Many Tacks to Attack the Stack

When faced with a multi-axis motion application, many users stack motion stages, and in fact that is
a fine approach for assemblies of just a few axes.  But as applications become more complex, so do the equivalent stacks-of-stages, and very real and practical considerations begin to come into play:
  • Stiffness.  Some stage manufacturers publish stiffness specifications in terms of axial deviation per unit force, but this is of little utility in estimating the dynamic performance of a stage ...or a stack.  A more pertinent metric is the resonant frequency, as it integrates both the effective coefficient of stiffness of a mechanism and the summed mass of its construction.  (Accordingly, knowing Fres means you can easily estimate the possible step/settle time for a well-tuned closed-loop stage: approximately [3 Fres]-1).  In our experience, most high-quality conventional linear stages will exhibit resonant frequencies on the order of 75-120Hz, unloaded.  Stack them, and the resulting structure can have significantly limited responsiveness and long settling times.
  • Inconsistent dynamics.  The bottom stage in a stack carries the mass of the entire stack, and so on up to the top stage, which carries only the application load.  So tuning is a laborious, axis-by-axis process, with different settings for each axis... and consequently different responsiveness.
  • Inflexible rotation-centerpoint placement.  Stacked stages place the center of their tip/tilt and rotation motions at the geometric centers of each rotation-stage and goniometer bearing.  These can sometimes be arranged to coincide at a desired point in space (for example, at the focal point of a lens) via custom adaptor plates and fixtures, but this takes time and effort and is inflexible should application needs change.  And significant changes can alter the dynamics of the stack, necessitating a re-tuning of each axis... again.
  • Cabling.  Cables are a fact of life in motion control, and managing them deserves more attention than it often gets.  To begin, cables can be a conduit for vibration that can impact an entire application setup in un-obvious ways.  Even one's choice of draping the cables off an isolation platform can influence an application's overall stability and performance in profound ways.  As a stage moves, any cable being dragged along can contribute to parasitic motions and other errors.  Stiff cables can do so even if arranged in a non-dragging manner.  Cables can break and snag and come loose, contributing to premature failures that can be hard to diagnose.  And generally, these problems scale with the number of axes in a user-stacked system.  (Manufactured stacks sometimes benefit from integrated cable management.)  
  • Central aperture.  Many applications--especially in optics--benefit from transmissive construction of the motion stack.  This is difficult or impossible to achieve with a stacked structure of many axes.
  • Size, weight and fragility.  Simply stated, stacks can be substantial in height and mass.  And since the bottom stages bear the burden of the entire tall stack, their bearings are vulnerable to brinelling and other damage from inadvertent forces.  Besides inviting damage from elbow-knocks when set up, this often necessitates disassembly for shipping, adding cost and hassle and introducing variability when reassembled. 
  • Orthogonality and parasitic errors.  Stacked axes interact in complicated ways; for example, runout in the X axis is seen as unwanted motion in the Y and Z axes; angular deviation of an axis similarly imparts motion in the travel-directions of the other axes, with magnitude proportional to the distance to the moving axis.  And in stacks, that multiplicative lever-arm can be large.

Solution: Attack the stack

It may seem like hyperbole, but all these issues can be avoided by utilizing principles of parallel
kinematics.  Instead of a tall stack of all the necessary axes with the workpiece perched on top, such systems support a single workpiece in parallel by a tripod or hexapod structure, forming a much stiffer yet lighter-weight structure than is possible by stacking.  The best examples of the breed utilize non- or minimally-moving internal cables with conveniently integrated cabling to the controller.  User tuning requirements can be eliminated while providing precision and accuracy that can surpass the performance of some of the best available single-axis stages.

Today's easy-to-use controls

In prior years, the main obstacle to choosing this class of mechanism was the challenge of controlling the workpiece in a user-friendly way, using familiar Cartesian coordinates (X, Y, Z, θX, θY, θZ).  This changed with the introduction of PI's first hexapod two decades ago.  That instrument utilized a fully-integrated industrial PC-based digital controller running clever firmware that transparently managed the coordinate transformation process, providing unprecedentedly flexible control in all six degrees of freedom with a programmable rotational center-point, settable by a single software command.

One Stop, Many Solutions 

These innovations set the tone for PI miCos' broad array of parallel kinematic mechanisms: innovative solutions that can actually cost less than stacks of six stages of commensurate performance.  Today's offering benefits from years of continuous advancement in mechanical design and controls engineering.  Our newest controller integrates an ultra-modern, industrial-class real-time operating system and provides such features and options as TTL motion triggers, analog position-waveform definition, standard internal data recorder with optional analog input, and a high-speed network interface for integration into factory automation systems and remote access.  Its sophisticated software support includes comprehensive LabVIEW libraries, MATLAB support, a convenient GUI for setup and test, and well-documented dynamic libraries for Windows, Linux and OS X.

Two families of parallel kinematics

Today, PI miCos offers two basic architectures for six-DOF mechanisms: six-legged hexapods, and three-legged SpaceFabs:

Hexapods

The hexapods utilize a variety of motion technologies for the actuator legs, ranging from brushed or brushless DC servo-motors to high-force PiezoWalk™non-magnetic actuators.  Both fixed- and extendable-strut designs are utilized depending on application needs.

SpaceFabs

These innovative tripod mechanisms utilize three fixed-length legs and three XY actuation modules which provide extended transverse travels for the assembly.  Motion technologies can include piezomotors, rotary and linear DC servomotors, and stepper motors.

Many roles for stacks

Let's be clear: we like stage stacks.  We sell lots of stage stacks.  Stage stacks are entirely appropriate for applications of all kinds.  But that's the benefit of having a deep toolbox and a global team with broad and deep experience across a multitude of disciplines: we draw on that experience in consultation with our customers, choosing (or custom-developing) optimal solutions and cross-pollinating from related applications in other fields.

Bring us your "impossible" requests!

Maybe they are impossible... or maybe they just require a fresh approach, or a trick from another application field.  Mission-critical PI miCos technology is at the heart of much of today's highest tech, from semiconductor manufacturing, to photonics packaging and test, to genomics, to single-molecule biophysics, to ultra-resolution microscopy.  Put our experience to work on your problems!




PI miCos: A high density of solutions for the rarified world of vacuum

Since the last millenium, serving research and industrial needs for positioning equipment for vacuum and cryogenic applications has been a key focus for PI miCos.  These many years of expertise have yielded some of the most sophisticated design, manufacturing and support capabilities for the field available anywhere.  A host of off-the-shelf and customized configurations are offered, as are highly configured integrated assemblies of multiaxis motion for environments to 10-9 mbar.  A small sampling is presented below.

In addition to having built impressive design and fabrication capabilities, PI miCos has deployed dedicated applications teams to support key fields, such as our recently-announced Beamline Instrumentation Group, which has its own field-specific website at BeamlineInstrumentation.com. Also see our earlier post, Little Motions for Big Physics.

Besides the PI miCos positioning equipment currently hard at work on Mars, a host of more Earthly applications can benefit from these capabilities, including:
  • Diffractometry
  • Ellipsometry
  • Particle beam instrumentation
  • Synchrotron beamline equipment
  • Microlithography
  • Nanoimprint lithography
  • E-beam, ion beam and Auger microscopy...
As high-vacuum applications continue to spread throughout the research and technology world, constant innovation and continuous improvement of design and manufacturing processes are needed just to keep up.  Partnering with a source of acknowledged expertise will help your application be productive sooner and keep costs down.  Have a tough vacuum positioning application?  See our web resources and tutorials here and here, and consult your local PI miCos applications professional today.


Ultra-High-Vacuum Rotation Stage with High Precision
  • Reproducibility to 0.0002 deg (bidirectional)
  • Rotational Velocity to 200 deg/second
  • Payload to 50 kg
  • Optional High Resolution Encoder
  • Clear aperture up to 120 millimeters
Ultra-High-Vacuum Positioning Table with Extreme Accuracy UPM-160 UHV
  • Positioning Range to 205 millimeters (8 inches)
  • Unidirectional Position Reproducibility to ±0.02 microns
  • Max. Velocity 100 mm/second
  • Payload to 35 kg
  • Linear scale encoder (center mounted)
Ultra-High-Vacuum Positioning Table LS-180-UHV
  • Positioning Range to 508 millimeters (20 inches)
  • Unidirectional Position Reproducibility to ±0.05 microns
  • Max. Velocity 200 mm/second
  • Payload to 100 kg
  • Precision Limit Switches
  • Available linear scale encoder (center-mount)
High-Vacuum Positioning Table LS-110-UHV
  • Positioning Range to 305 millimeters (12 inches)
  • Unidirectional Position Reproducibility to ±0.05 microns
  • Max. Velocity 90 mm/second
  • Payload to 10 kg
  • Precision Limit Switches
  • Available linear scale encoder (center-mount)
High-Vacuum Precision Positioning Table PLS-85-HV
  • Positioning Range to 155 millimeters (6 inches)
  • Unidirectional Position Reproducibility to ±0.05 microns
  • Max. Velocity 100 mm/second
  • Payload to 10 kg
  • Precision Limit Switches
  • Available linear scale encoder
Ultra-High-Vacuum Positioning Table MT-65-UHV
  • Positioning Range to 155 millimeters (6 inches)
  • Unidirectional Position Reproducibility to ±0.05 microns
  • Max. Velocity 100 mm/second
  • Payload to 10 kg
  • Precision Limit Switches
  • Available linear scale encoder
High-Vacuum Micropositioning Stage MTS-65 HV
  • Positioning Range to 52 millimeters (2 inches)
  • Unidirectional Position Reproducibility to ±0.1 microns
  • Max. Velocity 8 mm/second
  • Payload to 2 kg
  • Precision Limit Switches
  • Available linear scale encoder
Ultra-High-Vacuum Positioning Table VT-80-FV-HV-UHV
  • Positioning Range to 300 mm
  • Unidirectional Position Reproducibility to ±0.2 microns
  • Max. Velocity 20 mm/second
  • Payload to 5 kg
  • Precision Limit Switches
Ultra-High-Vacuum Precision Actuator Pusher MP-20-L-UHV
  • Positioning Range to 75 mm
  • Unidirectional Position Reproducibility to ±0.5 microns
  • Max. Velocity 3.5 mm/second
  • Force max. 125 N
  • Precision Limit Switches
  • High resolution
  • With MP-B & MP-F inserts
Ultra-High-Vacuum Precision Linear Actuator MP-20-S-UHV
  • Travel range 12.5 mm
  • Unidirectional Position Reproducibility to ±1 microns
  • Max. Velocity 3 mm/second
  • Force max. 20 N
  • Precision Limit Switches
  • High resolution

A resource for the latest for imaging

There's a lot going on in the microscopy-imaging world.  It seems that each month brings publication
Long-travel coarse/fine specimen
positioners in a super-resolution
microscope, courtesy of the
Bewersdorf Lab at Yale University
of another clever technique that casts our eyes ever deeper into the nanoscale world.  The post-Rayleigh era of microscopy is in full bloom, and to support it have come new tools and technologies: new objectives of extreme capabilities; new cameras that waste not a single photon; new software which provides usability, enhances productivity and teases details out of the murk; and new motion technologies of surpassing precision and stability.

A current article in Microscopy Today serves as a compendium of recent motion technologies of specific interest to imaging scientists and engineers.  Its central focus is piezo ceramic technology in general and its burgeoning application in one novel design after another.  Familiar from layered stack actuators of astonishing resolution but limited travel, piezo ceramics are now utilized in long-travel designs spanning several broad mechanical classes.

The article discusses some of the most promising of these developments for imaging applications:

  • High-stiffness piezo walking actuators for objective positioning over 2mm with picometer positionability; 
  • High-stability resonant piezomotors for fast sample positioning with submicron precision over centimeters of travel; 
  • New controls techniques for ever-finer linearity and controllability of flexure-guided piezo-stack mechanisms... 

Each of these newly-developed technologies represents a response to seemingly impossible application challenges.  Each is an enabler of new avenues of investigation, new discoveries, and new breakthroughs.  Perhaps yours will be among them!  Just let us know what impossible application challenges you'd like addressed next.


Little motions for Big Physics

China's ultra-modern Shanghai SynchrotronRadiation Facility
PI was recently honored to participate in the 7th International Conference on Mechanical Engineering Design of Synchrotron Radiation Equipment and Instrumen-tation-- the biennial MEDSI conference of users and engineers at the world's particle accelerators. We'd been invited to present a half-day tutorial on "Nano-Precision Mechanisms for Beamline Components" --a surprisingly broad and nuanced field of nanopositioning technology.

The meeting was hosted in the stunning city of Shanghai by the Shanghai Institute of Applied Physics (SINAP), and our first words should be to thank the Institute again for their extraordinary hospitality as well as for their polished management of an informative and enjoyable technical conference and exhibition.
Big Physics indeed-- the view from inside
the Shanghai Synchrotron Radiation Facility.

Today's synchrotron applications are notable for their diversity and importance across fields as varied as semi-conductor process development and life sciences. As one example, their intense X-ray output is now a fundamental tool for investigating fine, complex structures in 3D at an atomic scale.  In fact, groundbreaking research in protein crystallography using synchrotron radiation to reveal the architecture of proteins won Stanford professor Roger Kornberg the Nobel Prize in Chemistry in 2006.  This is proving to be foundational to our advancing understanding of disease processes, with import ranging from biophysics to the design of advanced antibiotics.

Kornberg's Nobel-winning
investigation of protein structure
was published in the April, 2001 issue of Science.
Synchrotron facilities are a global aspect of Big Physics today, with major operations on all continents.  We were happy to see familiar customers from around the world attending the conference-- this is truly a field of global importance spanning many scientific specialties.  Here on the Internet, notable reading resources include the Australia Synchrotron's tutorial on macromolecular crystallography and the New Zealand Synchrotron Group's concise overview of synchrotron techniques... and much more.

Precise optic and sample positioning--often in vacuum and often requiring sub-nanoscale controllability--is fundamental to the science performed at synchrotron facilities.  Indeed, the MEDSI presenters' applications incorporated a wide range of motion mechanisms and methodologies including scanning, wavelength selection, gap adjustment, steering and focusing, seismic isolation and alignment automation.

Many of these applications involve exacting and specialized requirements: extreme stability, non-magnetic construction and complex configurations of multiple axes including mixes of motorized and piezoelectric actuation.  Accordingly, as we were preparing our tutorial we included input and content from our global organization, which includes focused domain expertise at PI miCos.  The resulting tutorial was truly the result of an intensive worldwide effort spanning many weeks.
From the PI miCos catalog, a 5m spectrometer
subassembly for synchrotron
applications.  Click to enlarge.

Our thanks to our hosts, the PI and miCos colleagues who contributed, and to the many dozens of conference participants who shared their expertise and enthusiasm for this exciting, important and productive field.  Special thanks are also due to our colleagues at PI Shanghai for all their helpfulness, knowledge and friendship. For the first-time visitor, it was a treat to see how this member of PI's worldwide network has evolved and expanded into a formidable design and manufacturing resource for our customers in the area and worldwide.  And finally, thanks to the sparkling city of Shanghai, with its friendly, industrious people and endless fascinations.


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.