Absolute, high-resolution opt
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more common optical rotary encoders, optical linear
position encoders are preferably attached to high-
,5 µm.precision linear bearings.
Most commercially available optical linear posi-
tion encoders are based on a precise glass scale
where a binary grating of an approximately 10–
40-µm period is etched into the chromium layer that
covers the glass. The glass scale is transillumi-
nated, and, with an array of secondary gratings and
with the Moire´
1
effect, the shadow of the grating is
analyzed with photodiodes that are large compared
with the grating period. Four of these diodes are
employed, generating photocurrents that are approxi-
mately sinusoidally shaped as a function of position
and phase shifted by p@2 with respect to each other
for the determination of the actual position of the
encoder head to the glass scale, within one period of
In the next generation of machine tools, a preci-
sion of 1 µm will be standard,
4
so for critical axes
position encoders must provide a resolution of 0.1
µm, ,10 times better than that obtainable with
present standard shadow linear encoders. High-
resolution encoders are available from various suppli-
ers,
5,6
exhibiting resolutions down to 10 nm. These
systems are based on the diffraction of an illuminat-
ing beam at a grating of short period and the
detection of the interference pattern of selected
diffraction orders. As with standard optical linear
encoders, these systems measure position incremen-
tally.
If power failures or mechanical shocks occur in the
manufacturing process, CNC machines must be re-
set to certain reference positions in each axis, which
takes time and requires a special procedure to avoid
damage. Moreover, to proceedwith themanufactur-
ing process, the positional state at the moment of
interrupt must be reestablished, which is difficult to
achieve. A more favorable option would be to em-
The authors are with Paul Scherrer Institute Zu¨rich, Badener-
strasse 569, CH-8048 Zu¨rich, Switzerland.
Received 28 June 1995.
0003-6935@96@010201-08$06.00@0Kai Engelhardt and Peter Seitz
Modern computer-controlled manu
measurement of the linear positio
here. It is based on the transi
diode. The scale has two code tra
determination of position and an
single-lens telecentric optical syste
custom photodetector. This speci
1complex2 Fourier transform for the
encoder shows a resolution of 10
distances, verified with a commerci
Key words: Position encoder, d
application specific integrated circu
1. Introduction
Position encoders are key elements in computer-
controlledmanufacturing [computer numerically con-
trolled 1CNC2]. CNC machine tools, for example,
CNC grinding and turning machines, provide high
precision and repeatability because the positions of
all axes are monitored and controlled throughout the
manufacture of a work piece. Compared with ther
1996 Optical Society of Americaical position encoder
facturing machinery requires the absolute and highly accurate
. Such an absolute, optical linear position encoder is described
luminance of a glass scale with an inexpensive light-emitting
ks, one based on a pseudo-random binary sequence for the coarse
other periodic code for accurate fine-position measurement. A
images the code tracks in a mechanically insensitive way onto a
l detector IC is capable of determining the components of the
patial frequency of the periodic code. The absolute optical position
nm and an absolute accuracy of better than 100 nm over short
l laser interferometer.
stance measurement, telecentric imaging, diffractive lens, photo-
it.
r
1996 Optical Society of America
the grating. With this principle of relative position
measurement, standard linear position encoders
2
provide a resolution of approximately 10 down to 0.5
µm. They are limited to an incremental determina-
tion of position because the number of scale periods
are counted up from a certain reference position.
Linear encoders based on a magnetic scale
3
also
work incrementally, and they show a resolution ofploy an absolute position encoder with the help of
1 January 1996 @ Vol. 35, No. 1 @ APPLIED OPTICS 201

which the manufacturing process can be immedi-
ately continued after the interruption.
A variety of absolute position encoders
7,8
have
image of the absolute track and the image of the
incremental track. The major advance with this
approach is the simplified alignment in the assemblybeen developed that acquire position information by
decoding a coarse pattern on a scale with a high-
resolution line sensor and subpixel algorithms to
achieve position resolutions of 10–0.1 µm despite the
coarse code pattern. In general, these systems are
expensive, and they are no economical alternative to
standard shadow encoders. Interferometers pro-
vide an extremely high resolution, of the order of
some nanometers, but these instruments prove to be
impracticable in a workshop: Over long distances a
useful accuracy of the order of 1 µm can be achieved
only if the refractive index of the air and its variation
along the path of the measurement are taken into
account. As a result, accurate interferometric posi-
tion control on amachine tool requires a complex and
expensive setup, which is not ideal in any case and
incapable of handling turbulence and air inhomoge-
neities.
We describe a low-cost, absolute position encoder
that provides a position resolution and short-range
accuracy of the order of 0.1 µm. As used by stan-
dard optical position encoders, our system is based
on a glass scale that is transilluminated. The scale
exhibits two code tracks running in parallel, the
incremental track that is a grid with a 20-µm period,
responsible for the high resolution, and an absolute
track for the absolute position readout in registra-
tion with the incremental code.
2. System Design
At first sight, similar encoder systems
9,10
that em-
ploy a double track on a glass scale have been
described in the literature. These systems are as-
sembled as a combination of two independent conven-
tional encoders, an incremental and an absolute.
In contrast to these systems the proposed encoder,
shown schematically in Fig. 1, is designed as a single
integrated system that detects the position from the
simultaneous readout of both tracks. A single light-
emitting diode 1LED2 transilluminates the scale, and
a single imaging system projects both code tracks
onto a single photodetector chip that acquires the
Fig. 1. Schematic diagram of the high-resolution, absolute opti-
cal position encoder system.
202 APPLIED OPTICS @ Vol. 35, No. 1 @ 1 January 1996of the encoder system, which is a key factor for
inexpensivemanufacturing and robustness inmount-
ing and operation.
To identify the position uniquely, the absolute code
track must provide a resolution of one period of the
incremental track, which was chosen to be 20 µm.
As the absolute code pattern a pseudo-random bi-
nary sequence 1PRBS2 was chosen that is a popular
coding scheme in absolute position encoders.
11–14
The PRBS is written along the code track, and for an
n-bit PRBS any consecutive sequence of n bits repre-
sents a new code pattern that cannot be found at any
other position along the scale within the range of
unambiguity given by the number n of bits.
In the proposed application the absolute track
should provide a resolution of one period in the
incremental track of 20 µm. An 18-bit PRBS imple-
mented with marks of 20-µmwidth for each bit has a
range of unambiguity of 5.24 m. To detect the
absolute position unambiguously within this range,
a field of 360-µm width must be sensed on the
absolute track. Compared with other absolute en-
coding schemes, such as the binary or the Gray code,
the PRBS code pattern can be acquired with a
detection system with ,10 times lower spatial reso-
lution, resulting in an increased robustness against
pollution of the scale and increased mechanical
tolerances. In addition, PRBS’s can easily be gener-
ated electronically by simple shift registers, e.g., as
shown in Ref. 14.
3. Optical System
In a low-cost position encoder the optical system
must be simple and inexpensive. For mass produc-
tion it would be desirable to fabricate the system by
injection molding of plastics. To reduce cost in
assembly and alignment, the optical system should
be largely insensitive to mechanical tolerances.
To meet the simplicity requirement it was decided
to employ a singlet to perform the imaging of the
code tracks onto the detector. In the proposed setup
a grating of 20-µm period with an ,0.5-mm field
width must be magnified by a factor of 4 in the
detection plane to match the detector size. To meet
the robustness requirement, the imaging system is
optimized for maximum depth of focus at the object
side, to be insensitive to variations in the distance
from the scale to the imaging system. Amechanical
tolerance value of only 610 µm is allowed with
standard linear encoders based on a scale of 20-µm
period and moire´ detection. This low figure serves
as a reference for our achievements.
The modulation of the photocurrents from the
incremental detector on the photo-application spe-
cific integrated circuit 1ASIC2, which is discussed in
Section 4, depends on both the contrast and the
period of the grating image. To obtain a large
modulation depth the grating image must be of high