REFLECTION TYPE PHOTOELECTRIC SWITCH

Abstract: A reflection type photoelectric switch (1) including a light emitting member (3) for emitting a substantially single light beam having first and second different wavelengths, a reflector (7) for reflecting the emitted light beam at different reflection rates with respect to the first and second and wavelengths, a light receiving member (4) for sensing rays at the first and second wavelengths, and means for analyzing outputs generated from the light receiving member (4) receiving the reflected light beam at the first and second wavelengths to generate an output representing the existence of an object (8) to be detected by the photoelectric switch.

Description: This invention relates to a reflection type photoelectric switch including a light projecting member for projecting a light beam to a reflector and a light receiving member for receiving a light beam reflected from the reflector to produce a light path, whereby an object is detected when it blocks the light path, and more particularly to an improved photoelectric switch which can precisely detect even an object having a mirror surface. A reflection type photoelectric switch housing light projecting and receiving members is well known in which the light projecting member projects a light beam to a retro-type reflector and the receiving member receives a light beam reflected from the reflector whereby a detection signal is obtained when the projected light beam is blocked by an object to be detected. Such a conventional photoelectric switch, however, has the disadvantage that an object having a metallic or mirror surface cannot be detected thereby because the light beam projected from the projecting member is reflected on the mirror surface and received by the receiving member. In order to overcome such a disadvantage, in EP-A-0 105 528 there has been proposed a photoelectric switch according to the preamble of claims 1 and 7, in which a pair of light projecting members respectively project light of different wavelengths to a retro-type reflector and only one of the wavelengths is reflected by the reflector so that the discrimination between the reflector and an object to be detected by the switch can be performed by the comparison of the levels of the reflected light at the respective wavelength. This conventional switch needs two pairs of projecting and receiving members, so that the number of optical components is unnecessarily increased. It is a primary object of this invention to provide a reflection type photoelectric switch which can precisely detect the object having a mirror surface by utilizing a relatively simple optical system. Is is a further object of this invention to provide a reflection type photoelectric switch employing a pair of projecting and receiving members to provide a single optical axis utilizing different waveform lights. The invention is as claimed in claims 1 and 7. According to this switch, one projecting member projects light of a first and second or a plurality of wavelengths towards the reflector on a single optical axis. The reflector is designed to differently reflect different wavelength lights while the light receiving element housed within the switch is so designed to sense the reflected lights at the different wavelengths, so that the detection of the object passing the light path between the light emitting means and the reflector is by the comparison between the levels of the reflected light at the respective wavelengths. Neither Light at no wavelength is received by the receiving member when an object having no mirror surface blocks the light path, while light at all wavelengths is received by the receiving member when an object having a mirror surface blocks the light path and itself reflects the light at the different wavelengths toward the receiving element, whereby objects with and without a mirror surface can precisely be detected by comparison of the levels of the reflected light at different wavelengths. These and other objects and advantages of this invention will become apparent from the following detailed description in connection with the accompaning drawings in which: Fig. 1 is a schematic view illustrating an optical construction of a reflection type photoelectric switch of a first embodiment according to this invention; Fig. 2(a) is a sectional side view of a light emitting diode of a multicolor luminous type employed as a light projecting element; Fig. 2(b) is a graph illustrating a luminous characteristic of the light emitting diode; Fig. 3 is a graph illustrating a wavelength response of a retro-type reflector; Fig. 4(a) is a sectional side view of a color sensor as a light receiving element; Fig. 4(b) is an equivalent circuit of the color sensor; Fig. 4(c) is a graph illustrating a wavelength response of the sensor; Fig. 5 is a schematic block diagram illustrating an electric construction of a photoelectric switch of this embodiment; Fig. 6(a) is a sectional side view of a light receiving element as a modification of the first embodiment; Fig. 6(b) is an equivalent circuit of the element of Fig. 6(a); Fig. 6(c) is a graph illustrating a response curve of the element; Fig. 7 is a schematic block diagram illustrating an electric construction of a reflection type photoelectric switch as a second embodiment of this invention; Fig. 8 is a time chart illustrating waveforms at the respective portions; and Fig. 9 shows a construction of a retro-type reflector as a modification of the first embodiment of this invention. Referring to Fig. 1, there is shown a construction of an optical system employing a reflection type photoelectric switch in accordance with a first embodiment of this invention. The switch includes a housing 1, a printed circuit board 2 carrying a light emitting element 3 such as a light emitting diode and a light receiving element 4 such as a photodiode which are mounted on the board 2. The light emitting element 3 may employ a single light emitting diode having a GaAlAs hetero-conjunction shown in Fig. 2(a) and a spectrum characteristic wherein different wavelengths of light are emitted as shown in Fig. 2(b). Exemplarily, the light emitting diode may be the diode which simultaneously emits a strong red radiation at 660 nm and a weak infrared radiation at 880 nm as a substantially single light beam. A pair of converging lenses 5 and 6 are respectively disposed corresponding to the light emitting and receiving elements 3 and 4. The converging lens 5 guides the radiation emitted from the emitting element 3 to a retro-type reflector 7 positioned apart from the main body of the photoelectric switch by a predetermined interval. The reflector 7 is designed to absorb only one of the radiations, e.g. red light, and reflect only another radiation, e.g. infrared, or vice versa. The reflector 7 in this embodiment employs a single mold consisting of combination of corner cubes made of acrylic resin mixed with dyes which absorb red light and passes infrared light, and is designed to have spectrum characteristics wherein red light at 700 nm is reflected at a substantially zero rate but infrared light at wavelengths longer than 800 nm is reflected at a 90% or higher rate as shown in Fig. 3. The converging lens 6 disposed on a front wall of the photoelectric switch converges the light reflected from the reflector to the light receiving element 4. The light receiving element 4 employs a color sensor separating light having different wavelengths to generate signals. Exemplarily, the sensor is constructed to have a pair of photo diodes each having a different conjunction depth in a single chip as shown in Fig. 4(a) to discriminate color by utilizing the difference between the respective spectrum characteristics of the diodes. The diodes are represented by an equivalent circuit having a pair of photodiodes PD-a and PD-b as shown in Fig. 4(b) which respectively have response curves PD-a and PD-b shown in Fig. 4(c). Referring to Fig. 5, a detection circuit section in this embodiment will be described hereinafter. The outputs of the pair of photodiodes PD-a and PD-b are applied to non-inverting amplifiers 10 and 11 which respectively amplify the outputs from the photodiodes at the same amplifying rate. The output from the amplifier 10 is applied to an adder 12 connected to a voltage source 13. The adder 12 sums a minute voltage level V off of the source 13 and the output from the amplifier 10, and its output is applied to the inverting input terminal of an operational amplifier representing a comparator 14. The output from the amplifier 11 is applied to the non-inverting input terminals of the respective operation amplifiers representing comparators 14 and 15 as comparing means. The comparator 14 compares the output from the amplifier 10 added to the minute voltage V off with the output from the amplifier 11, and the voltage source 13 is disposed to fix a comparison output to a low "L" level when both outputs from amplifiers 10 and 11 are equal. The comparator 15 at its inverting input terminal is connected with a reference voltage source 16 providing a reference voltage V ref to compare the voltage V ref with an input voltage thereof. The outputs from comparators 14 and 15 are applied to AND circuit 17 to apply their logic product to output circuit 18 which is designed to produce a object detection singal when its output is a low "L" level. The light emitting diode employed in the light emitting element 3 generates the spectrums represented by the graph of Fig. 2(b), that is, visible red light and infrared light, which are radiated through the lens 5 to the retro-type reflector 7 remote from the main body of the photoelectric switch. The reflector 7 absorbs red light and reflects only infrared light back into the same direction. Accordingly, only infrared light is applied to the light receiving element 4 through converging lens 6. The operation states at the respective portions of the photoelectric switch are represented by the following Table 1: (I) Object non-exists (II) Ordinal object exists (III) Object having mirror surface exists Photo Current PD-a 1 0 2 PD-b 5 0 1 Comparator 14 H L L 15 H L H AND output H L L Switch output non exist exist When there is no object to block the light path (I), the photodiode PD-a produces a relatively low level photo-current while the photodiode PD-b produces a high level photo-current since the photodiodes PD-a and PD-b of the light receiving element 4 have the response curves shown in Fig. 4(c). The Table 1 expresses the respective relative values of photo-currents as "1" and "5" when there is no object. The signals corresponding to such photo-currents are amplified by non-inverting amplifiers 10 and 11 for application to the comparators 14 and 15. In this occasion, the output of amplifier 11 is large, so that comparator 14 is a high "H" level. The comparator 15 also produces a high "H" level since the signal is larger than a determined threshold level V ref . Thus, AND circuit 17 produces a high "H" level and output circuit 18 does not produce any object detection signal. When an ordinary object 8 to be detected blocks the light path between the photoelectric switch and the reflector 7 (II), neither red nor infrared light is applied to the light receiving element 4. As expressed in Table 1, the photocurrents of the photodiodes PD-a and PD-b are zero, and the comparator 15 produces a "L" level. The comparator 14 also produces a "L" level since the non-inverting input terminal of comparator 14 is supplied with the minute voltage V off of the voltage source 13 by adder 12. Thus, the output of AND circuit 17 becomes a "L" level, so that an object detection signal is produced through output circuit 18. When an object 8 having a mirror surface blocks the light path but reflects light by the mirror surface (III), radiation having spectrums shown in Fig. 2(b) is applied to the light receiving element 4 through converging lens 6 as it is. The respective photodiodes PD-a and PD-b of the element 4 have spectrum characteristics shown in Fig. 4(c), so that the relative values of photocurrents of the photodiodes PD-a and PD-b are exemplarily expressed as "2" and "1" shown in Table 1. These photocurrents are amplified as they are each to be compared by the comparators 14 and 15. The comparator 14 is "L" because the output from the diode PD-a is large, while the comparator 15 becomes "H" because the input to the non-inverting input terminal of comparator 15 is higher than the reference voltage V ref . Thus, the logic product from AND circuit 18 becomes "L", and an object detection signal is produced. The adder 12 and the AND circuit 17 are employed in this embodiment, but detection signals may be produced by only one of them whether or not the object to be detected has any mirror surface. Referring to Figs. 6(a) through 6(c), there is shown a light receiving element modified with respect to the element 4 of the above-mentioned first embodiment. As shown in Fig. 6(a), the light receiving element is constructed to have a pair of photodiodes PD-a and PD-b disposed in a single chip at different light receiving areas. Further, thin interference filters 21a and 21b having different characteristics are disposed on the respective light receiving areas so as to provide different spectrum characteristics. The equivalent circuit of the element of Fig. 6(a) is represented by the circuit of Fig 6(b) similar to Fig. 4(b), and the respective spectrum characteristics follow the transparency characteristics of the filters 21a and 21b shown by the curves PD-a and PD-b of Fig. 6(c). The receiving element of this embodiment is applied to the element 4 and the photoelectric switch of the first embodiment, and any object can be detected whether or not the object has a mirror surface as described in the first embodiment. Referring to Fig. 7, there is shown a circuit diagram of a photoelectric switch as a second embodiment of this invention, in which a light emitting element 3 emits light pulses to avoid affection by external light and improve the detection efficiency in comparison to a light emitting diode 3 continuously emitting light as in the first embodiment. In the second embodiment, the light emitting element 3 emits light pulses by signals from an oscillation circuit 22. Capacitors C1 and C2 are respectively connected between photodiodes PD-a and PD-b and amplifiers 10 and 11, so that only a.c. components of the outputs from the diode Pd-a and PD-b are applied to the amplifiers 10 and 11 and further applied to comparators 14 and 15 through capacitor C3 including adder 12 or capacitor C4. Moreover, the output from AND circuit 17 is applied to integrating circuit 23 to apply its output to output circuit 18 after holding it for a predetermined interval. As shown in Fig. 8, if the light signals are emitted from the light emitting diode 3 as illustrated in Fig. 8(a) when there is no object to be detected (I), an ordinary object 8 blocks the light path (II) or an object 8 having a mirror surface blocks the light path (III), the respective photodiodes PD-a and PD-b of the light receiving element 4 produce outputs shown in Figs. 8(b) and 8(c) which are amplified by amplifiers 10 and 11 as photo-converted electric signals to be compared by comparators 14 and 15, so that output signals shown in Figs. 8(d) and 8(e) are produced by the comparators 14 and 15. Thus, the logic product of the output signals shown in Fig. 8(f) is obtained from AND circuit 17. The AND circuit 17 produces an output at "H" level when any object does not exist or at "L" level when it exists whether or not it has a mirror surface. The output from circuit 17 is held for a predetermined interval by integrating circuit 23 for application to output circuit 18 which generates output signals shown in Fig. 8(g) according to the non-existence or existence of the object to be detected. Though the retro-type reflector 7 in the first embodiment is constituted with combination of corner cubes made of acrylic resin absorbing red light, the reflector 7 may be replaced by a reflector in which an infrared filter 25 passing infrared light only but blocking red light is applied on a front wall of a unit 24 consisting of transparent corner cubes, if desired, as shown in Fig. 9.

Claims: A reflection type photoelectric switch which detects the presence of an object, comprising a light emitting means (3, 5) for emitting a substantially single light beam having first and second different wavelengths into a space in which the object may be detected, a reflector (7) for reflecting said light beam at different reflection rates with respect to said first and second wavelengths, a light receiving means (4) for sensing said first and second wavelengths of the reflected light beam and which generates outputs in response to said first and second wavelengths, and means for analyzing said outputs generated by said light receiving means (4) at said first and second wavelengths to generate an output representing the existence in the space of the object detected by the photoelectric switch, characterized in that said light emitting means (3, 5) comprises a single light emitting diode (3) which emits light at said first and second wavelengths. A reflection type photoelectric switch according to claim 1, in which said light emitting means (3, 5) includes a converging lens (5). A reflection type photoelectric switch according to claim 1 or 2, in which said light emitting means emits light through a single transparent window. A reflection type photoelectric switch according to anyone of the preceding claims, in which said means for analyzing said outputs includes comparing means (14, 15) for comparing the outputs produced by said light receiving means (4). A reflection type photoelectric switch according to anyone of the preceding claims, in which said reflector (7) has a reflection rate where light at said first wavelength is absorbed but light at said second wavelength is reflected by the reflector (7). A reflection type photoelectric switch according to anyone of the preceding claims, in which said light emitting diode (3) emits light in a pulse mode. A photoelectric switch which detects the presence of an object, comprising a light emitting means for emitting a substantially single light beam into a space in which the object may be detected, a reflector for reflecting the said light beam at different reflection rates with respect to different wavelengths, a light receiving means for sensing said different wavelengths and receiving the light reflected from said reflector and which generates outputs in response to said different wavelengths, means for processing said outputs generated by said light receiving member to produce unique signals respectively corresponding to said different wavelengths, and means for analyzing said unique signals to discriminate whether the received light beam is reflected from the reflector or from an object to be detected by the photoelectric switch, characterized in that said light emitting means comprises a single chip light emitting element emitting light at a plurality of wavelengths and said different wavelengths are said plurality of wavelengths. A photoelectric switch according to claim 7, in which said light emitting means includes a light emitting element and a converging lens. A photoelectric switch according to claim 7 or 8, in which said light receiving means includes a single chip of a light receiving element to sense light at said plurality of wavelengths. A photoelectric switch according to anyone of the claims 7 to 9, in which said light emitting element emits light in a pulse mode.

Inventors: NAKAMURA, ARATA 647-3, FURUKAWA-CHO
Applicants: OMRON TATEISI ELECTRONICS CO.
Application year: 1987
Main classification: G01V9
IPC codes: G01V8, H01H35