MOTION SYSTEMS HANDBOOK
Positioning systems: An overview Positioning stages and tables are integrated systems consisting of motors and mechanical power transmission devices, or linear motors and actuators complete with encoders, sensors, and controllers. Better mechanical components and feedback and control options are enabling stages capable of motion that’s more accurate than ever. So, positioning stages today can execute tasks with tighter synchronization than in the past, useful for complicated axis commands. Stages and tables are used in a range of highperformance applications, such as industrial robots, fiber optics and photonics, vision systems, machine tools, semiconductor equipment, medical component laser machining, micromachining and electronic manufacturing. Stages can 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. The drive mechanisms for positioning stages and tables can also vary significantly depending on a number of factors including cost and desired accuracy. For instance, stages can be direct-drive types driven by linear servomotors or by a combination of motors and gearing and couplings, and can be linear or rotary actuator driven (either using electric actuators, or even pneumatic or hydraulic actuation). Other methods can 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.
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DESIGN WORLD — MOTION
Positioning Stages & Gantries & XY Tables — Motion Control HB 08-19.indd 116
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One kind of component used in stages requiring reliability and high accuracy are air bearings. Air bearings support a load with a thin film of pressurized air between the fixed and moving elements. They are 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.
X-Y TABLES 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 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.
The new long travel linear motor stage from PI (Physik Instrumente) features travel length of 32 in. (813 mm). The direct-drive stage achieves high velocities to 79 in./sec (2 m/sec) based on a direct drive ironless linear servo motor. High accuracy, repeatability, and functionality are guaranteed by an integrated absolute-measuring linear position encoder featuring 1-nm sensor resolution.
motioncontroltips.com | designworldonline.com
8/19/19 3:35 PM
