For our anniversary, a wealth of new solutions for you

What better way to celebrate the one-year anniversary of PI's marriage with miCos than to announce a host of popular miCos stages newly compatible with PI's cost-effective and easy-to-use Mercury DC servo- and stepper-motor controllers?

This development brings the write-once benefits of PI's General Command Set (GCS) to the burgeoning world of miCos' superb linear and rotary stages.  GCS is the command set used across PI's modern product line of motion, nanopositioning and hexapod controllers, and it allows code built for one  PI model to be used alongside or ported readily to other controller and stage models.  In addition,
  • Initial setup is greatly facilitated by automatic configuration of standard parameters based on an extensive database indexed by stage model numbers, while custom parameters remain easy to implement in non-volatile form.  
  • PI's standard tuning and diagnostic tools are supported out of the box.  This makes customization straightforward and safe.
  • Your productivity will benefit from PI's extensive and well-documented support for programming environments as diverse as LabVIEW, MATLAB and textual languages like C and VisualBasic.  Rich and well-supported options for Linux as well as Windows are also offered as part of the Mercury's standard feature-set.
Since many applications involve a variety of motion form-factors, having a single programming platform to deal with and a single global support resource to rely upon has proven to be highly beneficial to our customers.  And since software is the face of instrumentation when architecting an application and as well as in everyday use, having a consistent, well-wrung-out set of development and support tools is increasingly important.  So we think this is big news-- and good news, especially for anyone contemplating a complex application, or one that might need to scale.

The Mercury controllers deserve a special spotlight of their own.  Highly popular for their combination of performance and value, they feature responsive RS-232 and USB interfaces for snappy communications, with built-in networking for painlessly creating multi-axis configurations.

C-663 supports stepper motor stages while C-863 supports DC servo-motor stages.  Each features a bank of programmable TTL lines that let you trigger motion or trigger external equipment or processes, and you can easily configure them via a single GCS command to provide deterministic, real-time position increment indication or instantaneous in-motion/on-target status.  The Mercury controllers' integrated amplifiers support mechanisms with surprisingly powerful motors while also providing compatibility with our stages featuring onboard ActiveDrive™ amplifiers.

PI miCos stages newly compatible with C-663 and C-863 include:



As ever, contact your local PI miCos applications professional for guidance and world-class support.

Enabling Curiosity with PI and PI miCos

Landing a rover on Mars is an amazing accomplishment.  Landing one the size of a car, stuffed with scientific instrumentation, is downright astonishing.
Illustrations courtesy NASA

The initial excitement and pride over the Mars Curiosity rover's inventive descent to the Martian surface has given way to continuous, methodical scientific exploration using a variety of sensors and instrumentation.  The cameras get most of the press, with their stunning panoramic views and occasional amusing curious sightings.  But science is a patient discipline, and experiments which peel back the layers of Mars' composition and history are underway each day now.

Performance and reliability are essential for all research and industrial applications, but the prospect of a service call more than hundreds of millions of kilometers away poses special challenges, so it was critical for every component of the Curiosity rover to have proven reliability and robust performance.  We are thrilled that PI and PI miCos products are not only part of the rover's instrumentation package but already performing important science on Mars.

"Only through curiosity can we discover opportunities, and only by gambling can we take advantage of them."
--Clarence Birdseye
PI's award-winning PICMA® low-voltage ceramic actuators have been the gold standard for reliability in nanopositioning and are the heart of PI's nanopositioning stage products since their introduction several years ago.  They are the heart of our nanopositioning equipment.  NASA's testing of these actuators validated their performance over 100 billion cycles, which aided their qualification for use as the foundation for the Chemistry & Mineralogy (CheMin) instrument, now hard at work in Gale Crater.  

Sixteen PICMA actuators operate a precision oscillatory material delivery system feeding transmissive X-ray diffraction and fluorescence spectrometry experiments.  Thirty-two sample chambers (including five containing fixed references) are arrayed around a sample wheel; the chambers are arranged in pairs with a PICMA actuator coupling each pair. The actuators are used to load sample powder into the chambers and to unload it once metrology is concluded.  Clearly, their reliability is crucial to the mission's success.  Metrology is performed during the Martian night so that the CCD sensor can be efficiently cooled.  This rover never sleeps!

While the PICMA actuators are hard at work performing spectrometry on Martian mineral samples, another experiment called ChemCam is busy performing the first interplanetary Laser Induced Breakdown Spectrometry (LIBS).  This all-optical, non-contact technique utilizes a powerful, pulsed infrared laser to induce optical emission from interesting samples.  The visible, sparking flash that results from each laser pulse is evaluated by a fiber-coupled spectrometer, with chemometric analysis providing the material breakdown of the sample.

One key advantage of this methodology is that the geologic sample being tested can be a distance from the rover.  But it requires exacting control of the optical focus, and that's where a space-qualified variant of the PI miCos MT Series stage comes in.  This high-precision stepper-motor stage axially translates the secondary mirror of the telescope which collects the optical return from the sample while providing imaging information to place the sample within geologic context.

The shocks and vibration of launch and landing necessitated that every component in the stage from the stepper motor to the crossed roller bearings be validated and optimized to eliminate the possibility of failure or degradation.  The autofocus process places stringent demands on the stage's performance-- resolution, backlash, trajectory quality and stability are all crucial for responsive and predictable operation and reliable data.  The wide temperature excursions to which the stage was to be exposed in flight and on the Martian surface added significantly to the challenge.  Modeling and thermal compensation technologies and sophisticated vacuum-compatible components, coatings and lubricants were utilized.  This specialized variant of the commercial-off-the-shelf (COTS) stage passed all preflight tests and validated the cost-containment strategy of leveraging COTS designs.

Of course, performance, reliability and cost-effectiveness have their place here on Earth as well. Contact your local PI sales engineer for the finest products and applications advice in the solar system.

Diving boards are for the Olympics

It's important to have a well-functioning nanopositioning device.  But it's also important to have a well-thought-out supporting structure for it.  After all, any shortcomings of the structure will confer drift and instabilities onto the application.

It may seem basic, but we'd encourage even longtime nanopositioning users to read this through.  We've learned some important lessons alongside some very experienced users; perhaps your application might benefit from some overlooked detail.

To begin, a stable structure means rigidity, flatness and close mechanical coupling are requirements.  Let's review each of those in turn:

  • Rigidity.  Mounting hardware must be substantial and solid.  Massiveness per se isn't necessarily desirable, as the resonant frequency of the structure goes as sqrt(stiffness/mass), so clearly it is this stiffness-to-mass ratio that should be maximized.  So a flanged, triangulated, "I-beam" or boxed supporting element would be preferable to a solid component of the same dimensions or the same mass.  (On the other hand, nanopositioning devices in high-dynamic applications need a significant reactive mass to push against-- you don't want to eliminate all mass in the supporting structure.)

    It's also important that your nanopositioning and other equipment be mounted per their manufacturers' recommendations.  In particular, don't skip bolts, and don't use double-sided tape in lieu of bolting.  The material stability of structural elements is also worth considering if drift over the very long term is important.  And note that replacing screw-driven mechanisms with piezomotor-driven mechanisms has proven to significantly improve long-term stability by eliminating the gradual flow of lubricants in the drive assembly.

    Structural rigidity issues often need special attention in microscopy applications, as high-throughput motion and nanopositioning equipment is often retrofitted into these systems.  Thin mounting platforms, tall (sometimes hinged) structures, and long risers with little triangulation can all contribute to instability.

  • Flatness.  A nanopositioner or other motion device will conform to the platform it's bolted-to.  Poor trajectory can be one consequence of a non-flat mounting, but other odd effects can occur.  We've seen stages with drift issues and "hunting" problems perform perfectly once their mounting issues were resolved.

    Similarly, over-torquing the mounting bolts can warp or even damage a stage.  There's usually no reason to torque beyond "snug tight." In fact, when a motion issue occurs after a system is set up, over-tightening of the mounting bolts is one of the first things that should be checked.

  • Close mechanical coupling.  Motion devices and their loads should be mounted rigidly together with as little cantilevering as possible.  Anyone who's walked the length of a diving board is familiar with how the magnitude of its flexing and bouncing increases and their frequency diminishes with distance from the platform.  Similarly, long brackets and extenders are rarely a good idea in a nanopositioning application, though sometimes they're necessary for various reasons.  In such cases it's important to have realistic expectations about application throughput and settling performance. 

    Often, mounting extenders and offset loads not only lower resonant frequencies but introduce additional resonant drivers such as torque moments.  An interesting example was a high-speed scanning application we reviewed not long ago; it required the device-under-test to be cantilevered off the side of a fast piezoelectric nanopositioning stage.  The rapid linear motions of the stage drove a pendular oscillation of the load due to nanoscale rotational ringing the stage platform.  Fortunately, the cure in this case was easy: counterbalancing the load to eliminate the torque moment proved an instant cure, even though mass was consequently being added.  (Note that optimizing the counterbalance to place the center-of-inertia rather than the center-of-mass on the stage's centerline is needed for best results.) 

In addition, pay attention to cable draping (the subtle physics of which was discussed in an earlier post), and minimize the use of shielding boxes on your vibration-isolation platform.  Cables will efficiently transfer vibrations from fans and other sources directly to your application, while shields couple acoustics and room modes from the air to your table.  Recently, Steve Ryan, Vice President of Technical Manufacturing Corporation, shared some insights on that point at a meeting we'd arranged with an accomplished customer working at the forefront of picoscale metrology.  Essentially, the room acts as an air column of fundamental frequency F0 = v/2L.  He noted that a typical laboratory might have a wall-to-wall dimension L on the order of perhaps 10m, and with the speed of sound v at about 340m/sec that would imply a room fundamental of approximately 17Hz-- below the range of human hearing. The customer's whole-table shielding box, at the time mounted directly to the table, was coupling that subacoustic standing wave directly to the application.  Sure enough, moving the shielding box to its own supporting frame significantly improved the application's stability.

See you at the Biophysical Society conference in San Diego

The reasons we spotlight biophysical applications so often are straightforward: These applications are challenging and beautiful; they have resulted in many innovative approaches of possible utility to other fields, and their teachings are crucial for the advancement of science and the improvement of human life.  
Fascinating depiction of DNA translocating
through a solid-state nanopore.
Courtesy Biophysics Group at the Kavli Institute
of NanoScience, Delft University of Technology.

Applications demanding resolutions at the nanoscale are becoming commonplace across many industries, ranging from semiconductor manufacturing to materials science to photonics, but biophysical applications often also require positional stabilities spanning unusually long periods of time.  Sophisticated lasers, cameras, modulators or steering mirrors, position-sensitive detectors, a high-end microscope, coarse and fine stages and a host of ancillary instruments--plus a powerful computer--complete the typical setup, and everything must be meshed and coordinated, and it all must perform with superb resolution and nanoscale stability over the long duration of experiments.

This has posed significant challenges in motion technology, driving innovation on our side.  One challenge has been that observation of long-term nanoscale stabilities was beyond the capabilities of classical position-metrology instrumentation.  Measuring nanoscale positions dependably over many minutes remained an elusive goal until some clever work, in a biophysics laboratory, enabled its direct observation.

Ingeniousness characterizes this field.  By definition interdisciplinary, it has served to vividly demonstrate the process of recombinant innovation, in which tricks and technologies from different arenas get mashed together to propel advancement.  Out of the biophysical field have come significant breakthroughs in optical trapping, super-resolution imaging, and atomic force microscopy, leading to revelations about cellular structure and biological molecular machines and the uncloaking of cell-membrane pores-- the mysterious gateways targeted by half of all drugs.  The bustle of individual transport molecules has been directly observed as they ferry their cargoes from place to place along gossamer fibrils, their gaits and forces characterized, their startling talent for editing their own work revealed.  The stuff of miracles.

A beautiful, mysterious and consequential field, full of innovation.  We hope you can join us at the Biophysical Society annual meeting, 25-29 February, to see what's new.

The Nanometer: No Fan of Fans

When you work in the nano world, 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.

Executive summary
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 tieing-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.

Other fundamentals
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.

Read More Articles relating to nanopositioning

Punching Out A Novel Optical-Tweezer Calibration Technique

Consider: A punching bag is suspended by a spring.  A boxer pummels it, banging it from side to side in a chaotic, random way.  The boxer sways slightly from side to side.  You know the frequency and amplitude of his swaying.  From observing the anarchic path of the punching bag, can you determine the stiffness of the spring?

This is an exact analogy to a clever, insightful and astonishingly simple recent technique for calibrating the stiffness of an optical tweezer, the force-well formed by a tightly focused laser beam which is used to trap, manipulate and track microscopic dielectric beads in fluid-filled micro-chambers in single-molecule biophysics.  The coated beads can be hitched to molecular motors and DNA molecules, allowing the machinery of life to be illuminated.

Clearly, in order for an optical tweezer to be a quantitative tool, its force characteristics must be known.  One classical technique for calibrating a trap is to sweep the fluid chamber at constant velocity, back and forth, and observe the deviation of a trapped bead as the fluid drags it.  The trap force can be calculated by applying Stokes’ familiar hydrodynamic equation to the observed deflection of the bead.

Dynamic Digital Linearization enables Stokes
calibration of optical tweezers by
eliminating following errors.
In theory, only the drag coefficient of the fluid and the diameter of the bead need to be known.  But this technique requires a sophisticated level of control of the piezoelectric microscope stage on which the fluid chamber is mounted.  The stage must perform a triangle wave motion, but the finite system bandwidths inherent in any electromechanical system ordinarily cause distortion of the actual waveform.  The corners of the positioning waveform become rounded, and following-error accumulates, meaning the stage position deviates from the desired instantaneous position as the waveform proceeds.  The overall amplitude of the waveform is rolled-off, and most importantly: the critical constant-velocity portion of the waveform isn’t as constant-velocity as needed.  Fortunately, the availability of Dynamic Digital Linearization has salvaged this technique for a generation of researchers.  With this unique technology, an advanced nanopositioning controller can virtually eliminate following-error in motions using its built-in waveform generator.  The Stokes calculations can be applied with confidence.

But back to our punching-bag.  Reduce it to the nanoscale, and it is our dielectric bead.  The pummeling boxer is Brownian motion, and his slight side-to-side sway is a nanoscale, sinusoidal position waveform applied to the fluid chamber by the piezoelectric microscope stage.  The spring is the optical trap.

If this can serve, then several advantages emerge versus the Stokes-calibration approach.  First, the sinusoidal waveform is a single frequency and can be selected to avoid driving structural resonances in the microscope assembly that might otherwise be problematic given the high-frequency Fourier components of the sharp-cornered triangle wave used in Stokes calibration.  Its amplitude is very small.  No constant-velocity region is required, and moderate following errors and rolloff are no issue since all needs to be known is the position-waveform amplitude in all axes, which a parallel-kinematic stage with direct motion metrology can provide.  And the analysis can be performed entirely in the frequency domain using the positional trace of the bead.  Brilliant!

From Fig. 2 of the Tolić-Nørrelykke paper, showing the
power spectral density of the bead's motion.
The technique was published as “Calibration of optical tweezers with positional detection in the back focal plane” in the Review of Scientific Instruments by Tolić-Nørrelykke, Schäffer, Howard, Pavone, Jülicher and Flyvbjerg after a collaboration which spanned Europe.  As they detail in their paper, the forces acting on the bead are well understood and separable: there is the chaotic Brownian motion, and then there is the drag-induced sinusoidal motion as the fluid chamber is oscillated at the nanoscale.  Observe the motion and calculate its power spectral density (PSD).  The PSD will show the white-noise spectrum of the Brownian motion, with a spike at the sinusoidal frequency.  From the power in the spike, the stiffness of the trap can be derived from the straightforward physics of a damped, simple harmonic oscillator.  The bead dimensions and fluid viscosity need not be known.

As an aside, there is a tantalizing resemblance between this PSD and the frequency-domain plot sometimes proffered as illustrating the positional resolution of some nanopositioning devices.  The difference is that this plot comes from actual motion data, not sensors indirectly inferring position via flexural strain deep in the mechanics.  (One could immobilize the platform of such a stage, and the sensors would still see strains as the piezos actuate.)  And the plot floor also represents actual limits—in this case, the inescapable Brownian motion of the bead—rather than filtered electrical strain-sensor noise, which is sometimes hyped to imply sub-picometer positional stability that is, unfortunately, not a possibility in the real-world ambient environment of any laboratory on Earth.  At the nano scale, we’re all punching bags.

(The author would like to thank Armin Hoffmann of the University of Alberta for bringing the Tolić-Nørrelykke paper to our attention.)