JP3223528B2 - Photoelectric sensor - 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 sensor having a feature in an automatic correction circuit for a projected light amount in the photoelectric sensor.

[0002]

2. Description of the Related Art A light emitting diode, a laser diode, or the like is used as a light emitting element in a photoelectric sensor. The luminous efficiency of these light emitting elements changes depending on the temperature, and the detection distance fluctuates due to the change. Therefore, an automatic projection light amount correction circuit (APC circuit) is conventionally used so that the light emission efficiency of the light emitting element is kept constant even when the temperature changes. This automatic circuit for correcting the amount of emitted light is described in, for example,
Opt Device Application Know-how, edited by Hiroshi Ito, CQ Publishing Company, page 109, FIG. 4-34, etc.
A light receiving element for monitoring is provided near the light emitting element, and the light emitting element is driven so that the light receiving level of the light receiving element is constant.

[0003]

By adding an automatic light emission correction circuit to the photoelectric sensor as described above, it is possible to correct the light emission of the light emitting element due to a change in temperature, a change over time, or the like.
However, the photoelectric sensor detects an object by amplifying the output of the light receiving element and performing signal processing on the amplified output. Therefore, if the amplification degree of the amplifier circuit fluctuates due to the temperature, the detection distance will fluctuate. Therefore, in the conventional photoelectric sensor, only the inside of the light emitting element driving circuit is a feedback loop, and the amount of emitted light can be kept constant. However, considering the detection circuit as the photoelectric sensor, it is an open loop. Therefore, the photoelectric sensor has a drawback that it depends on the temperature characteristics of the amplifier circuit and the like.

[0004] The present invention has been made in view of such conventional problems, and has as its technical object to eliminate the temperature-dependent characteristics of a photoelectric sensor.

[0005]

According to a first aspect of the present invention, there is provided a light projecting element for irradiating an object detection area with light, and a reflected light obtained from the object detection area among the light emitted from the light projection element. A first light receiving element for receiving light, a second light receiving element for directly receiving light from the light emitting element, and a first signal switching for alternately switching inputs from the first and second light receiving elements according to a clock signal. Means, amplifying means for amplifying the output of the first signal switching means, and signal processing for identifying a detected object in the object detection area by the output of the amplifying means when the output of the first light receiving element is selected by the first signal switching means. Means, an oscillating circuit for outputting a clock signal to the first signal switching means, an output of the amplifying means when the output of the second light receiving element is selected by the first signal switching means, and an output of the amplifying means having a constant level. According to the output of the oscillation circuit It is characterized in that it comprises driving means for driving the intermittently projecting element.

According to a third aspect of the present invention, there is provided a light projecting element for irradiating light to an object detection area, and a first light receiving element for receiving reflected light obtained from the object detection area out of the light emitted from the light projection element. An element, a second light receiving element for directly receiving light from the light emitting element, first signal switching means for alternately switching inputs from the first and second light receiving elements in response to a clock signal, Amplifying means for amplifying the output of the signal switching means; a first sample and hold circuit for holding the amplified output immediately before driving the light emitting element; and a second sample and hold circuit for holding the amplified output immediately after the light emitting element emits the light.
And a differential circuit for obtaining an output difference between the first and second sample-and-hold circuits, and an output of the differential circuit when the output of the first light receiving element is selected by the first signal switching means. Signal processing means for identifying an object to be detected, an oscillation circuit for outputting a clock signal to the first signal switching means, and holding the output of the differential circuit when the output of the second light receiving element is selected by the first signal switching means. And a driving means for intermittently driving the light emitting element in accordance with the output of the oscillation circuit so that the output becomes a constant level.

[0007]

According to the first aspect of the present invention having such a feature, the light emitting element is driven intermittently, the light is reflected by the object in the detection area, and the first light receiving element and the second light Provided to the light receiving element. The output is switched by the first signal switching means, the output of the first light receiving element is given to the signal processing circuit, the output of the second light receiving element is given to the driving means via the amplifying means, Is controlled to be a constant level. Therefore, the output obtained through the same amplifier circuit is controlled so as to be constant, so that the influence of the temperature characteristics of the light emitting element and the amplifier circuit, aging, and the like can be eliminated. In the invention of claim 3 of the present application, the object detection signal and the signal for adjusting the light amount are determined based on the difference between the amplified outputs before and after the light is projected by the light emitting element. Therefore, the influence of low frequency noise and the like can be reduced.

[0008]

FIG. 1 is a block diagram showing the overall configuration of a photoelectric sensor according to an embodiment of the present invention. In the figure, V / I
The converter 1 converts an input voltage into a current, and a light emitting element, for example, a light emitting diode 2 is connected to an output terminal thereof. In the vicinity of the light emitting diode 2, as shown in FIG.
The monitor photodiode 3, which is a light receiving element, is mounted in close proximity. The light from the light projecting element 2 is applied to the object detection area, and if there is an object that reflects light in the object detection area, the reflected light is transmitted to the photodiode 4 as the first light receiving element. An analog multiplexer 5 is connected to the photodiodes 3 and 4. The photoelectric sensor is provided with an oscillation circuit 6 that oscillates a clock signal at a predetermined cycle. The oscillation circuit 6 includes a clock signal CK1 having a constant cycle T and a clock signal CK2 obtained by dividing the output of the clock signal CK1.
have. The first clock signal CK1 is supplied to the analog switch 7, and the second clock signal CK2 is supplied to the delay circuit 8 and the sample / hold circuit (S / H circuit) 9. The delay circuit 8 is an off-delay type delay circuit for delaying the fall of its output, and its output is given to an analog multiplexer 5 and an analog demultiplexer 10 as first and second signal switching means.

FIG. 2A shows a specific example of the circuit of the analog multiplexer 5.
a and 5b are connected to the input 1 and the input 2 and their outputs are connected in common and connected to the output terminal. These analog switches 5a and 5b selectively transmit the input 1 or 2 to the output terminal by the output of the delay circuit 8 and the inverter 5c for inverting the output. The output selected in this way is provided to the I / V converter 11. The I / V converter 11 converts the light receiving current of the photodiodes 3 and 4 as light receiving elements into a voltage, and its output is transmitted to the analog demultiplexer 10 via the amplifier circuit 12. The demultiplexer 10 is obtained by inverting the input and output of FIG. 2A and outputs an amplified output to the comparator 13 or the sample and hold circuit 9 alternatively. Here, the photodiode 3 for monitoring is controlled by the analog multiplexer 5.
Is selected, the output of the demultiplexer 10 is transmitted to the sample and hold circuit 9, and when the photodiode 4 for detection is selected by the analog multiplexer 5, the output of the amplifier circuit 12 is transmitted to the comparator 13. The comparator 13 incorporates a gate circuit (not shown), which discriminates an input signal at a predetermined threshold level in synchronization with an oscillation output.
The output is given to the signal processing circuit 14. The comparator 13 and the signal processing circuit 14 constituting the signal processing means are the same as the ordinary photoelectric sensor, and output an object detection signal when a signal exceeding a predetermined level is continuously obtained.

The sample and hold circuit 9 is shown in FIG.
As shown in (1), the analog switch 9a is operated by the clock signal CK2 of the oscillation circuit 6, and a capacitor C for holding its output and a voltage follower 9b are provided. The differential / integration circuit 15 is an operational amplifier in which a feedback capacitor is connected between the input and output terminals, and the reference voltage source Vref is connected to the non-inverting input terminal. The differential / integration circuit 15 outputs the reference voltage Vre
The differential voltage signal is output so as to be equal to f, and the output is supplied to the V / I converter 1 via the analog switch 7. The V / I converter 1 drives the light emitting diode 2 by converting this voltage value into a current. Here, the sample hold circuit 9, the differential / integration circuit 15, the analog switch 7, and the V / I converter 1 hold the output of the amplifier circuit 12 when the light receiving output of the second light receiving element is selected. Driving means for intermittently driving the light emitting element so that the output becomes constant is configured.

FIG. 3C is a circuit diagram showing an example of the delay circuit 8, in which a clock signal CK2 is supplied to a transistor Q1.
And a signal is delayed for a predetermined time by a CR time constant circuit, and the output is inverted and output by an inverter 8a.

Next, the operation of this embodiment will be described with reference to a time chart. FIG. 3 is a time chart showing the waveform of each part of the present embodiment, and FIGS.
3 shows waveforms of respective portions a to l. The oscillation circuit 6 oscillates a clock CK1 having a predetermined period and a clock CK2 obtained by dividing the output thereof as shown in FIGS. 3A and 3B. When the clock signal CK1 is "H", the analog switch As shown in FIG. 3 (l), the output of the differential / integration circuit 15 is transmitted to the V / I converter 1, and the light emitting diode 2 emits light intermittently. Now, it is assumed that light-receiving signals as shown in FIGS. 3D and 3E are obtained from the monitoring photodiode 3 and the light-receiving photodiode 4 which receive the light, respectively. The clock CK2 of the oscillation circuit 6 is supplied to the delay circuit 8 as shown in FIGS.
, And the photodiodes 3 and 4 are switched by the delay output. When the output (c) of the delay circuit 8 is "H", the photodiode 3 is selected and the I / O
3 (f) through the V converter 11 and the amplifier circuit 12,
It is amplified as shown in (g). Then, as shown in FIG. 3 (i), the output is transmitted to the sample and hold circuit 9 by the analog demultiplexer 10. On the other hand, when the output of the delay circuit 8 is “L”, the analog multiplexer 5 and the demultiplexer 10
Is supplied to the comparator 13 via the I / V converter 11, the amplifier circuit 12, and the demultiplexer 10.
Here, the switching timing of the multiplexer 5 and the demultiplexer 10 is such that the interval between the monitoring photodiodes is short, but this may be reversed.

The light-receiving signals alternately taken in are amplified through the I / V converter 11 and the amplifier circuit 12. The comparator 13 and the signal processing circuit 14 perform the same processing as a normal photoelectric sensor, and output an object detection signal according to the level of an input signal. The sample hold circuit 9 is sampled when the clock signal CK2 is "H" as shown in FIGS. 3 (i) and 3 (j), and is held at the falling edge so that the DC voltage can be compared with the reference voltage Vref. Converted to a signal. This signal is supplied to the differential / integration circuit 15
Is compared with the reference voltage Vref, and feedback is applied so that the light emission amount becomes constant. The output is converted into a pulse signal by a clock signal CK 1 by an analog switch 7, and the light emitting diode 2 is driven via the V / I converter 1. In this case, the monitor signal and the detection signal pass through the same amplifier circuit.
Differences and variations in temperature characteristics due to differences in the amplifier circuits of the C circuit can be eliminated.

FIG. 4 is a block diagram showing a configuration of a photoelectric sensor according to a second embodiment of the present invention. In the above-described first embodiment, the outputs of the photodiodes 3 and 4 for receiving light are led to the I / V converter 11 via the analog multiplexer 5. However, if they pass through the multiplexer, the S / N ratio may deteriorate. obtain. Therefore, in the second embodiment, the outputs of the photodiodes 3 and 4 are supplied to the I / V converters 21 and 22 to convert them into voltage signals as shown. Then, the outputs of these I / V converters are connected to the analog multiplexer 5, and their outputs are switched and applied to the amplifier circuit 12. The other configuration is the same as that of the first embodiment described above, and a detailed description thereof will be omitted. In this embodiment, it is preferable to use parts having the same characteristics such as pair transistors so that the characteristics of the I / V converters 21 and 22 match. This has the effect of eliminating the difference in temperature characteristics between the detection circuit, the APC circuit, and the amplifier circuit without deteriorating the S / N ratio.

FIG. 5 is a block diagram showing a third embodiment of the present invention. When the light emitting cycle is short or when a low-frequency noise component is superimposed on the signal, the signal from the light emitting element is
It may fluctuate as shown in (e). Therefore, if the signal is switched by the analog multiplexer as it is, a correct light receiving signal cannot be sent to the subsequent circuit.
Therefore, in the third embodiment, the difference between the level immediately before the light receiving signal is input and the peak of the light receiving signal is detected.

In FIG. 5, the same parts as those in the first embodiment are denoted by the same reference numerals, and the detailed description is omitted. In this embodiment, the oscillation circuit 6A uses the same clocks CK1 and CK2 as those in the first embodiment.
In addition, a third clock signal CK3 that falls when the clock CK1 rises is output. CK2 is applied to the analog multiplexer 5 via the delay circuit 8.
Also, the photocurrent of the photodiode 3 or 4 selected by the analog multiplexer 5 is converted into a voltage signal by the I / V converter 11 and amplified by the amplifier circuit 12, as in the first embodiment described above. In the present embodiment, first and second sample hold circuits 32 and 31 are connected to the output side of the amplifier circuit 12. The sample and hold circuit 31 samples and holds the input by a clock signal CK 1, and the sample and hold circuit 32 samples and holds the input by a clock signal CK 3, and each output is given to a differential circuit 33. The differential circuit 33 supplies the differential output to the comparator 13 and the sample hold circuit 9. The clock signal CK1 of the oscillation circuit 6A is supplied to the frequency divider 34. Divider 3
4 divides the clock signal by 、 and generates the clock signal CK
A gate signal at a timing different from that of the comparator 2
3 is output. Other configurations are the same as those of the first
Since this is the same as the embodiment, a detailed description is omitted.

Next, the operation of this embodiment will be described. FIGS. 6A to 6K are waveform diagrams showing waveforms of the respective parts a to k in FIG. In this figure, the oscillation circuit 6A is shown in FIGS.
As shown in FIG. 6C, the clock signals CK1 to CK3 are oscillating, and the light emitting diode 2 is turned on by the clock signal CK1 in FIG. This signal is received by the photodiodes 3 and 4, and is passed through the analog multiplexer 5, the I / V converter 11 and the amplifier circuit 12,
It is assumed that the signal shown in (e) is obtained. As described above, low-frequency noise is superimposed on this signal. The sample-and-hold circuit 31 outputs the clock signal C shown in FIG.
Since the signal is sampled by K1 and held at the falling edge, the peak value of each light receiving signal is held as shown in FIG. The sample and hold circuit 32 outputs a clock signal CK3 falling at the rising edge of CK1.
Since the signal is sampled and held at the falling edge, the level immediately before the light is input is held as shown in FIG. Accordingly, by taking the difference between the preceding and succeeding signals in the differential circuit 33, a pure light receiving signal level can be obtained as shown in FIG. This signal is supplied to the comparator 13 and the sample hold circuit 9. As described above, since the comparator 13 is gated by the signal obtained by dividing the signal CK1 of the oscillation circuit 6A, it is necessary to extract only the differential signal when the photodiode 4 for detection is selected by the multiplexer 5. Can be. The sample and hold circuit 9 samples with the clock signal CK2 and holds the signal.
The differential / integration circuit 15 converts only the differential signal when
Can be given to The other processes are the same as those in the first embodiment, and thus detailed description is omitted. In this way, even when the received light signal fluctuates due to low frequency noise or the like, a pure detection signal component and a monitor signal component can be extracted, and the same effect as in the first embodiment can be obtained. .

In the first embodiment, the output of the amplifier circuit 1 is switched to the comparator 13 and the sample-and-hold circuit 9 by using the demultiplexer 10, but a gate signal is supplied to the comparator 13 as in the third embodiment. Thus, the multiplexer 4 allows the photodiode 4
May be provided to the comparator 13 when is selected. Also, in the third embodiment, the output of the differential circuit 33 is connected to the demultiplexer 1 as in the first embodiment.
It is also possible to switch between the comparator 13 and the sample-and-hold circuit via 0 to output.

[0019]

As described above in detail, according to the first and second aspects of the present invention, the correction amount of the automatic projection light amount correcting circuit for adjusting the light amount of the light emitting element includes the light receiving element and its amplification circuit. Feedback is applied to make the light emission level constant. Accordingly, the output level of the light emitting element can be adjusted as a whole of the photoelectric sensor, and the effect of reducing the fluctuation of the detection distance due to temperature change, aging, and the like can be obtained. According to the third and fourth aspects of the present invention, even when noise such as low-frequency noise is superimposed on the received light signal, the effect is obtained that only the received light signal can be extracted and the level of the projected light signal can be corrected. .

[Brief description of the drawings]

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

2A is a circuit diagram illustrating an example of a configuration of an analog multiplexer; FIG. 2B is a circuit diagram illustrating a configuration example of a sample-hold circuit; and FIG.

FIG. 3 is a time chart showing waveforms at various points in the first embodiment.

FIG. 4 is a block diagram showing a configuration of a second exemplary embodiment of the present invention.

FIG. 5 is a block diagram showing an overall configuration of a photoelectric sensor according to a third embodiment of the present invention.

FIG. 6 is a time chart showing waveforms at various points in the third embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 V / I converter 2 Light emitting diode 3, 4 Photodiode 5 Analog multiplexer 6, 6A Oscillation circuit 7 Analog switch 8 Delay circuit 9, 31, 32 Sample hold circuit 10 Analog demultiplexer 11, 21, 22 I / V converter 12 amplifying circuit 13 comparator 14 signal processing circuit 15 differential / integrating circuit 33 differential circuit 34 frequency divider

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-1-283725 (JP, A) JP-A-3-261219 (JP, A) JP-A-59-119646 (JP, U) (58) Investigation Field (Int.Cl. ⁷ , DB name) H03K 17/78

Claims (4)

(57) [Claims] A light-emitting element for irradiating the object detection area with light; a first light-receiving element for receiving reflected light obtained from the object detection area among light emitted from the light-emitting element; A second light receiving element for directly receiving light from the element, a first signal switching means for alternately switching an input from the first and second light receiving elements in accordance with a clock signal, and a first signal switching means Amplifying means for amplifying the output of the first light-receiving element; signal processing means for identifying a detected object in an object detection area by an output of the amplifying means when the output of the first light receiving element is selected by the first signal switching means; An oscillation circuit for outputting a clock signal to the signal switching means, and an output of the amplifying means being held when the output of the second light receiving element is selected by the first signal switching means, and the output being at a constant level. The oscillation times The photoelectric sensor characterized by comprising driving means for driving intermittently the light emitting element, the according to the output of. 2. An output of said amplifying means is provided, and said first and second signals are switched in response to signal switching by said first signal switching means.
2. The photoelectric sensor according to claim 1, further comprising second signal switching means for separating the output of said light receiving element and supplying it to a signal processing means and a driving means, respectively. 3. A light projecting element for irradiating the object detection area with light, a first light receiving element for receiving reflected light obtained from the object detection area out of light emitted from the light projecting element, and A second light receiving element for directly receiving light from the element, a first signal switching means for alternately switching an input from the first and second light receiving elements in accordance with a clock signal, and a first signal switching means Amplifying means for amplifying the output of the light emitting element, a first sample and hold circuit for holding an amplified output immediately before driving the light emitting element, and a second sample and hold circuit for holding an amplified output immediately after light emitting of the light emitting element A differential circuit for obtaining an output difference between the first and second sample-and-hold circuits; and an object detection area based on an output of the differential circuit when an output of the first light receiving element is selected by the first signal switching means. Signal processing for identifying detected objects Means, an oscillation circuit for outputting a clock signal to the first signal switching means, and an output of the differential circuit when an output of the second light receiving element is selected by the first signal switching means. Driving means for intermittently driving the light emitting element in accordance with the output of the oscillation circuit so that the output becomes a constant level. 4. An output of said differential circuit is provided, and said first and second signals are output in response to a switching signal of said first signal switching means.
4. The photoelectric sensor according to claim 3, further comprising second signal switching means for providing an output from said light receiving element to a signal processing means and a driving means, respectively.