JP3265663B2 - Photoelectric switch - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectric switch of a transmission type or a retroreflection type, and more particularly to a photoelectric switch for detecting an object such as a transparent film having a high light transmittance.

[0002]

2. Description of the Related Art Conventionally, a transmission type or recursive reflection type photoelectric switch includes a light emitting circuit for irradiating light to an object detection area and a light receiving circuit for receiving the irradiated light. The change of the light receiving level due to the presence or absence of the detected object during the period is detected by the light receiving circuit. Then, the output of the light receiving circuit is amplified and discriminated by a predetermined threshold, and the threshold level is set by a variable resistor for sensitivity adjustment or the like to determine the presence or absence of an object.

[0003]

In such a conventional photoelectric switch, an object can be easily detected if the change in the light receiving level due to the presence or absence of the object to be detected is large. However, when detecting objects with high light transmittance, such as transparent films and glass, it is difficult to adjust the threshold level because the change in the light reception level is small. There was a drawback that it could not be performed.

[0004] The present invention has been made in view of such a conventional problem, and has as its technical object to surely detect an object having a good light transmittance.

[0005]

SUMMARY OF THE INVENTION The present invention provides a light projecting means for driving a light projecting element at a constant level, a first light receiving element for receiving light directly radiated from the light projecting element, and an object receiving light from the light projecting element. A second light receiving element for receiving light emitted to the detection area,
A differential amplifier circuit for outputting the difference between the input signals, a first light receiving circuit for amplifying the output of the first light receiving element, and a second light receiving circuit for amplifying the output of the second light receiving element and providing the output to the differential amplifier circuit Circuit, a gate means for extracting a differential output of the differential amplifier circuit in synchronization with the light emission pulse of the light emission circuit, a comparison circuit for outputting an output when the output of the gate means exceeds a predetermined level, And a level adjusting means for adjusting the output level of the first light receiving circuit so that the output of the differential amplifier circuit becomes 0 level at the time of detection, and applying the output level to the differential amplifier circuit.

[0006]

According to the present invention having the above features, the light projecting element is driven by the light projecting means, a part of which is received by the first light receiving element and is supplied to the differential amplifier circuit via the level adjusting means. Is added to The light received through the object detection area is applied to the differential amplifier circuit through the second light receiving circuit. When the object is not detected, the output of the differential amplifier circuit is set to 0.
Leave as a level. In this case, when an object arrives and the light receiving level of the second light receiving circuit decreases, the output of the differential amplifier circuit changes greatly depending on the amplification factor of the differential amplifier circuit, and the light transmittance is large. An object can be easily detected.

[0007]

FIG. 1 is a block diagram showing the configuration of a transmission type photoelectric switch according to an embodiment of the present invention. In this embodiment, a transmission type photoelectric switch using an optical fiber is shown.
In this embodiment, the light emitting circuit 1 first generates a periodic light emitting pulse, and its output is supplied to an automatic power control circuit (hereinafter, referred to as an APC circuit) 2. The APC circuit 2 is a circuit for keeping the light emitting level of the light emitting element 3 constant, and constitutes a light emitting means together with the light emitting circuit 1. A light receiving element (first light receiving element) 4 for directly receiving a part of the irradiation light is attached near the light projecting element 3, and the light receiving output is given to a first light receiving circuit 5. On the output side of the light receiving circuit 5, a level adjusting means for adjusting the output level, in this embodiment, a variable resistor 6
Is connected. The output of the light receiving circuit 5 is directly applied to the APC circuit 2 as a light receiving signal for monitoring, and the light emitting level of the light emitting element 3 is regulated by the APC circuit 2 so that the light receiving level becomes constant.

Most of the light emitted from the light projecting element 3 enters the end of the optical fiber 7. The optical fiber 7 is an optical fiber for guiding the irradiated light to the object detection area, and a light receiving optical fiber 9 is disposed at the other end of the optical fiber so as to face the object detection area 8. The light receiving element 10 is arranged close to the other end of the light receiving optical fiber 10. The light receiving element 10 is a second light receiving element that receives the light irradiated on the object detection area 8, and its output is given to the second light receiving circuit 11. The light receiving circuit 11 is a circuit for amplifying the output from the light receiving element 10, and the output is provided to one input terminal of the differential amplifier circuit 12. The other input terminal of the differential amplifier circuit 12 is connected to the middle point of the variable resistor 6 for level adjustment described above. The differential amplifier circuit 12 is a circuit for detecting a difference between these signals, and its output is provided to an analog switch 13.

On the other hand, a waveform shaping circuit 14 is connected to the output terminal of the light projecting circuit 1. The waveform shaping circuit 14 is for shaping the waveform except for portions where the output of the rising and falling of the light emission pulse is not stable, and the output is given to the analog switch 13 as a switch signal. Analog switch 1
Numeral 3 is to output the output of the differential amplifier circuit 12 to the next-stage comparison circuit only while the light emission pulse is output. Here, the waveform shaping circuit 14 and the analog switch 13 constitute gate means for extracting the differential output of the differential amplifier circuit 12 in synchronization with the light emission pulse. The output terminal of the analog switch 13 is connected to a comparison circuit 15 and comparison circuits 16a and 16b. A predetermined threshold value Vref1 is set as a threshold value in the comparison circuit 15, and if there is an output exceeding this level, it is output to the outside as an object detection signal. In the comparison circuits 16a and 16b, predetermined positive and negative values Vref2 and Vref3 are set as threshold values, which are close to the 0 level.
The comparison circuits 16a and 16b constitute a window comparator for detecting that the output of the analog switch 13 is near the 0 level, and both outputs are given to the AND circuit 17. The AND circuit 17 detects the coincidence by the logical product, and its output is given to a light emitting diode 18 for display. The window comparator, the AND circuit 17, and the light emitting diode 18 constitute a zero level display means for displaying that the output of the analog switch is at the zero level.

Next, the operation of this embodiment will be described. First, the light projecting circuit 1 inputs a light projecting pulse to the APC circuit 2 and the waveform shaping circuit 14 at a constant cycle, and the light projecting element 3 is driven by the APC circuit 2. Part of irradiation light is light receiving element 4
And is fed back to the APC circuit 2 via the light receiving circuit 5. Then, the driving level of the light emitting element 3 is automatically controlled so that the level is always constant. On the other hand, light from the light projecting element 3 is transmitted to the light receiving optical fiber 9 via the optical fiber 7 and the object detection area 8. If there is no object in the object detection area 8, almost all light is transmitted to the light receiving optical fiber 9, converted into an electric signal by the light receiving element 10, and amplified by the light receiving circuit 11. The output of the light receiving circuit 11 and the output of the light receiving circuit 5 are transmitted to the differential amplifier circuit 12 via the variable resistor 6 for level adjustment. The differential output of the differential amplifier circuit 12 is the analog switch 1
3 is output in synchronization with the transmission pulse. Here, in a state where there is no object, the signal output from the analog switch 13 is set to be almost 0 level, that is, as shown in FIG.
It is assumed that the level of the variable resistor 6 is adjusted in advance so that both outputs of the comparison circuits 16a and 16b are turned on as shown in FIG. At this time, if the AND condition of the AND circuit 17 is satisfied and the light emitting diode 18 is turned on, it can be recognized that the level is 0 level.

In this state, the object 19 is located in the object detection area 8.
Arrives, the light is blocked and no light is incident on the light receiving element 10, so that the output level of the differential amplifier circuit 12 rises. Therefore, the light emitting diode 18 is turned off.
When the differential amplification output exceeds the threshold value Vref1 of the comparison circuit 15, the output is turned on as shown in FIG. 2A and an object detection signal is output to the outside.

The output of the light receiving circuit 11 in the absence of an object is set to V1 (for example, 100 mV), the gain of the differential amplifier circuit 12 is set to A (for example, 50 times), and the highly transparent object 19 arriving at the object detection area 8 Let α (for example, 0.8) be the transmittance of the light of .DELTA.V, and the change .DELTA.V in the output of the differential amplifier circuit 12 is expressed by the following equation. ΔV = V1 × (1−α) × A When the values of V1, A, and α are substituted as described above,
ΔV is about 1 V, and a large output change is obtained.
The level error when there is no detected object is, for example, ± 50 mV
Then, the S / N ratio at this time is about 10, and stable detection is possible.

In this embodiment, a photoelectric switch using an optical fiber is described. However, the present invention can be applied to a normal transmission type photoelectric switch and a retroreflective type photoelectric switch using a retroreflective plate. It goes without saying that you can do it. In this embodiment, the 0 level display means is constituted by the comparison circuits 16a and 16b, the AND circuit 17, and the light emitting diode so that the 0 level can be displayed. However, the output of the differential amplifier circuit can be displayed by various other methods. Zero level can be detected.

In this embodiment, the output of the differential amplifier circuit 12 is adjusted to be 0 by using the variable resistor 6 as a level adjusting means. , And may be input to the differential amplifier circuit 12. Then, the amplification factor is automatically set so that the output of the differential amplifier circuit 12 becomes 0 level. This eliminates the need for level adjustment using a variable resistor, and allows no adjustment.

[0015]

As described above in detail, according to the present invention, the change in the output level is amplified, so that the change in the light receiving level is extremely large. Therefore, even in the case of an object to be detected such as a transparent film or glass having a high light transmittance, an excellent effect that the presence or absence of the object can be detected with high sensitivity is obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an overall configuration of a photoelectric switch according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating an output of a differential amplifier circuit and an output change of a comparison circuit according to the embodiment.

[Explanation of symbols]

 Reference Signs List 1 light emitting circuit 2 APC circuit 3 light emitting element 4, 10 light receiving element 5, 11 light receiving circuit 6 sensitivity setting variable resistor 7, 9 optical fiber 12 differential amplifier circuit 13 analog switch 14 waveform shaping circuit 15, 16a, 16b Comparison circuit 17 AND circuit 18 Light emitting diode 19 Object to be detected

Claims (2)

(57) [Claims] 1. A light projecting means for driving a light projecting element at a constant level, a first light receiving element for receiving light directly emitted from the light projecting element, and a light projecting element irradiating an object detection area from the light projecting element. A second light-receiving element for receiving the light, a differential amplifier circuit for outputting a difference between input signals, a first light-receiving circuit for amplifying an output of the first light-receiving element, and a second light-receiving element. A second light receiving circuit that amplifies an output and provides the amplified light to the differential amplifier circuit; a gate unit that extracts a differential output of the differential amplifier circuit in synchronization with a light emitting pulse of the light emitting circuit; A comparison circuit for outputting an output when the output exceeds a predetermined level; and adjusting the output level of the first light receiving circuit so that the output of the differential amplifier circuit becomes 0 level when no object is detected,
And a level adjusting means for giving to the differential amplifier circuit. 2. The photoelectric switch according to claim 1, further comprising a zero-level display means for displaying that the output of said gate means is at a zero level when an object is not detected.