Nanopositioning Terms & Definitions

What is a nanopositioner?

A nanopositioner is a mechanical stage that consists of a movable component inside a rigid frame. The movable and static portions are electrodischarge machined (EDM) from a monolithic block and connected by flexure "hinges." The movable component can move in any or all of three translational axes, as well as in angular axes such as rotation or tilt. The flexure mounting used by nPoint insures that motion in one dimension does not cause motion in any other dimension. A closed-loop control system allows the user to provide inputs to the positioner and to monitor the positioner parameters.

What are the components of a nanopositioner?

An nPoint motion control or positioning system is comprised of four elements:

• Piezo actuators providing motion
• Mechanical translation mechanism (stage)
• Position sensor
• Control electronics to maintain the desired position

To realize positioning at nanometer levels of precision, these four elements need to be carefully designed and optimized. The desired attributes of a nanopositioner are extremely high resolution, accuracy, stability, and fast response. Conventional motion control technologies are unable to meet these requirements and provide nanometer positioning.

The key factors are accurate position sensing, careful mechanical design, and closed-loop control of the motion. nPoint nanopositioning devices have capacitance position sensors, piezo actuators, and flexure motion control integrated into a single system. Stages that control motion in one, two or three axes are available, with virtually zero cross-talk between motion directions.

nPoint technology provides a wide range of motion, combined with precise location and high scan speed. nPoint features a patented use of materials for a unique combination of high-load capacity and high-speed response. nPoint employs Finite Element Analysis in the development of these ultra precise positioning systems which are based on flexure-guiding mechanisms. Flexure-guiding mechanisms achieve a pure, independent, single-axis (x-, y- or z-) motion with minimal parasitic errors.

What is the benefit of kinematic mounting?

The use of kinematic mounting virtually eliminates the effect of distortions that the piezo movement causes to the frame of the stage. It is assumed that the frame is rigid with respect to the part of the stage that moves; this is not true and can be detrimental to achieving true nanometer precision in positioning. The use of kinematic mounting also diminishes the effect that temperature variations can have on the drift of the stage.

What are the benefits of flexure stages?

Flexure stages employ a non-conventional 'flexure bearing' mechanism in the system, in which the moving platform is linked to a static base by flexure hinges. The platform's movement, driven by piezo actuators, is guided by the flexure mechanism. The guiding motion is generated by elastic deformation of the flexure material. Therefore, the linkage is friction and stiction free, resulting in smooth motion. Optimization of the flexure design mechanism results in high stiffness and load capacity while simultaneously neutralizing the parasitics errors, such as out-of-plan motion, titling errors and crosstalk.

nPoint flexure designs employ a multiple amplification factor to the piezo actuator's travel range enabling shorter piezos to be used for greater travel ranges. Stages can be built smaller and more economically than without the advantage of flexure amplification. Flexures do not experience frictional wear and operate within the elastic range of the material eliminating the need for routine maintenance while performance remains consistent over the operational lifetime of the stage.

What materials are your stages available in?

nPoint's stages are available in Aluminum, Super Invar and Stainless Steel.

Assuming the same mechanical design, an aluminum stage will have the highest resonant frequency, which will translate to higher scanning speeds and faster settling times. Aluminum exhibits a high strength to weight ratio making it an excellent choice for weight critical applications (e.g. aerospace). However, an aluminum stage is more susceptible to thermal drift.

The primary advantage of the Super Invar stages is the superb thermal stability. The thermal expansion co-efficient of Super Invar is approximately 50 times smaller than that of aluminum. When a stage is to be integrated into a larger instrument, the materials of the rest of the system's components need to be taken into consideration.

nPoint has even employed multiple materials into a single stage design, thus utilizing the benefits of each material for optimum performance. Understanding the requirements and operating environment the stage will operate in will enable us to recommend the right material for your project.

How do you achieve "closed-loop?"

Our nanopositioners use piezo actuators to generate motion. The expansion of a piezo actuator is normally proportional to the driving voltage. In an open-loop system a linear voltage is applied for motion control. However, piezo actuators exhibit nonlinearity, hysteresis and creeping, which will be translated as positioning errors in the positioning system. The term "closed-loop" in nanopositioning indicates that a position sensor is used to monitor the position of the stage in real time and feed back the readings to the controller for error correction. It guarantees that the nanopositioner reaches its precise commanded position. The position sensor is most commonly a capacitor, a LVDT or a strain gage. The control electronics are either analog or digital. nPoint nanopositioners use capacitor sensors coupled with digital control electronics to achieve closed-loop control.

What is the lead time for a custom design?

90 days from completion of the product design. The timeline for a product design is determined by the individual customer. The intricacy of performance specifications, the uniqueness of the footprint requirements, and the level of integration into the customer's system all factor into the product design time. nPoint's design team has worked with a wide range of OEM and custom research end users to achieve their specific product needs. nPoint's experience will help prioritize your systems operational parameters and achieve a system that will best fit your product specifications.

What is the lead time for a standard product?

45 days ARO

Can I purchase a stage without the controller?

nPoint nanopositioners are available as stand alone units. Some researchers prefer to develop their own electronic controls for use with our stages. Other researchers want to drive the stage in open loop. To achieve maximum benefit with nPoint nanopositioners, we recommend purchasing an nPoint system which includes the positioner, electronic controller, and operating software. nPoint systems are mated at our factory where the positioner is calibrated and optimized to operate with the nPoint controller.

Do you offer vacuum compatible stages?

Yes, all nPoint stages are compatible at 10 ^-6 Torr, we also offer certain stages compatible at UHV, 10 ^-12 Torr.

Do your stages use piezo actuators?

Yes, our nanopositioners use high quality multi-layer piezoelectric actuators for movement. nPoint purchases its piezos from the industry leader in multi-layer piezoelectric solutions. Our relationship ensures that the piezos used in our nanopositioners are optimized for our specific product needs.

How fast can I scan with your stage?

What does bandwidth mean?

Bandwidth is the frequency at which the actual oscillation amplitude is attenuated by -3dB from the commanded oscillation amplitude. This is approximately equal to the frequency at which the actual oscillation amplitude attenuates by 30% from the commanded oscillation amplitude. The bandwidth has to be tested with small driving signal, which guarantees that the attenuation of the actual oscillation amplitude is caused by bandwidth limitation, not by driving current limitation.

As an example: when a stage is driven to oscillate with a 1µm amplitude at 1Hz it will respond by oscillating at 1Hz with a 1µm amplitude. If its bandwidth is 80Hz, when it is driven with an 80Hz signal it will respond by oscillating with an amplitude of 0.7µm.

What is the maximum scanning range for your stages?

How does the step response specification relate to scanning speed?

A nanopositioner with a high resonant frequency will have a fast step response specification. A nanopositioner with a fast step response specification allows the user to increase the bandwidth of the control loop. A high bandwidth control loop reduces phase delay, which in turn allows the user to scan at an increased rate. Phase delay induces tracking errors that are especially noticeable at the corners of common scanning waveforms.

What interface options are available on nPoint controllers?

How can I control an nPoint positioner from my own PC program?

If your application simply requires the stage to move from one static position to another, you can position the stage through the standard USB interface. We offer both an ActiveX control and Labview drivers to communicate with the nPoint C300 controller by calling simple functions. The ActiveX control comes with an example project in Microsoft Visual Basic 6.0, and the Labview drivers come with an example Labview Virtual Instrument.

If you would like to control the stage from your own PC program for a scanning application, the easiest way to do this is by adding a high speed DAC card to your PC. The DAC card will come with drivers and program examples from the manufacturer. The most common high speed DAC cards are 16 bit resolution. This gives you a positioning resolution of approximately 1.5 nm. Positioning resolution is not the same as noise. When scanning the stage with a constant velocity or smooth acceleration, the bandwidth of the control loop and the mechanical momentum of the stage will tend to smooth out the 1.5nm steps output by a DAC card.

If you would like to control the stage from your own PC program and require the best possible speed and accuracy, we offer a High Speed Parallel interface. This interface can communicate with the controller at full loop speed. This allows you to command position and read sensor data for up to three channels every 24 microseconds. This equates to approximately 41 kHz, and 20 bit positioning resolution. The user can communicate via the parallel interface by either installing one of two National Instruments parallel cards in their PC, or by designing/modifying custom microprocessor based hardware. nPoint provides an example Labview Virtual Instrument using built in Labview functions for use with National Instruments PCI cards. The example is complex and requires an experienced Labview programmer to modify. There are no simple to use drivers available due to the complexity of managing buffers and speed in a Microsoft Windows™ environment. For more detailed information on the C300 High Speed Parallel interface, please request a PDF copy of the interface specification by emailing sales@npoint.com

How fast can I position the stage or read back sensor data with the USB interface?

The USB interface for the nPoint C300 controller can operate at a maximum command rate of approximately 500 commands per second. These commands can be split evenly between positioning and sensor monitoring (250 per second for each), or they can be dedicated to one function. The USB interface command rate is suitable for static positioning applications. Scanning applications should utilize the analog controller inputs or the High Speed Parallel interface.

Which DAC card do you suggest for a scanning application?

Abbe error

A positioning or measurement error caused by parasitic rotations when a misalignment exists between the measurement axis and the point of interest. By reducing either parasitic rotations or the offset of misalignment, or both, the abbe error can be minimized. The abbe error can be estimated as: δ = ι * α where δ is positioning error, ι is the offset of misalignment between the measurement axis and the point of interest, and α is the angle of parasitic rotation.

Accuracy

Represents how close the actual position of a nanopositioner is to the theoretical position to which it is expected to move. It is affected (or determined) by linearity error, hysteresis, abbe error, scale factor error and positioning noise, etc.

Backlash

The motion error produced by direction reversing of the motion. It is presented as a constant hysteresis over the range and is an inherent problem in conventional motion translation mechanisms such as screw/nut, gears, trains and bearings, etc. Normally it is related to machining tolerance, wear, contact stiffness, temperature and loads, etc. nPoint's flexure motion translation mechanism and piezo actuator designs are inherently backlash free.

Bandwidth

The frequency range to which the amplitude of the stage's motion is dropped by 3dB with a small input scanning signal. It reflects how well the stage can follow the driving signal for a particular frequency range.

Crosstalk

The positioning error along one axis generated while the nanopositioner moves in other axes, such as the stage's response in the X axis when the stage is driven in the Y axis. Occasionally, the static linear crosstalk error can also be interpreted as orthogonal error.

Drift

A position change over time, which includes the effects of temperature change and other environmental effects. The drift may be introduced from both the mechanical system and electronics.

Hysteresis

The positioning error between forward scan and backward scan. A closed-loop control is an ideal solution for the problem. Capacitance sensors are normally used in nPoint's nanopositioners to provide feedback signals. It is a non-contact displacement measurement technique, which is hysteresis free.

Linearity error

The error between the actual position and the first-order best fit line (straight line). nPoint's nanopositioning products are calibrated with laser interferometry and the non linearity errors are compensated with a fourth order polynomial.

Position noise

The amplitude of the stage shaking when it is on a static command. It is usually measured and specified with RMS (1 σ) value. It is commonly used to define the resolution of the nanopositioners and is a combination of sensor noise, driver electronics noise and command noise, etc.

Orthogonality error

The angular offset of two defined motion axes from being orthogonal to each other. It can be interpreted as a part of crosstalk.

Range

The maximum displacement of the nanopositioners with the performance specified.

Resolution

The minimum step size the stage can move. It is usually defined by the position noise.

Resonant frequency

The first (or the lowest) resonant frequency of a nanopositioner. The resonant frequency could be of the mode along the motion axis or in other axes including rotation and other complex modes. In general, the higher the resonant frequency of a system, the higher the stability and the wider working bandwidth the system will have. The resonant frequency of a mechanical mechanism is determined by the ratio of stiffness and mass. When selecting a nanopositioner to move large samples it is important to understand how the resonant frequency will change when the nanopositioner is loaded.

Scale factor

The displacement per unit of command. The scale factors of nPoint's nanopostioners are calibrated with a laser interferometer.

Settling time

The time for stage to move to a commanded position and settle to within 2% of its final value of the step size. A small signal step response reflects the dynamic characteristics of the system in more detail. Therefore, small signal settling time is normally used in the specifications of nPoint's nanopositioning products — the settling time for the response to 1 micrometer step signal.

Slew rate

The highest rate of the position change of a nanopositioner. It is measured from the response to a large step signal.