CHOOSING A VIBRATION ISOLATION SYSTEM
Vibration isolation systems differ from other general measuring or test equipment. Most test equipment performance is quantified by its accuracy, resolution, or error, but a vibration isolation system does not possess these characteristics. No vibration isolation systems provide 100% isolation, they can only approach 100% isolation. The only vital characteristic of a vibration isolation system is whether or not it can be used for your particular application. The only test is if the system isolates vibrations such that it is possible to achieve the desired results from the equipment that it supports. Therefore the choice of the proper isolation system is critical.
There are several attributes that define the performance of an isolation system but transmissibility is the dominant attribute that is used. Unfortunately, transmissibility is not the entire story. The amplitude of vibration, the support structure, and the load may change the transmissibility curve. So the only true method in determining if an isolation system will do the job for your application is to test it in your facility with your instrument. That is why Herzan is happy to provide systems on a demo basis, so that you can be assured our tables work for your application before you decide to buy.
Some factors to consider when choosing an isolation system are the weight, size and center of gravity of the equipment to be isolated, the required resolution of the measuring equipment, the measurement conditions, such as a moving stage or changing loads, location of feature in space such as the eyepiece of a microscope, access to the instrument for service or setup and budget. |
Herzan provides two main types of isolation systems: active and passive isolation.
Active vibration isolation is an isolation system that detects vibration levels through sensors that send signals into a feedback or feed forward controller, which then sends a signal to actuators to counteract the forces. In a feedback system, a sensor will detect vibrations affecting the payload and react to these vibrations via force transducers to reduce the vibration levels at the payload. So in effect, the system is not only absorbing energy entering the system from the floor, it can also effectively absorb vibrations from the payload. The active systems can effectively absorb all forms of vibrations including translation and rotations, making it an isolation system that truly works in all six degrees of freedom. Typically active systems do not use air, so they are inherently stiffer, up to 500 times, than an air table.
In summary, an active system is continuously sensing and reacting to vibrations.
Passive vibration isolation systems employ a seismic mass supported on a soft spring. The spring can be made of air, metal, or rubberized materials. The spring and associated damping will absorb vibrations above the resonance of the spring. As vibrations enter the system in a passive system, they are passed through, amplified, or reduced. What actually occurs depends upon the disturbing frequency and the resonant frequency of the isolation system. A key problem of passive systems is that they actually amplify vibrations around the resonance of the spring. So in some cases the isolators act as a resonator, increasing the strength of vibrations instead of dampening them. Passive systems are usually quite good at absorbing vertical energy, but much less effective at reducing vibrations perpendicular |
to the plane of gravity. Because makers of passive systems try to lower their resonant frequency in order to increase the effective isolation range, most passive systems are very soft. This softness is a problem for moving or changing loads because systems with a resonance of < 1 Hz are extremely soft and have long settling time before measurements can be made. In other words, passive tables with low resonant frequency can be difficult to work with because any disturbance will require a few minutes of delay while the system settles.
A passive system simply waits for vibrations to enter the system and reduces the amplitude of any vibrations higher than the system resonance.
When to choose an active system over a passive:
The active system offers many good reasons to use this technology, but it is critical to many applications or conditions.
(Active Vs. Passive Isolation Table)
Site Considerations:
If the building has low frequency resonances < 10 Hz, a passive system will most likely cause some amplification of energy as opposed to isolation. Buildings which typically exhibit lower resonances are:
Buildings built near ocean and rivers can mirror the slow movement of tides and currents in building noise.
Buildings that are several stories tall, can easily have building resonances < 5 Hz.
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