chapter six transient and steady state responses in ...gajic/psfiles/transr_ssr.pdf · actual and...
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Chapter Six
Transient and Steady State Responses
In control system analysisand design it is important to consider the
completesystemresponseandto designcontrollerssuchthata satisfactory
responseis obtainedfor all time instants � , where � standsfor the
initial time. It is known that the systemresponsehas two components:
transientresponseand steadystateresponse,that is
��� ���
Thetransientresponseis presentin theshortperiodof time immediately
after the systemis turnedon. If the systemis asymptoticallystable,the
transientresponsedisappears,which theoreticallycanbe recordedas
261
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��� ��However, if the systemis unstable,the transientresponsewill increase
very quickly (exponentially)in time, andin themostcasesthesystemwill
be practically unusableor even destroyedduring the unstabletransient
response(ascanoccur, for example,in someelectricalnetworks).
Even if the system is asymptotically stable, the transient response
shouldbecarefullymonitoredsincesomeundesiredphenomenalike high-
frequencyoscillations(e.g. in aircraft during landing and takeoff), rapid
changes,and high magnitudesof the output may occur.
Assuming that the systemis asymptoticallystable, then the system
responsein the long run is determinedby its steadystatecomponentonly.
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For control systemsit is important that steadystateresponsevaluesare
as closeas possibleto desiredones(specifiedones)so that we have to
studythecorrespondingerrors,which representthedifferencebetweenthe
actualanddesiredsystemoutputsat steadystate,andexamineconditions
underwhich theseerrorscanbe reducedor eveneliminated.
In Section6.1wefind analyticallytheresponseof asecond-ordersystem
due to a unit step input. The obtainedresult is usedin Section6.2 to
define importantparametersthatcharacterizethesystemtransientresponse.
For higher-ordersystems,only approximationsfor the transientresponse
parameterscan be obtainedusinga computer.The steadystateerrorsof
linearcontrolsystemsaredefinedin Section6.4,andthefeedbackelements
which help to reducethe steadystateerrorsto zeroare identified.
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6.1 Response of Second-Order Systems
Considerthe second-orderfeedbacksystemrepresented,in general,by
the block diagramgiven in Figure 6.1, where representsthe system
staticgain and is the systemtime constant.It is quite easyto find the
closed-looptransferfunction of this system,that is
� � �� � �
The closed-looptransferfunction canbe written in the following form
� � � � �
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U(s)
+ - s(Ts+� 1)K� Y(s)
Figure 6.1: Block diagram of a general second-order system
Quantities and � arecalled,respectively,the system damping ratio
andthe system natural frequency. The systemeigenvaluesaregiven by
����� � � � � �where � is the system damped frequency.
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The location of the systempoles and the relation betweendamping
ratio, naturalanddampedfrequenciesaregiven in Figure6.2.
+
+
Re{ s}
Im�
{ s}
λ�
2
λ�
1
ζωn
ω� n
θ� ω� d = ωn 1−ζ 2
− ζcos� θ =
Figure 6.2: Second-order system eigenvalues in terms of parameters � �
In the following we find the closed-loopresponseof this second-order
systemdue to a unit step input.
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Sincethe Laplacetransformof a unit stepis we have ! ! !
Dependingon the value of the dampingratio three interestingcases
appear:(a) the critically dampedcase, ; (b) the over-dampedcase,
; and (c) the under-dampedcase, .
Thesecasesaredistinguishedby the natureof the systemeigenvalues.
In case(a) the eigenvaluesaremultiple andreal, in (b) they are real and
distinct, and in case(c) the eigenvaluesarecomplexconjugate.
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(a) Critically Damped Case
For , wehaveadoublepoleat " . Thecorrespondingoutputis#"" # "
"" #
which after taking the Laplaceinverseproduces
$&%('*) " $&%+',)
The shapeof this responseis given in Figure6.3a,wherethe locationof
the systempoles - - # # is also presented.
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(b) Over-Damped Case
For the over-dampedcase,we havetwo real andasymptoticallystable
polesat . / . The correspondingclosed-loopresponseis easily
obtainedfrom
0. /
1. /
as
0 2436587(9;:<7>=@?6A 1 2B3C5D7(9E2F7(=G?CA
It is representedin Figure 6.3b.
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0 2 4 6 8 10 12 14 160
0.5
1
1.5
(c)
(a)
(b)
−2 −1 0−1
−0.5
0
0.5
1
(a)
p1=p2
−4 −2 0−1
−0.5
0
0.5
1
(b)
p1 p2
−2 0 2−2
−1
0
1
2
(c)
p1
p2
Figure 6.3: Responses of second-order systems and locations of system poles
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(c) Under-Damped Case
This caseis the most interestingand importantone. The systemhasa
pair of complexconjugatepolesso that in the -domainwe have
H IJ K
LIJ K
Applying the Laplacetransformit is easyto show(seeProblem6.1) that
the systemoutput in the time domainis given byM<N8O(P*Q
I J I
where from Figure 6.2 we have
I I
The responseof this systemis presentedin Figure6.3c.
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Theunder-dampedcaseis themostcommonin control systemapplica-
tions. A magnifiedfigureof thesystemstepresponsefor theunder-damped
caseis presentedin Figure 6.4.
y(t)R
tp ts ttr
0.10
OS
0S
0.900.951.001.05
Figure 6.4: Response of an under-damped second-order system
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6.2 Transient Response Parameters
The most important transientresponseparametersare denotedin Figure
6.4 asresponseovershoot,settling time, peaktime, andrise time.
Theresponseovershootcanbeobtainedby finding themaximumof the
function with respectto time. This leadsto
T U V WYX+Z\[ ] ]U V W@X(Z�[ ]
or
T ] ] ]
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which implies
^From this equationwe have
^The peak time is obtainedfor , i.e. as
_ ^ ` aand times for other minima and maximaare given by
b _ ^ ` aSince the steadystatevalue of is cYc , it follows that the
response overshoot is given by
_ cYc dfeYg+h*ikj d<e@g(h,ilj dm�noqp m�r
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Overshootis very often expressedin percent,so that we can define the
maximum percent overshoot ass tquv�w t�x
y
FromFigure6.4, theexpressionfor the response5 percentsettling time
can be obtainedas
z sf{Y|+}*~���
which for the standardvaluesof leadsto
z � � �Note that in practice .
The responserise time is definedas the time requiredfor the unit step
responseto changefrom 0.1 to 0.9 of its steadystatevalue.
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Therise time is inverselyproportionalto thesystembandwidth,i.e. the
wider bandwidth,the smallerthe rise time. However,designingsystems
with wide bandwidthis costly,which indicatesthatsystemswith very fast
responseare expensiveto design.
Example 6.1: Considerthe following second-ordersystem
�
We have
�� � �� � �
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The peak time is obtainedas
� �
and the settling time is found to be
� �
The maximumpercentovershootis equal to
� �q���� ���
The step response obtained using the MATLAB functions
[y,x]=step(num,den,t); t=0:0.1:5 is given in Figure6.5.
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0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
0.2
0.4
0.6
0.8
1
1.2
time t [sec]
y(t)
Figure 6.5: System step response for Example 6.1
It canbeseenthattheanalyticallyobtainedresultsagreewith theresults
presentedin Figure6.5. From Figure6.5 we areable to estimatethe rise
time, which in this caseis approximatelyequalto � .
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6.3 Transient Response of High-Order Systems
In theprevioussectionwehavebeenableto preciselydefineanddetermine
parametersthat characterizethe systemtransientresponse.This hasbeen
possibledue to the fact that the systemunderconsiderationhasbeenof
order two only. For higher-ordersystems,analyticalexpressionsfor the
systemresponseare not generallyavailable. However,in somecasesof
high-ordersystemsone is able to determineapproximatelythe transient
responseparameters.
A particularly important is the casein which an asymptoticallystable
systemhasa pair of complexconjugatepoles(eigenvalues)muchcloserto
the imaginaryaxis thanthe remainingpoles. This situationis represented
in Figure 6.6.
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Thesystempolesfar to theleft of theimaginaryaxishavelargenegative
realpartsso that theydecayvery quickly to zero(asa matterof fact, they
decayexponentiallywith ����� , where � are negativereal parts of the
correspondingpoles).Thus,the systemresponseis dominatedby the pair
of complexconjugatepolesclosestto the imaginaryaxis. Thesepolesare
called the dominant system poles.
Im�
{ s}
Re�
{ s}0�
λd
λd
Figure 6.6: Complex conjugate dominant system poles
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This analysiscan be also justified by using the closed-loopsystem
transferfunction. Consider,for example,a systemdescribedby its transfer
function as
Sincethe polesat –60and–70 arefar to the left, their contributionto the
systemresponseis negligible (they decayvery quickly to zero as �f�@�D�and � �G�Y� ). The transferfunction canbe formally simplified asfollows
¡�@�
¡�8�
¢
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Example 6.2: In this examplewe useMATLAB to comparethe step
responsesof the original andreduced-ordersystems.The resultsobtained
for and £ are given in Figure 6.7. It can be seenfrom this
figure thatstepresponsesfor theoriginal andreduced-order(approximate)
systemsalmost overlap.
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
0.05
0.1
0.15
0.2
0.25
time t [sec]
(1):
y(t)
, (2
):yr
(t)
(1)
(2)
Figure 6.7: System step responses for the original
(1) and reduced-order approximate (2) systems
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The correspondingMATLAB programis:
z=-1; p=[-3 —10 —60 —70]; k=12600;
[num,den]=zp2tf(z,p,k);
t=0:0.05:5; [y,x]=step(num,den,t);
zr=-1; pr=[-3 —10]; kr=3;
[numr,denr]=zp2tf(zr,pr,kr);
[yr,xr]=step(numr,denr,t);
plot(t,y,t,yr,’- —’);
xlabel(’time t [sec]’);
ylabel(’(1):y(t), (2):yr(t)’);
grid; text(0.71,0.16,’(1)’);
text(0.41,0.13,’(2)’);
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6.4 Steady State Errors
The responseof an asymptoticallystablelinear systemis in the long run
determinedby its steadystatecomponent.During the initial time interval
the transientresponsedecaysto zero so that in the remaining part of
the time interval the systemresponseis representedby its steadystate
componentonly. Control engineersare interestedin having steadystate
responsesas closeas possibleto the desiredonesso that we define the
so-calledsteadystateerrors,which representthedifferencesat steadystate
of the actualand desiredsystemresponses(outputs).
Beforewe proceedto steadystateerroranalysis,we introducea simpli-
fied versionof the basiclinear control systemproblemdefinedin Section
1.1.
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Simplified Basic Linear Control Problem
As definedin Section1.1 the basiclinear control problemis still very
difficult to solve. A simplified versionof this problemcanbe formulated
asfollows. Apply to thesysteminput a time functionequalto thedesired
systemoutput. This time function is known as the system’sreference
input and is denotedby . Note that . Comparethe
actualanddesiredoutputsby feedingbacktheactualoutputvariable.The
difference representsthe error signal. Use the error
signal togetherwith simple controllers(if necessary)to drive the system
underconsiderationsuchthat is reducedasmuchaspossible,at least
at steadystate.If a simplecontroller is usedin the feedbackloop (Figure
6.11) the error signal hasto be slightly redefined.
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In the following we use this simplified basic linear control problem
in order to identify the structureof controllers(feedbackelements)that
for certaintypesof referenceinputs(desiredoutputs)producezerosteady
stateerrors.
Considerthe simplestfeedbackconfiguration of a single-inputsingle-
output systemgiven in Figure 6.11.
G(s)
H(s)¤
U(s) = R(s) E(s)¥
+ -
Y(s)Controller - Plant
Feedback Element
Figure 6.11: Feedback system and steady state errors
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Let the input signal representtheLaplacetransformof the
desiredoutput (in this feedbackconfigurationthe desiredoutputsignal is
usedasan input signal); then for , we seethat in Figure6.11
the quantity representsthe differencebetweenthe desiredoutput
andthe actualoutput . In orderto be ableto reduce
this errorasmuchaspossible,we allow dynamicelementsin thefeedback
loop. Thus, as a function of hasto be chosensuchthat for the
given type of referenceinput, the error, now definedby
is eliminatedor reducedto its minimal valueat steadystate.
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From the block diagramgiven in Figure6.11 we have
so that the expressionfor the error is given by
Thesteadystateerror componentcanbe obtainedby usingthefinal value
theoremof the Laplacetransformas
¦Y¦ §©¨ ª ¦ ¨ « ¦ ¨ «This expressionwill be used in order to determinethe nature of the
feedbackelement such that the steadystate error is reducedto
zero for different typesof desiredoutputs. We will particularly consider
step,ramp,andparabolicfunctionsasdesiredsystemoutputs.
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Beforewe proceedto theactualsteadystateerroranalysis,we introduce
one additional definition.
Definition 6.1 The type of feedback control system is determinedby
the numberof polesof the open-loopfeedbacksystemtransferfunction
locatedat the origin, i.e. it is equalto , where is obtainedfrom
¬ ® ¬ ¯ °²±³®Now we considerthe steadystateerrorsfor different desiredoutputs,
namelyunit step,unit ramp,and unit parabolicoutputs.
Unit Step Function as Desired Output
Assumingthat our goal is that the systemoutput follows as close as
possiblethe unit stepfunction, i.e. , we have
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´Y´ ´�µ ¶
´Yµ ¶ ·
where · is known as the position constant
· ´�µ ¶It follows that the steadystateerror for the unit stepreferenceis reduced
to zero for · . We seethat this conditionis satisfiedfor .
Thus,we canconcludethat the feedbacktype systemof order at least
oneallows thesystemoutputat steadystateto track theunit stepfunction
perfectly.
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Unit Ramp Function as Desired Output
In this casethe steadystateerror is obtainedas
¸Y¸ ¸º¹ » ¸Y¹ » ¼
¸Y¹ » ½
where
½ ¸�¹ »is known asthe velocity constant. It follows that ½ , i.e. ¸�¸for . Thus, systemshaving two and more pure integrators(
terms)in the feedbackloop will be able to perfectly track the unit ramp
function as a desiredsystemoutput.
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Unit Parabolic Function as Desired Output
For a unit parabolicfunction we have ¾ so that
¿�¿ ¿�À Á ¾ ¿YÀ Á  Ãwherethe so-calledacceleration constant,
Ã, is definedby
à ¿YÀ ÁÂ
We can concludethatÃ
for , i.e. the feedbackloop must
have three pure integratorsin order to reducethe correspondingsteady
stateerror to zero.
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Example 6.5: The steadystateerrorsfor a systemthat hasthe open-
loop transfer function as
are Ä Å�ÅÆ Å�ÅÇ Å�Å
Sincetheopen-looptransferfunctionof this systemhasoneintegratorthe
outputof the closed-loopsystemcanperfectly track only the unit step.
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Example 6.6: Consider the second-ordersystem whose open-loop
transfer function is given by
Thepositionconstantfor this systemis È so that thecorrespond-
ing steadystateerror is
É�É È
The unit step responseof this systemis presentedin Figure 6.12, from
which it can be clearly seenthat the steadystateoutput is equal to ;
hencethe steadystateerror is equalto .
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0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
time t [sec]
y(t)
Figure 6.12: System step response for Example 6.6