Abbe Error

**Abbe Error** - Abbe Error is the
linear off-axis error introduced through the
amplification of tilt by an Abbe offset moment arm
(depicted as *h* in the figure at right).
This type of error increases with distance from the
axis of motion. For example, the Abbe error is
approximately 0.01 µm of error based on a 10mm
moment arm and a tilt of 1µradian.

Accuracy

**Accuracy** - Accuracy is the
departure of the actual position from the commanded
position (although, more precisely, it is the
inaccuracy). Accuracy may be expressed per unit
distance of travel or over the full travel of the
stage.

For example, a linear stage may be specified as having an accuracy of 3 micrometers per inch of travel. This specification is sometimes considered the "linearity."

Axes

**Axes** - The convention used for NAI
stages is the travel (longitudinal) axis is X, the
cross-travel (lateral) axis is Y, and the vertical
axis is Z as shown in figure at right.

Axial thrust

**Axial thrust** - For a rotary stage,
the axial thrust refers to the hold-down strength of
the rotor or the force required to release the rotor
from its kinematic footing.

Axial Wobble

**Axial wobble** - For a rotary stage,
axial wobble is the repeatable component of total
axial angular deviation resulting from machining
tolerances on mating surfaces.

Backlash

**Backlash** - Backlash in an error in
positioning experienced upon reversal of travel
direction. Backlash is the portion of commanded
motion that produces no change in position upon
reversal of travel direction and is caused by
clearance between elements in the drive train and
mechanical deformation. Backlash also affects
bidirectional repeatability. NAI manual stages do
not exhibit measurable backlash because they are
spring preloaded.

Direct Top Load

**Direct Top Load** - Direct top load
is the maximum downward vertical load or force which
can be applied to a stage slider perpendicular to
the mounting surface (without deformation or
associated motion error). If the force or center of
gravity of the load is located off-center of the
mounting surface, there will be an additional moment
load (refer to moments).

Eccentricity

**Eccentricity** - This is the radial
(perpendicular to the axis of rotation) deviation of
the center of rotation (lateral shift of the center
of rotation) from its mean position as a stage
rotates through one revolution.

Moment Load

**Moment Load** - Moment load is the
maximum torque sustainable (without deformation or
associated motion error) when a force is applied to
a rotary table parallel to the rotation axis of a
rotary table. The load therefore is applied at a
moment arm (distance) from the table center of
rotation. The torque moment is computed as the
product of the applied load and the moment arm.

Pitch (moment) Load

**Pitch (moment) Load** - Pitch load is
the maximum torque sustainable when the force or
center of gravity of a vertically applied load is
longitudinally offset from the center of the slider
mounting surface (without deformation or associated
motion error). The load therefore is applied at a
longitudinal moment arm (distance) from the center
of the mounting surface. The pitch moment is
computed as the product of the applied load and the
longitudinal moment arm.

Planar Shift

**Planar Shift** - For a rotary stage
table, planar shift is the maximum out of plane
excursion of the table during rotation. This shift
is partially correlated with wobble, but a component
is due to local variation in table motion.

Radial Load

**Radial Load** - Radial load is the
maximum force sustainable (without deformation or
associated motion error) when the force is applied
radially to a rotary table.

Repeatability

**Repeatability** - Repeatability
represents the maximum difference between actual
position values and commanded values, obtained in
repetitive moves of a positioning stage to the
commanded position. There are two types of
repeatability: unidirectional and bidirectional.
They are not the same as accuracy. Thus, a system
may be very repeatable yet lack in accuracy. The
distinction between accuracy and repeatability is
illustrated in the figure at right.

Return Force

**Return Force** - Return force refers
to manual stage preload spring force. It exhibits a
maximum value at full compression and a minimum
value at full extension positions of the slider.
When a manual stage is mounted with the travel axis
oriented vertically, these extreme values indicate
what loads will remove the preload. The values
correspond to the thumbscrew below the slider with
the spring at full compression and the thumbscrew
above the slider with the spring at full extension.

Roll (moment) Load

**Roll (moment) Load** - Roll load is
the maximum torque sustainable (without deformation
or associated motion error) when the force or center
of gravity of a vertically applied load is laterally
off-set from the center of the slider mounting
surface. The load therefore is applied at a lateral
moment arm (distance) from the center of the
mounting surface. The roll moment is computed as the
product of the applied load the lateral moment arm.

Rotor Stall Torque

**Rotor stall torque** - The rotor
stall torque corresponds to the stall (zero
velocity) torque of the drive motor, modified by the
gear ratios of the specific drive train connecting
the motor to the rotor.

Runout

**Runout** - Runout pertains to the
linear (as opposed to angular) portion of off-axis
error present during slider translation. It is the
maximum departure from desired, ideal straight line
motion and consists of two orthogonal components,
flatness and straightness. Runout is specified as
the larger of the maxima of these two components.

Torque Load

**Torque load** - Torque load is the
maximum torque sustainable (without deformation or
associated motion error) when a force is applied
tangentially to a rotary table (perpendicular to a
radial from the center of rotation of a rotary
table). The load therefore is applied at a moment
arm (distance) from the table center of rotation.
The torque moment is computed as the product of the
applied load and the moment arm.

Total Axial Angular Deviation

**Total axial angular deviation** - In
addition to axial wobble, this includes
non-repeatable (stochastic) axial deviation due to
relative bearing motion and other mechanical
contributions in the drive mechanism.

Vacuum Compatibility

**"Vacuum compatibility"** - This
refers to the ability of selected products to
operate at 10^{-6} Torr pressure. This is
achieved by selection of device materials and
lubrication means that exhibit limited outgassing
behavior.

Wobble (linear stage)

**Wobble (linear stage)** - In the
context of a linear stage, this refers to the larger
of the maxima of slider roll, pitch, and yaw
encountered during slider translation.

Yaw (moment) Load

**Yaw (moment) Load** - Yaw load is the
maximum torque sustainable (without deformation or
associated motion error) when the force or center of
gravity of a laterally applied load is
longitudinally offset from the center of the slider
mounting surface. The load therefore is applied at a
longitudinal moment arm (distance) from the center
of the mounting surface. The yaw moment is computed
as the product of the applied load and the
longitudinal moment arm.