Robot Mechanisms and Mechanical Devices Illustrated - Profe Saul

Chapter 1 Motor and Motion Control Systems 13
Figure 1-14 This single-axis linear
guide for load positioning is
supported by air bearings as it
moves along a granite base.
the tolerance, wear, and compliance in the mechanical components
between the carriage and the position encoder that can cause deviations
between the desired and true positions. Consequently, this feedback
method limits position accuracy to ballscrew accuracy, typically ±5 to 10
µm per 300 mm.
Other kinds of single-axis stages include those containing antifriction
rolling elements such as recirculating and nonrecirculating balls or
rollers, sliding (friction contact) units, air-bearing units, hydrostatic units,
and magnetic levitation (Maglev) units.
A single-axis air-bearing guide or stage is shown in Figure 1-14. Some
models being offered are 3.9 ft (1.2 m) long and include a carriage for
mounting loads. When driven by a linear servomotors the loads can reach
velocities of 9.8 ft/s (3 m/s). As shown in Figure 1-7, these stages can be
equipped with feedback devices such as cost-effective linear encoders or
ultra-high-resolution laser interferometers. The resolution of this type of
stage with a noncontact linear encoder can be as fine as 20 nm and accuracy
can be ±1 µm. However, these values can be increased to 0.3 nm resolution
and submicron accuracy if a laser interferometer is installed.
The pitch, roll, and yaw of air-bearing stages can affect their resolution
and accuracy. Some manufacturers claim ±1 arc-s per 100 mm as the
limits for each of these characteristics. Large air-bearing surfaces provide
excellent stiffness and permit large load-carrying capability.
The important attributes of all these stages are their dynamic and
static friction, rigidity, stiffness, straightness, flatness, smoothness, and
load capacity. Also considered is the amount of work needed to prepare
the host machine’s mounting surface for their installation.