Once viewed as "guideways for the rich and famous," air bearings use in the last two years has been driven by explosive growth in two application areas: flat panel displays and hard disk drives. But according to Kevin McCarthy, chief technology officer for Danaher Motion/Dover (www.danahermotion.com), fundamental technology advantages are also moving air bearings away from their place as exotic, ultra-high precision motion control solutions.
"What I see going forward, especially given new low-cost air bearing designs, is penetration into applications that don't necessarily require extreme precision," says McCarthy. He notes that the real win in these potential applications is the fact that air bearing systems are totally non-contact and, as a result, offer essentially infinite, maintenance-free service life.
"When you have high speed applications operating at two to five m/sec, conventional guides and ways produce noise, wear, and friction. It's my conviction that air bearings will find their way into moderate precision applications, where the real benefits will be infinite service life, low noise, and no wear."
Back to Basics
According to McCarthy, all moving objects possess six degrees of freedom—three linear and three rotary. The task of a linear motion guideway is to eliminate, as closely as possible, five of these degrees of freedom, leaving a single rotary or linear axis of motion. He says that air bearings are the purest and highest performance means of defining linear or rotary motion, and have substantial advantages over conventional mechanical guideways.
The key for engineers designing precision motion control systems is to understand the possibilities of air bearing designs, and how their fundamental technology advantages may contribute to the success of individual applications.
Completely Non-Contact Bearing Ways, Motor, and Encoder
A main advantage of direct drive air bearing stages is that the three critical components of the stage—the guideways, the motor, and position feedback—are all completely non-contact. According to McCarthy, an air bearing, direct drive stage has a mass and current is passed through it. In response, it develops a force in an extremely linear and predictable manner. This force, and the resulting acceleration, is singly and doubly integrated by the velocity and position loops of the servo control, using position data from an equally non-contact linear encoder.
Stated simply, contact is corruption, he says. The presence of numerous and over-constrained contacts in traditional stages, along with lubricants, preload variations, leadscrews with torque variations, recirculating ball cogging, and retainer creep prevents them from achieving the static or dynamic performance levels of air bearing systems.
Extreme Resolution Capability
Since air bearing, direct drive stages have no intrinsic resolution limit, McCarthy says they produce air bearing positioning systems with resolutions as fine as 31 picometers (that's less than the classical Bohr radius of the hydrogen atom).
He believes that leadscrews begin to compromise system performance at resolution levels at or below 100 nm (0.1 micron). And while mechanical bearing systems can be pushed below 100 nm, issues such as bearing friction, preload variations, recirculator cogging, and lubricant issues can become problems.
One area where air bearing stages provide higher performance than traditional mechanical bearing is throughput. The absence of friction allows for substantially shorter settling times, since there is no need to wait for the servo loop integrator term to overcome friction. Feed forward terms in the servo filter can be applied much more accurately, again because of the absence of friction.
Unlimited Service Life
Since the absence of contact means absence of wear, McCarthy says an air bearing will operate without change for decades. He notes that the company's AirBeam product series has been deployed in the thousands in digital printing systems worldwide over the last ten years, and has demonstrated intrinsically high production reliability. "We are seeing applications in which high precision (the traditional driver for the use of air bearings) is not required, and air bearings are being selected strictly for their unlimited, maintenance-free service life."
Extremely Straight Motion
Air bearing guideways provide straight motion and tend to integrate minor errors in the surface over which they run, so the resulting errors are very low and of long period. As an example, Danaher Motion's AirBeam 2000 has 2000 mm of travel, but pitch and yaw over this range of travel are held to less than 10 arc-seconds. Linear departures from straight-line motion are less than 10 microns over the 2000 mm of travel.
Ultra Constant Velocity
McCarthy says there are a number of applications that require extremely precise constant velocity motion. Residual errors in high-end designs are primarily due to Abbe errors resulting from very small angular errors, thermal effects, and environmental vibration. With suitable component selection, tracking errors during motion can be held to levels as low as ±2 nm at low speeds.
Millinewton Level Force Measurement and Generation
The absence of friction, together with the linear conversion of current to force in the direct drive linear motor, means that these stages, with no additional changes, also function as both precise force generators and force transducers at the Millinewton level. With linear motor air bearing positioners, load cells are essentially built into each stage by design, and can perform very sensitive touch-point detection at extremely low force levels. McCarthy says that the ability to detect part locations at the 100 nm level offers new process control variables that would otherwise be poorly controlled.
Extremely Low Particle Counts
Due to the non-contact nature of the bearing ways, linear motor, and linear encoder, these stages are nearly perfect from the perspective of particulate contamination. As long as the air used to pressurize the bearing is dust free (and very conventional filters offer 99.7 percent filtration of all particles above 0.1 micron), the only particle source component is the cable plant. Proper selection of qualified materials in the moving cable set has permitted stages to be used in ultra-critical applications such as wafer steppers. In some cases, pure dry nitrogen is used to operate the air bearings, producing a nearly undetectable level of particulate generation.
Accurate, Sub-Micron Dither Motion
Some air bearing systems offer a servo-to-optical-peak-power alignment solution which determines the gradient of the optical power curve using small amplitude sine and cosine dither motions in the plane perpendicular to the optical axis. McCarthy says this is a far better method than software-based search algorithms, and requires the purity and friction-free attributes of air bearing, direct drive stages to succeed.
Friction non-linearities and lubricant effects in conventional stages prevent the production of accurate, sub-micron dither motion, and eliminate this means of peak locking. He says that piezo stages are capable of producing dither motions, but their limited travel is a disadvantage, typically requiring an additional set of stages below them to achieve macro travels.