by Matt Reck, Product Line Manager, Air Bearings Systems,Physik Instrumente
It’s easy to overlook air bearings. Most of the time, mechanical bearings work well enough for a motion application. Most, but not all. Submicron bearing rumble not an option? Is geometric performance and angular repeatability paramount? Here are tips that will smooth out the specification process.
Air bearings offer distinct advantages in precision positioning, including no backlash or static friction; they are also suitable for most high-speed applications. But some conditions are not conducive for the use of air bearings. Knowing air bearing parameters can be the key to a truly optimized bearing system application.
Friction is the enemy
Frictionless drive and bearing technology is a prerequisite for vibration-free precision motion with nano-scale resolution, repeatability and guiding accuracy. There are several ways to achieve frictionless motion. Piezo drives and flexure guidance are well established for short travel ranges. Another principle is based on magnetic levitation (magnetic bearings). These motion systems are more exotic, but not travel limited, and are often designed for multi-axis motion applications. Another solution for long travel ranges consists of air bearings driven by electromagnetic linear and torque motors.
Here is a six-axis stage with magnetic bearings—magnetic levitation.
Image courtesy of PI
Air-bearing stages are rotary or linear positioners that float on a cushion of air, using one of several preload mechanisms, nearly eliminating mechanical contact, and thus wear, friction and hysteresis effects. They deliver high throughput and precision, especially for multi-axis motion.
Benefits for motion-control applications
Here are some of the most common factors supporting the use of air-bearing stages for motion-control applications:
1. Frictionless, accurate positioning
A direct-drive motor and high-resolution encoder can position a moving carriage supported by an air bearing to within nanometers in a linear application, or within tenths of arc-seconds in rotational applications. The lack of friction and mechanical contact means there is minimal hysteresis or reversal error, making it highly repeatable and suitable for many inspection and manufacturing operations. Stiction is virtually eliminated, improving resolution capabilities and reducing in-position “hunting“ (limit cycling). Position repeatability can also be obtained within a few encoder counts. Similar precision can be obtained by piezo flexure guided stages, however over much smaller travel ranges. Magnetic levitation is another option.
2. Velocity stability and scanning
The lack of mechanical bearing elements means there is nothing to get in the way of smooth, controlled velocity (stability to better than 0.01%). Experiments and processes, like inertial sensor testing, tomography, wafer scanning and surface profiling—which require continuous motion at tightly controlled speeds—are best served by air-bearing systems.
Flatness and repeatability of flatness measurement of a PIglide LC 230-mm travel stage. The 2-sigma repeatability of the flatness error is less than 50 nm in this case.
3. Low error motions due to surface averaging effect
Linear air-bearing stages have straight and flat travels; pitch, roll and yaw errors can be measured in tenths of arc-seconds. Rotary stages can have tilt (wobble) errors less than 1 arc-second. Additionally, the angular performance of an air bearing is repeatable. This capability guarantees part quality and measurement reliability for applications like mirror and optics inspection, semiconductor inspection and medical device manufacturing.
Surface Averaging effect of the air bearing. The result is elimination of high-frequency bearing rumble and much better straightness and flatness of motion compared to mechanical bearings.
Image courtesy of PI
4. Long travel requirements
Piezo-driven flexure stages and actuators can satisfy many high-precision positioning applications. However, these designs are usually limited to a few millimeters of travel. Air-bearing linear stages can be used for travels of 25 mm or more. Manufacturers can provide linear air bearings with travels up to 1 m or more with custom design.
A flexure-guided, UHV-compatible, XYZ piezo nanopositioning stage.
5. Wobble-free or high-speed rotary motion
Rotary air bearings are exceptionally stiff and can deliver highly precise rotary motion. Radial, axial and wobble error motions are much smaller than most mechanical bearing systems can provide, and the rotary motion is exceptionally smooth since there are no roller elements. Rotary positioning stages generally can achieve speeds up to 600 rpm, while air bearing spindles are used in higher-speed applications. Rotary bearing designs can be mounted with the plane of the table in either the horizontal (a turntable, for example) or vertical orientations.
An example of a rotary air bearing.
6. Minimal maintenance
There are no contacting parts to undergo wear and tear, and no regular maintenance procedures to be performed, like lubrication. An air-bearing stage is essentially maintenance-free. Further, the system is highly stable; because there is no wear, the performance characteristics should not change over the life of the system. There is little need for recalibration. Moving cables and hoses are often the only wear items in an air-bearing system.
7. Cleanliness
Because air bearings are wear-free, they generate virtually no particulates that can become airborne. This feature makes them an option for cleanroom applications like optics inspection, wafer inspection, bio-pharma research and flat-panel display inspection. For extremely clean applications, it is recommended that the air bearings operate using 99.9 % pure nitrogen.
8. Precise force control and sensing
Air bearings are frictionless, which means when they are coupled with a direct-drive motor or voice coil, they work well in micro- and nano-Newton force control applications. Such applications can include pick-and-place of delicate items, materials testing and coordinate measuring applications.
Air Flow in a planar XY air bearing stage. The bearings are vacuum-preloaded for higher stiffness.
Image courtesy of PI
Precision applications
Linear, planar, spherical and rotation stages with air bearings are optimal motion-control components for industry and research.
Shown here is a three-motor planar XY air-bearing stage with active yaw control. Image courtesy of PI
Air bearings ensure frictionless motion, a fact that makes them the preferred choice for zero-gravity simulations. Their guiding accuracy (straightness and flatness of motion) is well below 1 µm over long travel ranges. This precision guarantees optimal part quality and measurement reliability for applications such as flat panel inspection, optics inspection, semiconductor inspection and medical device manufacturing.
Spherical air bearings can be used to simulate zero gravity.
Environments unsuitable for air bearings
By the nature of their design, air-bearings are not suitable for all operating environments
1. Vacuum environments
While it is not impossible to operate an air bearing in a vacuum, it is challenging. Vacuum applications should generally be avoided. Instead, stages based on mechanical bearings, magnetic levitation or flexure guiding systems should be used.
2. Dirty, dusty applications
Air bearings are generally used in clean environments. Applications where heavy amounts of dust, dirt, debris and fluids are present should generally be avoided.
3. Unavailable pressurized air or nitrogen
Air bearings require a continuous supply of clean compressed air or nitrogen. If the application does not allow for such a supply to be present, an air bearing can not be used.
Physik Instrumente
www.physikinstrumente.com
For information on non locating bearings click here.
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