways in Measuring elements usually,the direct action way and the reverse action way.When the algebra value of the input signal,the controlled variable,enlarging and the algebra value of the outpu
16、t signal,the measuring value,enlarges immediately in a measuring element,the measuring element is the direct action measuring element.The direct way of measuring element is described with the symbol of positive,“+”.When the algebra value of the input signal decreasing and the algebra value of the ou
17、tput signal decreases immediately in measuring element,the measuring element is reverse action measuring element.The reverse action way of measuring element is described with the symbol of negative,“”.4.1.2 Criteria equation of symbol rule(controller“”)(manipulating element“”)(process“”)(measuring e
18、lement“”)=“”Notice:This equation is mainly used to determine the action way of a controller,because the symbol of a process lies on impersonal process and the action way of a manipulating element lies on the security of a process and measuring element is generally positive.When the process is positi
19、ve and the valve installed is an air-to-open in a system,the controller must be a direct action way controller.In other words,when the symbol of the process is same with the symbol of the valve in a control system,the symbol of the controller must be“”.Whereas the symbol of the process is reverse wo
20、th the symbolof the valve,the symbol of the controller is“+”.4.3 Objective of setting the controller.Objective:The objective of“setting the controller”is that we must choose proper value of the parameters of the control modes employed by the controller after the connection of a control system comple
21、ted in order to obtain a stable control system and the higher control quality that meets the control objectives.The control objectives may specify some or all of the following characteristics:1.The accuracy and speed of response of the measuring transmitter;2.The residual error allowed by the contro
22、ller after a load change;3.The response of the control system to a step change in load or setpoint.Quarter amplitude decay or critical damping may be specified;4.The maximum frequency limit.The control system is able to follow setpoint changes and disturbances with frequencies less than the maximum
23、limit;5.The response time,the rise time,and the settling time of the control system;6.Minimum cost.The initial cost of the hardware is only one part of this objective.Maintenance cost,operating cost,reject product costs,and environmental costs are examples of the other costs that may be a factor.Thi
24、s objective requires a judgment based on economic and social conditions.In tuning the controller,the objective of the choice parameters is to choose the reasonable controller parameters to make controller can attain above part or all of control target in control process.Stable MarginA control system
25、 not only is stable but also has the stable margin.The stable margin of a control system is described by and which are the damp-coefficient and decay-ratio of a control system.Assume,there is a stable oscillating function:,While:decay ratio:=.costy tAet22213111ttmmmtmyyeeeeye Where:the m is the deca
26、y exponent,m=/.The is the real part and the is the imaginative part in a pair conjugate complex root.The is the real part and the is the imaginative part in a pair conjugate complex root.This shows that the greater the is,the greater the m is.In other words,the greater the m is(or the is),the quicke
27、r the decay-function decays.Thus the rate of decay speed in decay response process can be described with the.The is oscillating decay frequency of the decay function.Because the greater oscillating frequency,the,can cause that the action of the actuator is frequently,the is not the greater value.In
28、the root complex plane,we may draw some lines to form the certain district limited,as above Figure,which is the objective district in that the closed-loop characteristic roots of the control system tuned should sit through“setting the controller”course.The objective district represented the request
29、to the control system.In the figure,the equal-line represents the quickness of a control system in regulating course,all of control systems have the same quickness,which have the closed-loop characteristic roots at the equal-line,and the equal-line represents the most high oscillating-frequency in t
30、he transient of a control system,all of control systems have the most high oscillating-frequency,which have the closed-loop characteristic roots at the equal-line,and the equal-m line represents the decay-rate in the transient of a control system,all of control systems have the same decay-rate in th
31、e transient,which have the closed-loop characteristic roots at the equal-m line.4.4 Tuning ways 4.3.1 The experience wayITDT parameterrangcontrolsystem(%)(min)(min)liquid-levelpressureflow-ratetemperature2080%3070%40100%2060%3100.110.430.31The experience way is method based on designer experience of
32、 control system.Taking value rang of three control parameters is listed in above table in various types of control systems.Taking value rang is a basis to make experience method establishes every kind of control parameter.Notice:In the table,the proportional control action is described with the para
33、meter,.is called the proportional-band and equal:=1/.From here then,the smaller the value of is,the stronger the proportional control action is.PKThe steps of the experience method are:1.The P controller:2.The PI controller:80%;(canceling the integral control mode);IT0(canceling the derivative contr
34、ol mode).Then:Gradually increasing =4:1(Quarter amplitude decay response course).again increasing 20%;0(canceling the derivative control mode).DT Then:taking grater value Gradually decreasing =4:1(Quarter amplitude decay response course).DTIT 3.The PID controller:again decreasing(1020)%;maintaining
35、the value;Then:0 Gradually increasing =4:1(Quarter amplitude decay response course).ITDT4.3.2 Critical Proportional-Degree Way(or Stable Boundary Way)In critical proportional-degree way,to set various control parameters is based on the critical proportional-degree and the critical equal-amplitude os
36、cillating cycle when the transient of a control system is critical oscillatory.parameterrangControl modeKProportional mode P ProportionalPlus Integral PIProportional plusDerivative plusIntegral PID22.2KK1.78 (min)(min)ITDTKT0.850.50.13KTKTThe steps of critical proportional-degree way:1.When a proces
37、s is stable,the control system tuned is taken from manual-control state into automatic-control state.The should is the greater value;2.Then the value of gradually decreases.(Oscillation gradually aggrandizes);3.Then,to change the setpoint-value with step change.(The change rang of the step change ap
38、proximately equal the 1030%of the stable value of a process.);4.To observe the response of controlled variable of the control system and record change value of the controlled variable and the value of,and then to determine whether the transient of the control system is the equal-amplitude oscillatin
39、g according to the dates recorded.If the transient of the system is not equal-amplitude oscillating one,we must repeat the steps(2)and(3)until the equal-amplitude oscillating transient appears;5.When the equal-amplitude oscillating transient appears,the value of then is the ,and the period of oscill
40、ation is the ;KKT6.We can determine the value of three control parameters corresponding to each control mode according to the table x.xx and the value of the and acquired through the above test.KTK4.3.3 Decay Response Graph Way In decay response graph way,to set various control parameters is based o
41、n the quarter amplitude decay oscillation proportional-degree and the quarter amplitude decay oscillating cycle when the transient of a control system is the quarter amplitude decay oscillating response.DK ProportionalPlus Integral PIProportional plusDerivative plusIntegral PID22.21.780.850.50.13DKD
42、K (min)(min)ITDTD KTD KTDKTParameterrangcontrolmodeProportional PThe steps of decay response graph way:1.When a process is stable,the control system tuned is taken from manual-control state into automatic-control state.The should is the greater value;2.Then,to change the setpoint-value with step cha
43、nge.(The change rang of the step change approximately equal the 1030%of the stable value of a process.);3.Then the value of gradually decreases.(Oscillation gradually aggrandizes);4.To observe the response of controlled variable of the control system and record change value of the controlled variabl
44、e and the value of,and then to determine whether the transient of the control system is the quarter-amplitude decay oscillatory according to the dates recorded.If the transient of the system is not quarter-amplitude decay oscillating one,we must repeat the steps(2)and(3)until the quarter-amplitude d
45、ecay oscillating transient appears;5.When the quarter-amplitude decay oscillating transient appears,the value of then is the ,and the period of oscillation is the ;DKDKT 6.We can determine the value of three control parameters corresponding to each control mode according to above table and the value
46、 of the and acquired through the above test.DKTDK4.3.4 dynamic characteristic parameters methodThe so-called dynamic state characteristic parameter method,it is the method in which the optimum control parameter of a controller is chosen through a set of empirical formulas according as the step-respo
47、nse of the control-channel of the broad sense open-process in a control system.The way has been brought by Ziegler and Nichols at 1942 year Wc(S)Wv(S)Wo(S)Wm(S)SPYThere is a closed-loop control system showed in figure.At input pots of manipulating-element and measuring-transmitter to break conjuncti
48、on of the system,we are bring to bear a step-signal at the input pots of manipulating-element and measure and record the output signal of the measuring-transmitter in the control system.According to these data of the output-signal obtained,we draw the step-response characteristic curve,and find out
49、the characteristic parameters of the process,0T -time delay,-time constant,and -response speed basing on the step-response characteristic curve.Thus According to the characteristic parameters of the process obtained,we may calculate the optimum setting control parameters of a controller basing on th
50、e formulas listed below.1.Non-regulating processWhen a broad sense open-process is a non-regulating process,of which step-response curve is illustrated in the following figure,we may use the setting formulas listed in the table 1 to calculate the setting-parameters of a controller.t0y(t)=tg0tx(t)The
