The Software Is the Hardware Capability

Author(s):
Scott Jordan - Physik Instrumente

Industry:
Industrial Controls/ Devices/ Systems, Electronics

Products:
LabVIEW,

The Challenge:
Preserving high resolutions for the latest generation of long-travel nanopositioners for customers using popular NI analog I/O cards to command piezo position

The Solution:
Using National Instruments LabVIEW and a broad array of NI multifunction data acquisition devices to develop, demonstrate, document, and deploy a new technology for increasing DAC resolution, which virtually eliminates the travel-vs.-resolution tradeoff.

As the world’s leading nanopositioner manufacturer, we partner closely with our customers to solve the toughest positioning challenges in research and industrial fields as diverse as life sciences, defense, semiconductor manufacturing, and photonics. Nanopositioners are based on the exquisitely fine dimensional expansion of piezoelectric ceramics when a voltage is applied. Sophisticated, frictionless lever mechanisms multiply this expansion to provide usable travel ranges for the customer’s load.

Recently, compact yet very long travel piezo flexure mechanisms have been introduced. Because the piezo actuation is approximately proportional to the applied voltage, the number of addressable positions of a nanopositioner is equal to the number of distinct voltage steps available from the digital-to-analog converter (DAC) in the system: 2^N where N is the number of bits for the DAC. As travels have lengthened, DAC granularity causes positioning resolution to worsen proportionally. This was an increasing problem for our many users of popular NI multifunction analog I/O devices, which provide high speeds and accuracies and the ability to synchronously sample multiple analog lines while outputting position command waveforms of arbitrary complexity.

Customer Requirements

A customer’s laser nanopatterning application posed a special conundrum. Their 3-dimensional nanopatterning process required tight coordination between the piezo actuation and laser control, and their patterns were extremely detailed and complicated. Analog interfacing under the control of LabVIEW would be ideal, and legacy LabVIEW code existed which would greatly facilitate the customer’s work. But a high-quality 16-bit DAC would yield 65,536 addressable positions across the minimum target travel range of 300 mm, meaning a resolution of about 5 nm, exceeding the customer’s positioning resolution tolerance. Alternate I/O cards with additional DAC resolution available at the time were unacceptable for reasons including functionality, compatibility, or speed.

A solution came to mind: although nanopositioners are very fast, NI DACs are faster, with many thousands or even millions of analog updates per second, controlled by precision on-board clocks. Could the least significant bits (LSBs) of a DAC be pulse-width modulated at high rates? If so, then the unused time-domain capability of the DAC would be converted by the nanopositioner into additional sub divisional positioning resolution, effectively adding M “virtual bits” to the N physical bits of the DAC, with no loss of system bandwidth, accuracy or stability. And perhaps more importantly, could this be done in a practical and user-friendly or even transparent fashion?

A LabVIEW Solution

We began to work on sample LabVIEW code for LSB PWM. We created a quick laboratory setup: a PI nanopositioner, the laptop from the airplane, a NI DAQCard 6036E analog I/O card with BNC-2110 connector box, and a Polytec laser interferometric vibrometer. The DAQCard was used to generate the commanding voltage to the nanopositioner while synchronously monitoring its position. The ability of the LSB PWM technique to add many bits of resolution was instantly demonstrated, with each added bit representing a doubling of performance.

With LabVIEW we quickly packaged the code in a modular subVI that could be dropped into existing VIs to provide additional bits of analog resolution with no other change to the customer’s system and with fully transparent compatibility with both traditional NI-DAQ and NI-DAQmx. A review of the literature prompted us to apply for U.S. and international patents on the principle for both software and hardware implementations: U.S. Patent #6,950,050 was quickly awarded; foreign patents are pending.

NI-DAQ Ensures Hardware Compatibility, Flexibility

Because the LabVIEW code interacts with the NI-DAQ or NI-DAQmx driver, differences between hardware choices are managed automatically. We verified the addition of 2 to 12 additional bits of resolution across a variety of NI analog I/O hardware, including R Series FPGA-based boards. M, the number of added virtual bits of resolution, has been shown as:

M ≈ log₂(Update_Rate ÷ PWM_Frequency)

where PWM_Frequency is typically chosen to lie well above the mechanical resonant frequency of the device to prevent any excitation.

The high speed of most NI analog I/O hardware effectively means that DAC resolution ceases to be a performance bottleneck in nanopositioning applications (though of course other possible bottlenecks such as ambient disturbances or amplifier noise remain).

In theory, virtually any DAC can benefit. Applications are not limited to nanopositioning. For example, electro-optic devices such as cavities and modulators gain resolution in the same way. Hardware implementations are also possible; even consumer products such as high-end audio and video devices can benefit from additional bits of resolution in their drive circuitry. The technique has also been demonstrated in the actuation of MEMS mechanisms; in that case the ultimate benefit is the reduction of die complexity and cost since complex high-bitness DACs need not be integrated in order to achieve good resolution.

Conclusion

LabVIEW played a foundational role in the development, demonstration, documentation and deployment of this new technology, tradenamed HyperBit™. By multiplying the resolution of popular NI analog I/O hardware while maintaining compatibility with LabVIEW and customers’ existing code, we preserved the option of using flexible and responsive analog interfacing for our customers who make use of the new generation of long-travel nanopositioners and other devices whose performance may become bitness-limited as applications advance.

For more information, contact:

Scott Jordan

PI (Physik Instrumente)

6537 Fall River Dr.

San Jose, CA 65120

Tel: (408) 268-9486

E-mail: Scottj@pi-usa.us

To read more about this application, visit: http://www.physikinstrumente.com/en/news/fullnews.php?VID=&newsid=107

 

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