When you work with nanometer resolution motion equipment, details matter. Instabilities can be caused by a variety of things, ranging from floor disturbances leaking past isolation systems, to acoustic energy coupled into the application structure, to tuning issues with the motion system embedded in the application, to resonant excitation of structural elements of the setup. In real life applications, instabilities can lead to smeared images and other noisy data.
Fourier analysis is particularly helpful in revealing the root cause of the vibration problem and often points out one troublesome frequency: 60Hz — the mains frequency here in North America and many other locales — or its integer multiples.
Now, PI nanopositioning equipment is highly shielded and CE certified, meaning it has passed a gauntlet of tests including validation of its immunity to electromagnetic interference (EMI) and of its own low EMI emissions. For example, PI controllers perform reliably in high-EMI environments like synchrotrons where other equipment fails. And PI engineers armed with a laser Doppler vibrometer and other instrumentation frequently consult with customers to identify tough environmental noise sources and develop solutions that enable application productivity. With such extensive testing experience, we have developed some insight into the possible sources of instabilities and the following template for approaching these situations.
Over the years, we’ve learned that issues at the mains frequency or an integer multiple are almost always an indication of electrically-driven vibration from some element embedded somewhere in the equipment in the application or adjacent to it. Even elements not ordinarily considered “moving parts” can cause problems, such as transformers.
Of course, stating that fact is a lot easier than chasing down the root cause of such an issue. That, unfortunately, is usually quite the snipe hunt. There are several places to start:
1) Deactivate motion equipment
Though EMI-driven motion of PI piezo nanopositioning systems is rare, it can be helpful to eliminate it as a potential cause. Sometimes you can unplug the drive amp from the piezo, allowing its sensors to remain active while eliminating any possible stimulation. (Check your system documentation– while most PI piezo nanopositioning systems can be safely operated with the piezo drive voltage disconnected or even hot-unplugged, others can be damaged.) If the piezo or other motion equipment cannot be disconnected or turned off, comparing the system behavior with the servo off and on can sometimes illuminate matters.
2) Check onboard and ancillary equipment
This is where most problems originate. Go through the local environment, disconnecting each piece of equipment from the AC mains, one by one, to see its impact on the disturbance at the mains frequency and its multiples. Disconnection is preferred to simply turning off since supplies and chargers can remain powered when their equipment is shut down.
Start the hunt with equipment mounted to the structure or resting on the table. Possible culprits include transformers, chargers and “wall wart” AC adapters (which can vibrate at twice the mains frequency), fans (AC-powered fans generally rotate at an integer multiple of the mains frequency), disk drives (which, although not AC-powered, often coincidentally spin at multiples of 60Hz, such as 7200 or 15000 rpm), even fluorescent-light ballasts (which can vibrate at 2X the mains frequency). Pumps for fluid-cooled lasers are a possibility, too, since they are often powered by motors spinning at an integer multiple of the mains frequency, and vibrations can be transferred by the piping or fluid.
Cast an especially suspicious eye at any fan-cooled equipment sitting on the table… illuminators and oscilloscopes are frequent culprits. It almost goes without saying that computers, even laptops, should not sit on the optical table if they contain fans or spinning disk drives.
3) Review the isolation-system configuration
Placement of cables and hoses. Well-designed vibration isolation systems place their isolators along the node-line of the first bending mode of the platform, generally about 20% in from the short edges of the platform. This minimizes structural excitation from the residual floor vibration transmitted through the isolators. Ideally, cables and hoses should be tied-down and draped off the table along these same lines to minimize structural excitation from cable-borne vibrations. (NOTE: isolators are often placed further out, at the corners, to maximize the stable load envelope height and to improve knee room, especially for small tables and isolated workbenches. The node-lines are still the best places for tying down and draping cables, though.)
Tautness of cables and hoses. Cables and hoses should be draped with generous loops and support. Taut cables transmit vibration efficiently, as anyone who has played with a tin-can-and-string telephone can attest. Clamp or tie them to the structure, ideally at a point along a node-line.
Isolator installation. Ensure all isolators are floating freely in the middle of their travel. Adjust as necessary.
4) Get rid of multiple-outlet strips
Eliminate as many multiple-outlet strips as you can. These are frequent offenders in forming ground-loops in the laboratory. Where possible, connect your AC-powered instrumentation to the same outlet or circuit. Ensure good grounding of all equipment.
5) Eliminate acoustic and electromagnetic sources
Transformers and motors which draw substantial current can be significant sources of electromagnetic interference (EMI), including components at the mains frequency. This can couple into command and control lines, potentially corrupting instrumentation signals. Coupled low-frequency EMI can even cause direct vibrational excitation of metallic structural elements through ferromagnetism or by Eddy-current generation. For example, a power supply containing a large transformer and residing underneath an optical table can cause sub-micron-scale vibrations by coupling electromagnetically into the bottom skin of the table.
Needless to say, regardless of any mains frequency issues, the environment for nanoscale work must be quiet overall. Siting is important, since even the best vibration isolators provide non-zero transmissibility. Basement locations well away from HVAC equipment are preferred. Foot-traffic and noise sources (including conversation) should be minimized. We have seen boom-boxes sitting on table-mounted shelving — never a good idea! A few years back, Prof. Steven M. Block of Stanford University vividly demonstrated the impact of acoustic noise on nanoscale stability by using his advanced optical tweezer setup to record “The Girl from Ipanema” being played in the next room: see his demonstration of “the world’s most expensive low-fidelity sound system” starting around 18:50 of his engaging NIH lecture, Optical Tweezers: Biophysics, One Molecule at a Time.
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