PI Piezo Nanopositioning Systems: Superior Performance Piezo Stages
PI is the world-leading supplier of piezo stage nanopositioning alignment and scanning systems.
PI piezo systems with frictionless drives and flexure guidance are decidedly superior to conventional positioning stages with crossed roller bearings, etc. in terms of resolution, reproducibility, straightness and
Achieving nanometer and subnanometer precision requires more than a piezo-driven stage capable of making moves on this precision scale. The stage internal metrology system must also be capable of measuring motion on the nanometer scale. The primary characteristics to consider when selecting a stage metrology system are linearity, sensitivity (resolution), stability, bandwidth, and cost. Other factors include the ability to measure the moving platform directly and contact vs. noncontact metrology. Three types of sensors are typically used in piezoelectric nanopositioning applications — capacitive, strain, and LVDT. See Table 1 below for a summary of the characteristics of each sensor type.
PI capacitive sensors measure the gap between two plates based on electrical capacitance. These sensors can be designed to become an integral part of a nanopositioning system, with virtually no effect on size and mass (inertia). Capacitive sensors offer the highest resolution, stability, and bandwidth. They enable direct measurement of the moving platform and are noncontact. Capacitive sensors also offer the highest linearity (accuracy). PI's capacitive sensors / control electronics use a high-frequency AC excitation signal for enhanced bandwidth and drift-free measurement stability (subnanometer stability over several hours, see here). PI’s exclusive ILS linearization system further improves system linearity. If used with PI’s digital controllers, digital polynomial linearization of mechanics and electronics makes possible overall system linearity of better than 0.01%. Capacitive sensors are the metrology system of choice for the most demanding applications.
A strain gauge sensor is a resistive film bonded to a piezo stack or—for enhanced precision—to the guiding system of a flexure stage. It offers high resolution and bandwidth and is typically chosen for cost-sensitive applications. As a contact type sensor, it measures indirectly, in that the position of the moving platform is inferred from a measurement at the lever, flexure or stack. PI employs full-bridge implementations with multiple strain gauges per axis for enhanced thermal stability. PI's PICMA® drive technology also enables higher performance of actuator-applied strain gauge sensors.
LVDT sensors measure magnetic energy in a coil. A magnetic core attached to the moving platform moves within a coil attached to the frame producing a change in the inductance equivalent to the position change. LVDT sensors provide noncontact, direct measurements of position. They are cost-effective and offer high stability and repeatability.
Table 1
Sensor Type
Sensitivity* (Resolution)
Linearity*
Stability* / Repeatability
Bandwidth*
Metrology Type
Excitation Signal
Capacitive
Best
Best
Best
Best
Direct / Noncontact
AC
Strain
Better
Good
Good
Better
Inferred ** (Infect) / Contact
DC
LVDT
Good
Good
Better
Good
Direct / Noncontact
AC
* Note. The ratings describe the influence of the sensor on the performance of the complete nanopositioning system. Resolution, linearity, repeatability, etc. specifications in the PI product data sheets indicate the performance of the system including the controller, mechanics and sensor. They are verified using advanced external nanometrology equipment such as Zygo Interferometers. It is important not to confuse these figures with the theoretical performance of the sensor alone.
** Strain type sensors (metal foil, semiconductor, or piezoresistive) infer position information from strain.
Ultra-Low-Inertia Nanopositioning Systems, Ultra-High Precision Position Sensors
Ultra-low-inertia
solid-state piezo stages can repeatedly move bidirectional nanometer level steps, at up to hundreds of Hz if required.
PI offers the largest variety of custom and standard ultra-low inertia Nano-Positioning solutions.
Examples of Parallel-Kinematics / Metrology NanoPositioning Systems
Capacitive position sensors are the metrology system of choice for the most demanding applications.
Two plate capacitive sensors ensure highest precision, linearity and longterm stability.
These absolute-measuring, non-contact sensors detect motion at sub-nanometer levels directly (direct metrology)
and provide linearity, accuracy, resolution, stability and bandwidth superior to strain gauge type sensors (piezo resistive
sensors), LVDT sensors and incremental encoders (glass scale type encoders). If used in parallel-kinematics multi-axis piezo stages
they can also provide the information for automatic runout-compensation.
Minimized recoil forces are a by-product of the ultra-low-inertia approach. Classical motorized stages, even when equipped
with high-resolution encoders cannot achieve this precision.
Their higher inertia, friction, and servo dither prevent fast motion at the nanometer level.
Because flexure motion is based on the elastic deformation (flexing) of a solid material, friction and stiction are entirely eliminated. Flexures exhibit high load capacity, stiffness,
and resistance to shock and vibration. Flexures are maintenance free
and not subject to wear. They are vacuum compatible, operate over a wide temperature
range and require neither lubricants nor compressed air for operation, as air bearings do.
Not all flexures are created equal! PI multi-axis nanopositioning systems are based on
FEA calculated wire-EDM-cut parallel-kinematics flexure designs.
These multilink flexure guiding systems eliminate cosine errors and
provide bidirectional flatness and straightness in the nanometer or microradian range.
This high precision means that even the most demanding positioning tasks can be run bidirectionally
for higher throughput.
Wire-EDM cutting process
provides highest-accuracy flexure guiding systems in compact nanopositioning stages.
Typical 0.5 µrad bidirectional trajectory repeatability (P-752.11C
piezo stage) means processes may be performed bidirectionally for twice the productivity
(graph, right)
PI nanopositioning systems employ the award-winning PICMA® piezoelectric actuators, the only actuators with co-fired
ceramic encapsulation. The PIMCA® piezo technology was specifically developed by PI’s piezoceramic division to
provide higher performance and lifetime in nanopositioning applications.
Piezoelectric multilayer actuators are similar to ceramic capacitors and are not affected by wear and tear.
PI nanopositioning systems are designed to be driven at lower voltages than most other piezo systems (100 V vs. 150 V).
The research literature, PI’s own test data and 30+ years of experience, all confirm that lower average electric fields
lead to longer lifetime.
Active Trajectory Control is avail- able on single-module parallel-metrology nanopositioning systems.
It can improve straight- ness and flatness to sub-nano- meter precision. Digital control- lers with advanced
coordinate transformation algorithms allow active trajectory control for up to 6 DoF.
Active Trajectory Control significantly improves guid- ing precision.
It requires a Parallel Metrology Sensor setup.
Elliptical scan in a laser micro-drilling application with XY piezo stage,
conventional controller. The outer curve describes the target position,
the inner curve shows the actual motion of the piezo stage.
Same scan as before, with Dynamic Digital Linearization.
The tracking error has been reduced to a few nanometers, real and target
positions are indiscernable in the graph
Preshaping™ algorithms and dynamic digital linearization can increase the dynamic linearity and effective
bandwidth of high-speed piezo nanopositioning systems by up to 3 orders of magnitude. This translates into higher
dynamic accuracy, and increased throughput.
Conventional PID (proportional-integral-derivative) piezo controllers cannot
completely eliminate phase lag and tracking errors (the difference between actual and target positions) in dynamic operation. This is due to the nonlinear nature of piezoelectric material (PZT), the limited control bandwidth, and the fact that a PID controller needs to see an error before it attempts to correct it.
DDL solves this problem. PI offers DDL as an option for digital controllers such as the E-710. This PI-exclusive technology reduces tracking errors (the difference between the commanded position and actual position) and phase lag in dynamic applications to virtually indiscernible levels. The result is an improvement in dynamic linearity and usable bandwidth of up to three orders of magnitude. Dynamic Digital Linearization works both in single-axis and multi axis applications (see graphs).
The example above shows ringing of a poorly damped component on a high-speed piezo nanopositioning stage.
While the closed-loop piezo stage settles perfectly, the component cannot keep up. Conventional
solutions to this problem would involve slowing down the Nanopositioning stage. Mach™ eliminates ringing without sacrificing speed.
It does not even require retuning of the servo system.
The exclusive Mach™ Throughput Processor™ eliminates resonant ringing, allowing rapid motion without a
settling phase.This technique also eliminates resonances excited in neighboring components, outside the
nanopositioning system's servo loop. The result is significantly increased throughput.
Self-generated vibration affects:
The load and fixturing that
the nanopositioner actuates
The supporting structure on
which the nanopositioner is mounted
All other components attached
to the supporting structure
The example above shows vibrations induced at the beginning of a saw-tooth scan, typical in image acquisition applications. The vibration results in lower image quality. Mach™ improves the image quailtiy; there is no need to reduce the scanning frequency or chang the mechanical components in the system.
Mach™ is available as a firmware option for several PI Digital Piezo Controllers and also as an upgrade option for analog controllers.
This technology is protected by one or more of the following US
and foreign Patents licensed from Convolve, Inc.: US 4,916,635; US 5,638,267;
0433375 Europe; 067152 Korea, and other Patents pending. Mach™, Throughput Coprocessor™
and NanoAutomation® are trademarks of Polytec PI, Inc. Input Shaping™ is a trademark of Convolve, Inc.
Serial Kinematics vs. Parallel Kinematics in Nanopositioning Systems
Stacked Serial Kinematics Flexure
Nanopositioning System
Simple Design, but:
Slower response (lower stage
carries inertial mass of upper stage);
Non-symmetric resonant frequencies
(lower stage is slower than upper stage, requires different servo settings).
Orthogonality error is mounting-angle
dependant.
Runout in Y cannot be monitored/compensated
by the sensor in the X stage or vice versa.
Nested Serial Kinematics Flexure
Nanopositioning System
Thinner and better response than Stacked Serial Kinematics, but no other advantages
Parallel Kinematics / Metrology
Flexure Nanopositioning System
Same ultra-low inertia for
X and Y motion, providing higher responsiveness and axis-independent performance.
Excellent, mounting independent
orthogonality.
Parallel Metrology for reduced runout: X sensor (PI
uses non-contact two plate capacitance sensors) can monitor and correct
for Y runout and vice versa (Active
Trajectory Control).
Additional rotation axis (Theta
Z) feasible with 3 actuators / sensors and digital controller.
* Ask about custom sizes, sensors or special designs.
Capacitive and LVDT sensors are direct metrology devices.
Capacitive sensors provide the highest accuracy, bandwidth and linearity.
Introduction to Piezoelectric Nanopositioning Actuators (Nano-Transducers)
Piezo Actuators (PZT) are ultra-high-resolution Nano-Transducers for a variety of applications from Nanotechnology
to Aerospace and Biotechnology. PI offers the largest selection Piezo Actuators and Translators (linear actuators) worldwide, for scientific and and industrial applications.
In addition to the hundreds of standard models, we manufacture custom designs tailored to customers’ requirements. PI is highly vertically integrated, controlling each manufacturing step from piezo raw materials to finished systems.
* Ask about custom sizes, sensors or special designs.
** Preloading increases tensile force capacity
SGS = high-resolution strain gauge sensor LVDT = Linear Variable Differential Transformer
Further information about piezoceramics can be found in the PI Ceramic catalog or at www.piceramic.de.
