Beiträge

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

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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

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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.