Why VC curves are probably the most important criteria to compare the performance of vibration isolators

Typically, different vibration isolators are compared using the transmissibility curve and resonance frequency as measures. Both these specifications are extremeny important for any isolator, since they basically define how much vibration reduction the isolator is capable of, depending on the frequency of disturbance. As most vibration isolators can be modelled in a first approximation as a mass suspended on a spring and damper, the transmissibility has the following characteristics, and isolation starts above 1.41 times the resonance frequency.

Clearly, the resonance frequency should be as low as possible, since it would shift the whole curve to the left, realizing isolation in a broader frequency range, and also to a stronger amount.

However, especially in active vibration isolators, one cannot neglect certain nonlinearities and further sources of excitation. Namely this is the maximum allowable force from the actuators, which limits the performance towards large amplitudes.

Equally important is the performance under low excitation sources. It is a fact that all electronic components in an active systems generate some self-noise. Even piezoelectric sensors include to a certain amount noise in the sensor signal. The sensor signal is amplified multiple times in the feedback loop to generate the strong vibration isolation performance. As the control cannot distinguish between the noise and the real signal, also the noise reaches the actuators and excites the isolated platform with a certain noisy vibration spectrum. This noise is independend from the vibration excitation, and represents a constant vibration spectrum on the isolated plate. It is the lower limit of the absolute amplitudes, which the isolator is capable of.

It is therefore a hard fact to compare different isolators.

Vibration Criteria curves (VC-curves) are a common industry standard to classify existing vibration levels. They are based on a set of one-third octave band absolute velocity spectra:

Workshop: Distinctly perceptible vibrations
Office: Perceptible vibration
Residential Day: Barely perceptible vibration. Appropriate to sleep areas in most instances Adequate for semiconductor probe test equipment and microscopes less than 40x
Op. Theatre: Vibration not perceptible. Suitable in most instances for microscopes to 100x
VC-A: Adequate in most instances for optical microscopes to 400x
VC-B: Adequate for inspection and lithography to 3 µm line widths
VC-C: Appropriate for optical microscopes to 1000x, inspection and lithography inspection equipment
VC-D: Suitable even for the most demanding equipment including eletron microscopes
VC-E: Assumed to be adequate for the most demanding of sensitive systems including long path, laser-based, small target systems, E-Beam lithography systems working at nanometer scales

Between each VC-curve, limit for the maximum allowed vibration amplitudes is halved.
As VC-E is currently the toughest criterion, all further levels F, G etc. are for evaluation purpose only, and not used in industry standards.

To figure out the limitations concerning the noise floor of the Seismion Reactio, we have tested it on a place with as little vibration excitation as possible, and then measure the spectrum on the isolated top-plate. Following is the test result.

Measured absolute vibration velocities with Seismion Reactio

It can be seen that our Reactio achieves VC-F level in the whole frequency range starting below 1 Hz. That means it easily fulfills all industry standards even for the most sensitive systems. Above 2 Hz even VC-G level is realized.

How Seismion realizes industry leading noise floor levels

One key specification in the development of Seismion Reactio isolators has been to realize the lowest noise level amoung all currently available active vibration isolators in its class. The control feedback loop purely consists of analog components, which are selected also based on their noise characterists. The noise level of our piezoelectric sensors are calculated based on scientific publications, and its properties are chosen accordingly.

Often, the transmissibility curves given in datasheets are measured under controlled shaker excitation, which is so large that noise does not have any influence. However, transmissibility curves given by Seismion are all measured only under ambient excitation in laboratory, which should very much agree to the vibration level that the isolator normally experiences under operation.

How Seismion evaluates the performance of the vibration isolators by transmissibility measurements

The previous acticle has considered modeling aspects for optimizing the performance of active vibration isolators. But equally important is the test under real conditions. For this part, we at Seismion have developed our own measurement kit. It consists of two sensor units, that both detect vibrations in vertical and horizontal direction. These signals are processed in the frequency domain, and the vibration spectra of each of the four sensors are determined.

Subsequently, the transmissibility is calculated as the ratio of top-plate spectrum divided by base spectrum, both for vertical and horizontal directions. The following is a screenshot of our program, which in real-time displays the spectra and transmissibilities.

The top figure displays the 4 spectra of the sensors. It can be seen that there is a growing gap between the two sensors placed on the base (S2) and the two placed on the isolated top-plate (S1). This difference is exactly the reduction caused by the isolator, which reaches up to -30 dB in this example.

It is important to mention, that we are measuring the transmissibility without artificial shaker excitation, like it is normally done. Instead, we simply place the vibration isolator on a rigid table, which only receives the ambient vibrations as in a typical laboratoriy environment. In this way, we are measuring the isolation performance as it really matters for the end-user, and not with an artificial excitation which might be best suited to show the performance the isolator.

Based on these measured spectra, one can also realize the importance of low-frequency isolation. The largest vibration amplitudes in this measurement are located in the range 1-10 Hz, with an distinct peak at 10 Hz, after which it is strongly reduced. The 10 Hz is most likely the resonance of the table where the isolator is placed on. Therefore, the isolation performance is most urgently needed below 10 Hz. This is the range where active isolators outperform air spring isolators the most.

Comparison with the calculated transmissibility curve shows a very good agreement, which validates our computer model. In the above example it can be noticed that the measured transmissibilities are rising for higher frequencies approximately above 30 Hz. This is not the actual performance of the isolator, but it is caused by the noise floor of the sensors. Since the sensors on the top-plate receive much less vibration signal due to the isolation, the remaining signal is dominated by the noise.

To emphasize this, the noise limit of the sensors are included in the upper window. It can be seen that the measured spectrum approaches this noise floor, but does not go below it. Since the spectrum of base excitation gets smaller for high frequencies, the gap between these spectra is eventually diminished to zero, which is interpreted as transmissibility of the isolator, but in fact here is due to the sensor noise.

Repeating the same measurement with a stronger excitation will result in a different (better) transmissibility, since the noise floor is not reached that soon.

The Seismion measurement box of course can also be used to evaluete different placements of the isolator and the application, and choose the one with lowest excitation level.

How Seismion optimizes the performance of Reactio vibration isolator using multiphysics models and transmissibility curves

The transmissibility is probably the most important property of vibration isolators. Basically the transmissibility can be regarded as the amount of vibration that gets through the isolator divided by the vibration that is present on the base where the isolator is placed.

Based on the mechanical properties like the stiffness, damping and the mass upon the isolator, as well as the sensitivity of the sensors, the actuator constant, and – most importantly – the control circuitry, we have built multiphysics computer models that allow us to calculate the transmissibility of an isolator with a given parameter set. This is an important part in our development and design of our vibration technology products, since it allows us to figure out interdependencies between the different subsystems, determine stability limits, perform parameter studies, and to make optimizations of the overall system before actually building up an isolator.

With the help of these models, we can even shape the transmissibility curve according to the needs of the end user and its application. Depending on the specific requirements, some applications might need a stronger isolation already in the very low frequency range like 1-2 Hz, while other applications can to some amount sacrifice this low frequency performance for an even higher isolation in the range 10-100 Hz, for example. The natural stability limits of feedback-controlled systems require to make a trade-off between conflicting goals, which can ideally be solved by our computer models.

However, beside linear model behaviour, several other influences need to be studied. Using rather simple linear models, the calculated transmissibility curve would be always the same for a given vibration isolator, no matter what kind of vibration excitation you apply to the system. But testing an isolator in real world will reveal many more factors that matter.

Probably easy to understand is the influence of the maximum forces, that the vibration isolator can generate. Due to the limits of the control circuitry and the actuator itself, every active system is limited to some extend. In cases with strong excitations, the feedback control is saturated, and the generated control force does not grow linearly with the excitation anymore. As a result, the isolation performance is reduced.

At the other extremum, also very small excitation is a challenge for vibration isolators. Since every electronics, and even the piezoelectric sensors, exhibit a certain amount of noise, this noise translates into a noisy actuator force, which excites the top-plate to vibrations. This noise is always present, but normally it is multiple times smaller than the real signal from the sensors and therefore it is no concern. But high-precision applications are typically already placed in a vibration-free surrounding, and in this case the signal-to-noise ratio gets worse, and as a result also the isolation performance. Actually, the noise in the control loop determines the lowest vibration levels, that the top-plate can fulfill.

Seismion Reactio vibration isolators are designed especially with these interdependencies in mind. Our sensors are developed in-house and offer the very high sensitivity that we need. Together with the dedicated, low-noise electronics we have realized an exceptional good signal-to-noise ratio. As a result, the isolator also works perfectly even under low excitation levels. This can easily be proven by the Vibration Criteria curves (VC-curves), that the isolator can realize. From our measurements, VC-F is met already from 1 Hz, and VC-G already from 2 Hz. These vibration criteria levels are extremely tight, and currently not even used as design criteria, but only for evaluation. VC-E is in fact the design criterion for most demanding sensitive equipment like E-beam lithography at nanometer scales, which our Seismion Reactio isolator easily fulfills.

Measured Vibration Criteria curve of Seismion Reactio isolators

It should be noted that even passive isolators like air springs have significantly lower performance (or even no performance at all) under low excitation levels, since the air springs do not elastically deform under small vibration amplitudes.

Concerning the maximum forces, the Seismion Reactio are higher than most of the competitor products, meaning that they can also isolate strong disturbances.

Such complex and sophisticaled systems like active vibration isolators also have to be tested thoroughly beside the computer optimization. This will be discussed in the next blog entry.

The advantages of active vibration isolators compared to passive air spring systems

Passive vibration isolators are widely known and used in various fields of industry. They can be as simple as a rubber block, or rubber-metal-pieces. Or they are based on air spring isolators. In both cases the isolation performance is based on a soft and elastic connection between the vibrating base and the isolated top-plate.

However, such systems all have a clear resonance, in which they amplify the unwated vibrations, instead of reducing them. Isolation only occurs at frequencies higher than the resonance. Therefore it is maybe the important design criterion to have the resonance frequency as low as possible. While ruber blocks have resonances in the range of 8 Hz and higher, state-of-the-art air spring isolators can go down to 2.5 Hz. Active vibration isolators like Seismion Reactio, however, have resonances well below 1 Hz. The effect of such a low resonance frequency is shown in the following figure. The isolation performance of active Seismion Reactio is a factor 7 to 37 higher than air spring systems!

Improvement of isolation performance using active vibration isolators

Another important advantage of active isolators is the fact, that they actually stabilize the system, and not only isolate it from the base. The negative effect of the soft and weakly damped characteristics of air spring systems lead to a very long settling time, since the system is vibrating many cycles until eventually coming to a stop. Compared to this, the active Seismion Reactio isolator nearly instantly stops the vibrations due to the stabilizing characteristics. As a result, Seismion Reactio isolators are a perfect solution for production or inspection systems with moving XY-tables, since the cycle times can be strongly reduced.

Settling time of air spring systems and active Seismion isolators

The difference between vibration isolation and vibration stabilization

There is often some misunderstanding between vibration isolation and vibration stabilization, which one is more important for your application, and wether the vibration isolator product can fulfill this requirement. This article explains the technical background and explains the advantages of Seismion Reactio as vibration stabilizators.

Vibration isolation in this contect typically refers to ground isolation (but can also mean acoustic isolation for example). The source of excitation is the base, on which the application or the isolator is placed. These disturbances are building vibrations, which are excited by foot-fall, air-conditioner, wind, nearly traffic or elevators, to name just a few.

Typical vibration isolation constellation

The goal of a vibration isolator is to minimize the transfer path from ground to the isolated top-plate. Passive isolators like air springs or rubber mounts are therefore designed to be rather soft and weakly damped. The softer it is, the less force is reaching the top-plate, F=cx. Ideal for isolation would be zero stiffness, however practically this is limited by the payload capacity requirements and geometry. The following figure shows transmissibility curves for 3 isolators with different stiffness values.

Effect of lower stiffness upon the isolation performance

The lower stiffness can be obtained in the resonance frequency, which is shifted to smaller values. Since isolation is only achieved above the resonance frequency, both the frequency range with isolation as well as the amount of isolation is improved.

Active isolators often also rely on a soft mount design, but they also feature an active feedback control, which further reduces the resonance frequency of the passive system. Therefore, Seismion active vibration isolators are based on a more rigid mount, which strongly helps for the stabilizing function, as we will see in the following.

Stabilization is understood as the ability of the system to counteract against direct excitations. Such excitation is acting directly on the isolated part. Typically it is coming from direct user interaction, for example pressing a button or looking through a microscope. Also the application itself can generate vibrations, like changing lenses or moving XY-stages for surface inspection.

Typical vibration stabilization constellation

For vibration stabilization, the goal is to minimize the settling time of the system. Settling time refers to the time period, which is needed for the system to come to a rest. The following figure shows the excited vibrations after an impuls, which occurs for example after acceleration or deceleration of a moving table.

Settling time of air spring systems and active Seismion isolators

These graphs clearly shows the big advantage of active Seismion isolators compared to air springs. The soft mount design of passive isolators is in fact counterproductive for stabilization, since the system performs several cycles of the weakly damped and low frequency vibration. The active isolator however, offers a strong stabilization performance. This is partly due to the skyhook-damper design principle, and to a stronger extend due to the stiffer and higher damped mount, which efficiently counteracts the direct disturbance.

Seismion Reactio isolators are therefore also the perfect choice for inspection and production devices with moving stages.