Optical distance meter

Abstract

This invention aims to create a simpler, shutterless phase-difference optical distance meter while reducing temperature phase drift. [Solution] The optical distance meter 100 includes a light-emitting element 20 that emits distance measuring light modulated at multiple modulation frequencies, a first light-receiving element 40 that receives distance measuring light reflected from the object to be measured, a second light-receiving element 40 that receives a portion of the light as reference light, a first frequency converter group 43 connected to the first light-receiving element 40 that converts the received signal of the distance measuring light into a distance measuring intermediate frequency signal using a local signal with a frequency close to the modulation frequency, a second frequency converter group 53 connected to the second light-receiving element 50 that converts the received signal of the reference light into a reference intermediate frequency signal using a local signal with a frequency close to the modulation frequency, and a calculation processing unit 60 that calculates the distance from the distance measuring intermediate frequency signal and the reference intermediate frequency signal to the object to be measured, wherein the multiple modulation frequencies are 10 MHz or less.

Description

[Technical Field]

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

[0002] An optical distance meter measures distance by emitting a distance-measuring light from a light-transmitting unit toward the object to be measured, and receiving the reflected distance-measuring light, which is reflected by the object, via a distance-measuring optical path. When distance is measured using only the distance-measuring light, temperature phase drift in the light-transmitting drive circuit and the light-receiving circuit, as well as drift due to the electrical circuit, appear as measurement errors. Therefore, in a phase-difference type optical distance meter, a reference light is used, which enters the light-receiving unit from the light-transmitting unit via a reference optical path, and accurate distance measurement is performed from the phase difference between the distance-measuring light and the reference light. This utilizes the fact that the phase difference changes with distance. If the phase difference is Δφ, the distance to the object to be measured 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 such, and the distance D can be determined by measuring the phase difference Δφ. In actual measurements, modulated light modulated with two or more carrier signals of different frequencies is used, and each digit of the distance measurement value is determined according to the resolution of each.

[0003] Therefore, conventional phase-difference optical distance meters were equipped with a shutter to switch between the distance-measuring optical path and the reference optical path. However, in this case, the measurement time is longer due to the shutter switching. Also, because the distance-measuring light and the reference light cannot be measured simultaneously, a difference in temperature phase drift occurs in the light-emitting and light-receiving elements and electrical components of the light-transmitting and light-receiving units, which causes 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, the optical distance meter described in Patent Document 1 has been proposed. The optical distance meter described in Patent Document 1 comprises two light-emitting elements and one light-receiving element. Light emitted from the first light-emitting element is incident on the light-receiving element via a distance-measuring optical path, and light emitted from the second light-emitting element in synchronization with the light emission from the first light-emitting element is incident on the first light-receiving element via a reference optical path.

[0005] Patent Document 1 describes how the temperature phase drift between two light-emitting elements can be approximated and eliminated by using light modulated at a main modulation frequency of 10 MHz or less, where the temperature phase drift is small, and light modulated at a para-modulation frequency close to the main modulation frequency. This reduces measurement errors. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2023-142441 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the optical distance meter in Patent Document 1 has two light-emitting elements. Since the internal temperature of a light-emitting element rises when it emits light, having two of them means that the effect of the temperature rise is greater, and the temperature phase drift tends to be larger. For this reason, there was a need to further reduce the phase drift due to the temperature rise. In addition, although the optical distance meter in Patent Document 1 uses two light-emitting elements, it requires a high-precision PLL (Phase Locked Loop) circuit for synchronization and a high-precision oscillator for the second light-emitting element, so there was a need for a cheaper and simpler configuration.

[0008] This disclosure is made in view of the circumstances described herein, 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 the reference optical path and the 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 light-emitting element that sends light modulated at multiple modulation frequencies as distance measuring light to a distance measuring optical path that travels back and forth to the object to be measured; a first light-receiving element that receives the distance measuring light reflected by the object to be measured; a second light-receiving element that receives a portion of the light sent from the light-emitting element as reference light via the reference optical path; and a distance measuring frequency converter corresponding to each modulation frequency, which is connected to the first light-receiving element and multiplies the received signal based on the distance measuring light by a first local signal of a local frequency close to the corresponding modulation frequency to convert the received signal into a distance measuring intermediate frequency signal. The device comprises a group of frequency converters, a second group of frequency converters each comprising a reference frequency converter corresponding to each modulation frequency, connected to the second photodetector, and multiplying the photodetector signal based on the reference light by a second local signal of a local frequency close to the corresponding modulation frequency to convert the photodetector signal based on the reference light into a reference intermediate frequency signal, and a calculation processing unit that calculates the distance to the object to be measured based on the distance measuring intermediate frequency signal and the reference intermediate frequency signal, wherein the calculation processing unit calculates the distance to the object to be measured by subtracting the reference intermediate frequency signal from the distance measuring intermediate frequency signal, and the plurality of modulation frequencies are 10 MHz or less. [Effects of the Invention]

[0010] According to the above embodiment, a phase-difference optical distance meter that does not use a shutter can be made simpler than conventional ones while reducing temperature phase drift. [Brief explanation of the drawing]

[0011] [Figure 1] This is a block diagram of an optical distance meter according to an embodiment of the present disclosure. [Figure 2] This figure shows a variable density filter, which is a means for adjusting the amount of light received by the distance measuring light of the same optical distance meter. [Figure 3] This table shows examples of the main modulation frequency, local frequency, and intermediate frequency of the optical distance meter. [Figure 4] (A) to (C) are diagrams schematically showing the spectrum of the ranging intermediate frequency signal of the same optical distance meter, and are diagrams for explaining the improvement of the S / N (signal-to-noise) ratio by the settings of the amplifier and the received light amount adjusting means. [Figure 5] (A) to (C) are diagrams schematically showing the spectrum of the reference intermediate frequency signal of the same optical distance meter, and are diagrams for explaining the improvement of the S / N ratio by the settings of the amplifier and the received light amount adjusting means. [Figure 6] This is a block diagram of an optical distance meter according to Modification 1 of the present embodiment. [Modes for Carrying Out the Invention]

[0012] Hereinafter, preferred embodiments of the present disclosure will be described with reference to the drawings, but the present disclosure is not limited thereto. In each embodiment, elements having the same mechanical configuration are denoted by the same reference numerals, and overlapping descriptions are omitted as appropriate.

[0013] (Embodiment) Figure 1 is a block diagram of the optical distance meter 100 according to the embodiment. The optical distance meter 100 is a phase difference type optical distance meter that calculates the distance to the measurement target 22 from the phase difference between the ranging light and the reference light. The optical distance meter 100 mainly includes a light emitting element 20, a first light receiving element 40 and a second light receiving element 50, an arithmetic processing unit 60, and a storage unit 70.

[0014] The light emitting element 20 is, for example, a semiconductor light emitting element such as a laser diode, and emits visible or infrared laser light. The light emitting element 20 emits light modulated at a plurality of modulation frequencies F1, F2, and F3. The modulation frequencies F1, F2, and F3 are in descending order of frequency (F1>F2>F3). For example, the modulation frequency F1 is higher than 10MHz, the modulation frequency F2 is 10MHz to 1MHz, and the modulation frequency F3 is 1MHz or lower.

[0015] The first light-receiving element 40 and the second light-receiving element 50 are, for example, photodetectors such as avalanche photodiodes, and output the received distance-measuring light and reference light as electrical signals (distance-measuring signal and reference signal), respectively. The first light-receiving element 40 and the second light-receiving element 50 are equivalent light-receiving elements.

[0016] The arithmetic processing unit 60 is a control arithmetic unit comprising, for example, at least one processor (e.g., a CPU (Central Processing Unit)) and at least one memory (SRAM (Static Random Access Memory), DRAM (Dynamic RAM), etc.). The arithmetic processing unit 60 realizes its function by having the processor read a program for performing that function into memory and execute it.

[0017] 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. When using these CPLDs or FPGAs, circuits and other components configured on these elements realize their functions.

[0018] The arithmetic processing unit 60 controls the emission of light from the light-emitting element 20 and calculates the distance to the object to be measured 22 based on the light signals received by the first light-receiving element 40 and the second light-receiving element 50. Further details will be described later.

[0019] The storage unit 70 is a non-temporary computer-readable storage medium such as flash memory or a hard disk drive. The storage unit 70 stores programs for the arithmetic processing unit 60, which is described below, to perform measurements, adjust the amount of received light, calculate distance values, etc., as well as initial setting data, measurement data, etc.

[0020] The configuration of the optical distance meter 100 and the details of distance measurement are described below. First, the oscillator 1 generates a signal with modulation frequency F1 in accordance with instructions from the arithmetic processing unit 60. The frequency divider 2 divides the signal with modulation frequency F1 to generate signals with modulation frequencies F2 and F3. These signals with modulation frequencies F2, F3 and the signal with modulation frequency F1 are input to the drive circuit 4 via the frequency superposition circuit 3. The light-emitting element 20 is driven by the drive circuit 4 and emits light modulated with modulation frequencies F1, F2, and F3.

[0021] The frequency generation circuit 12 generates a local signal (first local signal) with local frequencies F2+a·F2,F3+a·F3 and a local signal (second local signal) with local frequencies F2+b·F2,F3+b·F3. Here, the coefficient a is a value that is sufficiently small with respect to 1 (|a|<<1), and the coefficient b is a value that is sufficiently small with respect to 1 (|b|<<1).

[0022] The first and second local signals are input to the distance frequency converters 44 and 47 and the reference frequency converters 54 and 57, respectively, as will be described later. The difference between each modulation frequency F2 and F3 and the local frequencies F2+a·F2, F2+b·F2; F3+a·F3, F3+b·F3, |a|·F2,|b|·F2;|a|·F3,|b|·F3 is in the range of several hundred kHz to several tens of kHz.

[0023] Light emitted from the light-emitting element 20 passes through the distance measuring optical path 21, is reflected by the object to be measured 22, and enters the first light-receiving element 40 via the distance measuring optical path 23.

[0024] In the distance measuring optical path 23, a variable density filter 24 and a light receiving optical system 25, which are the first light receiving light intensity adjustment means, are arranged in front of the first light receiving element 40. The variable density filter 24 is, for example, a thin, disc-shaped neutral density filter as shown in Figure 2.

[0025] 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 attenuation rate is 0 to 100% (i.e., the transmittance is 100 to 0%). 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 light attenuation rate (transmittance) when the filter drive motor 24a is rotated according to the instructions of the calculation processing unit 60.

[0026] A portion of the light emitted from the light-emitting element 20 is split by the beam splitter 33 and enters the second light-receiving element 50 via the reference optical path 31. In the reference optical path 31, a density filter 32, which is a second light-receiving intensity adjustment means, is positioned before the second light-receiving element 50. As the density filter 32, for example, a neutral density filter with the same configuration as the variable density filter 24 may be fixed and used. Alternatively, a neutral density filter with a fixed density may be used. The density filter 32 adjusts the level of light-receiving intensity of the reference light by attenuating the reference light to achieve a preset transmittance.

[0027] The first light-receiving element 40 is connected to the first amplifier 41, and the first amplifier 41 is connected to the first frequency converter group 43 via the first band filter 42. The first frequency converter group 43 comprises two identical frequency converters, the distance measuring frequency converters 44 and 47.

[0028] The first photodetector 40 converts the received light into signals with three frequencies F1, F2, and F3. The first amplifier 41 amplifies the amplitudes of these signals. Of these signals, the signal with the high frequency modulation frequency (hereinafter also referred to as the high frequency modulation frequency) F1 is not used in the calculation of the distance measurement value. Therefore, the amplified signal is input to the distance measurement frequency converters 44 and 47 after the signal with frequency F1 is removed by the first band filter 42.

[0029] The distance measuring frequency converter 44 performs frequency multiplication using the first local signal of local frequency F2 + a·F2. The multiplied signal is output to the distance measuring band filter 45. The distance measuring band filter 45 removes the high-frequency region and outputs an intermediate frequency signal (distance measuring intermediate frequency signal) of frequency a·F2 to the A / D converter 46.

[0030] Similarly, the distance measuring frequency converter 47 performs frequency multiplication using the first local signal of local frequency F3 + a·F3. The multiplied signal is output to the distance measuring band filter 48. The distance measuring band filter 48 removes the high-frequency region and outputs an intermediate frequency signal (distance measuring intermediate frequency signal) of frequency a·F3 to the A / D converter 49. In this way, the first frequency converter group 43 includes a number of distance measuring frequency converters corresponding to the number of modulation frequencies used in calculating the distance measurement value.

[0031] Meanwhile, the second light-receiving element 50 is connected to the second amplifier 51, and the second amplifier 51 is connected to the second frequency converter group 53 via the second band filter 52. The second frequency converter group 53 includes two frequency converters, reference frequency converters 54 and 57. The second amplifier, the second band filter 52, the second frequency converter group 53, and the reference frequency converters 54 and 57 are of the same specifications as the first amplifier 41, the first band filter 42, and the first frequency converter group 43, respectively.

[0032] Similar to the first photodetector 40, the second photodetector 50 converts the received light into signals with three frequencies F1, F2, and F3. The second amplifier 51 amplifies the amplitudes of these signals. The signal with high-frequency modulation frequency F1 is not used in the calculation of the distance measurement value. The amplified signal is then input to the reference frequency converters 54 and 57 after the signal with frequency F1 is removed by the second band filter 52.

[0033] The reference frequency converter 54 performs frequency multiplication using a second local signal of local frequency F2 + b·F2. The multiplied signal is output to the reference band filter 55. The reference band filter 55 removes the high-frequency region and outputs an intermediate frequency signal (reference intermediate frequency signal) of frequency b·F2 to the A / D converter 56.

[0034] Similarly, the reference frequency converter 57 performs frequency multiplication using a second local signal of local frequency F3 + b·F3. The multiplied signal is output to the reference band filter 58. The reference band filter 58 removes the high-frequency region and outputs an intermediate frequency signal (reference intermediate frequency signal) of frequency b·F3 to the A / D converter 59. In this way, the second frequency converter group 53 also includes a number of frequency converters corresponding to the number of modulation frequencies used in calculating the distance measurement value.

[0035] The A / D converters 46, 49, 56, and 59 are each connected to the arithmetic processing unit 60. The A / D converters 46, 49, 56, and 59 convert analog signals into multi-level digital signals and output the signal waveform as a sine wave to the arithmetic processing unit 60, enabling the arithmetic processing unit 60 to acquire signal amplitude information and phase information. The signal amplitude information and phase information are used to calculate the distance measurement value. The signal amplitude information is also used for adjusting the amount of light received.

[0036] The arithmetic processing unit 60 analyzes the signal amplitude and phase information of the intermediate frequency signals with frequencies a·F2 and a·F3 to calculate the measured distance along the optical path using the ranging light at modulation frequencies F2 and F3. The arithmetic processing unit 60 also analyzes the signal amplitude and phase information of the reference intermediate frequency signals with frequencies b·F2 and b·F3 to calculate the measured reference optical path using the reference light at modulation frequencies F2 and F3. Then, by subtracting the measured reference optical path values ​​from the measured distance along the optical path, the arithmetic processing unit 60 calculates the measured distance to the target reflector using the ranging light at modulation frequencies F2 and F3, respectively.

[0037] Here, we will explain the details of the method for calculating the distance value to the object 22 in the arithmetic processing unit 60, and the reason why the effect of temperature phase drift can be reduced in this calculation method. Note that temperature phase drift occurs in the same way for both modulation frequencies F2 and F3, but the method for calculating the distance value and the reason why the effect can be reduced are the same for both modulation frequencies, so we will explain the signal for modulation frequency F2 as a representative example.

[0038] Let F2 = (1 + 0)f2 be the frequency of the electrical signal applied to the light-emitting element 20, and let F2 + a·F2 = (1 + a)f2 be the frequency of the first local signal applied to the distance measuring frequency converter 44. Here, a > 0 and b > 0.

[0039] The equation for the waveform y1 at the first photodetector 40 is given by equation 2 below.

[0040]

number

[0041] Next, let's consider the equation of the waveform after frequency conversion. If we let y2 be the waveform of the first local signal of local frequency (1+a)·F2 which is multiplied with y1 by the distance measuring frequency converter 44, then y2 is It can be expressed as shown in Equation 3.

[0042]

number

[0043] Next, in the distance measuring frequency converter 44, the waveform y3 of the intermediate frequency signal with frequency a·F2 based on the distance measuring light that has passed through the distance measuring band filter 45 after y1 has been frequency-converted (multiplied) by y2 can be expressed as shown in Equation 4.

[0044]

number

[0045] On the other hand, the equation for the waveform at the second photodetector 50 is given by equation 5 below.

[0046]

number

[0047] Next, let's consider the equation for the waveform after frequency conversion. If we let y5 be the waveform of the second local signal of local frequency (1+b)·F2 which is multiplied with y4 by the reference frequency converter 54, then y5 can be expressed as shown in equation 6.

[0048]

number

[0049] Next, in the reference frequency converter 54, the waveform y6 of the reference intermediate frequency signal with frequency b·F2 based on the reference light that has passed through the reference band filter 55 after y4 has been frequency-converted (multiplied) by y5 can be expressed as shown in Equation 7.

[0050]

number

[0051] Next, the distance measurement value d is determined. The distance measurement value d can be converted by the following general formula (Equation 8).

[0052] [Mathematical Formula] where θ is the initial phase obtained by measurement, f is the modulation frequency, and c is the speed of light.

[0053] Let d0 be the measured value of the ranging optical path for the phase component of the waveform y3 of the ranging intermediate frequency signal of frequency a·F2 based on the ranging light, and d1 be the measured value of the reference optical path for the phase component of the waveform y6 of the reference intermediate frequency signal of frequency b·F2 based on the reference light, then d0 and d1 can be expressed by Equation 9 and Equation 10 below, respectively.

[0054] [Mathematical Formula]

[0055] When these two equations are subtracted, the result is as shown in Equation 11.

[0056] [Mathematical Formula] where φ APD1 (f2) is the temperature phase drift of the first light receiving element 40 due to the signal of frequency F2, and φ APD2 (f2) is the temperature phase drift of the second light receiving element 50 due to the signal of frequency F2. It is known that when the modulation frequency is 10 MHz or lower, the temperature phase drift of the light receiving element is negligibly small. Therefore, φ APD1 (f2), φ APD2 (f2) can be neglected, so Equation 11 can be expressed as Equation 12.

[0057] [Mathematical Formula]

[0058] From Equation 12, the distance measured from the optical distance meter 100 to the object being measured 22 can be determined. From Equation 12, the temperature phase drift φ originating from the light-emitting element 20, the first light-receiving element 40, and the second light-receiving element 50 can be determined. LD (f2), φ APD1 (f2), φ APD2 It can be seen that (f2) has been removed, and the temperature phase drift has been reduced. In Patent Document 1, the temperature phase drift difference between the two light-emitting elements was removed by approximation. However, in the optical distance meter 100, since there is only one light-emitting element, the temperature phase drift difference can be calculated to zero, and the phase drift error can be reduced even further.

[0059] Furthermore, the distance measurement value from the optical distance meter 100 to the object being measured 22, which is obtained by Equation 12, is affected by a temperature phase drift φ originating from the electronic components on the light-receiving side (distance measurement band filter 45, reference band filter 55). AMP1 ,φ AMP2 This includes [something]. Generally, it is known that the temperature phase drift that occurs in elements such as light-emitting elements and photodetectors, and in electronic components, is larger as the applied signal frequency increases.

[0060] The temperature phase drift occurring in the distance measuring band filter 45 and the reference band filter 55 is a temperature phase drift caused by the low-frequency (intermediate-frequency) signal after frequency conversion, and its effect is smaller than, for example, the temperature phase drift that occurs in the light-emitting element and photodetector 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.

[0061] Furthermore, there is a temperature phase drift φ originating from these light-receiving electronic components (distance measurement band filter 45, reference band filter 55). AMP1 ,φ AMP2To achieve the effect of reducing the influence of , the conditions a<0 and b>0, a>0 and b<0, and a<0 and b<0 are also acceptable, in addition to a>0 and b>0. The modulation frequency, the first and second local frequencies, and the intermediate frequency for each condition are shown in Figure 3 using α=|a| and β=|b|. As shown in Figure 3, in all conditions, the intermediate frequency after the distance measuring band filter 45 is α·F2, and the intermediate frequency after the reference band filter 55 is β·F2, and in both cases the temperature phase drift φ originating from the light-receiving electronic component is similar. AMP1 ,φ AMP2 It can be seen that the impact can be reduced.

[0062] Here, if |a|=|b|, then from equation 12, the temperature phase drift φ originating from the light-receiving electronic components (distance band filter 45, reference band filter 55) is as follows. AMP1 ,φ AMP2 This is particularly advantageous because it allows for cancellation and elimination. The case where |a|=|b| is when the absolute value of the difference between the modulation frequency F2 and the local frequency F2+a·F2 of the first local signal corresponding to that frequency is equal to the absolute value of the difference between the modulation frequency F2 and the local frequency F2+b·F2 of the second local signal corresponding to that frequency. When |a|=|b|, it is possible to approximate it as shown in Equation 13.

[0063]

number

[0064] With this approximation, equation 12 can be transformed into equation 14.

number

[0065] As a result, the temperature phase drift φ originating from the electronic components on the light-receiving side is detected. AMP1 ,φ AMP2 It becomes possible to eliminate the effects of here.

[0066]

number

number

number

[0067]

number

[0068] If we let dx be the corrected distance measurement value after correcting for the distance difference Δ, then dx can be calculated as shown in Equation 16.

[0069]

number

[0070] Thus, with the optical distance meter 100 according to this embodiment, by simultaneously receiving light emitted from one light-emitting element with two photodetectors and using light modulated by a signal with a modulation frequency of 10 MHz or less (F2, F3), the temperature phase drift occurring in the light-emitting element and the photodetectors can be ignored. Furthermore, since the temperature phase drift difference of the electronic components on the photodetector side can be calculated to be zero, errors caused by the temperature phase drift difference can be reduced, and the accuracy of the distance measurement can be improved.

[0071] As a result, compared to the optical distance meter described in Patent Document 1, which does not use a shutter to switch between the reference optical path and the distance measuring optical path and has two light-emitting elements and one photodetector, it becomes possible to omit high-precision oscillators, high-precision PLL circuits, etc., that are required to synchronize the emission of the two light-emitting elements. Therefore, the overall configuration can be simplified while reducing temperature phase drift compared to conventional designs. This makes it possible to reduce costs.

[0072] Furthermore, the optical distance meter 100 according to this embodiment can measure the distance measuring optical path and the reference optical path simultaneously, enabling faster distance measurement than conventional shutter-type optical distance meters. Additionally, the absence of a shutter reduces costs. Conventional shutter-type optical distance meters kept the power to the light-emitting element ON during continuous distance measurement to minimize temperature phase drift differences due to measurement time differences. However, with the optical distance meter 100, the power to the light-emitting element can be turned ON / OFF for each distance measurement without being affected by temperature phase drift due to time differences, thus saving power. In this way, it is possible to achieve effects equivalent to those of the optical distance meter described in Patent Document 1.

[0073] Furthermore, in the optical distance meter 100, if the absolute value of the difference between the modulation frequency and the local frequency of the corresponding first local signal is equal to the absolute value of the difference between the modulation frequency and the local frequency of the corresponding second local signal for the same modulation frequency, then the temperature phase drift originating from the light-receiving electronic components (distance measurement band filter 45, reference band filter 55) will be equal, making it possible to set the temperature phase drift difference to zero in the distance measurement formula (Equation 12). This further improves the accuracy of the distance measurement.

[0074] Furthermore, by storing the distance difference measurement value corresponding to the phase difference between the first local signal and the second local signal in the storage unit 70, and by having the arithmetic processing unit 60 subtract the distance difference measurement value from the measured distance value to correct the measured distance value, the accuracy of the measured distance value can be further improved.

[0075] 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 first band filter 42 and the second band filter 52 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 make the oscillator 1 oscillate at a high-frequency modulation frequency F1 greater than 10 MHz. However, when the light-emitting element 20 is a laser diode, 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 light-emitting element 20 at modulation frequency F1, because this stabilizes the phase of the signal.

[0076] Incidentally, it is generally known that using a signal with a high modulation frequency in an optical distance meter results in higher accuracy of the measured distance. For this reason, in the optical distance meter 100 according to this embodiment, which does not use a high-frequency modulation frequency F1, the accuracy of the measured distance may be low. In such cases, the optical distance meter 100 can ensure accuracy equivalent to or better than that when using a signal with modulation frequency F1 by setting the amplification of the first amplifier 41 and the second amplifier 51 and the light receiving light intensity adjustment means (variable density filter 24 and density filter 32) as follows, as needed.

[0077] In a typical optical distance meter, the received light signal is amplified by an amplifier and used to calculate the distance. The amplification factor of the amplifier is set to an appropriate value according to the specifications of the optical distance meter (light-emitting element, light-receiving element, and other components) such that the amount of received light for distance measurement, i.e., the level of the intermediate frequency signal for distance measurement, is the minimum magnitude that satisfies the limit distance of the distance measurement light. Specifically, it is the signal level at which the A / D converter count becomes 1 at the limit distance. In addition, the light-receiving light intensity adjustment means is set to an appropriate transmittance value so that, with the amplification factor at an appropriate value, the amplitude of the intermediate frequency signal becomes the maximum value within the input range of the subsequent A / D converter.

[0078] Therefore, in the optical distance meter 100, the appropriate value for the amplification of the first amplifier 41 (appropriate amplification value) is set to the minimum magnitude at which the level of the distance measuring intermediate frequency signal satisfies the limit of the reach of the distance measuring light, and the appropriate value for the transmittance of the variable density filter 24 (appropriate transmittance value) is set to the value at which the distance measuring intermediate frequency signal when the light-emitting element 20 is emitted reaches the appropriate light intensity level when the amplification is set to the appropriate amplification value. In addition, the appropriate value for the second amplification 51 is set to the same value as the appropriate amplification value.

[0079] However, in the optical distance meter 100, the amplification of the first amplifier 41 is set lower than the appropriate amplification value, and with the amplification set lower than the appropriate amplification value, the transmittance of the variable density filter 24 is adjusted to a value such that the intermediate frequency signal for distance measurement when the light-emitting element 20 emits light reaches the appropriate light intensity level. In addition, the amplification of the second amplifier 51 is set lower than the appropriate amplification value, and with the amplification set lower than the appropriate amplification value, the transmittance of the density filter 32 is adjusted so that the reference intermediate frequency signal when the light-emitting element 20 emits light reaches the appropriate light intensity level of the reference light. As a result, the signal-to-noise ratio of the intermediate frequency signal for distance measurement and the reference intermediate frequency signal is improved, and the measurement accuracy is improved.

[0080] Furthermore, it is preferable that the reduction rate of the amplification of the second amplifier 51 is greater than the reduction rate of the amplification of the first amplifier 41. This is because the signal-to-noise ratio of the reference optical path 31 can be improved compared to the signal-to-noise ratio of the distance measuring optical path. In this way, the variation in the measured distance can be reduced and the accuracy improved compared to the optical distance meter described in Patent Document 1.

[0081] This explains why accuracy improves. Figure 4(A) schematically shows the signal spectrum of the intermediate frequency signal (for example, the intermediate frequency signal with frequency a·F2 in Figure 1) in the optical distance meter 100 when the amplification of the first amplifier 41 is set to the appropriate amplification value and the transmittance of the variable density filter 24 is set to the appropriate transmittance value. The amplified intermediate frequency signal contains both signal and noise, but when the amplification is set lower than the appropriate amplification value, the signal and noise are reduced in the same ratio, as shown in Figure 4(B).

[0082] In this state, the transmittance of the variable density filter 24 is set to increase the amount of light transmitted so that the received signal level input to the A / D converter 46 becomes the appropriate light amount equivalent to when the amplification of the first amplifier 41 is set to an appropriate value. For example, if the amplification of the first amplifier 41 is reduced by 1 / x (x>0) from the appropriate value, the transmittance of the variable density filter 24 is set to x times. For example, in a conventional optical distance meter, if the appropriate light amount is obtained when the transmittance of the variable density filter is 1% when the appropriate amplification is 10 times, the transmittance of the variable density filter 24 is set to 10 percent (10 times) to obtain a similar received signal level when the amplification is set to 1 (1 / 10). In this way, the received signal level increases, but the level of noise generated on the light-emitting element 20 side does not change. In addition, the level of noise generated on the first light-receiving element 40 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 4(C), the signal level can be increased without increasing the noise level, improving the signal-to-noise ratio. This makes it possible to improve accuracy.

[0083] On the other hand, Figure 5(A) schematically shows the signal spectrum of the reference intermediate frequency signal (for example, the reference intermediate frequency signal with frequency b·F2 in Figure 1) in the optical distance meter 100 when the amplification of the second amplifier 51 is set to the appropriate amplification value and the transmittance of the variable density filter 24 is set to the appropriate transmittance value. The amplified reference intermediate frequency signal also contains both signal and noise, but when the amplification is set lower than the appropriate amplification value, the signal and noise are reduced in the same ratio, as shown in Figure 5(B).

[0084] In this state, the transmittance of the density filter 32 is set to increase the amount of light transmitted so that the received signal level input to the A / D converter 56 becomes the same as the appropriate light amount when the amplification of the second amplifier 51 is set to an appropriate value. In other words, when the amplification of the second amplifier 51 is reduced to an appropriate value of 1 / y (y>0, and preferably y>x), the transmittance of the density filter 32 is set to y times. For example, in a conventional optical distance meter, if the appropriate light amount is achieved when the transmittance of the density filter 32 is 1% (i.e., attenuation rate of 99%) when the appropriate amplification is 10 times, the transmittance of the density filter 32 is set to 20 percent (20 times) in order to achieve a similar received signal level with an amplification of 0.05 times (1 / 20). In this way, the received signal level increases, but the level of noise generated on the light-emitting element 20 side does not change. Furthermore, as the level of the received light signal increases, the level of noise generated on the second light-receiving element 50 increases somewhat, but it is negligible compared to the signal.

[0085] As a result, as shown in Figure 5(C), the signal level can be increased without increasing the noise level, and the signal-to-noise ratio (S / N ratio) of the reference intermediate frequency signal is improved. This makes it possible to improve the accuracy of the reference optical path measurement value d1.

[0086] Since the distance measurement value is calculated from the difference between the distance measurement optical path measurement value d0 and the reference optical path measurement value d1, improving the accuracy of the reference optical path measurement value d1 also improves the accuracy of the distance measurement value. In Patent Document 1, there was only one photodetector, so reducing the amplification of the amplifier in the distance measurement optical path would reduce the amplification of the reference optical path to a similar extent. In this embodiment, since a configuration with two photodetectors is used, it becomes possible to independently change the amplification of the distance measurement intermediate frequency signal and the amplification of the reference intermediate frequency signal, making it possible to further improve the accuracy of the distance measurement value compared to the optical distance meter in Patent Document 1.

[0087] Incidentally, the arithmetic processing unit 60 is configured to drive the filter drive motor 24a according to the amount of light received during measurement (signal level (signal amplitude) of the intermediate frequency signal output from the A / D converter 46) so that the amount of light received becomes the maximum value within the input range of the A / D converter 46. The A / D converter 46 does not measure the signal amplitude in the saturation region that exceeds the input range of the measurable signal level. Therefore, the arithmetic processing unit 60 drives the filter drive motor 24a so that the amount of light received becomes the maximum value within the input range if the amount of light received is greater than the input range of the A / D converter 46, and so that the amount of light received becomes the maximum value within the input range if it is within the input range of the A / D converter 46. Also, if it is smaller than the input range of the A / D converter 46, the filter drive motor 24a is driven so that the amount of light received becomes the maximum value within the input range. Therefore, the appropriate light intensity level for distance measuring light is the maximum value within the input range if the amount of light received for distance measuring light is greater than the input range of the A / D converter 46, and the value of the amount of light received if it is within the input range of the A / D converter 46.

[0088] Furthermore, if the amplification of the first amplifier 41 is set lower than the appropriate amplification 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 may result in the signal being mistakenly digitized to 0, making signal detection impossible and potentially shortening the maximum range achievable by the measuring light. However, this problem can be resolved by increasing the resolution of the A / D converter used, that is, by using a high-resolution A / D converter to make the detection limit when the amplification is set low the same as the detection limit when the amplification is set to the appropriate value. In addition, in the above explanation, both the amplification of the first amplifier 41 on the measuring optical path side and the transmittance of the variable density filter 24, and the amplification of the second amplifier 51 on the reference optical path side and the transmittance of the density filter 34 were adjusted. However, adjusting only one of them can also be expected to improve accuracy to some extent.

[0089] (Variation 1) Figure 6 is a block diagram of the optical distance meter 100A according to Modification 1. The optical distance meter 100A has a substantially the same configuration as the optical distance meter 100, but differs in that the light-emitting element 20A is an LED (Light-Emitting Diode) instead of a laser diode. Furthermore, the oscillator 1A oscillates at a frequency F2 between 10MHz and 1MHz, rather than a frequency F1 greater than 10MHz. The frequency divider 2 divides the signal of frequency F2 and outputs a signal of frequency F3 of 1MHz or less.

[0090] Therefore, the drive circuit 4A is driven only by signals of frequencies F2 and F3. Since the received signals from the first photodetector 40 and the second photodetector 50 do not contain any signals originating from the signal of frequency F1, the first band filter 42 and the second band filter 52 are also unnecessary. In this way, when the light-emitting element 20A is an LED, even without transmitting light modulated at a high frequency modulation frequency F1, the stability of the signal phase is not affected, and it is possible to achieve the same effect as the optical distance meter 100.

[0091] The embodiments of this disclosure have been described above with reference to the drawings, but these are illustrative examples, and various other configurations can be adopted. Furthermore, the embodiments described above can be combined to the extent that they do not conflict with each other.

[0092] Furthermore, while the flowcharts used in the above description show multiple steps (processes) in sequence, the execution order of the steps performed in each embodiment is not limited to the order in which they are described. In each embodiment, the order of the illustrated steps can be changed to the extent that it does not impair the content.

[0093] Some or all of the above embodiments may also be described as follows, but are not limited to the following:

[0094] 1. A light-emitting element that transmits light modulated with multiple modulation frequencies as a distance-measuring light to a distance-measuring optical path that travels back and forth to the object to be measured, A first light-receiving element that receives the distance-measuring light reflected from the object to be measured, A second light-receiving element receives a portion of the light emitted from the aforementioned light-emitting element as reference light via a reference optical path, A group of first frequency converters, each comprising a distance measuring frequency converter corresponding to a different modulation frequency, connected to the first light-receiving element, which multiplies the received light signal based on the distance measuring light by a first local signal of a local frequency close to the corresponding modulation frequency to convert the received light signal based on the distance measuring light into a distance measuring intermediate frequency signal, A second group of frequency converters, each comprising a reference frequency converter corresponding to a modulation frequency, connected to the second photodetector, and multiplying by a second local signal of a local frequency close to the corresponding modulation frequency to convert the photodetector signal based on the reference light into a reference intermediate frequency signal, The system includes a calculation processing unit that calculates the distance to the object to be measured based on the distance measuring intermediate frequency signal and the reference intermediate frequency signal, The calculation processing unit subtracts the reference intermediate frequency signal from the distance measurement intermediate frequency signal to calculate the distance value to the object to be measured. An optical distance meter characterized in that the plurality of modulation frequencies are 10 MHz or less. 2. For the same modulation frequency, The absolute value of the difference between the modulation frequency and the local frequency of the corresponding first local signal, The absolute value of the difference between the modulation frequency and the local frequency of the corresponding second local signal is: The optical distance meter described in item 1 above, characterized by being equal. 3. The system further includes a storage unit that stores distance difference measurements corresponding to the phase difference between the first local signal and the second local signal. The optical distance meter according to claim 1 or 2 above, characterized in that the calculation processing unit corrects the distance measurement value by subtracting the distance difference measurement value from the distance measurement value. 4. An amplifier that amplifies the received light signal based on the reference light and inputs it to the second group of frequency converters, A light-receiving light intensity adjustment means for adjusting the level of light received by the reference light, Furthermore, The amplification of the amplifier is set lower than the appropriate amplification value, and the transmittance of the light receiving light intensity adjustment means is set such that the reference intermediate frequency signal when the light-emitting element emits light reaches the appropriate light intensity level when the amplification is set lower than the appropriate amplification value. The optical distance meter according to claim 1 or 2 above, characterized in that the appropriate amplification value is the minimum value at which the level of the distance measuring intermediate frequency signal satisfies the limit distance reachable by the distance measuring light. 5. A first amplifier that amplifies the received light signal based on the distance measuring light and inputs it to the first group of frequency converters, A first light-receiving light intensity adjustment means for adjusting the level of light received by the distance measuring light, A second amplifier that amplifies the received light signal based on the aforementioned reference light and inputs it to the second group of frequency converters, A second light-receiving light intensity adjusting means for adjusting the level of light received by the reference light, Furthermore, The amplification of the first amplifier is set to be lower than the appropriate amplification value, and the transmittance of the first light receiving light intensity adjustment means is set such that, when the amplification of the first amplifier is set to be lower than the appropriate amplification value, the level of the distance measuring intermediate frequency signal when the light-emitting element is emitted becomes the appropriate light intensity level of the distance measuring light. The amplification of the second amplifier is set to be lower than the appropriate amplification value, and the transmittance of the second light receiving light intensity adjustment means is set such that, when the amplification of the second amplifier is set to be lower than the appropriate amplification value, the level of the reference intermediate frequency signal when the light-emitting element is emitted is at the appropriate light intensity level of the reference light. The appropriate amplification value is the minimum value at which the level of the intermediate frequency signal for distance measurement satisfies the limiting distance of the distance measurement light. The optical distance meter according to claim 1 or 2, characterized in that the reduction rate of the amplification of the second amplifier from the appropriate amplification value is greater than the reduction rate of the amplification of the first amplifier from the appropriate amplification value. 6. The light-emitting element is a laser diode, and in addition to the plurality of modulation frequencies, it sends light modulated with a high-frequency modulation frequency that is higher than the plurality of modulation frequencies to the distance measuring optical path and the reference optical path. The optical distance meter according to any one of claims 1 to 5, characterized in that the distance measurement signal based on the distance measurement light modulated with the high frequency modulation frequency and the reference signal based on the reference light modulated with the high frequency modulation frequency are configured not to be input to the first and second frequency converter groups. 7. The optical distance meter according to any one of items 1 to 5 above, characterized in that the light-emitting element is an LED (Light-Emitting Diode). [Explanation of Symbols]

[0095] 20: Light-emitting element 20A: Light-emitting element 21: Distance measurement optical path 22: Object to be measured 23: Distance measurement optical path 31: Reference optical path 40: First photodetector 41: First Amplifier 43: First group of frequency converters 44: Distance measuring frequency converter 47: Distance measuring frequency converter 50: Second photodetector 51: Second Amplifier 53: Second group of frequency converters 54: Reference frequency converter 57: Reference frequency converter 60: Processing Unit 70: Storage section 100:Lightwave distance meter 100A:Light wave distance meter

Claims

1. A light-emitting element that transmits light modulated with multiple modulation frequencies as a distance-measuring light to a distance-measuring optical path that travels back and forth to the object to be measured, A first light-receiving element that receives the distance-measuring light reflected from the object to be measured, A second light-receiving element receives a portion of the light emitted from the aforementioned light-emitting element as reference light via a reference optical path, A group of first frequency converters is provided, each corresponding to a modulation frequency, connected to the first light-receiving element, and multiplies the received light signal based on the distance-measuring light by a first local signal of a local frequency close to the corresponding modulation frequency to convert it into a distance-measuring intermediate frequency signal. A second group of frequency converters, each comprising a reference frequency converter corresponding to a modulation frequency, connected to the second photodetector, and multiplying by a second local signal of a local frequency close to the corresponding modulation frequency to convert the photodetector signal based on the reference light into a reference intermediate frequency signal, The system includes a calculation processing unit that calculates the distance to the object to be measured based on the distance measuring intermediate frequency signal and the reference intermediate frequency signal, The calculation processing unit subtracts the reference intermediate frequency signal from the distance measurement intermediate frequency signal to calculate the distance value to the object to be measured. An optical distance meter characterized in that the plurality of modulation frequencies are 10 MHz or less.

2. For the same modulation frequency, The absolute value of the difference between the modulation frequency and the local frequency of the corresponding first local signal, The absolute value of the difference between the modulation frequency and the local frequency of the corresponding second local signal is: The optical distance meter according to claim 1, characterized in that they are equal.

3. The system further includes a storage unit that stores distance difference measurements corresponding to the phase difference between the first local signal and the second local signal. The optical distance meter according to claim 1 or 2, characterized in that the calculation processing unit corrects the distance measurement value by subtracting the distance difference measurement value from the distance measurement value.

4. An amplifier that amplifies the received light signal based on the aforementioned reference light and inputs it to the second group of frequency converters, A light-receiving light intensity adjustment means for adjusting the level of light received by the reference light, Furthermore, The amplification of the amplifier is set lower than the appropriate amplification value, and the transmittance of the light receiving light intensity adjustment means is set such that the reference intermediate frequency signal when the light-emitting element emits light reaches the appropriate light intensity level when the amplification is set lower than the appropriate amplification value. The optical distance meter according to claim 1 or 2, characterized in that the appropriate amplification value is the minimum value at which the level of the intermediate frequency signal for distance measurement satisfies the limiting distance of the distance measurement light.

5. A first amplifier that amplifies the received light signal based on the distance measuring light and inputs it to the first group of frequency converters, A first light-receiving light intensity adjustment means for adjusting the level of light received by the distance measuring light, A second amplifier that amplifies the light-receiving signal based on the aforementioned reference light and inputs it to the second group of frequency converters, A second light-receiving light intensity adjusting means for adjusting the level of light received by the reference light, Furthermore, The amplification of the first amplifier is set to be lower than the appropriate amplification value, and the transmittance of the first light receiving light intensity adjustment means is set such that, when the amplification of the first amplifier is set to be lower than the appropriate amplification value, the level of the distance measuring intermediate frequency signal when the light-emitting element is emitted becomes the appropriate light intensity level of the distance measuring light. The amplification of the second amplifier is set to be lower than the appropriate amplification value, and the transmittance of the second light receiving light intensity adjustment means is set such that, when the amplification of the second amplifier is set to be lower than the appropriate amplification value, the level of the reference intermediate frequency signal when the light-emitting element is emitted becomes the appropriate light intensity level of the reference light. The appropriate amplification value is the minimum value at which the level of the intermediate frequency signal for distance measurement satisfies the limiting distance of the distance measurement light. The optical distance meter according to claim 1 or 2, characterized in that the reduction rate of the amplification of the second amplifier from the appropriate amplification value is greater than the reduction rate of the amplification of the first amplifier from the appropriate amplification value.

6. The light-emitting element is a laser diode, and in addition to the plurality of modulation frequencies, it sends light modulated with a high-frequency modulation frequency that is higher than the plurality of modulation frequencies to the distance measuring optical path and 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 with the high frequency modulation frequency and the reference signal based on the reference light modulated with the high frequency modulation frequency are configured not to be input to the first and second frequency converter groups.

7. The optical distance meter according to claim 1 or 2, characterized in that the light-emitting element is an LED (Light-Emitting Diode).

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