The Air Bearing Advantage in High-Precision Positioning
- Saturday, 01 April 2006
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 direct-drive stages include:
- Added infrastructure 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 over-constrained contacts in traditional stages (not to mention lubricants, pre-load 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.