Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-06-18
2001-09-04
Font, Frank G. (Department: 2877)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C250S221000, C250S2140AL
Reexamination Certificate
active
06285017
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to photoelectric sensors, and in particular to mechanisms for determining and indicating alignment of a photoelectric receiver with a light beam.
BACKGROUND OF THE INVENTION
Photoelectric sensors are widely used in industry for a variety of measurement and control functions, based upon breaking or restoring of a light beam emitted and received by the sensor system. Examples include counting objects moving along a conveyor, determining the size of objects, detection of a machine part moving into a predetermined position, and countless other applications in many different industries.
Photoelectric sensors can be of two general types. In one type, a single housing contains both a light emitter and a light detector. A reflective target is used to rebound the beam from the emitter back to the detector across the path of objects of be detected. The other type employs an emitter and a detector in separate housings which are placed on opposite sides of the objects' path, with the beam being transmitted directly from the emitter across the path to the detector.
In all types of photoelectric systems some method of indicating accurate alignment of the optical path is required to aid in the correct positioning of the elements for proper operation. A common alignment technique utilizes an indicating lamp or LED with circuitry that illuminates the indicator when the scanning beam from the emitter is aligned with the receiver. The lamp or LED turns off when the beam is broken or misaligned with the receiver. When installing a system, the emitter, receiver and reflecting target, if any, are positioned so that the indicating lamp or LED turns on, at which time the components are secured in place.
While this type of mechanism is useful in achieving alignment in many instances, it is unfortunately subject to certain disadvantages and difficulties. The most common difficulty is that the indicator does not signal the degree of beam alignment with the receiver, but instead only indicates that the alignment is adequate to permit light beam detection under the conditions prevailing at that time. However, it is common to have a situation where the alignment is only marginal at the time of the installation, and thereafter a small amount of vibration, dirt buildup or drift of the amplifier electronics causes a system malfunction.
U.S. Pat. No. 4,356,393 discloses an alignment system that provides an indication of the degree of alignment of the photo-sensing components. Specifically, a light or sound source is pulsed at a frequency that varies with the intensity of the received light from the sensing beam. Thus the greater the frequency, i.e. the faster the light flashes or the beeps occur, the better is the alignment of the sensing components.
Another alignment approach is found in U.S. Pat. No. 5,243,181 in which the intensity of the received sensing signal reducing towards the detection threshold of the system is detected by a logic circuit which provides an indication of the marginal signal. This process only operates near the threshold of detection and is facilitated by the inevitable noise in any low level signal.
SUMMARY OF THE INVENTION
The present invention overcomes these difficulties by providing a mechanism that is operative to indicate the degree of alignment of the receiver to the beam, so that optimal alignment can be achieved and situations of marginal alignment can be avoided.
This application varies the duration of indicator activation at a constant frequency relative to the intensity of the received light. The process of varying duration of a pulse while holding the frequency constant is termed Pulse Width Modulation, referenced hereinafter as PWM.
This is accomplished by a photoelectric detecting and indicating apparatus which includes a light detector that receives light and responds by producing a detector signal indicative of intensity of the received light. A control circuit processes the detector signal and produces an output signal that has a constant frequency and pulses which vary in duration with the intensity of the received light. An indicator is driven by the output signal and produces a repetitive indication whereby the intensity of the received light may be discerned.
In the preferred embodiment, the indicator may be either a light or sound emitter. In the case of a light emitter, flashes of light are produced which occur at a constant rate, but which vary in duration as a function of the intensity of the received light. Similarly the sound emitter produces audible pulses at a constant rate, but which vary in duration as a function of the received light intensity.
Thus with this approach the output of the indicator is pulse width modulated to indicate the intensity of the receiver light and thus the degree of alignment of the photoelectric detecting and indicating apparatus.
REFERENCES:
patent: 4356393 (1982-10-01), Fayfield
patent: 5255117 (1993-10-01), Cushman
patent: 5644730 (1997-07-01), Fayfield
patent: 5777557 (1998-07-01), Fayfield
patent: 5862907 (1999-01-01), Taylor
Engineer's Notebook: A Handbook of Integrated Circuit Applications,Forrest M. Mims, III (Copyright 1979).
Eaton Corporation
Font Frank G.
Hass George E.
Punnoose Roy M.
Quarles & Brady LLP
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