Target detector capable of rejecting close-in objects
FIELD OF THE INVENTION 
     This invention relates generally to target detection electro-optic apparatus and, in particular, to an improved target detector having a receiver element that includes an auxiliary photodetector for reducing an excessive sensitivity to a target located within a predetermined range of concern. 
     BACKGROUND OF THE INVENTION 
     In commonly assigned U.S. Pat. No. 4,788,439, issued Nov. 29, 1988, entitled &#34;Multiple Detector Fog Suppression and Edge Enhancement&#34; A. H. Hardy et al. describe a proximity sensing system that detects target objects while protecting against false target detection due to fog, clouds, or other aerosols. One or more target detectors are alternately disposed adjacent to one or more guard detectors in such a manner that the edges of fields of view are immediately adjacent. Positive signals generated from target detectors are summed with negative signals from the guard detectors. The summing action steepens the slope of the summed signal that is produced when a target image moves onto the edge of a target detector. 
     In U.S. Pat. No 4,479,053, issued Oct. 23, 1984, entitled &#34;Focal Plane Array Optical Proximity Sensor&#34; A. R. Johnson discloses an optical proximity sensor that includes an illuminator assembly and a detector assembly. Lenses associated with each assembly are spaced apart in order to define a predetermined detection volume. 
     One problem associated with some target detection systems of the prior art is that an excessive sensitivity is experienced for objects located within a predetermined distance range. This excessive sensitivity results from optical side lobes and multiple reflections. 
     It is therefore an object of the invention to provide for a substantial elimination of excessive response within a predetermined range of a receiver element associated with a target detector system. 
     SUMMARY OF THE INVENTION 
     The foregoing and other problems are overcome and the object of the invention is realized by a target detector having a reduced or eliminated sensitivity to a target within a predetermined range of interest. The target detector includes a transmitter assembly and a receiver assembly. The receiver assembly includes a first photodetector positioned upon a receiver element. The receiver assembly receives light rays reflecting from a target viewed by the receiver assembly. A second photodetector is located upon the receiver assembly at a position where light rays reflecting from a target, within the predetermined range of interest, fall on the second photodetector. An electrical signal generated by the second photodetector is combined with an electrical signal generated by the first photodetector. A gain of the second signal is established such that the combined signal is maintained at an acceptable level over the range of interest. 
     In accordance with the invention there is disclosed object detection apparatus that includes a transmitter assembly and a receiver assembly. The receiver assembly includes a receiver element having an input aperture for receiving electromagnetic radiation reflecting from an object. The receiver element further includes an output aperture and a curved surface disposed relative to the input aperture for reflecting substantially all electromagnetic radiation, reflecting from an object at a range in excess of a range of interest, to the output aperture. A first radiation detector is disposed at the output aperture for receiving the object radiation reflected thereto and for generating a first electrical signal having a magnitude that is a function of a magnitude of the electromagnetic radiation received by the first radiation detector. A second radiation detector is disposed at a location upon the receiver assembly for receiving object radiation reflected thereto that reflects from an object located at a range that is within the range of interest. The second radiation detector generates a second electrical signal with a magnitude that is a function of a magnitude of the electromagnetic radiation received by the second radiation detector. The apparatus further includes circuitry for combining the first electrical signal with the second electrical signal for substantially eliminating a component of the first electrical signal that is due to electromagnetic radiation reflecting from an object within the range of interest. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The above set forth and other features of the invention are made more apparent in the ensuing Detailed Description of the Invention when read in conjunction with the attached Drawing, wherein: 
     FIG. 1a is cross-sectional view of a target detector receiver assembly of the prior art, the view showing light rays reflecting from a target at a range outside of a range of interest and a single photodetector disposed for detecting these rays; 
     FIG. 1b is cross-sectional view of the target detector receiver assembly of FIG. 1a, the view showing light rays reflecting from a target at a range within the range of interest; 
     FIG. 2 is an elevational view showing the prior art target detector receiver assembly of FIGS. 1a and 1b; 
     FIG. 3a illustrates a target detector constructed and operated in accordance with the invention and including a transmitter assembly and a receiver assembly; 
     FIG. 3b is a top view of the target detector of FIG. 3a illustrating a transmitted fan beam radiation pattern; 
     FIG. 3c is a top view of the target detector of FIG. 3a illustrating a transmitted pencil beam radiation pattern; 
     FIG. 3d is cross-sectional view of the receiver assembly associated with the target detector of the invention, the view showing light rays reflecting from a target located at a range within the range of interest and the interception thereof by a second photodetector; 
     FIG. 4 is a graph that depicts the improvement in in-range response that is achieved through the use of the invention; and 
     FIG. 5 is cross-sectional view of the receiver assembly associated with the target detector of the invention, the Figure showing a further embodiment of signal combining circuitry. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1a, 1b, and 2 depict a portion of a prior art target detector receiver assembly 10. Receiver assembly 10 includes a receiver element 12 and a photodetector 14, both of which are disposed for receiving light rays reflecting a target (not shown). A filter 14a is interposed between the detector 14 and the receiver element 12. Also not shown in FIGS. 1a, 1b and 2 is a transmitter assembly that projects a beam of light for illuminating objects in front of the receiver assembly 10. The reflected light rays are focussed and directed to the photodetector 14 by reflective surfaces associated with the receiver assembly 10. 
     The shape of receiver element 12, in a presently preferred embodiment of the invention, is similar to that disclosed in FIGS. 7 and 8 of commonly assigned U.S. Pat. No. 4,325,633, issued Apr. 20, 1982, entitled &#34;Apparatus for Determining of Angle of Incidence of Electromagnetic Radiation&#34; by r Leland V. Gardener. The disclosure U.S. Pat. No. 4,325,633 is incorporated herein by reference in its entirety. As employed therein a receiver 10&#39; is used in an angle of arrival detection system and incorporates a plurality of photodiodes. 
     In FIG. 1a, 1b and 2 the receiver element 12 is comprised of a single block of glass. The receiver element 12 has a planar transparent front surface, this surface functioning as an input aperture 16. Planar surfaces 18 and 20 delimit the sides of the receiver element 12. These surfaces are clad and blackened and define therebetween a portion of a chamber. The receiver element 12 has an elliptical cylinder reflector 22 functioning as a rear chamber surface. The elliptical curvature of the reflector 22 is described by the equation: 
     
         X.sup.2 /A.sup.2 +Y.sup.2 /B.sup.2 =1 
    
     where 
     X=coordinate on X axis, 
     Y=coordinate on Y axis, 
     A=major semi-axis, and 
     B=minor semi-axis. 
     In the case where A is very large relative to B, the ellipse approximates a parabola so closely that the difference is not significant. 
     A planar transparent surface 24 provides an output aperture. A further planar surface 26 is made reflective and serves to fold the optical path such that rays that are reflected from surface 22 are reflected from surface 26 to the output aperture 24. The curvature of the reflector 22 is chosen such that input rays (A, B, C) reflecting from a target outside of a range of interest are focussed at the output aperture 24. 
     As is also shown in the perspective view of FIG. 2 the photodetector 14 is mounted at the output aperture 24 to detect the focussed target image and, hence, the presence of the target. The photodetector 14 is preferably a silicon PIN-type device. 
     In the angle of arrival receiver 10 disclosed in the above referenced U.S. Pat. No. 4,325,633 a slot is cut across the rear surface. The slot is provided to block rays resulting from undesirable reflections due to targets at angles outside the range of the detector. This is said to be important in minimizing detection of the sun or sunlit clouds in the vicinity of the detector surface. 
     FIG. 1b illustrates the operation of the receiver assembly 10 for a target that enters the range of interest. For this embodiment the target range of interest is considered to be within a range of zero inches to approximately 12 inches from the surface of the input aperture 16. As can be seen, beams A&#39;, B&#39; and C&#39; are incident upon the input aperture 16 at an angle such that the focussed beam is no longer primarily incident on the detector 14. As the target approaches the input aperture 16 the angle between the rays (A&#39;, B, and C,) and the surface of the input aperture 16 decreases, resulting in an increase in an elevation angle referenced to a line normal to the input aperture 16 surface. As the elevation angle increases, the focussed rays move off of the output aperture 24 in a direction away from the input aperture 16 and towards a slot 28. By example, for a target range of less than approximately 12 inches a significant portion of the reflected rays can be seen to be incident upon the slot 28, as shown in FIG. 1b. This is a desired result in that a substantial rejection of close-in targets is achieved. 
     However, it has been found that there is a sufficient amount of scattering both within and outside of the receiver assembly 10 such that when the target is close in, and when a magnitude of the returned signal is high, a significant optical signal is still received by the photodetector 14. This optical signal incident on the photodetector 14 produces an unwanted in-range response. This problem is due primarily to optical side lobes and multiple reflections, as indicated by the ray (R). 
     In order to eliminate this problem of undesirable in-range response, and in accordance with the embodiment of the invention depicted in FIGS. 3a-3d, a second photodetector 30 is positioned on the receiver assembly 10 at a location such that the signal from the photodetector 30 is operable to reduce the undesirable in-range response of the photodetector 14. The use of the second photodetector 30 may also be employed to generate a true in-range &#34;hole&#34;, that is, a range in which a diffuse target cannot be sensed regardless of how bright the diffuse target appears. Preferably, the photodetector 30 is also a silicon PIN-type device having spectral response characteristics that are similar or identical to those of the photodetector 14. A filter 30a may be interposed between photodetector 30 and the receiver element 12. 
     Referring to FIG. 3a there is illustrated a target detector system 40 constructed and operated in accordance with the invention. A housing 42 having transparent windows 40a and 40b encloses a transmitter assembly 44 and a receiver assembly 10&#39;. As can be seen in the top views of FIG. 3b and FIG. 3c the transmitter assembly 44 transmits through the window 40a a fan-shaped beam 44a or a pencil-shaped beam 44b. The transmitted beam is preferably a pulsed beam of radiation having wavelengths within the infrared spectrum. It should be realized, however, that the teaching of the invention also applies to a radiation source having a continuous wave modulation. The transmitted beam reflects from an object or target disposed in front of the system 40. The reflected beam enters the system 40 through the window 40b and impinges on the input aperture of the receiver assembly 10&#39;. 
     For the embodiment shown in FIG. 3a and FIG. 3d the second photodetector 30 is positioned at the slot 28. That is, the second photodetector 30 is positioned at a region of the receiver assembly 10&#39; where rays are directed that reflect from a target within the range of interest. Photodetector 30 thus receives a strong optical return from the direct rays only when the target is close to the input aperture 16, that is, within the range of interest. 
     In the embodiment depicted in FIG. 3d the output signal from the second photodetector 30 is applied to a gain block 32 and thence to an inverting input of an amplifier 34. The amplifier 34 operates to subtract the signal generated by the photodetector 30 from the signal generated by the photodetector 14, thereby effectively eliminating the component of target radiation (R) that is received by the photodetector 14 due to internal reflection and scattering within the receiver assembly 10&#39;. The gain block 32 is selected to provide an amount of gain to the output signal of the photodetector 30 such that, for example, a threshold comparator block 36 does not generate a target detected signal when the target is within the range of interest. For a given application the gain of block 32 may be greater than or less than unity. 
     FIG. 4 depicts the output of the photodetector 14 (solid trace B) and the output of the second photodetector 30 (dashed trace A) as a function of target range from the input aperture 16. Subtracting the signal A from the signal B substantially eliminates any response due to a target within the range of interest, or 12 inches or less for this embodiment. 
     It should be realized that a number of modifications may be made to the teaching of the invention while still accomplishing the desired result. For example, the shape of the receiver assembly 10&#39; may be other than that depicted in FIG. 3a and FIG. 3d. Also, the signal from the photodetector 30 may be combined in a manner other than subtracting the signal from the output of photodetector 14. For example, the electrical signal from photodetector 30 may be employed to vary the gain of the amplifier 34 such that the signal from photodetector 14 is attenuated for close-in targets. 
     In this regard, and referring to FIG. 5, there is shown another embodiment of the signal processing electronics for combining the outputs of the photodetectors 14 and 30. In FIG. 5 the output of the signal combining amplifier 34 drives a further amplifier 35, a noninverting (+) output of which drives the threshold circuit 36. An inverting (-) output of the amplifier 35 drives the input of a blanking threshold circuit 37 which detects a positive voltage excursion due to a signal from the detector 30. A large signal from the detector 30 will be differentiated by the electronic circuitry and may cause a false target detection at threshold 36. The output of the blanking threshold circuit 37 drives a blanking gate 38 which operates to shut-off the amplifier 35 by diverting to ground, for a predetermined interval of time, the signal from amplifier 34. This arrangement prevents a false target detection due to differentiation of a large signal from detector 30. To further enhance performance, and to partially compensate for signal delay caused by gain block 32, the signal from detector 14 may be delayed by a predetermined amount from the signal generated by detector 30. This is accomplished by delay element 39. 
     Thus, while the invention has been particularly shown and described with respect to embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope and spirit of the invention.