Multi-axis stages are usually constructed in either an XY or a gantry configuration. Within the category of XY stages, there are stacked and monolithic designs. But both XY and gantry stages can be designed as planer types.
Stacked XY stages consist of two individual axes mounted together, and they offer a high degree of design flexibility. Monolithic designs combine the base of the Y (top) axis with the carriage of the X (bottom) axis for a compact design with high orthogonality between the axes. Gantry stages have two X (bottom) axes that support a Y (cross) axis, providing high load capacity and high stiffness.
When a gantry or XY stage is designated as a “planar” type, this indicates that the system has been designed to provide extremely high travel accuracy. Planar stages are typically constructed of air bearing guides and linear motor drives. The key to providing flatness and straightness of travel is that the air bearings ride directly on the base of the stage, which is often made of granite (although aluminum can also be used). Air bearings in planar stages are also preloaded, which provides extremely high stiffness and fast settling times for highly dynamic moves.
Some manufacturers use crossed roller guides rather than air bearings, but while they provide very high travel accuracy and flatness, smoothness of motion is typically less with crossed roller guides than it is with air bearings. For the drive mechanism, precision ball screws can be used in place of linear motors, but again, smoothness of motion is lower for ball screws than for linear motors.
Air bearings support a load along their entire length, in contrast to mechanical rolling element bearings, which support a load only at the contact points of the balls or rollers. So, where the travel of mechanical bearings is influenced by every imperfection in the bearing surface, an air bearing averages all the imperfections of the surfaces across the air bearing length. This is sometimes referred to as the “surface-averaging” effect.
Another type of planar stage is often referred to as a planar motor. In this design, the base of the stage serves as the motor stator, and the forcer incorporates an air bearing, allowing it to glide over the stator. The stator and forcer are constructed much like a linear motor, with the stator containing multiple teeth and the forcer containing windings.
Multiple forcers can be used on the same stator, with the ability to move independently from one another. And, forcers can move not only in the X and Y directions, but also in the theta direction, with a small amount of rotation possible. Because of the strong magnetic attraction between the forcer and the stator, planar motors can be operated in wall-mounted or upside-down orientations.
The primary application areas for planar stages and motors are commonly found in the semiconductor industry, in processes such as wafer scribing and lithography, as well as scanning and inspection systems.
This video from PI shows the internal construction and operating principles of a planar stage.