Optical distance meter

Abstract

To provide a phase difference-type light wave rangefinder without using a shutter for switching between a reference optical path and a distance-measuring optical path, with reduced temperature phase drift and a simpler configuration.SOLUTION: A light wave rangefinder 100 comprises: a first light-emitting element 20 sending distance measuring light modulated by main modulation frequencies F2, F3 to a distance-measuring optical path 21; a second light-emitting element 30 sending reference light modulated by side modulation frequencies F2+b*F2, F3+b*F3 adjacent to the main modulation frequencies F2, F3 to a reference optical path 31; a light receiving element 40 receiving the distance measuring light and the reference light; and frequency converters 44, 49 inputting a light receiving signal based on the distance measuring light and the reference light and signals of local frequencies 2+a*F2, F3+a*F3, and generating a distance measuring intermediate frequency signal based on the distance measuring light and a reference intermediate frequency signal based on the reference light. Distance to a target reflecting object 22 is calculated by subtracting the reference intermediate frequency signal from the distance measuring intermediate frequency signal. The main modulation frequencies F2, F3 are 10 MHz or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical distance meter, and more particularly to a phase difference type optical distance meter. [Background technology]

[0002] In such phase-difference optical distance meters, if distance is measured using only the measuring light, temperature phase drift in the light-transmitting and receiving circuits, as well as delays caused by the electrical circuits, will result in measurement errors. Therefore, it is common practice to accurately measure distance using the phase difference between the measuring light and the reference light. This utilizes the fact that the phase difference changes with distance. If the phase difference is Δφ, the measured distance is D, the modulation frequency is f, and the speed of light is C, then the phase difference Δφ is: Δφ = 4πfD / C (Equation 1) It is expressed as , and the distance D can be determined by measuring the phase difference Δφ. In practice, modulated light modulated with carrier signals of two or more different frequencies is used for measurement, and each digit of the distance value is determined according to the resolution of each.

[0003] For this reason, phase-difference optical distance meters are equipped with a shutter to switch between the distance-measuring optical path and the reference optical path. However, this method has the problem that the observation time is longer due to the shutter switching, and because the distance-measuring light and the reference light cannot be measured simultaneously, a difference in temperature phase drift occurs due to the light-emitting and light-receiving elements and electrical components of the light-transmitting and receiving units, causing errors. For this reason, optical distance meters that do not use a shutter have been proposed.

[0004] As an optical distance meter that does not use a shutter, for example, the optical distance meter described in Patent Document 1 has been proposed. The optical distance meter described in Patent Document 1 is equipped with two light-emitting elements and two light-receiving elements. Light emitted from the first light-emitting element is split by a beam splitter, one part enters the first light-receiving element via a distance-measuring optical path, and the other part enters the second light-receiving element via a first reference optical path. Light emitted from the second light-emitting element is split by a beam splitter, one part enters the second light-receiving element via a second reference optical path, and the other part enters the first light-receiving element via a third reference optical path.

[0005] Furthermore, light modulated with two types of signals—a main modulation frequency and a paramodulation frequency close to the main modulation frequency—is simultaneously transmitted from the first and second light-emitting elements to the distance measuring optical path and the first to third reference optical paths. The light is simultaneously received by the first and second photodetectors, and the distances of the distance measuring optical path and the first to third reference optical paths are measured simultaneously. The temperature phase drift in each optical path is calculated and canceled out by the processing unit, thereby reducing measurement errors. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2011-013068 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the optical distance meter described in Patent Document 1 had a complex configuration because it used two light-emitting elements and two light-receiving elements, and had three reference optical paths (optical fibers). Therefore, there was a need for an optical distance meter that did not use a shutter, with a simpler configuration while reducing temperature phase drift.

[0008] This invention has been made in view of the above circumstances, and aims to provide a simpler configuration than conventional ones in a phase difference type optical distance meter that does not use a shutter to switch between a reference optical path and a distance measuring optical path, while reducing temperature phase drift. [Means for solving the problem]

[0009] To achieve the above objective, an optical distance meter according to one aspect of the present invention includes: a first light-emitting element that sends light modulated at a plurality of main modulation frequencies as a distance measuring light to a distance measuring optical path that travels back and forth to a target reflector; a second light-emitting element that sends light modulated at paramodulation frequencies adjacent to each of the main modulation frequencies as a reference light to a reference optical path; a light-receiving element that receives the light sent from the first and second light-emitting elements; a group of frequency converters connected to the light-receiving element that convert the received signals based on the distance measuring light and the reference light, respectively, into intermediate frequency signals; and a calculation processing unit that calculates the distance to the target reflector based on the intermediate frequency signals. The frequency converter group comprises the same number of frequency converters as the main modulation frequency, each frequency converter receives a signal of a local frequency corresponding to each main modulation frequency, each local frequency is a frequency close to both the corresponding main modulation frequency and a paramodulation frequency close to the main modulation frequency, each frequency converter generates a distance measuring intermediate frequency signal based on the distance measuring light and a reference intermediate frequency signal based on the reference light, the calculation processing unit calculates the distance to the target reflector by subtracting the reference intermediate frequency signal from the distance measuring intermediate frequency signal, and the plurality of main modulation frequencies are 10 MHz or less.

[0010] In the above embodiment, it is also preferable that the difference between each main modulation frequency and the local frequency corresponding to each main modulation frequency is 10 times or more greater than the difference between each paramodulation frequency and the main modulation frequency adjacent to each paramodulation frequency.

[0011] In addition, in the above aspect, an amplifier that amplifies the received optical signal and inputs it to the frequency converter group, and a received light amount adjustment means for adjusting the received light amount level detected by the light receiving element are provided. The amplification degree of the amplifier is set lower than an appropriate value at which the level of the ranging intermediate frequency signal becomes the minimum magnitude that satisfies the reach limit flight distance of the ranging light. The received light amount adjustment means is preferably set such that the level of the ranging intermediate frequency signal becomes the minimum magnitude that satisfies the reach limit flight distance in a state where the amplification degree is set lower than the appropriate value.

[0012] In addition, in the above aspect, the first and second light emitting elements are laser diodes. The first light emitting element sends out light modulated by a high-frequency main modulation frequency higher than the plurality of main modulation frequencies to the ranging optical path in addition to the plurality of main modulation frequencies. The second light emitting element sends out light modulated by a high-frequency side modulation frequency close to the high-frequency main modulation frequency to the reference optical path in addition to the side modulation frequencies close to the plurality of main modulation frequencies. It is also preferable that the ranging signal based on the ranging light modulated by the high-frequency main modulation frequency and the ranging signal based on the reference light modulated by the high-frequency side modulation frequency are configured not to be input to the frequency converter group.

Advantages of the Invention

[0013] According to the optical wave distance meter according to the above aspect, in a phase difference type optical wave distance meter that does not use a shutter, it is possible to reduce the temperature phase drift and have a simpler configuration than before.

Brief Description of the Drawings

[0014] [Figure 1] It is a block diagram of an optical wave distance meter according to a first embodiment of the present invention. [Figure 2] It is a diagram showing a variable density filter which is a received light amount adjustment means for the ranging light of the optical wave distance meter. [Figure 3](A) to (C) are diagrams showing the received light signal spectrum of the same optical wave distance meter, and are diagrams explaining the improvement of the S / N (signal-to-noise) ratio by the settings of the amplifier and the received light amount adjustment means. [Figure 4] It is a table showing examples of the main modulation frequency, side modulation frequency, local frequency, and intermediate frequency of the same optical wave distance meter. [Figure 5] It is a block diagram of an optical wave distance meter according to one modification of this embodiment.

Embodiments for Carrying Out the Invention

[0015] Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited thereto. Also, in each embodiment, elements having the same mechanical configuration are denoted by the same reference numerals, and duplicate explanations are omitted as appropriate.

[0016] (First Embodiment) FIG. 1 is a block diagram of an optical wave distance meter 100 according to the first embodiment. The optical wave distance meter 100 is a phase difference type optical wave distance meter that calculates the distance to the target reflector 22 from the phase difference between the distance measuring light and the reference light. The optical wave distance meter 100 mainly includes two light emitting elements (that is, the first light emitting element 20 and the second light emitting element 30), a light receiving element 40, an arithmetic processing unit 60, and a storage unit 70.

[0017] The first light emitting element 20 and the second light emitting element 30 are, for example, laser diodes, and emit visible or infrared laser light. The first light emitting element 20 emits light modulated by a plurality of main modulation frequencies F1, F2, F3. The second light emitting element 30 emits light modulated by side modulation frequencies F1 + b·F1, F2 + b·F2, F3 + b·F3 close to each of the main modulation frequencies F1, F2, F3. Here, the coefficient b satisfies 0 < b, and |b| is a value sufficiently small with respect to 1 (|b| << 1).

[0018] The main modulation frequencies F1, F2, and F3 are in descending order of frequency (F1 > F2 > F3). For example, frequency F1 is greater than 10 MHz, frequency F2 is between 10 MHz and 1 MHz, and frequency F3 is 1 MHz or less. Furthermore, the deviations |b|·F1, |b|·F2, and |b|·F3 between each main modulation frequency F1, F2, and F3 and the adjacent paramodulation frequencies F1+b·F1, F2+b·F2, and F3+b·F3 are preferably between several tens of kHz and several kHz.

[0019] The light-receiving element 40 is, for example, an avalanche photodiode, and outputs the received distance-measuring light and reference light as electrical signals (distance-measuring signal and reference signal), respectively.

[0020] The arithmetic processing unit 60 is, for example, at least one of Processor (e.g., CPU (Central Processing Unit)) and memory (SRAM (Static Random Access Memory), DRAM (D y It is a control arithmetic unit equipped with (memory, RAM, etc.). The arithmetic processing unit 60 realizes its function by having the processor read a program for executing that function into memory and execute it.

[0021] Alternatively, at least a portion of the arithmetic processing unit 60 may be configured in hardware using a CPLD (Complex Programmable Logic Device), FPGA (Field Programmable Gate Array), etc. Furthermore, when using these CPLDs or FPGAs, circuits configured on these elements realize their functions.

[0022] The arithmetic processing unit 60 controls the emission of light from the first light-emitting element 20 and the second light-emitting element 30, and calculates the distance to the target reflector based on the light-receiving signal from the light-receiving element 40. Further details will be described later.

[0023] The storage unit 70 is a computer-readable storage medium such as flash memory or a hard disk drive. The storage unit 70 stores programs for performing measurement, adjustment of received light intensity, calculation of distance values, etc., by the arithmetic processing unit 60, as described below, as well as initial setting data, measurement data, etc.

[0024] The configuration of the optical distance meter 100 and the details of distance measurement are described below. First, the oscillator 1 generates a signal at the main modulation frequency F1 based on instructions from the arithmetic processing unit 60. This signal at the main modulation frequency F1 is input to the frequency divider 2 and also to the oscillator 5 and the local signal oscillator 11 via PLLs 9 and 10. The PLLs 9 and 10 are used to ensure that the oscillator 5 and the local signal oscillator 11 oscillate in precise synchronization with the main modulation frequency F1.

[0025] The frequency divider 2 divides the signal at the main modulation frequency F1 to generate signals at the main modulation frequencies F2 and F3. These signals at the main modulation frequencies F2 and F3, along with the signal at the main modulation frequency F1, are input to the drive circuit 4 via the frequency superposition circuit 3. The first light-emitting element 20 is driven by the drive circuit 4 and emits light modulated at the main modulation frequencies F1, F2, and F3.

[0026] Oscillator 5 generates a signal with paramodulation frequency F1+b·F1. This signal with paramodulation frequency F1+b·F1 is further divided by frequency divider 6 to obtain signals with paramodulation frequencies F2+b·F2 and F3+b·F3. These signals with paramodulation frequencies F1+b·F1, F2+b·F2 and F3+b·F3 are input to drive circuit 8 via frequency superposition circuit 7. The second light-emitting element 30 is driven by drive circuit 8 and emits light modulated at paramodulation frequencies F1+b·F1, F2+b·F2 and F3+b·F3.

[0027] The local signal oscillator 11 generates a signal having a local frequency of F1 + a·F1. Here, the coefficient a satisfies 0 < a, and |a| is a value sufficiently small with respect to 1 (|a| << 1). The signal having the local frequency of F1 + a·F1 is divided by the frequency generation circuit 12 to generate a signal having a local frequency of F2 + a·F2 and a signal having a local frequency of F3 + a·F3. These signals having the local frequencies of F2 + a·F2 and F3 + a·F3 are input to the first frequency converter 44 and the second frequency converter 49, respectively, as will be described later. The deviations |a|·F1, |a|·F2, |a|·F3 between each main modulation frequency F1, F2, F3 and the corresponding local frequencies F1 + a·F1, F2 + a·F2, F3 + a·F3 are several hundreds of kHz to several tens of kHz. Therefore, the local frequencies F1 + a·F1, F2 + a·F2, F3 + a·F3 are frequencies close to both the corresponding main modulation frequencies F1, F2, F3 and the sideband modulation frequencies F1 + b·F1, F2 + b·F2, F3 + b·F3.

[0028] In particular, it is preferable that the deviations |a|·F1, |a|·F2, |a|·F3 between each main modulation frequency F1, F2, F3 and the main modulation frequencies of the corresponding local frequencies F1 + a·F1, F2 + a·F2, F3 + a·F3 are more than 10 times the deviations |b|·F1, |b|·F2, |b|·F3 between each main modulation frequency F1, F2, F3 and the corresponding sideband modulation frequencies F1 + b·F1, F2 + b·F2, F3 + b·F3, that is, |a| ≥ 10|b|. The reason will be described later.

[0029] The light emitted from the first light-emitting element 20 is reflected by the target reflector 22 through the ranging optical path 21 and reaches the light-receiving element 40 through the ranging optical path 23.

[0030] In the ranging optical path 23, a variable density filter 24, which is a light-receiving light amount adjusting means, and a light-receiving optical system 25 are arranged in front of the light-receiving element 40. The variable density filter 24 is, for example, a thin disk-shaped neutral density filter as shown in FIG. 2.

[0031] The variable density filter 24 has an opening 241, and is configured such that the filter density continuously increases in the circumferential direction, indicated by arrow X in Figure 2, from the opening 241 toward the other end 242 in the circumferential direction, so that the light intensity attenuation rate ranges from 0 to 100%. The variable density filter 24 is connected to the rotation axis of the filter drive motor 24a at its center 243. The variable density filter 24 adjusts the level of light received by the distance measuring light by adjusting the attenuation rate of the transmitted light when the filter drive motor 24a is rotated according to the instructions of the calculation processing unit 60.

[0032] Meanwhile, the light emitted from the second light-emitting element 30 reaches the photodetector 40 via the reference optical path 31.

[0033] In the reference optical path 31, a density filter 32, which is a means for adjusting the amount of received light, is positioned in front of the photodetector 40. The density filter 32 is, for example, a fixed neutral density filter with the same configuration as the variable density filter 24. Alternatively, a neutral density filter with a fixed density may be used. The density filter 32 adjusts the level of received light from the reference light by attenuating the reference light at a preset attenuation rate.

[0034] The light-receiving element 40 is connected to the amplifier 41, and the amplifier 41 is connected to the frequency converter group 43 via the band filter 42. The frequency converter group 43 consists of two frequency converters: a first frequency converter 44 and a second frequency converter 49.

[0035] The light received by the photodetector 40 is converted into signals with six frequencies F1, F1+b·F1, F2, F2+b·F2, F3, and F3+b·F3. These signals are amplified by the amplifier 41. Of these signals, the high-frequency main modulation frequency (hereinafter also called the high-frequency main modulation frequency) F1 and the paramodulation frequency (hereinafter also called the high-frequency paramodulation frequency) F1+b·F1 are not used in the calculation of distance values. Therefore, the signals with frequencies F1 and F1+b·F1 are removed from the amplified signals by the bandpass filter 42 and input to the first frequency converter 44 and the second frequency converter 49, respectively.

[0036] In the first frequency converter 44, a local signal of local frequency F2 + a·F2 is input, and frequency multiplication is performed. The multiplied signal is output to band filter 45 and band filter 47. The band filter 45 removes the high frequency range, and the intermediate frequency signal (distance measuring intermediate frequency signal) of frequency a·F2 is input to the A / D converter 46. The band filter 47 removes the high frequency range, and the intermediate frequency signal (reference intermediate frequency signal) of frequency (ab)·F2 is input to the A / D converter 48.

[0037] In the second frequency converter 49, a local signal of local frequency F3 + a·F3 is input, and frequency multiplication is performed. The multiplied signal is output to band filter 50 and band filter 52. The band filter 50 removes the high-frequency range, and the intermediate frequency signal (distance measuring intermediate frequency signal) of frequency a·F3 is input to A / D converter 51. The band filter 52 removes the high-frequency range, and the intermediate frequency signal (reference intermediate frequency signal) of frequency (ab)·F3 is input to A / D converter 53.

[0038] Thus, the frequency converter group 43 includes a number of frequency converters corresponding to the number of main modulation frequencies used in calculating the distance value.

[0039] The A / D converters 46, 48, 51, and 53 convert analog signals into multi-level digital signals, and by making the signal waveform a sine wave, the arithmetic processing unit 60 can acquire signal amplitude information and phase information. The signal amplitude information and phase information are used to calculate distance values. In addition, the signal amplitude information is used for adjusting the amount of light received.

[0040] The A / D converters 46, 48, 51, and 53 are each connected to the arithmetic processing unit 60. The arithmetic processing unit 60 analyzes the signal amplitude and phase information of the intermediate frequency signals of frequencies a·F2 and a·F3 to calculate the measurement value of the distance-measuring optical path using the distance-measuring light of modulation frequencies F2 and F3. The arithmetic processing unit 60 also analyzes the signal amplitude and phase information of the intermediate frequency signals of frequencies (ab)·F2 and (ab)·F3 to calculate the measurement value of the reference optical path using the reference light of modulation frequencies F2+b·F2 and F3+b·F3. Then, the arithmetic processing unit 60 calculates the distance value to the target reflector using the distance-measuring light of frequencies F2 and F3, respectively, by subtracting the reference optical path measurement value from the distance-measuring optical path measurement value.

[0041] Here, we will explain why the effect of temperature phase drift can be reduced in the calculation of the distance value to the target reflector 22 in the arithmetic processing unit 60. Although temperature phase drift occurs similarly for both frequencies F2 and F3, the reason why its effect can be reduced is the same, so we will explain the signal at the main modulation frequency F2 as a representative example.

[0042] Let F2 = (1+0)f2 be the frequency of the electrical signal applied to the first light-emitting element 20, F2 + b·F2 = (1+b)f2 be the frequency of the electrical signal applied to the second light-emitting element 30, and F2 + a·F2 = (1+a)f2 be the frequency of the local signal applied to the first frequency converter 44, where a > 0 and b > 0, and it is preferable that the absolute value of |a| is 10 times or more larger than |b| (|a| ≥ 10|b|).

[0043] The equation for the waveform of the light-receiving element 40 is given by equation 2 below.

number

[0044] Next, consider the equation of the waveform after frequency conversion. Let the waveform of the local signal of the local frequency (1 + a)·F2 multiplied by y1 in the first frequency converter 44 be y2. Then, y2 can be expressed as in Equation 3.

[0045] y2 = y LO COS{2π(1 + a)f2t + φ} (Equation 3) However, y LO is the amplitude of the local signal, and φ is the phase of the local oscillation signal.

[0046] Next, in the first frequency converter 44, after frequency conversion (multiplication) of y1 and y2, the waveform y3 of the intermediate frequency signal of frequency a·F2 based on the ranging light that has passed through the band filter 45 can be expressed as in Equation 4.

Equation

[0047] Also, after frequency conversion (multiplication) of y1 and y2 in the first frequency converter 44, the waveform y4 of the intermediate frequency signal of frequency (a - b)·F2 based on the reference light that has passed through the band filter 47 can be expressed as in Equation 5.

Equation

[0048] Next, obtain the distance value. The ranging value d can be converted by the following general formula (Equation 6).

number

[0049] If d0 is the measurement along the optical path of the phase component of the waveform y3 of the intermediate frequency signal with frequency Δf2 based on the ranging light, and d1 is the measurement along the reference optical path of the phase component of the waveform y4 of the intermediate frequency signal with frequency 2Δf2 based on the reference light, then d0 and d1 can be expressed by the following equations 7 and 8, respectively.

number

[0050] Here, since |b| is sufficiently small with respect to 1, we can approximate it as 1+b≈1. Also, φ LD1 (f2) is the temperature phase drift of the first light-emitting element 20 due to a signal of frequency F2, and φ LD2 ((1+b)f2) is the temperature phase drift of the second light-emitting element 30 due to a signal of frequency F2-Δf2, and φ APD (f2) is the temperature phase drift of the photodetector 40 due to a signal of frequency F2, and φ APD ((1+b)f2) is the temperature phase drift of the photodetector 40 due to a signal of frequency F2-Δf2, but the modulation frequency is 10MHz. below In this case, the temperature phase drift of the light-emitting element and the photodetector is known to be negligibly small. Therefore, φ LD1 (f2), φ LD2 ((1+b)f2), φ APD (f2), φ APD Since ((1+b)f2) can be ignored, equations 7 and 8 can be expressed as equations 9 and 10, respectively.

[0051]

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[0052] From Equation 11, the distance from the optical distance meter 100 to the target reflector 22 can be determined, but Equation 11 does not account for the temperature phase drift φ originating from the first light-emitting element 20, the second light-emitting element 30, and the photodetector 40. LD1 (f2), φ LD2 ((1+b)f2), φ APD (f2), φ APD It can be seen that ((1+b)f2) has been removed, and the temperature phase drift has been reduced.

[0053] The distance value from the optical distance meter 100 to the target reflector 22, calculated by Equation 11, includes a temperature phase drift φ originating from the electronic components (band filters 45, 47) on the light-receiving side. AMP1 ,φ AMP2 This includes the following. Generally, it is known that the temperature phase drift that occurs in electronic components, light-emitting elements, and photodetectors is larger the higher the frequency of the applied signal. The temperature phase drift that occurs in the band filters 45 and 47 is the temperature phase drift caused by the low-frequency (intermediate frequency) signal after frequency conversion, and its effect is smaller than that of the temperature phase drift that occurs in light-emitting elements and photodetectors based on the modulation frequency F1 signal in conventional optical distance meters that use a high-frequency modulation frequency F1. Therefore, in the optical distance meter 100, it is possible to reduce the effect of temperature phase drift by using a modulation frequency of 10 MHz or less, which results in small temperature phase drift.

[0054] Here, in the case of |a|≧10|b|, from equation 11, the temperature phase drift φ originating from the light-receiving electronic components (band filters 45, 47) is as follows. AMP1 ,φ AMP2 This is particularly advantageous because it allows you to eliminate [the enemy].

[0055] When |a|≧10|b|, |b| is sufficiently small compared to |a|, so it is possible to approximate it as shown in Equation 12.

number

[0056] With this approximation, equation 11 can be transformed into equation 13.

number

[0057] This temperature phase drift φ originates from the electronic components on the light-receiving side (band filters 45, 47). AMP1 ,φ AMP2 In order to achieve the effect of reducing the influence, in addition to a>0 and b>0, the conditions a<0 and b>0, a>0 and b<0, and a<0 and b<0 may also be used. The main modulation frequency, paramodulation frequency, local frequency, and intermediate frequency under these conditions are shown in Figure 4 using α=|a| and β=|b|. Under the above conditions, if |a|≧10|b|, then |b| is sufficiently small compared to |a|, α·F2 ≈ (α-β)·F2 (Equation 14) or α·F² ≈ (α+β)·F² (Equation 15) It is possible to approximate it as follows.

[0058] Thus, in this embodiment, two light-emitting elements are made to emit light simultaneously, and the modulation frequency is 10 MHz. below By using light modulated by the (F2, F3) signals, it becomes possible to calculate the distance measurement value while ignoring the effects of errors caused by temperature phase drift in the light-emitting element and photodetector, particularly the difference in temperature phase drift between two light-emitting elements.

[0059] As a result, compared to conventional optical distance meters that do not use a shutter and have two light-emitting elements and two light-receiving elements, SecondThis makes it possible to omit the photodetector and the optical fibers that constitute the reference optical path between the first light-emitting element and the second photodetector, and between the second light-emitting element and the first photodetector, thereby simplifying the overall configuration. Furthermore, the simplified configuration allows for cost reduction.

[0060] Furthermore, the optical distance meter 100 according to this embodiment can measure distance faster than conventional shutter-type optical distance meters because it simultaneously determines the initial phase of each intermediate frequency signal related to the distance measuring optical path and the reference optical path without using a shutter to switch between the distance measuring optical path and the reference optical path. In addition, cost reduction is possible by not using a shutter. Moreover, conventionally, the power supply of the light-emitting element was kept ON during continuous distance measurement in order to reduce the temperature phase drift difference due to the time difference in measurement. However, with the optical distance meter 100, simultaneous distance measurement of the distance measuring optical path and the reference optical path is possible, so it is not affected by temperature phase drift due to the time difference, and the power supply of the light-emitting element can be turned ON / OFF for each distance measurement, thereby saving power. This achieves the same effect as the optical distance meter described in Patent Document 1.

[0061] In the optical distance meter 100, the oscillator 1 generates a signal with frequency F1, and drives the drive circuit 4 with frequencies F1, F2, and F3. However, the received signal based on frequency F1 is removed by the band filter 42 and is not used in the calculation of the distance measurement value. As explained in the modified example below, it is not necessarily required to oscillate the oscillator 1 at a high frequency F1 greater than 10 MHz. However, in the case where the first light-emitting element 20 and the second light-emitting element 30 are laser diodes, as in this embodiment, it is advantageous to configure the oscillator 1 to oscillate at a modulation frequency F1 higher than the modulation frequencies F2 and F3, and to drive the first light-emitting element 20 at the main modulation frequency F1 and the second light-emitting element 30 at the paramodulation frequency F1+b·F1, because this stabilizes the phase of the signal.

[0062] Incidentally, it is generally known that using a signal with a higher modulation frequency results in higher accuracy of the distance measurement. For this reason, in the optical distance meter 100 according to this embodiment, which does not use the high-frequency modulation frequency F1, the accuracy of the distance measurement may be low. Therefore, in the optical distance meter 100, by setting the amplification of the amplifier 41 and the amount of light received as follows as needed, it is possible to ensure the same accuracy as when using a signal with the main modulation frequency F1.

[0063] Conventional optical distance meters (see, for example, Patent Document 1, Figure 4) also use an amplifier to amplify the received light signal and use it to calculate the distance value, similar to optical distance meter 100. In conventional optical distance meters, the amplification factor of the amplifier is set to an appropriate value according to the specifications of the optical distance meter, which is the minimum amount of received light (specifically, the signal level at which the A / D converter count becomes 1) that satisfies the limit range of the measuring light. In addition, in optical distance meter 100, the amplification factor of amplifier 41 is set lower than the appropriate value that satisfies the minimum amount of received light that satisfies the limit range. Furthermore, in optical distance meter 100, the attenuation rate of the light receiving light intensity adjustment means is set lower than the attenuation rate of the light receiving light intensity adjustment means set under the same conditions.

[0064] The reason why accuracy improves in this way will be explained. Figure 3(A) schematically shows the signal spectrum of the intermediate frequency signal (for example, the intermediate frequency signal with frequency Δf2 in Figure 1) based on the distance measuring light in the optical distance meter 100 when the amplification of the amplifier 41 is set to an appropriate value. The amplified intermediate frequency signal contains both signal and noise, but when the amplification is set lower than the appropriate value, the signal and noise are reduced in the same ratio, as shown in Figure 3(B).

[0065] In this state, A / DThe attenuation rate of the variable density filter 24 is set to increase the amount of transmitted light so that the received signal level input to the converter becomes the appropriate light intensity. In other words, when the amplification of the amplifier 41 is reduced to 1 / N (where N is a natural number), the attenuation rate of the variable density filter 24 is set to N times the appropriate value. For example, in a conventional optical distance meter, if the appropriate light intensity is achieved when the transmittance of the variable density filter is 9% (i.e., attenuation rate of 91%) when the amplification is 10 times, the transmittance of the variable density filter 24 is set to 90 percent of 10 times (attenuation rate of 10 / 91 times, or 10%) in order to achieve a similar received signal level with the amplification at 1 time (1 / 10). In this way, the received signal level increases, but the level of noise generated on the light-emitting element side does not change. In addition, the level of noise generated on the light-receiving element side increases slightly with the increase in the received signal level, but it is negligible compared to the signal. As a result, as shown in Figure 3(C), the signal level can be increased without increasing the noise level, improving the signal-to-noise ratio (S / N ratio). This makes it possible to improve accuracy.

[0066] Incidentally, the arithmetic processing unit 60 is configured to drive the filter drive motor 24a according to the detected amount of received light (signal level (signal amplitude) of the intermediate frequency signal output from the A / D converters 46 and 51) during measurement, so that the amount of received light becomes the maximum value within the input range of the A / D converters 46 and 51. In the A / D converters 46 and 51, the signal amplitude is not measured in the saturation region that exceeds the input range of the measurable signal level. Therefore, if the amount of received light is greater than the input range of the A / D converters 46 and 51, the arithmetic processing unit 60 drives the filter drive motor 24a so that the amount of received light becomes the maximum value within the input range, and if it is within the input range of the A / D converters 46 and 51, it drives the filter drive motor 24a so that the amount of received light becomes the maximum value within the input range. Also, if it is smaller than the input range of the A / D converters 46 and 51, it drives the filter drive motor 24a so that the amount of received light becomes the maximum value within the input range. In this case, the attenuation rate of light is set to be lower than when the same variable density filter is used, provided that the amplification is at an appropriate value.

[0067] Furthermore, by reducing the amplification of the amplifier 41, the signal amplitude of the intermediate frequency signal based on the reference light also decreases. Therefore, the attenuation rate of the density filter 32, which is a means for adjusting the amount of light received, is also set to be lower than when the same density filter is used with an appropriate amplification level.

[0068] Furthermore, if the amplification of amplifier 41 is set lower than the appropriate value, the amplitude will not be properly decomposed by the A / D converter, and the signal amplitude of less than 1 count will not be detected during signal conversion. This will result in the signal being mistakenly digitized to 0, making signal detection impossible and shortening the maximum range that the measuring light can reach. However, this problem can be solved by increasing the resolution of the A / D converter used, that is, by using a high-resolution A / D converter so that the detection limit when the amplification is set low is about the same as the detection limit when the amplification is set to the appropriate value.

[0069] (modified version) Figure 5 is a block diagram of an optical distance meter 100A according to a modified example of this embodiment. The optical distance meter 100A has a substantially the same configuration as the optical distance meter 100, but differs in that the first light-emitting element 20A and the second light-emitting element 30A are LEDs (Light-Emitting Diodes) instead of laser diodes. The oscillator 1 oscillates at a frequency F2 between 10MHz and 1MHz, not at a frequency F1 greater than 10MHz. The period division unit 2 divides the signal at frequency F2 and outputs a signal at frequency F3 of 1MHz or less. The oscillator 5 generates a signal at paramodulation frequency F2+b·F2. This signal at paramodulation frequency F2+b·F2 is divided by the frequency divider unit 6 to become a signal at paramodulation frequency F3+b·F3. The local signal oscillator 11 generates a signal at local frequency F2+a·F2. This local frequency F2+a·F2 signal is divided by the frequency generation circuit 12 to generate a local frequency F3+a·F3 signal.

[0070] Therefore, the drive circuit 4 is driven only by signals of frequencies F2 and F3, and the drive circuit 8 is driven only by signals of frequencies F2+b·F2 and F3+b·F3. Since the light received by the photodetector 40 does not contain any signals originating from the signal of frequency F1, the band filter 42 is also unnecessary. In this way, when the first light-emitting element 20A and the second light-emitting element 30A are LEDs, the stability of the signal phase is not affected even if light modulated at the high-frequency modulation frequency F1 is not transmitted, and thus it is possible to achieve the same effect as the optical distance meter 100.

[0071] Although preferred embodiments of the present invention have been described above, these embodiments are merely examples of the present invention, and it is possible to combine them based on the knowledge of those skilled in the art, and such forms are also included within the scope of the present invention. [Explanation of symbols]

[0072] 20,20A: First light-emitting element 21: Distance measurement optical path 22: Target reflector 23: Distance measurement optical path 24: Variable concentration filter (means for adjusting the amount of light received) 30,30A: Second light-emitting element 31: Reference optical path 32: Density filter (means for adjusting the amount of light received) 40: Photodetector 41: Amplifier 43: Frequency converter group 44: First frequency converter (frequency converter) 49: Second frequency converter (frequency converter) 60: Arithmetic Processing Unit 100,100A:Light wave distance meter

Claims

1. A first light-emitting element transmits light modulated with multiple main modulation frequencies as a distance measuring light to a distance measuring optical path that travels back and forth to the target reflector, A second light-emitting element sends light modulated at a paramodulation frequency close to each of the aforementioned main modulation frequencies into a reference optical path as reference light, A light-receiving element that receives light emitted from the first light-emitting element and the second light-emitting element, A group of frequency converters connected to the light-receiving element, which convert the received light signals based on the distance-measuring light and the reference light, respectively, into intermediate frequency signals, The system includes a calculation processing unit that calculates the distance to the target reflector based on the intermediate frequency signal, The frequency converter group consists of the same number of frequency converters as the main modulation frequency, and each frequency converter is input to a signal of a local frequency corresponding to each main modulation frequency. Each local frequency is a frequency that is close to both the corresponding main modulation frequency and a paramodulation frequency close to the main modulation frequency. Each frequency converter generates a distance measuring intermediate frequency signal based on the distance measuring light and a reference intermediate frequency signal based on the reference light. The calculation processing unit calculates the distance to the target reflector by subtracting the reference intermediate frequency signal from the distance measurement intermediate frequency signal. The aforementioned multiple main modulation frequencies are 10 MHz or less. An amplifier that amplifies the received light signal and inputs it to the frequency converter group, The system further comprises a light-receiving element and a light-receiving element The amplification of the amplifier is set lower than the appropriate value at which the level of the intermediate frequency signal for distance measurement meets the minimum range limit of the distance measurement light. The optical distance meter is characterized in that the light receiving light intensity adjustment means is set so that the level of the distance measuring intermediate frequency signal is the minimum size that satisfies the maximum reachable distance, with the amplification level set lower than the appropriate value.

2. The difference between each main modulation frequency and the local frequency corresponding to each main modulation frequency is: The optical distance meter according to claim 1, characterized in that the difference between each paramodulation frequency and the main modulation frequency adjacent to each paramodulation frequency is 10 times or more.

3. The first light-emitting element and the second light-emitting element are laser diodes, The first light-emitting element sends light modulated with a high-frequency main modulation frequency that is higher than the plurality of main modulation frequencies, in addition to the plurality of main modulation frequencies, to the distance measuring optical path. The second light-emitting element sends light modulated at a high-frequency paramodulation frequency close to the high-frequency main modulation frequency, in addition to the paramodulation frequencies close to the plurality of main modulation frequencies, to the reference optical path. The optical distance meter according to claim 1 or 2, characterized in that the distance measurement signal based on the distance measurement light modulated at the high-frequency main modulation frequency and the distance measurement signal based on the reference light modulated at the high-frequency paramodulation frequency are configured not to be input to the frequency converter group.

4. The optical distance meter according to claim 1 or 2, characterized in that the first light-emitting element and the second light-emitting element are LEDs (Light-Emitting Diodes).

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