3.2 bode plot - binils.com
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3.2 BODE PLOT
The Bode plot is a frequency response plot of the sinusoidal transfer function of a
system. One is a plot of the magnitude of a sinusoidal transfer function versus log ฯ. The
other is a plot of the phase angle of a sinusoidal transfer function versus log ฯ. The main
advantage of bode plot is that multiplication of magnitudes can be converted into
addition. Also, a simple method for sketching an approximate log-magnitude curve is
available. A Bode plot is a (semilog) plot of the transfer function magnitude and phase
angle as a function of frequency.
The gain magnitude is many times expressed in terms of decibels (dB) = 20 log10A.
Semilog sheet
Two sets of axes: gain on top, phase below (identical)
Logarithmic frequency axes
Gain axis is logarithmic โ either explicitly or as units of decibels(dB)
Phase axis is linear with units of degrees
Figure 3.2.1 Magnitude and phase plots of Bode plot
[Source: โLinear Control System Analysis and Design with MATLABโ by John J DโAzzo, Constantine, Stuart, Page: 318]
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BASIC FACTORS OF G(jฯ)
The basic factors that are very frequently occur in a typical transfer function G(jฯ) are,
1. Constant gain, K
2. Integral and derivative factors (๐๐)โ1
3. First-order factors (1 + ๐๐๐)โ1
4. Quadratic factors (1 + 2๐ (๐
Constant Gain, K
Let G(s)=K,
๐
๐๐
) + (
โ1 ๐ ) ) ๐๐
๐บ(๐๐) = ๐พ = ๐พโ 0๐
๐ด = |๐บ(๐๐)|๐๐ ๐๐ = 20 log ๐พ
๐ = โ ๐บ(๐๐) = 0๐
The magnitude plot for a constant gain K is a horizontal straight line at the magnitude of
20log K db. The phase plot is a straight line at 0o.
Integral Factor
Let G(s)=K/s,
Figure 3.2.2 Bode plot of constant gain, K
[Source: โControl Systemsโ by A Nagoor Kani, Page: 3.10]
๐พ ๐บ(๐๐) =
๐๐
= ๐พ โ โ 90๐ ๐
2
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๐พ ๐ด = |๐บ(๐๐)|๐๐ ๐๐ = 20 log ( )
๐
๐ = โ ๐บ(๐๐) = โ90๐
The magnitude plot of the integral factor is a straight line with the slope of -20db/dec and
passing through zero db when ฯ=K. The phase plot is a straight line at -90o.
Derivative factor
Let G(s)=Ks,
Figure 3.2.3 Bode plot of integral factor, K/jฯ
[Source: โControl Systemsโ by A Nagoor Kani, Page: 3.11]
๐บ(๐๐) = ๐พ๐๐ = ๐พ๐โ 90๐
๐ด = |๐บ(๐๐)|๐๐ ๐๐ = 20 log(๐พ๐)
๐ = โ ๐บ(๐๐) = +90๐
The magnitude plot of the integral factor is a straight line with the slope of 20db/dec and
passing through zero db when ฯ=K. The phase plot is a straight line at +90o.
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Figure 3.2.4 Bode plot of derivative factor, K x jฯ
[Source: โControl Systemsโ by A Nagoor Kani, Page: 3.11]
First order factor in denominator
Let ๐บ(๐ ) = 1
1+๐ ๐
1 ๐บ(๐๐) =
1 + ๐๐๐
1 =
โ1 + ๐2๐2 โ โ tanโ1 ๐๐
1
๐ด = |๐บ(๐๐)|๐๐ ๐๐ = 20 log ( ) โ1 + ๐2๐2
๐ = โ ๐บ(๐๐) = โ โ tanโ1 ๐๐
The magnitude plot of the first order factor can be approximated by two straight lines,
one is a straight line at zero db for the frequency range, 0<ฯ<1/T, and the other is a
straight line with slope -20db/dec for the frequency range, 1/T<ฯ<โ. The corner
frequency is ฯc=1/T and the loss in db at the corner frequency is -3db. The phase angle
of the first order factor varies from 0o to -90o as ฯ is varied from zero to infinity. The
phase plot is a curve passing through -45o at ฯc.
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Figure 3.2.5 Bode plot of first order factor in denominator, 1/(1+jฯT)
[Source: โControl Systemsโ by A Nagoor Kani, Page: 3.13]
First order factor in numerator
Let ๐บ(๐ ) = 1 + ๐ ๐
๐บ(๐๐) = 1 + ๐๐๐ = โ1 + ๐2๐2โ tanโ1 ๐๐
๐ด = |๐บ(๐๐)|๐๐ ๐๐ = 20 log (โ1 + ๐2๐2)
๐ = โ ๐บ(๐๐) = โ tanโ1 ๐๐
The magnitude plot of the first order factor can be approximated by two straight lines,
one is a straight line at zero db for the frequency range, 0<ฯ<1/T, and the other is a
straight line with slope 20db/dec for the frequency range, 1/T<ฯ<โ. The corner
frequency is ฯc=1/T and the loss in db at the corner frequency is +3db. The phase angle
of the first order factor varies from 0o to +90o as ฯ is varied from zero to infinity. The
phase plot is a curve passing through +45o at ฯc.
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๐
Figure 3.2.6 Bode plot of first order factor in numerator, (1+jฯT)
[Source: โControl Systemsโ by A Nagoor Kani, Page: 3.14]
Quadratic factor in denominator
Second order closed loop transfer function is given by
๐บ(๐ ) =
2 ๐
๐ 2 + 2๐๐๐๐ + ๐2
( ๐ 2
1 ๐
๐ ) ๐๐
1
+ 2๐ ๐๐ + 1 1
๐บ(๐๐) = ๐๐ 2
( ) ๐๐
+ 2๐
๐๐
๐๐ + 1
= ๐ 2
โ (๐๐)
+ 2๐ ๐๐
+ 1 ๐๐
๐บ(๐๐) = 1
โ โtanโ1 (
2๐ ๐
๐๐ )
๐2
โ(1 โ ๐2
) + 4๐2 ๐2
1 โ ๐ 2
๐๐2 ๐๐
2
At low frequencies when ฯ<<ฯn, the magnitude is,
๐ด = โ20 log โ1 โ ๐2
(2 โ 4๐2) + ๐๐
๐4
4 โ โ20 log 1 = 0
๐
At high frequencies when ฯ>>ฯn, the magnitude is,
๐ด = โ20 log โ1 โ ๐2
(2 โ 4๐2) + ๐๐
๐4
๐๐4
๐
2 ๐
2
=
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๐ด โ โ20 log โ
๐4
4 = โ 20 log
๐2 ๐ 2
2 = โ20 log ( )
๐๐ ๐๐
2๐
๐
๐๐
๐ = โ ๐บ(๐๐) = โ tanโ1 ( ๐๐ )
1 โ ๐2
๐๐2
The magnitude plot of the quadratic factor in the denominator can be approximated by
two straight lines, one is a straight line at zero db for the frequency range, 0<ฯ< ฯn, and
the other is a straight line with slope -40db/dec for the frequency range, ฯn<ฯ<โ. The
frequency at which the two asymptotes meet is called the corner frequency. For the
quadratic factor, the frequency, ฯn is the corner frequency, ฯc. The phase angle of the
quadratic factor varies from 0o to -180o as ฯ is varied from zero to infinity. The phase
plot is a curve passing through -90o at ฯc. At the corner frequency, phase angle is -90o
and independent of ๐, but at all other frequency it depends on ๐.
Figure 3.2.7 Bode plot of quadratic factor in denominator
[Source: โControl Systemsโ by A Nagoor Kani, Page: 3.15]
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๐
Quadratic factor in the numerator
๐ 2 + 2๐๐๐๐ + ๐2
๐ 2 ๐
๐บ(๐ ) =
๐๐ 2
2 ๐
๐๐
๐ = ( ) ๐๐
๐
+ 2๐
2
๐๐
+ 1
๐๐
๐บ(๐๐) = ( ) ๐๐
+ 2๐
๐๐
+ 1 = โ ( ) ๐๐
+ 2๐ + 1 ๐๐
๐2 ๐บ(๐๐) = โ(1 โ
๐2
) + 4๐2
โ tanโ1 ( 2๐
๐
๐๐ )
๐๐2 ๐๐
2 1 โ
๐2
๐๐2
At low frequencies when ฯ<<ฯn, the magnitude is,
๐ด = 20 log โ1 โ ๐2
(2 โ 4๐2) + ๐๐
๐4
4 โ 20 log 1 = 0
๐
At high frequencies when ฯ>>ฯn, the magnitude is,
๐ด = 20 log โ1 โ ๐2
(2 โ 4๐2) + ๐๐
๐4
๐๐4
๐4 ๐2 ๐ 2
๐ด โ 20 log โ ๐๐
4 = 20 log
๐ 2
= 20 log ( ) ๐
2๐ ๐
๐ = โ ๐บ(๐๐) = tanโ1 ( ๐๐ )
1 โ ๐2
๐๐2
The magnitude plot of the quadratic factor in the denominator can be approximated by
two straight lines, one is a straight line at zero db for the frequency range, 0<ฯ< ฯn, and
the other is a straight line with slope +40db/dec for the frequency range, ฯn<ฯ<โ. The
frequency at which the two asymptotes meet is called the corner frequency. For the
quadratic factor, the frequency, ฯn is the corner frequency, ฯc. The phase angle of the
quadratic factor varies from 0o to +180o as ฯ is varied from zero to infinity. The phase
plot is a curve passing through +90o at ฯc. At the corner frequency, phase angle is +90o
and independent of ๐, but at all other frequency it depends on ๐.
๐
2 ๐
2
๐
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Figure 3.2.8 Bode plot of quadratic factor in numerator
[Source: โControl Systemsโ by A Nagoor Kani, Page: 3.16]
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PROCEDURE FOR MAGNITUDE PLOT OF BODE PLOT
Step 1: Convert the transfer function into Bode form or time constant form.
Step 2: List the corner frequencies in the increasing order and prepare a table as shown
Term Corner frequency
rad/sec
Slope
db/dec
Change in Slope
db/dec
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In the above table, enter K or K/(jฯ)n or K(jฯ)n as the first term and the other terms in the
increasing order of corner frequencies. Then enter the corner frequency, slope contributed
by each term and change in slope at every corner frequency.
Step 3: Choose an arbitrary frequency ฯl which is lesser than the lowest corner frequency.
Calculate the db magnitude of K or K/(jฯ)n or K(jฯ)n at ฯl and at the lowest corner
frequency.
Step 4: Then calculate the gain (db magnitude) at every corner frequency one by one by
using the formula,
Gain at ฯy = change in gain from ฯx to ฯy + Gain at ฯx
Ay = (Slope from ฯx to ฯy x log(ฯy/ ฯx) + Gain at ฯx
Step 5: Choose an arbitrary frequency ฯh which is greater than the highest corner
frequency. Calculate the gain at ฯh by using the formula in step 4.
Step 6: In a semilog graph sheet mark the required range of frequency on x-axis (log
scale) and the range of db magnitude on y-axis (ordinary scale) after choosing proper
units.
Step 7: Mark all the points obtained in steps 3, 4, 5 on the graph and join the points by
straight lines. Mark the slope at every part of the graph.
PROCEDURE FOR PHASE PLOT OF BODE PLOT
The phase plot is an exact plot obtained with exact phase angles of G(jฯ) computed
for various values of ฯ and is then tabulated. The choice of frequencies are preferably the
frequencies chosen for magnitude plot. Usually the magnitude plot and phase plot are
drawn in a single semilog sheet on a common frequency scale. Take another y-axis in the
graph where the magnitude plot is drawn and, in this y-axis, mark the desired range of
phase angles after choosing proper units. From the tabulated values of ฯ and phase angles,
mark all the points on the graph. Join the points by a smooth curve.
DETERMINATION OF GAIN MARGIN AND PHASE MARGIN FROM BODE PLOT
The gain margin in db is given by the negative of db magnitude of G(jฯ) at the
phase crossover frequency, ฯpc. The ฯpc is the frequency at which phase of G(jฯ) is 180o.
if the db magnitude of G(jฯ) at ฯpc is negative then gain margin is positive and vice versa.
Let ฮฆgc be the phase angle of G(jฯ) at gain cross over frequency, ฯgc. The ฯgc is
the frequency at which the db magnitude of G(jฯ) is zero. Now the phase margin, ฮณ is
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given by, ฮณ = 180o+ ฮฆgc. If ฮฆgc is less negative than -180o, then phase margin is positive
and vice versa. The positive and negative gain margins and phase margins are illustrated
in figure 3.2.9.
Figure 3.2.9 Gain margin and Phase margin in Bode plot
[Source: โControl Systemsโ by A Nagoor Kani, Page: 3.20]
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๐
๐
๐
๐
2
3.5 CORRELATION BETWEEN FREQUENCY DOMAIN AND TIME DOMAIN
SPECIFICATIONS
For a second order system,
Put s=jฯ
๐ถ(๐ )
๐ (๐ )
2
= ๐
๐ 2 + 2๐๐๐๐ + ๐2
๐ถ(๐๐)
๐ (๐๐)
2
= ๐
โ๐2 + 2๐๐๐๐๐ + ๐2
๐ถ(๐๐) = ๐ (๐๐)
1
๐2 ๐
Let ๐ข =
๐ , then
๐๐
โ ๐2 + 2๐๐ ๐๐
+ 1
๐ถ(๐๐) 1
We know,
Now,
๐ (๐๐) =
(1 โ ๐ข2) + 2๐๐๐ข
๐(๐๐) = |๐(๐๐)|โ ๐(๐๐)
1 |๐(๐๐)| =
โ(1 โ ๐ข2)2 + (2๐๐ข)2
2๐๐ข ๐ = โ tanโ1 ( )
1 โ ๐ข
1
๐๐ =
2๐โ1 โ ๐2
๐๐ = ๐๐ โ1 โ 2๐2
๐๐ = ๐๐ โ1 โ 2๐2 + โ4๐4 โ 4๐2 + 2
๐๐ = โ180๐ + ๐
where,
2๐
๐
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๐ = tanโ1
(
โโ4๐2 + 1 โ 2๐2
)
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(
)
3.4 DETERMINATION OF CLOSED LOOP RESPONSE FROM OPEN LOOP
RESPONSE
M and N circles
Peak magnitude
๐ถ(๐๐) ๐๐ = 20 log |
๐ (๐๐)| ๐๐ต
where, 3 dB is considered good.
M-CIRCLES
๐(๐๐) =
๐บ(๐๐)
1 + ๐บ(๐๐)
๐บ(๐๐) = ๐ + ๐๐
๐(๐๐)
=
๐ + ๐๐
1 + ๐ + ๐๐
โ๐2 + ๐2โ tanโ1 ๐
= ๐
โ(1 + ๐)2 + ๐2โ tanโ1 ( ๐
1 + ๐
โ๐2 + ๐2 = โ tanโ1
โ(1 + ๐)2 + ๐2
Let, M = Magnitude of M(jฯ)
๐ ( ) โ tanโ1 ๐
๐ ( )
1 + ๐
โ๐2 + ๐2 |๐(๐๐)| =
โ(1 + ๐)2 + ๐2
๐2(1 + ๐)2 + ๐2๐2 = ๐2 + ๐2
๐2(1 โ ๐2) + (1 โ ๐2)๐2 โ 2๐2๐ = ๐2
๐2 + ๐2 โ 2
๐2
(1 โ ๐2)
๐ = ๐2
(1 โ ๐2)
Adding (
๐2
2
2 ) on both sides, we get,
(1โ๐ )
(๐ โ
๐2
2
๐ 2
) + ๐2 = ( )
(1 โ ๐2) (1 โ ๐2)
The above equation represents a family of circles with its
)
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centre at ( ๐2
(1โ๐2)
, 0) and radius ๐ (1โ๐2)
Family of M-circles corresponding to the closed loop magnitudes, M of a unit feedback
system is given by the figure 3.4.1.
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N-CIRCLES
Figure 3.4.1 Constant M-circles in the polar co-ordinates
[Source: โAutomatic Control Systemsโ by Benjamin C. Kuo, Page: 487]
โ ๐บ(๐๐)
โ ๐(๐๐) = ๐ผ = โ (1 + ๐บ(๐๐))
๐ผ = tanโ1 ๐
โ tanโ1 ๐
๐ 1 + ๐
tan ๐ผ = ๐ = tan (tanโ1 ๐
โ tanโ1 ๐
)
We know,
๐ 1 + ๐
tan(๐ด โ ๐ต) =
tan ๐ด โ tan ๐ต 1
+ tan ๐ด tan ๐ต
๐ N = (
๐2 + ๐ + ๐2)
1 2
(๐ + ) 2
+ (๐ โ
1 2
) 2๐
1 = + (
4
1 2
) 2๐
The above equation represents the family of circles with its
Centre at (โ 1 ,
1 ) and radius โ
1 + (
1 )
2 2๐ 4 2๐
2
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Figure 3.4.2 Constant N-circles in the polar co-ordinates
[Source: โAutomatic Control Systemsโ by Benjamin C. Kuo, Page: 490]
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3.1 FREQUENCY RESPONSE
The response of a system for the sinusoidal input is called sinusoidal response. The
ratio of sinusoidal response to sinusoidal input is called sinusoidal transfer function of
the system and in general, it is denoted by, T(jฯ). The sinusoidal transfer function is the
frequency domain representation of the system and so it is also called frequency domain
transfer function.
The frequency domain transfer function T(jฯ) is a complex function of ฯ. Hence,
it can be separated into magnitude function and phase function. Now, the magnitude and
phase functions will be real functions of ฯ and they are called frequency response.
The frequency response can be evaluated for open loop system and closed loop
system. The frequency domain transfer function of open loop and closed loop systems
can be obtained from the s-domain transfer function by replacing โsโ by jฯ as shown:
Open loop transfer function: ๐ฎ(๐๐) = |๐ฎ(๐๐)|โ ๐ฎ(๐๐)
Loop transfer function: ๐ฎ(๐๐)๐ฏ(๐๐) = |๐ฎ(๐๐)๐ฏ(๐๐)|โ ๐ฎ(๐๐)๐ฏ(๐๐)
Closed loop transfer function: ๐ด(๐๐) = |๐ด(๐๐)|โ ๐ด(๐๐)
The advantages of frequency response analysis are the following:
1. The absolute and relative stability of the closed loop system can be estimated
from the knowledge of their open loop frequency response.
2. The practical testing of systems can be easily carried with available sinusoidal
signal generators and precise measurement equipments.
3. The transfer function of complicated systems can be determined experimentally
by frequency response tests.
4. The design and parameter adjustment of the open loop transfer function of a
system for specified closed loop performance is carried out more easily in
frequency domain.
5. When the system is designed by the use of frequency response analysis, the
effects of noise disturbance and parameter variations are relatively easy to
visualize and incorporate corrective measures.
6. The frequency response analysis and designs can be extended to certain non-
linear control systems.
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The frequency response of a system is a frequency dependent function which expresses
how a sinusoidal signal of a given frequency on the system input is transferred through
the system. Time-varying signals at least periodical signals โ which excite systems, as
the reference (set point) signal or a disturbance in a control system or measurement
signals which are inputs signals to signal filters, can be regarded as consisting of a sum
of frequency components. Each frequency component is a sinusoidal signal having
certain amplitude and a certain frequency. (The Fourier series expansion or the Fourier
transform can be used to express these frequency components quantitatively.) The
frequency response expresses how each of these frequency components is transferred
through the system. Some components may be amplified, others may be attenuated, and
there will be some phase lag through the system. The frequency response is an important
tool for analysis and design of signal filters (as low pass filters and high pass filters), and
for analysis, and to some extent, design, of control systems. Both signal filtering and
control systems applications are described (briefly) later in this chapter. The definition of
the frequency response โ which will be given in the next section โ applies only to linear
models, but this linear model may very well be the local linear model about some
operating point of a non-linear model. The frequency response can be found
experimentally or from a transfer function model. It can be presented graphically or as a
mathematical function.
FREQUENCY DOMAIN SPECIFICATIONS
The performance and characteristics of a system in frequency domain are measured
in terms of frequency domain specifications. The requirements of a system to be designed
are usually specified in terms of these specifications.
The frequency domain specifications are,
a) Resonant peak, Mr
b) Resonant frequency, ฯr
c) Bandwidth, ฯb
d) Cut-off rate
e) Gain margin, Kg
f) Phase margin, ฮณ
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๐
๐
๐
๐
๐
๐
โ
FREQUENCY DOMAIN SPECIFICATIONS OF SECOND OREDER SYSTEM
Resonant peak, Mr
The maximum value of the magnitude of closed loop transfer function is called the
resonant peak, Mr. A large resonant peak corresponds to a large overshoot in transient
response. Consider the closed loop transfer function of second order system,
๐ถ(๐ )
๐ (๐ )
= ๐(๐ ) =
2 ๐
๐ 2 + 2๐๐๐๐ + ๐2
The sinusoidal transfer function M(jฯ) is obtained by letting s=jฯ.
๐2
๐(๐๐) = ๐
(๐๐)2 + 2๐๐๐(๐๐) + ๐2
2
= ๐
โ๐2 + 2๐๐๐(๐๐) + ๐2
2
= ๐
๐2 (โ ๐2
+ 2๐๐ ๐
+ 1)
๐ 2 ๐
1
๐๐
1 โ ( ๐
๐๐
2 + 2๐๐ ๐
๐๐
Let normalized frequency, ๐ข = ( ๐
), ๐๐
1
๐(๐๐) = 1 โ ๐ข2 + 2๐๐๐ข
Let, M โ Magnitude of closed loop transfer function
ฮฑ โ Phase of closed loop transfer function
๐ = |๐(๐๐)| = [(1 โ
๐ข2)2
+ 4๐2 1
๐ข2] 2
๐ผ = โ ๐(๐๐) = โ tanโ1 2๐๐ข
1 โ ๐ข2
The resonant peak is the maximum value of M. The condition for maximum value of M
can be obtained by differentiating the equation of M with respect to u and letting
(dM/du=0) when (u=ur) with normalized frequency, ๐ข๐ = ๐๐. ๐
๐
)
=
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โ
On differentiating โMโ with respect to โuโ, we get,
๐๐
๐๐ข
๐ = ๐๐ข
[1 โ ๐ข2
1
+ 2๐๐๐ข] 2
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1 = โ [1 โ ๐ข2
2
+ 2๐๐๐ข]
3
2[2(1 โ ๐ข2 )(โ2๐ข) + 8๐2
๐ข]
[โ4๐ข(1 โ ๐ข2) + 8๐2๐ข] = โ 3
2[(1 โ ๐ข2)2 + 4๐2๐ข2]2
[4๐ข(1 โ ๐ข2) โ 8๐2๐ข] = โ 3
2[(1 โ ๐ข2)2 + 4๐2๐ข2]2
Replacing u by ur and equating dM/du to zero,
[4๐ข๐(1 โ ๐ข๐ 2) โ 8๐2๐ข๐]
3 = 0
2[(1 โ ๐ข๐ 2)2 + 4๐2๐ข๐
2]2
4๐ข๐(1 โ ๐ข๐ 2) โ 8๐2๐ข๐ = 0
4๐ข๐ โ 4๐ข๐ 3 โ 8๐2๐ข๐ = 0
4๐ข๐ โ 4๐ข๐ 3 = 8๐2๐ข๐
4๐ข๐ 3 = 4๐ข๐ โ 8๐2๐ข๐
๐ข๐ 2 = 1 โ 2๐2
๐ข๐ = โ1 โ 2๐2
Therefore, the resonant peak occurs when ๐ข๐ = โ1 โ 2๐2
On substituting for M with M=Mr and u=ur,
1 1 ๐๐ = 1
=
[(1 โ ๐ข๐ 2)2 + 4๐2๐ข๐
2]2
[(1 โ
(1 โ 2๐
2))
+ 4๐
2(1 โ 2๐
1
2)]2
1 = 1 =
1 1
= 1 1
1 =
2๐โ1 โ ๐2
[4๐4 + 4๐2 โ 8๐4]2 [4๐2 โ 4๐4]2
1
[4๐2(1 โ ๐2)]2
Resonant frequency, ฯr
๐๐ = 2๐โ1 โ ๐2
The frequency at which the resonant peak occurs is called resonant frequency, ฯr.
This is related to the frequency of oscillation in the step response and thus it is indicative
โ
2
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of the speed of transient response.
Normalized resonant frequency,
๐ข๐
๐๐
= ๐๐
= โ1 โ 2๐2
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Bandwidth, ฯb
๐๐ = ๐๐โ1 โ 2๐2
The bandwidth is the range of frequencies for which the system normalized gain is
more than -3db. The frequency at which the gain is -3db is called cut-off frequency.
Bandwidth is usually defined for closed loop system and it transmits the signals whose
frequencies are less than the cut-off frequency. The bandwidth is a measure of the ability
of a feedback system to reproduce the input signal, noise rejection characteristics and rise
time. A large bandwidth corresponds to a small rise time or fast response.
Let, normalized bandwidth,
๐๐
๐ข๐ = ๐
When u=ub, the magnitude M, of the closed loop system is 1/โ2 or (-3db) On
substituting for M with u=ub and equating it to 1/โ2
๐ = 1 1
1 =
โ2
[(1 โ ๐ข๐2)2 + 4๐2๐ข๐2]2
On squaring and cross multiplying, we get,
(1 โ ๐ข๐2)2 + 4๐2๐ข๐
2 = 2
1 + ๐ข๐4 โ 2๐ข๐
2 + 4๐2๐ข๐2 = 2
๐ข๐4 โ 2๐ข๐
2(1 โ 2๐2) โ 1 = 0
Let ๐ฅ = ๐ข๐2,
Hence,
๐ฅ2 โ 2๐ฅ(1 โ 2๐2) โ 1 = 0
๐ฅ = 2(1 โ 2๐2) ยฑ โ4(1 โ 2๐2)2 + 4
= 2
2(1 โ 2๐2) ยฑ 2โ(1 + 4๐4 โ 4๐2) + 1
2
Let us take only the positive sign,
๐ฅ = 1 โ 2๐2 + โ(2 + 4๐4 โ 4๐2)
But, ๐ข๐ = โ๐ฅ
1
๐ข๐ = [1 โ 2๐2 + โ(2 + 4๐4 โ 4๐2)]2
Also, ๐ข๐ = ๐๐
๐๐
๐
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๐๐
๐๐
Cut-off rate
1
๐๐ = ๐๐ [1 โ 2๐2 + โ(2 + 4๐4 โ 4๐2)]2
The slope of the log-magnitude curve near the cut-off frequency is called cut-off
rate. The cut-off rate indicates the ability of the system to distinguish the signal from
noise.
Gain margin, Kg
The gain margin, Kg is defined as the value of gain, to be added to system, in order
to bring the system to the verge of instability. The gain margin is given by the reciprocal
of the magnitude of open loop transfer function at phase crossover frequency.
The frequency at which the phase of open loop transfer function is 180o is called
the phase crossover frequency, ฯpc.
1 ๐พ๐ =
|๐บ(๐๐ )|
1 ๐พ๐๐๐ ๐๐ = 20 log ๐พ๐ = 20 log
|๐บ(๐๐ )|
The gain margin in db is given by the negative of the db magnitude of G(jฯ) at
phase crossover frequency. The gain margin indicates the additional gain that can be
provided to system without affecting the stability of the system.
[Note: The gain margin of second order system is infinite].
Phase margin, ฮณ
The phase margin, ฮณ is defined as the additional phase lag to be added at the gain
crossover frequency in order to bring the system to the verge of instability.
The gain crossover frequency, ฯgc is the frequency at which the magnitude of the
open loop transfer function is unity (or it is the frequency at which the db magnitude is
zero).
The phase margin is obtained by adding 180o to the phase angle, ๐ of the open loop
transfer function at the gain crossover frequency. The phase margin indicates the
additional phase lag that can be provided to the system without affecting stability.
๐2
๐บ(๐ ) = ๐
๐ (๐ + 2๐๐๐)
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Put s=jฯ,
๐2
๐2 1
๐บ(๐๐) = ๐
= ๐ ๐ ๐ = ๐ ๐
๐๐(๐๐ + 2๐๐๐)
Let normalized frequency, ๐ข = ๐
๐๐
๐๐(๐ ๐๐ )๐๐(๐
๐๐
1
+ 2๐) (๐ ๐๐ )(๐ ๐๐
+ 2๐)
Magnitude of G(jฯ),
๐บ(๐๐) =
1
(๐๐ข)(๐๐ข + 2๐)
1
|๐บ(๐๐)| = =
Phase of G(jฯ),
(๐ข)โ(๐ข2 + 4๐2) โ(๐ข4 + 4๐ข2๐2)
โ ๐บ(๐๐) = โ90๐ โ tanโ1 ๐ข
2๐
At the gain crossover frequency ฯgc, the magnitude is unity.
Hence, at u=ugc,
Let x= ugc
2
1 |๐บ(๐๐๐๐)| = = 1
โ(๐ข๐๐4 + 4๐ข๐๐
2๐2)
(๐ข๐๐4 + 4๐ข๐๐
2๐2) = 1
(๐ข๐๐4 + 4๐ข๐๐
2๐2) โ 1 = 0
๐ฅ2 + 4๐ฅ๐2 โ 1 = 0
โ4๐2 ยฑ โ16๐4 + 4
๐ฅ = 2
Let us take only the positive sign,
= โ2๐2 ยฑ โ4๐4 + 1
Hence,
๐ฅ = โ2๐2 + โ4๐4 + 1
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1
Phase margin,
๐ข๐๐ = [โ2๐2 + โ4๐4 + 1]2
๐พ = 180๐ + ๐๐๐
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๐พ = 180๐ + โ ๐บ(๐๐๐๐ ) = 180๐ + (โ90๐ โ tanโ1
๐ข๐๐) 2๐
1
[โ2๐2 + โ4๐4 + 1]2 ๐พ = 90๐ โ tanโ1
2๐
Figure 3.1.1 Typical magnification curve of a feedback control system
[Source: โAutomatic Control Systemsโ by Benjamin C. Kuo, Page: 463]
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3.3 POLAR PLOT
The polar plot of a sinusoidal transfer function G(jฯ) is a plot of the magnitude
G(jฯ) versus the phase angle of G(jฯ) on polar coordinates as ฯ is varied from zero to
infinity. Thus, the polar plot is the locus of vectors |G(jฯ)| < as ฯ is varied from zero to
infinity. The polar plot is also called Nyquist plot. It is a graphical method of determining
stability of feedback control systems by using the polar plot of their open-loop transfer
functions. Polar plot is a plot to be drawn between magnitude and phase. Polar plot is a
plot of magnitude of G(jฯ) versus the phase of G(jฯ) in polar co-ordinates. But the
magnitudes are presented with normal values only. The Polar plot is a plot, which can be
drawn between the magnitude and the phase angle of G(jฯ) H(jฯ) by differentiating g ฯ
from zero to โ. The polar graph sheet is described in below mentioned image. This graph
sheet includes various concentric circles and radial lines. The concentric circles and the
radial lines are considered as the magnitudes and phase angles.
โข Angles are highlighted with positive values in anti-clock wise direction.
โข Mark angles with negative values in clockwise direction.
The polar plot is usually plotted on a polar graph sheet. The polar graph sheet has
concentric circles and radial lines. The circles represent the magnitude and the radial lines
represent the phase angles. Each point on the polar graph has a magnitude and phase
angle. The magnitude of a point is given by the value of the circle passing through that
point and the phase angle is given by the radial line passing through that point. In polar
graph sheet a positive phase angle is measured in anticlockwise from the reference axis
(0ยบ) and a negative angle is measured clockwise from the reference axis (0ยบ). In order to
plot the polar plot, magnitude and phase of G(jฯ) are computed for various values of ฯ
and tabulated. Usually the choice of frequencies are corner frequencies and frequencies
around corner frequencies. Choose proper scale for the magnitude circles. Fix all the
points on polar graph sheet and join the points by smooth curve, write the frequency
corresponding to each point of the plot. Alternatively, if G(jฯ) can be expressed in
rectangular coordinates as,
G(jฯ) = GR(jฯ) + jGi(jฯ)
where, GR(jฯ) = Real part of G(jฯ), Gi(jฯ) = Imaginary part of G(jฯ)
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Then the polar plot can be plotted in ordinary graph sheet between GR(jฯ) and Gi(jฯ) by
varying ฯ from 0 to infinity. In order to plot the polar plot on ordinary graph sheet, the
magnitude and phase if G(jฯ) are computed for various values of ฯ. Then convert the
polar coordinates to rectangular coordinates using P โ R conversion (polar to rectangular
conversion) in the calculator. Sketch the polar plot using rectangular coordinates. For
minimum phase transfer function with only poles, type number of the system determines
the quadrant at which the polar plot starts and the order of the system determines quadrant
at which the polar plot ends. The minimum phase systems are systems with all poles and
zeros on left half of s-plane. The start and end of polar plot of all pole minimum phase
system are shown in figures respectively. Some typical sketches of polar plot are shown
in table. The change in shape of polar plot can be predicted due to addition of a pole or
zero.
1. When a pole is added to s system, the polar plot end point will shift by -90ยบ.
2. When a zero is added to s system, the polar plot end point will shift by +90ยบ.
Figure 3.3.1 Start and end of polar plot of all pole minimum phase system
[Source: โControl Systemsโ by A Nagoor Kani, Page: 3.38]
RULES FOR DRAWING POLAR PLOT
โ Substitute, s=jฯ in the open loop transfer function.
โ Write the expressions for magnitude and the phase of G(jฯ) H(jฯ).
โ Find the starting magnitude and the phase of G(jฯ) H(jฯ) by substituting
ฯ=0. So, the polar plot starts with this magnitude and the phase angle.
โ Find the ending magnitude and the phase of G(jฯ) H(jฯ) by substituting
ฯ=โ. So, the polar plot ends with this magnitude and the phase angle.
โ Check whether the polar plot intersects the real axis, by making the
imaginary term of G(jฯ) H(jฯ) equal to zero and find the value(s) of ฯ.
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โ Check whether the polar plot intersects the imaginary axis, by making real
term of G(jฯ) H(jฯ) equal to zero and find the value(s) of ฯ.
โ For drawing polar plot more clearly, find the magnitude and phase of G(jฯ)
H(jฯ) by considering the other value(s) of ฯ.
DETERMINATION OF GAIN MARGIN AND PHASE MARGIN FROM POLAR
PLOT
The gain margin is defined as the inverse of the magnitude of G(jฯ) at phase
crossover frequency. The phase crossover frequency is the frequency at which the phase
of G(jฯ) is 180ยบ. Let the polar plot cut the 180ยบ axis at point B and the magnitude circle
passing through the point B be GB. Now the gain margin, Kg = 1/ GB. If the point B lies
within unity circle, the gain margin is positive otherwise negative. If the polar plot is
drawn in ordinary graph sheet using rectangular coordinates then the point B is the cutting
point of G(jฯ) locus with negative real axis and Kg = 1/|GB| where GB is the magnitude
corresponding to point B). The phase margin is defined as, phase margin, ฮณ = 180o+ ฮฆgc
is the phase angle of G(jฯ) at gain crossover frequency. The gain crossover frequency is
the frequency at which the magnitude of G(jฯ) is unity. Let the polar plot cut the unity
circle at point A as shown in figures. Now the phase margin, ฮณ is given by โ AOP, i.e.,
โ AOP is below -180ยบ axis then the phase margin is positive and if it is above -180ยบ axis
then the phase margin is negative.
Figure 3.3.2 Polar plot with positive and negative gain and phase margins
[Source: โControl Systemsโ by A Nagoor Kani, Page: 3.41]
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GAIN ADJUSTMENT USING POLAR PLOT
To determine K for specified GM
Draw G(jฯ) locus with K = 1. Let it cut the -180ยบ axis at point B corresponding to gain
of GB. Let the specified gain margin be x db. For this gain margin, the G(jฯ) locus will
cut -180ยบ at point A whose magnitude is GA.
1 20 log
1
log
๐บ๐ด
=
= ๐ฅ
๐ฅ
๐บ
๐ด
1
20
๐ฅ
๐บ๐ด
= 1020
1
๐บ๐ด =
Now the value of K is given by, K = GA/GB
๐ฅ
1020
If, K>1, then the system gain should be increased.
If K<1, then the system gain should be reduced.
To determine K for specified PM
Draw G(jฯ) locus with K = 1. Let it cut the unity circle at point B. (The gain at point B
is GB and equal to unity). Let the specified phase margin be xยบ. For a phase margin of xยบ,
let ฮฆgcx be the phase angle of G(jฯ) at gain crossover frequency.
๐ฅ๐ = 180๐ + ฯ๐๐๐ฅ
ฯ๐๐๐ฅ = ๐ฅ๐ โ 180๐
In the polar plot, the radial line corresponding to will cut the locus of G(jฯ) with K = 1
at point A and the magnitude corresponding to that point be GA.
Now, K = GB/GA = 1/GA (since GB = 1)