A simplified method of designing a phase lead compensator to improve the m-s-d system's phase margin

IJRET: International Journal of Research in Engineering and Technology
____________________________________________________________________________
Volume: 02 Issue: 07 | Jul-2013, Available @
A SIMPLIFIED METHOD OF DESIGNING A PHASE
COMPENSATOR TO IMPROVE THE M
Muawia Mohamed Ahmed Mahmoud
AL Neelain University, Faculty of Engineering, Head of Control Engineering Department,
Compensators are used to alter the response of a control system in order to accommodate the set design criteria. This is done
introducing additional poles and zeros to the system. Improving the mass
improve its performance. The paper aims to describe short steps to design a phase
system to achieve the desired level of phase margin for system.
----------------------------------------------------------------
1. INTRODUCTION
The compensator is an additional device or component used to
improve the system performance. Phase-lead compensator
alters the transient response of the system. Phase
compensator alters the steady state characteristics of the
response. The cascade compensator is usually used
with the plant to achieve the required phase margin
system [1]. Figure (1) shows the block diagram of a cascaded
compensator with the plant. The cascade compensator's
transfer function is denoted as Gc(s), and the plant as G(s).
Input and output are denoted as R(s) and C(s) respectively.
Figure (1) Cascade compensator in the system
The open loop transfer function of the system
equation (1). Equation (2) computes the overall
system transfer function [2].
Open-loop = Gc(s).G(s) (1)
Closed-loop=
.
.
The plant G(s) used in the paper was the mass
system. The problem could be simplified in how the phase
margin of the system will be increased to improve the system
performance.
IJRET: International Journal of Research in Engineering and Technology eISSN: 2319
____________________________________________________________________________
2013, Available @ http://www.ijret.org
A SIMPLIFIED METHOD OF DESIGNING A PHASE
TO IMPROVE THE M-S-D SYSTEM
Muawia Mohamed Ahmed Mahmoud
University, Faculty of Engineering, Head of Control Engineering Department,
Abstract
Compensators are used to alter the response of a control system in order to accommodate the set design criteria. This is done
introducing additional poles and zeros to the system. Improving the mass-spring-damper phase margin is an essential so as to
mprove its performance. The paper aims to describe short steps to design a phase-lead compensator for the mass
system to achieve the desired level of phase margin for system.
---------------------------------------------------------------------***---------------------------------------------------------------------
The compensator is an additional device or component used to
lead compensator
alters the transient response of the system. Phase-lag
compensator alters the steady state characteristics of the
response. The cascade compensator is usually used in series
phase margin of the
Figure (1) shows the block diagram of a cascaded
compensator with the plant. The cascade compensator's
(s), and the plant as G(s).
s R(s) and C(s) respectively.
Cascade compensator in the system
of the system is given in
the overall closed loop
loop = Gc(s).G(s) (1)
(2)
The plant G(s) used in the paper was the mass-spring-damper
system. The problem could be simplified in how the phase
margin of the system will be increased to improve the system
2. M-S-D TRANSFER FUNCTION
A typical arrangement of the mass
(ignoring the friction force)
the transfer function, the differential equation was written
using Newton's law as[3]:
M

b kx
Where M= Mass
b = damping constant
k = spring stiffness constant
x = displacement
Typical values used for M, b, and k are:
M= 10 kg, b= 11 n.s/m and k
With a force (F) applied to the mass, the transfer function
(G(s)) of the system taking the force as input and the
displacement x as output will be:
G(s) =
This a second order system
The phase margin of the system could be found using the
Bode plot[4]. A phase margin (PM) of about 45
for the system to have a stable performance.
The design problem is how to improve the phase margin of the
system?
eISSN: 2319-1163 | pISSN: 2321-7308
__________________________________________________________________________________________________
135
A SIMPLIFIED METHOD OF DESIGNING A PHASE-LEAD
D SYSTEM'S PHASE MARGIN
University, Faculty of Engineering, Head of Control Engineering Department,muawia15858@hotmail.com
Compensators are used to alter the response of a control system in order to accommodate the set design criteria. This is done by
damper phase margin is an essential so as to
lead compensator for the mass-spring-damper
-------------------------------------
D TRANSFER FUNCTION
A typical arrangement of the mass-spring-damper system
(ignoring the friction force) is shown in figure (2). To derive
the transfer function, the differential equation was written
F (3)
b = damping constant
k = spring stiffness constant
x = displacement
Typical values used for M, b, and k are:
n.s/m and k = 1 n/m.
With a force (F) applied to the mass, the transfer function
of the system taking the force as input and the
displacement x as output will be:
(4)
This a second order system with a gain of 100.
The phase margin of the system could be found using the
A phase margin (PM) of about 450 is suitable
for the system to have a stable performance.
The design problem is how to improve the phase margin of the

IJRET: International Journal of Research in Engineering and Technology
____________________________________________________________________________
Volume: 02 Issue: 07 | Jul-2013, Available @
Figure (2) Mass-spring-damper system
3. COMPENSATOR DESIGN STEPS
At first the current phase margin of the system
compensation) was investigated using the Bode plot. Matlab
command (margin) was used for checking the current phase
margin. This is shown in figure (2).
Figure (2) Phase margin of the system before compensation
Figure (2) shows that the phase margin of the mass
damper system equal to 19.8 degree ≈ 200.
The design aims to increase the phase margin to 45
next step done was to calculate the phase margin difference
between the current phase and the desired phase margin
namely 450 - 200 = 250.
A phase margin of 50 is added as a safety margin
phase margin to be compensated will be 30
compensator standard form of the phase-
takes the form shown in equation (5) as shown below
∝
∝ 1
Where α = p/z , the ratio of pole to zero.
10
-3
10
-2
10
-1
10
0
10
1
-180
-135
-90
-45
0
Phase(deg)
Bode Diagram
Gm = Inf dB (at Inf rad/sec) , Pm = 19.8 deg (at 3.08 rad/sec)
Frequency (rad/sec)
-60
-40
-20
0
20
40
Magnitude(dB)
IJRET: International Journal of Research in Engineering and Technology eISSN: 2319
____________________________________________________________________________
2013, Available @ http://www.ijret.org
damper system
3. COMPENSATOR DESIGN STEPS
irst the current phase margin of the system (without
was investigated using the Bode plot. Matlab
command (margin) was used for checking the current phase
before compensation
Figure (2) shows that the phase margin of the mass-spring-
The design aims to increase the phase margin to 450. The
was to calculate the phase margin difference
between the current phase and the desired phase margin
is added as a safety margin 5]. The
phase margin to be compensated will be 300. The
lead compensator
takes the form shown in equation (5) as shown below [6].
(5)
Note that if α < 1 the general form of the compensator
represents a phase-lag compensator.
Values of α and T must be found to complete the phase
compensator design.
To find α value the sine rule[7] was used as shown in
equation (6)
sin(PM) =
α
α
.
Thus sin(3) =
α
α
from which
function takes the form shown in equation (7)

The next step is to find the value of T.
of the compensator at maximum phase
(α) dB [6]. Thus
-20*Log(3) = 9.54 dB
The max magnitude of the
between poles and zeros frequencies therefore
ωm =
√α
ωm meets the Half of the magnitude of the max phase
shift.
The half magnitude = (0.5)(9.54) =
plot this value meets ωm
equation (9) yields:
T =
√
= 0.14 (10)
Substituting values of α
compensator shown in equation (5) yields the transfer function
of the required phase-lead compensator as shown in equation
(11).
Gc(s) =
.
.
Referring to figure (1) this compensator is inserted in
before the M-S-D transfer function.
The loop transfer function as explained in eq
found as:
G(s).Gc(s) =
.
The Bode plot of the system's loop transfer was found as
shown in figure (3).
10
2
eISSN: 2319-1163 | pISSN: 2321-7308
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136
1 the general form of the compensator
lag compensator.
and T must be found to complete the phase-lead
value the sine rule[7] was used as shown in
(6)
from which α = 3. Now the transfer
the form shown in equation (7).
(7)
The next step is to find the value of T. We have the magnitude
of the compensator at maximum phase shift equal to -20*Log
20*Log(3) = 9.54 dB (8)
magnitude of the phase Lead occurs half way
between poles and zeros frequencies therefore[7]:
(9)
meets the Half of the magnitude of the max phase-lead
magnitude = (0.5)(9.54) = -4.77 dB . From Bode
= 4 rad/s .substituting this value in
= 0.14 (10)
and T in the standard form of the
ensator shown in equation (5) yields the transfer function
lead compensator as shown in equation
(11)
Referring to figure (1) this compensator is inserted in cascade
D transfer function.
The loop transfer function as explained in equation (1) is

.! .
(12)
The Bode plot of the system's loop transfer was found as

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
__________________________________________________________________________________________________
Volume: 02 Issue: 07 | Jul-2013, Available @ http://www.ijret.org 137
Figure (3): The 450 required PM of the system
Note that the required phase margin was obtained.
4. RESULT DISCUSSION
The steps followed in the design obtained exactly the required
phase margin of M-S-D system. Matlab command (margin)
was very useful in the design procedure. The steps taken in the
design procedure are summarized as:
Step 1: Check the frequency response of the system before
starting the compensation procedure and note the phase
margin.
Step 2: Find the difference between the system phase margin
and the required phase margin.
Step 3: Add five degree to the difference as safety margin and
find the required phase margin to be compensated.
Step 4: Use the sine law to find α then find the half of the
maximum magnitude from -20*Log α and find the
corresponding frequency ωm and use equation (9) to find T.
Step 5: Construct the compensator, insert it in cascade with the
system, and check the desired phase margin of the system'
loop.
The step response of the compensated system can also be plot
to study the transient response characteristics. The
compensator has added a new pole to the two poles of the M-
S-D system. The three poles were found by using the Matlab
command (pole).
P1 = -7.1429
P2 = -1.0000
P3 = -0.1000.
Note that since the first pole (-7.1429) is located to the left of
the other two poles, the characteristics of the transient
response will get better.
5. CONCLUSIONS
In this paper the transfer function of the mass-spring-damper
system was modelled and its phase margin was checked. The
phase margin was probably narrow. A lead compensator was
designed and added to the system to increase its phase margin
to 450. After the compensator was designed and inserted in
cascade with the system, the desired phase margin was
obtained. PID controller may designed for the system to
improve its closed loop characteristics.
6. REFERENCES
[1] Katsuhiko Ogata, "Modern Control Engineering", Prentice-
Hall, Inc., 1997 pp 405-407.
[2] Chi-Tsong, "Analog and Digital Control System Design",
state university of New York, pp 95-96.
[3] Hugh Jack, "Dynamic System Modelling and Control ",
Copyright 1993-2003 Hugh Jack, pp 354-355.
[4] Rife, D.D. and Vanderkooy, J. (1989) Transfer Function
Measurement with Maximum-Length Sequences, J. Audio
Eng. Soc., Vol. 37, No. 6, pp.419-443.
[5] what is meant by frequency response, available at
http://www.dsp.stackexchange.com/questions/536/what-is-
meant-by-a-systems-impulse-response-and-frequency-
response, accessed on 30/6/2013.
[6] S. K. Bhattacharya, "Control System Engineering",
Pearson Education India, 2008,pp 10.1-10.2.
[7] Norman S. Nise, "Control Systems Engineering",John
Wiley and Sons, 2004, pp 703-704.
-100
-50
0
50
Magnitude(dB)
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
-180
-135
-90
-45
0
Phase(deg)
Bode Diagram
Gm = Inf dB (at Inf rad/sec) , Pm = 45.1 deg (at 4.09 rad/sec)
Frequency (rad/sec)

A simplified method of designing a phase lead compensator to improve the m-s-d system's phase margin

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A simplified method of designing a phase lead compensator to improve the m-s-d system's phase margin