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.
A motion system's dynamic performance will always be limited by the stiffness of the structure on which it is built. Any forces created by the acceleration and deceleration of the stage must be counteracted by the support structure. These forces can be quite large and transient. All that energy must be transmitted to the floor by the structure, and if the structure is not rigid enough, the whole system can flex and vibrate, which will have a negative impact on scanning performance.
Encoder Resolution and Signal Quality
The type and quality of linear encoder will have a direct impact on the performance of the scanning system. The two fundamental parameters of concern when selecting the best encoder are its resolution and its periodic signal error.
Resolution is determined by the measuring increment (or pitch) of the linear scale and the interpolation (or multiplication) factor in the encoder electronics. The finer the pitch, the better the resolution (and thus fidelity) that is available to the control system.
Signal interpolation, which is essentially the analog-to-digital conversion of the raw encoder signal, can either be performed in the encoder itself or in the controller electronics. In the case where the controller performs the interpolation, the encoder signal being sent back from the stage will be an analog differential sinusoidal signal. When the encoder self-interpolates, the encoder signal will typically be a digital RS-422 (TTL-level) signal. Modern controllers offer higher levels of interpolation and support faster speeds than encoder-based interpolation. However, RS-422 signals can be more noise-immune than analog signals, so in some cases encoder-based interpolation is preferred.
For high-end scanning applications, designers must also be concerned with the periodic signal error of the encoder. This is an error that repeats every signal period of the scale and usually cannot be calibrated or corrected by the controller. This periodic error will directly print through to the motion of the system and will be seen as a following error in the scanning performance data.
Servo Controls and Drives Selection
Driving an air bearing system and maintaining excellent scanning performance is not an easy task for a servo control system. The lack of friction, and therefore damping, in the mechanical system makes controlling the motion especially difficult. The controller must have high bandwidth, high-speed processing, advanced control algorithms, and excellent I/O fidelity to process and produce the high-speed signals coming in and out of the controller. The drives must be incredibly quiet and have a wide dynamic range, excellent current control at low power levels, and high bandwidth.
Cable Track Design
Management of moving cables in a motion system may be the most overlooked and underappreciated aspect of good motion system design. Aside from the obvious aspects of wear, failure, and electrical noise, the design of the cable management system can have direct impacts on the motion performance itself.
Cables and cable tracks exert forces on the moving parts of a motion system. In high-performance scanning applications, traditional plastic e-chains can create audible clicks as their hinges rotate, and those perturbations can affect scanning performance. The best cable loop designs for scanning systems use flat-flex cables with no carriers, or moving cables loops are avoided altogether.
Vibrations generated outside of the motion system can be transmitted from the floor up through the machine structure and can adversely impact scanning performance. To counteract this, most high-performance motion systems use a vibration isolation system. Isolation systems can range from simple rubber mounts, to passive pneumatic systems, to fully active magnetic levitation systems. Passive systems offer excellent isolation performance but can suffer from an inability to rigidly support a system with aggressive dynamics. Fully active systems can be extremely expensive, so a balance between budget and performance must be found. For systems with slow speeds and accelerations, passive systems will typically work well.
Air bearings deliver superior performance for scanning applications, but have other advantages as well. There is minimal maintenance because there are no contacting parts to undergo wear and tear and cleanliness. Because of the frictionless and lubricant free design, virtually no particulates are generated to become airborne. Air bearings also deliver superior geometric performance, such as straightness/flatness for linear motion and eccentricity/runout in the case of rotary motion.
This article was written by Matt Reck, Air Bearing Product Line Manager, PI (Physik Instrumente) LP, Auburn, MA. For more information, Click Here .