Upcoming paper: statistical descriptors in the use of vibration critiera

If you are a member of IEST and have some interest in low-vibration environments in research settings, I will encourage you to sit in on a session on "Nanotechnology Case Studies" at ESTECH. I'll be presenting some work on the statistical methodologies that we have been developing over time. I know of at least three other presentations, and they all look intriguing. So, I hope that this will be well-attended. See the abstract for my paper below.

"Nanotechnology Case Studies", ESTECH 2017, May 10 from 8AM to 10AM

Consideration of Statistical Descriptors in the Application of Vibration Criteria

Byron Davis, Vibrasure

Abstract:

Vibration is a significant “energy contaminant” in many manufacturing and research settings, and considerable effort has been put into developing generic and tool-specific criteria. It is important that data be developed and interpreted in a way that is consistent with these criteria. However, the criteria do not usually include much discussion of timescale or an appropriate statistical metric to use in determining compliance. Therefore, this dimension in interpretation is often neglected.

This leads to confusion, since two objectively different environments might superficially appear to meet the same criterion. Worse, this can lead to mis-design or mis-estimation of risk. For example, a tool vendor might publish a tighter-than-necessary criterion simply because an “averaging-oriented” dataset masks the influence of transients that are the actual source of interference. On the other hand, unnecessary risk may be encountered due to lack of information regarding low- (but not zero-) probability conditions or failure to appreciate the timescales of sensitivities. 

In this presentation, we explore some of the ways that important temporal components to vibration environments might be captured and evaluated. We propose a framework for data collection and interpretation with respect to vibration criteria. The goal is to provide a language to facilitate deeper and more-meaningful discussions amongst practitioners, users, and toolmakers. 
 

Can we isolate this microscope from floor vibrations?

For projects that house sensitive instruments and activities – like nanotech labs or vivariums – vibration and noise impacts from the outside world can interfere with research productivity. When it comes to these environmental (rather than locally-generated) building vibrations, location is often the single most important variable. Usually, the farther you can get from external sources -- like major roadways or rail alignments -- the better. 

Of course, most projects don't have the luxury of avoiding the sound and vibration sources that come with civilization: you have to put your building somewhere, and mostly due to cost and convenience, that somewhere is almost always going to be in a populated area. 

Since there's only so much you can do about the environment, we are often asked about local vibration isolation systems that act right at the tools themselves. These are devices like active isolation pads that sit under electron microscopes as well as passive systems like pneumatically-floated-slabs (sometimes built into a pit in the foundation) or spring-based systems that cradle the tool in a height-saving outrigger. Conceptually, they are similar to air tables but are designed to sit below an otherwise floor-mounted tool. These systems are only getting better as technology improves, and you can't ignore the possibilities that they offer when it comes to micro-vibration problems in sensitive buildings. 

An example transmissibility curve for a vibration isolation pad like those used to protect sensitive electron microscopes. Here, "transmissibility" can be thought of as the fraction of floor vibrations that get through the system and affect the microscope. Therefore, on this plot, lower is better: you would prefer that a lower fraction of building vibrations get through.

Of course, you'd probably prefer to avoid using these receiver-based vibration isolation systems in the first place: they are expensive; require at least a little maintenance; create elevation and/or footprint problems; and limit your flexibility when the system is “designed to the tool” or built into the foundation. What’s more, if you had to rely on an isolation system to meet a micro-vibration criterion for your nanotech lab, then what are you going to do when you buy or develop a new tool, with a more-demanding criterion? Anyway, quieter is almost always better, both for routine shared imaging suites as well as for lab groups who build or modify instruments.

From a technical perspective, though, the biggest thing to keep in mind is that these isolation schemes can only attenuate -- not eliminate -- floor vibrations. Furthermore, they aren’t equally effective at all frequencies: they universally work better at higher frequencies than at lower frequencies. Even the most-sophisticated receiver-based vibration isolation systems don't work very well below a few Hertz. This is important because many common imaging tools, like electron microscopes, are often more sensitive to micro-vibrations at lower frequencies than at higher frequencies.

As before, lower transmissibility means that less building vibration gets past the isolation system and into our microscope. Universally, isolation systems perform better at higher frequencies than at lower frequencies. This is important, because imaging tools are not equally sensitive to all frequencies, and also because lab environments do not exhibit uniform micro-vibration across the spectrum.

So, when you ask whether a marginal (but desirable) site could be made workable by putting an isolation pad under your SEM or TEM, you have to have some data describing the frequency content of that building vibration environment. If there’s a lot problematic floor vibration at very low frequencies, then your investment might not pay off the way you had hoped. On the other hand, if all the biggest problems are at middle and high frequencies, then an isolation system might be just the answer. 

What this all means is that tool-based vibration isolation schemes aren't silver bullets. They can't remedy all building vibration deficiencies. Of course, they can be very useful in the right situations and can even rescue an otherwise impossible laboratory site. Just be aware of these limitations, and make sure you are working with good data as you proceed with design of your lab.

Reciprocity: vibration isolation works the same, regardless of which way you look

Last month, I wrote about vibration isolator frequency, and why we have to pay attention to it when isolating rotating machinery (especially in highly-sensitive settings). That discussion centered around the notion of tuning between the spring and the machine it supports. This explains why -- in vibration-sensitive labs and fabs -- neoprene mounts are probably a terrible isolator choice for a 900RPM fan. Here, the isolation frequency is too close to the primary fan vibration frequency; they're "tuned" to each other, and the isolator acts like an amplifier instead of an attenuator.

It's worth pointing out that other tunings can happen, too. And they can be equally problematic. Not only can the isolator be tuned to the machine frequency; the system could also end up tuned to the natural frequency of the structure itself. 

You can think of these systems from two different directions. That earlier post looked at the machine isolation problem of a vibrating payload isolated from a sensitive structure. Now, however, let's invert the problem and say that the structure is the vibration source while the payload is sensitive; imagine an optical microscope sitting on a lab bench. The Principle of Reciprocity insures that all the same concepts apply to both.

Isolation systems act the same, whether you're trying to isolate the building from the payload (like a pump) or trying to isolate the payload from the building (like a microscope). If you understand one, then you'll understand the other. You can thank Maxwell for figuring this out.

In our microscope isolation system, the same kind of problematic tuning can arise. If the structure's natural frequency matches the isolated system's natural frequency, then we're going to have problems.

Imagine that you're installing a microscope in a lab, and you choose mounts that result (via pad stiffness and microscope mass) in a 12Hz system resonance. That means that if you bump into the microscope, the entire isolated system will "ring," bouncing back and forth 12 times per second. What if the laboratory is on an upper floor of the building, and the structure -- unbeknownst to you -- also exhibits a natural frequency at 12Hz?  

Every time someone walks by, that 12Hz floor resonance is going to get excited greatly; since your isolation system is itself tuned to 12Hz, all that vibratory motion very efficiently finds it way into the microscope. In fact, those vibrations will end up getting amplified rather than attenuated, and your images are probably going to get a lot worse. The same thing would happen even if the frequencies aren't so perfectly aligned; the common wisdom is that the frequencies have to be separated by at least 40% to avoid strong interaction. 

So, even when you're isolating microscopes rather than machines, frequency still matters. I didn't choose 12Hz randomly; that's a common number for rubber-type mounts, and it's also common for vibration-designed laboratory floors. So, this isn't just a theoretical risk.

Everything has a natural frequency: the structural floor, the lab bench, the vibration-isolated system. Even the microscope itself has internal resonances; these are the reason why the instrument is sensitive to vibrations in the first place. And when it comes to vibration isolation, allowing these resonances line up (in frequency) is usually not what we want.