Air bearings for increased scanning performance Air bearings perform well when compared to other options for one main reason: air bearings have no friction. An air bearing has no physical contact between moving parts and oats on a cushion of air. Therefore, air bearings do not need to overcome static friction in order to produce motion, and do not need to overcome dynamic friction to maintain motion. With the minor exception of drag forces created by aerodynamic drag from the surrounding atmosphere and viscous drag created by the shearing of the air bearing film, an air bearing system behaves exactly as Newton’s three laws of motion predict. Line scanning Mechanical bearings still have to contend with 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. This makes friction behave in unpredictable and non-linear ways. Further, the friction levels can vary widely from unit to unit. 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 affects the motion pro le. Both friction and noise make it impossible for the servo- controls to maintain ideal scanning performance. Frictionless Motors Air bearing stages designed for scanning can be driven by a direct-drive linear motor. 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. By coupling non-contact air bearings with non-contact linear motors, air bearing stages create friction-free motion perfect for scanning. PIglide LC linear air bearing stage, model A-110 Other Impacts on Scanning Performance Not all air bearing motion systems are created equal. Yes, the use of an air bearing and linear motor are necessary to scanning, but other design decisions will impact system performance as well. These factors must be considered and tradeoffs made during system design to determine how to best meet scanning performance specifications. These factors include: payload; system stiffness; motor design; encoder selection, resolution, and signal type; servo-controls and drives selection; cable track design; electrical noise isolation and wiring design; and vibration isolation and control. Following error data for model A-110 stage. Scanning performance levels The level of achievable scanning performance will vary from system to system. However, the following is performance data captured from PIglide air bearing stages.This set of data is taken from a model A-110 This set of data is taken from a model A-110 PIglide LC linear air bearing stage with a 2μm incremental optical sine/cosine analog encoder with a glass scale. This encoder was chosen for its signal-to-noise quality. The stage is operated with an ACS SPiiPlus EC motion controller. Following errors are below +-20 nanome- ters at a scan speed of 2 mm/sec. Over a small window of 20μm of travel, the following error is below +-10 nanometers. This set of data is taken from a model A-322 PIglide HS Planar XY air bearing stage with absolute optical encoder, 1nm BISS output, and steel scale. The stage is operated with an ACS SPiiPlus EC motion controller. Following errors are below +-25 nanometers at a scan speed of 20 mm/sec, and below +-100 nanometers at a scan speed of 250 mm/sec. It can be seen that following error does increase with scanning speed, but not at the same rate. In the plots above, the speed is increased 12.5x while the following error only increased 4x. Performance levels, such as this, are simply not possible with mechanical bearing technologies. PI www.pi-usa.us