MOTION SYSTEMS HANDBOOK
Positioning stages and tables One of the most common types of integrated motion systems
TABLES
is positioning stages or tables. These systems consist of a number of
So-called X-Y tables are similar to X-Y Cartesian systems, in that they have two axes (X and Y, as their name implies) mounted on top of each other, and typically have strokes of one meter or less. But the key difference between X-Y Cartesian systems and X-Y tables lies in how the load is positioned. Instead of being cantilevered as in a typical Cartesian system, the load on an X-Y table is almost always centered on the Y axis, with no significant moment created on the Y axis by the load. X-Y tables generally work only within their own footprint, meaning the load does not extend beyond the Y axis. This makes them best suited for applications where a load needs to be positioned in the horizontal plane (X-Y). A typical example is a semiconductor wafer being positioned for inspection, or a part being positioned for a machining operation to take place. Designs referred to as
common motion components including motors (either rotary or linear) and linear actuators, as well as controllers, encoders and other sensors.
POSITIONING STAGES Positioning stages provide one of several different types of motion. They can be linear, rotary or even lift types (Z-axis positioning stages). Among these, they can be configured in many different ways including movement in one direction (or axis) only, in multiple directions (X-Y positioning), or for extremely small and precise movements, as in nanopositioning applications where moves are in the micro- or nanometer range. Depending on a number of factors including cost and desired accuracy, the drive mechanisms for positioning stages and tables can vary significantly. For instance, stages can be direct-drive types driven by linear servomotors or by a combination of motors, gears and couplings. They can be linear or rotary actuator driven, either using electric actuators or other types. Some other common methods include belt and pulley systems, ball screws or lead screws. Precision and accuracy requirements can also dictate design decisions such as what components to use in assembling a positioning stage. For stages requiring reliability and high accuracy, air bearings are often used to minimize friction. Air bearings support a load with a thin film of pressurized air between the fixed and moving elements. They’re typically referred to as aerostatic bearings, because a source of pressure rather than relative motion supplies the film of air. For instance, so-called planar stages are typically constructed of air bearing guides and linear motor drives. Unlike ordinary bearings, the surfaces of an air bearing do not make mechanical contact, so these systems don’t need lubrication. Because the surfaces do not wear, the systems don’t generate particulates, which makes them suitable for clean-room applications. When supplied with clean, filtered air, the bearings can operate without failure for many years.
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DESIGN WORLD — MOTION
8 • 2020
Many manufacturers offer software selection tools to help engineers size and pick the right stage or table for a given application. For example, simulation software from PI (Physik Instrumente) helps determine if a hexapod (or Stewart platform) is suitable for a specific positioning task, in terms of workspace, load, center of mass, and operating orientation.
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