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Glossary - Vibration Isolation
| Active
isolation system |
| A
system which produces equal and opposite vibrations in
an attempt to cancel the effects of unwanted vibrations.
These systems can be subject to electronic problems such
as excessive feedback, an improper response to an impulse
in which the system overcompensates and actually creates
a vibration problem. |
| Air
table |
| An
older type of isolation table which uses compressed air
as a cushion to reduce the transmission of vibration from
the floor to the payload. |
| Amplitude
(Vibration) |
| The
amount of displacement of the vibrating object. (Sometimes
the amount of velocity or acceleration of the vibration
is used.) |
| Anti-vibration |
| Any
isolation mount or device which acts to reduce vibration
to a chosen object is an anti-vibration device. |
| Auto-adjust |
| An
electromechanical method of adjusting the Minus K isolator
for varying weight loads or the effects of temperature
change. This is used in situations in which the operator
will not have access to the isolator for extended periods
of time, and mass or temperature change is present |
| Ballast |
|
Extra
weights which are added on top of the isolator with
the payload for one of four purposes:
1) To bring the center of mass of the payload close
to the center of the isolator top plate. This is helpful
when the payload center of mass is offset from the center
of its footprint. It is important to have the center
of mass of the payload over the center of the isolator
in order to attain the best performance from the isolator.
Sometimes ballast is also used to raise or lower the
CG vertically to reduce the possibility of horizontal
movement inducing tilting in the payload. The ideal
vertical position for the center of mass is at the level
of the isolator top plate.
2) To add inertia to the payload. This is helpful when
there are stiff cables and hoses which connect items
in the payload with items external to the isolator.
Also this is helpful when there is movement in the payload
itself-the effect of the movement is less when the overall
mass is greater.
3) To raise the total payload weight closer to the maximum
allowed, for the purpose of obtaining the best horizontal
isolation (lowest horizontal natural frequency).
4) To compensate for changes in weight of the payload
without the need for readjustment to keep the isolator
floating. For instance, this might be helpful with a
SEM with a cryogen system to compensate for cryogen
boil-off.
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| Beam-column |
| A
column which has both an eccentric axial compression load
and a transverse bending load at the same time. |
| Bench
top |
| A
type of isolator which is designed to sit on a laboratory
bench or table. |
| Center
of Gravity (CG) |
| For
an object or group of objects in a particular arrangement
in space, the CG is the point at which the mass of the
whole group acts as if it were concentrated at that point
with respect to gravity. This is specified in 3 dimensions.
It is important to have a good idea of the location of
the CG of the payload you want to isolate. This can be
specified by giving the 'CG in height and plan view.'
The height CG is the vertical distance to the CG of the
payload from the base of the payload. The 'plan view'
CG is the horizontal location of the CG as if you were
looking down on it from above (like in an architectural
blueprint). |
| Center
of Mass (CM) |
| For
Minus K's purposes, this is the same as 'CG.' |
| Damping |
| Energy
removed from a vibrating system to decrease the amplitude
of vibration. This can be in the form of an elastomer
or a fluid with a piston. Damping in a system will decrease
the amplitude of movement at resonance, but will tend
to decrease isolation efficiency. Damping also helps to
offset the effects of air currents, payload motion, or
stiff cables and hoses connecting the payload to an unisolated
object. |
| Decibel
(dB) |
|
A
logarithmic unit of measure which compares two quantities.
When the quantities involved are amplitudes, dB is defined
as 10 multiplied by the base 10 logarithm of the ratio
of the squares of the output and input amplitudes. Note
that when output = input, dB = 0. This is equivalent
to a transmissibility of 1 (100%).
When
the output is half of the input, that is a change of
-6 dB. When the output is 1/10 of the input, that is
-20 dB. When the output is 1/100 of the input, that
is -40 dB.
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| Degree
of Freedom (DOF) |
| This
refers to the number of free directions of movement which
are allowed in a mechanical system. There are three axes
x, y, z along which translation (linear movement) can
occur or about which rotational movement can occur. Thus
there are six degrees of freedom possible. A six DOF isolation
system, such as those from Minus K, reduces vibration
in all possible directions for both translational and
rotational movement. A single DOF system would only reduce
vibration with respect to one axis in either linear or
rotational movement. The vibrations present in a particular
environment along with the requirements of the instrument
or experiment determine how many DOFs are needed. |
| Dynamic
load |
| A
force exerted on one object by another due to linear acceleration
or angular change. An example would be the forces placed
on a person in a vehicle due to acceleration, stopping,
or turning the vehicle. A static load is just the effect
of gravity, i.e. of one object on another when the objects
are at rest. The momentum of the dynamic load causes a
force which is much greater than the weight of the object,
and correspondingly greater displacement. For instance,
a paperweight held in your hand might be heavy, but does
not injure your hand (static load). But if you drop it
on your foot, it may have enough force to break your foot
(dynamic load). |
| Elastomer |
| A
class of man-made materials which are elastic, similar
to rubber. Elasticity means that it can be stretched or
compressed but will return to its original shape when
released. |
| Footprint |
| The
horizontal area taken up by the payload where it is supported.
The overall dimensions are relevant, but it is also helpful
to know the dimensions and placement of supporting feet
of an instrument. |
| Frequency |
|
This is the rate, in cycles per second, of periodic
(recurring) phenomena, such as vibration.
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| Frequency
adjust |
| This is a feature on most Minus K isolators
which allows fine-tuning of the vertical resonant frequency
of the isolator for a given load. Our isolators are adjusted
for 0.5 Hz resonant frequency at the maximum allowable
load before shipping. In most cases, it is not necessary
to adjust the frequency. When frequency adjustment is
made, be sure to remove the plastic cap from the adjustment
screw, use the appropriate hex wrench or socket, and adjust
in increments of 1/8 turn or less, followed by repositioning
with the Load Adjust each time.
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| ½
Hz performance |
| Most
Minus K isolators are designed to have a ½ Hz resonant
frequency. This very low resonant frequency means that
isolation starts at a lower frequency than other types
of isolation systems, and so at any frequency above resonance,
isolation performance is better (see the transmissibility
curve). |
| Hertz
(Hz) |
| The
unit of frequency measurement, in cycles per second (cps).
1 Hz = 1 cps. |
| Inertia |
| A
property of any object which has mass (in the sense of
weight, not size). The greater the inertia, the more the
object resists changes in movement. Greater payload inertia
can be helpful in reducing the effects of air currents,
acoustic noise, stiff hoses and cables running from the
payload to an object not isolated, or movements in the
payload itself. |
| Isolation
efficiency |
| This
is the ratio of output (post-isolation) vibration to input
vibration, expressed as a decimal or percent. This naturally
varies over a range of frequencies so that better efficiency
is generally obtained at higher frequencies. For instance,
Minus K isolators typically achieve 99% isolation efficiency
at 5 Hz. This means that the output vibration has only
1% of the amplitude of the input vibration. |
| Isolator |
| A
mechanism for reducing the amplitude of vibrations transmitted
from one object to another. Multiple isolators plus a
platform may be used to create one isolation system. |
| k |
| The
stiffness constant defined by Hooke's Law, which states
that the reaction force from a spring operating in its
elastic range is proportional to the deflection caused
in the spring. The reaction force is in the opposite direction
from the deflection, thus F = -kx, where k is positive
and the deflection is x. |
| Load
capacity |
| Each
isolator is designed to operate for payloads within a
certain range, which is specified for each isolator model.
The range has both a minimum and a maximum. |
| Low
Frequency |
| As
in ‘low frequency vibration isolation.’ These
are vibrations below 10 Hz which are common in buildings.
The reduction of low frequency vibrations is critical
for obtaining the maximum performance of sensitive instruments. |
| Natural
frequency |
| Any
object has a characteristic frequency at which mechanical
energy is most efficiently transferred in a periodic way.
The frequency depends on the mass, material, and geometry
of the object. A yo-yo or a hula hoop are examples-you
have to interact with the toy at its natural frequency
to keep it moving, pushing at the right time in each cycle
of movement. Energy transfer is fairly efficient at lower
frequencies than the natural frequency, but becomes very
inefficient at higher frequencies, which is why an object
with a low natural frequency can be used as a mechanical
low-pass filter to isolate vibration from source to payload.
At resonance, the amplitude of the object's movement will
increase without bound unless there is sufficient damping
or a mechanical stop. |
| Negative
stiffness |
| A
state of a spring system in which deflection results in
a force in the direction of the deflection. |
| Negative
stiffness adjustment |
|
See Vertical stiffness adjustment.
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| Noise |
| For
Minus K's purposes, this refers to unwanted vibration
which is at such a frequency and magnitude that it interferes
with imaging or data acquisition. Imperceptible environmental
vibrations cause noise which adversely affects the use
of sensitive instruments and experiments |
| Nominal
load capacity |
| The
load specified in the name of the isolator model. For
instance, the 100BM-8 has a nominal load of 100 lbs. The
nominal load is generally about 5 lbs. less than the actual
maximum load of the isolator. |
| Passive
isolation system |
| A
mechanical system which operates by virtue of its passive
reaction to excitation. This has the advantage of requiring
no electrical power, and, consequently, not introducing
extraneous problems. |
| Payload |
|
The
entire collection of objects which are to be placed
on the isolator.
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| Performance |
|
Isolation
performance is measured by transmissibility or its inverse,
isolation efficiency. Transmissibility is how much vibration
gets through the system, whereas isolation efficiency
is how much is attenuated.
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| Plan
view |
| A term from architectural drafting which
refers to the view as if you were looking straight down
on an object or room. |
| Platform |
| The
part of an isolation system on which the object to be
isolated is placed, usually used for multiple isolator
systems. Usually this is an appropriately sized metal
plate. |
| Pneumatic |
| Refers to mechanical systems which use compressed
air actuators or shock absorbers. |
| Resonance |
|
The
condition of a system when it is excited at its natural
frequency. The excitation increases the energy in the
system more and more unless there is damping or a mechanical
stop added to the system.
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| Resonant
frequency |
| See
'Natural frequency.' |
| Shock |
|
A
strong force of short duration applied to an object,
an impact.
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| Stiffness |
|
See
'k'. The property of a mechanical part or system which
reacts with an opposing force against bending, compression,
etc.
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| Supercritical |
|
For
Minus K's purposes, this term refers to a state of the
isolator such that readjustment is required to restore
the isolator's function. When too much negative stiffness
is applied to the isolator for a particular load, the
isolator becomes unstable and the vertical position
indicator pin will swing between high and low positions.
The remedy for this is 1) Remove the rubber cap from
the frequency adjust screw on the isolator. 2) Using
the required hex wrench or socket, increase the frequency
(turn to the left as the arrow shows) by a 1/8 turn
of the screw. 3) Turn the crank handle for the load
adjust to bring the VPI to the center. This may take
many turns of the handle, and you may have to try both
directions. 4) If necessary, repeat steps 2 and 3 several
times.
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| Table |
| An
isolation table, such as the MK-26, which has a support
frame which raises the isolated surface to working height.
Also called ‘anti-vibration table.' |
| Transmissibility |
| The
ratio of vibration out over vibration in at a particular
frequency. This is used as a measure of performance of
an isolator. The input is at the base of the isolator,
which is the floor or table the isolator is placed on.
The output is on top of the isolator top plate, where
the isolated instrument is positioned. Transmissibility
can be expressed in % or as a decimal or dB. |
| Transmissibility
curve |
| The
plot of transmissibility vs. frequency over a range of
frequencies. |
| Vertical
stiffness adjustment |
|
The
feature on Minus K isolators (except the BM-6) which
allows the operator to raise or lower the vertical natural
frequency of the isolator within design limits.
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| Vertical
position indicator (VPI) |
|
The
feature included on all Minus K standard isolators which
allows the user to know when the isolator is at its
optimum vertical position, i.e. vertically floated and
centered in its vertical range of travel. The indicator
consists of an oval slot with a line to mark the center
and a pin which travels in the slot to indicate position.
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| Vibration |
| Repetitive,
cyclic movement of an object or structure. |
| Vibration-free
Table |
| See
‘isolator’, ‘workstation,’ ‘table.’
Technically, no vibration-free table exists. Vibrations
can be greatly attenuated but not completely eradicated. |
| Vibration
isolation |
| The
technology of vibration reduction in which an isolation
system is placed between the source of unwanted vibration
and an item which needs to be shielded from the vibration.
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| Workstation |
| An
isolation table, such as the MK-26 or WS-4, which is free-standing
and elevates the isolated surface to working height. |
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