The Air Bearing Advantage in High-Precision Positioning

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. Air bearings are the purest and highest-performance means of defining linear or rotary motion, and have substantial advantages over conventional mechanical guideways. These advantages become more pronounced as the desired resolution increases, and many aspects of high-precision positioning are uniquely enhanced when air bearing guideways are chosen.

The advantages that air bearing direct- drive stages offer include:

• Completely non-contact bearing ways, motor, and encoder

• No friction

• High resolution and throughput

• Straight motion and constant velocity

• Consistent servo tuning

• Exploiting of feedforward terms Disadvantages of air bearing directdrive stages include:

• Added infrastruture requirements to provide an air supply

• Lower stiffness than rolling element steel bearing ways

• Increased susceptibility to amplifier-induced or environmental vibration

• Perceived higher cost

The Non-Contact Factor

There is a tremendous advantage in having the three critical components of the stage — the guideways, the motor/actuator, and the position feedback — all completely non-contact. All of the subsequent advantages mentioned above stem from this simple fact. In stark contrast to conventional stages, the air bearing direct-drive stage is a nearly perfect physics package — it has a mass, and you pass a current 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.

The presence of numerous and overconstrained contacts in traditional stages (not to mention lubricants, preload variations, leadscrews with torque variations, recirculating ball cogging, retainer creep, etc.) prevents them from coming close to achieving either the static or dynamic performance levels of air bearing direct-drive stages. In a number of single-axis air bearing stages, there are no moving cables, and all connectors are mounted in the stationary base. In multi-axis stacks, there inevitably will be some moving cables, and these should be as supple and non-influencing as possible. What residual cable effects remain are small forces (not friction), for which the servo integrator develops an equal and opposing force.

Friction is a highly non-linear effect, and degrades the performance of servo control loops, since they are based upon linear system theory. The absence of friction from air bearing direct-drive stages permits much higher static and dynamic performance to be achieved.

Resolution and Throughput

Leadscrews tend to run out of gas for resolution levels at or below 100 nanometers (0.1 micron). While there are tricks that can be used to make leadscrews cooperate in this regime, they require fairly high-strung tuning, and may compromise dynamic performance. Similarly, while a few mechanical bearing systems can be pushed below 100 nanometers, issues such as bearing friction, preload variations, recirculator cogging, and lubricant issues make this an uphill battle, with the need to wait while the servo loop integrator term-papers over the problems. Air bearing direct-drive stages have no intrinsic resolution limit, and positioning systems with resolutions of 31 picometers can be created. In terms of throughput, air bearing stages provide higher performance than traditional mechanical bearings. The absence of friction in air bearings allows a substantially shorter settling time, since there is no need to wait for the effect of the servo loop integrator term to overcome friction. In addition, feedforward terms in the servo filter can be applied much more accurately because of the absence of friction.

Given the totally non-contact nature of the bearing ways, motor, and encoder, the resulting service life of air bearing direct-drive stages is essentially unlimited. The absence of contact means the absence of wear, and the air bearing will operate without change over decades. Despite the unlimited life of an air bearing stage, there is a class of “traumas” that can damage the stage. Dropping a vise on the air bearing surfaces, for example, will leave a dent that may prevent motion. Pumping oil instead of air into the compressed air line is another failure mechanism, as is putting 20 amps into a 5-amp linear motor coil, or a 24- volt supply on a 5-volt encoder. Most of these issues can be easily prevented using fuses, I2T current limiting, coalescing filters, and voltage clamps.

Motion and Velocity

The job of a guideway is to eliminate, as much as possible, five of the six degrees of freedom. Air bearing guideways do this extremely well. They tend to integrate minor errors in the surface over which they run, and the resulting errors are very low, and of long period.

There are a number of applications that require extremely precise constant velocity motion. Due to the intrinsic purity of air bearing direct-drive technology, stages based upon this design offer the highest possible performance in constant velocity applications. 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 nanometers at low speeds.

Servo Tuning and Feedforward

Traditional stages, with their assortment of mechanical bearings, leadscrews, and nuts, require careful servo tuning to achieve optimum performance. Attempts to minimize move and settle times often lead to tunings that are marginally stable, and are individually hand-tuned for each axis. Since tuning is dependent on physical parameters such as leadscrew torque, linear bearing preload, and lubricant properties, and these vary along travel in any given unit, from unit to unit, and over time, the result is often unsatisfactory.

Air bearing direct-drive stages have no contacting parts to wear. The servo tuning drops directly out of a spreadsheet; for any given stage, the only free variables are the desired servo bandwidth, the payload mass, and any structural resonance. Stages with the same payload mass are identically tuned; there is no need to fine-tune each stage to match any unit to unit variations.

Simple PID loops exhibit significant position lag during acceleration, and position lead during deceleration. At 10 meters per second squared acceleration (~1G) and a servo bandwidth of 50 Hz, the terminal following error is 0.4 millimeters (400 microns). The servo loop must then reverse direction and eliminate this terminal error. This error can be reduced by acceleration feedforward, which adds and subtracts a current command during acceleration to cancel the normal position lag and lead. Friction guideways can benefit from feedforward terms in the servo filter, but the variable and imprecise level of friction yields only moderate benefit. Direct-driven air bearing stages can benefit much more from the use of feedforward terms. Terminal following error in air bearing systems can be cut to a few percent of the nominal value by using feedforward terms, which provides direct benefits in settling time and throughput.

Disadvantages

There is a modest increment in system cost and complexity for air bearings over conventional stages due to the need to provide a supply of clean, dry air. In most facilities, compressed air is generally distributed, and the added cost for spot regulation and filtration are low. If house air is not provided, there can be a higher level of burden imposed by the need to add a compressed air station, and if it is adjacent to the stage system, the added noise and vibration can be an issue.

Air bearing guideways are less stiff than the rolling steel bearing guideways used by conventional stages. This could lead to a lower first resonance and a lower servo bandwidth. The numerous mechanical elements present in traditional staging usually are the elements that set a limit to the servo bandwidth, and they are absent in air bearing directdrive stages. One additional consequence of the use of air bearing guideways relates to torques due to overhung loads. While the load capacity for centered loads of air bearing stages is quite high, there is a distinct limit to the allowable magnitude of torques due to cantilevered loads. There are a number of other reasons (Abbe error, for example) why overhung loads are not a good idea in general, but if they cannot be avoided, conventional bearings may be indicated.

Conventional stages have significant amounts of friction, which is, in nearly all cases, a distinct disadvantage. One positive aspect to friction, however, is that it provides position stability in the presence of external stimuli, and does so without the intervention of the servo loop. In a frictionless, direct-drive stage, the servo loop has the sole responsibility of suppressing axial vibration. Vibration sources can be due to either the environmental background, or by the servo amplifier, especially if that is of PWM design.

Air bearings have long been considered “stages for the rich and famous.” The number of suppliers was very small, and if accuracy or dynamic performance requirements dictated air bearings, the cost was high. Today, more lower-cost options are available, and there are applications with moderate cost and performance targets using air bearings for their unlimited service life and high-speed capability.

This article was written by Kevin McCarthy, Chief Technology Officer, at Danaher Motion, Wood Dale, IL. Contact Mr. Mc- Carthy in Salem, NH, at 603-893-0588 or Kevin.McCarthy@danahermotion.com. For more information on Danaher Motion, visit http://info.ims.ca/5657-323.