JPH05152923A - Photoelectric switch - Google Patents

Abstract

PURPOSE:To effectively eliminate burst noise consecutive for plural light emitting pulses in the photoelectric switch driving intermittently a light emitting element. CONSTITUTION:The switch is provided with a sample-and-hold circuit 12 holding a light receiving level just before a light emitting clock pulse is emitted. Based on the output, threshold level Vth of a comparator 14 is set to discriminate a light receiving signal. The output is given to a gate circuit 15 to extract a signal coincident with a light emitting pulse and the signal is processed to obtain an object detection section.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectric switch, and more particularly to a photoelectric switch characterized by processing received light signals thereof.

[0002]

2. Description of the Related Art FIG. 5 is a block diagram showing an example of a conventional photoelectric switch. In the figure, the photoelectric switch has an oscillating circuit 1 for generating a clock signal, and its output is given to a frequency dividing circuit 2. The frequency dividing circuit 2 divides this signal to generate a light emitting pulse, and its output is given to the current amplifying circuit 3. The current amplifying circuit 3 is for illuminating the light projecting element 4 with a pulse. Now, the light receiving element 5 is provided at a position facing the light projecting element 4 or at a position where the reflected light from the object is received, and the waveform of the signal is shaped through the amplifier circuit 6 and the comparator 7. Then, the light emitting pulse of the frequency dividing circuit 2 is used as a gate signal, and the output is integrated and waveform shaped by the signal processing circuit 8 to output an object detection signal.

[0003]

However, since the output obtained from the light receiving element 5 is gated by the light projecting pulse as shown in FIG. 6A, the output shown in FIGS. 6B and 6C is obtained. Impulsive noise is removed. However, when burst noise such that light is continuously applied is applied to the light receiving element 5, the light receiving signal is the same as that for which a light receiving signal is continuously obtained for several periods. Therefore, as shown in FIG. 6 (c), there is a drawback that it malfunctions and outputs an incorrect object detection signal.

The inventions according to claims 1 to 3 of the present application have been made in view of the above problems of the conventional photoelectric switch, and accurately detect an object by the light reception level before and after the projection of the projection pulse. Making it possible is a technical issue.

[0005]

According to a first aspect of the present invention, an oscillation circuit that periodically oscillates a light emitting pulse, a light emitting element, and a current amplifier circuit that drives the light emitting element by the light emitting pulse. A light receiving element that receives the light emitted from the light emitting element, a sample hold circuit that holds the output of the light receiving element at the timing immediately before the irradiation of the light emitting pulse, and a reference determined based on the output of the sample hold circuit. A comparator for comparing the electric potential and the output of the light receiving element,
A gate circuit that opens and closes the output of the comparator by a signal corresponding to the light emission clock of the oscillation circuit.

According to a second aspect of the present invention, a shift register which receives an output of the gate circuit and shifts by a signal corresponding to a light emitting clock pulse, and an AND circuit which performs a logical product of parallel outputs of the shift register, It is characterized by including.

Further, the invention of claim 3 of the present application is characterized by comprising an integrating circuit for integrating the output of the gate circuit, and a comparator for discriminating the output of the integrating circuit by a predetermined threshold value.

[0008]

According to the invention of claim 1 of the present application having such a feature, the output of the light receiving element is held at the timing immediately before the irradiation of the light projecting pulse, and the threshold value of the comparator is set based on the level. ing. Then, the comparator compares this threshold value with the level of the received light signal. Therefore, when a burst signal is continuously applied, the light receiving level does not change significantly due to the light emitting pulse.
No output is obtained from the comparator. Even if a signal is obtained at the start of the burst signal or the like, since the signal processing is performed through the gate circuit that uses the signal corresponding to the light projection clock as the gate signal, an erroneous object detection signal will not be obtained.

[0009]

1 is a block diagram showing the overall construction of a photoelectric switch according to a first embodiment of the present invention. In this figure, the same parts as those in the conventional example described above are designated by the same reference numerals, and detailed description thereof will be omitted. In this embodiment, the output of the oscillator circuit 1 is given to the frequency divider circuit 11. The frequency dividing circuit 11 frequency-divides the output of the oscillation circuit 1 to generate a light emitting pulse and also generates a switching signal immediately before. The light projection pulse is given to the current amplification circuit 3, and the light projection element 4 is driven. Then, the light receiving element 5 is arranged at a position opposed to this or at a position for receiving the reflected light via the object detection area.
The light receiving element 5 converts an optical signal into an electric signal, and its output is given to the sample hold circuit 12 via the amplifier circuit 6 and the capacitor C1. The sample hold circuit 12 is composed of a switch SW that is opened and closed by a switching signal from the frequency divider circuit 11 and a capacitor connected between its output and the ground terminal. The sample hold circuit 12 holds the light reception output immediately before the rise of the light projection pulse, and the output is given to one end of the resistor R2 via the voltage follower 13. Further, a constant voltage V is supplied from a stabilized voltage source (not shown).
Power is supplied to each part by being supplied with cc, and a series circuit including resistors R1 and R2 is connected to one end of this power supply.
The midpoint of the resistors R1 and R2 is given to the inverting input terminal of the comparator 14 as a threshold value Vth. The terminal of the capacitor C1 is connected to the non-inverting input terminal of the comparator 14 via the input resistor. The comparator 14 compares the threshold value Vth with the received light signal, and its output is given to the gate circuit 15. The gate circuit 15 uses the light emitting pulse of the frequency dividing circuit 11 as a gate signal and supplies a comparison signal to the shift register 16 only while the light emitting pulse is obtained. The shift register 16 determines whether or not the signal is continuous with the light emitting pulse as a shift pulse, and the output thereof is determined by the AND circuit 17 and output.

Next, the operation of this embodiment will be described with reference to the time charts of FIGS. 2 (a) to 3 (e) are waveform charts showing the waveforms of the respective portions a to e in FIG. 2 is a time chart when there is no object, and FIG. 3 is a time chart when the object is close to the detection area. Figure 2 (a)
2 shows the light projection pulse output from the frequency dividing circuit 11, and FIG. 2B shows the switching signal of the switch SW of the sample hold circuit 12. Now, when there is no object in the object detection area and impulsive noise or burst noise is applied, the light receiving signal at the output end of the amplifier circuit 6 is shown by the solid line in FIG. 2C, for example. Here, as shown in FIG. 2B, the amplified output is transmitted to the sample hold circuit 12 only when the switch SW is in the ON state, and is held by the voltage follower 13. Then, this level is raised to a high level by the voltage dividing circuit of the resistors R1 and R2 and added as a threshold value Vth of the comparator 14. The alternate long and short dash line shown in FIG. 2C shows the change in the threshold value Vth. Now, the output from the comparator 14 is obtained as shown in FIG. 2D only when the received light signal is higher than the threshold value Vth. Since this signal is input to the gate circuit 15 and opened / closed by using the light projection pulse as a gate signal, a signal cannot be obtained for spike noise and burst noise as shown in FIG.

When an object arrives at the detection area as shown in FIG. 3C, a light reception signal corresponding to the light projection pulse is obtained. In this case, the value held by the sample hold circuit 12 is always constant at the level immediately before the emission pulse is emitted. Therefore, the threshold Vth is shown in FIG.
It is fixed at the level shown by the alternate long and short dash line in (c). Therefore, the signal shown in FIG. 3D is continuously obtained from the comparator 14. Since this signal is synchronized with the projection pulse, it passes through the gate circuit 15 and shift register 16
A signal is continuously obtained. Therefore, the logical product condition is satisfied after the signals are detected from the predetermined number of light emitting pulses, and the object detection signal can be output from the AND circuit 17.

In this embodiment, the output of the comparator is identified through the gate circuit, and the continuity of the signal is detected by the shift register and the AND circuit. However, as shown in FIG. 4, the output of the gate circuit 15 is integrated circuit. 21 may be given. It is also possible to configure so that the object detection signal is output when the integrating circuit 21 integrates and the comparator 22 exceeds a predetermined threshold value.

In the above-mentioned embodiment, the threshold value Vth of the comparator is pulled up to the power supply side based on the outputs of the sample and hold circuit 12 and the voltage follower 13 so that a slightly higher level becomes the threshold value. A threshold voltage may be applied to the output by adding a constant voltage. It is also possible to divide the output of the voltage follower 13 into a threshold value of the comparator 14 and to input the input of the amplifier circuit 6 via the resistance voltage dividing circuit to the input circuit thereof.

[0014]

As described in detail above, according to the present invention, a stable operation can be performed without malfunction due to bursty noise that continues for a long time and without malfunction due to spike noise. be able to. In this way, there is no malfunction even with respect to burst noise, continuous noise, and ambient light, and only the object can be detected.

[Brief description of drawings]

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

FIG. 2 is a time chart showing the operation of the present embodiment when noise is added.

FIG. 3 is a time chart showing the operation of this embodiment when an object is detected.

FIG. 4 is a block diagram showing an overall configuration of a photoelectric switch according to a second embodiment of the present invention.

FIG. 5 is a block diagram showing an example of a conventional photoelectric switch.

FIG. 6 is a time chart showing the operation of a conventional photoelectric switch.

[Explanation of symbols]

 1 oscillation circuit 3 current amplification circuit 4 light emitting element 5 light receiving element 6 amplification circuit 11 frequency dividing circuit 12 sample hold circuit 13 voltage follower 14 comparator 15 gate circuit 16 shift register 17 AND circuit 21 integrating circuit 22 comparator

Claims (3)

[Claims] 1. An oscillation circuit that periodically oscillates a light projecting pulse, a light projecting element, a current amplifier circuit that drives the light projecting element by the light projecting pulse, and a light emitted from the light projecting element. A light receiving element for receiving light, a sample hold circuit for holding the output of the light receiving element at a timing immediately before irradiation of the light projecting pulse, a reference potential determined based on the output of the sample hold circuit, and an output of the light receiving element. A photoelectric switch, comprising: a comparator for comparison; and a gate circuit that opens and closes an output of the comparator by a signal corresponding to a light emission clock of the oscillation circuit. 2. A shift register provided with an output of the gate circuit and shifted by a signal corresponding to a light-emission clock pulse, and an AND circuit for taking a logical product of parallel outputs of the shift register. The photoelectric switch according to claim 1. 3. The photoelectric switch according to claim 1, further comprising: an integrator circuit that integrates the output of the gate circuit, and a comparator that discriminates the output of the integrator circuit with a predetermined threshold value.