Two dimensional optical position indicating apparatus

An optical position indicating apparatus having a light source, a detector for detecting the presence of light and producing an electrical output in response thereto, a reflecting structure for reflecting light from the light source across a target zone, and, a retroreflecting structure for reflecting the light incident thereupon along a path substantially coincident with the path of incidence of the light. The retroreflecting structure comprises a plurality of retroreflective assemblies, the reflecting structure and the plurality of retroreflective assemblies are substantially disposed about the target zone and a limiting structure limits the detector to a finite viewing range so that rotation of the detector effects scanning of the target zone. The detector is capable of measuring angular displacement from a reference so that presence of an opaque object within the target zone will register upon the detector as a plurality of shadows thereby resulting in absence of electrical output from the detector for each of the plurality of shadows, whereby the location of the opaque object within the target zone can be determined in two dimensions by geometric calculations involving angular displacements of the plurality of shadows.

BACKGROUND OF THE INVENTION 
The present invention is directed to an apparatus for determining the 
location of an object along one or more axes and in particular to an 
apparatus for accomplishing such location by optical means. 
In today's world the increasing prevalence of computers and devices using 
computer-like apparatus has given rise to a recognition of a need for 
simplifying the human operator-to-computer interface to facilitate the 
input of data to the computing apparatus. Numerous devices and apparatus 
have been produced for such interface enhancement such as keyboards, 
joystick controls, and various types of touch screen inputs. The various 
types of touch screen inputs include screen overlays superimposed upon 
cathode ray tube displays of computing devices, which apparatus can be 
capacitive, resistive, ultra-sonic or consist of a conductive grid. There 
are various disadvantages to such touch screens: the overlay in front of 
the cathode ray tube degrades the contrast of the display as it appears to 
an operator and may also degrade the resolution of the display on the 
cathode ray tube, such overlays attract and trap dirt to further 
contribute to degradation of contrast and resolution, such overlays are 
often made of materials which are easily scratched or otherwise optically 
degraded with time or use, and with such overlays non-glare cathode ray 
tube finishes are greatly reduced in effectiveness. Moreover, such 
overlays touch screens generally increase in complexity and expense of 
manufacture with increases of resolution in their detection capabilities. 
Optical touch screen input apparatus overcome the aforementioned 
shortcomings of touch screen overlays by creating a light curtain in front 
of a cathode ray tube or other display, penetration of which curtain is 
detectable by the apparatus and interpreted to fix the location of the 
penetration in the requisite number of dimensions for the particular 
application. Examples of types of optical touch screen input devices 
include the system disclosed in U.S. Pat. No. 4,267,443 (Carroll) and the 
device disclosed in U.S. Patent Application Ser. No. 183,357, filed Sept. 
2, 1980 (Barlow). The Carroll system employs arrays of light emitting 
diodes and photo-detectors along opposite sides, switching of the light 
emitting diodes in a sequential manner and sensing the presence of light 
sequentially in the opposing photo-detectors. With appropriately situated 
arrays of light emitting diodes and corresponding opposing photo-detectors 
it is possible by such a device to determine the location of an object 
within a two dimensional location field. The Carroll apparatus, however, 
is disadvantageous because the large numbers of light emitting diodes and 
photo-detectors required by that apparatus render it expensive to 
construct. Further, the large number of discreet elements renders the 
device more prone to material breakdown than a device with a lesser number 
of components. Still further, resolution of the Carroll apparatus is 
limited by the number and size of the light emitting diodes and 
photo-detectors, and any increase in resolution of detection by such a 
device is necessarily accompanied by a commensurate increase in the number 
of components, thereby increasing the cost of construction and the 
probability of breakdown. 
The Barlow device employs a single continuous light source in a corner of a 
touch field and a photo-detector scanning the touch field, which 
photo-detector is located in the opposite corner from the light source. 
Along all four sides of the touch field there are stepped mirrors arranged 
so that light from the light source is reflected across the touch field in 
two perpendicular arrays and subsequently directed toward the rotating 
detector. Thus the light from each beam arrives at the detector from a 
slightly different angle and, because the detector is rotatively scanning 
the touch field, the pattern of light of the detector is interpretable to 
determine the location of an object within the touch field in two 
dimensions. Such a device as is disclosed by Barlow overcomes some of the 
disadvantages of Carroll in that it is potentially lower in cost of 
production and maintenance, however, a high degree of precision is 
required in building a workable Barlow system since the mirrors must be 
precisely aligned for proper operation to occur. Such a requirement for a 
high degree of precision in alignment necessarily adds to cost of 
production and sensitivity to physical shock, both of which are 
disadvantageous in a commercial environment. 
SUMMARY OF THE INVENTION 
The present invention is a device for locating an object in two dimensions 
within a target zone by detection of interruption of light paths dispersed 
across the target zone. The present invention, therefore, provides a 
direct human operator-to-computer interface which requires little or no 
familiarity with any intervening system such as a keyboard or the like. Of 
course, a plurality of devices incorporating the present invention could 
be arranged adjacent to each other to provide three dimensional location 
of an object within a target space. The present invention incorporates 
extremely low cost materials which facilitate simple, low cost 
construction which does not require critical tolerances and is therefore 
easy to manufacture. Further, the materials incorporated in the present 
invention facilitate construction of the invention with narrow borders and 
provide, theoretically, infinite resolution with no additional cost or 
complexity. 
This flexibility, ease and low cost of construction is accomplished in 
large part through the employment of material which is configured so that 
a high percentage of light received within given angular displacement 
limits from a perpendicular to the surface of the material at the point of 
incidence will be reflected back along the path of incidence. Thus, the 
term retroreflecton is applied to such material. Light incident at angles 
outside the angular limits will be retroflected as well but to a sharply 
lesser degree as those angular limits are exceeded. Moreover, such 
retroreflective material is disposed within the present invention in a 
manner whereby light which is incident upon that material at any given 
point is within the angular displacement limits from a perpendicular to 
ensure a high degree of retroreflection. 
It is therefore an object of this invention to provide an optical position 
locating apparatus of simple low cost, easily maintained rugged 
construction. 
A further object of the invention is to provide an optical position 
locating apparatus which is easy to manufacture with narrow borders 
disposed about the target zone. 
Still a further object of this invention is to provide an optical position 
locating apparatus of theoretically infinite resolution without high cost 
or critical tolerance requirements.

DETAILED DESCRIPTION OF THE INVENTION 
The preferred embodiment of an optical location apparatus 10 is shown in 
perspective at FIG. 1. An optical position location apparatus 10 is 
comprised of a housing 12 which serves to maintain the various elements of 
the apparatus 10 in proper relative relation as well as to protect those 
elements from environmental incursions in an operating situation. The 
housing 12 further serves to define a target zone 14 within which target 
zone 14 the location of objects is to be determined. Disposed within the 
housing 12 about the target zone 14 are a flat reflector, such as a mirror 
16, a retroreflector 18 and a retroreflector assembly 20. The 
retroreflector assembly 20 is comprised of a retroreflector strip 22 and a 
plurality of retroreflector elements 24 arranged in echelon adjacent to 
the retroreflector strip 22. In one corner of the optical position 
location apparatus 10 there is light directing means, such as a beam 
splitter 26, a detector assembly 28 and a light source 30. 
Referring now to FIGS. 4, 5 and 6, the detector assembly 28, light source 
30 and the beam splitter 26 will be discussed in greater detail. For ease 
of understanding of the invention, like reference numerals will be used to 
identify like elements in the various drawings. 
In FIG. 4 the detector assembly 28 is shown situated in spaced relation 
from the target zone 14 with the beam splitter 26 interposed between the 
detector assembly 28 and the target zone 14. The light source 30 is 
positioned below the beam splitter 26 in a manner allowing transmitted 
light beams such as 32 to be distributed by the beam splitter 26 across 
the target zone 14. Returning light beams such as 34, having traversed the 
target zone in a manner to be described in more detail hereafter, are 
transmitted through the beam spliter 26 to the detector assembly 28. 
FIG. 5 shows the light source 30 situated adjacent the beam splitter 26 and 
the detector assembly 28 situated behind the beam splitter 26. The beam 
splitter 26 is situated at a 45 degree angle to the housing 12 so that 
transmitted light beams such as 32 travel from the light source 30, 
reflect from the beam splitter 26 to traverse the target zone 14 and 
return as returning light beams such as 34 to be transmitted through the 
beam splitter 26 to the detector assembly 28. The detector assembly 28 is 
comprised of a drive motor 36 which has a shaft 38 and a detector housing 
40, which detector housing 40 is supported on the shaft 38 so that the 
detector housing 40 rotates in response to rotation of the drive motor 36. 
Fixedly contained within the detector housing 40 is a photo-detector 42, 
and formed in the wall of detector housing 40 is an aperture 44 (best seen 
in FIG. 6). Aligned with the aperture 44 and the photo-detector 42 and 
affixed to the detector housing 40 is a lens 46. The sensitive face 48 of 
the photo-detector 42 is a plane which contains the focal point 50 of the 
lens 46. A mask 49 is affixed to the sensitive face 48, which mask 49 
provides a view-limiting aperture (not shown) centered about the focal 
point 50 of the lens 46 to enhance resolution of detection of light 
interruptions by objects with the target zone 14. Thus, by rotating the 
detector housing 40 and its associated aperture 44 and lens 46, the 
photo-detector 42 scans the target zone 14 for the presence of returning 
light, such as beams 34, and produces an electrical signal in response to 
the presence of such light, which signal is provided to and processed by 
external electronic circuitry (not shown). 
Referring now to FIG. 2, the operation of the present invention will be 
explained in greater detail. There are generally three groups of light 
paths employed by the present invention: a first group, such as 52, 
emanates from the light source 30, is reflected by the beam splitter 26 
across the target zone 14, and is reflected from the mirror 16 to the 
retroreflector 18. The retroreflector 18 is fashioned from a material 
having the characteristic that for light received by that retroreflector 
18 within given angular displacement limits from a perpendicular to the 
surface of the retroreflector 18 at the point of incidence, a large 
percentage of that light will be reflected back along the path of 
incidence, thus the term retroreflection is applied to such material. Some 
light is retroreflected at angles of incidence outside the established 
angular limits from a perpendicular, but retroreflected light as a 
percentage of incident light, falls off rapidly as angles of incidence 
exceed those angular limits. The retroreflector 18 is curved as shown in 
FIG. 2 appropriately to ensure that light incident upon retroreflector 18 
which is reflected from the mirror 16 will, along the length of 
retroreflector 18, have an angle of incidence within the prescribed 
angular limits for the material comprising retroreflector 18 appropriate 
to ensure a high degree of retroreflection. A second group of light paths, 
such as 54, emanates from the light source 30, is reflected by the beam 
splitter 26 across the target zone 14, is reflected by the mirror 16 to 
arrive at the retroreflector assembly 20. The retroreflector elements 24 
of the retroreflector assembly 20 are arranged in echelon to ensure angles 
of incidence of light paths such as 54 which arrive at the retroreflector 
elements 24 via reflection from the mirror 16 at an angle of incidence 
within the limits required for retroreflection of the light back along the 
path of incidence, thence to be reflected from the mirror 16 and return to 
the photo-detector 42 via transmission through the beam splitter 26 and 
the lens 46. Of course, some portion of light paths 54 reflected from 
mirror 16 will be incident upon retroreflector strip 22; however, angles 
of incidence of light paths 54 upon retroreflector strip 22 will be 
outside the angular limits for a high degree of retroreflectivity. The 
retroreflector elements 24 are situated to ensure a high degree of 
retroreflectivity of light paths 54 so that any retroreflected light paths 
54 from retroreflector strip 22 will have negligable impact upon operation 
of the invention. A third group of light paths, such as 56, emanates from 
the light source 30, is reflected by the beam splitter 26 across the 
target zone 14 directly to retroreflector strip 22 of the retroreflector 
assembly 20. Retroreflector strip 22 is curved to ensure angles of 
incidence of light paths such as 56 are within the limits required for a 
high degree of retroreflection back along the path of incidence to the 
photo-detector 42 via transmission through the beam splitter 26 and lens 
46. The presence of an object such as 58, 59 or 60 within the target zone 
14 would interupt such light paths as 52, 54 or 56; such interuptions 
would be detected as an absence of light by the photo-detector 42 and, 
through geometrical calculations to be discussed in greater detail 
hereafter, the position of the object causing such an absence of light can 
be determined with precision. 
FIG. 3 illustrates the relative positions of the various elements of the 
invention and provides clarity as to the construction of the device. It is 
worthy of note that the inner boundary 62 of the housing 12 is open to the 
target zone 14 to facilitate free passage of light across the target zone 
14 to the various reflective and retroreflective elements bordering 
thereon. Of course, colored filters could be interposed at the inner 
boundary 62 of the housing 12 to limit responsiveness of the device to 
specific ranges of the light spectrum as may be appropriate in a given 
operational environment. 
Referring now to FIG. 7, a schematic diagram of the preferred embodiment of 
the present invention is illustrated with the inner boundary 62 bordering 
the target zone 14 and the detector assembly 28 located with the sensitive 
face 48 (not shown) of the photo-detector 42 situated at an origin 64 of a 
coordinate axis, the x axis of which is comprised of side 66 of the inner 
boundary 62 and the y axis of which is comprised of side 68 of the inner 
boundary 62. 
In the schematic diagram of the preferred embodiment of the present 
invention illustrated in FIG. 7, the mirror 16 is located opposite the 
origin 64, and beyond the mirror 16 is depicted, in broken lines, the 
virtual image of the target zone 14, 14'. Thus an object 70 located within 
the target zone 14 will have a virtual image object 70' located within the 
virtual image target zone 14'. Similarly there will be a virtual image 
origin 64' and a virtual image detector assembly 28'. The blocking of 
returning light beams (not shown) within the target zone 14 will produce 
two shadows at the origin 64. A first shadow 72 is associated with the 
object 70 and the target zone 14 and is angularly displaced from the x 
axis 66 by an angle a. A second shadow 74 is associated with the virtual 
image 70' of the virtual image target zone 14', which second shadow 74 is 
angularly displaced from the x axis 66 by an angle b. As can be seen by 
FIG. 7, the x displacement of the object 70 from the origin 64 may be 
expressed by the equation: 
EQU x=y tan (90.degree.-a)=y CTN a, 
where y is the displacement of the object from the x axis. 
It should also be noted that x may be expressed as: 
EQU x=(2y-y) tan (90.degree.-b)=(2y-y) ctn b. 
Simultaneous solution of the above equations yields the result: 
EQU x=2y/(tan a+tan b) 
The y displacement of the object 70 within the target zone 14 may be 
directly calculated as follows: 
EQU y=x tan a 
Thus the x and y coordinates of the object 70 within the target zone 14 
with respect to the origin 64 can be precisely geometrically calculated as 
the detector 28 rotates about the origin 64 and detects shadows caused by 
the object 70 and its virtual image 70'. External electronic circuitry 
known in the art (not shown) is employed to accomplish this geometric 
calculation with sufficient speed and accuracy to accomplish any desired 
resolution of x, y location of an object within the target zone 14. 
It is to be understood that, while the detailed drawings and specific 
examples given describe preferred embodiments of the invention, they are 
for the purpose of illustration only, that the apparatus of the invention 
is not limited to the precise details and conditions disclosed and that 
various changes may be made therein without departing from the spirit of 
the invention which is defined by the following claims.