A REFLECTION TYPE PHOTO-ELECTRIC SWITCH

Abstract: A reflection type photoelectric switch adapted to project light to an object to be detected and sense the object on the basis of reflected light therefrom, which comprises a split-type composite light receiving unit consisting of two light receiving segments, a dif­ ferential amplifier receiving the outputs of the two light receiving segments, and a waveform shaping device for discriminating the differential amplification output of said differential amplifier with a pre­ determined level and generating a detection output for the object.

Description: FIELD OF THE INVENTION This invention relates to a reflection type photoelectric switch. More particularly, this invention relates to a reflection type photoelectric switch adapted to detect markings on a sheet or the like which is driven by a roller device, for instance. DESCRIPTION OF THE PRIOR ART In the conventional photoelectric switch of the reflection type for detection of markings on a sheet, a condenser lens is used to focus a light beam from a light emitting element to a predetermined position on the sheet and a marking on the sheet is detected from a change in the level of reflected light. Since the reflected light level is changed by delicate displacement of the sheet in the direction of the optical axis, such a conventional reflection type photoelectric switch is liable to detect a marking in a false position. To overcome this drawback, there has been proposed a mark sensor such that two distinct light beams are projected to different positions on the sheet and a marking on the sheet is detected from the differences from the respective reflected light levels. However, such a technique has the disadvantage that the construction of the optical system is complicated. US-A- 3 423 597 discloses a photo-electric switch according to the preamble of claim 1. OBJECTS OF THE INVENTION It is therefore an object of the invention to provide a photoelectric switch which is capable of detecting markings on a sheet with accuracy even if the sheet to be detected is displaced somewhat in the direction of the optical axis. SUMMARY OF THE INVENTION The photoelectric switch according to this invention is defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing the construction of an embodiment of the photoelectric switch; Fig. 2 is a schematic view showing its optical system; Fig. 3 is a time chart showing changes in signal level in various parts of the photoelectric switch according to the movement of a sheet; and Fig. 4 is a schematic view showing the optical system of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 1, the reflection type photoelectric switch comprises a light receiving means consisting of two light receiving units 11, 12, a differential amplifier 13 to which the outputs of the respective light receiving units 11, 12 are applied, a waveform shaping circuit 14 for discriminating the output of the differential amplifier with a predetermined level, and an output circuit. In Fig. 2 which shows the optical system of the same photoelectric switch, a light source 1 which may for example be a light emitting diode is positioned in the focusing position of a condenser lens 3 with the interposition of a half mirror 2. The condenser lens 3 condenses the light from the light emitting diode 1 and projects it to the surface of a sheet 4 which is driven in a direction perpendicular to the optical axis. The sheet 4 is provided with markings 5 at predetermined spaced intervals and the optical system is so disposed that the markings come into the focusing position of the condensor lens 3. The reflected light from the sheet 4 passes through the condenser lens 3 and reaches the half mirror 2, where a portion of the reflected light passes through the half mirror 2 and is incident on a composite photodiode 6. The composite photodiode 6 consists of two photodiodes PD-a and PD-b which have the same light receiving area. Referring to the block diagram of the photoelectric switch in Fig. 1, the photodiodes PD-a and PD-b of the composite photodiode 6 are fed to amplifiers 11 and 12, respectively. The amplifiers 11 and 12 amplify these outputs with a predetermined amplification factor and the resultant outputs are fed to the differential amplifier 13. The differential amplifier 13 amplifies the difference between the outputs of the two amplifiers 11 and 12 and the output of this differential amplifier is fed to the waveform shaping circuit 14. The waveform shaping circuit is provided with a predetermined threshold level Vth and when the threshold is exceeded, the detection output is outputted through the output circuit 15. The operation of the photoelectric switch according to this embodiment is described below with reference to Figs. 2 and 3. The light beam from the photodiode 1 which is incident on the sheet 4 hits the marking 5 as the sheet 4 travels, and the projected image passes to the composite photodiode 6 through the condenser lens 3 and half mirror 2. As shown, as the marking 5 on the sheet 4 is shifted in the direction of division of the photodiode 6, for example in the direction of the arrowmark from the right-hand side, the image of the marking 5 is laterally reversed so that the level of light incident on photodiode PD-a is first altered and then the level of light incident on photodiode PD-b is then altered. Therefore, light reception signals as shown in Fig. 3 (a) and (b) are obtained from the two amplifiers 11 and 12, respectively. As these signals are applied to the differential amplifier 13, this amplifier 13 generates a differential amplification output as shown in Fig. 3 (c). This output is discriminated with the predetermined threshold level Vth and, as a result, a detection signal for marking 5 is obtained from the waveform shaping circuit 14 as shown in Fig. 3 (d). When the focusing position of the condenser lens 3 is traversed by the marking 5 on the sheet 4, it may happen that the sheet 4 is displaced in the direction of the optical axis, altering the distance from the center of the marking. In this case, the image of marking 5 on the photodiode 6 is distorted so that there occurs a decrease in the output level. Therefore, the output level of the differential amplifier is also varied but as long as the level is over the threshold Vth applied to the waveform shaping circuit 14, a detection signal for marking 5 can be outputted. This means that when the amplitude of the differential output is sufficiently large, there can be obtained a detection signal for marking 5 on sheet 4 even if the sheet is displaced in the direction of the optical axis. Referring to Fig. 4 which shows the optical system of a photoelectric switch according to this invention, a light emitting diode 21 embedded in the center of a condenser lens 20 is used as a light source. In the position where the reflected light from marking 5 is received along the optical axis, a split-type composite photodiode 6 similar to that used in the preceding embodiment is provided. In this second embodiment, too, the outputs of photodiodes Pd-a and PD-b are amplified and fed to a differential amplifier 13 for detection of marking 5 on the sheet 4. This arrangement dispenses with a half mirror so that the optical system is simplified. While both of the above embodiments were explained with reference to the use of the light source 1 which emits continuous light, it is possible to drive the light emitting diode in pulses. In this case, the reflected light from the marking can be amplified and processed by opening a gate at driving. Such an arrangement helps eliminate the noise due to external light and thereby reduces the possibility of false operation to a substantial extent. It should also be understood that many other changes and modifications may be easily made by one skilled in the art without departing from the scope of the invention.

Claims: A reflection type photoelectric switch adapted to project light to an object to be detected and to sense the object on the basis of reflected light therefrom comprising two light receiving means, a differential amplifier (13) receiving the outputs of said two light receiving means, characterized by said two light receiving means being a split type composite light receiving means having two light receiving segments (PD-a, Pd-b), the projection of light being by a means comprising a light source (21) embedded between portions of a condensing lens (10) receiving the light reflected from said object, and a single threshold waveform shaping means (14) directly connected to said differential amplifier (13) for discriminating the difference amplification output of said differential amplifier (13) with a single predetermined level and generating a single detection output for said object during the time between the crossings by said difference amplification output of the single threshold level. A reflection type photoelectric switch according to claim 1 wherein said light receiving segments (PD-a, PD-b) have equal light receiving areas.

Inventors: NAKAMURA, ARATA OMRON TATEISI ELECTRONICS CO., ONJI, NORIO OMRON TATEISI ELECTRONICS CO.
Applicants: OMRON TATEISI ELECTRONICS CO.
Application year: 1987
Main classification: G01D5
IPC codes: G01D5, H01H35, G06K7, H03K17