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Tables & Breadboards

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VIBRATION ISOLATION BASICS

Supports Optical Tables Table Supports Non-Magnetic

Making the right choicescan make all the differ- ence when it comes to iso- lating vibrations, For more information contact us directly.

Introduction
â–  The severity of the ENVIRONMENTand associate this with the selection ofthe ISOLATION SYSTEM.After both aspects have been consideredseparately, the overall requirements can be reviewed together, to ensure that the table-top is compatible with the mounting sys-tem.

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his theory section on the vibrationproperties of optical tables, bread-boards and their support systemshas been written for the scientist or engi- neer who is not overly familiar with vibra- tion theory. It serves to introduce some ofthe key parameters and characteristics ofoptical tables. More importantly, it pro- vides a basis on which products can be se- lected; ensuring that money is not wasted on over specified product purchases orcritical work time is not lost due to lowerspecification products being selected.In the wind, tall buildings may sway witha horizontal frequency of 1 to 10 Hz. Vibrations due to equipment and machin-ery often have a somewhat higher frequen-cy, but usually below 200 Hz. Vibrations, both natural and manmade, are always with us and produce relative motions of objects that may be imperceptible to thecasual observer and yet are often disastrousto a wide range of precision experiments.Therefore, before choosing a vibration iso-lation system it is important to determine:a) the severity of the environment wherethe table is going to be placed, (i.e. in a basement or on the upper floor of a steelframe building).b) the sensitivity of the application - whatis the intended application to be con- ducted on the table (i.e. how sensitive isthe set up).It is useful to simplify vibration isolationproblems into two parts: k = stiffness c = damping f = frequency m = massIf we restate the above equation in wordsthen:A plot of compliance versus frequency inaccordance with this formula, shows that the compliance of a rigid body can be sep- arated into three parts; stiffness, resonanceeffects and mass effects. Selecting a Tabletop The absence of relative motion betweenany two (or more) components on the sur- face of an optical tabletop is the primary goal of optical table design. An optical tabletop is an example of acommon problem in engineering andphysics, namely the deformation of a body or structure in response to external forces. These forces may be static, such as the sag-ging of a tabletop due to a large localizedmass being placed upon it. Or they may be dynamic, for example acoustic vibrations in the air, the vibrations of a small motor sitting on top of the table or vibration in-duced from the building into the tabletopthrough its mounting points. Compliance The most widely used transfer function forthe vibrational response of an optical table is compliance. In the case of a constant (static) force, this is defined as 'the ratio of the linear or angular displacement to themagnitude of the applied force'. In thecase of a dynamically varying force (vibra- tion), the compliance is 'the ratio of the excited vibrational amplitude (angular or linear displacement) to the magnitude ofthe forcing vibration'.Any deflection of the tabletop is evident by the change in relative position of thecomponents mounted on the table surface.Therefore by definition, lower compliancevalues means a better table, because deflec- tion of the surface on which components are mounted is minimized. Compliance is used to measure deflectionat different frequencies and is measured inunits of displacement/unit force, i.e. me- ters per Newton.To understand compliance, consider a hy-pothetical structure with only one vibra- tional degree of freedom, i.e. a structurewith only one direction of deformation.This could be a steel bar that is firmly an- chored at one end and which can vibrate only in one direction (i.e. can bend only inone plane).The general expression for compliance of asystem such as this is given by:Where:
â–  The sensitivity of the SET UP and as-sociate this with the selection of theTABLE.
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