Achieving Exceptional Precision with Air Bearing Motion Systems

While traditional mechanical bearings and guidance systems are suitable for most motion applications, they have many drawbacks, especially when it comes to high performance motion control. In applications where lifetime, minimal vibrations, optimal precision, repeatability, or geometric performance are essential, air-bearing guided mechanics can provide superior performance and deserve a closer look.

Air-bearings and air bearing motion stages utilize a cushion of air to eliminate mechanical contact (Figure 1), thereby effectively mitigating common problems associated with traditional bearings such as friction, wear, vibration, hysteresis effects, and particle generation. These factors serve as key indicators that air-bearing stages may be the ideal choice for high precision, 24/7 test automation and metrology applications.

Figure 1. 3-axis air bearing stage for laser processing consisting of a rotary air bearing stage (1) and a compact planar XY stage (2). (Image: PI)

How Their Exceptional Precision is Achieved

Air bearing surfaces are usually hard coated and meticulously ground to achieve exceptional tolerances. High bearing stiffness is achieved by frictionless magnetic or vacuum preloading. The surface averaging effect helps to provide significantly better geometric performance, such as straightness and flatness because the large area of the bearing together with the precision ground surfaces can deal much better with small imperfections than ball bearings or roller bearings (Figure 2).

Figure 2. Surface averaging effect of an air bearing versus conventional bearings. (Image: PI)

The effect can be compared to the difference between a hover craft and a conventional truck riding over a pot-hole-covered highway. That averaging effect is also comparable to the smoothness of a magnetic levitation train versus a conventional train running on steel wheels.

With the absence of wear, and no need for lubrication and maintenance, air bearings offer considerable benefits in high-speed motion applications and precise positioning, particularly in high throughput 24/7 automation scenarios that are demanding high uptime and reliability.

Air Bearing Advantages for Precision Motion and Positioning Applications

Virtually Unlimited Lifetime, Maintenance-Free, Clean Room Compatible: Since air bearings work without mechanical contact between components, they do not deteriorate and thus do not require periodic inspections, maintenance, or relubrication cycles. Also, unlike with cross-roller bearings, there is no risk of cage migration, especially when small repetitive motion cycles are executed. Due to the absence of friction and lubricants, these systems also fulfill the requirements for clean room applications. The F-142 photonics alignment system, from PI takes advantage of these features (Figure 3).

Figure 3. The F-142 compact photonics alignment system provides multi-axis motion with high speed and nanometer resolution. It is based on miniaturized air bearing stages with direct drive motors and automatic gravity force compensation on the Z-axis. (Image: PI)

Motion with Excellent Geometric Performance and Extremely Small Straightness, Flatness, and Eccentricity Errors: Air bearings provide high accuracy, owing to the high-precision manufacturing process of their components. Due to the surface averaging effect, linear air bearings feature exceptionally flat and straight travel (0.75 μm over 500 mm) with minimal roll, pitch, and yaw errors (10 μrad), and rotary air bearings are also superior in terms of eccentricity, wobble and tip/tilt. This makes them highly suitable for manufacturing and measurement processes such as optical inspection, providing excellent repeatability of the same procedure.

This exceptional precision matters for advanced manufacturing processes where every nanometer counts: for example, modern semiconductors are produced with line widths of single-digit nanometers, and even mechanical components in the latest generation of automotive engines can require submicron precision.

True-Planar Multi-Axis Motion: Planar XY stages and XY-theta stages cannot be designed with traditional mechanical bearings. Here, individual axes are usually stacked on top of another. In some instances, XY stages are combined into a single assembly and falsely called planar stages, however, the X and Y axis do not use the same reference plane. The approach of building a multi-axis motion system by stacking individual axes is simple but has several drawbacks. As the upper axis moves to the extremes, it will generate torque loads on the lower axis, leading to geometric errors.

In air-bearing designs, fully planar XY and XY-Theta positioning systems are feasible, where all degrees of freedom reference to the same base plane and are fully supported over the full travel range. The A-322 XY-theta stage, from PI, for example (Figure 4), uses one common plane for both X and Y motion, and a flexure joint on the cross axis even allows for 1 degree of rotation around Z, a great advantage in applications where small alignment errors need to be corrected. A special gantry control algorithm with a separate Theta-Z control loop in the motion controller takes full advantage of that feature.

Figure 4. The A-3 22 planar XY air bearing stage, from PI, provides 1 degree of Theta-Z motion, ideal for small corrections, for example when a workpiece and a camera or laser system need to be aligned. (Image: PI)

Motion with Highly Constant Velocity, Vibration-Free and Very High Dynamic Range: The fluid film in air bearings can readily accommodate high velocity, and some air bearings can even improve efficiency at high speed, due to aerodynamic lift effects. Certain processes and experiments, such as semiconductor wafer scanning, 3D tomography, and inertial sensor testing, demand constant motion at precisely controlled speeds, where mechanical bearing rumble would introduce unwanted errors. In these instances, air bearing systems are the most appropriate solution to provide the necessary continuous motion at minimum speed fluctuations, and they also last longer than mechanical bearings (Figure 5).

Figure 5. The A-523 parallel-kinematic Z-Tip-Tilt air bearing stage, from PI, features a very low profile and can handle loads to 10 kg with rapid acceleration. (Image: PI)

Long Travel Ranges of 1 m and More Feasible: Air bearings are not limited to short travel ranges, in contrast to another well-known frictionless positioning technology, often used in nanopositioning applications — piezo flexure guided mechanisms can provide sub-nanometer precision, high scanning speeds and share many of the advantages of air bearings, however their travel ranges are limited to micrometer and the lower millimeter range. With air bearings, motion ranges beyond one meter are easily achievable and all the advantages of smaller air bearings are carried over.

High Accuracy, Frictionless Motion: In linear motion applications, precise positioning of a moving carriage within a few nanometers is achievable by using a non-contact, direct-drive motor and high-resolution optical encoder in combination with an air bearing (Figure 6). For rotational applications, angular resolutions to tenths of arc-seconds are achievable. Here, slotless torque motors are used as the driving force.

Figure 6. A high-speed voice-coil linear actuator with air bearing guides. (Image: PI)

Air bearings are often preferred in many inspection, metrology, and manufacturing applications due to their minimal hysteresis effect or reversal error, which arises from a lack of mechanical contact and friction. The elimination of friction makes it possible to minimize hysteresis and improve repeatability and accuracy significantly.

Another technology to be considered in precision positioning, with similar performance to air bearings is called magnetic levitation. Here, magnetic fields replace the function of air as a supporting medium, but control electronics are significantly more complex because all six degrees of freedom have to be monitored and controlled all the time.

Rotary Motion with Minimal Eccentricity and Wobble: Rotary air bearings are highly effective in providing precise rotary motion due to their high stiffness and the aforementioned surface averaging effect. In air bearing rotary stages, wobble or tilt errors typically occur within the range of 0.1 to 1 arc-second – significantly smaller than with mechanical bearing-based rotary stages.

Cost-Efficiency: Air bearing mechanisms can also use air pressure to provide actuation in addition to guidance. This design eliminates additional electric motors, reducing complexity, size, and cost. In general, absence of wear and tear and the need for maintenance is a great advantage reducing operating cost and improving ROI. This may be the most important factor when designing high-throughput automation equipment with 24/7 operation and stringent uptime requirements for years to come.

Overall, air bearings offer a number of advantages over traditional bearings, such as being more precise, accurate, and durable. They also operate more quietly and produce less vibration. Air bearings are environmentally friendly and cost-effective. Additionally, they can be designed to meet a wide variety of requirements in various industries, including semiconductor manufacturing, precision machining, metrology, aerospace, and scientific research.

This article was written by Stefan Vorndran, VP Marketing for Physik Instrumente L.P. (Auburn, MA). For more information, visit here .