Industrial Machining Embraces Nano Positioning
|Linear motors based on the converse piezoelectric effect have a multitude of “limbs” attached to a stationary frame.|
Forty years ago, in the early days of computer numeric control (CNC) machine tool equipment, standard machining tolerances were a few thousandths of an inch. High precision reached an order of magnitude better—to a few ten thousandths of an inch. Today, advances in automated machining allow tolerances to reach below one micron (1 mm). that’s 0.00004 in!
Descending to length scales on the order of a fraction of a micron and smaller brings us into the world of nanotechnology—the world of objects a few hundred nanometers and smaller. Here, the rules change. Conventional measurement techniques become physically impossible. Actuators relying on fine-pitch precision ballscrews simply can’t do the job.
It is impossible for human beings to get an intuitive feel for nanometer sizes. The nano world is simply too far removed from our macroscopic experience. Scientists, who have increasingly dealt with the nano world for decades, simply learn the relative sizes of objects in that world:
Ten hydrogen atoms fit in 1 nm almost exactly.
The wavelength of green light is approximately 350 nm.
Viruses (or, more correctly, virii) vary in size from a few tens to a few hundreds of nanometers.
A DNA molecule is 2.2 to 2.6 nm wide.
Entire DNA molecules, however, are almost big enough to see, being up to 73 million nm, or about .075 mm, long.
Widths of the smallest structures on the latest microprocessors are about 45 nm.
If one micron were blown up to the width of a two-page spread in this magazine, 1 nm would be 1/32 in. wide.
Below 1 mm, applicable technology changes significantly. The limit of feedback devices are easily reached, as are the kinds of motors that can actually move in that realm. These are real concerns for growing numbers of engineers, as some applications already require holding positions to a tenth of a nanometer.
Issues when working at that scale include working slowly, at the rate of a nanometer per second. For reference, this is slower than your hair grows. Such precision requires a different level of controls capability as well as feedback devices. When moving that slowly, you’re not getting very much information between each position movement, so you have to be able to create linear motion with little information.
Ancient abilities, new technologies
Despite the recent media hype surrounding the word “nanotechnology,” preparation of materials at the nanometer level is one of the oldest technologies known. For example, the exquisite colors and detail of cave paintings at Lascaux in southwestern France, which were created some 16,000 years ago, were made possible by ancient humans’ ability to consistently grind metal oxides into nanometer-sized powder particles.
Nanopositioning began to have practical importance with the invention of the scanning tunneling microscope (STM) by IBM Zurich researchers Gerd Binnig and Heinrich Rohrer in 1981, and later the atomic-force microscope (AFM) by Binnig, Calvin Quate, and Christoph Gerber in 1986.
These instruments actually use the converse piezoelectric effect, where certain crystalline materials change shape or dimension when acted on by an electrostatic potential. The converse piezoelectric effect can be used to create nanoscale motion in two ways:
Direct motion, in which a structure made of piezoelectric material pushes directly on the object to be moved;
Piezo motors, also called ultrasonic motors, in which a number of small crystals energized at ultrasonic frequencies create motion by alternately pushing and retracting like the legs of a caterpillar.
|CNC machine tools capable of nanometer-level precision fabricate exquisite mechanical components. Source: ACS Motion Control|
The converse piezoelectric effect is actually quite small. Changing the thickness of a 10 mm thick piece of lead zirconate titanate (PZT, the most commonly used piezoelectric material) by 100 nm would require a potential of 2,670 V!
Practical direct-motion piezoelectric actuators require stacking many elements. Instead of using one piece, stacking 100 wafers 0.1 mm thick energized in parallel reduces the voltage by a factor of 100 to 27 V.
Direct motion actuators are limited to very small total motions on the order of 100 nm, but can achieve very high resolution. For the 100-wafer actuator above, changing the applied voltage by 2.67 V changes the thickness by 10 nm.
It is interesting to note that such piezoelectric actuators can provide forces up to the order of 1,000 lb and accelerations on the order of 1,000 g.
Piezo motors solve the problem of limited travel while maintaining the system’s exquisite resolution. They can be made for linear or rotational action.
Linear versions are the easiest to understand. Imagine a two parallel stationary structures with a plunger able to move freely along the centerline between them. A row of “limbs” are attached to one of the stationary structures and extend into the space between so they can contact the plunger. A second row attaches to the second stationary structure to contact the plunger’s other side. Both rows have an even number of limbs.
Each limb comprises two parts: a longitudinal “leg” whose length changes when voltage is applied, surmounted by a shear-moving “foot” that twists to one side or the other when voltage is applied. The limbs then “walk” the plunger in the desired direction in nanometer-size steps. By repeating these steps at high frequency, it is possible to rapidly move the plunger in either direction, with unlimited range and nanometer precision.
Rotary piezo motors can be made in a similar way. Instead of limbs lined up on either side of a straight plunger, rotory units place limbs around the circumference inside a circular stator, which contact a rotating shaft.
At such levels of positioning accuracy and precision, motor and mechanical stage characteristics play a large role. The ability of stage manufacturers to meet fabrication tolerance requirements, including straightness, resonance decoupling, and mechanical dampening are important. Motor
|Human ancestors used powdered oxides ground to nanometer sizes to create beautiful works of art 16,000 years ago. Source: GettyImages|
issues such as cogging, phase imbalances, and other non-linear behavior degrade performance. Electrical noise and bus-voltage ripple causes additional disturbance to ideal positioning. Temperature and friction also can be detrimental to a motion system. Specifically, variations in these parameters change mechanical characteristics and will induce disturbances in the position control system.
The problem with piezo motors is that they behave very differently from conventional motors. Overcoming non-linear effects requires a motion controller with high bandwidth, a lot of processing power, and great flexibility to implement control algorithms specific for nanopositioning.
Whereas loop frequencies for normal machining are 1-4 kHz, loop frequencies for nanometer-level motion need to be more like 32 kHz. While the motor control needs high bandwidth to provide stability, making that stable motor move extremely slowly requires predictive control.
Sensors with nano precision
To achieve nanometer precision, position sensing must also have nanometer precision. A number of position sensing technologies are available, including capacitive, magnetic, linear voltage displacement transducers (LVDTs), and optical encoders. Laser interferometer feedback devices are also used for feedback, particularly in photonic applications and for space-qualified systems. In this article, we’ll concentrate on capacitive and optical technologies.
Capacitive sensors use the variation in capacitance between two parallel plates set a small distance apart. Because such a structure’s capacitance varies conversely with the distance between the plates, it is not difficult to achieve nanometer precision measuring displacements through a small range.
Typically, the capacitor is used to modify the output frequency of a crystal-controlled oscillator, and modern frequency measuring circuits are capable of very high precision. If the oscillator shifts through the frequency range of 10-11 MHz as the plates move a distance of 1 mm, and the frequency measuring equipment can sense variations of 1 Hz, the system provides a sensitivity of 1 nm/Hz, Therefore, the overall resolution is 1 nm.
Optical encoders, on the other hand, can be made with scales on the order of a few hundred nanometers. Achieving lower resolution requires interpolating between fiducials to very high accuracy. Suppose fiducials are separated by 500 nm. Interpolating by a factor of 1,000 provides 0.5 nm resolution.
Nanopositioning systems have not only found application in, but also make possible, numerous industries including data storage, semiconductors, microelectronics, precision mechanics, life science, medical technology, optics, photonics, and nanometrology.
The semiconductor industry is well into having tens of nanometers be the critical dimension of an integrated circuit. When you pick up your iPhone or iPod today, 32 nm is the length of the transistor gates. The next step in semiconductor process evolution will be to 18 nm.
|Nanopositioning stages move and locate macroscopic objects for machining, inspection, and other manufacturing tasks. Source: Physik Instrumente|
For wafer inspection, where defects need to be observed at less than one micron in size, imaging with pixel sizes of less than a micron requires nanometer-level positioning to keep a constant speed. The more counts, the more constant that speed becomes and the better the images appear.
As the hard disk drive industry moves to increasingly higher density, testing requires nanometer-level resolution. Hard drive manufacturers need nanometer-level positioning to accurately place tracks. At 500,000 tracks per inch, tracks are separated by only 50 nm.
In molecular biology, for cell research and genomic research, being able to do something with a cell requires precision at the 10 nm level. Emerging chemical industry applications include constructing new chemicals by positioning atoms, and combining molecules through nanopositioning.
Diamond-turning machines are basically very high-end lathes used for machining hard materials, such as glass and ceramics, to nanometer precision. In the materials sciences, people looking at very small nanostructures in their materials use scanning electron microscopes to step every 25 nm or so. There are also photonic applications where people align lasers or lenses with nanometer precision, such as bandwidth filtering.
The majority of these applications incorporate electronically automated controls. Nanometer-level measurements are done in a temperature controlled environment, and keeping humans out is key, because humans generate heat and can affect the measurement.
The machine tool industry is starting to use nanoscale CNC equipment for very high precision applications, such as in space-qualified mechanisms, and physical instruments for measurement. Nanopositioning in machine tools is a nascent industry, however. A recent show near Hoffman Estates, IL, included exhibits from all the people in the U.S. building desktop CNC nanopositioning tools; it fit in the lobby of a hotel. In five or ten years, however, it will be a very different industry.
Control Engineering wishes to thank the following individuals, who provided the information for this article: Kevin Steele, sales and marketing manager for semiconductor and medical industries at Bosch Rexroth ( www.boschrexroth-us.com ). Kevin Kaufenberg, national sales and product manager for the electronics, semiconductor and medical markets at Heidenhain Corp. ( www.heidenhain.com ). Stefan Vorndran, director of marketing communications at Physik Instrumente, LP ( www.physikinstrumente.com ). Jason Goerges, control and applications engineer, ACS www.acsmotioncontrol.com ).
|Blue-green light has a wavelength of 500 nm.|
|C.G. Masi is a senior editor with Control Engineering. Contact him via email at firstname.lastname@example.org|
Piezoelectricity and nanopositioning technology
TEXT: Nanopositioning began to have practical importance with the invention of the scanning tunneling microscope (STM) by IBM Zurich researchers Gerd Binnig and Heinrich Rohrer in 1981, and later the atomic-force microscope (AFM) by Binnig, Calvin Quate, and Christoph Gerber in 1986.
These instruments actually used the converse piezoelectric effect, where certain crystalline materials change shape or dimension when acted on by an electrostatic potential. This effect was predicted by Gabriel Lippmann in 1881 and immediately confirmed by Pierre and Jacques Curie, who had previously demonstrated the direct piezoelectric effect (where applying mechanical force to deform certain crystals resulted in an electric potential appearing across its faces).
The converse piezoelectric effect can be used to create nanoscale motion in two ways:
Direct motion where a structure made of piezoelectric material pushes directly on the object to be moved;
Piezo motors , also called ultrasonic motors, where a number of small crystals energized at ultrasonic frequencies alternately push and retract like the legs of a catarpillar to push a rotor or linear element to create motion.
A simple, one-element direct-motion piezoelectric actuator uses a single piezoelectric crystal cut into a rectangular parallelepiped, with electrically conducting coatings vacuum deposited on two opposing sides. Applying an electric potential across the coatings creates an electric field within the crystal, pushing charges in the crystal unit cells out of position. The net effect is a deformation of the crystal’s macroscopic dimensions — strain.
Depending on how the crystal is cut, and which sides are coated, the deformation may be longitudinal (where the strain is normal to the coated sides) or shear (where the coated sides shift parallel to each other). In any case, the strain is proportional to the electric field applied to the crystal.
Such crystals can be, and often are, cut with shapes other than rectangular parallelpipeds. A common shape is a flat cylinder, with its length much shorter than its radius. In any case, the strain field’s shape depends on the relative geometries of the crystal and electrode coatings. The strain-field amplitude depends linearly on the applied electric field strength and the piezoelectric strain coefficient of the material.
The converse piezoelectric effect is actually quite small. To change the thickness of a 1 mm thick wafer of lead zirconate titanate (PZT– the most commonly used piezoelectric material) by 100 nm would require a potential of 2,670 V, which exceeds the breakdown voltage of the ceramic!
Clearly, practical direct-motion piezoelectric actuators require stacking many elements. If instead of using one wafer 1 mm thick, we use 10 wafers 0.1 mm thick energized in parallel, we could reduce the voltage by a factor of 10 to 267 V. While this is still high, it is well below the PZT’s breakdown field.
Direct motion actuators are limited to very small total motions, but can achieve very high resolution. For the 10-wafer actuator above, changing the applied voltage by 2.67 V changes the thickness by 1 nm.
Piezo motors solve the problem of limited travel while maintaining the system’s exquisite resolution. They can be made for linear or rotational action. Linear versions are the easiest to understand. Imagine a two parallel stationary structures with a plunger able to move freely along the centerline between them. A row of “limbs” are attached to one the stationary structures and extend into the space between so they can contact the plunger. A second row attaches to the second stationary structure to contact the plunger’s other side. Both rows have an even number of limbs.
Each limb comprises two parts: a longitudinal “leg” whose length changes when voltage is applied surmounted by a shear “foot” that twists to one side or the other when voltage is applied.
The action occurs in five steps:
Voltage is applied to the odd-numbered legs in both rows so that they extend toward the plunger until their attached feet contact it. At the same time, opposite voltage is applied to the even-numbered feet to contract them away from the plunger.
Voltage is applied to the odd-numbered feet so that they all twist so as to push the plunger in the desired direction. Simultaneously, voltage is applied to the even-numbered feet (no longer in contact with the plunger) to move them in the opposite direction.
Voltage is applied to the even-numbered legs to extend them until their feet contact the plunger. Now, all of the feet are in contact with the plunger to prevent slippage caused by outside forces.
Voltage is applied to the odd-numbered legs to retract them away from the plunger.
Voltage is applied to the odd-numbered feet to push the plunger in the desired direction.
By repeating these steps at ultrasonic speeds with large numbers of limbs, it is possible to rapidly move the plunger in either direction with unlimited range, and nanometer precision. The force such a motor is capable of applying is also virtually unlimited. Piezoelectric materials are basically rock capable of sustaining very high stresses. To increase the net force, simply use more and/or larger limbs. Doubling the number of limbs, for example, effectively doubles the force. To achieve multiaxis motion, simply stack stages in various orientations.
Rotary piezo motors can be made in a similar way. Instead of limbs lined up on either side of a straight plunger, rotory units place limbs around the circumference of a rotating shaft. The motor’s stator is now a cylinder to which the limbs’ stationary ends are fixed.
Other arrangements are in use, but this is the easiest to describe.
Early STMs and AFMs used direct motion to move a scanning tip over a surface to be imaged. The very earliest devices used a set of three actuators to move the tip along three orthogonal axes, creating nanoscale motions in three space. Later versions used a ceramic tube to accomplish the same thing.
Today, most nanopositioning actions keep the tool (whether it’s a cutting tool or a scanning laser microscope beam) fixed and move the piece being worked on. Such mechanisms are called “stages” after similar mechanisms used with light microscopes. To achieve two-dimensional positioning, stack two stages oriented at right angles to each other. To get three, bolt on a third oriented vertically.
C.G. Masi, senior editor