Piezoelectric Nanopositioning and Scanning Systems with Frictionless Drives Offer Superior Performance

PI Nanopositioning and scanning systems with frictionless flexure guidance are decidedly superior to positioners with conventional guiding systems (crossed roller bearings, etc.) in terms of resolution, reproducibility, straightness and flatness. Due to their inherent friction and limited guiding precision, traditional positioners are best used in applications requiring repeatability on the order of 0.1 µm, even though encoder readout may indicate much higher resolution. In contrast, PI piezo-driven flexure nanopositioners can easily achieve repeatability and minimum incremental motion in the subnanometer realm.

Higher Speed

With piezo drives, capable of accelerations of up to 10,000 g, and their low moved mass, such piezo stages can provide significantly higher scanning speeds than motorized systems.

Why Flexures?

Flexure motion is based on the elastic deformation (flexing) of a solid material. Friction and stiction are entirely eliminated, and flexures exhibit high stiffness, load capacity 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.

Figure 1. Long-travel (10 mm) multi-dimensional, Roberts-linkage flexure system prevents parallelogram errors in XYZ positioning applications.

Excellent Guiding Accuracy

The multilink flexure guiding systems employed in most PI piezo nanopositioners (Fig. 2) 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.

Figure 2. Wire-EDM cutting process provides highest-accuracy flexure guiding systems in compact piezo nanopositioning stages

Lifetime / PICMA Piezo Actuators

PI PZT nanopositioning systems employ the award-winning PICMAR piezo actuators, the only actuators with cofired ceramic encapsulation. The PIMCA piezo technology was specifically developed by PI's piezoceramic division to provide higher performance and lifetime in nanopositioning applications.

Multilayer piezo 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.

Measuring Nanometers: Stage Metrology Selection

Achieving nanometer and subnanometer precision requires more than a piezo 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 five 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 measurement. Three types of sensors are typically used in piezo nanopositioning applications- capacitive, strain, and LVDT.

Table 1 summarizes the characteristics of each sensor type.

Table 1. Sensor characteristics

* Note. The ratings describe the influence of the sensor on the performance of the whole nanopositioning system. Resolution, linearity, repeatability, etc. specifications in the PI product data sheets indicate the performance of the complete system and include the controller, mechanics and sensor. They are verified using external nanometrology equipment (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.

Capacitive Sensors

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. 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.

Strain Gauge Sensor

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 fullbridge 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

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.

Figure 3. Response of a PI Nanopositioning stage to a square wave control signal clearly shows the true sub-nm positional stability, incremental motion and bidirectional repeatability. Measured with external capacitive gauge, 20 pm resolution.

Parallel and Serial Designs, Controller Choice

Parallel and Serial Kinematics Mechanisms

There are two ways to achieve multi-axis motion: parallel and serial kinematics. Serial kinematics (nested or stacked systems) are simpler and less costly to implement, but they have some limitations compared to parallel kinematics systems.

In a multi-axis serial kinematics system, each actuator (and usually each sensor) is assigned to exactly one degree of freedom. In a parallel kinematics multi-axis system, all actuators act directly on the same moving platform (relative to ground), enabling reduced size and inertia, and the elimination of microfriction caused by moving cables (Fig. 4). This way, the same resonant frequency and dynamic behavior can be obtained for both the X and Y axes. The advantages are higher dynamics and scanning rates, better trajectory guidance as well as better reproducibility and stability.

Figure 4. Principle of a PI XYΘ_z, minimum inertial-mass, monolithic, parallel kinematics nanopositioning system. Accuracy, responsiveness and straightness/flatness are much better than in stacked multi-axis (serial kinematics) systems.

Direct Parallel Metrology

Multi-Axis Measurements Relative to a Fixed Reference

Parallel kinematics facilitates implementation of Direct Parallel Metrology - measurement of all controlled degrees of freedom relative to ground. This is a more difficult design to build but it leads to clear performance advantages. A parallel metrology sensor sees all motion in its measurement direction, not just that of one actuator. This means that all motion is inside the servo-loop, no matter which actuator may have caused it, resulting in superior multi-axis precision, repeatability and flatness, as shown in Fig. 5. Direct parallel metrology also allows stiffer servo settings for faster response. Offaxis disturbances - external or internal, such as induced vibration caused by a fast step of one axis - can be damped by the servo.

Figure 5. Flatness of an active-trajectory controlled nanopositioning stage over 100 x 100 µm scanning range is about 1 nm.

Analog and Digital Controllers

PI manufactures a large variety of analog and digital nanopositioning controllers. State-of-the-art PI digital control systems offer several advantages over analog control systems: coordinate transformation, real-time linearity compensation and elimination of some types of drift.

Digital controllers also allow virtually instant changes of servo parameters for different load conditions, etc. However, not all digital controllers are created equal. Poor implementations can add noise and lack certain capabilities of a well-designed analog implementation, such as fast settling time, compatibility with advanced feed-forward techniques, stability and robust operation. PI digital controllers can download device-specific parameters and calibration information from ID-chipequipped nanopositioning stages, facilitating interchangeability of nanomechanisms and controllers.

All PI nanopositioning controllers (analog and digital) are equipped with one or more user-tunable notch filters. A controller with notch filter can be tuned to provide higher bandwidth because sideeffects of system resonances can be suppressed before they affect system stability. For the most demanding step-and-settle applications, PI's exclusive Mach(TM) InputShaping implementation is available as an option.

Controllers / Interfacing

Digital Dynamic Linearization (DDL)

The E-710 digital controller features an internal algorithm that can eliminate tracking errors and nonlinearity in repetitive waveforms to increase linearity and effective bandwidth in scanning applications by up to 1000-fold (Fig. 6a, 6b).

Figure 6a. Six-axis digital piezo controller with Super Invar 6D- nanopositioning stage. All PI nanopositioning systems and controllers are fully CE compliant.

Figure 6b. Rapid scanning motion of a P-621.1CD (commanded rise time 5 ms) with the E-710 controller and DDL option. Digital Dynamic Linearization virtually eliminates the tracking error (<20 nm) during the scan. The improvement over a classical PID controller is up to 3 orders of magnitude, and increases with the scanning frequency.

Choice of Interface: Digital or Analog?

Analog interfacing provides high bandwidth and remains the most common way of commanding piezoelectric motion systems. It is usually the choice when the control signal in the application is provided in analog form. A key advantage of analog interfacing is its intrinsic deterministic (realtime) behavior, contrasted to the difficulty of accurately timing high-bandwidth communications on present-day multitasking PCs.

However, when analog control signals are not available, or when a significant distance between the control signal source and the nanopositioning controller would affect signal quality, digital interfacing, which must not be confused with digital control, is the preferred choice.

Digital signals can be transferred through copper wires, or for complete EMI immunity, through optical fibers.

Five types of digital interfaces are typically used in piezo-nanopositioning applications: parallel-port, RS-232, IEEE 488, USB, Fiber Link and, with some digital controllers, direct DSP links. For dynamic, high-precision applications, the exact timing of an interface is more important than the data transfer rate.

Interface Bandwidth vs. Timing

Piezo-driven stages can respond to a motion command on a millisecond or microsecond time scale.

That is why synchronization of motion commands and data acquisition have a high impact on the quality of many applications, like imaging or micromachining. The USB, for example, was designed to transfer huge blocks of data at high speeds, but exact timing was a much lesser concern. While insignificant in less responsive positioning systems, this kind of non-deterministic behavior may not be tolerable in high-speed tracking or scanning applications. Each motion command - comprising just a few bytes - must be transferred instantaneously and without latency. A lower-bandwidth bus with higher timing accuracy may perform better in many applications.

There are several factors that affect the response of a digital interface: the timing accuracy of the operating system on the controlling computer; the bus timing protocol; the bandwidth of the bus; and, the time it takes the digital interface (in the piezo controller) to process each command.

Parallel-port interfaces do not require command parsing and offer the best combination of throughput and timing accuracy.

In addition, to the interface properties, the bandwidth of the nanopositioning system (mechanics and servo) matters. A slow system (e.g. 100 Hz resonant frequency) will not benefit from a responsive interface as much as a high-speed mechanism.

CE Compliance

All standard PI nanopositioning systems are fully CE compliant.

Test and Calibration - Why High-Quality Nanometrology Equipment Matters

Piezo nanopositioning systems are significant investments and PI believes in optimizing the performance of every customer's system. PI individually calibrates every stage and optimizes the dynamic performance for the customer's application. Furthermore, PI makes significant continuing investments in improved-quality, higher-performance nanometrology equipment so that we can deliver better value to our customers.

Because a nanomechanism can only be as accurate as the equipment it was tuned and tested with, PI closed-loop stages are calibrated exclusively with the prestigious Zygo ZMI-2000 and ZMI-1000 interferometers. PI's nanometrology calibration laboratories are seismically, electromagnetically and thermally isolated, with temperatures controlled to better than 0.25 C° / 24hr. We are confident that our calibration capabilities and procedures are the benchmark for the industry.

Additionally, as part of our philosophy of continuous improvement, formal NIST traceability is scheduled to be established by the end of 2006.

This information has been sourced, reviewed and adapted from materials provided by PI (Physik Instrumente) LP, Piezo Nano Positioning.

For more information on this source, please visit PI (Physik Instrumente) LP, Piezo Nano Positioning.