US 3551683 A
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Description (OCR text may contain errors)
Unlteu Dlflltb [docile Inventor Gordon R. Tothill Chatham, Ontario, Canada Appl. No. 761,987 a I Filed Sept. 24, 1968 Patented Dec. 29, 1970 Assignee Electro Dynamics 8; Telecom Limited Chatham, Ontario, Canada a corporation of Canada APPARATUS FOR DETECTING THE PRESENCE OF AN OBJECT IN A ZONE IIAVING PHOTOCELLS AND A SERIES OF STAGGERED AND OVERLAPPING LENSES FOR PRODUCING MULTIPLE IMAGES ON SAID PI-IOTOCELLS 18 Claims, 7, Drawing Figs.
U.S.Cl 250/221, 350/ i 67 Int. Cl 606m 7/00 Field oi'Search.... 250/221,
 References Cited UNITED STATES PATENTS 2,915,843 12/1959 Pabst et al. 350/167 3,018,313 1/1962 Gattone 350/167 3,l67,74l 1/1965 Brown 350/167 3,183,499 5/1965 Cowen 250/230 Primary Examiner-James W. Lawrence Assistant Examiner-Martin Abramson Attorney-Wolfe, Hubbard, Voit & Osann ABSTRACT: Apparatus for detecting the presence of an object in a zone traversed by a beam of light which includes a longitudinal light source with a series of overlappingly staggered lenses arranged opposite the source and photodetectorl means behind the lenses for responding to shadows cast by the object. Associated circuitry includes provision for triggering at predetermined threshold levels of light.
' FIG 2b INVEN'IOR;
' GORDON RTOTHILL K Arrvs.
APPARATUS FOR DETECTING THE PRESENCE OF AN OBJECT IN A ZONE HAVING PHOTOCELLS AND A SERIES OF STAGGERED AND OVERLAPPING LENSES FOR PRODUCING MULTIPLE IMAGES ON SAID PHOTOCELLS It is an object of the invention to provide detecting apparatus for a protected zone at a machine or the like which has high inherent sensitivity for reliable response to small objects in the zone, which provides uniform sensitivity and absence of blind spots throughout the zone, and which, once set, is highly stable in its operation and substantially nonresponsive to changes in the level of ambient light.
It is another object to provide detecting apparatus for a protected zone which is easy to set up, which is capable of withstanding hard industrial usage, and which may be more or less universally used with a wide range of types and sizes of machines, even those requiring a zone of protection up to 12 feet or more in width.
In one of its aspects, it is an object to provide detecting apparatus of the light beam type having associated circuitry which can be set to be nonresponsive to objects intended to be passed through the zone, such as strip stock, but which responds reliably upon any increase in light obstruction due to the presence of the fingers or hand of the operator.
It is finally an object to provide protective means as above which is inherently simple and inexpensive, which requires little or no maintenance, and which may be used for analogous purposes such as counting or surveillance without modification.
Other objects and advantages of the invention will become apparent upon reading the attached detailed description and upon reference to the drawings in which? FIG. 1 is a perspective view showing a control system installed at a machine;
FIG. 2a is a perspective view, partly broken away, showing a photoelectric reciever according to the invention;
FIG. 2b is a perspective view of a light source;
FIG. 3 is a sideview, partly broken away, taken along the line 3-3 of FIG. 2;
FIG. 3a is a perspective view of one of the lenses;
FIG. 4 is a diagrammaticview showing several shadows of an object formed on adjacent photodetectors; and
FIG. 5 is a block diagram of an electronic circuit used with the receiver of FIG. 2a.
While the invention has been described in connection with a certain preferred embodiments, it will be understood that the invention is not limited to such embodiments and that I intend to cover the various alternative and equivalent constructions included within the spirit andscope of the appended claims.
Turning now to the drawings there is disclosed in FIG. I a light source 11 mounted upon the bed of a typical machine 12 for which the invention is well suited, in this case a power press. Arranged opposite the light source is a photoelectric control unit or reciever 13, the light source and control unit together defining a protected zone in front of the working region of the press having limits indicated by the dot-dash lines 130. The light source 11 preferably includes a conventional fluorescent tube 14 and a curved reflector 15 which directs the light emanating from the tube I4 in a vertically wide beam toward the control unit 13. As explained below, the control unit 13 is adapted to interrupt power for promptly deenergizing the machine 12 whenever any portion of the operator's body or any other object is placed in the protected zone.
In accordance with the present invention the control unit includes a plurality of lenses spaced closely' in a longitudinal array and witha longitudinally distributed photodetector, the adjacent lenses in the array being staggered and overlapping so that a light-obstructing object in the zone creates a plurality of shadows on the photodetector to produce a decisive change in output signal for shutting off the machine or otherwise denoting a condition of alarm. Thus, referring to FIGS. 2a and 3, the control unit 13 includes a housing 16 having a lens array 17 which focuses the light from the source onto photodetectors in the form of photocells 18. The latter are electrically connected together, preferably in series, to provide a pair of output terminals which are connected in one. leg of a bridge circuit as will be described in connection with FIG. 5. The photodetectors are preferably photoconductive cells which exhibit a passive change in resistance upon varying the amount of incident light, although the invention is not limited thereto but would include use of active or potential-producing cells where connected to suitable responsive circuitry. Other detector cells which may be employed include photodiodes. phototransistors, photoemissive cells. photovoltaic cells, photomultipliers and the like.
More specifically in accordance with the invention the lenses are in the form of chordal segments of cylindrical lenses arranged end to end in rows of equal width, with the rows being closely adjacent one another and with the lenses in each row being incrementally offset with respect to the lenses in an adjacent row. A typical lens 19, as shown in FIG. 3a, has a convex, i.e., cylindrical surface 19a, a flat surface 19b opposed thereto and fiat, parallel, sides 190. The length and width dimensions are indicated at I and w respectively. A pair of spaced longitudinal light shields 20 are provided, one along each edge of the lens array. The lenses are preferably arranged with their flat faces arranged in a common plane facing the light source. The individual lenses may be readily mounted by cementing them, by means of a light-transmitting film of cement, to a transparent facing member 19d which may be made of glass or plastic. A main advantage of this arrangement is that a flat, smooth surface is less likely to collect dirt and may be easily cleaned when cleaning does become necessary. The lenses may be mounted convex side out if desired. however.
The individual photodetectors 18 preferably have a width or span which is substantially equal to, or in excess of, the width of the lens array, and the photocells, rather than being evenly staggered, are staggered in groups with the photocells being centered under the respective groups. This is brought out in FIG. 3 in which adjacent lenses, indicated at 19, are shown offset from one another by a distance d and centered, as a group of three, along the centerline 18d of the associated photodetector 18. The preferred width relationship between the lens array and the photodetectors is shown in FIG. 4.
With the lenses and photodetectors relatedas shown in FIGS. 3 and 4, it is found that interposition of even a small object, such as the finger of a machine operator, in the light beam produces multiple shadows on the photodetector 18 as indicated at 18a, 18b, and in FIG. 4. Such multiplicity of shadows brings about a marked change in the resistance or output signal of the affected photocell and a corresponding large change in the resistance, or output signal, at the terminals of the photocells connected as a group. It is found that even a small object in the protected zone will cause shadows to occur not only on the photodetector which is in general alignment with the object but on the adjacent photodetectors as well to provide an even more decisive change in total output, although the shadows will normally be off center on the adjacent detectors as shown at the right-hand side of FIG. 4. It is found, rather surprisingly, that the sensitivity is highly uniform over the entire protected zone with freedom from blind spots; in other words, there is no point in the zone in which a light obstructing object may be inserted without producing a triggering change in the output of the connected photodetectors. It has been found that three rows of lenses 19 provide excellent results and might be considered optimum from an economic point of view. However, it will be understood that more than three rows oflenses could be employed, either with or without additional photodetectors, and with the span of the detectors being substantially equal to the width of the lens array, without departing from the invention. Use of two rows of lenses might be justified in certain cases where operation is limited to the detection of larger objects. As to the number of photodetectors, 6 or 7 appear optimum with a lens assembly having a length a of about 17 inches. Reasonably good results can be achieved with as few as four photodetectors or as many as or 12 photodetectors in a length of 17 inches. It may be particularly noted that an object in the vicinity of the lenses produces approximately the same overall change in resistance, or output, of the photodetectors as the same object in the vicinity of the light source. Thus when an obstruction is closely adjacent the lenses darker and more well defined shadows will be produced on a limited number of the photodetectors, whereas the same object located at a greater distance from the lenses will produce lighter shadows, not as well defined, but which cover a greater total area on the photodetectors so that the change in resistance or output of all the photodetectors taken as a group remain substantially the same.
In carrying out the invention, all of the lenses may be molded together as a single piece of transparent plastic. However, it is preferred to employ separate lenses with the interfaces at the sides of adjacent lenses treated to inhibit reflection of stray light within a given lens as well as to inhibit passage of stray light across the flat from one lens to the next. This is accomplished by roughening the sidewalls of the individual lenses. If the lenses are cemented together a cement should be employed having an index of refraction which differs from the index of the lens material itself. If it is desired to reduce lateral passage of stray light from one lens to the next to a minimum, thin opaque separators may be provided between the adjacent lenses. in general, use of separate lenses will tend to make the detector unit less susceptible to changes in ambient light than where a single molded lens assembly is employed.
In accordance with one of the aspects of the invention, the photodetector means, comprising all of the photodetectors 18 connected together, are placed in one leg of a bridge circuit, the bridge circuit being coupled to an amplifier for actuating a power control device as, for example, a relay for shutting off the protected machine, the bridge circuit having means for shifting the control point and with detenting means being provided to prevent fluttering of the relay under marginal conditions. Thus turning to FIG. 5 there is shown a bridge circuit 21 consisting of fixed resistors 22, 23 forming two of the arms, coarse and fine rheostats 24, 25 forming a third arm and the photodetectors 18, preferably connected in series with one another, forming the fourth arm. A direct current source 26 is connected to the power terminals 27, 28 of the bridge while the output terminals 29, 30 are connected to a DC amplifier 31. The output of the DC amplifier is fed to an integrator or filter 32 having a time constant sufficient to smooth the output of the DC amplifier and to prevent the passage of noise impulses. The output of the integrator 32 is coupled to a switching amplifier 33 which in turn controls a relay 34 having normally open contacts 34a which, upon dropout, are utilized to turn off the protected machine. Power is supplied for the relay from a suitable source 35. In order to insure that the relay opens and closes positively and without indecision, even under marginal conditions, electrical detenting may be obtained by providing a positive feedback loop between the switching amplifier 33 and DC amplifier 31. Such loop, indicated at 37, utilizes the voltage drop across a resistor 36. Adjustment of the amount of feedback, as by varying the value of the resistor 36, is a matter well within the skill of the art.
To protect the amplifier 33 from the voltage generated by collapse of the field in the relay 34 when the relay is deenergized, a suitable transient suppressor 38 may be used formed, for example, by an appropriately faced diode and series resistor.
It will be apparent that the "sense of the relay operation depends upon the amplifier circuitry and either closure or opening of the relay, contacts 34a may be utilized to provide the protective function. Thus, depending upon the sense of the relay, a relay bypass switch having either normally open or normally closed contacts may be connected to the contacts 341: to permit jogging of the machine during setup.
In operation, the light source 11 and control unit 13 are arranged in alignment at the respective sides of the protected zone. With the lamp l4 turned on, the bridge is balanced by adjusting the coarse and fine rheostats 24, 25 to achieve the null condition. When an object is placed in any part of the beam, the resulting unbalance signal is fed to the amplifiers to cause deenergization of the relay and opening of the no contacts 34a to produce a condition of dropout in the power source for the machine to bring about prompt stoppage. A conventional power press of modern design is equipped with an automatic brake to minimize coasting. g
In a practical form of the invention, the light source 11 is a conventional fluorescent tube having a length of 24 inches,
and the lens assembly is about 17 inches in length. The width (measured between the two plane parallel surfaces 19c) of each lens 19 is 0.3125 inch, the chord length (measured along the planar face 1%) is 3 inches and the radius of curvature of the curved side 19a is 2 inches. The focal length of the lenses 19 is 4.02 inches. The spacing between the front (planar) side of the lens assembly 19 and the photodetectors 18 is about 3 k inches. The photodetectors 18 are photoconductive cells manufactured by Sigma Instruments, Inc. and identified as Model No. 5HC2. The dark resistance of these photodetectors is 0.5 megohms, and the resistance decreases with increasing light (1.5 kolohms at a light intensity of 10-foot candles). The center to center separation of the photodetectors 18 is approximately 3 inches and the thickness of each lens 19 at the center is about fifteen-sixteenths inch. The offset distance d is 0.3 l 25 inch. It was found experimentally that best results were achieved with a minimum separation of light source and control unit of about 2 feet, and that adequate sensitivity was achieved with separations of up to about 12 feet. The vertical coverage may be extended as desired by extending the light source and lens array. A high degree of reliability has been obtained with good stability in the face of changes in ambient light making the device well suited to rough industrial usage.
Although the electronic circuit described above is suitable for use with photoconductive cells, it will be apparent to one skilled in the art that analogous circuitry maybe employed to utilize the signals produced by other photosensitive devices for actuating the relay or otherwise controlling the machine. Where the system is to be used in environments subject to wide variations in ambient light, it will be understood that the effect of natural ambient light may be minimized by making use of modulated light at the light source and by employing an AC amplifier which is sharply tuned to double the frequency of the light source, for example Hz where the light source is a fluorescent tube energized from the regular 60 Hz supply line. Preferably a higher modulation frequency should be employed, one nonintegrally related to the frequency of the supply line.
While the invention has been described in connection with a preferred embodiment employing spaced photodetectors of circular contour, it will be apparent that the invention is not limited thereto and photodetectors of rectangular configuration may be used, either spaced from one another or longitudinally extended to provide a substantially continuous photosensitive surface under the lens array. lt will be apparent, also, that the invention in its broader aspects is not limited to use with separate detectors,,but includes the possibility of employing a single extensive detector, in which case the lenses need not be arranged in offset groups but may be uniformly distributed using an distance of offset d equal to the length of the lens divided by the number of rows of lenses.
In the simplest aspect of the invention, the presence of any light obstructing object in the protected zone serves to turn off the machine. it is one of the features of the invention, however, that stock may be fed to the machine through the protected zone if desired simply by adjusting the control point of the bridge to make the circuit unresponsive to the stock but triggerable by any additional light obstruction as, for example, by the hand of the operator feeding the stock into the protected zone. Also while the described embodiment is particularly suitable for use in controlling a machine to protect per sonnel from injury, it will be understood that the invention can be used for analogous purposes where sensitivity to small objects in a broad beam can be used to advantage. For example, the apparatus may be used to count the number of workpieces passing from a production machine by settingit up at the output of the machine and by coupling the relay 34 to'a suitable counter. Then, too, the device may be employed for operating a door or the like upon entry of a person or object into the detection zone. Utility for safety purposes includes use as a burglar alarm upon passage of an intruder oruse'as a smoke detector. Consequently, the term condition of alarm" used in the claims, contemplates either the shutoff or actuation of any machine or controlled device upon entry of anobject into the light-traversed zone.
The term longitudinal array" as used herein refers to a group of lenses occupying an area which is strung out in a direction which conforms to the direction of an associated longitudinal light source. The term row" as used herein has to do with a line of direction, for example, the line of direction between lens centers or the line of direction of the long axis of a rectangular lens element regardlessof the number of lenses lying along the line of direction.
1. Apparatus for detecting the presence of an object in a zone traversed by light emanating from a longitudinal light source producing a beam of extended width comprising, in combination, a plurality of lenses spaced closely in a longitudinal array directed at said light source and in sufficient .number to span the width of the beam, longitudinally arranged photodetector means associated with the lenses and shielded from ambient light for receiving light originating at the source, (adjacent) the lenses being arranged in rows with lenses in adjacent rows being staggered and overlapping so that a light-obstructing object in the zone creates a plurality of shadows on the photodetector means to produce a decisive change in the output signal thereof, and means responsive to the change for denoting a condition of alarm.
2. The combination as claimed in claim 1 in which the lenses are flat sided and arranged flat-to-flat to reduce the width of the array.
3. The combination as claimed in claim 1 in which the lenses are in the form of chordal segments of cylindrical lenses.
4. The combination as claimed in claim 1 in which the lenses are in the form of chordal segments of cylindrical lenses arranged end to end in rows of equal width with the rows closely adjacent to one another and with the lenses in each row being incrementally offset with respect to the lenses in an adjacent row.
5. The combination as claimed in claim 4 in which there are at least three rows of lenses.
6. The combination as claimed in claim 4 in which the lenses have flat and convex faces and in which the flat faces are positioned in a plane facing the light source.
7. The combination as claimed in claim 4 in which the photodetector means has a width sufficient to substantially span the width of the lens array.
8. The combination as claimed in claim 1 in which the photodetector means consists of photodetectors spaced side by side along the length of the array and connected in series with one another.
9. The combination as claimed in claim 1 in which the lenses are in the form of separate chordal segments of cylindrical lenses of equal width having flat parallel sides and arranged in rows flat-to-flat with the lenses in each row being incrementally offset with respect to the lenses in an adjacent row so that the light obstructing object produces a plurality of shadows which are longitudinally distributed on the photodetector means.
10. The combination as claimed in claim 9 in which the flat sides of the lenses have interfaces such as to inhibit reflection of stray light within a given lens as well as to inhibit passage of stray light across the flat from one lens to the next.
11. The combination as claimed in claim 9 in which each of the lenses has a convex face and a flat face, a flat trans arent facing member, the flat faces being cemented to the acing member, the facing member being arranged in a plane facing the source.
12. The combination as claimed in claim 9 in which the amount of offset is limited so that the lenses fall into groups each comprising an optical unit and in which the photodetector means comprises a plurality of electrically connected photodetector units spaced longitudinally, each photodetector unit being centered with respect to the lenses in a group.
13. The combination as claimed in claim 1 in which the lenses are in the form of chordal segments of cylindrical lenses having a width dimension which is a small fraction of the length dimension and arranged closely side by side in longitudinal rows with the lenses of each row longitudinally offset from the lenses in an adjacent row, parallel light shields on each side of the lens array projecting forwardly in the direction of the source, and means for shielding the photodetector means so that the photodetector means is exclusively exposed tolight entering through the lenses.
14. Apparatus for detecting the presence of an object in a zone traversed by light emanating from a longitudinal light source producing a beam of extended width, comprising, in combination, a plurality of lenses spaced closely (end to end) in longitudinal array directed at said light source and in sufficient number to span the width of the beam, longitudinally arranged photodetector means associated with the lenses for receiving light originating atthe source and having a pair of output terminals, (adjacent) the lenses being arranged end to end in rows with lenses in adjacent rows being staggered and overlapping so that a light-obstructing object in the zone creates a plurality of shadows on the photodetector means to produce a decisive change in output signal at the terminals,'a bridge circuit, the output terminals of the photodetector means being included in a leg of said bridge circuit, power control means, and an amplifier interposed between the bridge circuit and the power control means.
15. The combination as claimed in claim 14 in which the power control means is in the form of a relay and in which detenting means are provided for insuring positive opening and closing of the relay under marginal conditions of light reaching the photodetector means.
16. The combination as claimed in claim 15 in which the detenting means is in the form of a positive feedback loop around the amplifier.
17. The combination as claimed in claim 14 in which means are provided for shifting the control point of the bridge to produce a null output signal when a light obstruction of predetermined obstructing area exists in the zone thereby to permit passage of predetermined workpieces therethrough while producing an effective output signal capable of operating the power control means when the light obstruction exceeds the predetermined area thereby to detect the presence of an additional and unwanted object in the zone.
18. The combination as claimed in claim 1 in which the iongitudinal light source is in the form of a fluorescent tube surrounded on three sides by a reflector extending the length thereof.