This blog spotlights innovative applications of precision motion control, positioning, nanopositioning and micropositioning. We hope it is an enjoyable and informative resource, and a starting-point for cross-pollination and recombinant innovation across disciplines. Please let us know your comments and suggestions!
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:
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.
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!
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.
When most people hear the word "piezo" in the context of motion control, they understandably think of the classical piezoelectric stack actuator, composed of hundreds of thin layers of specialized ceramic interleaved with electrodes and sintered together. When a voltage is applied, the stack expands. Expansion is limited to about 1% of the stack length-- thus, a 100 mm long stack provide about 100 microns of travel. Clever, frictionless lever amplifiers can be fabricated (usually using sophisticated wire electric discharge machining) to provide magnified travel. In this way a compact piezo stage can provide hundreds of microns of travel.
This basic approach has served the microscopy industry well over our many years of manufacturing our popular PIFOC™ objective positioners, specialized linear motion devices optimized to tuck unobtrusively into a turret assembly while providing fast and straight axial positioning of the objective. However, microscopes' dimensional constraints limit the amount of lever amplification such devices can incorporate. 400 microns has traditionally been the limit for PIFOCs in our catalog.
Until now. Over the past several years, we've engineered new piezo technologies which provide much longer travels. Rather than rely on the simple expansion and contraction of the ceramic element, our various piezomotors utilize either ultrasonic linear actuation or various approaches to walking actuation. Each piezomotor principle has its inherent strengths for target applications but all provide theoretically unlimited travel, fieldless operation, high stiffness and holding force, nanoscale position-hold stability over long periods, and compact size.
N-725 PIFOC® is the first piezo-objective
drive with integrated NEXACT®
Piezo Linear Motor, combining
smooth motion, long travel ranges
and fast response with extreme position stability
Our NEXACT® motors, part of our PiezoWalk® family of ceramic motors, are an excellent example of all the above, plus sub-nanometer resolution. Their small size and impressive force makes them ideal for long-travel objective positioning, and they are at the heart of our new N-725 NEXACT PIFOC Objective Positioner. Offering a full 1 mm of travel, this unique mechanism offers high speed and maintenance-free operation. Its long travel helps accommodate varying substrates and easy load/unload operations, making it ideal for automation applications. And now it is available in systems integrating the Motion X FocusTrac™Autofocus Sensor for especially responsive and crisp autofocus actuation.
N-725 tracking focus of
a disk spinning at 300 RPM
Meanwhile, autofocus is now a capability which spans almost all PI motion device and controller combinations. Ease-of-use, stability, speed, applications flexibility and reliable focus capture from extreme out-of-focus conditions were notable design targets accomplished with all configurations. Systems integrating N-725 meet all these criteria over the full 1mm range of the device. Its sophisticated, all-digital E-861 Controller/Driver offers USB and RS-232 connectivity together with TTL utility and trigger lines and a joystick port. And, as a PI General Command Set device, it is supported by a wealth of proven software development tools and the leading microscopy suites.
Fast, automatic focus capture
from 900 microns out of focus!
A special capability is Fast Focus & Freeze (F3), PI's exclusive ability to capture and track the focal plane and then bumplessly switch to nanoscale-stable position-hold, with the ability to precisely position the objective with respect to the focal plane using the device's integrated position sensor. This is an invaluable capability for high-throughput automated Z sectioning and other quantitative studies where the focal plane serves as a datum plane. With N-725,F3means the initial condition can be up to 1 mm out of focus.
Count the enablers: unprecedented travel, easy integration, high responsiveness, fast actuation, robust focus-capture and tracking, and Fast Focus & Freeze. N-725 is a revolutionary addition to the microscopist's toolkit.
