Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2000-04-19
2002-04-30
Kim, Robert H. (Department: 2882)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S559400
Reexamination Certificate
active
06380532
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to utilizing a beam of light to detect the presence of objects, and more particularly to optical sensing equipment which employ a plurality of photodetectors to receive the light which has been reflected by an object to be detected.
In manufacturing operations, it is desirable to detect the presence of an object moving down a conveyor. This enables material handling equipment to direct the object safely between conveyor sections or to a work station. It also is desirable to detect when objects become jammed along the conveyor.
An optical detector system often is used for these purposes. One type of detection apparatus, referred to as a retro-reflective system places an emitter-detector assembly on one side of the conveyor and a reflector on the opposite side. A beam of light is sent from the emitter across the conveyor to the reflector and then returns back across the conveyor to the detector. An object, moving along the conveyor, interrupts the beam of light, thereby providing an indication of the presence of the object. Retro-reflective systems have the disadvantage of requiring installation of a reflector on the opposite of the conveyor. Installation of the reflector in many situations is difficult or interferes with other operations being performed along the conveyor. Therefore, it is desirable to utilize an object detector apparatus that does not require devices on both side of the conveyor.
In response, sensing systems have been developed which detect the reflection of the light beam from objects moving alone the conveyor. However, such systems must address several potential problems. First, the reflectivity of the objects vary greatly from very specular in nature to ones that are very diffuse. In addition, black objects naturally absorb more light than white objects. The circuitry that processes the signal from the light detector can be designed with a relatively high sensitivity to detect low reflectivity objects. However, that high sensitivity often results in the signal processing circuitry being saturated in response to light from highly reflective objects.
In addition, high sensitivity sensing circuits can falsely respond to highly reflective objects on the opposite side of the conveyor. For example, a shiny metal object being transported on a cart next to the conveyor system can reflect enough light back to the photodetectors to be erroneously interpreted as an object moving down the conveyor. Therefore, it is desirable to have a detector system that has a relatively high sensitivity and a sharp cutoff at a distance equal to the far side of the conveyor.
FIG. 1
depicts a prior detection system of this type. In this system, an emitter
10
transmits a beam of light across the conveyor. One ray
12
of that light beam is illustrated passing through an output lens
14
. An object
16
reflects the ray
12
through another lens
18
onto a detector assembly
20
. The detector assembly
20
has an near detector
22
that receives light from objects which are relatively close to the detector assembly and has a far detector
24
that receives light from objects which farther away from the detector assembly. Note that the objects usually move in a direction that is orthogonal to the plane of the drawing. The farther an object is from the emitter
10
the smaller the angle of the reflected ray
12
. For example, the reflected beam from object
16
strikes the near detector
22
, whereas the reflected ray from a more distant object
26
, beyond a given cutoff distance
28
from the emitter, strikes the far detector
24
and not the near detector
22
.
It should be understood that the emitter
10
produces a beam of radiation comprising numerous rays. Thus, when the entire beam is reflected from an object, some of the rays may strike the near detector
22
and other reflected rays may strike the far detector
24
. If the object is within the cutoff distance
28
from the emitter, a greater amount of reflected light will strike the near detector
22
than the far detector
24
. Conversely, when the object is beyond the cutoff distance
29
, a greater amount of light is reflected onto the far detector
24
than onto the near detector
22
. The output signal produced by a detector corresponds to the amount of light which impinges that detector. Thus, by comparing the two detector signals, the object detection apparatus is able to distinguish an object moving along the conveyor from objects beyond the conveyor.
This dual detector system does well when a diffuse object fully blocks the beam of light from the emitter. However, when an out of range object blocks only a portion of the emitted beam or is specular, light from that out of range object can be falsely interpreted as being from an object on the conveyor because the near detector
22
may receive more light than the far detector
24
.
SUMMARY OF THE INVENTION
An object detection system has an emitter which produces a light beam that will be reflected by objects to be detected. First and second photodetectors are located on one side of the emitter and a third photodetector is located on an opposite side of the emitter. The first, second and third photodetectors respectively produce first, second and third signals in response to being struck by the emitter light that is reflected by an object.
The three photodetectors have separate fields of view that are aimed so that the second signal from the second photodetector will be greater than the combined signals from the first and third photodetectors when an object is within a given distance of the emitter. This given distance referred to as the cutoff distance defines the sensing range of the object detection system. When an object is beyond the cutoff distance, the combined first and third signals from the first and third photodetectors will be greater than the second signal. Therefore, a processing circuit is able to determine whether an object is within the cutoff distance by arithmetically combining the three photodetector signals. The three photodetectors are aimed so that even a specular object that is beyond the cutoff distance will not produce signals that are falsely interpreted as coming from an object within range.
In the preferred embodiment, the three photodetectors are photodiodes and the first and second photodetectors are connected inversely in parallel. This results in the signal from the first photodetector being subtracted from the signal from the second photodetector at a first input node to which both photodetectors are connected. An amplifier has an input coupled to the first input node and an output coupled to a summing node. The third photodetector is coupled to an input of another amplifier that has an output also coupled to the summing node. Thus the signal produced at the summing node indicates presence of an object within range of the object detection system.
REFERENCES:
patent: 3885872 (1975-05-01), Howe, Jr. et al.
patent: 4767934 (1988-08-01), Stauffer
patent: 5617174 (1997-04-01), Mikami
Banner Specification sheet for QMT 42 Series Fixed field sensor.*
Opcon Application Note for Non-Contact Sensors, Feb. 1990.
Eaton Installation Instructions for 6200 and 6210 Retroreflectors (No date).
Haas George E.
Kim Robert H.
Quarles & Brady LLP
Song Hoon K.
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