Positioning systems: An overview

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.

Positioning stages and tables are integratedsystems consisting of motors and mechanical powertransmission devices, or linear motors and actuators completewith 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.

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.

Z-Axis stages from Optimal Engineering Systems, Inc. (OES) are driven by an 8-mm diameter ball screw with a 1-mm lead, and guided by cross roller bearings. They feature vertical travel distances up to 20 mm with resolutions of 0.1 micron or 0.05 microns with 10/20 micro-steps per step motor drive, and repeatability of +/- 0.5 microns.

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.

This is where the principle of “how the system is used” helps distinguish between the various types of multi-axis systems. 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 “openframe” or “open aperture” have a clear opening through the center of the table. This allows them to be used in applications where light or objects need to pass through, such as back-lit inspection applications and insertion processes.

Because X-Y tables are primarily used for high-precision applications, the guideway of choice is crossed roller slides, which provide extremely smooth and flat travel. Drive mechanisms are typically ball screw or linear motor, although fine pitch lead screws are also common.