JP2007205775A - Photoelectric sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photoelectric sensor which minimizes the influence of interfered light even when a plurality of individual photoelectric sensors are used, arranged in a row. <P>SOLUTION: The photoelectric sensor comprises a light emitting element 10, a photodiode 20, a current-voltage conversion circuit 40, a coupling capacitor Co, an amplifier circuit 50, a CPU 60, and the like. An analog switch Sw is provided between the current-voltage conversion circuit and the coupling capacitor Co. The analog switch Sw is turned off during a period other than during a signal of self emission light is read, thereby controlling to allow the circuit not to capture any signals. When the photoelectric sensors are used, arranged in a row, even when light emitted from an adjacent photoelectric sensor enters as interfered light during a period other than the signal reading period during which a signal of self emission light is read, its influence is thereby excluded. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a photoelectric sensor.

Conventionally, a photoelectric sensor including a light projecting element and a light receiving element is widely known. In this device, light is emitted from a light projecting element toward an object to be detected, and light reflected on the object to be detected (or transmitted light) is received by the light receiving element. Then, based on the signal level of the light reception signal output from the light receiving element, the presence / absence, position, displacement, etc. of the detected object are measured.
FIG. 9 shows the light projection timing of the light projecting element and the waveform of the received light signal. In this way, the light reception signal is output with a slight delay with respect to the light projection timing. In addition, undershoot occurs at the signal end of the signal. This is because the capacitor (for example, for removing the DC component from the light receiving signal) included in the circuit is charged / discharged as the signal passes. Caused by waking up. As an example of this type of photoelectric sensor, the following patent document has been proposed.
Japanese Patent Laid-Open No. 10-19673

In recent years, as shown in FIG. 10, a plurality of the above-described photoelectric sensors (each of which is independent) may be used side by side. In this example, a light projecting element 1a and a light receiving element 2a, a light projecting element 1b and a light receiving element 2b, a light projecting element 1c and a light receiving element 2c are paired, and three photoelectric sensors are used.
When the photoelectric sensor is provided in this manner, light projected by one photoelectric sensor (light projecting element) to perform detection operation enters the other photoelectric sensor (light receiving element) adjacent thereto as interference light. This can lead to measurement errors.

In particular, even if the light projection timing between the sensors is shifted by some method, it is affected by the interference light in the following cases.
For example, in FIG. 11, the light projection timing of the light projecting element 1b and the light reception signal of the light receiving element 2b are shown in the upper stage, and the light projection timing of the light projecting element 1a and the light reception signal of the light receiving element 2a are shown in the lower stage, respectively. ing. Here, although the light projecting timing of the light projecting element 1a does not overlap the light projecting timing of the light projecting element 1b, as described above, the light reception signal is output with a slight delay from the light projecting timing, The end undershoots. Therefore, when the light emitted from the light projecting element 1a enters the light receiving element 2b (indicated by the alternate long and short dash line in the figure), the undershoot overlaps with the normal light receiving signal, and the light receiving level of the normal light receiving signal is lowered. .
The present invention has been completed based on the above situation, and provides a photoelectric sensor capable of minimizing the influence of interference light even when a plurality of independent photoelectric sensors are used side by side. Objective.

  As means for achieving the above-mentioned object, the invention of claim 1 is characterized in that a light projecting element that projects light toward the object to be detected at predetermined intervals, light received from the light projecting element, A light-receiving element that outputs a light-receiving signal according to the amount; and a light-receiving unit that captures the light-receiving signal at a synchronization timing synchronized with the light-projecting timing of the light-projecting element and detects the object to be detected based on the signal level The light receiving means is characterized in that the signal output from the light receiving element is blocked or the signal level is reduced in a predetermined period before the synchronization timing.

  According to a second aspect of the present invention, the light receiving means according to the first aspect is based on a filter that removes a frequency component of a predetermined band from the received light signal and a level of the received light signal that has passed through the filter. A processing unit for detecting a detection object, an analog switch provided in an input stage of the filter, and a predetermined period before the synchronization timing are output from the light receiving element by controlling the analog switch to be in an open state. And a control unit for blocking signal input.

  According to a third aspect of the present invention, in the first or second aspect of the present invention, the light receiving means interferes based on the presence or absence of a signal output from the light receiving element in the interference light detection period immediately before the synchronization timing. Interference light detecting means for detecting light, and interference avoiding means for performing a predetermined interference avoiding operation when interference light is detected by the interference light detecting means, and excluding the interference detection period from the predetermined period. In this period, the signal output from the light receiving element is blocked or the signal level is lowered.

<Invention of Claim 1>
According to the first aspect of the present invention, even if there is interference light incident and a signal is output from the light receiving element in a predetermined period before the synchronization timing, the light receiving means blocks the input of the signal or Reduce the level. In this way, if the signal due to the interference light is suppressed, the undershoot of the signal due to the interference light is inevitably suppressed, so that it is emitted from the pair of light projecting elements and received at the synchronization timing. The level of the received light signal is kept at the correct value. Thereby, the interference prevention effect with respect to excessive interference light and the interference prevention effect when the light projection period is short are obtained, and it is possible to provide a photoelectric sensor with excellent reliability. Excessive interference light occurs, for example, when a light projecting element of an adjacent photoelectric sensor is arranged closer than a pair of light projecting elements.

<Invention of Claim 2>
According to the invention of claim 2, the analog switch is controlled to be in the open state by the control unit for a predetermined period before the synchronization timing. As a result, it is possible to prevent a signal caused by the interference light from being input to the filter, and hence the subsequent circuit.

<Invention of Claim 3>
According to the invention of claim 3, in the interference light detection period, the interference light detection means performs the interference light detection operation, and when the interference light is detected, the interference avoidance means performs a predetermined interference avoidance operation. . Thereby, even if the interference light is incident immediately before the synchronization timing, the influence can be eliminated. In addition, during the period excluding the interference detection period from the predetermined period, the light receiving means performs a process of blocking the input of the signal or reducing the level of the input signal. Even if interference light enters during the period excluding the interference detection period, the influence can be eliminated.

<Embodiment 1>
A first embodiment of the present invention will be described with reference to FIGS.
FIG. 1 is a block diagram showing the electrical configuration of a photoelectric sensor. In FIG. 1, reference numeral 10 denotes a light projecting element, reference numeral 20 denotes a photodiode as a light receiving element, and reference numeral 30 denotes a signal processing unit (in the light receiving means of the present invention). Equivalent). The photodiode 20 is connected to the power supply line Vc through a resistor R while being reverse-biased.

  The signal processing unit 30 includes a current-voltage conversion circuit 40, a coupling capacitor Co, an amplification circuit 50, and a CPU 60. The input terminal of the current / voltage conversion circuit 40 is connected to the cathode terminal K of the photodiode 20. The coupling capacitor Co corresponds to the “filter for removing a frequency component of a predetermined band from the light reception signal” of the present invention.

  The CPU 60 controls the entire photoelectric sensor, and has a function of determining the light projection timing of the light projecting element 10 and the timing for reading the light reception signal output from the photodiode 20 (hereinafter referred to as synchronization timing). . An analog switch Sw is interposed between the output stage of the current-voltage conversion circuit 40 and the input stage of the coupling capacitor Co. The analog switch Sw is supplied with a gate signal G from the CPU 60. During a period in which the gate signal G is input, the analog switch Sw is turned ON to allow the input of a signal to the coupling capacitor Co. During a period when no signal is input, the signal is turned off to prohibit the input of a signal to the coupling capacitor Co.

  Incidentally, the opening / closing control function of the analog switch Sw by the CPU 60 realizes a “control unit that blocks input of a signal output from the light receiving element by controlling the analog switch to an open state” of the present invention. .

Hereinafter, a specific detection operation will be described.
When the detection operation is started, as shown in FIG. 2, the CPU 60 generates a light projection timing signal Sc repeated at a predetermined cycle, and outputs this to the light projecting element 10. Thereby, detection light is emitted from the light projecting element 10 at a predetermined period.

  And the light radiate | emitted from the light projection element 10 is reflected by the workpiece | work W as a to-be-detected object, and the reflected light enters into the photodiode 20. FIG. As a result, a photocurrent io corresponding to the amount of received light flows from the cathode terminal of the photodiode 20, and this is converted into a voltage signal (hereinafter referred to as a light reception signal) by the current-voltage conversion circuit 40.

  Thereafter, the DC component of the received light signal is removed by the coupling capacitor Co. FIG. 2 shows the waveform at the point a in FIG. 1 (the waveform at the time of output from the coupling capacitor Co) as the waveform of the received light signal. As shown in the figure, the waveform at point a (hereinafter referred to as light reception signal Sr) has an undershoot at the signal termination portion, which is the charge accumulated in the coupling capacitor Co (DC component of the light reception signal). This is because the discharge occurs after the signal passes.

  The light reception signal Sr from which the DC component has been removed is then input to the amplifier circuit 50, amplified there, and then input to the input port 61 of the CPU 60. Although not shown in the figure, an A / D converter is provided at the input stage of the CPU 60, and the amplified received light signal Sr is converted into a digital value and then taken into the CPU 60. Yes.

  Then, the CPU 60 reads the input port 61 as shown in FIG. 2 for a predetermined period (hereinafter referred to as signal reading) from the timing (synchronous timing) shifted by the light receiving operation delay α with respect to the light projecting timing on the light projecting side. Called a period).

  As a result, the received light signal can be captured at a timing synchronized with the light projecting side. Then, the CPU 60 performs processing for determining the presence / absence of a workpiece based on the level of the received light reception signal. Once the detection is started, a series of such detection operations are repeated. Hereinafter, the light emitted from the light projecting element 10 paired with the photodiode 20 is referred to as self-projected light. Further, the processing function performed by the “processing unit” of the present invention is realized by the determination processing by the CPU 60.

  As described above, the CPU 60 performs a light receiving signal reading operation in accordance with the light projecting timing on the light projecting side. For this reason, even if a plurality of photoelectric sensors are used in combination, only a signal incident at a timing synchronized with its own light projection timing can be taken in as a light reception signal.

  However, for example, as shown in the lower part of FIG. 2, when adjacent photoelectric sensors are projected at a timing close to their own projection timing and incident as interference light, the received light signal Sz output from the photodiode 20 There is a possibility that the terminal portion (undershoot) overlaps the signal reading period on the self-projecting side.

  Therefore, in the present embodiment, the analog switch Sw is turned OFF (the input of the gate signal G is prohibited by the CPU 60) during a period other than the signal reading period on the self-projecting side (F period in FIG. 2). Therefore, no signal is input to the subsequent circuit after the analog switch Sw (corresponding to “blocking input of a signal output from the light receiving element” of the present invention). As a result, the coupling capacitor Co is charged during a period other than the signal reading period and does not discharge itself during the signal reading period.

  Accordingly, the signal level of the light reception signal Sr can be correctly maintained at the original value, and measurement errors caused by interference light can be eliminated. Especially, when the interference prevention effect against excessive interference light and the light projection period are short. An interference preventing effect can be obtained, and a highly reliable photoelectric sensor can be provided. Excessive interference light occurs, for example, when a light projecting element of an adjacent photoelectric sensor is arranged closer than a pair of light projecting elements.

  Although the description will be repeated, it is indispensable to provide the analog switch Sw before the coupling capacitor Co in order to obtain the above effect. If the analog switch Sw is provided at the subsequent stage of the coupling capacitor Co, the coupling capacitor Co is charged if a signal is input during the period even if the analog switch Sw is turned off. Then, when the analog switch is turned on in order to capture the light reception signal of the self-projection light, the coupling capacitor Co is in a charged state and discharge of the charge starts, so that the light reception signal Sr of the self-projection light is affected. Because it gives.

<Embodiment 2>
Next, Embodiment 2 of the present invention will be described with reference to FIG.
In the second embodiment, an interference light detection function and an interference avoidance function are newly added to the photoelectric sensor of the first embodiment, and other configurations are the same as those of the first embodiment.

  Specifically, in the first embodiment, the analog switch Sw is turned OFF for the entire F period in FIG. 3, but in the second embodiment, in each F period, the H period immediately before the synchronization timing ( With respect to the interference light detection period of the present invention, the analog switch Sw is turned on, and the signal reading operation for the input port 61 is performed by the CPU 60 during the same H period. ing.

  With such a configuration, a timing (for example, as shown in FIG. 3) is obtained by detecting whether or not a light reception signal is input to the input port 61 during the H period (detected by the CPU 60). It is possible to determine the presence or absence of incoming interference light in the lower part.

  The CPU 60 “detects the presence / absence of a light reception signal during the H period” to realize the “function fulfilled by the interference light detection means” of the present invention.

  When the interference light is detected, the CPU 60 stops the output of the light projection timing signal Sc for a predetermined period, and thereafter, delay processing (interference avoiding operation) for outputting the light projection timing signal Sc is performed again. It should be noted that the delay function of the CPU 60 realizes “the function performed by the interference avoiding means” of the present invention.

  As a result, the self-projected light is shifted to a position where it does not overlap the interference light. Therefore, after the light projection timing is adjusted, the workpiece W can be detected without being affected by the interference light.

  Even after the light projection timing is adjusted by the previous interference avoiding operation, the interference light itself does not disappear, and therefore the portion other than the H period in the F period is the analog switch Sw as in the first embodiment. Must be turned off to restrict the input of signals to the coupling capacitor Co. In this way, during other periods except for the signal processing period, the influence of the charging and discharging of the coupling capacitor Co (reduction in the level of the light reception signal Sr) can be eliminated by preventing the coupling capacitor Co from capturing the signal as much as possible.

<Embodiment 3>
A third embodiment of the present invention will be described with reference to FIGS.
In the third embodiment, noise countermeasures for disturbance light (for example, light of an inverter-type fluorescent lamp that is lit at a high frequency) are intended to further improve measurement accuracy. Specifically, the light projecting element is driven by a burst signal having a higher frequency than the frequency band of disturbance light. Thereby, the signal output from the light receiving element also becomes a high-frequency signal. Therefore, the output high-frequency signal is applied to a filter (amplifier circuit 120 in this embodiment) that includes the disturbance light frequency band in the stop band, and only the frequency component higher than the disturbance light frequency band is extracted, thereby the disturbance light component. Can be suppressed.

  In this embodiment, the circuit configuration of the signal processing unit 100 is changed from the configuration of the first embodiment in accordance with the burst driving of the light projecting element. That is, the signal processing unit 100 includes a current-voltage conversion circuit 110, a coupling capacitor C1, an amplifier circuit 120, a rectifier circuit 130, a low-pass filter (corresponding to a filter that removes a frequency component of a predetermined band from a light reception signal of the present invention) 140, and a CPU 150. is doing. An analog switch Sw is provided at the input stage of the low-pass filter 140 (see FIG. 4).

Hereinafter, a specific detection operation will be briefly described with reference to FIGS. 4 and 5.
When the detection operation is started, the CPU 150 generates the light projection signal St by the burst signal B having continuous pulses, and outputs this to the light projecting element 10. As a result, the light projecting element 10 is turned on at a high frequency.

  Then, the light emitted from the light projecting element 10 is reflected by the workpiece W and then received by the photodiode 20. As a result, a photocurrent corresponding to the amount of received light is output from the photodiode 20. The photocurrent is converted into a voltage signal (light reception signal) by the current-voltage conversion circuit 110, and then the direct current component is removed by the coupling capacitor C1.

  Thereafter, only a frequency component higher than the frequency band of the disturbance light is amplified by the amplification circuit 120 in the received light signal. Thereby, the signal component in the frequency band including disturbance light is suppressed (removal of noise components).

  However, since the light reception signal (shown as light reception signal 1 in FIG. 5) output from the amplifier circuit 120 is a high-frequency signal similar to the burst signal B, even if it is directly input to the CPU 150, the signal level Cannot be read correctly. For this reason, rectification is performed by the next rectifier circuit 130 and, after that, the high-frequency component is further removed by the low-pass filter 140, and then input to the CPU 150.

  FIG. 5 shows the waveform of the received light signal after passing through the low-pass filter 140 (the waveform at the point b in FIG. 4) as the rectified signal 2. Undershoot is seen at the end of the signal. Therefore, in this embodiment as well, as in the first embodiment, the analog switch Sw is turned off by the CPU 150 during the period before the synchronization timing in order to eliminate the influence of the interference light.

  Thus, for example, even if the adjacent photoelectric sensor performs burst light projection at the timing shown in the lower part of FIG. 5 and the photodiode 20 receives the burst light, the received light signal (interference light is received and output). Light reception signal) is not input to the low-pass filter 140.

  As shown in FIG. 4, the signal processing unit 100 also includes a coupling capacitor C1. However, since the signal passing through the signal processing unit 100 is a high-frequency signal, undershoot is small and the influence is small. In other words, the undershoot at the signal end appears larger as the frequency becomes lower, so the analog switch Sw may be provided at least before the input stage of the low-pass filter 140.

  In this embodiment, the analog switch Sw is disposed immediately before the low-pass filter 140, but this suppresses the influence of switching noise when the analog switch Sw is opened and closed as much as possible (assuming that the analog switch Sw is connected to the circuit). This is because noise is amplified by the amplifier circuit 120 or the like if it is provided near the input stage.

<Embodiment 4>
A fourth embodiment of the present invention will be described with reference to FIG.
In the third embodiment, an analog switch Sw is provided between the rectifier circuit 130 and the low-pass filter 140, and is turned off for a predetermined period before the synchronization timing, so that the signal to the low-pass filter 140 can be transmitted during the synchronization period. Although the input is restricted, the fourth embodiment adopts a configuration in which the signal itself is suppressed by controlling the amplification factor instead of the ON / OFF control of the analog switch Sw.

  Specifically, two types of gain are set in the amplifier circuit 160 in FIG. 6, and the switching can be performed by a control signal output from the CPU 150. That is, by reducing the amplification factor for a predetermined period before the synchronization timing, it is possible to suppress a signal input before the synchronization timing (a signal due to interference light). As an example of the configuration for switching the gain, when the amplifier circuit 160 is configured by a grounded emitter circuit, the collector resistance may be varied.

<Other embodiments>
The present invention is not limited to the embodiments described with reference to the above description and drawings. For example, the following embodiments are also included in the technical scope of the present invention, and further, within the scope not departing from the gist of the invention other than the following. Various modifications can be made.

(1) In the first embodiment, the level of the signal input during the F period is lowered by controlling the ON / OFF of the analog switch Sw, and in the fourth embodiment, the gain of the amplifier circuit 120 is adjusted. In addition, the signal can be lowered by turning ON / OFF the reverse bias of the photodiode 20 (see FIG. 7, reverse bias OFF in the F period). That is, if the reverse bias is removed, the signal output from the photodiode 20 becomes smaller than when the reverse bias is applied.
In addition, if the light projecting element performs burst projection as in the third embodiment, the signal level can be lowered by switching the filter. That is, two types of filters having different passbands are prepared in advance, and when a signal is to be passed, a filter including the band of the same signal in the passband is selected. A filter including the band in the stop band may be selected.

(2) In the first embodiment, the analog switch Sw is turned off for the entire other period (F period) excluding the signal reading period. The analog switch Sw may be turned on. For example, as shown in FIG. 8, it is a predetermined period (J period in the figure) immediately after the signal reading period.

1 is a block diagram showing an electrical configuration of a photoelectric sensor according to Embodiment 1. FIG. Diagram showing operation timing of analog switch The figure which shows the operation timing of the analog switch which concerns on Embodiment 2. The block diagram which shows the electric constitution of the photoelectric sensor based on Embodiment 3. FIG. The figure which shows the state where the undershoot has occurred at the end of the signal after passing through the low pass filter The block diagram which shows the electrical constitution of the photoelectric sensor based on Embodiment 4. FIG. Block diagram of an embodiment for turning ON / OFF reverse bias The figure which shows the example which operated the analog switch Sw about the part with little influence even if interference light enters. The figure which showed the light emission timing of a light projection element, and the waveform of a received light signal The figure which shows the state which used multiple photoelectric sensors side by side The figure which shows the relation between the light reception signal which originates in interference light and the light reception signal of self-projection

Explanation of symbols

10 ... Photoelectric sensor 20 ... Photodiode (light receiving element)
30 ... Signal processing unit (light receiving means)
50 ... Amplification circuit 60 ... CPU
Sw ... Analog switch Co ... Coupling capacitor (filter)

Claims (3)

A light projecting element that projects light at predetermined intervals toward the object to be detected;
A light receiving element that receives light from the light projecting element and outputs a light reception signal corresponding to the amount of light received;
In a photoelectric sensor comprising: a light receiving unit that captures the light reception signal at a synchronization timing synchronized with the light projection timing of the light projecting element, and detects the detected object based on the signal level;
The light receiving means is
A photoelectric sensor, wherein input of a signal output from the light receiving element is blocked or signal level is reduced in a predetermined period before the synchronization timing. The light receiving means is
A filter for removing a frequency component of a predetermined band from the received light signal;
A processing unit for detecting the detected object based on the level of the received light signal that has passed through the filter;
An analog switch provided in the input stage of the filter;
2. The control unit according to claim 1, further comprising: a control unit configured to block an input of a signal output from the light receiving element by controlling the analog switch in an open state during the predetermined period before the synchronization timing. Photoelectric sensor. The light receiving means is
Interference light detection means for detecting interference light based on the presence or absence of a signal output from the light receiving element in the interference light detection period immediately before the synchronization timing;
An interference avoidance means for performing a predetermined interference avoidance operation when interference light is detected by the interference light detection means,
3. The photoelectric device according to claim 1, wherein an input of a signal output from the light receiving element is blocked or a signal level is lowered during a period excluding the interference detection period from the predetermined period. Sensor.

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