Aluminium ousts granite to slash air bearing costs

Engineers work for Anorad in the Netherlands have come up with new way of building linear air bearing motors which should make them much cheaper, and thus suitable for a far wider range of applications.

Air bearings with direct-drive linear motors are one of the most accurate ways of providing rapid motion accurately over long distances. Because the moving parts "float" on air without making contact, friction is negligible and maintenance costs are low. They are also extremely fast and accurate, with good repeatability and controllability, and low power consumption.

But traditionally, air bearings have had to be built using a base formed from ground and lapped slabs of granite, which are costly and heavy. This has restricted their use to a handful of specialist applications.

Now, the Dutch engineers have come up with a way of replacing the granite with extruded aluminium beams, which serve as the platform for the linear motor. The developers are coy about the details, but the beams are understood to have precise ribbing and webbing geometries. They are said to be easy to manufacture and are rigid enough to achieve repeatabilities of a few microns. They can carry payloads up to 15kg, at speeds of up to 3m/s, with 3G of acceleration, and with a velocity stability said to better than 1%.

Anorad`s designers have also tackled the problem of small particles becoming trapped in the gap and grinding away at the beam`s surface. A slightly porous puck is used which absorbs any particles into its pores. The 4 bar air pressure on which the puck rides also helps to clear any debris from the bearing surfaces.

The ironless motor has an epoxy core and uses a linear optical encoder for position feedback. It works, in effect, like a brushless servo motor and can be controlled by standard amplifiers and motion controllers.

The new axis design made its debut at the recent Drives & Controls Show. The first commercial version has a 2m travel and is aimed at applications such as semiconductor manufacturing, digital printing and biomedical machines. Anorad`s Andy Holmes says that although the aluminium-based axes are not quite as accurate as the granite versions, they will deliver a much higher performance than conventional linear bearings at a similar cost. He therefore expects them to make inroads into the general automation market.