Lecture abstract EE C128 / ME C134 – Feedback Control Systemssojoudi/EEC128-chap10.pdf · 10 FR...

EE C128 / ME C134 – Feedback Control SystemsLecture – Chapter 10 – Frequency Response Techniques

Alexandre Bayen

Department of Electrical Engineering & Computer ScienceUniversity of California Berkeley

September 10, 2013

Bayen (EECS, UCB) Feedback Control Systems September 10, 2013 1 / 64

Lecture abstract

Topics covered in this presentation

I Advantages of FR techniques over RL

I Define FR

I Define Bode & Nyquist plots

I Relation between poles & zeros to Bode plots (slope, etc.)

I Features of 1st- & 2nd-order system Bode plots

I Define Nyquist criterion

I Method of dealing with OL poles & zeros on imaginary axis

I Simple method of dealing with OL stable & unstable systems

I Determining gain & phase margins from Bode & Nyquist plots

I Define static error constants

I Determining static error constants from Bode & Nyquist plots

I Determining TF from experimental FR data


Chapter outline

1 10 Frequency response techniques10.1 Introduction10.2 Asymptotic approximations: Bode plots10.3 Introduction to Nyquist criterion10.4 Sketching the Nyquist diagram10.5 Stability via the Nyquist diagram10.6 Gain margin and phase margin via the Nyquist diagram10.7 Stability, gain margin, and phase margin via Bode plots10.8 Relation between closed-loop transient and closed-loopfrequency responses10.9 Relation between closed- and open-loop frequency responses10.10 Relation between closed-loop transient and open-loopfrequency responses10.11 Steady-state error characteristics from frequency response10.12 System with time delay10.13 Obtaining transfer functions experimentally


10 FR techniques 10.1 Intro




Advantages of frequency response (FR) methods, [1, p.534]

In the following situations

I When modeling TFs from physical data

I When designing lead compensators to meet a steady-state errorrequirements

I When finding the stability of NL systems

I In settling ambiguities when sketching a root locus



The concept of FR, [1, p. 535]

I At steady-state, sinusoidalinputs to a linear systemgenerate sinusoidal responsesof the same frequency withdi↵erent amplitudes and phaseangle from the input, each ofwhich are a function offrequency.

I Phasor – complexrepresentation of a sinusoid

I ||G(!)|| – amplitudeI \G(!) – phase angleI M cos(!t+ �) ... M\� Figure: Sinusoidal FR: a. system; b.

TF; c. IO waveforms



The concept of FR, [1, p. 535]

I Steady-state output sinusoid

Mo(!)\�o(!)

= Mi(!)M(!)\(�i(!) + �(!))

I Magnitude FR

M(!) =Mo(!)

Mi(!)

I Phase FR

�(!) = �o(!)� �i(!)

I FRM(!)\�(!)

Figure: Sinusoidal FR: a. system; b.TF; c. IO waveforms



Analytical expressions for FR, [1, p. 536]

I General input sinusoid

r(t) = A cos(!t) +B sin(!t)

=

pA2

+B2cos

�!t� tan

�1�BA

��

I Input phasor formsI Polar, Mi\�i

Mi =

pA2

+B2

�i = � tan

�1�BA

�

I Rectangular, A� jBI Euler’s, Miej�i

Figure: System withsinusoidal input



Analytical expressions for FR, [1, p. 536]

I Forced response

C(s) =As+B!

s2 + !2G(s)

I Steady-state forced response after partial fraction expansion

Css(s) =MiMG

2 e�j(�i��G)

s+ j!+

MiMG2 ej(�i��G)

s� j!

where MG = ||G(j!)|| and �G = \G(j!)I Time-domain response

c(t) = MiMG cos(!t+ �i + �G)

I Time-domain response in phasor form

Mo\�o = (Mi\�i)(MG\�G)

I FR of systemG(j!) = G(s)|s!j!


10 FR techniques 10.2 Asymptotic approximations: Bode plots




History interlude

History (Hendrik Wade Bode)

I 1905 – 1982

I American engineer

I 1930s – Inventor of Bode plots,gain margin, & phase margin

I 1944 – WWII anti-aircraft(including V-1 flying bombs)systems

I 1947 – Cold War anti-ballisticmissiles

I 1957 – Served on NACA (nowNASA) with Wernher vonBraun (inventor of V-1 flyingbombs & V-2 rockets)

Figure: Hendrik Wade Bode



General Bode plots, [1, p. 540]

G(j!) = MG(!)\�G(!)

I Separate magnitude and phase plots as a function of frequencyI Magnitude – decibels (dB) vs. log(!), where dB = 20 log(M)

I Phase – phase angle vs. log(!)



Bode plots approximations, [1, p. 542]

I TFG(s) = s+ a

I Low frequencies

G(j!) ⇡ a\0�

I High frequencies

G(j!) ⇡ !\90�

I Asymptotes – straight-lineapproximations

I Low-frequencyI Break frequencyI High-frequency

Figure: Bode plots of s+ a: a.magnitude plot; b. phase plot



Simple Bode plots, [1, p. 542]

0

10

20

30

40

Ma

gn

itud

e (

dB

)

10−2

10−1

100

101

102

0

30

60

90

Ph

ase

(d

eg

)

Frequency (Hz)

Figure: Bode plot of s+aa

−40

−30

−20

−10

0

Ma

gn

itud

e (

dB

)

10−2

10−1

100

101

102

−90

−60

−30

0

Ph

ase

(d

eg

)

Frequency (Hz)

Figure: Bode plot of as+a




0

10

20

30

Ma

gn

itud

e (

dB

)

10−1

100

101

8989.289.489.689.8

9090.290.490.690.8

91

Ph

ase

(d

eg

)

Frequency (Hz)

Figure: Bode plot of s

−30

−20

−10

0

Ma

gn

itud

e (

dB

)

10−1

100

101

−91−90.8−90.6−90.4−90.2

−90−89.8−89.6−89.4−89.2

−89

Ph

ase

(d

eg

)

Frequency (Hz)

Figure: Bode plot of 1s




0

20

40

60

80

Ma

gn

itud

e (

dB

)

10−2

10−1

100

101

102

0

45

90

135

180

Ph

ase

(d

eg

)

Frequency (Hz)

Figure: Bode plot of s2+2⇣!ns+!2n

!2n

−80

−60

−40

−20

0

Ma

gn

itud

e (

dB

)

10−2

10−1

100

101

102

−180

−135

−90

−45

0

Ph

ase

(d

eg

)

Frequency (Hz)

Figure: Bode plot of !2n

s2+2⇣!ns+!2n



Detailed 2nd-order Bode plots, [1, p. 550]

Figure: Bode plot of s2+2⇣!ns+!2n

!2n

Figure: Bode plot of !2n

s2+2⇣!ns+!2n


10 FR techniques 10.3 Intro to Nyquist criterion




History interlude

History (Harry Theodor Nyquist)

I 1889 – 1976

I American engineer

I 1917 – 1934 AT&T

I 1934 – 1954 Bell TelephoneLabs

I 1924 – Nyquist-Shannonsampling theorem

I 1926 – Johnson–Nyquist noise

I 1932 – Nyquist stabilitycriterion Figure: Harry Theodor Nyquist



Introduction, [1, p. 559]

I Relates the stability of a CLsystem to the OL FR and theOL poles and zeros

I# CL poles in RHP

I Provides information on thetransient response andsteady-state error

Figure: CL control system



Derivation concepts, [1, p. 560]

G(s) =NG

DGand H(s) =

NH

DH

G(s)H(s) =NGHG

DGDH

F (s) = 1 +G(s)H(s) =DGDH +NGNH

DGDH

T (s) =G(s)

1 +G(s)H(s)=

NGNH

DGDH +NGNH

I Poles of 1 +G(s)H(s) are the same as the poles of the OL system,G(s)H(s)

I Zeros of 1 +G(s)H(s) are the same as the poles of the CL system,T (s)




I Map – function

I Contour – collection of points

I For our particular scenario,assume

F (s) =(s� z1)(s� z2)...

(s� p1)(s� p2)...

and a clockwise direction formapping the points on thecontour A

Figure: Mapping contour A throughfunction F (s) to contour B




I If F (s) has only zeros or onlypoles that are not encircled bythe contour then contour Bmaps in a clockwise direction

Figure: Contour mapping – withoutencirclements




I If F (s) has only zeros that areencircled by the contour thencontour B maps in a clockwisedirection

I If F (s) has only poles that areencircled by the contour thencontour B maps in acounterclockwise direction

I If F (s) has only poles or onlyzeros that are encircled by thecontour then contour B mapdoes encircle the origin

Figure: Contour mapping – withencirclements




I If F (s) has #poles

= #

zeros

that are encircled by thecontour then contour B mapdoes not encircle the origin Figure: Contour mapping – with

encirclements




I Each pole or zero of1 +G(s)H(s) whose vectorundergoes a complete rotationof contour A must yield achange of 360� in theresultant, R, or a completerotation of contour B

I A zero inside a CW contour Ayields a CW rotation ofcontour B

I A pole inside a CW contour Ayields a CCW rotation ofcontour B

I N = P � ZI N , # CCW rotations of

contour B about the originI P , # poles of 1 +G(s)H(s)

inside contour AI Z, # zeros of 1 +G(s)H(s)

inside contour A




Adjustment – extend the contour A to includethe entire RHP

I Z, # RHP CL polesI CL stability!

I P , # RHP OL polesI Easy

I N , # CCW rotations of contour B aboutorigin

I Di�cult

Adjustment – map G(s)H(s) instead of1 +G(s)H(s)

I N , # CCW rotations of contour B about�1

I Less di�cult

Figure: Contour enclosingRHP to determinestability



Definition, [1, p. 563]

Definition (Nyquist stability criterion)

I If a contour, A, that encircles the entire RHP is mapped through theOL system, G(s)H(s), then the # of RHP CL poles, Z, equals the #of RHP OL poles, P , minus the # of CCW revolutions, N , around�1 of the mapping.

Z = P �N

I The mapping is called the Nyquist diagram of G(s)H(s).

I FR technique because the mapping of points on the positive j!-axisthrough G(s)H(s) is the same as substituting s = j! into G(s)H(s)to form the FR function G(j!)H(j!).



Applying the Nyquist stability criterion, [1, p. 563]

I No RHP CL polesI P = 0

I N = 0

I Z = 0

I CL system is stable

I 2 RHP CL polesI P = 0

I N = �2

I Z = 2

I CL system is unstable Figure: Mapping examples – withencirclement: a. contour does notenclose CL poles; b. contour doesenclose CL poles


10 FR techniques 10.4 Sketching the Nyquist diagram




Method



Example, [1, p. 564]

Example (general)

I G(s) = 500(s+10)(s+3)(s+1)

Figure: Vector evaluation of theNyquist diagram: a. vectors on contourat low frequency, b. vectors on contouraround 1; c. Nyquist diagram




Example (poles on contour)

I G(s) = s+2s2

Figure: a. contour, b. Nyquist diagram


10 FR techniques 10.5 Stability via the Nyquist diagram





Example (general)

I G(s) = K(s+3)(s+5)(s�2)(s�4)

Figure: a. system; b. contour, c.Nyquist diagram




Example (general)

I G(s) = Ks(s+3)(s+5)

Figure: a. contour; b. Nyquist diagram



Stability via mapping only the positive j!-axis, [1, p. 571]


10 FR techniques 10.6 Gain margin & phase margin via the Nyquist diagram



10 FR techniques 10.6 Gain margin & phase margin via the Nyquist diagram

Definitions, [1, p. 574]

Two quantitative measures of howstable a system is

I Gain margin, GM – the changein OL gain, expressed in dB,required at 180� of phase shiftto make the CL systemunstable

I Phase margin, �M – thechange in OL phase shiftrequired at unity gain to makethe CL system unstable

Figure: Nyquist diagram showing gainand phase margins


10 FR techniques 10.7 Stability, gain margin, & phase margin via Bode plots




Stability via Bode plots, [1, p. 576]

Method

I Draw a Bode log-magnitude plot

I Determine the range of the gain that ensures that the magnitude isless than 0 dB (unity gain) at that frequency where the phase is±180�




Example (general)

I Use Bode plots to determinethe range of K within whichthe unity FB system is stable.

G(s) =k

(s+ 2)(s+ 4)(s+ 5)

Figure: Bode log-magnitude and phasediagrams



Gain & phase margin via Bode plots, [1, p. 578]

MethodI Gain margin

I Phase plot !!GM = !|�=180�

I At !GM , magnitude plot !gain margin, GM , which isthe gain required to raise themagnitude curve to 0 dB

I Phase marginI Magnitude plot !

!�M = !|G=0dBI At !�M , phase plot ! phase

margin, �M , which is thedi↵erence between the phasevalue and 180�

Figure: Gain and phase margins on theBode diagrams




Example (general)

I If K = 200, find the gain andphase margins.

G(s) =k

(s+ 2)(s+ 4)(s+ 5)

Figure: Bode log-magnitude and phasediagrams


10 FR techniques 10.8 Relation between CL transient & CL FRs




Damping ratio & CL FR, [1, p. 580]

Peak magnitude of the CL FR

Mp =1

2⇣p1� ⇣2

Frequency of the peak magnitude

!p = !n

p1� 2⇣2

Figure: 2nd-order CL system

Figure: CL FR peak vs. %OS for a 2pole system



Response speed & CL FR, [1, p. 581]

I Bandwidth of a 2-pole system

!BW = !n

q(1� 2⇣2) +

p4⇣4 � 4⇣2 + 2

I !n–Ts relation

!n =

4

Ts⇣

I !n–Tp relation

!n =

⇡

Tp

p1� ⇣2

I !n–Tr relationI Found using look-up table

Figure: Representativelog-magnitude plot



Response speed & CL FR, [1, p. 582]

Figure: Normalized bandwidth vs. damping ratio for:a. Ts, b. Tp; c. Tr


10 FR techniques 10.9 Relation between CL & OL FRs



10 FR techniques 10.9 Relation between CL & OL FRs

Constant M circles & constant N circles, [1, p. 583]

Skip for now


10 FR techniques 10.10 Relation between CL transient & OL FRs




Damping ratio from M circles, [1, p. 589]

Skip for now



Damping ratio from phase margin, [1, p. 589]

Skip for now



Response speed from OL FR, [1, p. 591]

Skip for now


10 FR techniques 10.11 Steady-state error characteristics from FR




Position constant, [1, p. 593]

Type 0 system

G(s) = K

Qni=1(s+ zi)Qmj=1(s+ pj)

Initial log-magnitude value

20 logM = 20 logKp

Position constant

Kp = K

Qni=1 ziQmj=1 pj

Figure: Typical unnormalized andunscaled Bode log-magnitude plotsshowing the value of static errorconstants



Velocity constant, [1, p. 594]

Type 1 system

G(s) = K

Qni=1(s+ zi)

sQm

j=1(s+ pj)


20 logM = 20 log

Kv

!0

Velocity constant

Kv = K

Qni=1 ziQmj=1 pj

Frequency axis intersect

! = Kv




Acceleration constant, [1, p. 595]

Type 2 system

G(s) = K

Qni=1(s+ zi)

s2Qm

j=1(s+ pj)

Acceleration constant

Ka = K

Qni=1 ziQmj=1 pj


20 logM = 20 log

Ka

!20

Frequency axis intersect

! =

pKa



10 FR techniques 10.12 System with time delay



10 FR techniques 10.12 System with time delay

Modeling time delay, [1, p. 597]

Time delay – delay between thecommanded response and the startof the output response

G0(s) = e�sTG(s)

FR

G0(j!) = e�j!TG(j!)

= |G(j!)|\[�!T + \G(j!)]Figure: E↵ect of delay upon FR


10 FR techniques 10.13 Obtaining TFs experimentally




Obtaining transfer functions experimentally, [1, p. 602]

Skip for now



Method

1. Estimate the pole-zero configuration from the Bode diagramsI Initial slope ! system typeI Phase excursions ! #poles & #zeros

2. Look for obvious 1st- & 2nd-order pole or zero FR characteristics

3. Peaking & depressions ! underdamped 2nd-order pole & zero,respectively

4. Extract 1st- & 2nd-order characteristicsI Overlay ±20 or ±40 dB/decade lines on magnitude curve &

±45�/decade lines on the phase curveI Estimate break frequenciesI For 2nd-order poles & zeros, estimate ⇣ & !n

5. Form a TF of unity gain using the poles & zeros foundI Subtract the FR of the model from the measured FR and repeat the

process if necessary



Bibliography

Norman S. Nise. Control Systems Engineering, 2011.


Lecture abstract EE C128 / ME C134 – Feedback Control Systemssojoudi/EEC128-chap10.pdf · 10 FR...

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