VIBRATION ISOLATION OBSERVATIONS ON THE LIMITATIONS ON PASSIVEACTIVE VIBRATION CONTROL Mike Brennan

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VIBRATION ISOLATION OBSERVATIONS ON THE
LIMITATIONS ON PASSIVE/ACTIVE VIBRATION
CONTROL Mike BrennanInstitute of Sound and
Vibration ResearchUniversity of
SouthamptonUnited Kingdom
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Active vibration isolation

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Feedback Control of a Single-degree-of-freedom
System
accelerometer
Can feed back displacement, velocity or
acceleration
equipment
X
m
H(j?)
controller
c
Fs
k
actuator
Feedback gains
Closed-loop response is given by
Y
vibrating base
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Feedback Control of a Single-degree-of-freedom
System base excitation
accelerometer
equipment
X
m
H(j?)
controller
c
fs
k
actuator
Y
vibrating base

• Constant gain feedback control

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Active Vibration Isolation Feedback Control

• Objective
• To isolate the delicate piece of
• equipment using active vibration control

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The Control Problem
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Overall Performance
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Active Vibration Isolation Feedback Control
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Active Vibration Isolation Feedback Control

• How will the system perform with
• Metallic Isolators
• low damping
• Wave effects in the isolator
• ???

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Active Vibration Isolation isolator considered
as a continuous system


Equipment (mass)
isolator
Distributed mass and stiffness


Base


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Active Vibration Isolation isolator considered
as a continuous system
Transmissibility
Frequency ratio
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Active Vibration Isolation Absolute Velocity
Feedback Control


Equipment (mass)
isolator


Base
controller


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Active Vibration Isolation Absolute Velocity
Feedback Control
Increasing gain (velocity feedback)
Transmissibility
Frequency ratio
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Active Vibration Isolation Absolute Velocity
Feedback Control

• Only effective at controlling the fundamental
• resonance frequency
• Not effective at controlling the wave effects
  in
• the isolator

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Active Vibration Isolation Relative Velocity
Feedback Control


Equipment (mass)
isolator


Base
controller


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Active Vibration Isolation Relative Velocity
Feedback Control
Increasing gain (velocity feedback)
Transmissibility
Frequency ratio
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Active Vibration Isolation Relative Velocity
Feedback Control

• Only effective at controlling the fundamental
• resonance frequency
• Has a significant detrimental effect at higher
• frequencies

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Active Vibration Isolation - observations

• Relatively easy to control the fundamental
• resonance frequency
• Cannot use velocity feedback to control
• isolator resonances
• Possible solution is to
• add damping (metallic)
• use acceleration feedback at high frequencies
• modify the passive system

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Passive vibration isolation

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Passive Vibration isolation






Transmissibility, T dB
Freq. Hz
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The Trade-Off between natural frequency and
static displacement
Frequency Hz
Static Displ. cm
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Non-linear Stiffness
Force
Displacement, x
k(x)
Force
Quasi Zero Stiffness
Displacement
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Non-linear Stiffness

• Geometrical non-linearity

Negative Stiffness
Positive Stiffness
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A system with negative stiffness
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Simple(st?) QZS Model 2 oblique linear springs
in parallel with a vertical linear spring
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Force-displacement characteristic
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Optimal Configuration
1) How to achieve zero stiffness at the static
equilibrium position?
2) What is the displacement range within which
the stiffness is lower than a desired value?
3) Are there any drawbacks?
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1) Zero Stiffness at the equilibrium position
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2) Displacement and low stiffness
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3) Drawbacks

• Nonlinear dynamic behaviour for large
  displacements

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CONCLUSIONS

• Active Vibration Isolation has some benefits
  and some limitations
• Passive vibration Isolation using non-linear
  elements has some
• benefits and some drawbacks
• Possibility to combine active isolation with
  non-linear stiffness