HEIDENHAIN Length Gauges - Manual (Page 10)

5 µm
10
Measuring principles
Measuring standard
HEIDENHAIN length gauges are character-
ized by long measuring ranges and consis-
tently high accuracy. The basis for both is
the photoelectrical scanning principle.
HEIDENHAIN length gauges use material
measuring standards consisting of abso-
lute or incremental graduations on sub-
strates of glass or glass ceramic. These
measuring standards permit large measur-
ing ranges, are insensitive to vibration and
shock, and have a defined thermal behav-
ior. Changes in atmospheric pressure or
relative humidity have no influence on the
accuracy of the measuring standard—
which is the prerequisite for the high long-
term stability of HEIDENHAIN length
gauges.
HEIDENHAIN manufactures the precision
graduations in specially developed,
photolithographic processes.
• AURODUR: matte-etched lines on gold-
plated steel tape with typical graduation
period of 40 µm
• METALLUR: contamination-tolerant
graduation of metal lines on gold, with
typical graduation period of 20 µm
• DIADUR: extremely robust chromium
lines on glass (typical graduation period
of 20 µm) or three-dimensional chromi-
um structures (typical graduation period
of 8 µm) on glass
• SUPRADUR phase grating: optically
three dimensional, planar structure;
particularly tolerant to contamination;
typical graduation period of 8 µm and
finer
• OPTODUR phase grating: optically three
dimensional, planar structure with
particularly high reflectance, typical
graduation period of 2 µm and less
Along with these very fine grating periods,
these processes permit a high definition
and homogeneity of the line edges.
Together with the photoelectric scanning
method, this high edge definition is a
precondition for the high quality of the
output signals.
The master graduations are manufactured
by HEIDENHAIN on custom-built high-
precision dividing engines.
DIADUR phase grating with
approx. 0.25 µm grating height DIADUR graduation
Measurement procedure
With the incremental measuring
method , the graduation consists of a
periodic grating structure. The position
information is obtained by counting the
individual increments (measuring steps)
from some point of origin. Since an
absolute reference is required to ascertain
positions, the measuring standard is
provided with an additional track that bears
a reference mark. The absolute position
on the scale, established by the reference
mark, is gated with exactly one signal
period.
The reference mark must therefore be
scanned to establish an absolute reference
or to find the last selected datum.
With the absolute measuring method,
the position value is available from the en-
coder immediately upon switch-on and can
be called at any time by the subsequent
electronics. There is no need to move the
axes to find the reference position. The ab-
solute position information is read from
the graduated disk , which is formed from
a serial absolute code structure. A separate
incremental track is interpolated for the
position value and at the same time—de-
pending on the interface version—is used
to generate an optional incremental signal.
Photoelectric scanning
Most HEIDENHAIN encoders operate
using the principle of photoelectric
scanning. Photoelectric scanning of a
measuring standard is contact-free, and as
such, free of wear. This method detects
even very fine lines, no more than a few
microns wide, and generates output
signals with very small signal periods.
The finer the grating period of a measuring
standard is, the greater the effect of diffrac-
tion on photoelectric scanning. HEIDEN-
HAIN uses two scanning principles with
linear encoders:
• The imaging scanning principle for
grating periods of 20 µm and 40 µm
• The interferential scanning principle
for very fine graduations with grating
periods of, for example, 8 µm