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Lecture 20

AER 403 Lecture Notes - Lecture 20: Optical Table, Hovercraft, Aircraft EnginePremium

5 pages71 viewsWinter 2015

Department
Aerospace
Course Code
AER 403
Professor
Fengfeng Xi
Lecture
20

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AER 403 W20
Motion transmission isolation
• )n motion transmission isolation, there is no internal vibration source.
The excitation is the external support motion.
• (ence we need to look at the support motion vibration. |X/Y|=
[+ξ/)2]/[(1−/)2)2+(2ξ/)2]
when/n>> ,amplituderatio<,X/Y=//n 0
• This means that the isolation system requires a small n.
• A small n can be realized by a soft spring and/or a relative heavy
base.
• A typical example is an optical table which is made of marble
supported by an air cushion system.
• Looking at the equation of motion
md2x/dt2 = -k(x-y) - c(dx/dt-dy/dt)
md2x/dt2 + cdx/dt + kx = ky+ cdy/dt
Equivalent transmitting force through spring & damper
22, | 17
Motion transmission isolation
• /n>> , > X/Y=//n 0 Transmitting force is used
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to overcome the inertia force. • /n=, =
X/Y=1
Transmitting force is used
to overcome the spring and damping force. • /n=, =9
X/Y= /ξ+ 1/2
22,
| 18
Optical table
22, | 19
Transmissibility & isolation efficiency
• Transmissibility
|X/Y| |output amplitude / input amplitude|
• )solation efficiency [Y-X)/Y]×100%
• Decibel p.
dB = 20 log (X/Y)
Example
|X/Y| = 0.1 [(1-0.1)/1]100%=90% 20 log 0.1 = -20 dB
|X/Y| = 0.015 [(1-0.015)/1]100%=98.5% 20 log 0.015 = -36 dB
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