JP5234653B2 - Light wave distance meter - Google Patents

Description

  The present invention relates to a phase difference type lightwave distance meter.

  As a phase difference type optical wave distance meter, the one disclosed in Patent Document 1 below is known. FIG. 14 shows a block diagram of this light wave distance meter.

  In this light wave distance meter, distance measuring light L emitted from a light source 3P such as a laser diode is placed on a measurement point via a light transmission optical system such as a prism 10P, 12P, a mirror 4P, and an objective lens 5P. The light is emitted toward a target (prism or the like) 6P. The light source 3P is connected to the modulator 2P, the modulator 2P is connected to the reference signal oscillator 1P, and the distance measuring light L is modulated by the reference signal K generated by the reference signal oscillator 1P.

  The distance measuring light L reflected by the target 6P is incident on a detector (light receiving element) 7P such as a photodiode through a light receiving optical system including an objective lens 5P and a mirror 4P. Then, the distance measuring light L is converted into an electric signal as a distance measuring signal M by the detector 7P. The phase difference between the distance measurement signal M and the reference signal K sent from the modulator 2P is measured by the phase meter 9P, and the distance to the target 6P is determined from the phase difference.

  On the other hand, the distance measuring light L emitted from the light source 3P switches the position of the shutter 8P so that the distance measuring light L passes through an external optical path that reciprocates to the target 6P, and without such an external optical path, It is possible to switch between the case where the light beam immediately enters the detector 7P as the reference light R through the internal optical path constituted by the prisms 10P, 11P, 12P. When the distance is measured using the reference light R that has passed through the internal optical path in the same manner as the distance measuring light L that has passed through the external optical path, an error inherent to the light wave rangefinder can be known. In this way, by alternately performing the measurement with the distance measuring light L and the measurement with the reference light R, the error inherent in the lightwave distance meter is corrected from the distance measured using the distance measuring light L, and the accurate measurement up to the target 6P The distance can be determined.

Japanese Patent No. 3236941

Generally, in the above-described phase difference type lightwave distance meter, the distance measuring light L is modulated using a plurality of reference signals K. Here, if the modulation frequencies F ₁ , F ₂ , and F ₃ based on the reference signal K are 75 MHz, 3.75 MHz, and 250 kHz, respectively, the respective wavelengths are 4 m, 80 m, and 1200 m, and each measurable range is 2 m, 40m and 600m. As can be seen, it is necessary to use a low modulation frequency for long distance measurements. However, when a modulation frequency of several hundred kHz or less is used, there is a problem in that the amount of reflection of the distance measuring light L from the fine particles in the air increases and the measurement error increases. In particular, in recent high-accuracy lightwave rangefinders, the amount of reflection of ranging light L from fine particles in the air at the time of long-distance measurement has reached a level that cannot be ignored.

  The present invention has been made in view of the above problems, and an object of the present invention is to reduce measurement errors in long-distance measurement using a phase-difference optical wave distance meter.

  In order to solve the above problems, in the invention according to claim 1, a frequency superimposing circuit for superimposing a plurality of modulation frequencies, and a light emitting element for emitting light modulated at a plurality of modulation frequencies superimposed by the frequency superimposing circuit, In the lightwave distance meter comprising a light receiving element that receives light emitted from the light emitting element and a plurality of frequency converters connected to the light receiving element, the frequency superimposing circuit includes a modulation other than the lowest modulation frequency. An AND circuit to which a frequency is input and an XOR circuit to which an output of the AND circuit and the lowest modulation frequency are input are characterized.

  The invention according to claim 2 is characterized in that, in the invention according to claim 1, the local oscillation signal of the lowest frequency is a frequency somewhat different from the difference or sum of the second lowest modulation frequency and the lowest modulation frequency. To do.

  In the invention according to claim 3, in the invention according to claim 1, a local oscillation signal having a different frequency is added to each of the plurality of frequency converters, and the local oscillation signal having the lowest frequency has a frequency other than the lowest modulation frequency. The frequency is somewhat different from the difference or sum of the two modulation frequencies and the lowest modulation frequency.

  In the invention according to claim 4, in the invention according to claim 1, 2, or 3, the duty ratio of the modulation signal having the lowest modulation frequency is 18 to 23% or 77 to 85%, and the modulation signal other than the lowest modulation frequency is used. The duty ratio is 50%.

According to the first aspect of the present invention, the frequency superimposing circuit includes an AND circuit to which a modulation frequency other than the lowest modulation frequency is input, and an XOR circuit to which the output of the AND circuit and the lowest modulation frequency are input. because is, the signal of the lowest modulation frequency F ₃ is, it becomes stronger than in the case of only the conventional one of the aND circuit, thereby improving the accuracy and reliability of long-distance measurement.

  According to the invention according to claim 2, in the invention according to claim 1, the local oscillation signal having the lowest frequency is set to a frequency somewhat different from the difference or sum of the second lowest modulation frequency and the lowest modulation frequency. The difference between the second lowest modulation frequency and the lowest modulation frequency from the intermediate frequency signal obtained by multiplying the frequency component that is the difference between the second lowest modulation frequency and the lowest modulation frequency by the local oscillation signal of the lowest frequency. A distance measurement value is obtained when using a modulation frequency of Since the difference between the second lowest modulation frequency and the second lowest modulation frequency is close to the second lowest modulation frequency, the lowest modulation frequency is not detected by the proximity method without detecting the lowest modulation frequency component from the ranging signal. A distance measurement value using the frequency can be obtained. Thereby, it is possible to reduce the influence of the reflection of the ranging light by the fine particles in the air. In addition, the frequency component of the difference or sum of the second lowest modulation frequency and the lowest modulation frequency can be maintained at a sufficient level, and a sufficient S / N ratio can be obtained. Can be improved.

  According to the invention of claim 3, in the invention of claim 1, the frequency of the local oscillation signal of the lowest frequency is somewhat different from the difference or sum of one modulation frequency other than the lowest modulation frequency and the lowest modulation frequency. Therefore, as in the invention according to claim 2, the distance measurement value when the lowest modulation frequency is used can be obtained by the proximity method without detecting the lowest modulation frequency component from the distance measurement signal. it can. Moreover, the difference or sum frequency component between one modulation frequency other than the lowest modulation frequency and the lowest modulation frequency can maintain a sufficient level, and a sufficient S / N ratio can be obtained. Thus, the same effect as in the second aspect of the invention can be achieved.

  According to the invention according to claim 4, in the invention according to claim 1, 2, or 3, the duty ratio of the modulation signal having the lowest modulation frequency is 18 to 23% or 77 to 85%, and other than the lowest modulation frequency. By setting the duty ratio of the modulation signal to 50%, signals related to all modulation frequencies can be maintained at a predetermined level or higher, and the accuracy and reliability of measurement can be further improved in all measurement ranges.

It is a block diagram of the light wave distance meter concerning one example of the present invention. It is a figure which shows the frequency component contained in the light radiate | emitted from the light emitting element of the said light wave distance meter. It is a figure explaining the method of obtaining the frequency of the difference of both frequencies from two adjacent frequencies. It is a figure explaining the proximity method which obtains a ranging value using two adjacent modulation frequencies. It is a figure explaining the frequency superposition circuit of the said light wave distance meter. For the output of the AND circuit when the signals of frequencies F ₁ (75 MHz), F ₂ (3.75 MHz), and F ₃ (250 kHz) are all input to one AND circuit, the vicinity of the frequency F ₁ is expanded in the Fourier series. It is a result of simulation. Similarly, it is a result of a simulation in which the vicinity of the frequency F ₂ is expanded by Fourier series. Wherein the output from the frequency superposing circuit, the result of a simulation Fourier series expansion near frequencies F ₁ when the signal of frequency F _1, F _2, F ₃ was input to the frequency superimposing circuit of the light wave light wave rangefinder . Similarly, it is a result of a simulation in which the vicinity of the frequency F ₂ is expanded by Fourier series. 10 is a table for explaining the main points of the results shown in FIGS. It is a diagram showing a relationship between the duty ratio and the frequencies F ₁ of the signal level of the frequency F _3. It is a diagram showing a relationship between the duty ratio and the frequency F ₂ of the signal level of the frequency F _3. It is a diagram showing a relationship between the duty ratio and the frequency F ₃ of the signal level of the frequency F _3. It is a block diagram of the conventional lightwave distance meter.

  Hereinafter, an embodiment of the lightwave distance meter of the present invention will be described with reference to the drawings.

  First, the lightwave distance meter will be described in detail based on the block diagram shown in FIG.

In the oscillator 1 generates a signal of a modulation frequency F _1. The signal having the modulation frequency F ₁ is input to the frequency divider 2 and also input to the oscillator 6 via the PLL 5. The PLL 5 is used to accurately synchronize the signal output from the oscillator 6 with the signal output from the oscillator 1.

The frequency divider 2 divides the signal of the modulation frequency F ₁ and generates signals of the modulation frequencies F ₂ and F ₃ . The signals of the modulation frequencies F ₂ and F _{3 and} the signal of the modulation frequency F ₁ are input to the drive circuit 4 through the frequency superimposing circuit 3. The light emitting element 11 connected to the load resistor 8 is driven by the drive circuit 4 and emits light modulated by the modulation frequencies F ₁ , F ₂ and F ₃ .

The oscillator 6 generates a signal with a local oscillation frequency F ₁ + Δf _{1 that} is somewhat different from the modulation frequency F ₁ . The signal of the local oscillation frequency F ₁ + Δf ₁ is divided by the frequency generation circuit 12 and has local oscillation frequencies F ₂ + Δf ₂ and F ₂ −F ₃ + Δf ₃ that are somewhat different from the modulation frequencies F ₂ and F ₃ , respectively. A local oscillation signal is also generated. These local oscillation signals of local oscillation frequencies F ₁ + Δf ₁ , F ₂ + Δf ₂ and F ₂ −F ₃ + Δf ₃ are input to frequency converters 32, 35 and 38, respectively, as will be described later.

  The light emitted from the light emitting element 11 is divided into two by the beam splitter 20, and by switching the shutter 28, one is emitted as distance measuring light from a light transmission optical system (not shown) and reciprocates to the target reflector 22. The light enters the light receiving element 30 through the distance measuring optical path 23, and the other enters the light receiving element 30 as the reference light through the reference light path 26 inside the lightwave distance meter. In the distance measuring optical path 23, a light receiving optical system 24 and a variable density filter 25 for adjusting the amount of light are disposed in front of the light receiving element 30. Also in the reference optical path 26, a density filter 27 for attenuating the amount of light is arranged in front of the light receiving element 30.

  A distance measurement signal output from the light receiving element 30 is divided into three through an amplifier 31, the first being input to the first frequency converter 32 and the second being input to the second frequency converter 35. The third is input to the third frequency converter 38.

The first frequency converter 32 generates a local oscillation somewhat different from the frequency F ₁ described above in the frequency F ₁ component of the ranging signal obtained from the ranging light that has passed through the ranging optical path 23 or the reference light that has passed through the reference optical path 26. Multiply the signal of frequency F ₁ + Δf ₁ to generate an intermediate frequency signal of frequency Δf ₁ . The second frequency converter 35 generates a local oscillation that is somewhat different from the frequency F ₂ described above, in the frequency F ₂ component of the ranging signal obtained from the ranging light that has passed through the ranging optical path 23 or the reference light that has passed through the reference optical path 26. Multiply the signal of frequency F ₂ + Δf ₂ to generate an intermediate frequency signal of frequency Δf ₂ .

By the way, when the light emitting element 11 is modulated with the frequencies F ₁ , F ₂ , and F ₃ , when the frequency F ₁ > F ₂ > F ₃ , the components of the frequencies F ₁ , F ₂ , and F ₃ as shown in FIG. In addition, components of frequencies F ₁ + F ₃ , F ₁ -F ₃ , F ₂ + F ₃ , and F ₂ -F ₃ are also included. Although components of frequencies F ₁ + F ₂ and F ₁ -F ₂ are also generated, they are not shown in FIG. 2 because the frequency is far from the frequency F ₁ or F ₂ .

The third frequency converter 38 is provided with a local oscillation frequency F ₂ -F ₃ + Δf ₃ that is somewhat different from the above-described frequency F ₂ -F ₃ , so that the distance measuring light or reference light path via the distance measuring optical path 23 is added. 26, the frequency F ₂ -F ₃ component of the ranging signal obtained from the reference light that has passed through 26 is multiplied by the signal of the local oscillation frequency F ₂ -F ₃ + Δf ₃ to generate an intermediate frequency signal of frequency Δf ₃ . Here, as described above, was used somewhat different local oscillation frequency _F 2 -F ₃ + _{Δf 3} and the frequency _F 2 -F _3, can also be used _F 2 _{-F 3 -Δf} 3 as the local oscillation frequency It is.

  The intermediate frequency signals output from the frequency converters 32, 35, and 38 are respectively removed from high-frequency components by the low-pass filters 33, 36, and 39.

  The intermediate frequency signals that have passed through the low-pass filters 33, 36, and 39 are input to A / D converters 34, 37, and 40, respectively. Then, the initial phase and amplitude of these intermediate frequency signals are obtained by a CPU (arithmetic control unit) (not shown). When the initial phase of each intermediate frequency signal is obtained, the distance to the target reflector 22 is calculated by correcting an error generated inside the light wave rangefinder. Further, when the amplitudes of the intermediate frequency signals are obtained, these amplitudes are used for light amount adjustment by the variable density filter 25.

According to the present embodiment, ranging values at the modulation frequencies F ₁ and F ₂ can be obtained in the same manner as in the past. Further, a distance measurement value d ₃ at the modulation frequency F ₃ that is the difference between the two adjacent modulation frequencies F ₂ and F ₂ −F ₃ is also obtained by the proximity method. Hereinafter, explaining the reason why the distance value at the modulation frequency F ₃ is obtained.

The modulation frequencies F ₂ and F ₃ are generally generated by dividing the modulation frequency F ₁ . For example, to make a frequency _{F 2} to the frequencies _{F 1} and circumferential 20 minutes, and made a frequency _{F 3} and the frequencies _{F 1} 300 divided. For example, if the _{F 1} and 75 MHz, _{F 2} is 3.75 MHz, _{F 3} becomes 250 kHz. Therefore, a component of frequency F ₂ −F ₃ = 3.5 MHz can be detected from the distance measurement signal. As shown in FIG. 3, a component of the frequency F _{3 that is} the difference between the _two frequencies F ₂ and F ₂ -F ₃ is generated from both components of the adjacent frequencies F ₂ and F ₂ -F ₃ by a beat. . As can be seen from this example, the following equation generally holds.
_{F 3 = F 2 - (F} 2 -F 3) (1)

Assuming that the wavelength at the frequency F ₃ is λ ₃ , the wavelength at the frequency F ₂ is λ ₂ , the wavelength at the frequency F ₂ -F ₃ is λ ₂₃ , and the speed of light is c, the equation (1) can also be expressed as I can write.
c / λ ₃ = c / λ ₂ −c / λ ₂₃ or 1 / λ ₃ = 1 / λ ₂ −1 / λ ₂₃ (2)

FIG. 4 shows the relationship between distance and phase when the frequencies F ₂ and F ₂ -F ₃ are close to 3.75 MHz (wavelength 80 m) and 3.5 MHz (wavelength about 86 m). At a distance of 600 m (roundtrip 1200 m), the phase difference between the two becomes 2π. On the other hand, the phase of the frequency F ₃ (wavelength 1200 m) that is the difference between the two frequencies F ₃ and F ₂ −F ₃ is also 2π. Now, the phase of the frequency _{F 3,} it is seen equal the phase difference between the frequency _{F 3} phase and frequency _F 2 -F ₃ phases. In general, when the distance measurement value at the frequency F ₃ is d ₃ , the distance measurement value at the frequency F ₂ is d ₂ , and the distance measurement value at the frequency F ₂ -F ₃ is d ₂₃ , the following equation is established.
2d ₃ / λ ₃ = 2d ₂ / λ ₂ -2d ₂₃ / λ ₂₃ = θ / (2π) (3)

However, θ is the phase of the frequency F _3. Therefore, the phase θ can be calculated from the phase difference of the phase 2π _{(2d 23} / λ ₂₃₎ of the phase 2π (2d ₂ / λ ₂₎ and the frequency _F 2 -F ₃ at the frequency _{F 2.} In Equation (3), 2d ₃ , 2d ₂ , and 2d ₂₃ are set because the distance measuring light reciprocates to the target reflector 22. If the value of 2d ₂ / λ ₂ −2d ₂₃ / λ ₂₃ is negative, 2π is added to calculate this phase angle θ. When the phase angle θ is thus calculated, the distance measurement value d ₃ at the frequency F ₃ can be calculated from the following equation.
d ₃ = λ ₃ · θ / (4π) (4)

Thus, if the 3.75MHz and 3.5MHz for the frequency _{F 2} and _F 2 -F ₃ is close, the frequency _{F 3} which is a difference between the two frequencies _{F 3} and _F 2 -F ₃ is a 250kHz The wavelength is 1200 m. In this way, it is possible to obtain the distance value d ₃ when the extremely low modulation frequency F ₃ is used without detecting the extremely low frequency F ₃ component from the distance measurement signal, and it becomes possible to measure a long distance, and moreover than before. Can also reduce the error.

Incidentally, in this embodiment, the it is conceivable to simply use AND circuit as the frequency superimposing circuit 3, as compared with the general frequency F ₂ -F ₃ component frequency F ₃ component in the output of frequency superimposing circuit 3 becomes smaller If for (see FIG. 10), in particular long distance measurement, not sufficient S / N ratio can be obtained, as compared with measurement using as a modulation frequency F _3, there is a possibility that the accuracy and reliability is not particularly improved. Therefore, in this embodiment, in order to increase the frequency F ₂ -F ₃ component as much as possible, the frequency superimposing circuit 3 has an AND circuit 3A to which the modulation frequencies F ₁ and F ₂ are inputted, as shown in FIG. consist XOR circuit (exclusive OR circuit) 3X the output modulation frequency _{F 3} of the circuit 3A are inputted.

The effect of the frequency superimposing circuit 3 in this embodiment will be described with reference to FIGS. FIG. 6 shows the output of the AND circuit when all signals having a duty ratio of 50% are input to a single AND circuit at frequencies F ₁ (75 MHz), F ₂ (3.75 MHz), and F ₃ (250 kHz). This is a result of a simulation in which the vicinity of the frequency F1 is expanded in the Fourier series. FIG. 7 also shows the result of a simulation in which the vicinity of the frequency F ₂ (3.75 MHz) is expanded in the Fourier series. FIG. 8 shows the frequency superposition of this embodiment with a signal having a duty ratio of 50% at frequencies F ₁ (75 MHz) and F ₂ (3.75 MHz) and a signal having a duty ratio of 77.17% at frequency F ₃ (250 kHz). It is the result of the simulation which carried out the Fourier series expansion | deployment of the vicinity of the frequency F1 about the output from the frequency superimposition circuit 3 when making it input into the circuit 3. FIG. FIG. 9 also shows the result of a simulation in which the vicinity of the frequency F ₂ (3.75 MHz) is expanded in the Fourier series. FIG. 10 is a table for explaining the main points of the results shown in FIGS. FIG. 11 is a diagram illustrating the relationship between the duty ratio of the frequency F ₃ and the signal level of the frequency F ₁ . Figure 12 is a graph showing the relationship between the duty ratio and the frequency F ₂ of the signal level of the frequency F _3. Figure 13 is a diagram showing the relationship between the duty ratio and the frequency F ₃ of the signal level of the frequency F _3.

As can be seen from FIG. 10, in this embodiment, the signal level of the frequency F ₁ is stronger than when the frequency superimposing circuit 3 is only one AND circuit. Signal level of the frequency F ₂ is becomes somewhat weaker than in the case of only one of the AND circuit, the difference is no problem within 1 dB. What should be noted in the present embodiment is that the signal relating to the frequency F ₂ ± F ₃ , especially the signal relating to F ₂ -F ₃ is considerably stronger than the case of only one AND circuit, and the signal of the frequency F ₃ It is not much inferior to. Therefore, according to the present embodiment, a sufficient S / N ratio can be obtained with a signal related to the modulation frequency F ₂ -F ₃ , and the accuracy and reliability of long-distance measurement can be improved.

As can be seen from FIGS. 11 to 13, the duty ratio of the frequency F ₃ is 18 to 23% or 77 to so that the signal levels related to the frequencies F ₁ , F ₂ , and F ₃ are all about −10 dB or more. About 85% is appropriate.

  By the way, the present invention is not limited to the lightwave distance meter of each of the above embodiments, but can be widely used in surveying instruments having a built-in lightwave distance meter, for example, a total station or other distance measuring devices.

Further, instead of the frequency _F 2 -F ₃ as described above, the frequency _F 2 ₊ F 3 shown in FIG. 2 (4 MHz, the wavelength _{75m), F 1 -F 3 (} 75.25MHz, wavelength 5.99m), _{F 1} + F ₃ (74.75 MHz, wavelength 4.01 m) may be used. In these cases, local oscillation signals of frequencies F ₂ + F ₃ ± Δf ₃ , F ₁ −F ₃ ± Δf ₃ , and F ₁ + F ₃ ± Δf ₃ are added to the frequency converter 38 in the embodiment of FIG. Then, an intermediate frequency signal having a frequency Δf ₃ is obtained. Distance value at the frequency F ₃ is obtained by the proximity method any case.

Furthermore, the environmental reflections is less of the distance measuring light by particles in the air (high altitude or space, etc.), be measured directly at the modulation frequency F ₃ without using a proximity method, the accuracy and reliability of long-distance measurement Can be improved. In this case, in the embodiment of FIG. 1, the local oscillation frequency applied to the frequency converter 38 is changed to F ₃ ± Δf ₃ .

3 frequency superimposing circuit 3A AND circuit 3X XOR circuit 11 light emitting element 22 target reflector 23 distance measuring optical path 26 reference optical path 28 shutter 30 light receiving elements 32, 35, 38 frequency converters F ₁ , F ₂ , F ₃ modulation frequency F ₁ + ΔF ₁ , F ₂ + ΔF ₂ , F ₂ -F ₃ + ΔF ₃ Local oscillation frequency

Claims (4)

A frequency superimposing circuit that superimposes a plurality of modulation frequencies, a light emitting element that emits light modulated at a plurality of modulation frequencies superimposed by the frequency superimposing circuit, and a light receiving element that receives the light emitted from the light emitting element; In a lightwave distance meter comprising a plurality of frequency converters connected to the light receiving element and applied with local oscillation signals of different frequencies,
The frequency superimposing circuit is composed of an AND circuit to which a modulation frequency other than the lowest modulation frequency is input, and an XOR circuit to which an output of the AND circuit and the lowest modulation frequency are input, Total.   2. The optical rangefinder according to claim 1, wherein the local oscillation signal having the lowest frequency has a frequency somewhat different from the difference or sum of the second lowest modulation frequency and the lowest modulation frequency.   The plurality of frequency converters are applied with local oscillation signals of different frequencies, and the local oscillation signal of the lowest frequency is somewhat different from the difference or sum of one modulation frequency other than the lowest modulation frequency and the lowest modulation frequency. The light wave distance meter according to claim 1, wherein the light wave distance meter is a frequency.   The duty ratio of the modulation signal having the lowest modulation frequency is 18 to 23% or 77 to 85%, and the duty ratio of the modulation signal other than the lowest modulation frequency is 50%. Lightwave distance meter as described in 1.

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