Reflection type photoelectric switch
This application is a continuation of application Ser. No. 55,286, filed May 29, 1987, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. FIELD OF THE INVENTION 
     This invention relates to a reflection type photoelectric switch. More particularly, this invention relates to a reflection type photoelectric switch adated to detect markings on a sheet or the like which is driven by a roller device, for instance. 
     2. 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 in 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 an optical axis, such as conventional reflection type photoelectrical switch is liable to detect a marking in a false position. To overcome this drawback, there has been proposed a mark sensor wherein two distinct light beams are projected to different positions on the sheet and a marking on the sheet is detected in the differences from the respective reflected light levels. However, such a technique has the disadvantage that the construction of the optical system is complicated. 
     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 accurately even if the sheet to be detected is displaced somewhat in the direction of the projection light axis. 
     SUMMARY OF THE INVENTION 
     The photoelectric switch according to this invention comprises a light receiving means having two distinct light receiving segments and detects the marking of a reflective object on the basis of the output of a differential amplifier. Since in this arrangement the outputs of the respective light receiving segments both vary when the sheet to be detected is displaced in the direction of optical axis, the change is offset in the differential amplifier, with the result that markings on the sheet can be accurately detected without fail. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the construction of an embodiment of the photoelectric switch according to this invention; 
     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 a second embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 which illustrates one embodiment of the invention, 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, the projection image passes 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 to alter the distance from the center of the marking. In this case, the image of marking 5 on the photodiode 2 is distorted so that a decrease in the output level occurs. 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, a detection signal for marking 5 on sheet 4 can be obtained even if the sheet 4 is displaced in the direction of the optical axis. 
     Referring to FIG. 4 which shows the optical system of a photoelectric switch according to a second embodiment of 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 noise due to external light to thereby reduce 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 spirit and scope of the invention.