Feinmess Dresden catalogue - Armstrong Optical Ltd

Definitions
Absolute accuracy:
The absolute accuracy is defined by
the deviation of the actual position
from the rated one (real designation:
Inaccuracy). If a system is to
move by 100 mm but the system
really does move by 99.99 mm only
(measured through an ideal scale),
then the inaccuracy amounts to
0.01 mm. The permanent positioning
error along an axis is designated
as accuracy, after all the other
linear deviations have been eliminated.
Linear (or systematic) deviations
are, for example, cosine deviation,
screw pitch error, angular
error at the measuring point, and
deviation due to thermal expansion.
In graphic representation, these
deviations are shown in a protocol
of position and deviation by the
slope of a compensating straight line.
Referring to the slope of this
straight line the absolute accuracy
can be approximated by calculation
in the following way:
Absolute accuracy =
Accuracy along the axis of
movement + slope x traverse
In positioning units equipped with,
for example, glass scales, the slope
is close to zero, and the absolute
accuracy equals the accuracy along
the axis of movement. The stated
values apply only under the condition
of a clamping area according
to DIN 576 /00 and a torque of
the mounting screw of 0.15 Nm
(at PMT 160 two center screws).
Acceleration:
Speed variation per unit time.
Acceleration = Speed /time (a = v/t)
Accuracy:
Accuracy describes the expected
deviation of the actual position
from the rated one. Positioning
accuracy depends, among others,
on the measuring method applied
for detecting the actual position.
The given value is half peak to
peak value.
Backlash:
Backlash means that positioning
error occurring with change of direction.
Backlash can be caused by
insufficiently pre-loaded thrust or
pic. 1: Guiding accuracy
inaccurate meshing of drive components,
for example, gear teeth. In
most cases, backlash occurs uniformly
and can be compensated for
through installing suitable electronic
equipment. Steinmeyer-made
ballscrews and ballscrew assemblies
are characterised by an extremely
small backlash.
Concentricity runout and error
of drunkenness:
Considering a rotary table, concentricity
(eccentricity) defines the
runout of the centre of rotation
from its middle position detected
over a single turn. In a perfectly
centred rotary measuring table
there would be no eccentricity
while rotating. Drunkenness of a
rotary measuring table is the angular
runout of the axis of rotation
detected over a single turn.
Control sensitivity:
This parameter stands for the minimum
control quantity, which initiates
movement, also described
as the ratio of resulting movement
and causing drive.
Deviation from straightness/
flatness:
Straightness/flatnes is defined as
a sum of the maximum upward
and maximum downward deviation
from the baseline. The stated values
apply only under the condition
of a clamping area according to
DIN 576 /00 and a torque of the
mounting screw of 0.15 Nm
(at PMT 160 two center screws).
Friction:
Friction has been defined as resistance
between contacting surfaces
while a movement is running.
Friction may be constant or depend
on speed. Various factors have an
impact on the total friction of a system,
for example, sliding friction
or wear, and lubricant viscosity.
Guiding error:
The guiding error is the linear portion
of a deviation from the axis
of movement, and composed of
two orthogonal components:
straightness (deviation inside of
the plane of the slide) and flatness
(deviation outside of the plane of
the slide).
Guiding accuracy: (pic.1)
Due to inaccuracies in the guideways
(for example, tolerances of
the rolling elements) the clamping
surface of the measuring table
does not move along an ideal
straight line. Typical deviations are
the yaw error Gz and tilting error
Gy. These errors are also defined as
absolute and relative deviations.
The relative deviation is the deviation
related to the medium straight
line (compensating straight line).
Error of straightness Gz (yawing).
Error of flatness Gy (tilting) .
Hysteresis:
Hysteresis characterises different
movements released by same input
signals, which occur with various
directions. Hysteresis is caused by
elastic forces in various compo-
Test certificate indicating the straightness measured on
a standard table PMT 160-50-DC-R
nents of the drive chain (for example,
distortion of the drive screw).
Hysteresis is often mistaken for
backlash, which can, however,
precisely be determined and compensated
for by suitable control
electronics. The given value is half
peak to peak value.
Inertia and inertia moment:
Inertia and inertia moment represent
the measure of resistance of a
mass against a rotational velocity
variation. The higher the inertia or
inertia moment the greater is the
force (torque) required for acceleration
or deceleration of a load.
The inertia (inertia moment) depends
on the mass (and shape) of
the load.
Life cycle:
Depends from constructions, actual
loads, actual speeds, continuous
operation, environmental factors
and drives.
Lifting-off deviation:
Lifting-off deviation is an enlargement
of the linear deviation of the
axis of movement caused by the
arm due to tilting or drunkenness
of the guiding axis. This deviation
comes into effect especially when
gauging takes place relatively far
away from the guiding axis. The
deviation is to be considered similar
to a guiding variation, but in
contrast to that variation, the lifting-off
deviation can be diminished
by shortening the arm.
FEINMESS DRESDEN GmbH (+49 351) 8 85 85-0 www.feinmess.de 01 |2007