JP5667337B2 - Reflective photoelectric switch and object detection method - Google Patents

Description

  The present invention relates to a reflection type photoelectric switch, and more particularly to a reflection type photoelectric switch and an object detection method for detecting whether the distance to an object is far or close to a predetermined reference distance.

  Conventionally, as one of the reflection type photoelectric switches, a distance setting reflection type (Background Suppression, hereinafter abbreviated as BGS) photoelectric switch for detecting whether the distance from the photoelectric switch to an object is greater than or close to a predetermined reference distance has been proposed. Known (for example, refer to Patent Document 1 and Patent Document 2). According to such a BGS photoelectric switch, only an object can be detected without detecting the background.

  On the other hand, a laser measuring instrument using interference (self-coupling effect) in the semiconductor laser between the output light of the laser and the return light from the measurement object has been proposed as a distance meter using the interference of light by the laser ( For example, refer nonpatent literature 1, nonpatent literature 2, and nonpatent literature 3). FIG. 12 shows a composite resonator model of an FP type (Fabry-Perot type) semiconductor laser. In FIG. 12, 101 is a semiconductor laser, 102 is a wall opening of a semiconductor crystal, 103 is a photodiode, and 104 is an object to be measured.

When the oscillation wavelength of the laser is λ and the distance from the wall open surface 102 closer to the measurement target 104 to the measurement target 104 is L, the return light from the measurement target 104 and the resonator 101 are satisfied when the following resonance condition is satisfied. The inner laser beams strengthen each other, and the laser output increases slightly.
L = qλ / 2 (1)
In Formula (1), q is an integer. This phenomenon can be sufficiently observed even if the scattered light from the measurement object 104 is very weak, because the apparent reflectance in the resonator 101 of the semiconductor laser increases, causing an amplification effect.

  Since the semiconductor laser emits laser beams having different frequencies according to the magnitude of the injection current, an external modulator is not required when modulating the oscillation frequency, and direct modulation is possible by the injection current. FIG. 13 is a diagram showing the relationship between the oscillation wavelength and the output waveform of the photodiode 103 when the oscillation wavelength of the semiconductor laser is changed at a certain rate. When L = qλ / 2 shown in Expression (1) is satisfied, the phase difference between the return light and the laser light in the resonator 101 becomes 0 ° (the same phase), and the return light and the resonator 101 When L = qλ / 2 + λ / 4, the phase difference becomes 180 ° (opposite phase), and the return light and the laser light in the resonator 101 are the weakest. Therefore, when the oscillation wavelength of the semiconductor laser is changed, a place where the laser output becomes strong and a place where the laser output becomes weak appear alternately, and the laser output at this time is detected by the photodiode 103 provided in the resonator 101. As shown in FIG. 13, a stepped waveform having a constant period is obtained. Such a waveform is generally called an interference fringe.

  Each stepped waveform, that is, each interference fringe is called a mode pop pulse (hereinafter referred to as MHP). MHP is a phenomenon different from the mode hopping phenomenon. For example, if the number of MHPs is 10 when the distance to the measurement object 104 is L1, the number of MHPs is 5 at half the distance L2. That is, when the oscillation wavelength of the semiconductor laser is changed for a certain time, the number of MHPs changes in proportion to the measurement distance. Therefore, if the MHP is detected by the photodiode 103 and the frequency of the MHP is measured, the distance can be easily measured.

  A BGS photoelectric switch can be realized by using the self-coupled laser measuring instrument as described above. The BGS photoelectric switch may be determined on / off based on whether the object is at a short distance or a long distance compared to a predetermined reference distance. Therefore, when a self-coupled laser measuring instrument is used as a BGS photoelectric switch, whether the average period of the measured MHP is longer or shorter than the known reference period of the MHP when the object is at the reference distance. Can be judged. When the average period of the measured MHP is longer than the known reference period of the MHP when the object is at the position of the reference distance, it is determined that the object exists at a shorter distance than the reference distance, and the measurement is performed. When the cycle of the MHP is short, it is determined that the object is present at a distance longer than the reference distance.

JP-A 63-102135 Japanese Unexamined Patent Publication No. 63-187237 Tadashi Ueda, Satoshi Yamada, Susumu Shito, “Distance Meter Using Self-Coupling Effect of Semiconductor Laser”, Proceedings of the 1994 Tokai Branch Joint Conference of Electrical Engineering Society, 1994 Satoshi Yamada, Susumu Shito, Norio Tsuda, Tadashi Ueda, “Study on a small rangefinder using the self-coupling effect of a semiconductor laser”, Aichi Institute of Technology research report, No. 31 B, p. 35-42, 1996 Guido Giuliani, Michele Norgia, Silvano Donati and Thierry Bosch, “Laser diode self-mixing technique for sensing applications”, JOURNAL OF OPTICS A: PURE AND APPLIED OPTICS, p. 283-294, 2002

  As described above, a BGS photoelectric switch can be realized by using a self-coupled laser measuring instrument. However, simply determining the average period of MHP and comparing it with the reference period results in poor determination accuracy. Therefore, using the method proposed by the inventor in Japanese Patent Application No. 2007-015020, the frequency distribution of the MHP period is obtained, the representative value of the distribution such as the median or the mode value is obtained, and the representative value of the distribution of this period is obtained. If the distance to the object is calculated based on the frequency distribution of the period and the calculated distance is compared with the reference distance, the determination accuracy can be improved. However, such a method requires a memory and a computer, and there is a problem that the cost of the BGS photoelectric switch increases.

  When an object is present closer to the reference distance, the MHP cycle distribution shifts to a longer side than the reference cycle Th as shown by a distribution 40 in FIG. On the other hand, when an object is present at a position farther than the reference distance, the distribution of the MHP cycle shifts to a shorter one than the reference cycle Th as shown by a distribution 41 in FIG. Therefore, if the number Nlong of MHPs having a longer period than the reference period Th is compared with the number Nshort of MHPs having a shorter period than the reference period Th, the distance of the object can be determined with a simple and inexpensive configuration. In this determination method, when Nlong> Nshort is satisfied, it is determined that the object is present at a shorter distance than the reference distance. When Nlong <Nshort is satisfied, it is determined that the object is present at a longer distance than the reference distance. .

  However, in the determination method for comparing the number Nlong of MHPs having a longer period than the reference period Th with the number Nshort of MHPs having a shorter period than the reference period Th, noise such as disturbance light is counted as MHP, or the signal teeth Since there is an MHP that cannot be counted due to missing, an error may occur in the period of the MHP to be measured. Therefore, there is a possibility that the determination is erroneous when the object is located near the reference distance.

The present invention has been made to solve the above-described problems, and an object of the present invention is to realize a highly accurate reflective photoelectric switch with a simple and inexpensive configuration using a self-coupled laser measuring instrument.
Another object of the present invention is to realize a reflective photoelectric switch that can correct a measurement error of a cycle and can accurately determine the distance of an object.

  The reflective photoelectric switch of the present invention includes a semiconductor laser that emits laser light, a laser driver that operates the semiconductor laser, a laser light emitted from the semiconductor laser, and a return from an object that exists in front of the semiconductor laser. Detecting means for detecting an electrical signal including an interference waveform caused by a self-coupling effect with light; and a period measuring means for measuring the period of the interference waveform included in the output signal of the detecting means every time the interference waveform is input; When the period of the interference waveform when the object is at a reference distance is defined as a reference period, the frequency of the period of the interference waveform measured by the period measuring unit is a first predetermined number of the reference period. A frequency N1 of a period less than twice, a frequency N2 of a period not less than a first predetermined number times the reference period and less than the reference period, and a second place of the reference period and not less than the reference period Counting means for classifying the frequency into a frequency N3 having a period less than several times (first predetermined number <second predetermined number) and a frequency N4 having a period not less than a second predetermined number times the reference period; Compare the frequencies N1 and N4 and the sum of frequencies (N2 + N3). If the frequency N1 is the largest, it is determined that the object is located at a distance farther than the reference distance. If the frequency N4 is the largest, When the object is determined to be present at a shorter distance than the reference distance, and the sum of the frequencies (N2 + N3) is the largest, the magnitudes of the frequencies N2 and N3 are compared, and when the frequency N2 is large, It is determined that an object is present at a longer distance than the reference distance, and a determination unit that determines that the object is present at a shorter distance than the reference distance when the frequency N3 is large is provided. is there.

In addition, one configuration example of the reflective photoelectric switch of the present invention further includes a frequency correction unit that calculates a correction value N2 ′ of the frequency N2 by N2 ′ = N2−N1, and the determination unit includes the frequency N1 and N4 are compared with the sum of the frequencies (N2 + N3), and when the frequency N1 is the largest, it is determined that the object exists at a distance farther than the reference distance, and when the frequency N4 is the largest, When it is determined that the object is present at a shorter distance than the reference distance and the sum of the frequencies (N2 + N3) is the largest, the correction value N2 ′ of the frequency is compared with the magnitude of the frequency N3, and the correction value N2 ′ is compared. Is larger than the reference distance, it is determined that the object is located at a longer distance than the reference distance. If the frequency N3 is greater, it is determined that the object is located at a shorter distance than the reference distance.
In addition, one configuration example of the reflective photoelectric switch of the present invention further includes a frequency correction unit that calculates the correction value N3 ′ of the frequency N3 by N3 ′ = N3 + N1, and the determination unit includes the frequencies N1 and N4. The magnitudes of the sums of frequencies (N2 + N3) are compared. When the frequency N1 is the largest, it is determined that the object is located at a distance farther than the reference distance, and when the frequency N4 is the largest, the object is When it is determined that the distance is closer than the reference distance, and the sum of the frequencies (N2 + N3) is the largest, the frequency N2 is compared with the correction value N3 ′ of the frequency, and when the frequency N2 is large, It is determined that the object exists at a longer distance than the reference distance, and when the correction value N3 ′ is large, it is determined that the object exists at a shorter distance than the reference distance.

In one configuration example of the reflective photoelectric switch of the present invention, the laser driver includes a first oscillation period including at least a period in which the oscillation wavelength continuously increases monotonously and a period in which the oscillation wavelength continuously decreases monotonously. The semiconductor laser is operated so that at least the second oscillation period including at least alternately exists. Further, the frequencies N1, N2, N3, and N4 obtained by the counting means are calculated for each oscillation period. Normalizing means for obtaining the sum N1 ″, N2 ″, N3 ″, N4 ″ of the frequencies of the first oscillation period and the second oscillation period for each normalized frequency N1, N2, N3, N4 And frequency correction means for calculating the correction value N2 ′ of the frequency N2 ″ by N2 ′ = N2 ″ −N1 ″, and the determination means is the sum of the frequencies N1 ″ and N4 ″ and the frequency (N2 ″ + N3) )) When the frequency N1 ″ is the largest, it is determined that the object is at a distance farther than the reference distance, and when the frequency N4 ″ is the largest, it is determined that the object is at a shorter distance than the reference distance. When the sum of the frequencies (N2 ″ + N3 ″) is the largest, the correction value N2 ′ of the frequency is compared with the magnitude of the frequency N3 ″, and when the correction value N2 ′ is large, the object is the reference When the frequency N3 ″ is larger than the distance, it is determined that the object exists at a shorter distance than the reference distance.
In one configuration example of the reflective photoelectric switch of the present invention, the laser driver includes a first oscillation period including at least a period in which the oscillation wavelength continuously increases monotonously and a period in which the oscillation wavelength continuously decreases monotonously. The semiconductor laser is operated so that at least the second oscillation period including at least alternately exists. Further, the frequencies N1, N2, N3, and N4 obtained by the counting means are calculated for each oscillation period. Normalizing means for obtaining the sum N1 ″, N2 ″, N3 ″, N4 ″ of the frequencies of the first oscillation period and the second oscillation period for each normalized frequency N1, N2, N3, N4 And frequency correction means for calculating the correction value N3 ′ of the frequency N3 ″ by N3 ′ = N3 ″ + N1 ″, and the determination means is the sum of the frequencies N1 ″ and N4 ″ and the frequency (N2 ″ + N3 ″) ) Compare the size of When the frequency N1 ″ is the largest, it is determined that the object is at a distance farther than the reference distance, and when the frequency N4 ″ is the largest, it is determined that the object is at a shorter distance than the reference distance. When the sum of frequencies (N2 ″ + N3 ″) is the largest, the frequency N2 ″ is compared with the frequency correction value N3 ′, and when the frequency N2 ″ is large, the object is the reference distance. If the correction value N3 ′ is large, it is determined that the object is present at a shorter distance than the reference distance.
In one configuration example of the reflective photoelectric switch of the present invention, the first predetermined number is 0.5, and the second predetermined number is 1.5.

  The object detection method of the present invention includes an oscillation procedure for operating a semiconductor laser by supplying a drive current to the semiconductor laser, a laser beam emitted from the semiconductor laser, and an object existing in front of the semiconductor laser. A detection procedure for detecting an electrical signal including an interference waveform caused by a self-coupling effect with return light, and a period of the interference waveform included in an output signal obtained by the detection procedure is measured every time the interference waveform is input. When the period of the interference waveform measured by the period measurement procedure and the period of the interference waveform measured by the period measurement procedure is defined as a reference period when the period of the interference waveform when the object is at the position of the reference distance is a reference period, A frequency N1 having a period less than the first predetermined number of times, a frequency N2 having a period not less than the first predetermined number of times and less than the reference period, and a second not less than the reference period and the second reference period. A counting procedure for classifying the frequency N4 into a frequency N3 of a period less than a predetermined number of times (first predetermined number <second predetermined number) and a frequency N4 of a period equal to or greater than a second predetermined number of times of the reference period; When the frequencies N1 and N4 are compared with the sum of the frequencies (N2 + N3), and the frequency N1 is the largest, it is determined that the object is located farther than the reference distance, and the frequency N4 is the largest Determines that the object is present at a shorter distance than the reference distance, and when the sum of the frequencies (N2 + N3) is the largest, the magnitudes N2 and N3 are compared, and if the frequency N2 is large, It is determined that the object is present at a longer distance than the reference distance, and a determination procedure for determining that the object is present at a shorter distance than the reference distance when the frequency N3 is large is provided. It is.

  According to the present invention, it is possible to correctly determine whether the distance from the reflective photoelectric switch to the object is longer or closer than the reference distance by eliminating the influence of missing mode hop pulses and excessive noise detection during period measurement. . Further, in the present invention, the period measuring means, the counting means, and the determining means can be realized with a simple configuration, and a highly accurate reflective photoelectric switch can be realized with a simple and inexpensive configuration.

  Further, in the present invention, by providing the frequency correction means, it is possible to more effectively remove the influence of excessive noise detection during period measurement, and it is possible to further improve the accuracy of determining the distance of an object.

[First Embodiment]
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing a configuration of a BGS photoelectric switch according to a first embodiment of the present invention.
The BGS photoelectric switch in FIG. 1 collects and emits a semiconductor laser 1 that emits laser light, a photodiode 2 that converts the optical output of the semiconductor laser 1 into an electrical signal, and the light from the semiconductor laser 1. A lens 3 that collects the return light from the object 10 and makes it incident on the semiconductor laser 1, a laser driver 4 that drives the semiconductor laser 1, and a current-voltage conversion that converts the output current of the photodiode 2 into a voltage and amplifies it. The amplification unit 5, the filter unit 6 that removes the carrier wave from the output voltage of the current-voltage conversion amplification unit 5, the MHP cycle included in the output voltage of the filter unit 6 is measured, and the frequency of the MHP cycle is used as a reference cycle. A periodic classification unit 7 that performs classification based on a value based thereon, a determination unit 8 that determines whether the object 10 is closer or farther than a predetermined reference distance based on the classification result of the periodic classification unit 7, and a determination unit 8 And a display unit 9 for displaying the determination result.

  The photodiode 2 and the current-voltage conversion amplification unit 5 constitute detection means. Hereinafter, for ease of explanation, it is assumed that a semiconductor laser 1 of a type that does not have a mode hopping phenomenon (VCSEL type, DFB laser type) is used.

  The laser driver 4 supplies a triangular wave drive current that repeatedly increases and decreases at a constant change rate with respect to time to the semiconductor laser 1 as an injection current. As a result, the semiconductor laser 1 has a first oscillation period in which the oscillation wavelength continuously increases at a constant change rate in proportion to the magnitude of the injection current, and a first oscillation period in which the oscillation wavelength continuously decreases at a constant change rate. It is driven to alternately repeat the two oscillation periods. FIG. 2 is a diagram showing the change over time of the oscillation wavelength of the semiconductor laser 1. In FIG. 2, P1 is the first oscillation period, P2 is the second oscillation period, λa is the minimum value of the oscillation wavelength in each period, λb is the maximum value of the oscillation wavelength in each period, and Tt is the period of the triangular wave. In the present embodiment, the maximum value λb of the oscillation wavelength and the minimum value λa of the oscillation wavelength are always constant, and the difference λb−λa is also always constant.

  Laser light emitted from the semiconductor laser 1 is collected by the lens 3 and enters the object 10. The light reflected by the object 10 is collected by the lens 3 and enters the semiconductor laser 1. However, condensing by the lens 3 is not essential. The photodiode 2 is disposed in the semiconductor laser 1 or in the vicinity thereof, and converts the optical output of the semiconductor laser 1 into a current. The current-voltage conversion amplification unit 5 converts the output current of the photodiode 2 into a voltage and amplifies it.

  The filter unit 6 has a function of extracting a superimposed signal from the modulated wave. FIG. 3A is a diagram schematically illustrating an output voltage waveform of the current-voltage conversion amplification unit 5, and FIG. 3B is a diagram schematically illustrating an output voltage waveform of the filter unit 6. In these figures, the oscillation waveform (carrier wave) of the semiconductor laser 1 in FIG. 2 is removed from the waveform (modulation wave) in FIG. 3A corresponding to the output of the photodiode 2, and the MHP in FIG. A process of extracting a waveform (interference waveform) is shown.

The period discriminating unit 7 measures the period of MHP included in the output voltage of the filter unit 6, and uses the MHP known period (hereinafter referred to as the reference period Th) when the object 10 is located at a predetermined reference distance. The frequency of the MHP period is classified based on the value based on it.
FIG. 4 is a block diagram showing the configuration of the period sorting unit 7. The period sorting unit 7 includes a period measurement unit 70 and a counting unit 71. The period measurement unit 70 includes a rising edge detection unit 72 and a time measurement unit 73.

  FIG. 5 is a diagram for explaining the operation of the period measurement unit 70, and schematically shows the output voltage waveform of the filter unit 6, that is, the MHP waveform. In FIG. 5, H1 is a threshold value for detecting the rising edge of MHP.

  The rise detection unit 72 detects the rise of MHP by comparing the output voltage of the filter unit 6 with the threshold value H1. The time measuring unit 73 measures a time tuu (that is, the MHP cycle) from the rising edge of the MHP to the next rising edge based on the detection result of the rising edge detecting unit 72. The time measurement unit 72 performs such measurement every time the rising edge of MHP is detected.

  The counting unit 71 sets the frequency of the MHP cycle T measured by the cycle measuring unit 70 to a frequency N1 of a cycle less than 0.5 times the reference cycle Th (0.5Th> T) and 0. 0 of the reference cycle Th. The frequency N2 of the cycle of 5 times or more and less than the reference cycle Th (0.5Th ≦ T <Th) and the cycle of the cycle of the reference cycle Th or more and less than 1.5 times the reference cycle Th (Th ≦ T <1.5Th) The frequency N3 and the frequency N4 having a period 1.5 times or more (1.5 Th ≦ T) of the reference period Th are classified.

  As described above, the period separation unit 7 separates the frequency of the MHP period. The period separation unit 7 measures the MHP period for each measurement period (in the present embodiment, each of the first oscillation period P1 and the second oscillation period P2), and separates the frequency of the period.

  Next, the determination unit 8 determines whether the object 10 is closer or farther than the reference distance from the measurement result of the period sorting unit 7. The determination unit 8 compares the magnitudes N1 and N4 of the MHP period with the magnitude (N2 + N3) of the frequencies, and determines that the object 10 exists at a distance farther than the reference distance when the frequency N1 is the largest. When N4 is the largest, it is determined that the object 10 exists at a shorter distance than the reference distance. Further, the determination unit 8 compares the frequencies N2 and N3 when the sum of frequencies (N2 + N3) is the largest. If the frequency N2 is greater than the frequency N3, the object 10 is present at a distance greater than the reference distance. If the frequency N3 is greater than the frequency N2, it is determined that the object 10 exists at a shorter distance than the reference distance.

The determination unit 8 performs such determination for each measurement period (in the present embodiment, each of the first oscillation period P1 and the second oscillation period P2) in which the period classification unit 7 measures and classifies the MHP period. To do.
The display unit 9 displays the determination result of the determination unit 8.

  6 and 7 are diagrams for explaining the determination principle of the present embodiment. FIG. 6 is a diagram showing a frequency distribution of MHP periods when a waveform is missing, and FIG. 7 is a period due to noise. It is a figure which shows frequency distribution of the period of MHP at the time of being divided into two. 6 and 7, T0 is a representative value (median value or mode value, etc.) of the frequency distribution a of the original period of MHP.

For example, if the lack of MHP (detection omission) occurs during the period measurement because the intensity of the MHP is small, the MHP period at the place where the lack occurred is approximately twice the original period, and this was caused by the lack. The frequency distribution of the MHP cycle is a normal distribution centered on 2T0 (b in FIG. 6). This frequency distribution b is similar to the frequency distribution a of the original period of MHP.
On the other hand, if noise is erroneously detected as MHP during the period measurement, the MHP period is divided into two at a random rate. At this time, the frequency distribution of the period of the MHP divided into two as a result of excessively counting the noise becomes a symmetric distribution with respect to 0.5T0 (c in FIG. 7).

  In the present embodiment, the frequency of the MHP cycle is divided into four as described above, and when the frequency N1 of the cycle less than 0.5 times the reference cycle Th is the largest, the frequency N1 is not caused by noise. Assuming that the period is the original period of MHP, it is determined that the object 10 exists at a longer distance than the reference distance. Further, when the frequency N4 of the cycle 1.5 times or more the reference cycle Th is the largest, the frequency N4 is not caused by the lack of MHP but is regarded as the original cycle of the MHP, and the object 10 is the reference distance. It is determined that it exists at a short distance. If the distance of the object 10 cannot be determined by the frequency N1 or N4, the frequencies N1 and N4 are ignored, and the frequency N2 and the reference cycle Th which are 0.5 times the reference cycle Th and less than the reference cycle Th are exceeded. In addition, the perspective of the object 10 is determined by comparing the frequency N3 with a period less than 1.5 times the reference period Th.

Thus, in this embodiment, the influence of missing MHP and excessive noise detection during period measurement is removed, and the distance from the BGS photoelectric switch to the object 10 (more precisely, the distance from the semiconductor laser 1 to the object 10). It is possible to correctly determine whether is far from or near the reference distance.
The period separation unit 7 and the determination unit 8 may be realized by a computer including a CPU, a storage device and an interface, and a program stored in the storage device, or may be realized by hardware. In this embodiment, since only the measurement of the MHP period, the period classification, and the frequency comparison are sufficient, the period classification unit 7 and the determination unit 8 can be realized with a simple configuration.

[Second Embodiment]
Next, a second embodiment of the present invention will be described. Also in the present embodiment, since the entire configuration of the BGS photoelectric switch is the same as that of the first embodiment, description will be made using the reference numerals in FIG.
FIG. 8 is a block diagram showing a configuration of the period sorting unit 7 of the BGS photoelectric switch according to the second embodiment of the present invention. The period sorting unit 7 of the present embodiment is obtained by adding a frequency correction unit 74 to the configuration of the first embodiment. The operations of the period measuring unit 70 and the counting unit 71 are the same as those in the first embodiment.

The frequency correction unit 74 includes a correction value N2 ′ of a frequency N2 of a period not less than 0.5 times the reference period Th and less than the reference period Th, and a frequency having a period not less than the reference period Th and less than 1.5 times the reference period Th. A correction value N3 ′ for N3 is calculated as follows.
N2 '= N2-N1 (2)
N3 ′ = N3 + N1 (3)

  Then, the frequency correction unit 74 notifies the determination unit 8 of the frequencies N1, N2, N3, and N4 obtained by the counting unit 71 and the correction values N2 'and N3' obtained by itself. Similar to the first embodiment, the cycle sorting unit 7 sorts the frequency of the MHP cycle for each measurement period, and calculates the correction value of the frequency. The frequency correction unit 74 may calculate either one of the correction values N2 'and N3'.

  Next, the determination unit 8 of the present embodiment compares the magnitudes N1 and N4 of the MHP cycle with the magnitude (N2 + N3) of the frequencies, and when the frequency N1 is the largest, the object 10 is farther than the reference distance. When it is determined that the object exists at a distance and the frequency N4 is the largest, it is determined that the object 10 is present at a shorter distance than the reference distance. The determination unit 8 compares the frequency correction value N2 ′ with the frequency N3 when the sum of frequencies (N2 + N3) is the largest. If the correction value N2 ′ is greater than the frequency N3, the object 10 is the reference distance. If the frequency N3 is larger than the correction value N2 ′, it is determined that the object 10 exists at a shorter distance than the reference distance. The determination unit 8 performs such determination for each measurement period.

When the frequency correction unit 74 calculates the correction value N3 ′, the determination unit 8 performs the following determination. That is, the determination unit 8 compares the magnitudes N1 and N4 of the MHP cycle with the sum of the frequencies (N2 + N3), and determines that the object 10 exists at a distance greater than the reference distance when the frequency N1 is the largest. When the frequency N4 is the largest, it is determined that the object 10 exists at a shorter distance than the reference distance. The determination unit 8 compares the frequency N2 with the frequency correction value N3 ′ when the frequency sum (N2 + N3) is the largest, and when the frequency N2 is greater than the frequency correction value N3 ′, the object 10 is If it is determined that the object exists at a distance longer than the reference distance, and the correction value N3 ′ of the frequency is larger than the frequency N2, it is determined that the object 10 exists at a shorter distance than the reference distance.
Other configurations of the BGS photoelectric switch are as described in the first embodiment.

  As shown in FIG. 7, the frequency distribution c of the MHP period divided into two as a result of excessively counting noise during the period measurement often overlaps the frequency distribution a of the original period of the MHP. Therefore, in the present embodiment, the frequency N2 of the period that is 0.5 times or more of the reference period Th and less than the reference period Th is corrected as shown in Expression (2), and 1.5 or more of the reference period Th and 1.5 of the reference period Th. The frequency N3 having a period less than double is corrected as shown in Expression (3).

  Thus, in this embodiment, the influence of excessive noise detection at the time of period measurement can be more effectively removed, and the accuracy of determining the perspective of the object 10 can be further improved as compared with the first embodiment. Can do.

[Third Embodiment]
Next, a third embodiment of the present invention will be described. Also in this embodiment, since the configuration of the BGS photoelectric switch is the same as that of the first embodiment, description will be made using the reference numerals in FIGS.
The operation of the cycle measuring unit 70 of the cycle sorting unit 7 is the same as that of the first embodiment.

  As in the first embodiment, the counting unit 71 of the present embodiment sets the frequency of the MHP cycle T measured by the cycle measurement unit 70 to less than 0.5 times the reference cycle Th (0.5Th> T), the frequency N1 of the period 0.5 or more times the reference period Th and less than the reference period Th (0.5Th ≦ T <Th), and the frequency N1 of the reference period Th 1 or more. The frequency is divided into four: frequency N3 having a cycle less than 5 times (Th ≦ T <1.5Th) and frequency N4 having a cycle 1.5 times or more (1.5Th ≦ T) of the reference cycle Th.

Similar to the first embodiment, the determination unit 8 compares the magnitudes N1 and N4 of the MHP cycle with the magnitude (N2 + N3) of the frequencies, and when the frequency N1 is the largest, the object 10 is larger than the reference distance. Is determined to exist at a long distance, and when the frequency N4 is the largest, it is determined that the object 10 exists at a short distance from the reference distance. In addition, when the sum of frequencies (N2 + N3) is the largest, the determination unit 8 is in the vicinity of the cycle nTh and less than the cycle nTh with reference to a cycle nTh that is an integral multiple of the reference cycle Th (n is an integer of 2 or more). Is added to the frequency N2, and the frequency of the cycle T in the vicinity of the cycle nTh and not less than the cycle nTh is added to the frequency N3. For example, the determination unit 8 adds the frequency of a cycle 1.5 times or more and less than 2 times the reference cycle Th to the frequency N2, and adds the frequency of a cycle 2 times or more and less than 2.5 times the reference cycle Th to the frequency N3. Then, the determination unit 8 compares the magnitudes of the frequencies N2 and N3 after the addition, and when the frequency N2 is greater than the frequency N3, the determination unit 8 determines that the object 10 exists at a distance farther than the reference distance, and is greater than the frequency N2. When the frequency N3 is large, it is determined that the object 10 exists at a shorter distance than the reference distance.
Other configurations of the BGS photoelectric switch are as described in the first embodiment.

The present embodiment shows a correction method in the case where missing MHP occurs during measurement of the MHP cycle, and is effective when N3 <N2 <(N3 + N4) holds in the measurement result of the cycle measurement unit 70. is there.
When this embodiment is used in combination with the second embodiment, it is necessary to use the correction value N2 ′. The addition is performed on the correction value N2 ′ and the frequency N3.

[Fourth Embodiment]
In the first to third embodiments, it may be erroneously determined that the object 10 is not stationary. The reason is that if the object 10 existing in front of the semiconductor laser is moving in the direction approaching the BGS photoelectric switch during the oscillation period, the number of MHPs increases in the first oscillation period P1 (the MHP cycle is short). This is because the number of MHPs decreases (the MHP cycle becomes longer) in the second oscillation period P2.

  FIG. 9 shows the frequency distribution of the MHP period when there is a division of the MHP period due to noise or a missing waveform and the object 10 is not stationary. Assuming that the frequency ratio of the division of the MHP period due to noise and the loss of the waveform is substantially the same in the first oscillation period P1 and the second oscillation period P2, the object is obtained by determining the frequency after normalizing the frequency. Even if 10 moves, it is possible to correctly determine whether the distance from the BGS photoelectric switch to the object 10 (more precisely, the distance from the semiconductor laser 1 to the object 10) is far from or close to the reference distance.

Also in the present embodiment, since the entire configuration of the BGS photoelectric switch is the same as that of the first embodiment, description will be made using the reference numerals in FIG.
FIG. 10 is a block diagram showing a configuration of the period sorting unit 7 of the BGS photoelectric switch according to the fourth embodiment of the present invention. The period separation unit 7 of the present embodiment is obtained by adding a normalization unit 75 and a frequency correction unit 76 to the configuration of the first embodiment. The operations of the period measuring unit 70 and the counting unit 71 are the same as those in the first embodiment.

  In the present embodiment, the frequencies N1, N2, N3, and N4 obtained by the counting unit 71 in the first oscillation period P1 are N1 (P1), N2 (P1), N3 (P1), and N4 (P1), respectively. The frequencies N1, N2, N3, and N4 obtained by the counting unit 71 in the second oscillation period P2 are N1 (P2), N2 (P2), N3 (P2), and N4 (P2), respectively.

The normalizing unit 75 normalizes the frequencies N1, N2, N3, and N4 with the frequency of each oscillation period for each oscillation period, and then performs normalization for each frequency N1, N2, N3, and N4. The frequency sum N1 ″, N2 ″, N3 ″, N4 ″ of one oscillation period P1 and the second oscillation period P2 is obtained.
N1 "= {N1 (P1)
/ (N1 (P1) + N2 (P1) + N3 (P1) + N4 (P1))}
+ {N1 (P2)
/ (N1 (P2) + N2 (P2) + N3 (P2) + N4 (P2))}
... (4)
N2 "= {N2 (P1)
/ (N1 (P1) + N2 (P1) + N3 (P1) + N4 (P1))}
+ {N2 (P2)
/ (N1 (P2) + N2 (P2) + N3 (P2) + N4 (P2))}
... (5)
N3 "= {N3 (P1)
/ (N1 (P1) + N2 (P1) + N3 (P1) + N4 (P1))}
+ {N3 (P2)
/ (N1 (P2) + N2 (P2) + N3 (P2) + N4 (P2))}
... (6)
N4 "= {N4 (P1)
/ (N1 (P1) + N2 (P1) + N3 (P1) + N4 (P1))}
+ {N4 (P2)
/ (N1 (P2) + N2 (P2) + N3 (P2) + N4 (P2))}
... (7)

The frequency correction unit 76 calculates the correction value N2 ′ for the frequency N2 ″ and the correction value N3 ′ for the frequency N3 ″ as follows.
N2 '= N2 "-N1" (8)
N3 ′ = N3 ″ + N1 ″ (9)

  The frequency correction unit 76 notifies the determination unit 8 of the frequencies N1 ″, N2 ″, N3 ″, N4 ″ obtained by the normalization unit 75 and the correction values N2 ′, N3 ′ obtained by itself. In the present embodiment, in order to obtain the sum of the first oscillation period P1 and the second oscillation period P2, the period sorting unit 7 operates every oscillation period (period of a triangular wave). Note that the frequency correction unit 76 only has to calculate one of the correction values N2 'and N3'.

  Next, the determination unit 8 of the present embodiment compares the magnitudes N1 ″ and N4 ″ of the MHP period with the magnitude (N2 ″ + N3 ″) of the frequencies, and when the frequency N1 ″ is the largest, the object 10 If the frequency N4 ″ is the largest, it is determined that the object 10 exists at a shorter distance than the reference distance. Further, the determination unit 8 compares the frequency correction value N2 ′ with the frequency N3 ″ when the sum of frequencies (N2 ″ + N3 ″) is the largest, and when the correction value N2 ′ is larger than the frequency N3 ″, When it is determined that the object 10 exists at a distance farther than the reference distance, and the frequency N3 ″ is larger than the correction value N2 ′, it is determined that the object 10 exists at a distance closer than the reference distance. Is determined for each oscillation period (triangular wave period).

When the frequency correction unit 76 calculates the correction value N3 ′, the determination unit 8 performs the following determination. In other words, the determination unit 8 compares the magnitudes N1 ″ and N4 ″ of the MHP cycle with the sum of the frequencies (N2 ″ + N3 ″), and when the frequency N1 ″ is the largest, the object 10 is farther than the reference distance. When the frequency N4 ″ is the largest, it is determined that the object 10 exists at a shorter distance than the reference distance. Further, when the sum of frequencies (N2 ″ + N3 ″) is the largest, the determination unit 8 compares the frequency N2 ″ with the frequency correction value N3 ′, and the frequency N2 ″ is larger than the frequency correction value N3 ′. In this case, it is determined that the object 10 exists at a longer distance than the reference distance, and when the power correction value N3 ′ is larger than the frequency N2 ″, it is determined that the object 10 exists at a shorter distance than the reference distance.
Other configurations of the BGS photoelectric switch are as described in the first embodiment.

  In the present embodiment, as in the second embodiment, it is possible to effectively remove the influence of excessive noise detection during period measurement, and even if the object 10 moves, the distance of the object 10 can be reduced. It can be judged correctly.

[Fifth Embodiment]
In the first to fourth embodiments, the MHP waveform is extracted from the output signal of the photodiode that is the light receiver. However, it is also possible to extract the MHP waveform without using the photodiode. FIG. 11 is a block diagram showing a configuration of a BGS photoelectric switch according to the fifth embodiment of the present invention. The same reference numerals are given to the same configurations as those in FIG. The BGS photoelectric switch of this embodiment uses a voltage detection unit 11 instead of the photodiode 2 and the current-voltage conversion amplification unit 5 of the first to fourth embodiments.

  The voltage detector 11 detects and amplifies the voltage between the terminals of the semiconductor laser 1, that is, the anode-cathode voltage. When interference occurs between the laser light emitted from the semiconductor laser 1 and the return light from the object 10, an MHP waveform appears in the voltage between the terminals of the semiconductor laser 1. Therefore, it is possible to extract the MHP waveform from the voltage between the terminals of the semiconductor laser 1.

Similar to the first to fourth embodiments, the filter unit 6 has a function of extracting a superimposed signal from the modulated wave, and extracts an MHP waveform from the output voltage of the voltage detection unit 11.
The operations of the semiconductor laser 1, the laser driver 4, the period sorting unit 7, the determination unit 8, and the display unit 9 are the same as those in the first to fourth embodiments.

  Thus, in this embodiment, the MHP waveform can be extracted without using a photodiode, and the parts of the BGS photoelectric switch can be reduced as compared with the first to fourth embodiments. The cost of the photoelectric switch can be reduced.

  The present invention can be applied to a reflective photoelectric switch.

It is a block diagram which shows the structure of the BGS photoelectric switch which concerns on the 1st Embodiment of this invention. It is a figure which shows one example of the time change of the oscillation wavelength of the semiconductor laser in the 1st Embodiment of this invention. It is a wave form diagram showing typically the output voltage waveform of the current-voltage conversion amplification part in the 1st embodiment of the present invention, and the output voltage waveform of a filter part. It is a block diagram which shows the structure of the period classification | category part of the BGS photoelectric switch which concerns on the 1st Embodiment of this invention. It is a wave form diagram which shows typically the output voltage waveform of the filter part of the BGS photoelectric switch concerning a 1st embodiment of the present invention. It is a figure which shows frequency distribution of the period of a mode pop pulse when the loss | disappearance arises in the waveform in the 1st Embodiment of this invention. It is a figure which shows the frequency distribution of the period of a mode pop pulse when a period is divided into 2 by noise in the 1st Embodiment of this invention. It is a block diagram which shows the structure of the period classification | category part of the BGS photoelectric switch which concerns on the 2nd Embodiment of this invention. It is a figure which shows the frequency distribution of the period of a mode hop pulse when there exists a division | segmentation of the period of a mode hop pulse by noise, a missing waveform, and the object is not stationary. It is a block diagram which shows the structure of the period classification part of the BGS photoelectric switch which concerns on the 4th Embodiment of this invention. It is a block diagram which shows the structure of the BGS photoelectric switch which concerns on the 5th Embodiment of this invention. It is a figure which shows the compound resonator model of the semiconductor laser in the conventional laser measuring device. It is a figure which shows the relationship between the oscillation wavelength of a semiconductor laser, and the output waveform of a built-in photodiode. It is a figure which shows the relationship between the distance of an object, and the frequency distribution of the period of a mode pop pulse.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser, 2 ... Photodiode, 3 ... Lens, 4 ... Laser driver, 5 ... Current-voltage conversion amplification part, 6 ... Filter part, 7 ... Period classification part, 8 ... Determination part, 9 ... Display part, 10 DESCRIPTION OF SYMBOLS ... Object, 11 ... Voltage detection part, 70 ... Period measurement part, 71 ... Counting part, 72 ... Rise detection part, 73 ... Time measurement part, 74 ... Frequency correction part, 75 ... Normalization part, 76 ... Frequency correction part.

Claims (12)

A semiconductor laser that emits laser light;
A laser driver for operating the semiconductor laser;
Detecting means for detecting an electrical signal including an interference waveform generated by a self-coupling effect between laser light emitted from the semiconductor laser and return light from an object existing in front of the semiconductor laser;
Period measuring means for measuring the period of the interference waveform included in the output signal of the detecting means every time the interference waveform is input;
When the period of the interference waveform when the object is at the position of the reference distance is a reference period, the frequency of the period of the interference waveform measured by the period measurement unit is a first predetermined number times the reference period. A frequency N1 of a cycle less than the first, a frequency N2 of a cycle greater than or equal to the first predetermined number times the reference cycle and less than the reference cycle, and a frequency less than a second predetermined number of times greater than the reference cycle and the reference cycle (first predetermined cycle) Counting means for separating the frequency into a frequency N3 of a number <a second predetermined number) and a frequency N4 of a cycle not less than a second predetermined number times the reference cycle;
When the frequencies N1 and N4 are compared with the sum of the frequencies (N2 + N3), and the frequency N1 is the largest, it is determined that the object is located farther than the reference distance, and the frequency N4 is the largest Determines that the object is present at a shorter distance than the reference distance, and when the sum of the frequencies (N2 + N3) is the largest, the magnitudes N2 and N3 are compared, and if the frequency N2 is large, A reflection unit comprising: a determination unit that determines that the object exists at a longer distance than the reference distance, and determines that the object exists at a shorter distance than the reference distance when the frequency N3 is large; Type photoelectric switch. The reflective photoelectric switch according to claim 1,
Furthermore, a frequency correction means for calculating the correction value N2 ′ of the frequency N2 by N2 ′ = N2−N1 is provided,
The determination means compares the frequencies N1 and N4 and the sum of frequencies (N2 + N3), and when the frequency N1 is the largest, determines that the object exists at a distance farther than the reference distance, and the frequency When N4 is the largest, it is determined that the object is present at a shorter distance than the reference distance, and when the sum of the frequencies (N2 + N3) is the largest, the correction value N2 ′ of the frequency is compared with the size of the frequency N3. When the correction value N2 ′ is large, it is determined that the object exists at a distance farther than the reference distance, and when the frequency N3 is large, it is determined that the object exists at a short distance from the reference distance. A reflective photoelectric switch. The reflective photoelectric switch according to claim 1,
Further, a frequency correction means for calculating the correction value N3 ′ of the frequency N3 by N3 ′ = N3 + N1 is provided,
The determination means compares the frequencies N1 and N4 and the sum of frequencies (N2 + N3), and when the frequency N1 is the largest, determines that the object exists at a distance farther than the reference distance, and the frequency When N4 is the largest, it is determined that the object is present at a shorter distance than the reference distance, and when the sum of frequencies (N2 + N3) is the largest, the frequency N2 is compared with the frequency correction value N3 ′. When the frequency N2 is large, it is determined that the object exists at a distance greater than the reference distance. When the correction value N3 ′ is large, it is determined that the object exists at a distance closer than the reference distance. A reflective photoelectric switch. The reflective photoelectric switch according to claim 1,
In the laser driver, a first oscillation period including at least a period in which the oscillation wavelength continuously monotonously increases and a second oscillation period including at least a period in which the oscillation wavelength continuously decreases monotonously exist alternately. Operating the semiconductor laser,
Further, after the frequencies N1, N2, N3, and N4 obtained by the counting means are normalized by the frequency of each oscillation period for each oscillation period, the first oscillation period is set for each normalized frequency N1, N2, N3, and N4. Normalizing means for obtaining the sum N1 ″, N2 ″, N3 ″, N4 ″ of the frequencies of the second oscillation period;
Frequency correction means for calculating the correction value N2 ′ of the frequency N2 ″ by N2 ′ = N2 ″ −N1 ″,
The determination means compares the frequencies N1 ″ and N4 ″ with the magnitude of the frequency (N2 ″ + N3 ″), and when the frequency N1 ″ is the largest, the object exists at a distance farther than the reference distance. If the frequency N4 ″ is the largest, it is determined that the object is closer than the reference distance. If the sum of the frequencies (N2 ″ + N3 ″) is the largest, the frequency is corrected. The value N2 ′ is compared with the frequency N3 ″, and when the correction value N2 ′ is large, it is determined that the object exists at a distance farther than the reference distance, and when the frequency N3 ″ is large, the object It is determined that is present at a shorter distance than the reference distance. The reflective photoelectric switch according to claim 1,
In the laser driver, a first oscillation period including at least a period in which the oscillation wavelength continuously monotonously increases and a second oscillation period including at least a period in which the oscillation wavelength continuously decreases monotonously exist alternately. Operating the semiconductor laser,
Further, after the frequencies N1, N2, N3, and N4 obtained by the counting means are normalized by the frequency of each oscillation period for each oscillation period, the first oscillation period is set for each normalized frequency N1, N2, N3, and N4. Normalizing means for obtaining the sum N1 ″, N2 ″, N3 ″, N4 ″ of the frequencies of the second oscillation period;
Frequency correction means for calculating the correction value N3 ′ of the frequency N3 ″ by N3 ′ = N3 ″ + N1 ″,
The determination means compares the frequencies N1 ″ and N4 ″ with the magnitude of the frequency (N2 ″ + N3 ″), and when the frequency N1 ″ is the largest, the object exists at a distance farther than the reference distance. If the frequency N4 ″ is the largest, it is determined that the object is present at a shorter distance than the reference distance. If the frequency sum (N2 ″ + N3 ″) is the largest, the frequency N2 ″ is determined. And the frequency correction value N3 ′ are compared. When the frequency N2 ″ is large, it is determined that the object is located at a distance farther than the reference distance, and when the correction value N3 ′ is large, the object It is determined that is present at a shorter distance than the reference distance. The reflective photoelectric switch according to any one of claims 1 to 5,
The reflection type photoelectric switch according to claim 1, wherein the first predetermined number is 0.5 and the second predetermined number is 1.5. In the object detection method for detecting whether the distance to the object is longer or closer than a predetermined reference distance,
An oscillation procedure for operating the semiconductor laser by supplying a driving current to the semiconductor laser;
A detection procedure for detecting an electrical signal including an interference waveform caused by a self-coupling effect between a laser beam emitted from the semiconductor laser and a return beam from an object existing in front of the semiconductor laser;
A period measurement procedure for measuring the period of the interference waveform included in the output signal obtained by the detection procedure every time the interference waveform is input;
When the period of the interference waveform when the object is at the reference distance is a reference period, the frequency of the period of the interference waveform measured by the period measurement procedure is a first predetermined number of the reference period. A frequency N1 of a period less than twice, a frequency N2 of a period not less than a first predetermined number of times of the reference period and less than a reference period, and a frequency not less than a second predetermined number of times of the reference period and greater than the reference period (first A counting procedure for sorting into four frequencies: frequency N3 of a predetermined number <second predetermined number) and frequency N4 of a period equal to or greater than a second predetermined number times the reference period;
When the frequencies N1 and N4 are compared with the sum of the frequencies (N2 + N3), and the frequency N1 is the largest, it is determined that the object is located farther than the reference distance, and the frequency N4 is the largest Determines that the object is present at a shorter distance than the reference distance, and when the sum of the frequencies (N2 + N3) is the largest, the magnitudes N2 and N3 are compared, and if the frequency N2 is large, A determination procedure for determining that the object is present at a distance greater than the reference distance and determining that the object is present at a distance closer than the reference distance when the frequency N3 is greater. Detection method. The object detection method according to claim 7.
Furthermore, a frequency correction procedure for calculating the correction value N2 ′ of the frequency N2 by N2 ′ = N2−N1 is provided,
The determination procedure compares the frequencies N1 and N4 and the sum of frequencies (N2 + N3), and when the frequency N1 is the largest, determines that the object exists at a distance farther than the reference distance, and the frequency When N4 is the largest, it is determined that the object is present at a shorter distance than the reference distance, and when the sum of the frequencies (N2 + N3) is the largest, the correction value N2 ′ of the frequency is compared with the size of the frequency N3. When the correction value N2 ′ is large, it is determined that the object exists at a distance farther than the reference distance, and when the frequency N3 is large, it is determined that the object exists at a short distance from the reference distance. An object detection method characterized by: The object detection method according to claim 7.
Furthermore, a frequency correction procedure for calculating the correction value N3 ′ of the frequency N3 by N3 ′ = N3 + N1 is provided,
The determination procedure compares the frequencies N1 and N4 and the sum of frequencies (N2 + N3), and when the frequency N1 is the largest, determines that the object exists at a distance farther than the reference distance, and the frequency When N4 is the largest, it is determined that the object is present at a shorter distance than the reference distance, and when the sum of frequencies (N2 + N3) is the largest, the frequency N2 is compared with the frequency correction value N3 ′. When the frequency N2 is large, it is determined that the object exists at a distance greater than the reference distance. When the correction value N3 ′ is large, it is determined that the object exists at a distance closer than the reference distance. An object detection method characterized by: The object detection method according to claim 7.
In the oscillation procedure, the first oscillation period including at least a period during which the oscillation wavelength continuously increases monotonously and the second oscillation period including at least a period during which the oscillation wavelength continuously decreases monotonously exist alternately. Operating the semiconductor laser,
Further, after the frequencies N1, N2, N3, and N4 obtained by the counting procedure are normalized by the frequency of each oscillation period for each oscillation period, the first oscillation period is obtained for each normalized frequency N1, N2, N3, and N4. And a normalization procedure for obtaining the sum N1 ″, N2 ″, N3 ″, N4 ″ of the frequencies of the second oscillation period;
A frequency correction procedure for calculating the correction value N2 ′ of the frequency N2 ″ by N2 ′ = N2 ″ −N1 ″,
The determination procedure compares the frequencies N1 ″ and N4 ″ with the sum of frequencies (N2 ″ + N3 ″), and when the frequency N1 ″ is the largest, the object exists at a distance farther than the reference distance. If the frequency N4 ″ is the largest, it is determined that the object is closer than the reference distance. If the sum of the frequencies (N2 ″ + N3 ″) is the largest, the frequency is corrected. The value N2 ′ is compared with the frequency N3 ″, and when the correction value N2 ′ is large, it is determined that the object exists at a distance farther than the reference distance, and when the frequency N3 ″ is large, the object It is determined that exists at a shorter distance than the reference distance. The object detection method according to claim 7.
In the oscillation procedure, the first oscillation period including at least a period during which the oscillation wavelength continuously increases monotonously and the second oscillation period including at least a period during which the oscillation wavelength continuously decreases monotonously exist alternately. Operating the semiconductor laser,
Further, after the frequencies N1, N2, N3, and N4 obtained by the counting procedure are normalized by the frequency of each oscillation period for each oscillation period, the first oscillation period is obtained for each normalized frequency N1, N2, N3, and N4. And a normalization procedure for obtaining the sum N1 ″, N2 ″, N3 ″, N4 ″ of the frequencies of the second oscillation period;
A frequency correction procedure for calculating the correction value N3 ′ of the frequency N3 ″ by N3 ′ = N3 ″ + N1 ″,
The determination procedure compares the frequencies N1 ″ and N4 ″ with the sum of frequencies (N2 ″ + N3 ″), and when the frequency N1 ″ is the largest, the object exists at a distance farther than the reference distance. If the frequency N4 ″ is the largest, it is determined that the object is present at a shorter distance than the reference distance. If the frequency sum (N2 ″ + N3 ″) is the largest, the frequency N2 ″ is determined. And the frequency correction value N3 ′ are compared. When the frequency N2 ″ is large, it is determined that the object is located at a distance farther than the reference distance, and when the correction value N3 ′ is large, the object It is determined that exists at a shorter distance than the reference distance. The object detection method according to any one of claims 7 to 11,
The first predetermined number is 0.5, and the second predetermined number is 1.5.

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