To ensure that a high-accuracy positioning system works optimally, the components that make up the system — bearings, position-measuring system, motor-and-drive system, and controller — must all work together as well as possible to meet the application criteria.

Base & bearing

To decide on the optimum system configuration, consider the mechanical part of the system first. For linear stages, these are the four common base-andbearing design choices:

Multiaxis system as used in microelectronics assembly. Horizontal axes include linear electric motor drives (red), linear encoders, and recirculating roller bearing blocks. Vertical axis is ball-screw driven. Base is granite; tables, aluminum.

• Aluminum base and slide with bolton ball-bearing ways.
• Aluminum or steel base and aluminum or steel side with four recirculating roller-bearing blocks on steel rails.
• Meehanite cast iron base and slide with integral roller-bearing ways.
• Granite guides with granite or cast iron slide and air bearings.

Aluminum is lighter than meehanite or steel but less stiff, less stable, less able to take a beating, and less stress-resistant. In addition, aluminum is much more sensitive to temperature changes. Cast iron is 150% stiffer than aluminum and 300% better in vibration damping. Steel is durable and stronger than iron. However, it suffers prolonged ringing, which is detrimental to fast move and settle times. See Figure 1.

Granite guides with air bearings provide the stiffest, most durable combination. Granite can be polished for flatness and straightness in the submicron range. The drawback to a granite table is that, because of the granite’s mass, it has a larger space envelope and weighs more than a steel or iron-based positioning system. However, because there is no contact between bearings and granite guide surfaces, there is no wear, and air bearings are largely self cleaning. Also, granite has excellent vibration damping characteristics and thermal stability.

In addition, design of the table itself is important in the table’s overall performance. Tables come in a variety of configurations from bolt-together units with many parts to simple cast bases and slides. Use of one material throughout the table generally provides more uniform response to temperature variations, leading to a more accurate system. Features such as ribbing provide damping, which enables swift settling.

Integral ways have an advantage over bolt-on ways in that even after a long time, no adjustment of ways for preload is needed.

Crossed roller bearings, Figure 2, have line contact between roller and raceway, whereas ball bearings, Figure 3, have point contact between ball and raceway. This generally results in smoother motion for roller bearings. There is less surface deformation (and wear) over the rolling surface and there is a greater contact area, so load is distributed more evenly. Loads up to 4.5 to 14 kg/roller are standard, along with high mechanical stiffness of about 150 to 300 Newtons/micron. Disadvantages include inherent friction from the line contact.

The small contact area that limits the ball bearing’s friction, however, also limits its load capacity. Roller bearings generally have longer lives than ball bearings. However, roller bearings cost more.

Standard table sizes of one manufacturer include 25 to 1,800-mm length and 100 to 600-mm slide width.

An air bearing configuration, Figure 4, consists of lift and guide bearings preloaded by opposing air bearings or by high-force rare-earth magnets imbedded in the guiding members. This noncontact design avoids the friction of other bearing designs. Also, air bearings suffer no mechanical wear. Furthermore, air bearings can be spaced widely apart. Thus, resulting geometric errors are averaged, producing angular deviations of less than 1 sec of arc and straightness of better than 0.25 micron over 200 mm.

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