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.

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.

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.

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.

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.

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.

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.

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.

Resonant frequency
See 'Natural frequency.'

Shock
A strong force of short duration applied to an object, an impact.

Stiffness
See 'k'. The property of a mechanical part or system which reacts with an opposing force against bending, compression, etc.

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.

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.

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.

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.

Workstation
An isolation table, such as the MK-26 or WS-4, which is free-standing and elevates the isolated surface to working height.