Focused optical beam encoder of position

A light beam, typically 780 nanometer coherent light generated by a laser diode, is focused, preferably by a wide angle graded index optics lens having a numerical aperture of 0.46 and a diameter of 1.8 mm, into a diffraction limited spot, of approximate diameter 0.039 to 0.3 mils, upon an encoder wheel. The encoder wheel is typically a photoetched radial reticular grating on Mylar.RTM. plastic having lines at 0.00025" width at a radius of 1". The focused light beam intercepts the encoder wheel at this 1" radius and is alternately transmitted and obstructed by the reticular grating of the rotating encoder wheel. The selectivity transmitted light beam is received at a detector, typically a phototransistor. Greater than 10,000 transitions per encoder wheel revolution are detectable.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to high resolution linear or rotary 
encoders used for determination of position and/or time derivatives of 
position. The present invention particularly relates to optical encoders 
of position. 
2. Description of the Relevant Art 
Encoders of position transform a physical position into an electrical 
signal corresponding to such position. Encoders of position which are 
optical employ a source of light, a detector of light from such source, 
and a grating which positionally moves relative to the path of light 
between the source and the detector in order, by such movement, to 
selectively interrupt the light path between the source and the detector. 
The optical path may be either through the grating or reflected from the 
grating. The position being encoded may either be angular (rotary) or 
linear. The position may be simultaneously encoded on either single or 
multiple channels of encoding. The absolute position relative to some 
predetermined reference position may be encoded, or only the incremental 
positional changes may be encoded. Most positional encoding is, however, 
of the incremental type wherein an alternating electrical signal is 
produced with successive incremental positional movements, each detectable 
increment of position producing one phase change of the electrical signal. 
The general fields of application for positional encoders, and particularly 
for optical position encoders, are diverse. They include inspections, 
measurements, and metrology; factory automation including machine tools, 
robotics, and assembly machinery; semiconductor processing equipment; 
medical therapuetic and diagnostic equipments; and computer peripherals 
including printers and disk drives. Specific applications include 
Coordinate Measuring Machines (CMM), Flexible Machining Centers (FMC), X-Y 
stages, Servo systems, rotary indexing tables, component insertion 
machines, photomask inspection machines, medical ultrasound systems, tape 
drives, printers, plotters, microfiche readers, and phototypesetters. 
An incremental optical positional encoder is simple in concept. It normally 
consists of four components. First there is a source of light, which 
source is normally either incandescent or a Light Emitting Diode (LED). In 
the optical transmission path is an assembly, normally an encoder strip 
for a linear encoder or an encoder wheel for a rotary optical encoder, 
which presents a pattern of alternating opaque and translucent segments. 
The light passing through (or reflected from) such segments is alternately 
transmitted and blocked (or, alternatively, reflected or absorbed) causing 
a light sensor, which is usually a phototransistor, to detect a light and 
dark modulation in the light beam. Finally, electrical signal conditioning 
circuitry is used to format the electrical signal generated by the light 
sensor (phototransistor) into usable information. The beam from a small 
light source, normally an L.E.D., which is collimated in an elementary 
optical encoder of this nature will normally have a positional resolution 
on the order of 10+ lines per inch. 
Attempts to extend this elementary concept of optical position encoders to 
high positional resolution levels within reasonably sized packages rapidly 
result in considerably increased design sophistication, and greatly 
increased cost on the order of hundreds of dollars per each high 
resolution optical encoder assembly. Particularly in order to obtain 
increased resolution in an optical position encoder (i) the light source 
is collimated and (ii) an additional element called a mask, or reticle, is 
added between the optical disk and the sensor. This mask, or reticle, 
produces a shuttering effect so that only when the translucent segments of 
both the encoder disk and the reticle are in alignment is light then 
permitted to pass in the path between the light source and the light 
sensor. Optical encoders employing both collimated optical beams and 
masks, or reticles, can typically resolve on the order of 144+ lines per 
inch. 
This improved level of resolution, and even that somewhat greater level of 
resolution attained by the very best prior art optical encoders which are 
of reasonable size (and which will shortly be specifically discussed), is 
very often inadequate in modern applications. For example, an important 
specification of robot performance and productivity is its 
acceleration/deceleration times. However, there is no good measurement 
standard to mathematically describe overshoot and settling times resultant 
from acceleration and deceleration of a robot arm. Despite the lack of 
quantification, the robot movement process is clearly visible. When a 
robot arm approaches a designated point that requires maximun 
repeatability under full payload, it is all too likely to exhibit an 
overshoot or settling time problem which causes it to do a "rain dance", 
jerking every which way. Another robot specification which correlates with 
the existence of these problems is encoder shaft pulses per second, or 
resolution. The more pulses per second generated by the encoder shaft at 
full speed, the more precise the possible control of the robot arm and the 
less likelihood of overshoot and/or settling time problems which, in the 
extreme case, result in crashes and damaged parts. 
Even when the positional encoder rate in pulses per second is very high, or 
even maximal, by the standards of the prior art, it should be recognized 
that electronic circuits which are carefully designed to apply time 
constants reflective of mechanical settling times are required to effect 
even approximately smooth movement. This is true even though the 
electronic circuits controlling mechanical motion may be many hundreds, 
and even thousands, of times faster-responding than the mechanical systems 
which are controlled in motion. Why then cannot electronic control of 
position be simplistically based on mere brute force positional feedback 
loop computation and resultant control?. The answer is simply that, in the 
past, the electronics needed to be sophisticated to effect control based 
on inadequately precise and inadequately timely positional information. 
The electronics must constantly extrapolate from imprecise and untimely 
information to predict where the mechanical system really is, and will be. 
It is generally insufficiently accurate and/or timely positional 
information, and not any intrinsic limitation of electronic control of 
mechanical motion, which imparts the jerkiness to robot motion which is so 
cleverly and amusingly satirized in a human pantomine of such robot 
motion. 
The present invention is intended, amongst other purposes, to render 
obsolete the stereotypical notion of jerky and spasmodic robot motion. It 
does so by providing low cost positional sensing of such high resolution 
that even quite crude electronic positional control circuits will suffice, 
in combination with improved positional encoders in accordance with the 
present invention, to so continuously apply accurate drive stimulus to 
mechanical motion so as to make such motion appear, in relation to the 
human senses, to be fluid and graceful. In other words, if position, and 
the first time derivative of position or velocity, and the second time 
derivative of position or acceleration, may all be readily known with 
astounding accuracy and time currency, then the most rudimentary equations 
relating actual future position to desired future position may be used to 
effect control of the motion of mechanical systems. Before positional 
encoders in accordance with the present invention are taught, however, it 
is useful to further consider specific prior art optical encoders of high 
performance. 
Representative highest-performing prior art optical rotary position 
encoders include the following. The Encoder Division of Dynamics Research 
Corporation offers in their Module 25 encoder a disk which can be provided 
with up to 3,000 lines, providing a maximun of 3,000 cycles per shaft 
revolution (exclusive of either internal or external cycle interpolation) 
in a 2.5 inch diameter package. The K3 series modular optical encoder from 
the Instrument Division of Dresser Industries is capable of up to 2500 
cycles per revolution resolution in a 2.1 inch diameter case. Finally, the 
HEDS-6000 series of incremental optical encoders from Hewlett Packard 
offers resolution of up to 1024 cycles per revolution in a 2.2 inch 
diameter case. The number of lines, or transitions, which are being 
resolved per inch in these encoders may be readily estimated from the fact 
that the circumference of a circle is pi times its diameter. 
All the prior art high resolution high optical encoders are very difficult 
and exacting of assembly, which contributes greatly to their cost. For 
example, the aforementioned HEDS-6000 series is available as a 
user-assemblable optical encoder kit. The number of major assembly steps 
is 8, each consisting of an average of 4 seperate substeps. Although in 
high volume applications using custom design tooling and automated 
equipments it is predicted by the manufacturer that encoder assembly can 
be accomplished in less than 30 seconds, assembly by more conventional 
manual means is a demanding task of many minutes which is normally 
performed by skilled technicians or assemblers. 
The difficulties, time demands, and resultant high cost of assembling 
high-resolution optical encoder assemblies stems from their basic and 
uneliminatable requirement for exacting mechanical and optical parts which 
must be aligned with great precision. Consider, for example, the 
sensitivity of a rotary optical encoder to motor shaft runout and woble. 
Motor shaft runout means that the optical encoder disk may be eccentric on 
its axis of rotation. This means that the alternating opaque and 
translucent segments, as detected at the radius of the encoder disk 
whereat it intercepts the light path, will not be of equal width in 
different sectors around the circumference of the optical disk. Unequal 
widths translate into unequal detected light energy, and unequal 
electrical signals resultant from such detection, for equal motion. 
Obviously positional resolution, and accuracy, is affected when the 
transition of the electrical signal may be a function of which sector of 
an eccentric encoder disk is being read as well as the motion and position 
of the encoder disk. 
Probably more important than motor shaft runout, which to some degree can 
be overcome with electrical signal shaping circuitry, is the problem of 
wobble, or that the plane of the disk may not be precisely perpendicular 
in all sectors thereof to the path of light through such encoded disk. If 
the alternating opaque and translucent segments on the disk are at a 
slight angle to the path of light intercepting such segments, than it 
should be envisioned that these segments, being of finite thickness, will 
intercept the light beam in a manner which causes more of a step function, 
or even a sine wave, in the light intensity detected at the light sensor 
as opposed to an on-off square wave of received light intensity. Thus, 
when the alternating segments, which are extremely narrow, are at a 
sufficient angle to the impingent light beam, then no modulation will be 
obtained at all, with the leading edge of one opaque segment overlapping 
the trailing edge of a predecessor opaque segment. In order to reduce this 
problem, the optical encoder disk may be made extremely thin. If it is 
thin and flexible then it may exhibit warp or systemic deformation, 
reintroducing wobble. If it is extremely thin and rigid then it may be 
readily subject to mechanical damage, especially from shock. 
Problems experienced with the demanding mechanical components, and 
alignment, of high resolution optical positional encoders are analogous to 
those experienced in phonographic reproduction of sound, in magnetic 
recording of disks, and in optical recording of disks. In order to obtain 
resolution performance at or near the limits of the prior art technology 
optical encoders, which limits are on the order of several hundred lines 
per inch resolution, cosiderable penalties may have to be paid in 
reliability, immunity to vibration and/or mechanical shock and/or 
temperature variation, and especially in cost. For these reasons, the 
present invention is embodied in a new apparatus for highest resolution 
optical position encoding which apparatus is both easy and low cost of 
assembly, and rugged and reliable in operation. 
SUMMARY OF THE INVENTION 
The present invention is an improvement to the apparatus of an optical 
position encoder having a source of light, a detector of light from such 
source, and a grating which moves relative to the path of light between 
the source and the detector in order, by such movement, to selectively 
interrupt the light path between the source and detector in a manner which 
may be detected by the detector. For a light path which is through the 
grating, as opposed to being reflected from the grating, the grating 
presents a pattern of alternating opaque and translucent segments. For the 
case of a rotary optical encoder encoding angular position, the grating is 
an encoded disk, or wheel. 
The improvement in accordance with the present invention is to convergently 
focus the light from the source of light onto the grating. The detector of 
light, nominally a phototransistor, is placed sufficiently close to the 
grating along the light path, and in the opposite direction along such 
path from the source of light and the focusing assembly, so that 
substantial of the light which has been focused onto the plane of the 
grading is intercepted. 
It is particularly desired, and realized in the preferred embodiment of the 
invention, that the focusing of the source of light should be to a 
principal focus at the approximate size of a diffraction limited spot upon 
the plane of the grating. Although this can be realized with normal glass 
lenses, it normally requires multiple lens elements of high cost. 
Correspondingly, the preferred embodiment method apparatus of the present 
invention uses a source of coherent light, nominally obtained from a low 
cost laser diode, which is focused into a diffraction limited spot by a 
graded index optics lens, also of low cost. A graded index optics lens is 
a lens in which the index of refraction is varied by changes in the 
optical index of the material of the lens obtained by doping of this 
material, nominally with thallium. The graded index optics lens which is 
particularly preferred for use in the apparatus of the present invention 
exhibits an index of refraction which decreases as the square of the 
radial distance from the optical axis. This graded index optics lens 
performs the same optical function as standard spherical lenses with the 
added feature that the end surfaces are flat and that the lens is of low 
cost. 
The preferred detector within the apparatus embodying the invention is 
simply a readily available phototransistor. At the preferred optical 
wavelength (approximately 780 nanometers) generated by the laser diode, a 
certain preferred wide angle graded index lens having a numerical aperture 
of approximately 0.46 and a diameter of approximately 1.8 millimeters is 
used to focus a diffraction limited spot on the plane of an encoder wheel. 
The diameter d of the diffraction limited spot is defined by optical 
theory as d 4.lambda.f/.pi.D wherein .lambda.=wavelength, f=focal length, 
and D=beam diameter. At the preferred configuration of the apparatus of 
the present invention, the diameter d of this diffraction limited spot at 
the 1/E.sup.2 power points will be on the order of 0.039 to 0.3 mils. 
Thus the typical resolution obtainable by the method and apparatus of the 
present invention is greater than 2500 lines per inch, or greater than 
10,000 transitions detectable on a photoetched radial reticular grating on 
mylar.RTM. plastic exhibiting lines at 2.5 ten-thousands line width when 
this grating is intercepted at a 1" radius by a focused light beam. This 
level of resolution is obtainable by an apparatus constructed in 
accordance with the present invention without high precision alignment of 
components, and without any high sensitivity of the apparatus so assembled 
to temperature, shock, vibration or other physical variables. The 
preferred embodiment apparatus costs less than approximately $30 to build, 
including assembly labor. The present invention may be directly further 
extended to the theoretical limits of optical performance based upon the 
smallest dimensions which can be optically resolved. If the inexpensive 
graded index optics lens is used, it typically has a cut-off wavelength as 
short as 0.380 micrometers.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention is an improvement to the method of optically encoding 
position, and to an apparatus for so optically encoding position. The 
position encoded may be either linear or angular. The time derivatives of 
the encoded position, i.e., velocity and acceleration, may be derived from 
the encoded position by conventional means including electronic circuitry. 
A first, rudimentary, prior art apparatus for optical encoding of rotary, 
angular, position is diagramatically shown in FIG. 1. This optical encoder 
employs a light source, nominally a Light Emitting Diode, or L.E.D. The 
light emitted by such L.E.D. is formed into COLLIMATED OPTICAL BEAM, 
either by the L.E.D. itself or by the use of standard optical elements 
(not shown), and transmitted in an optical path to an optical detector, 
nominally a PHOTOTRANSISTOR. An optical grading, herein for the detection 
of rotary position, in the form of an ENCODER WHEEL presents a pattern of 
alternating opaque and translucent segments between the L.E.D. light 
source and the detector PHOTOTRANSISTOR. When the ENCODER WHEEL moves 
angularly, such as in response to rotation of a shaft (not shown) to which 
it is affixed, then the light detector PHOTOTRANSISTOR will detect light 
and dark modulation in the received light beam, producing an output 
electrical signal which varies in level. Electrical circuitry (not shown) 
receives this output electrical signal, conditions it, and formats it into 
usable information representative of the angular position of the ENCODER 
WHEEL. The resolution of position so optically encoded is a function of 
the size of the COLLIMATED OPTICAL BEAM, and of the alignment of the 
components. Typically for the apparatus diagrammatically illustrated in 
FIG. 1 the positional resolution on the order of 10+ lines per inch. The 
angle which can be resolved depends on the radius of the ENCODER WHEEL. 
A second prior art optical encoder apparatus is diagramatically shown in 
FIG. 2. In order to obtain increased positional resolution, the light 
source originating at the L.E.D. is again formed into a COLLIMATED OPTICAL 
BEAM and is passed through the ENCODER WHEEL plus the additional element 
of the RETICLE, or mask. The RETICLE is added between the ENCODER WHEEL 
and the optical detector PHOTOTRANSISTOR in order to produce a shuttering 
effect. By operation of this shuttering effect light is permitted to pass 
from the L.E.D. to the PHOTOTRANSISTOR only when the translucent segments 
of both the ENCODER WHEEL and the RETICLE are in alignment. Since two 
translucent segments, or slits, must be lined up in order to allow 
completion of the optical transmission path, each segment may be 
individually narrower than the COLLIMATED OPTICAL BEAM. By such a 
technique, as illustrated in the prior art apparatus shown in FIG. 2, a 
resolution which is typically on the order of 144+ lines per inch may be 
obtained. 
Both the prior art apparatus shown in FIG. 1 and that shown in FIG. 2 are 
susceptible to eccentricity of the ENCODER WHEEL about its axis of 
rotation, also to any wobble of the ENCODER WHEEL in the plane 
perpendicular to the COLLIMATED OPTICAL BEAM, and also to any systemic 
misalignment of components. Particularly, it may be imagined what effect 
is derived if the plane of the ENCODER WHEEL is not precisely 
prependicular to the path of the COLLIMATED OPTICAL BEAM--either because 
of wobble in the ENCODER WHEEL or because of misaligment resulting in a 
constant tilt of the ENCODER WHEEL. In such a case, the alternating dark 
and light segments upon the ENCODER WHEEL will not intercept the 
COLLIMATED OPTICAL BEAM with sharp edges. Rather, such alternating opaque 
and translucent segments will only progressively obscure, and then 
progressively enable, transmission of the COLLIMATED OPTICAL BEAM. This 
progressive obscuration and unveiling causes that the light intensity at 
the light-detecting PHOTOTRANSISTOR, and the electrical signal developed 
from such PHOTOTRANSISTOR, will not be a perfect on/off function (an 
electrical square wave), but will rather be a progressive function (an 
electrical step or continuous wave) which will gradually deteriorate into 
a gray (black) or d.c. level indicative of a constantly blocked light 
path. Because of such sensitivity to the orthogonality between the 
COLLIMATED OPTICAL BEAM and the ENCODER WHEEL (and RETICLE), those PRIOR 
ART apparatus diagrammatically illustrated in FIG. 1 and FIG. 2 require 
precise initial alignment, and operational maintenance of such precise 
alignment. The method and apparatus of the present invention will be 
considerably simpler of initial alignment, and considerably more 
insensitive to any variation in this alignment, than are the prior art 
apparatus. 
A basic embodiment of an apparatus in accordance with the present invention 
is diagrammatically illustrated in FIG. 3. The fundamental addition to the 
prior art optical encoder shown in FIG. 1 is the FOCUSING LENS. The 
FOCUSING LENS convergently focuses light received from the light source, 
nominally the LASER DIODE, onto the plane of the ENCODER WHEEL. The 
focused light beam diverges after passing through the ENCODER WHEEL, and 
is substantially intercepted by the detector, nominally a PHOTOTRANSISTOR. 
Obviously the focused light beam intersects the ENCODER WHEEL at a smaller 
diameter spot at or near the principal focus, and thusly enables higher 
resolution, than is obtainable within prior art apparatus of equivalent 
dimension. At the threshold of considering the present invention it must 
be candidly admitted that if only a conventional glass focusing lens, 
without more and without any different means for focusing than is 
presented by such a conventional glass lens, is added to an apparatus of 
the prior art, then more problems may have been added than have been 
solved. Addition of the focusing element in the form of a glass lens(es), 
while totally effective to accomplish the present invention, is liable to 
be very expensive. 
Consequently the principles of the present invention are best implemented 
in a more sophisticated form than a somewhat simplistic (in execution, not 
in concept) addition of a FOCUSING LENS to the prior art apparatus. 
Particularly, the basic embodiment of the apparatus of the present 
invention shown in FIG. 3 needs not use a coherent light source, but is 
suggested to do so in the form of a LASER DIODE light source. When the 
light is coherent, then the FOCUSING LENS can focus such to the smallest 
spot which is possible within optical diffraction theory. This spot is 
called a DIFFRACTION LIMITED SPOT and it is this preferred spot which is 
shown to be focused on the plane of the ENCODER WHEEL in FIG. 3. The size 
of the DIFFRACTION LIMITED SPOT is dependent upon the wavelength of the 
light focused, the focal length of the FOCUSING LENS, and the diameter of 
the light beam which is focused by the lens. This dependency is expressed 
in the formula: 
EQU d=4.lambda.f/.pi.D 
wherein d is the diameter of the DIFFRACTION LIMITED SPOT in microns 
measured at the 1/E.sup.2 power points, wherein .lambda. is the wavelength 
of the focused light in microns, wherein f is the focal length in 
millimeters of the FOCUSING LENS, and wherein D is the diameter of the 
focused light beam in millimeters at the 1/E.sup.2 power points. 
Furthermore, the FOCUSING LENS accomplishing optical focusing even to the 
minute size of the preferred DIFFRACTION LIMITED SPOT may be made from 
glass. However, if the FOCUSING LENS is so made from glass then it will 
normally be required to be of multiple lens elements exhibiting extreme 
high precision in order to obtain focusing to such a minute size--the 
theoretical limit of optical focusing performance. In the preferred 
embodiment apparatus of the present invention, the FOCUSING LENS is not a 
conventional glass lens. It is preferably a relatively new development in 
optics called a GRADED INDEX OPTICS LENS. 
Graded index optics is a relatively new process for obtaining light 
bending, including focusing, from optical materials in which the index of 
refraction is altered by a doping of the material. In particular, a GRADED 
INDEX OPTICS LENS is an optical component in which the index of refraction 
changes as a function of the radius about the axis of the lens material. 
Even more particularly, one particular GRADED INDEX OPTICS LENS called the 
SELFOC.RTM. (trademarks of Nippon Sheet Glass Company). Micro Lens (SML) 
available from Nippon Sheet Glass Company, Limited is a cylindrical lens 
with an index of refraction which decreases as the square of the radial 
distance from the optical axis (which is also the axis of the cylinder). 
Particularly, the index of refraction n=n.sub.o (1-Ar.sup.2 /2) wherein 
n.sub.o and A are constants. Because of this parabolic index of 
refraction, the SML performs the same optical function as standard 
spherical lenses with the added feature that the end surfaces of the SML 
are flat. Such an SML is small and lightweight, offers simplified mounting 
and alignment, is available with adjustable focal lengths, and is 
extremely inexpensive in comparison to the glass lenses which it replaces. 
GRADED INDEX OPTICS LENSES, including the SML, are formed by thallium 
doping of an optical material followed by cleaving and polishing of the 
cylindrical ends. These graded index optics lenses are obtaining wide 
application in fiber optics, where they have a function to couple light to 
and from optical fibers. 
The two particular prior art GRADED INDEX OPTICS LENS, which are 
particularly two SELFOC Micro Lens (SML), which are preferred for use in 
the apparatus of the present invention are illustrated in FIG. 4, 
consisting of FIG. 4a and FIG. 4b. The SML illustrated in FIG. 4a is in a 
configuration for optical beam reduction, or contraction. Such a GRADED 
INDEX OPTICS LENS is obtainable as the type 0.23 Pitch SML. This 0.23 
pitch SML is designed so that its focal point, or FP, is always outside 
the lens when a collimated (0.63 micrometers to 1.56 micrometers 
wavelength range) beam is projected on the incident end surface. This 0.23 
pitch SML is typically used to change a diverging beam from a fiber or 
from a laser diode into a collimated beam. It is employed in the apparatus 
of the present invention to take a COLLIMATED LIGHT BEAM developed from a 
laser diode or other source and to focus such COLLIMATED LIGHT BEAM into a 
focal point (FP) at the plane of an ENCODER WHEEL. 
A second prior art preferred SML for use in the apparatus in the present 
invention is illustrated in FIG. 4b. This SML is available as a 0.29 Pitch 
lens which is used to change diverging beam from a laser diode into a 
converging beam. It is normally so used for coupling a laser diode to an 
optical fiber, or the light output of an optical fiber to a detector. It 
is used in the apparatus of the present invention to change a diverging 
beam originating at a SEMICONDUCTOR LASER into a converging beam focused 
to a DIFFRACTION LIMITED SPOT as the plane of an ENCODER WHEEL. 
A preferred embodiment implementation of the apparatus of the present 
invention which particularly employs a GRADED INDEX OPTICS LENS is shown 
in FIG. 5. The SEMICONDUCTOR LASER is a source of coherent light in the 
range which can be focused by the GRADED INDEX OPTICS LENS and detected by 
the PHOTOTRANSISTOR. The SEMICONDUCTOR LASER is nominally a Mitsubishi p/n 
4102 or Hitachi p/n 7801E each having an approximate wavelength of 780 
nanometers and a power consumption of 3 millowatts. The GRADED INDEX 
OPTICS LENS is normally a SELFOC.RTM. Micro Lens (SML) of the 0.29 Pitch 
type which was previously illustrated in FIG. 4b. Particularly, this 0.29 
Pitch SML is a Type SLW which is a wide angle device available from Nippon 
Sheet Glass Company, Ltd. This Type SLW SML has a numerical aperture of 
0.46 and a cut-off wavelength of approximately 0.380 micrometers. It is 
approximately 0.060 inches in diameter and 0.18 inches long. In the 
apparatus of the present invention the front surface of the GRADED INDEX 
OPTICS LENS is emplaced at a critical dimension of 0.020.+-.0.002 inches 
from the SEMICONDUCTOR LASER, and will form a DIFFRACTION LIMITED SPOT at 
a critical distance of 0.157.+-.0.002 inches from the rear surface of the 
GRADED INDEX OPTICS LENS whereat is located the ENCODER WHEEL PLANE. The 
detector is a simple PHOTOTRANSISTOR disposed on the other side of the 
ENCODER WHEEL PLANE at a distance whereat it can intercept substantially 
all of the radiation within the light beam which has been focused onto the 
ENCODER WHEEL PLANE. This distance is nominally made symmetrical with the 
separation of GRADED INDEX OPTICS LENS from the ENCODER WHEEL PLANE, or 
approximately 0.157 inches. The PHOTOTRANSISTOR may be any of the standard 
types which are all generally sensitive to 700 nanometers light, including 
types available from Mitsubishi, TRW, and Tandy Radio Shack amongst other 
suppliers. 
None of the dimensions or alignments within the preferred embodiment 
apparatus of the present invention shown in FIG. 5 are especially 
critical. The most important dimensions are the separation of the 
SEMICONDUCTOR LASER from a first end plane of the GRADED INDEX OPTICS 
LENS, and the separation of the ENCODER WHEEL PLANE from the second end 
plane of such GRADED INDEX OPTICS LENS. Both of these dimensions are 
preferably both accurate and fixed within .+-.0.002 inches. The angular 
tolerance of the GRADED INDEX OPTICS LENS through the ENCODER WHEEL to the 
PHOTOTRANSISTOR IS PREFERABLY WITHIN .+-.4.0 degrees. Likewise, the GRADED 
INDEX OPTICS LENS is also preferably coaxial to the PHOTOTRANSISTOR at 
.+-.0.020 inches. These readily achievable tolerances result in a depth of 
focus at the plane of the ENCODER WHEEL which is approximately .+-.0.005 
inches. In other words, a spot very close to the size of the DIFFRACTION 
LIMITED SPOT can be readily attained at the plane of the ENCODER WHEEL by 
a most elementary setup procedure of simply moving the components into 
proximity and visually or electronically observing the results. Thereafter 
these results are substantially maintained even in the presence of such 
shock, vibration, or temperature variations which do cause slight 
alterations in the separation of components. As may be envisioned, since 
the focus at the ENCODER WHEEL PLANE is at, or near, the size of a 
DIFFRACTION LIMITED SPOT, it does not significantly matter if such ENCODER 
PLANE is not perfectly orthogonal to the axis of the optical beam, or 
exhibits wobble or eccentricity during rotation. 
The performance level easily obtainable, and maintainable, with the 
preferred embodiment apparatus of the present invention shown in FIG. 5 is 
a resolution in excess of 2500 lines per inch. Particularly, a ENCODER 
WHEEL is prepared as a photo etched radial reticular grating on MYLAR.RTM. 
plastic (trademark of E. I. DuPont de Nemours and Company) or glass with 
radial lines at 2.5 ten-thousandths inch line width. The ENCODER WHEEL has 
a 1.25 inch radius (2.5 inch diameter) and is intercepted by the light 
beam at a 1 inch radius. At this radius over 10,000 line transitions are 
reliably resolved. The apparatus of the present invention is eventually 
capable of performance at the theoretically limited level which is 
determined by the size of the DIFFRACTION LIMITED SPOT. The ability to 
create such a DIFFRACTION LIMITED SPOT in the apparatus of the present 
invention should be compared to optical disk technology, and to the 
packing density obtainable on optical recording disks. Resolution of 
several thousand lines per inch in optical position encoders is expected 
to be readily feasible. 
The cost of one preferred embodiment apparatus of the present invention at 
quantities of 1000 is approximately $28.23. This is derived as $6.40 for a 
laser diode Hitachi type HL-7801E, $5.00 for a graded index lens NSG type 
W18-025-083, $2.00 for a mechanical housing including the disk of custom 
construction, $1.00 for a phototransistor Motorola type MRB630, $2.50 for 
shaft bearings SKF type W-0.625, $8.00 for an electronics and power supply 
board of custom construction, and approximately 1/3 hour assembly labor at 
$10.00 per hour. 
In accordance with the preceding discussion, the present invention should 
be recognized to show the focusing of the light path within an optical 
position encoder, and particularly the focusing of coherent light with a 
GRADED INDEX OPTICS LENS to obtain resolution at the limit of a 
DIFFRACTION LIMITED SPOT which is focused upon an ENCODER WHEEL. 
Correspondingly, the present invention should be interpreted in accordance 
with the following claims, only, and not merely in accordance with those 
particular embodiments within which the invention has been taught.