208322 Mechanical Vibrations Lesson 1 General Information Instructor: Name: Withit Chatlatanagulchai Office: 0901/12, 9th floor of Engineering Building Office Phone: 02-942-8555 ext 1858 Mobile Phone: 083-922-1112 E-mail: [email protected]Website: http://pirun.ku.ac.th/~fengwtc/ Lecture: Section 2: Monday 13.00 - 16.00 Room E3302 Section 401: Friday 16.00 - 19.00 Room E3402 Section 450: Tuesday 9.00 - 12.00 Room 6301 Downloadable Class Notes: Class notes for all lessons can be downloaded from my website above. Required Textbook: [1] Mechanical Vibrations, by Singiresu S. Rao, Prentice Hall, 2004 Recommended Textbooks: [2] Theory of Vibration with Applications, by William T. Thomson and Marie Dillon Dahleh, Prentice Hall, 1998 [3] Mechanical Vibration, by William J. Palm III, Wiley, 2007 References: [4] Fundamentals of Vibrations, by Leonard Meirovitch, Mc Graw Hill, 2001 [5] Principles and Techniques of Vibrations, by Leonard Meirovitch, Prentice Hall, 1997 [6] Vibration with Control, by Daniel J. Inman, Wiley, 2006 [7] Vibration Problems in Engineering, by William Weaver Jr, Stephen P. Timoshenko, Donovan H. Young, Wiley, 1990 [8] Dynamics and Control of Structures, by Leonard Meirovitch, Wiley, 1990 [9] Mechanical Vibration: Analysis, Uncertainties, and Control, by Haym Benaroya, Marcel Dekker, 2004 Grade Breakdown: Midterm Exam 45% Final Exam 55% Homework and Quiz 5% (Extra) Evaluation: 80% 75 80 60 75 55 60 45 55 40 45 30 40 30 A B B C C D D F + + + ≥ ≤ < ≤ < ≤ < ≤ < ≤ < ≤ < < Office Hours: You are welcome to come into my office at any time. Class Structure: Three hours are divided into three sections of one hour. In each hour, the class structure is as follows: 50 minutes: Study new materials (or discuss homework due that day) 10 minutes: Break Rules: 1) Close notes and books during exams. Open own notes and books during quizzes. 2) During exams and quizzes, only non-programmable calculator is allowed (Casio fx3800 or compatible.) 3) Homework problems are from the required textbook [1]. Each homework is due at next class. Homework is graded based on completion not correctness. Homework is an individual effort. You may consult your friends or me, but copying is strictly prohibited. 4) I do not accept late homework. 5) All in-class quizzes are random. 6) Grade sheet, which includes your score, ranking, and estimated grade, is posted on my website weekly. 7) Be punctual. If you are late, wait until next break to get in. 8) I do not accept students who transfer from other sections. 1 Copyright 2007 by Withit Chatlatanagulchai
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208322 Mechanical Vibrations Lesson 1
General Information Instructor: Name: Withit Chatlatanagulchai Office: 0901/12, 9th floor of Engineering Building Office Phone: 02-942-8555 ext 1858 Mobile Phone: 083-922-1112 E-mail: [email protected] Website: http://pirun.ku.ac.th/~fengwtc/ Lecture: Section 2: Monday 13.00 - 16.00 Room E3302 Section 401: Friday 16.00 - 19.00 Room E3402 Section 450: Tuesday 9.00 - 12.00 Room 6301 Downloadable Class Notes: Class notes for all lessons can be downloaded from my website above. Required Textbook: [1] Mechanical Vibrations, by Singiresu S. Rao, Prentice Hall, 2004 Recommended Textbooks: [2] Theory of Vibration with Applications, by William T. Thomson and
Marie Dillon Dahleh, Prentice Hall, 1998 [3] Mechanical Vibration, by William J. Palm III, Wiley, 2007 References: [4] Fundamentals of Vibrations, by Leonard Meirovitch, Mc Graw Hill,
2001 [5] Principles and Techniques of Vibrations, by Leonard Meirovitch,
Prentice Hall, 1997 [6] Vibration with Control, by Daniel J. Inman, Wiley, 2006 [7] Vibration Problems in Engineering, by William Weaver Jr, Stephen P.
Timoshenko, Donovan H. Young, Wiley, 1990 [8] Dynamics and Control of Structures, by Leonard Meirovitch, Wiley,
1990 [9] Mechanical Vibration: Analysis, Uncertainties, and Control, by Haym
Benaroya, Marcel Dekker, 2004
Grade Breakdown: Midterm Exam 45% Final Exam 55% Homework and Quiz 5% (Extra) Evaluation:
80% 75 8060 75 55 6045 55 40 4530 40 30
A BB CC DD F
+
+
+
≥ ≤ <
≤ < ≤ <≤ < ≤ <
≤ < < Office Hours: You are welcome to come into my office at any time. Class Structure: Three hours are divided into three sections of one hour. In each hour, the class structure is as follows: 50 minutes: Study new materials (or discuss homework due that day) 10 minutes: Break Rules: 1) Close notes and books during exams. Open own notes and books
during quizzes. 2) During exams and quizzes, only non-programmable calculator is
allowed (Casio fx3800 or compatible.) 3) Homework problems are from the required textbook [1]. Each
homework is due at next class. Homework is graded based on completion not correctness. Homework is an individual effort. You may consult your friends or me, but copying is strictly prohibited.
4) I do not accept late homework. 5) All in-class quizzes are random. 6) Grade sheet, which includes your score, ranking, and estimated grade,
is posted on my website weekly. 7) Be punctual. If you are late, wait until next break to get in. 8) I do not accept students who transfer from other sections.
1 Introduction to Vibrations Any motion that repeats itself after an interval of time is called vibration or oscillation. Examples of vibration are
• we hear because our eardrums vibrate • we see because light waves undergo vibration • breathing is vibration of lungs • walking involves oscillatory motion of legs and hands • we speak due to the oscillation of tongue • destruction of Tacoma Narrows bridge
Figure 1: Destruction of Tacoma Narrows bridge [1].
• explosion of space shuttle Challenger
Figure 2: Explosion of Challenger [2].
A vibratory system includes mass (to store kinetic energy), spring (to store potential energy), and damper (means by which energy is gradually lost.)
1.1 Degrees of Freedom Degrees of Freedom is the minimum number of independent coordinates required to determine completely the positions of all parts of a system at any instant of time. For example,
• a free particle undergoing general motion in space has three degrees of freedom (x,y,z)
• a rigid body in space has six degrees of freedom (three components for position and three components for orientation)
• a continuous elastic body has infinite number of degrees of freedom
The coordinates necessary to describe the motion of a system constitute a set of generalized coordinates. ☻ Example 1: [3] Determine number of degrees of freedom and generalized coordinates of the following systems. a)
Solution 1 DOF, .θ
4 Copyright 2007 by Withit Chatlatanagulchai
208322 Mechanical Vibrations Lesson 1
b)
Solution 3 DOF, 1 2 3, , .θ θ θ c)
Solution infinite degrees of freedom d)
Solution 2 DOF, , .x θ
e)
Solution 6 DOF, 3.1 2, , , , ,x y z θ θ θ f)
Solution 7 DOF, 1 2, , , , , , .r lx y z θ θ θ θ
5 Copyright 2007 by Withit Chatlatanagulchai
208322 Mechanical Vibrations Lesson 1
1.2 Categories of Vibrations • Discrete versus Continuous Systems
Systems with a finite number of degrees of freedom are called discrete systems or lumped parameter systems. Systems with an infinite number of degrees of freedom are called continuous systems or distributed systems. Most of the time, continuous systems are approximated as discrete systems, and solutions are obtained in a simpler manner.
• Free versus Forced Vibrations Free vibration is vibration of a system, after an initial disturbance, is left to vibrate on its own without external force acting on the system. Forced vibration is vibration of a system that is subjected to an external force. The system under free vibration will vibrate at one or more of its natural frequencies. When the system is excited (forced), the system is forced to vibrate at the excitation frequency.
• Undamped versus Damped Vibrations If no energy is lost or dissipated in friction or other resistance during oscillation, the vibration is known as undamped vibration. If any energy is lost in this way, it is called damped vibration.
• Linear versus Nonlinear Vibrations A linear system obeys principle of superposition shown in Figure 3. If the system does not obey principle of superposition, the system is said to be nonlinear system and hence having nonlinear vibration.
Vibrating System
Vibrating System
Vibrating System
1u 1y
2u 2y
1 2au bu+ 1 2ay by+
Figure 3: Principle of superposition of a linear
system.
• Deterministic versus Random Vibrations Vibration that results from excitation (force or motion) that can be predicted at any given time is called deterministic vibration. If the excitation at a given time cannot be predicted, the vibration is called nondeterministic or random vibrations.
Figure 4: Deterministic and random excitations
2 Harmonic Motion Harmonic motion is motion of an oscillating system that vibrates at one fixed frequency. Consider Figure 5, the harmonic motion can be represented by a vector of magnitude OP A rotating at a constant angular velocity ω . The projection of the tip of the vector OP on the vertical axis is given by
sin ,y A tω=
and the projection on the horizontal axis is given by
cos .x A tω=
If we replace the x-y plane with a complex plane where x-axis represents real axis and y-axis represents imaginary axis, we can write the vector in complex format as OP
cos sin ,Z x iy
A t i A tω ω= += +
where 1.i = −
6 Copyright 2007 by Withit Chatlatanagulchai
208322 Mechanical Vibrations Lesson 1
Figure 5: Harmonic motion as the projection
of the end of a rotating vector.
2.1 Exponential Form Using Euler’s equation
cos sin ,ie iθ θ θ= + (1)
we can write the vector OP in exponential form as
cos sin.i t
Z A t i A tAe ω
ω ω= +
=
2.1.1. Algebra of the Exponential Form Let 1
1 1i tZ Ae ω= and 2
2 2i tZ A e ω= be two different vectors in
exponential form, we have
( )
( )
( )
1 2
1 2
1 2
1 2
1
1 2 1 2
1 2 1 2
1 2 1 2
1 1
2 2
1 1
,
,
,
,
.
i t i t
i t i t
i t
i t
n in tn
Z Z Ae A e
Z Z Ae A e
Z Z A A e
Z A eZ A
Z A e
ω ω
ω ω
ω ω
ω ω
ω
+
−
+ = +
− = −
=
⎛ ⎞= ⎜ ⎟⎝ ⎠
=
7 Copyright 2007 by Withit Chatlatanagulchai
208322 Mechanical Vibrations Lesson 1
☻ Example 2: [3] Find the sum of the two harmonic motions ( )1 10cosx t tω= and ( ) ( )2 15cos 2 .x t tω= +
8 Copyright 2007 by Withit Chatlatanagulchai
208322 Mechanical Vibrations Lesson 1
Solution Let the sum of the two harmonic motions be
( ) ( )
( )( ) ( )( )cos
cos cos sin sin .
x t A t
t A t A
ω α
ω α ω
= +
= − α (2)
Then,
( ) ( ) ( )( )
( ) (3)
( )( ) ( )( )
1 2
10cos 15cos 2
10cos 15 cos cos 2 sin sin 2
cos 10 15cos 2 sin 15sin 2 .
x t x t x t
t t
t t t
t t
ω ω
ω ω ω
ω ω
= +
= + +
= + −
= + −
Comparing (2) with (3), we have
cos 10 15cos2,A α = + (4)
and
sin 15sin 2.A α = (5)
From (4) and (5), we have
( ) ( )2 2
1
10 15cos2 15sin 2
14.148,10 15cos 2cos
14.1481.3 .
A
rad
α −
= + +
=+
=
=
Therefore, we have the sum as
( ) ( )14.148cos 1.3 .x t tω= +
2.1.2. Velocity and Acceleration of Harmonic Motion Consider a rotating vector ,i tZ Ae ω= its derivatives with respect to time are
2
2 22
,
.
i t
i t
dZ i Ae i Zdtd Z Ae Zdt
ω
ω
ω ω
ω ω
= =
= − = − (6)
The projection of i tZ Ae ω= on the horizontal axis is cosx A tω= whose derivatives are given as
( )2
2 22
sin cos ,2
cos cos .
dx A t A tdt
d x A t A tdt
πω ω ω ω
ω ω ω ω π
⎛ ⎞= − = +⎜ ⎟⎝ ⎠
= − = +
(7)
We can see from both (6) and (7) that the acceleration vector leads the velocity vector by 90 degrees, and the velocity vector leads the displacement vector by 90 degrees as shown in Figure 6.
Figure 6: Displacement, velocity, and acceleration vectors as rotating vectors.
9 Copyright 2007 by Withit Chatlatanagulchai
208322 Mechanical Vibrations Lesson 1
3 Periodic Motion Periodic motion is motion of an oscillating system that vibrates at several different frequencies simultaneously. Examples of periodic motions are
• the vibration of a violin string is composed of the fundamental frequency f and all of its harmonics, 2 , 3 ,f f and so on
• free vibration of a multidegree-of-freedom system, to which the vibrations at each natural frequency contribute.
3.1 Fourier Series A French mathematician J. Fourier (1768-1830) showed that any periodic signal can be approximated by a series of sines and cosines that are harmonically related. Any periodic signal with period 2Lτ = can be approximated by the Fourier series
( ) 01
cos sin .n nn
n x n xf x a a bL Lπ π∞
=
⎛ ⎞= + +⎜ ⎟ (8) ⎝ ⎠
∑
0 , ,na a and are called Fourier coefficients with following equations nb
( )
( )
( )
01 ,
21 cos ,
1 sin .
L
L
L
n L
L
n L
a f x dxL
n xa f x dxL L
n xb f x dxL L
π
π
−
−
−
=
=
=
∫
∫
∫
(9)
☻ Example 3: [5] Find the Fourier series of the following periodic function
( ), 0,0
k xf x
k xπ
π− − < <⎧
= ⎨ < <⎩ with ( ) ( )2f x f xπ+ =
( )f x
k
k−
0 π 2π xπ−
10 Copyright 2007 by Withit Chatlatanagulchai
208322 Mechanical Vibrations Lesson 1
-6 -4 -2 0 2 4 6
-1
0
1
-6 -4 -2 0 2 4 6
-1
0
1
-6 -4 -2 0 2 4 6
-1
0
1
( )1S x
( )2S x
( )3S x
11 Copyright 2007 by Withit Chatlatanagulchai
208322 Mechanical Vibrations Lesson 1
Solution In (8), the period is . Since the signal has period 2L 2 ,π we have
2 2 ,.
LL
ππ
==
From (9), we have
( )0
0
0
12
12
0,
a f x dx
k dx k dx
π
π
π
π
π
π
−
−
=
⎛ ⎞= − +⎜ ⎟
⎝ ⎠=
∫
∫ ∫
( )1 cos
0,
nn xa f x dx
π
ππ π−
=
=
∫π
( )
( )
( )
0
0
0
0
1 sin
1 sin sin
1 cos cos
1 cos cos
2 cos 1 .
nn xb f x dx
k nx dx k nx dx
k knx nxn n
k k k kn nn n n n
k nn
π
π
π
π
π
ππ π
π
π
π ππ
π
3.1.1. Complex Form The Fourier series can be written in complex form using the Euler’s equation (1) as follows
( )
( )
/
/
,
1 .2
in x Ln
n
L in x Ln L
f x c e
c f x e dxL
π
π
∞
=−∞
−
−
=
=
∑
∫
3.1.2. Gibbs Phenomenon Figure 7 describes Gibbs phenomenon. As the number of terms of the Fourier series increases, the approximation can be seen to improve everywhere except near the discontinuity (point P.)
π
π
−
−
−
=
⎛ ⎞= − +⎜ ⎟
⎝ ⎠⎛ ⎞
= −⎜ ⎟⎜ ⎟⎝ ⎠⎛ ⎞= − − − +⎜ ⎟⎝ ⎠
= − +
∫
∫ ∫
Figure 7: Gibbs phenomenon.
12 Copyright 2007 by Withit Chatlatanagulchai
208322 Mechanical Vibrations Lesson 1
3.1.3. Frequency Spectrum Frequency spectrum or spectral diagram is plot of amplitude of each term in the Fourier series (8) with its corresponding circular frequency .ω If we expand (8), using ,2 /ω π τ= we have
( ) 0 1 1
2 2
3 3
cos sincos 2 sin 2cos3 sin 3
f x a a x b xa x b xa x b x
ω ωω ωω ω
= + +
+ ++ + +…
Figure 8 shows an example of the frequency spectrum (using and nd nφ
instead of and respectively.) na nb
Figure 8: Frequency spectrum of a typical periodic function of time.
The following plots show two examples of the frequency spectrum.
13 Copyright 2007 by Withit Chatlatanagulchai
208322 Mechanical Vibrations Lesson 1
4 Vibration Terminology Certain terminologies used in vibration analysis are as follows. Peak Value generally means the maximum value of a vibrating body. Amplitude is the maximum displacement of a vibrating body from its equilibrium position. Average Value or Mean Value indicates a steady or static value. It can be found by the time integral
( )0
1lim .T
Tx x t dt
T→∞= ∫
If the signal ( )x t is periodic, the formula above reduces to
( )0
1 ,x x t dtτ
τ= ∫ (10)
where τ is the period of the signal ( ).x t
Mean Square Value is the average of the squared values, integrated over some time interval T
( )2 2
0
1lim .T
Tx x t dt
T→∞= ∫
If the signal ( )x t is periodic, the formula above reduces to
( )2 2
0
1 ,x x t dtτ
τ= ∫ (11)
where τ is the period of the signal ( ).x t
☻ Example 4: [4] Find the mean value and the mean square value of the following signals:
(a) ) sinx t A t=
b) a rectified sine wave
( )x t
t
sinA t
14 Copyright 2007 by Withit Chatlatanagulchai
208322 Mechanical Vibrations Lesson 1
Solutiona) Since the signal ( ) sinx t A t= has period 2 ,π using (10), we get
2
0
1 sin20.
x A t dtπ
π=
=
∫
Using (11), we have
( )
( )
22 2
0
22 2
0
22
0
2 22
0 0
2
2
12
1 sin2
1 cos 22 2
1 1 sin 22 2 4
02
.2
x x t dt
A t dt
A t dt
A t t
A
A
π
π
π
π π
π
π
π
π
ππ
=
=
−=
⎛ ⎞= −⎜ ⎟⎜ ⎟
⎝ ⎠
= −
=
∫
∫
∫
b) Since the rectified sine wave has period ,π using (10), we get
( )0
0
1 sin
2cos .
x A t dt
A At
π
π
π
π π
=
= − =
∫
15 Copyright 2007 by Withit Chatlatanagulchai
208322 Mechanical Vibrations Lesson 1
Using (11), we get
( )2 2
0
2 2
0
2
0
2
0 0
2
2
1
1 sin
1 cos 22
1 1 sin 22 4
02
.2
x x t dt
A t dt
A t dt
A t t
A
A
π
π
π
π π
π
π
π
π
ππ
=
=
−=
⎛ ⎞= −⎜ ⎟⎜ ⎟
⎝ ⎠
⎛ ⎞= −⎜ ⎟⎝ ⎠
=
∫
∫
∫
Root Mean Square Value is the square root of the mean square value Decibel is a unit of measurement that is frequently used in vibration measurements. It is defined in terms of a power ratio
110
2
10log .pdBp
⎛ ⎞= ⎜ ⎟
⎝ ⎠
Since power is proportional to the square of the amplitude or voltage, we have
2
1 110 10
2 2
10log 20log .x xdBx x
⎛ ⎞ ⎛ ⎞= =⎜ ⎟ ⎜ ⎟
⎝ ⎠ ⎝ ⎠
Thus an amplifier with a voltage gain of 5 has a decibel gain of ( )1020log 5 14= + .
Octave is any frequency span with the maximum value twice the minimum value. For example, each of the ranges 80-160 Hz, 10-20 Hz, 30-60 Hz can be called an octave. Cycle describes the movement of a vibrating body from its undisturbed or equilibrium position to its extreme position in one direction, then to the equilibrium position, then to its extreme position in the other direction, and back to equilibrium position. Period of Oscillation is the time taken to complete one cycle of motion
2 ,πτω
=
when ω is called the circular frequency. Frequency of Oscillation is the number of cycles per unit time
1 .2
f ωτ π
= =
Phase Difference generally means the phase difference between two signals. Consider
( )1 1
2 2
sin ,sin ,
x A tx A t
ωω φ
=
= +
the phase difference is φ that means the maximum of the second vector would occur φ radians earlier than that of the first vector. Natural Frequency is the frequency of any system left to vibrate on its own without external force. A vibratory system having n degrees of freedom has n distinct natural frequencies of vibration. Beating Phenomenon describes motion resulting from adding two harmonic motions with frequencies close to each other. For example, if we add the two signals
16 Copyright 2007 by Withit Chatlatanagulchai
208322 Mechanical Vibrations Lesson 1
( )( ) ( )
1
2
cos ,
cos ,
x t A t
x t A t
ω
ω δ
=
= +
where δ is small value, using the trigonometry identity
cos cos 2cos cos ,2 2
A B A BA B + −⎛ ⎞ ⎛ ⎞+ = ⎜ ⎟ ⎜ ⎟⎠
we have ⎝ ⎠ ⎝
( ) ( ) ( )1 2
2 cos cos .2 2
x t x t x t
tA tδ δω
= +
⎛ ⎞= +⎜ ⎟⎝ ⎠
The plot of ( )x t is given in Figure 9.
Figure 9: Beating phenomenon.
Lesson 1 Homework Problems 1.43-1.47, 1.51, 1.52, 1.58, 1.66, 1.72 Homework problems are from the required textbook (Mechanical Vibrations, by Singiresu S. Rao, Prentice Hall, 2004)
References [1] http://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge [2] http://en.wikipedia.org/wiki/Space_Shuttle_Challenger [3] Mechanical Vibrations, by Singiresu S. Rao, Prentice Hall, 2004 [4] Theory of Vibration with Applications, by William T. Thomson and
Marie Dillon Dahleh, Prentice Hall, 1998 [5] Advanced Engineering Mathematics, by Erwin Kreyszig, Wiley, 2006