PI Physik Instrumente

Punching Out A Novel Optical-Tweezer Calibration Technique

2012-01-08T14:53:00.000-08:00

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.)

Best wishes--and a little gift!--from PI

2011-12-23T12:26:00.000-08:00

Thank you for a productive year filled with fascinating applications. Please enjoy our free Piezo University app from the Apple App Store and the Android Market!

"The Imaging Suite is the Microscope"

2011-12-01T21:52:00.000-08:00

Looking back at our publications in 2011, a favorite article was "Microscopy in 2011: The Imaging Suite is the Microscope," which appeared in American Laboratory. It touched on some important themes, including key applications in super-resolution microscopy, recent advancements in fast focus automation, and the importance of software in instrumentation.

That last point, regarding the importance of software, is familiar to the point of obvious for users. Software is the face of instrumentation to them. When great hardware supports great software and vice versa, productivity results. In contrast, hardware inadequately supported by software represents a frustrating waste of time for users.

Nowhere is this clearer than in the field of microscopy, where the classical limitations of optics are growing obsolete in the face of clever imaging and control techniques that tease resolution out of the application, and where the newly databased nature of discovery depends on coordinated, automated and networked control and communications of microscopes, staging and cameras.

We coined a phrase for this: "The Post-Rayleigh Era of Microscopy."

What this means: To the user, the face of the microscope is increasingly the computer screen. The knobs and buttons are increasingly virtual. The users themselves are increasingly remote. And the images are increasingly the result of sophisticated image acquisition and processing algorithms.

As the article discusses, a corresponding lesson is that today, equipment manufacturers and their users are both participants in a larger ecosystem which includes software vendors-- and in microscopy's case that means providers of imaging suites like MetaMorph, Micro-Manager and ScanImage. Accordingly, forging partnerships with software developers has been a priority for us. It's all in pursuit of productivity for our mutual customers.

Another reason the article was delightful for us was that American Laboratory, with its annual purchasing-plans surveys, was the very first publication to spotlight the advent of the personal computer as a top capital equipment line-item way back in the mid-'80s. The cycle was propelled by instrumentation programming suites like National Instruments' LabVIEW, introduced in 1986, and which instituted a clever graphical flowchart paradigm for composing automated applications called virtual instruments. NI's slogan was, "The Software Is The Instrument."

Indeed. And the imaging suite is the microscope today.

Industry's first piezo physics App-- Free

2011-11-30T13:02:00.000-08:00

Actual screen-snaps of the informative Piezo University app.
Available free for iOS & Android.

PI is the first nano/micropositioning company to offer its own informative, (mostly) non-commercial app for portable devices like the iPhone, iPad and Android phones and tablets.

Available free from the Apple App Store and the Android Market, the new Piezo University app offers illustrated glossaries, tutorials on piezo physics and mechanical design, and links to important industry resources.

All content is served live so it's always up-to-date and requires minimal storage. With its attractive, intuitive touch-enabled design and a wealth of thoughtful content, Piezo University deserves a place on your home screen.

Join the new Hexapods Discussion Group!

2011-10-28T07:58:00.000-07:00

Our first discussion group is live on Google Groups now and is open to all. The group can be viewed in a browser (http://groups.google.com/group/hexapods), or posts to it can be sent and received via email. Subscribe using the box below.

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We're looking forward to lots of informative and bi-directional applications and technology discussion. It's a great opportunity for cross-pollination and idea-sharing. See you there!

See you at the Pathology Visions Conference

2011-10-28T06:28:00.000-07:00

Digital pathology and telepathology are new weapons in fighting disease. Its enablers are the Internet and microscopy automation. At the Pathology Visions Conference in San Diego next week, we will be discussing key aspects of fast focus automation in particular. Here's the abstract:

Advances in high-throughput, high-reliability focus automation for digital pathology

Scott Jordan
Director, NanoAutomation Technologies
PI (Physik Instrumente) L.P.
scottj@pi-usa.us

Broad adoption of digital pathology depends upon reliable and repeatable slide digitization. In turn, repeatable/reliable whole slide imaging depends upon the ability to quickly find, hold and track focus. We discuss recent advances in piezoelectric focusing mechanisms and associated metrology of relevance to the community.

High-speed, high reliability focus optimization plays an important role in digital pathology by enabling faster capture of more repeatable images, by maintaining crisp focus during slide scanning motions, and by enabling real-time tracking over the acquisition intervals required by some emerging microscopy techniques. These attributes make focus automation a key variable in diagnostic concurrence.

Of the mechanical approaches available, piezo-actuator driven focus mechanisms combined with through-optic laser sensors offer the high-speed and high reliability required for meeting emerging demands. Piezo actuator driven mechanisms provide sub-millisecond response and can keep pace with throughput-driven methodologies. Thus they can improve process economics in digital pathology as they have in applications like gene sequencing, semiconductor lithography and interferometric metrology.

Here we review:

Four types of piezo actuators
Reliability and speed capability of piezo actuator driven focus mechanisms
Focus detection technologies often used with piezo mechanisms
Examples of piezo deployment for high speed focus in other industries
Key metrics for evaluating and selecting focusing technologies

New Technology Enables Focusing from Afar

2011-10-25T19:22:00.000-07:00

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.

Tip: How to Ensure the Best Resolution in Analog Interfacing

2011-09-05T13:47:00.000-07:00

A large portion of our customers' nanopositioning applications utilize analog interfacing for position-command. As we have discussed before, analog interfacing offers many compelling benefits including high speed, easy synchronizing, compatibility with our patented HyperBit DAC-resolution enhancement, straightforward generation of complex waveforms, and ready compatibility with external-sensor and tracking schemes.

But occasionally we will encounter a customer application where the analog-interfaced nanopositioning system isn't providing the resolution the customer expects. Most often, this is due to a simple issue: a mismatch between the voltage range of the customer's digital-to-analog converter (DAC) and the voltage range of the position-command input on the nanopositioner.

Consider the case where a 16-bit DAC (such as is common on multifunction cards installed in the customer's computer) offers a -10 to +10V range, but the input to the nanopositioner has a 0 to 10V range. The card's "16-bit"ness means that its 20V range is spread over 2¹⁶ steps... that's 65,536 steps. So the voltage resolution is 20 ÷ 65536 = 0.3mV. If the card were set to provide a range of 0 to 10V, then its resolution would be 10 ÷ 65536 = 0.15mV ...in other words, the resolution would be improved by a factor of two. The mismatch means an entire bit of resolution is lost!

Many (but unfortunately not all) multifunction analog I/O cards offer a configuration option (accessible, for example, via National Instruments' Measurement and Automation Explorer utility, NI-MAX, or via NI-DAQmx subVIs in LabVIEW) for setting the analog range of the DAC. Certainly, any application benefits from matching the analog ranges as closely as possible. For those occasions when it is not supported by the hardware, consider HyperBit as a way of recovering that lost bit... and many more.

Introducing Digital Control at an Analog Controller Price

2011-09-05T13:29:00.000-07:00

PI's groundbreaking E-709 Compact and Cost-Optimized Digital Piezo Controller leverages the very latest in digital electronics technology for dramatic value and performance. The unit features a very compact, panel-mount package and includes USB and analog interfaces for position commands and sensor monitoring as well as SPI for real-time interfacing in demanding industrial and research applications.

Digital control at an analog price: E-709

Despite its small size and analog-controller price, E-709 packs a host of features formerly found only on much costlier digital controllers:

A 10W peak-power amplifier
TTL utility interfaces for synchronization, triggering and signaling
A built-in data recorder
An internal waveform generator

This is a true digital controller, with a digital servo based on sophisticated, real-time algorithms. Beware the tendency of some to call any controller with communications interfaces "digital"! In a digital servo, gains and other parameters are software-settable, and the system is immune to DAC drift since the DAC resides inside the servo loop. And like all PI digital controllers, E-709 offers plug-and-play auto-calibration with our closed-loop nanopositioners. It is available in versions for PI nanopositioners with capacitive sensors or strain gauge and piezoresistive sensors, for which it offers unprecedented 5th-order digital linearization.

An unpackaged OEM version offers great performance
and value, with significant supportability benefits
from the plug-and-play auto-calibration and
remotely-accessible, software-based filter
parameters and diagnostic tools.

The E-709's digital servo offers a fast 10kHz update rate and integrates not just one but two notch filters. The importance of notch filters in providing responsive positioning performance deserves more mention than it's ordinarily given, as they desensitize the servo to observable resonances in the nanopositioner and its load and supporting structure. This allows significantly higher gains to be safely employed, which translates directly into crisp performance. All PI servocontrollers have notch filters; our digital controllers actually offer two. (Surprisingly, despite the manifest benefits of notch filters in nanopositioning, they remain uncommon in the marketplace.)

E-709 runs on any 24VDC source, making it ideal for OEM applications. In fact, an unpackaged version for OEM applications offers special cost-effectiveness. OEMs will also appreciate the supportability benefits of the software-settable servo parameters and plug-and-play automatic calibration. Research and industrial users alike will appreciate its utilization of PI's General Command Set, so applications written for any PI controller may be readily adapted to E-709 and vice versa. A host of software functionality is also supported, including comprehensive LabVIEW libraries and Windows .dll and Linux .so libraries.

E-709 is also ideal for autofocus applications ranging from research microscopy to industrial inspection, scanning and even the latest genomics applications. For example, it interfaces in real time with Motion X's superb FocusTrac through-optic focus sensors, providing precise, stable snap-in on the order of tens of milliseconds. It is compatible with our full line of classical PIFOC objective positioners and sample-positioning Z stages. It provides responsive real-time tracking, and it supports PI's unique Fast Focus & Freeze capability, where the unit can be bumplessly switched from external (focus) sensor to internal (capacitive, SGS or piezoresistive) sensor, allowing precise, calibrated, stable motions with respect to the focal plane.

E-709 offers a peek at the future of nanopositioning today, at an affordable price.

Attack the Stack

2011-05-15T22:37:00.000-07:00

Hexapods advance motion capabilities beyond convention

Multi-axis motion has conventionally been achieved by bolting-together multiple linear and rotary stages. And (speaking as a leading manufacturer of linear and rotary stages) this can certainly be an effective approach we thoroughly endorse.

But hexapods take multi-axis motion control to another level entirely:

PI hexapods' highly triangulated configuration and proprietary joint design results in greater stiffness than any stack of stages can provide. For example, the resonant frequency of our M-850 hexapod--our original model--with a substantial, 10kg load exceeds 90Hz transversely and 500Hz axially-- meaning this six-degree-of-freedom positioner when significantly loaded has a higher resonant frequency than many single-axis stages, unloaded!
Parallel kinematics means a PI hexapod’s six stiff linear actuators share the load of the moving platform, and their loading is purely axial, maximizing their stiffness. By comparison, with a stack of stages, only the top stage supports the load… the next stage supports the load and the top stage; the next stage supports the load and the top stage and the next stage… and so on, down to the bottom stage in the stack, which supports the load and all the other stages. This is a key reason for the superior stiffness and higher resonant frequency of hexapods, with direct impact on system responsiveness.
You need never tune a PI hexapod. On the other hand, each of the axes in a stack of servo stages will have its own tuning requirements, and optimizing for your load can be a significant chore for stacked configurations and can even risk damage.
Well-designed hexapods have no moving/sweeping cables to rub, wear and foul. After all, cabling tends to be the most unreliable part of conventional motion systems, so this advantage goes directly to reliability and MTBF. In addition, cable-borne vibration, rubbing and tugging will inevitably reduce the minimum incremental motion, repeatability and stability of stage stacks.

However, not all hexapods are created equal-- you will find some manufacturers whose cables drape and dangle, forming an untidy mess all the way to the controller. Besides raising reliability and stability issues, this is an invitation to electromagnetic interference. PI’s popular hexapods have exactly two cables going from high-quality connectors on the hexapod's base to the controller: one for the hexapod’s integrated, high-efficiency drive amplifier (no extra box required), and the other for control signals. Each cable has one connector on each end, for simple and reliable connections and painless setup.
The center-of-rotation for rotation stages and goniometers are fixed in space. By comparison, the center-of-rotation for PI hexapods may be placed anywhere in space with a single, simple software command. And, PI hexapods speak in human units: millimeters for the X, Y and Z axes (with resolution to 0.1 micron) and degrees for the pitch, yaw and roll axes (with resolution to 0.1 millidegree). This is all kept easy-to-use by the sophisticated digital controller which transparently handles all the coordinate transformations.
PI hexapod controllers offer advanced microrobotic capabilities like automatic vectoring and our General Command Set, which is both inherently multi-axis and easy-to-use. Powerful macro capabilities are built-in. Comprehensive and well-documented LabVIEW libraries, Windows .dll and Linux shared object libraries come standard and support the instrument’s high-level multi-axis capabilities, yet the sophisticated mnemonic command set can be utilized directly if desired. Up to two additional axes of servo-controlled motion can be optionally provided by the controller. Two channels of optical or analog data acquisition can similarly be provisioned internal to the controller. RS-232 and TCP/IP interfaces are standard, with GPIB a cost-effective option. And PI hexapods provide a repeatable absolute coordinate system that is consistent from power-up to power-up; beware alternative configurations which rely on springs to support the load or address hidden backlash issues in the drivetrain.

While PI hexapods offer many advantages over stacks of stages, one thing in common is a great diversity of available configurations. Specialized PI hexapods are available, ranging from vacuum-compatible models (for which the naturally low-surface-area hexapod configuration offers additional benefits compared to stage stacks), to ultra-stable models designed to carry high-bandwidth active-optic packages in professional telescopes, to the popular F-206, optimized for high-throughput photonic packaging automation.

PI has many years of experience in designing and manufacturing hexapods for the world's most demanding applications. As a consequence, PI makes more hexapods than all competitors combined. Our experience benefits your application with superior performance, reliability, software and global support. PI hexapods range in size from smaller than a coffee can to the size of a small car. If you don’t see what you need for your application, count on responsive support from a PI applications engineer at any of our worldwide offices.

How to Push a Rope: Enabling Accurate On-The-Fly Sectioning And Sampling with DDL

2011-04-07T14:07:00.000-07:00

Any physical or electronic mechanism has finite bandwidth. One practical consequence of this for nanopositioning is often seen in waveform actuation: sharp corners get rounded, and phase lags begin to accrue. This fundamentally limits the accuracy of sampling and sectioning techniques which infer position from time. In such situations, the diminished dynamic accuracy from the following error which occurs as a consequence of finite system bandwidth can be a significant limiter for the application. As the old saying goes, "You can't push a rope."

Except, you can. Read on.

For typical nanopositioning systems, the system bandwidth is limited by the resonant frequency of the loaded piezo stage; one-third of F_res is a reasonable rule-of-thumb for the bandwidth in such cases, and significant corner-rounding, attenuation and phase lags are seen well below this number. So, the obvious way to increase system bandwidth is to choose the stiffest (highest loaded F_res) stage (and, of course, reduce the load mass). However, the stiffness of the stage goes inversely as the square of the lever ratio of its integrated lever amplifiers, so high-F_res stages are typically limited in travel. The past few years have seen strikingly compact long-travel piezo stages become very popular; these achieve their long travel and small size through the use of novel lever amplifiers with high ratios, and one consequence is low F_res. An example is our P-629.1CD PIHera linear stage, which provides an amazing 1.5mm of closed-loop travel in a 100x100x22.5mm package, and a F_res of 110 Hz with a 120g load.

One way around the following error that inevitably results from finite system bandwidth is to sample position simultaneously with acquiring your other data using a deterministic, low-latency interface such as the analog, SPI or parallel I/O (PIO) interfaces offered on our controllers. This means you always know the exact position at which your data was acquired, so if the stage doesn't track your desired waveform perfectly at high speeds, it might not matter. A good example of a fast application enabled by this trick is our CyberAligner Modular Alignment Workstation targeted at characterization and packaging automation applications for waveguides and other fiber-coupled devices.

But this is not always an optimal solution. Perhaps the application demands that data be equally spaced in both position and time, or perhaps the quantity being measured can vary with instantaneous velocity (an example being current generated by moving a nanocoil probe over a sample containing small magnetic features). For such applications, there is no substitute to improving the fidelity of the position waveform. But conventional closed-loop servo technologies cannot address limited system bandwidth and often contribute significantly to it.

We offer two unique solutions:

Advanced Piezo Control, a proprietary servo algorithm which is optional on our top-of-the-line E-712 digital nanopositioning controller. This technology is ideal for virtually eliminating following error in tracking applications where the stage path is not predefined.
Dynamic Digital Linearization, a technology available in most of our digital controllers (the link is to the "Methods to Improve Piezo Dynamics" article in our Piezo University). This technology can reduce the following error of repetitive scan waveforms down to the system noise level. For highly-leveraged stages with inherently low resonant frequency, the improved dynamic accuracy can be remarkable.

Full 1.5mm piezo scan showing enhanced dynamic
accuracy/reduced following-error from Dynamic Digital Linearization

A recent customer application spotlights this. The application required rapid but very precise scanning over the full 1.5mm range of the P-629.1CD stage, with precise TTL signals from the nanopositioning controller at specific points in the waveform to trigger other instrumentation. We approached this application using LabVIEW and DDL: First, the controller's internal waveform generator is enabled and the waveform parameters and TTL trigger-output specifications downloaded via our comprehensive standard set of LabVIEW subVIs, then waveform generation is commenced. We used our digital controller's built-in data recorder capability to verify the impact of DDL in this application. The figure shows the tracking performance of the loaded P-629.1CD before and after enabling DDL.

This has proven to be an enabling technology for this and other applications in fields as diverse as semiconductor metrology, defense and clinical life sciences, where rapid scanning requirements increasingly include previously unapproachable dynamic accuracies over long travels.

The Original Low-Latency, High-Throughput, Real-Time Interface

2011-02-24T10:24:00.000-08:00

Choosing Analog Hardware

As the world grows more digital, increasingly advanced communications interfaces have been instituted in PI nanopositioning and micropositioning controllers and many other instruments. Even the new E-709 compact, cost-optimized digital nanopositioning controller comes complete with an SPI interface for command communications at the servo update rate of the controller (in addition to standard USB and RS-232 interfaces and a wealth of TTL trigger and synchronization lines). We discussed some of the newest novel interfaces and their applications in the presentation linked in our earlier blog post, Gleanings from SMB 2011.

But it's important not to overlook the benefits of the original fast, deterministic, real-time interface: analog. Most of our nanopositioning controllers are equipped with analog interfaces for position or autofocus command input, plus a feedback sensor monitor output (for closed-loop units). Even our current-generation digital nanopositioning controllers feature analog I/O options across-the-board.

There are many compelling advantages to analog interfacing:

Generating analog position steps and waveforms is very easy with today's software tools and powerful multifunction analog I/O hardware for personal computers.
The most popular analog I/O products, from National Instruments, share a common software interface between models. So software written for one model will work on other models and is readily transportable across operating systems.
Highly synchronous multichannel I/O is readily achieved in any of the popular programming languages, meaning nanopositioning processes can be performed in parallel with process metrology-- essential for improving throughput in scanning and other high-dynamic processes.

Programming analog I/O is straightforward enough for most users to do from scratch if they need to, but PI's Analog GCS LabVIEW library eliminates even that. This library is the industry's first plug-and-play LabVIEW library for instant productivity with analog-interfaced nanopositioning controllers. It is available without charge and allows the use of the same comprehensive and well-thought-out selection of subVIs as used with any of our other controllers. The GCS (General Command Set) approach means only the initialization subVI specifies an interface (for example RS-232, USB, TCP/IP, GPIB or in this case analog). So you can move from PI product to PI product or interface to interface simply by swapping out the initialization subVI. Functionalities in the Analog GCS LabVIEW library include point-to-point and waveform position generation with synchronous acquisition, a wealth of synchronization and triggering options, and much more.

As to hardware, this link will open a window to a National Instruments product selector page that presents a wide variety of analog multifunction devices suitable for most nanopositioning applications. The following criteria were used:

At least one analog input for process metrology or sensor acquisition
At least one analog output for commanding the position of open- or closed-loop nanopositioners
Waveform output capability (versus static-only updates for some other models.
Compatibility with the NI-DAQmx driver, the gold standard for analog I/O programming in any language.

Additional selectors are available to tailor the selection to your application needs and budget.

Although we can of course only test our Analog GCS LabVIEW library with a subset of NI's extremely broad offering, all the devices shown should work since all work with the NI-DAQmx driver.

If your budget allows, our top recommendation of all the current NI offerings is the top-of-the-line USB-6259 BNC (see photo). This is a portable, self-contained USB unit capable of astonishing throughput and microsecond-scale synchronization-- performance that frankly took us aback when we first used it, as we expected the USB interface would be a bottleneck. It has since been our recommendation for our CyberAligner high-throughput modular alignment engine since it also allows the workstation to be run from a laptop or virtual machine. It offers sixteen 16-bit differential analog inputs with an acquisition rate of 1.25 mega-samples/sec and four 16-bit analog outputs with a blistering update rate of 2.8 mega-updates/sec. That fast analog output speed makes it the ultimate choice for enhanced-resolution HyperBit applications, and we have achieved 27 bit positioning resolution using this unit. Its integrated BNC connectors are convenient and eliminate having to purchase a cable and BNC box.

Of course, all that functionality may be overkill for your application and budget. Fortunately there are plenty of other models with different form factors, fewer I/O options, different resolution, and more moderate speeds. There is even an unpackaged OEM version of the USB-6259.

There's lots of life left in analog, a venerable and truly real-time interface.

Celebrating Twenty Years of Digital Photonic Alignment

2011-02-12T13:25:00.000-08:00

A cost-effective, modular alignment engine is updated

The first digital gradient search technique was developed two decades ago to allow fiber optic devices to be efficiently aligned using the micropositioning devices of the day, which were low in speed, resolution and synchronization capabilities compared to today's piezoelectric nanopositioners. Systems based on this technology were the earliest of a decade-long wave of offerings targeted at industrial automated alignment applications.

As the telecom boom crested, we were approached by a leading industrial player to provide an especially cost-effective, robust and flexible alignment platform for coarse/fine alignment of photonic devices such as waveguides and laser diodes. The desire was for a simple stack of stages based on our NanoCube XYZ nanopositioning stage, which provides 100 microns of travel in three orthogonal axes with 2nm resolution. The customer specified that the software was to be modular, open-source and based on LabVIEW.

We reviewed this and similar applications, noting challenges like fiber-through-tube package designs and irregular coupling cross-sections. We decided that comprehensive application coverage together with highly time-efficient throughput was possible with a sequence of two operations:

A space-efficient double-spiral-scan, using motorized long-travel stages, for first-light capture and rough optimization, followed by
An extremely fast raster scan with synchronous data acquisition to compile the transverse coupling cross-section and identify the global maximum.

An advantage of the raster scan approach versus established gradient search techniques was its insensitivity to local maxima in the coupling-cross-section; this option had been unavailable a decade earlier due to the limitations of the motion devices of that day. Put plainly: since our piezo devices are so fast, why not collect lots of data to localize the global maximum directly rather than inferring the vector to it from the limited data older architectures could provide?

We called the result CyberAligner, and a YouTube video of it in action is still viewable (see below).

Recently this architecture enjoyed many advancements:

An upgrade to LabVIEW 2010,
Leveraging of the latest I/O capabilities provided by today's multifunction analog/digital hardware, including incredibly fast USB units,
Motion code based on PI's GCS General Command Set, allowing any type of motorized stages to be used for first-light seek and coarse alignment: cost-effective stepper-motor, robust DC servo-motor, stiff and stable NEXACT^®, swift PILine^®... and whatever will come next.

The update virtually doubles CyberAligner's speed from the already blazingly-fast version shown in the video, now allowing a full-field scan-and-align on the order of 250 milliseconds. The coupling cross-section data can be saved to a local or network drive, providing valuable process and device diagnostics in production. All-USB configurations are featured, cabling is simplified, and multiple workstations can be run off of one PC. Source-code, compiled and .dll versions of the modular workstation software are offered.

Today, as the twentieth anniversary of the first digital aligners dawns, the photonics industry is reawakening after years in the doldrums after the telecom bubble popped. With CyberAligner as part of PI's broad toolkit of solutions including the popular F-206 HexAlign six-degree-of-freedom hexapod alignment microrobot, we look forward to meeting a new generation of device and applications challenges.

Gleanings from SMB 2011

2011-02-07T13:51:00.000-08:00


PI's SMB 2011 presentation on fast interfacing techniques is available at this link.

Every two winters the leaders in the field of Single Molecule Biophysics gather at the Aspen Center for Physics for a fast-paced five days of presentations, networking, discussions and the occasional slalom down Buttermilk. The topic: the very latest in research and experimental approach in this most demanding and promising of fields.

Most of us view chemistry as the bulk behavior of macroscopic quantities of chemicals that interact according to sensible rules based on mass conservation, valence properties, and so on. Single Molecule Biophysics (SMB) takes chemistry quite a few steps deeper, combining clever optical, biochemical, instrumentation and computing approaches to allow observation of the activities of individual molecules as they go about their work making things happen... this thing called life, for example. SMB applications allow us to observe the transport, replication and transcription processes that occur continuously in every living cell. They also illuminate the fundamental processes of what can go wrong with life as well. Observing molecular behavior is already revealing the molecular foundations of many diseases, including dread afflictions like cancer and Parkinson's disease. With understanding will come cures.

Clearly this field is among the most "nano" of nanotechnologies, placing severe demands on performance and stability of the coarse and fine positioning equipment that is the foundation for many SMB applications. For the past two conferences, PI has been proud to be a sponsor and participant. For 2011 we presented a non-commercial review of fast interfacing techniques that have proven valuable for extending the capabilities of SMB-class applications. This paper has been reformatted for easy viewing and is now available at this link. Though it primarily spotlights SMB applications, we hope its information is valuable for any advanced application that can leverage interface throughput and determinacy.

2011's conference showed fascinating progress in the field, exemplified not only by the standing-room-only attendance and ongoing refinement of our understanding of the molecular basis of life, but also by several fascinating presentations from early industrial adopters of SMB techniques. These entrepreneurs are going to market with the first therapies and machines that leverage the field's teachings, and their endeavors represent the first commercial fruits of a highly important young field. Given the rapid pace of this energetic and innovative community, it is very safe to say that your life and those of your loved ones will benefit from SMB-rooted understanding and technologies.

Our deep thanks go to the organizers and participants of this fascinating conference.

Fascinating insights into sub-nm localization of molecules from Berkeley

2010-12-20T14:10:00.000-08:00

Subnanometre single-molecule localization, registration and distance measurements

Some attention-getting work performed by Alexandros Pertsinidis, Yunxiang Zhang and Nobelist Steven Chu (also the United States' Secretary of the Department of Energy) has revealed nanoscale error mechanisms believed to be universal in CCD-based wide-field imaging systems. The work was published in the prestigious journal, Nature.

The team developed mapping and active-stabilization techniques and demonstrated that these can combine to improve the optical localization of fluorescent molecules by an order of magnitude, to a resolution of 0.5nm and an accuracy of 0.77nm. This represents a significant advance for fields such as FRET microscopy --important for observing the shape-changes of proteins, as it reveals their mechanical action and thereby illuminates the foundation of many diseases.

A key revelation in the research was that much of the microscopy system's ability to localize molecules optically was limited by high-spatial-frequency non-uniformities in the inter-pixel photoresponse of the CCD. These nanoscale errors, which appear worse at the interstices between pixels, have the effect of a finely-patterned crinkling of the pixel distribution, as if the map of pixel locations had been wadded up and then re-flattened. Since fluorescent molecules are localized to sub-diffraction precisions by calculating the center of their (Gaussian) Airy disk image as it illuminates many pixels, this non-uniformity limits the precision and accuracy of the localization.

The stabilization of the imaging setup is achieved by imaging an illuminated pinhole onto the CCD. This serves as a bright fiducial and can be localized to 0.3% of a pixel. Its position is used as feedback to stabilize the optical system against perturbation, using a high-speed piezoelectric tip/tilt mount. The locked-in stability of the system was measured to be 0.64nm over several hours. A separate feedback system based on the optical position of a selected fluorescent molecule continuously adjusts the XYZ piezoelectric nanopositioning stage carrying the test sample with respect to the optics. These continuous, sub-nanometer-precision processes place a premium on interface throughput and responsiveness.

Combining these tools, the researchers developed a method of calibrating small regions of the CCD, allowing localization of molecules to 0.5nm-- a factor of ten better than had previously been achieved. The technique can be extended to cover larger areas of the detector, up to its full extent.

Acknowledging the importance of recombinant innovation across fields, the authors note:

"...Our methodology might also prove valuable to characterize/design precision photometric imaging systems in fields such as atomic physics or astronomy... the subnanometre closed-loop control and registration afforded by our technique could become essential concepts in design of future sub-10-nm optical lithography tools and may allow new nanometrology applications."

Tantalizingly, the researchers state that in an upcoming publication, the mapping technique is extended to the intra-pixel level with approximately 1 Angstrom precision, promising another round of groundbreaking innovation and further insights into the activities of the molecular machines that underly all life.

Memristors advance

2010-06-08T08:48:00.000-07:00

The challenge of keeping Moore's Law rolling as semiconductor linewidths dip to 20nm (only about eighty copper atoms wide) has driven such key new process technologies as nanoimprint lithography and EUV optical lithography. It has also driven quite a few nanopositioning developments, such as positioners capable of picometer resolution plus high holding forces and active vibration isolation systems which provide nanoscale-stable worksurfaces and foundations-- essential enablers for today's most advanced process tools.

But it is also driving development of fundamentally new materials and technologies. Graphene, a planar cousin to carbon nanotubes and buckyballs, is one of these, and is showing great promise for devising denser and faster microelectronics.

And then there are memristors, announced two years ago by HP and predicted in theory almost four decades ago by Prof. Raymond Chua of the University of California. Coverage in the IEEE Spectrum journal was particularly informative and is the source of the following two images.

First, these astonishing nanoscale devices are regarded as no less than the fourth fundamental passive electronic component (after resistors, capacitors and inductors):

Memristors are like resistors with memory. Think of them as nanoscale potentiometers that can be reversibly set and read in conventional binary 0/1 ways but which are also capable of achieving and maintaining "in between" states. Thus a single memristor only a few nanometers in extent can conceivably store many bits' worth of information, inviting a significant change in microelectronic architecture.

This means many things, most obviously some serious new competition for flash RAM and hard disks in the near future. Now Nature reports that these devices have been demonstrated as building-blocks for logic circuitry as well. Tantalizing recent developments by researcher R. Stanley Williams and colleagues are summarized in a recent IEEE Spectrum article,

The component’s use in computer memory was a foregone conclusion. The memristor can reversibly change its resistance depending on how much current flows through it. The researchers’ surprising new discovery is that a memristor can handle either data storage or logical computation depending on the amount and duration of the current sent through it. Three memristors can complete a NAND operation, the researchers report, so any Boolean function can be implemented if you string enough of the devices together.

Clearly, memristors herald a new chapter in semiconductor engineering, applications and fabrication techniques. But might there be applications beyond the microchip? A key purpose of this blog is to bring novel technologies to the attention of our diverse customer base in hopes of spurring cross-pollination and recombinant innovation across multiple fields. Perhaps memristors will play a role in enabling something entirely new: maybe a novel lab-on-a-chip, or miniaturized autonomous dataloggers powered by piezo scavenging, or nanoprobes with onboard signal processing and storage. More likely, it will be none of these-- who, at the birth of the any of the other three fundamental passive circuit elements, could have predicted the computer you're reading this on?

Williams is featured in a fascinating and disarmingly casual online video describing memristors, below.

Novel approach illuminates nm-scale stabilities over many minutes

2010-02-22T09:38:00.000-08:00

Researchers at the Block Lab at Stanford University devised a novel test which quantifies the stability benefits of long-travel sample positioning stages based on piezomotors as opposed to classical, screw-driven mechanisms. It has long been suspected that lubricant flow at the screw/nut interface would contribute to long-term settling and creep behavior and that a well-designed linear piezomotor would avoid this, but the necessary instrumentation to confirm this at the nanoscale over many minutes' time has not existed, as conventional interferometry and similar techniques are themselves insufficiently stable over the timeframes required.

Publication of the article followed the 5th Biennial Winter Workshop on Single Molecule Biophysics at the Aspen Center for Physics in 2009 and includes both an overview of nanopositioning terminology and specifications and a remarkable graph which visualizes the stability of a piezomotor and a good-quality screw-driven sample stage.

Piezo-based vibration isolation advances nanolithography

2010-02-22T09:24:00.000-08:00

So many of today's essential technological wonders would be impossible without the semiconductor industry's relentless and decades-long advancements in production techniques and capabilities. Today's powerful microprocessors and digital signal processors gain their capabilities and affordability from manufacturers' ability to repetitively and consistently replicate nanoscale structures with high yield using optical lithography. In addition to controlling the alignment and position of silicon wafers throughout their processing cycle, the machinery must be isolated from ambient vibrations from machinery, roadways, natural seismicity and even people walking around.

TMC has led the industry in leveraging the ability of piezoelectric actuators to achieve nanoscale stabilities which enable the next generations of nano-lithography. In an article in Semiconductor International, this capability is described, along with critical reliability advancements which ensure that semiconductor fabs can consistently and economically crank out the products our modern lives depend on.

Announcement: PI-USA is ITAR-compliant

2010-02-22T09:09:00.001-08:00

Many nanopositioning applications involve export-control considerations. PI-USA is pleased to note that we have instituted organizational structures and procedures to comply with ITAR requirements that often apply in these situations. Just advise your PI sales or applications engineer if your application involves export-controlled information.

A close-up look at Single-Molecule Biophysics

2010-02-22T09:09:00.000-08:00

XVIVO has composed a remarkable animated look at today's understanding of the molecular activities that go on in the living cell. This understanding is based on the quickly-evolving field of Single-Molecule Biophysics, which utilizes innovative tools such as optical tweezers and nanopores to illuminate the most fundamental processes of life: the dynamical mechanics of biochemistry. For us, this is a field which thrives at the furthest frontiers of nanoscale positioning control. It drives many advancements in resolution and functionality, and it depends on long-term nanoscale stabilities of every component in the system.

New Products: Family of USB nanopositioners with integrated, 24-bit DACs

2010-02-22T08:41:00.000-08:00

Our exciting, expanding line of digital and analog nanopositioning controllers is growing every month. As ever, our aim is to provide the broadest selection for the most optimal match between product and application. We hope you'll review our latest news on the controller front and contact your local PI applications professional to discuss your needs.

A fascinating video on Super-Resolution Microscopy

2010-02-22T08:37:00.000-08:00

One of the most astonishing and useful developments in the worlds of biology and biophysics in recent years has been the rapid advent of techniques such as Photoactivated Localization Microscopy (PALM) and Fluorescence Resonance Energy Transfer (FRET), which allow researchers to construct images with molecular-scale resolution, far finer than Abbe's classical diffraction limit allows. The field--collectively termed Super-Resolution Microscopy--is illuminating biological structures that are the fundamental scaffolding of life, the framework of disease, and the stepping-stones to cures.

The prestigious journal Nature has produced an instructive online video which provides a concise overview of this field:

PI (Physik Instrumente)

2010-02-18T12:49:00.001-08:00

PI (Physik Instrumente)