Displays ride on a cushion of air

Air bearing motion technology is helping manufacturers of flat panel displays meet customer demands for larger, higher resolution displays. Dean Palmer reports Producing flat panel displays (FPD), like many other manufacturing industries, requires ever-increasing production accuracy, with customers now demanding larger, higher resolution displays. The knock-on effect to the FPD manufacturers is that they now require higher accuracy positioning tables and motion control systems to produce these displays. In FPD laser-based micro machining, for example, control with sub-micron translational and sub-arc-second rotational errors is required. Typical travel ranges of up to 1,500mm together with vacuum chuck and substrate payloads of 95kg, are matched with speeds of more than 0.5m/s and 1g acceleration performance, which all adds up to quite staggering mass and inertia values. Compound errors between the axes, caused by the slightest misalignment or build up of pitch, roll, yaw and straightness/flatness errors, increase the likelihood of lost precision over such distances. So, for design engineers of mechanical systems and motion control suppliers, the problem of meeting, and exceeding, the customer's demands is quite daunting. The use of an air bearing stage is one way of achieving this high positioning accuracy for FPD manufacture. As Simon Smith, MD of motion control supplier Aerotech explained: "It allows precise and repeatable motion, is clean room compatible and offers extremely low maintenance. Also, it's non-contact so it reduces vibration. And other factors that affect repeatability in mechanical bearing alternatives, such as preload and ball engagement inconsistencies, are eliminated." Smith conceded that air bearings "are not new" but added that "traditional drive methods such as precision ball screws have now been surpassed by linear motor technology". He also said that the use of new materials, time-evolved design optimisation and more sophisticated software control have together provided the FPD industry with positioning systems that can match its stringent specifications. Aerotech's ABL9000 stage is based on an 'H Bridge' gantry configuration where two synchronously-driven Y-axis carriages ride along precision granite rails, which are supported on a granite base. Granite can be lapped to very high flatness tolerances and has excellent dimensional stability, so it's ideal reference surface for precision X-Y motion. A ceramic or aluminium cross beam, mounted between the Y-axis rails provides the X-axis air bearing rail. According to Smith, "this choice of material reduces moving mass and inertia loading and allows much higher performance while maintaining the required stiffness". He explained further: "The ABL9000 incorporates an active preload on both vertical and horizontal bearing surfaces. A combination of a micron-level air gap and a large air bearing surface to distribute the load over a wide surface area results in a stage that has outstanding stiffness, is ideal for heavy loading and has unsurpassed geometrical characteristics." Smith qualified this by stating that the air bearing stage uses a concept that is quite different from most competing technologies. "Other air bearing stages tend to rely on a 'puck' based design, derived from the pucks used in ice hockey. The bearing interaction relies on a number of small machined discs, with air holes arranged in one half of the bearing which deliver pressurised air and allow the two halves of the bearing to move freely." In contrast, Aerotech's air bearings do not rely on pucks. As Smith explained to Eureka: "The ABL9000 uses uniformly distributed air holes over the complete surface area. The advantage here is that the averaging effect of surface disturbance or imperfection is reduced over puck type systems. The thin film fills small surface voids and allows for other "irregularities" and a superior pitch, roll, yaw, straightness and flatness are all realised." With puck designs, Smith said the pucks are often made from anodised aluminium and the finishing process for this can cause surface flatness imperfections which, in turn, has a marked effect on the accuracy of the system. He continued: "For most flat panel display manufacture applications, customers purchase our BLM linear servo motors to drive the X and Y axes. There is zero cogging and no attractive forces, resulting in very smooth motion with high force and a bandwidth capable of positioning the stages to very small tolerances. For applications that require faster acceleration and increase load, the ABL9000 can be fitted with our BLMH or BLMX linear motors for even higher performance." Pointers * High accuracy production of large, high resolution flat panel displays, can be achieved using air bearing stages and linear servo motors * Most air bearing stages are of the 'puck' design, with a number of small machined discs with air holes arranged in one half of the bearing and which deliver pressurised air and allow the two halves to of the bearing to move freely * Aerotech's ABL9000 uses uniformly distributed air holes over the complete surface area, so that surface disturbance or imperfection is greatly reduced, and the system has superior pitch, yaw, roll and flatness over 'puck' designs