Air bearings and piezo actuators revolutionize the road to super-fine, super-fast movements.
Just a few years ago, systems able to position to within 10 min. (10 millionths of an inch) were considered state of the art. Today, the bar has been raised. Companies routinely require the ability to position to fractions of a micron, and nanometer (10-9 m, or 0.001 micron, or 0.000,000,039 in.) requirements are not unusual. They are increasingly common in areas such as biological cell separation, disk-drive manufacturing, and fiber-optic alignment.
The need for such positioning has given rise to considerable technology for making precision products with ultrahigh accuracy. Engineers in research and development labs have pushed far beyond the nano envelope. The next challenge is to measure in pico (10-12), femto (10-15), and atto (10-18) meters.
Today, manufacturers are using piezoelectric actuators or motors for fine, accurate, and fast movement. When positioning in nanometers, piezoelectric motors have many advantages over conventional magnetic motors. These advantages include high precision, repeatable nanometer and subnanometer-size steps, quick response (they are the fastest responding positioning elements available), and no wear and tear because piezoelectric motors are solid state.
Piezo comes from the Greek word for pressure (piesi). The piezoelectric motor operates using solid-state piezoelectric ceramic crystals, which can be used to convert electrical energy into mechanical energy and vice versa. The precise motion that results when an electric field is applied to a piezoelectric material is valuable in nanopositioning.
Nanometer-scale motion platforms typically use piezo actuators or stages for fine, accurate and fast movements. The piezo actuator usually sits on top of linear cross-roller or air-bearing stages that provide coarser positioning. This combination of elements lets the system use a primary stage to move into the general vicinity of interest, then employ the secondary piezoelectric stage for final subnanometer moves.
There are, however, problems when positioning in nanometers under real-world conditions. Factors such as temperature changes and noise (in mechanical and electronic components) cause vibration that makes nanometer measurements useless if not isolated adequately.
Subnanometer movements are also pointless unless they can be measured. The other challenge, then, is in selecting sensors for feedback loops that adequately tell the controller where the stage is. Encoder manufacturers have developed several types of sensing elements to meet such needs.
Another difficulty is that use of a piezo stage on top of another stage adds not only to the height of the system, but to its cost as well. This applies doubly to a dual-axis system (which is two times the height).
|The Nanostepper positioner has no moving parts. Its four axes of motion sit in a small 7-in.2 envelope just 1.2-in. high. Use of rare-earth permanent magnets as part of the linear stepper motor magnetics let it operate upside down if need be. One or more piezo-positioning stages sit on the linear-motor platform to provide 10 mm of travel with 20-nm resolution.|
|The magnetic stack of the linear step motor consists of two electromagnets with a rare-earth permanent magnet between them. Selectively energizing the electromagnets induces the forcer to move in steps. It takes four steps to move one pitch, the centerline distance between adjacent stator teeth on the platen. Microstepping allows motion inside of a full step to 0.0002 in. (5 microns).|
Vibration and stiffness both play important parts in nanometer measurements. The ideal system has a low profile and high stiffness, so the goal is to keep the stage height as low as possible. In applications demanding extremely straight or multiaxis motion, a stacked stage alone may not be up to the task.
An effective approach combines the submicron resolution of piezoelectric motors with the long travel of linear steppers. An example is the Nanostepper multiaxis positioner. It is essentially a dual-axis linear stepper motor with one or two piezoelectric stages mounted orthogonally in the housing, giving two coarse axes of motion and one or two fine axes in a single plane. The total thickness of these stages is 30.5 mm and they are designed for small and light loads.
The dual-axis linear stepper motor provides coarse positioning in X and Y and generally operates open loop. It includes integrated motor air-bearings and a positioning system. The (two or four-phase) dual-axis linear stepper motor consists of a moving forcer and a stationary platen. The platen, serving as the motor secondary, has a surface etched with a steel square-tooth pattern. The spaces between the teeth are filled with epoxy and the surface is lapped to a precision flatness. The platen is currently available in sizes up to 914 x 1,498 mm.
The forcer serves as the motor primary and is comprised of between four and eight or more magnetic stacks. Half of them mount orthogonally to the other half. This configuration is what lets the Nanostepper move in X and Y directions.
Each stack element contains two electromagnets and a rare-earth permanent magnet between them. Four sets of teeth on each forcer are spaced in quadrature so only one set at the time can align with platen teeth. Selectively energizing the electromagnets moves the forcer in steps. It takes four steps to move one pitch, the centerline distance between adjacent stator teeth.
The rare-earth magnets also preload the air bearings to the steel platen. They exert enough force to let the motor be inverted without having the forcer separate from the platen.
The magnetic flux passing between the forcer and platen gives rise to a strong force of attraction between the two pieces. The Nanostepper positioner can produce a holding force at any position, and when all power and air is removed, it magnetically detents in its last position with over 200 lb of holding force. This holding force helps to isolate any mechanical vibration and lets the piezo motor execute subnanometer moves.
The coarse positioning stage of the Nanostepper can hit 40 ips with up to 2 gs of acceleration. Forces available are from 2 to 80 lb in each axis. Practically, the two-phase stepper can reach a resolution of about 5 microns (0.0002 in.) open loop on coarse travel. Even better accuracy is available by mapping the platen, that is, measuring its minute peaks and valleys and writing software to compensate.
Finally, the Nanostepper positioner's air-bearing system lets the forcer glide over the platen. It is nothing more than a thin film of air that separates the two accurately machined surfaces. The bottom of the positioner housing is also the air-bearing surface, and it floats on 45 to 60 psi of filtered air. As air has a low viscosity, there is zero static and zero dynamic friction and the stage exhibits good shock and vibration resistance.
The piezoelectric motor uses solid-state piezoelectric ceramic crystals to produce motion. The ceramic crystals expand and contract as an electric field is applied. Expansion and contraction under an alternating electric field causes a controlled, sinusoidal vibration. Each vibration cycle produces a small step, as small as a few nanometers. The accumulation of these steps at a rate of 130,000 steps/sec lets the motor travel at speeds of up to 12 ips. The piezo stage also has a 10-mm travel in X and Y.
Position-sensing electronics are a key element. The piezoelectric stages operate closed-loop with linear optical encoders. Special optics let them resolve moves to 5 nm or less. Laser interferometers are the only alternative technology able to gauge such distances, but they are more expensive.
The sinusoidal signals are electronically interpolated to allow detection of displacement that is only a fraction of the optical fringe period. The encoder electronics amplify, normalize, and interpolate the output to the desired level of resolution. Signal-processing technology corrects for gain, phase, and offset errors to improve system performance.