Before we can discuss the best method for selecting an isolation system, we must define some basic terms.
A resonance is when a structure oscillates at a greater displacement than the input displacement. Resonances occur at different frequencies depending on the structure’s mechanical characteristics. The frequency where a structure’s dominant resonance occurs is called the natural frequency. Rigid structures have higher resonances than structures that are not well supported. Tall buildings have their natural frequency in the lower frequencies. Even vibration isolation systems have resonance at their natural frequency, a point at which they actually amplify incoming vibrations rather than damping them.
The resonances present in a support structure should be taken into account when selecting an isolator. For example, if a table has a natural resonance at 20 Hz and there are significant environmental vibrations at 20 Hz, an isolation system which shifts this resonance away from 20 Hz will reduce the system noise considerably. Conversely, if there are significant environmental vibrations at 2 Hz, using an isolation system with a natural resonance at 2 Hz would amplify the incoming vibrations instead of reducing them.
Damping is the reduction of the amplitude of a resonance. Any structure which has a damping effect can be called a damper, there are two general types. A tuned mass damper is designed to damp specific resonances in a structure. A dashpot used in automobile shocks is an example of a tuned mass damper.
Contrained layer damping is a type of damping which provides broadband damping. Constrained layer damping usually consists of a specialized flexible material constrained between two rigid surfaces. The sole of a sneaker is a prime example of constrained layer damping.
Isolation refers to impeding the transmission of troublesome noise. Isolation can be used to prevent harmful energy from entering into a system and disturbing it. Isolation can also be used on systems which generate noise to prevent this noise from being transmitted to the surrounding area or to other parts of a larger system. The concept of isolation can be applied to many different types of environmental noise. For example, simple vibration isolation systems consist of mass, a spring, and a damper. On another end of the spectrum are EMI isolation systems which employ highly sophisticated sensors and field generation devices to actively cancel EMI. Both systems are engaged in preventing the transmission of troublesome energy.
Transmissibility is a measure of the response of an isolator. Transmissibility is a ratio of the energy exiting a system divided by the energy entering the system. Thus, a measure of 1 means that there is ‘perfect’ transmission: no reduction or amplification is occurring and the energy is passing through unaltered. Transmissibility above 1 means that there is amplification happening; transmissibility below 1 means that there is reduction taking place. For example, a transmissibility of 0.01 means that only 1% of the incoming displacement is being transmitted, in other words there is 99% reduction at that point.
Transmissibility is usually expressed in a transmissibility graph. The transmissibility level is on the Y axis. Frequency, stated in hertz, is along the X axis. The transmissibility graph is the primary measure of an isolation system’s performance. It shows you how an isolation system performs across a broad frequency spectrum. The transmissibility graph tells you where a system’s natural resonance occurs, how much amplification occurs at that point, how quickly it picks up performance, and how much reduction it provides at maximum performance. A steep descent after the resonance, usually called a fast roll-off, indicates that the system achieves good isolation within a few hertz of the resonance. Some systems provide only a small resonance but do not achieve great reduction at peak performance, others provide a low natural resonance with a gradual roll-off. These performance characteristics will be summarized in a system’s transmissibility graph. The line on the graph is referred to as a system’s performance curve.
When selecting an isolation system, the first questions you should ask are the most basic: What isolation system will fit my instrument? Instruments come in a wide array of shapes, sizes, and weights. Many isolators won’t be appropriate considering these basic factors.
First, you should determine the weight of the instrument to be isolated. You will need to find an isolation system with adequate load capacity to handle this weight. Next, measure the dimensions of your instrument and make sure that the isolation systems you consider can accommodate its size. You will also need to take into account the center of gravity of the instrument. If the center of mass is located high up in the instrument structure, you’ll want to consider isolation systems with wider bases to ensure stability.
Another factor to consider at this point is the level at which you would like to isolate your instrument. It is easier to tune an isolator’s performance for a smaller mass and area. Also, intermediate structures can introduce resonances and noise. Thus it is best to position the isolation system as near the level of the sensitive elements of the system as possible. For example, if you have a large and complex instrument, but the stage and probe assembly is the only part that is sensitive to noise, you’ll want to locate the isolation system just under this assembly. Sometimes it won’t be practical to do this, often because it is a large instrument which can’t be disassembled or modified. It may be necessary to isolate the system at the floor or build an isolated structure around it.
At this point, you should also take into account the ergonomics of the system. How will the addition of an isolator change the way you use the system? For example, for a desktop microscope with optics that a user will need to look into, you won’t want an isolation system which significantly increases the height of the instrument. You would want a low-profile tabletop system. Another usability consideration is access points – an isolator shouldn’t interfere with sample loading or service access.
In order to understand which isolation system will best suit your application, you must understand your instrument’s sensitivity. Each instrument’s sensing mechanism has its own sensitivities to certain noise levels. Additionally, each instrument has its own mechanical structure and inherent resonances which can affect the transmission of incoming noise.
The most surefire way to determine an instrument’s sensitivity is to consult the allowable noise specifications developed by the manufacturer. These can usually be found in the instrument manual or the Installation Requirements document. If there are no specifications developed, consult with the instrument maker to get their input.
If you are lacking input from the instrument maker, you can try to extrapolate the instrument’s sensitivity level. Much of the instrument’s sensitivity is based on the degree of precision that it is operating at. Most imaging instruments which are operating at the nanoscale are quite sensitive to environmental noise. Another example of highly sensitive applications would be microbalances which are weighing masses in the microgram range.
It is safe to assume that an instrument being operated at its maximum level of precision will be more sensitive than one which is being used well within its range of capability. Pushing an instrument’s performance will bring you in close contact to its minimum noise floor and structural limitations. The introduction of any noise at this point can easily degrade the accuracy of measurements.
Lacking input from the instrument maker, the only ways to determine the frequency at which an instrument is sensitive is to do trial-and-error tests with vibration measurement equipment or by conducting exhaustive modal analysis on the instrument’s structure.
THE CENTRAL CHALLENGE
Every environment has different noise sources and mechanical characteristics. Even environments that seem identical to other locations can have vastly different vibration profiles. It is very difficult to predict the frequencies and displacement levels of vibrations that are present unless a site survey is performed. Similarly, every instrument is different. They have their own mechanical structure and inherent resonances. Additionally, the operational sensing mechanisms have their own sensitivities to certain vibration levels.
In short, the two critical factors in characterizing an isolation requirement—instrument sensitivity and noise levels—can be very hard to predict with great accuracy. Thus it is critical to gather as much information as possible before deciding on a system. One should always consider the critical information: the environment and probable noise sources, the instrument and application, and the nature of the problem. If site survey data is available, it should be analyzed alongside the allowable vibration specifications of the instrument and the transmissibility graph of the isolation systems in question.
Even when great care is taken in selecting an isolation system, it is difficult to tell precisely how an isolation system will affect a given set-up. At the end of the day, the only critical criterion for selecting an isolation system is whether or not it will work for your application in your environment. The only surefire way to determine if an isolation system is appropriate for your application is to test it in your facility with your instrument. That is why Herzan is happy to provide systems for on-site demonstrations, so that you can be assured our systems will work for your application before you decide to buy.
PASSIVE VIBRATION ISOLATION SYSTEMS
Passive vibration isolation systems operate by placing a spring between the incoming vibrations and the sensitive piece of equipment. These are referred to as passive systems because they do not react to noise levels, they simply provide isolation based on their mechanical characteristics. Mechanical springs take many forms and have a wide spectrum of performance characteristics. The most basic form of isolator is soft material, like sorbothane or rubber, sandwiched between two rigid surfaces. This construction combines both damping and isolation properties.
Another simple isolator would be a metal spring. These don’t provide damping, but can be effective at shifting resonances of machinery so they don’t negatively affect other nearby equipment. Bungees are another simple yet effective isolator. In addition to damping, they provide isolation by shifting the resonances to frequencies that have less effect on the instrument in question. Bungees are employed in some homebuilt isolation set-ups, but they are very soft so most instrument makers are uncomfortable placing their valuable instruments on bungee systems.
The next method of isolation in the hierarchy of complexity is the air-based isolation system. This is probably the most common isolation system in use with precision instruments. The typical large optical table usually employs an air-based isolation system. Air-based systems operate by supporting a mass, usually a breadboard or mounting plate, using a flexible diaphragm which is inflated by air. When a vibration enters the system, the diaphragm compresses down on the tank, which pushes air through an aperture, converting the vibration energy into heat energy. Air systems are generally soft, which moves their natural resonance to a low frequency and allows them to offer isolation over a broad frequency range. Thus, the air system acts as an isolator and damper. These systems are popular because they generally offer a high level of isolation over a broad frequency range at a moderate price.
ACTIVE VIBRATION CONTROL SYSTEMS
Active vibration control systems, also called active vibration isolation or active vibration cancellation, are isolation systems that dynamically react to incoming vibrations. There are two general types of active vibration cancellation systems: feedforward and feedback systems. Active isolation systems can be feed-forward systems which are programmed to compensate for regular periodic vibrations. Or they can be feedback systems which continually sense and react to incoming vibrations. Typical feedback systems have a sensing mechanism which senses incoming vibrations and a transducer which reacts to these vibrations, either by tuning an isolator to reduce the incoming vibrations or creating a signal which cancels them out. Active vibration control systems offer a number of benefits over passive systems, as discussed here.