Scanning is a common technique in applications ranging from high-resolution microscopy to industrial material processing. Scanning involves moving either a workpiece or an optic at a constant velocity while a reading or writing operation takes place. Air bearings are used for both purposes, especially when high precision and reliability are vital. While the physical act of writing an image or capturing an image differ by application and industry, all such applications share a common requirement — maintaining a constant velocity.
In most applications, the image capture or creation operation takes place at a constant frequency (i.e. frames per second in a camera). As the optic and workpiece move relative to each other, the spatial separation between each frame must be constant in order not to distort or blur the image (Figure 1). This means that the relative velocity between the two must remain constant.
The two most common methods of defining and specifying scanning performance are based on either velocity or position. A velocity-based specification is often defined as a percentage error at a specific velocity (e.g., 200 mm/sec scan speed, 0.01% error). A position-based specification is described in terms of position error at any point in time, also called following error. It means that the stage position cannot deviate from the planned motion trajectory by more than the error window during the entire scan (e.g., 200 mm scan distance, 200 mm/sec scan speed, +/-0.1 micron following error).
Following error is a common term used when discussing servo controls. It is a general term used to define the measured position error between the commanded trajectory vs. the actual trajectory. In this article, the term is used in the specific case of errors seen during the constant velocity scan move. Following errors seen during a contoured move are ignored where trajectory changes (and therefore accelerations) are commanded by the controller.
Air bearings provide superior scanning performance when compared to traditional mechanical technologies for one main reason: air bearings have no friction (Figure 2). An air bearing has no physical contact between moving parts, and floats on a cushion of air. They do not need to overcome static friction to produce motion and do not need to overcome dynamic friction to maintain motion.
Mechanical bearings suffer from two weaknesses that impact scanning performance: friction and noise. All mechanical bearings exhibit some level of static and dynamic friction that must be overcome by the drive and control system. Friction levels can change with temperature, lubrication, preload, speed, payload, and wear. Mechanical bearings, especially those with reciprocal elements, also suffer from noise. As rolling elements (balls or rollers) exit the bearing groove, recirculate in their carrier, and then re-enter the bearing groove, audible noise can be heard. This noise varies with speed, and the noise heard by the ear affects the motion profile by adding vibrations that the servo control must counteract. Non-recirculating bearings (such as cross-rollers) do not have this problem, but these usually offer limited travel lengths.
Another challenge faced by mechanical bearings is that the roller elements are never perfectly round. As the rollers or balls traverse the bearing ways, tiny oscillating errors created by this out-of-roundness will print through to the motion as a periodic error that can often be seen by the scanning system.
Air bearings exhibit another performance characteristic that makes them well suited for scanning applications: their motion is usually very straight and flat. Linear air bearings generate straightness and flatness error motions better than 1μm per 100 mm of travel. Straight-line performance is a benefit since progressive line scans can easily be overlaid, and flatness performance helps keep the image in focus without the need for expensive autofocus mechanisms.
The level of achievable scanning performance will vary from system to system. As a rule, a high-performance air bearing system will perform at an order of magnitude better than a comparable mechanical-bearing system.
Air bearing stages designed for scanning are driven by direct-drive ironless linear motors (Figure 3). Linear motors, like air bearings, have no contacting moving parts, and therefore do not suffer from noise and friction issues. Traditional mechanical drive mechanisms — such as screws, worms, belts, and gears — all suffer from the same weaknesses as mechanical bearings: friction and noise.
Ironless linear motors are used because they exhibit no attractive forces between the motor coil and the motor magnets, and therefore there is no cogging created as the coil moves relative to the magnets. Cogging is an effect seen when there are magnetic attractive forces between the coil and magnets, causing the coil to naturally want to rest centered between two magnet poles. As the coil moves relative to the alternating North/South magnet poles, the attractive forces increase and decrease sinusoidally. This oscillating force creates the cogging effect, which can be difficult for control systems to completely overcome. During constant velocity scanning, the cogging prints through to the motion as periodic following error.
Other factors must be considered and tradeoffs made during scanning system design. These include payload, system stiffness, encoder resolution and signal noise, servo controls and drives selection, cable track design, and vibration isolation.
The payload being moved by the motion system can have a significant impact on scanning performance. At the most basic level, the mass of the payload will help determine the bandwidth of the control system as well as the resonant frequencies of the system — the larger the mass, the lower the resonance and thus the lower the performance of the system. Attention must be paid to the location of the payload center of mass relative to bearings, the motor, and the encoder. Offsets of the center of mass can create torques in the system that must be overcome by the bearings during acceleration and deceleration. Such torques can excite vibrational modes that must be damped by the mechanical system or control system. These vibrations can degrade scanning performance and will show up as following errors. Finally, the payload must be fairly rigid. If the payload will flex or vibrate during motion, the control system will have a much more difficult time maintaining scanning performance.