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MIT OpenCourseWare http://ocw.mit.edu
2.72 Elements of Mechanical Design Spring 2009
For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.
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2.72
Elements of
Mechanical Design
Lecture 16: Dynamics
and damping
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Schedule and reading assignment Quiz
� None
Topics � Vibration physics
� Connection to real world
� Activity
Reading assignment � Skim last gear reading assignment… (gear selection)
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�
Resonance Basic Physics
� Exchange potential-kinetic energy
� Energy transfer with loss
Modeling
� 2nd order system model
� Spring mass damper
Transforms
Why Do We Care
� Critical to understanding motion of
structures- desired and undesired
� Generally not steady state
� Location error
� Large forces, high fatigue ω
Gain
ωn
mk
c x
F(t)
m
k n =ω
KE PE
Loss
Differential equations, Laplace
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Vibrations - Input Oscillation of System
A Why Categorize
� Different causes
� Different solutions
Input Form
� Forced – Steady State
• Command Signal
• Electrical (60 Hz) A � Free – Transient
• Impacts
t
Forced
t
Free
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Vibrations - Source Source
109 Undriven � Disturbance
1013 • Driven Disturbance
1017 Device Motors Spectrum Rotating Components
102 1 102 104
• Undriven
Electrical (60 Hz)
People (2 Hz)
Cars (10 Hz)
System
Driven Disturbance Nearby Equipment
� Command
• Step Response
A
t Vibration Command Response
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Vibrations: Command vs. Disturbance
Underdamped response
Noise
Free vibration
Golda, D. S., “Design of High-Speed, Meso-Scale Nanopositioners
Driven by Electromagnetic Actuators,” Ph.D. Thesis, Massachusetts
Institute of Technology, 2008.
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Vibrations - Example Example
A Forced A � Chinook
� Identify:
• Mode
t• Form
Free
t
• Source
• Response
109
1013 Spectrum
1017
104102 1021
Undriven Disturbance
t Response
System
A
Driven Disturbance
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Attenuating Vibrations Change System
� Mass, stiffness, damping
� Adjust mode shapes
Change Inputs
� Command: Input Modulation, Feedback
� Disturbance, reduce:
• undriven vibrations, e.g. optical table
• driven vibrations – alter device structure (damping on motors, etc.)
1
ms2+bs+k
ZF
t
A
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Change Input shaping
m, k, c
9© Martin Culpepper, All rights reserved
Modulating Command Vibrations
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Regimes � (1) Low Frequency (ω<ω n)
� (2) Resonance (ω≈ω n)
� (3) High Frequency (ω>ω n)
� Example – Spring/Mass demo
Behavior
x 1 1= =
F ms 2 + bs + k k(ω)
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ω/ωn
-1-1
1
1
10
10
Am
plifi
catio
n fa
ctor
(out
put/i
nput
)
1 2 3
1st order system
ζ = 0.05
ζ = 0.1
ζ = 0.2
ζ = 0.3
ζ = 0.4
ζ = 0.5ζ = 0.6ζ = 0.7
ζ = 0.8ζ = 0.9
ζ = 1.0
ζ = 2.0
Figure by MIT OpenCourseWare.
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Behavior – Low Frequency Frequency Response Regimes
� (1) Low Frequency (ω<ωn)
� (2) Resonance (ω≈ωn)
� (3) High Frequency (ω>ωn)
� Example – Spring/Mass demo
Low Frequency (ω<ω n) � System tracks commands
� Ideal operating range
� High disturbance rejection
1x
F k ≈
Resonance Frequency (ω≈ω n) � System Response >> command
� keff ↓ � Disturbances will cause very large response
� Quality factor = magnitude of peak x 1 � Damping ↑ = Q ↓
F =
2 ms bs k+ +
High Frequency (ω≈ω n) � System Response << command
� High disturbance rejection
© Martin Culpepper, All rights reserved 11
ω/ωn
-1-1
1
1
10
10
Am
plifi
catio
n fa
ctor
(out
put/i
nput
)
1 2 3
1st order system
ζ = 0.05
ζ = 0.1
ζ = 0.2
ζ = 0.3
ζ = 0.4
ζ = 0.5ζ = 0.6ζ = 0.7
ζ = 0.8ζ = 0.9
ζ = 1.0
ζ = 2.0
Figure by MIT OpenCourseWare.
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Behavior - Resonance
Frequency Response Regimes � (1) Low Frequency (ω<ωn)
� (2) Resonance (ω≈ωn)
� (3) High Frequency (ω>ωn)
� Example – Spring/Mass demo
Low Frequency (ω<ω n) � System tracks commands
� Ideal operating range
� High disturbance rejection
1
x Q < ≤Resonance Frequency (ω≈ω n)
k F k� System Response >> command
� keff ↓ � Disturbances will cause very large response
� Quality factor = magnitude of peak x 1 = � Damping ↑ = Q ↓
2F ms + bs + k
High Frequency (ω≈ω n) � System Response << command
� High disturbance rejection
© Martin Culpepper, All rights reserved 12
ω/ωn
-1-1
1
1
10
10
Am
plifi
catio
n fa
ctor
(out
put/i
nput
)
1 2 3
1st order system
ζ = 0.05
ζ = 0.1
ζ = 0.2
ζ = 0.3
ζ = 0.4
ζ = 0.5ζ = 0.6ζ = 0.7
ζ = 0.8ζ = 0.9
ζ = 1.0
ζ = 2.0
Figure by MIT OpenCourseWare.
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Behavior – High Frequency Frequency Response Regimes
� (1) Low Frequency (ω<ωn)
� (2) Resonance (ω≈ωn)
� (3) High Frequency (ω>ωn)
� Example – Spring/Mass demo
Low Frequency (ω<ω n) � System tracks commands
� Ideal operating range
� High disturbance rejection
Resonance Frequency (ω≈ω n) � System Response >> command
� keff ↓ � Disturbances will cause very large response
� Quality factor = magnitude of peak x 1 � Damping ↑ = Q ↓
F =
2 ms bs k+ +
x 1 High Frequency (ω≈ω n)
� System Response << command F ≈
2 mω
� Poor disturbance rejection
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ω/ωn
-1-1
1
1
10
10
Am
plifi
catio
n fa
ctor
(out
put/i
nput
)
1 2 3
1st order system
ζ = 0.05
ζ = 0.1
ζ = 0.2
ζ = 0.3
ζ = 0.4
ζ = 0.5ζ = 0.6ζ = 0.7
ζ = 0.8ζ = 0.9
ζ = 1.0
ζ = 2.0
Figure by MIT OpenCourseWare.
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Constitutive Relations Relevant equations
c � Damping ratio ζ =
2 km
k 2 � ω n ω = ωd = ωn 1− ζ
n m
1 1 � Gain G = ζ <P
2ζ 1−ζ 2
Q = 2ζ (1−ζ 2 )� Quality factor
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1
2
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Application of Theory Relate Variables to Actual Parameters
� Vibrational Mode
� Mass
� Stiffness mk
c x
F(t) � Damping
Transfer between Model and Reality Iteration � Iterative
� Start simple (1 mass, 1 spring)
Mode? � Add complexity
� Limits k?
Example – building (video) m?
c?
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Strategies for damping Material Pros and cons of each
� Grain boundary
� Internal lattice
� Viscoelastic (elastomers/goo)
Viscous � Air
� Fluid
Electromagnetic
Friction
Active
Combinations � Sponge
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Example: Couette flow relationships
Relevant equations
dx� τ = µ
dy
dx� x� (F , x�)F = τA = Aµ = Aµ
dy
F = cx� µ
Aµc =
h
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h
h
A
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Exercise (see next page too)
Perform a frequency analysis of the part� Develop & prove (FEA) how to increase nat. freq. via geometry change
� Any constraints you might have? Geometry changes can’t be unbounded
� Explain effect of your change on vibration amplitude (relative to outer base) at given ω, via sketches & plots
Xtra credit, assume: � Flexure is contained between two parallel plates (on top and bottom)
� Viscous air damping in the gaps on both sides
� 1 micron gap between the flexure sides and plates
� Elaborate on how well flexure is damped (don’t just use intuition)
Useful equations (c = damping coefficient, k = stiffness, m = mass)
Gain at peak amplitude Frequency at peak with max gain Damping ratio
1c G =ζ = ω p = ω n 1− 2ζ 2 p 4ζ 2 − 4ζ 4
2 km © Martin Culpepper, All rights reserved 18
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Flexure Flexure design
Constrained on 4 sides top
Bottom
See this diagram for extra credit:
19
Top plate
Flexure
Bottom plate © Martin Culpepper, All rights reserved
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Multiple ResonancesP
has
e L
ag (
deg
) G
ain
Z-Axis2 Center Stage Modes 10
0 10
Measured
-2 Model10
Fit
1 2 3 10 10 10
Frequency (Hz)
0
-100
-200
-300
1 2 3 10 10 10
Frequency (Hz)© Martin Culpepper, All rights reserved 20