JP2002181624A - Multi-optical-axis photoelectric sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a multi-optical-axis photoelectric sensor capable of carrying out stable detection without being influenced by noise even if a signal line for transmitting light receiving signals becomes long due to an increase in the number of optical axes. SOLUTION: The output of each light receiving circuit 22 is provided with an output buffer circuit 23 comprising a transistor Tr and an output resistance R1, which functions to lower the output impedance of the light receiving circuit 22. Both ends of the output resistance R1 are connected to a signal line L1 and a signal line L2 respectively through switching elements S1, S2. The respective control terminals of the switching elements S1, S2 are connected in common to a signal terminal of a shift register 24 driven in response to a driving signal from a CPU 25. The switching elements S1, S2 are normally in the OFF state, and when receiving a driving signal from the CPU 25 through the shift register 24, both switching elements S1, S2 are simultaneously operated ON synchronously with the driving timing of a light projecting element 11. Both signal lines L1, L2 are laid from the upper end side toward the lower end side of the multi- optical-axis photoelectric sensor according to a circuit pattern formed on a PC board and set to have the same shape.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-optical axis photoelectric sensor having an improved light receiving signal transmission system.

[0002]

2. Description of the Related Art Conventionally, as a multi-optical axis photoelectric sensor of this type, for example, a configuration disclosed in Utility Model Registration No. 2519310 is known. This means that light receiving signals from a plurality of light receiving elements arranged opposite to a plurality of light emitting elements arranged in a line in the vertical direction are sequentially transmitted to a detection unit (CPU) via one comparator. Performs the detection operation.

[0003]

However, in this type of multi-optical axis photoelectric sensor, the distances from the plurality of light receiving elements to the comparator are naturally different from each other. Some light receiving signals have to pass through a considerably long signal line. In a multi-optical axis photoelectric sensor having a large number of optical axes, its total length is, for example, 18
In some cases, the length of the signal line is 0 cm, and the length of a signal line disposed therein is considerably longer. At this time, a weak signal such as a light receiving signal is transmitted through a long signal line, and the influence of noise increases. The noise induced in a loop formed between the signal line and the ground level causes the signal to be transmitted to the comparator. There is a problem that the level of the input signal is not stable, so that the possibility of causing erroneous detection increases. On the other hand, a method is also conceivable in which a light-receiving amplifier provided in each light-receiving element amplifies a light-receiving signal to a level that is not affected by noise and then transmits the amplified signal to a comparator. Therefore, a degree of amplification that can cope with this is required, and the number of light receiving amplifiers increases, which causes problems such as an increase in cost and an increase in the size of the device.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to achieve stable detection without being affected by noise even when the number of optical axes increases and the signal line for transmitting a light receiving signal becomes longer. Is to provide a multi-optical axis photoelectric sensor capable of performing the following.

[0005]

In order to achieve the above object, a multi-optical axis photoelectric sensor according to the first aspect of the present invention includes a plurality of light emitting elements arranged in a line and driven sequentially, and the light emitting elements. A plurality of light receiving elements provided opposite to each other, a plurality of light receiving circuits for amplifying a signal from each of these light receiving elements and outputting a light receiving signal corresponding to the amount of received light, and a light receiving circuit for each of these light receiving circuits. A plurality of output buffer circuits provided with an output resistor between the output buffer circuit and a common line; and two balanced transmission signal lines connected via switching means to both ends of each output resistor of the output buffer circuit. And a terminating resistor connected between the ends of these two signal lines, and two switching means corresponding to each output resistor being simultaneously turned on in synchronization with the drive timing of each corresponding light emitting element, thereby providing an output. resistance A switch driving means for connecting both ends to both signal lines, a differential amplifier circuit which receives two signal lines as input and amplifies a difference between signals of both signal lines, and based on an output signal of the differential amplifier circuit It is characterized in that it comprises a detecting means for performing a detecting operation.

According to a second aspect of the present invention, there is provided a multi-optical axis photoelectric sensor, comprising: a plurality of light emitting elements arranged in a line and driven sequentially; and a plurality of light receiving elements provided to face each of the light emitting elements. And a plurality of light receiving circuits for amplifying the signal from each of these light receiving elements and outputting a light receiving signal corresponding to the amount of received light, and an output resistor provided between an output of each of these light receiving circuits and a common line. The output of each light receiving circuit is connected via a first switching means, and the common line is connected with two signal lines for balanced transmission connected via a second switching means and an impedance adjusting resistor. ,
By simultaneously turning on the terminating resistor connected between the ends of these two signal lines and the two switching means corresponding to each output resistor in synchronization with the drive timing of each corresponding light emitting element, the output resistance of the output resistor is reduced. A switch driving means for connecting both ends to both signal lines, a differential amplifier circuit which receives two signal lines as input and amplifies a difference between signals of both signal lines, and based on an output signal of the differential amplifier circuit It is characterized in that it comprises a detecting means for performing a detecting operation.

[0007]

According to the first aspect of the present invention, two switching means are simultaneously turned on by the switch driving means in synchronization with the driving timing of the opposing light projecting elements. Thus, both ends of the output resistor are connected to two balanced transmission signal lines. Then, the light receiving signal amplified according to the light receiving amount in the light receiving circuit is output as two signals of a level difference corresponding to the level of the light receiving signal from both ends of the output resistance via the output buffer circuit.
The signals are transmitted to the differential amplifier circuit through two balanced transmission signal lines with substantially the same input impedance. Then, the difference is amplified by the differential amplifier circuit, and a detection operation is performed by the detection means based on the output signal.
As described above, since the light receiving signal output from the light receiving circuit is so-called balanced transmission, even if the number of optical axes increases and the signal line for transmitting the light receiving signal becomes longer, the light receiving signal is stabilized without being affected by noise. Detection can be performed.

According to the second aspect of the present invention, the two switching means are simultaneously turned on by the switch driving means in synchronization with the driving timing of the opposing light projecting element, and the output resistance is reduced. Are connected to two balanced transmission signal lines. Then, the light receiving signal amplified according to the light receiving amount in the light receiving circuit is converted into two signals having a level difference corresponding to the level of the light receiving signal from both ends of the output resistor, and furthermore, each input impedance is substantially equal by the impedance adjusting resistor. The signal is transmitted to the differential amplifier circuit through two balanced transmission signal lines. Then, the difference is amplified by the differential amplifier circuit, and a detection operation is performed by the detection means based on the output signal. As described above, since the light receiving signal output from the light receiving circuit is so-called balanced transmission, even if the number of optical axes increases and the signal line for transmitting the light receiving signal becomes longer, the light receiving signal is stabilized without being affected by noise. Detection can be performed.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS <First Embodiment> A first embodiment of the present invention will be described with reference to FIG. The multi-optical axis photoelectric sensor according to the present embodiment includes a light projector 10 and a light receiver 20 which are arranged to face each other. On the side of the light emitter 10 facing the light receiver 20, a plurality of light emitting elements 11 made of, for example, LEDs are arranged in a line in the vertical direction, and on the side of the light receiver 20 facing the light emitter 10, A plurality of light receiving elements 21 formed of, for example, photodiodes, which make a pair with each light emitting element 11,
Again, they are arranged in a line along the vertical direction.

FIG. 1 shows an electrical configuration according to the multi-optical axis photoelectric sensor of the present embodiment. As shown in the figure,
The light emitting circuit 10 includes a light emitting circuit 1 to which the light emitting element 11 is connected.
The light projecting circuit 14 operates based on a predetermined clock pulse signal, and sequentially transmits a drive signal from the light projecting element 11 on the upper end side of the light projector 10 to the light projecting element 11 on the lower end side. And repeat this operation. As a result, optical signals are sequentially emitted from the light projecting elements 11 on the upper end side of the light projector 10.

The light receiver 20 is provided with a light receiving circuit 22 for each light receiving element 21 for amplifying a signal from the light receiving element 21 and outputting a light receiving signal corresponding to the amount of received light. The output of each light receiving circuit 22 is provided with an output buffer circuit 23 including a transistor Tr and an output resistor R1. More specifically, the base of the transistor Tr is connected to the output terminal of the light receiving circuit 22 and the emitter is connected to the output resistor R
This is a so-called emitter-follower circuit that is grounded via the first element 1 and functions to reduce the output impedance of the light receiving circuit 22. And the output resistance R
Both ends of 1 are connected to a signal line L1 and a signal line L2, respectively, via switch elements S1 and S2 formed of analog switches, for example. These switch elements S1, S2
These control terminals are commonly connected to signal terminals of a shift register 24 that receives and drives a drive signal from the CPU 25. That is, the transistor Tr of the output resistance R1
Are connected to the signal line L1 and to the switch elements S1 connected to the ground and ground sides.
2 are commonly connected to the signal line L2. Each of the switch elements S1 and S2 is always in an off state, and when receiving a drive signal from the CPU 25 via the shift register 24, both switch elements S1 and S2 are synchronized with the drive timing of the light projecting element 11. And turn on simultaneously. The two signal lines L1 and L2 are laid from the upper end to the lower end of the multi-optical axis photoelectric sensor by a circuit pattern formed on, for example, a PC base plate, and are set to have the same shape. .

The two signal lines L1 and L2 have terminal resistors R2 and R3 connected between their ends, and one end of the signal lines L1 and L2 is located below the light receiver 20 (the lower side in FIG. 1). They are connected to the input terminals of the differential amplifier OP, respectively. The differential amplifier OP amplifies the difference between the input signal lines L1 and L2. Terminating resistor R
Reference numerals 2 and R3 are for preventing distortion of the signal waveform transmitted through both signal lines L1 and L2. The output terminal of the differential amplifier OP is connected to a comparator 26 for comparing the input voltage level with a predetermined reference value, and the output thereof is connected to a CP.
It is provided to the input terminal of U25. Further, the CPU 25
Captures the clock pulse signal from the light emitting circuit 14 via a line 27 connecting the light emitting and receiving devices 10 and 20.

Next, the operation of the multi-optical axis photoelectric sensor configured as described above will be described. When the power of the multi-optical axis photoelectric sensor is turned on, as described above, the light emitting circuit 14 sequentially drives the light emitting element 11 on the upper end side of the light emitting element 10 to the light emitting element 11 on the lower end side at a predetermined frequency. The optical signal is sequentially emitted from the light projecting element 11 on the upper end side of the light projector 10 by giving a signal. At the same time, the CPU 25 takes in the clock pulse signal from the light emitting circuit 14 and
The shift register 24 is sequentially driven in synchronization with the clock pulse signal (that is, the light-emission timing of each light-emission element 11) to simultaneously turn on a predetermined pair of switch elements S1 and S2, and to turn on the pair. Switch element S1,
The S2 pair is sequentially moved from the upper side to the one corresponding to the lower light receiving element 21. Then, the switching elements S1, S2
When the pair is turned on, both ends of the output resistor R1 and the signal line L
1 and the signal line L2 are connected to each other, and each light receiving element 21 receives the light from the opposing light projecting element 11 and is amplified by the light receiving circuit 22 to generate a light receiving signal R1
Out of one end of the transistor Tr on the emitter side and a signal output from one end of the output resistor R1 on the ground side at the same time.
The signal is transmitted to the signal line L1 and the signal line L2 via S1 and S2. Here, both signal lines L1 and L2 have the same shape, the same distribution constant, and the output buffer circuit 2
3 and the input impedances are made substantially the same, so that even if both signal lines L1 and L2 are in an environment receiving noise, the signal lines L1 and L2 have the same effect on noise. Then, both signals are transmitted. Then, in the differential amplifier circuit OP, only the difference between the two signals is amplified and the noise component is canceled, so that only the light receiving signal from the light receiving circuit 22 is extracted from the differential amplifier circuit OP. Based on the output signal from the differential amplifier circuit OP, the comparator 26 determines and compares the signal with a predetermined level. When the light receiving signal changes beyond a predetermined value, for example, a detection indicator (not shown) blinks.

As described above, according to the multi-optical axis photoelectric sensor according to the present embodiment, since the light receiving signal output from the light receiving circuit 22 is so-called balanced transmission, even if the number of optical axes increases, the light receiving signal is transmitted. Even if the signal line becomes long, stable detection can be performed without being affected by noise.

<Second Embodiment> FIG. 2 shows a second embodiment (corresponding to the second aspect of the present invention), and shows only portions different from the first embodiment. That is, instead of connecting the output buffer circuit 23 to the output of the light receiving circuit 22 as in the first embodiment, a means for matching the input impedances of the signal line L1 and the signal line L2 is used.
A load resistor R4 having a load equivalent to the output impedance of the light receiving circuit 22 is connected to the input side of the second circuit 2 to adjust the condition of balanced transmission. With this configuration, the same effect as in the first embodiment can be obtained.

Third Embodiment In the first embodiment, one end of the signal line L2 is grounded (0 V). However, the present invention is not limited to this, and more stable amplification can be performed in the differential amplifier circuit OP. In order to perform this, in the third embodiment, as shown in FIG. 3, one end of the signal line L2 is connected to an arbitrary potential (2 in FIG. 3).
V). Thus, a signal having a somewhat high signal value is input to the differential amplifier circuit OP, and more stable amplification can be performed.

<Other Embodiments> The present invention is not limited to the above embodiments. For example, the following embodiments are also included in the technical scope of the present invention.
In addition to the following, various changes can be made without departing from the scope of the invention. (1) In the first embodiment, each of the light projecting element 11 and the light receiving element 21 is a four-optical axis photoelectric sensor of four each. However, the present invention is not limited to this. It may be. According to the present invention, even if the number of optical axes increases and a signal line for transmitting a light receiving signal becomes longer, stable detection can be performed without being affected by noise.

(2) In the first embodiment, the output signal from the differential amplifier circuit OP is determined using the comparator 26. However, the output signal from the differential amplifier circuit OP is
A configuration in which the detection operation is performed by directly taking in the U25 and making a software determination in the CPU 25 may be employed.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a multi-optical axis photoelectric sensor according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a second embodiment.

FIG. 3 is a circuit diagram showing a third embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 ... Light projecting element 21 ... Light receiving element 22 ... Light receiving circuit 23 ... Output buffer circuit 24 ... Shift register 25 ... CPU L1, L2 ... Both signal lines OP ... Differential amplifier circuit R1 ... Output resistance R2, R3 ... Terminating resistance R4 ... Load resistance S1, S2 ... Switch element

Continued on the front page F-term (reference) 2G065 AA04 AB28 BA33 BC04 BC40 CA12 DA15 5G055 AA01 AB01 AC02 AE49 5J050 AA11 AA13 BB18 DD00 EE24 EE31 EE35 EE39 FF04 FF10 5J092 AA01 AA21 AA21 HA26 FAA1919 HA02 HA45 KA01 KA02 KA17 KA18 KA26 KA28 KA32 MA01 MA19 SA01 TA01 UL02 UL07

Claims (2)

[Claims] 1. A plurality of light emitting elements arranged in a line and driven sequentially, a plurality of light receiving elements provided to face each of the light emitting elements, and amplifying signals from each of the light receiving elements. A plurality of light receiving circuits for outputting a light receiving signal corresponding to the amount of received light, a plurality of output buffer circuits provided for each of the light receiving circuits and provided with an output resistor between the common line and the output buffer circuit Two balanced transmission signal lines connected to both ends of each of the output resistors via switching means, a terminating resistor connected between the ends of these two signal lines, and Switch driving means for simultaneously turning on two corresponding switching means in synchronization with the driving timing of the corresponding light emitting elements to connect both ends of the output resistor to the two signal lines; A multi-optical axis comprising: a differential amplifier circuit that receives the two signal lines as input and amplifies a difference between signals of the two signal lines; and a detection unit that performs a detection operation based on an output signal of the differential amplifier circuit. Photoelectric sensor. 2. A plurality of light emitting elements arranged in a line and driven sequentially, a plurality of light receiving elements provided to face each of the light emitting elements, and amplifying signals from these light receiving elements. A plurality of light receiving circuits for outputting a light receiving signal corresponding to the amount of received light, an output resistor provided between an output of each of these light receiving circuits and a common line, and a A second switching means and two signal lines for balanced transmission connected to the common line via a second switching means and an impedance adjustment resistor; and an end portion between these two signal lines. The two ends of the output resistor are turned on at the same time by simultaneously turning on the two switching means corresponding to the respective output resistors in synchronization with the drive timing of the corresponding light emitting elements. Switch driving means for connecting to the two signal lines; a differential amplifier circuit for receiving the two signal lines as input and amplifying a difference between the signals of the two signal lines; A multi-optical axis photoelectric sensor comprising: a detecting unit that performs a detecting operation.