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Principle of the ABSOLUTE Linear Scale (example: AT300, 500-S/H)
AT500
MADE IN JAPAN
Signal cycle (interpolation) Resolution
COA 3768mm (512) 7.36mm
Electrostatic capacitance type MED 58.88mm (512) 0.115mm
FIN 0.92mm (512) Approx. 1.8µm Upon supply of power to a linear scale, position readings from
three capacitance-type sub-scales (COArse, MEDium and FINe)
and one from a photoelectric sub-scale (OPTical) are taken.
Photo-electronic type These sub-scales use such a combination of pitches, and are
so positioned relative to each other, that the readings at any
OPT 20µm (400) 0.05µm one position form a unique set and allow a microprocessor
(4096) 0.005µm
to calculate the position of the read head on the scale to a
resolution of 0.05 m (0.005 m).
3768mm
Specifying Linear Scale Accuracy
Positional indication accuracy (1) Unbalanced accuracy specification - maximum minus
minimum error
The accuracy of a linear scale is determined by comparing the
positional value indicated by the linear scale with the corresponding This method simply specifies the maximum error minus the minimum
value from a laser length measuring machine at regular intervals using error from the accuracy graph, as shown below. It is of the form:
H the accuracy inspection system as shown in the figure below. As the E = (+L)µm. L is the effective measuring range (mm), and and are
temperature of the inspection environment is 20C, the accuracy factors specified for each model.
of the scale applies only in an environment at this temperature. For example, if a particular type of scale has an accuracy specification
Other inspection temperatures may be used to comply with internal —103—0L—0
standards. of (3+ )m and an effective measuring range of 1000 mm, E is
6 µm.
Scale error at any point in range relative to start of range
Laser length measuring Computer Digital counter Error Maximum difference
machine counter in scale error: E(µm)
0 Effective positioning range
Laser source Interferometer Optical axis of Cube corner (2) Balanced accuracy specification - plus and minus about the
laser beam Fixture mean error
Scale unit This method specifies the maximum error relative to the mean error
Movable table from the accuracy graph. It is of the form: e = ±—2E (m). This is mainly
used in separate-type (retrofit) scale unit specifications.
Overview of Accuracy Scale error at any point in range relative to mean error over range
Inspection System
Error Mean error Maximum error about
The accuracy of the scale at each point is defined in terms of an error 0 mean error: ± —2E (µm)
value that is calculated using the following formula:
Effective positioning range
Error = Value indicated by the linear scale minus the
corresponding value of laser inspection system A linear scale detects displacement based on graduations of constant
pitch. Two-phase sine wave signals with the same pitch as the
A graph in which the error at each point in the effective positioning graduations are obtained by detecting the graduations. Interpolating
range is plotted is called an accuracy diagram. these signals in the electrical circuit makes it possible to read a
There are two methods used to specify the accuracy of a scale, value smaller than the graduations y generating pulse signals that
unbalanced or balanced, described below. correspond to the desired resolution. For example, if the graduation
pitch is 20 µm, interpolated values can generate a resolution of 1 m.
The accuracy of this processing is not error-free and is called
interpolation accuracy. The linear scale's overall positional accuracy
specification depends both on the pitch error of the graduations and
interpolation accuracy.
H-15
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