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 thatcomputers, 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.