Optical distance meter

Abstract

To provide a light wave rangefinder capable of effectively utilizing a light quantity range of distance measuring light.SOLUTION: A light wave rangefinder 100 comprises: a first light-emitting element 20 sending, as distance measuring light, light modulated by a plurality of main modulation frequencies to distance-measuring optical paths 21, 23 going to and returning from a target reflecting object; a second light-emitting element 30 sending, as reference light, light modulated by a side modulation frequency adjacent to each main modulation frequency to a reference optical path 31 in synchronization with the distance measuring light; a light receiving element 40 receiving the light emitted from the first and second light-emitting elements 20, 30 to output a distance measuring signal and a reference signal; and an arithmetic processing part 60 calculating a distance to the target reflecting object on the basis of phase difference between the distance measuring signal and the reference signal; and a light quantity adjustment circuit 80 adjusting light emission quantity of the first light-emitting element 20.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] In a phase difference type optical wave distance meter, a signal modulated in intensity is sent from a light emitting element as measurement light to a measurement optical path, reflected by a target reflector, received by a light receiving element, and converted into a measurement distance signal which is an electrical signal. Also, the light sent from the same light emitting element is switched to a reference optical path, received by the light receiving element as reference light, and converted into a reference signal which is an electrical signal. The measurement distance signal and the reference signal are input to an A / D converter after passing through an amplifier, a frequency converter, etc., and are converted from an analog signal to a digital signal. Then, a signal within the measurable range within the input range of the A / D converter is analyzed by an arithmetic processing unit for signal amplitude and phase information, and the phase of the measurement distance signal and the reference signal is calculated, thereby obtaining the straight-line distance (measurement distance value) to the target reflector. Also, the signal amplitude is used for determining the received light amount level.

[0003] Here, when the distance from the optical wave distance meter to the target reflector is short, the amount of received measurement light increases, and when it is long, the amount of measurement light decreases. Therefore, variations occur in the output level of the measurement distance signal according to the distance from the optical wave distance meter to the target reflector. When the amount of received measurement light is large and the measurement distance signal exceeds the maximum input value of the A / D converter (when the measurement distance signal is in the saturation region of the A / D converter), it does not become a signal within the measurable range and the measurement distance value cannot be calculated.

[0004] Conversely, when the amount of received measurement light is small, the amplitude of the received light signal is not appropriately resolved by the A / D converter, errors occur during signal conversion, and errors may occur in the measurement distance value. Therefore, it is necessary to adjust the received light amount so that the measurement distance signal is within the input range of the A / D converter. Thus, in Patent Document 1, a variable density filter driven by a motor or the like is provided between the target reflector and the light receiving element, and aperture adjustment is performed to adjust the amount of received reflected light (measurement light).

[0005] However, when adjusting the amount of light received using a variable density filter, it takes time to adjust the light intensity because the filter is operated mechanically. For this reason, Patent Document 2 proposes adjusting the amount of light emitted by changing the output of a light-emitting element by adjusting the resistance value of a digital potentiometer, instead of adjusting the amount of light received by a mechanical means such as a variable density filter. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Special Publication No. 51-8340 [Patent Document 2] Japanese Patent Publication No. 2011-013108 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, in the optical distance meter described in Patent Document 2, the distance measuring light and the reference light are emitted from the same light-emitting element, and the distance measuring light path and the reference light path are switched by a shutter and received by the same photodetector. Therefore, if the amount of light emitted by the distance measuring light is changed, the amount of light of the reference light changes accordingly. Consequently, if an attempt is made to adjust the amount of light of the distance measuring light to an appropriate level, the amount of light of the reference light may deviate from the input range of the A / D converter, and as a result, distance measurement may become impossible. Thus, the optical distance meter described in Patent Document 2 had the problem that the usable range of light intensity for the distance measuring light may be narrowed.

[0008] This invention has been made in view of the above circumstances, and aims to provide an optical distance meter that can effectively utilize the light intensity range of the distance measuring light. [Means for solving the problem]

[0009] To achieve the above objective, the optical distance meter according to the first embodiment of the present invention comprises: a first light-emitting element that sends light modulated at a plurality of main modulation frequencies as 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 a paramodulation frequency close to each of the main modulation frequencies as reference light to the reference optical path in synchronization with the distance measuring light; a light-receiving element that receives light emitted from the first and second light-emitting elements and outputs a distance measuring signal and a reference signal; a calculation processing unit that calculates the distance to the target reflector based on the phase difference between the distance measuring signal and the reference signal; and a light intensity adjustment circuit that adjusts the amount of light emitted by the first light-emitting element.

[0010] In the above embodiment, it is also preferable to include an A / D converter that performs A / D conversion of the distance measurement signal, wherein the arithmetic processing unit detects the amplitude of the distance measurement signal input to the A / D converter as the amount of light received by the light receiving element, and the arithmetic processing unit controls the light intensity adjustment circuit to adjust the amount of light emitted by the first light-emitting element so that the amount of light received falls within the input range of the A / D converter.

[0011] Furthermore, in the above embodiment, it is also preferable that the light intensity adjustment circuit includes a digital potentiometer, which is a variable resistor means, connected to the first light-emitting element and receiving a DC voltage, and that the arithmetic processing unit adjusts the amount of light emitted by the first light-emitting element by changing the resistance value of the digital potentiometer.

[0012] Furthermore, in the above embodiment, it is also preferable that the light intensity adjustment circuit includes a load resistor that applies a constant resistance to the first light-emitting element and a variable DC power supply that supplies a variable DC voltage to the load resistor, and the arithmetic processing unit adjusts the amount of light emitted by the first light-emitting element by varying the voltage supplied by the variable DC power supply. [Effects of the Invention]

[0013] According to the optical distance meter described above, it becomes possible to effectively utilize the light intensity range of the distance measuring light. [Brief explanation of the drawing]

[0014] [Figure 1] This is a block diagram of an optical distance meter according to the first embodiment of the present invention. [Figure 2] This is a flowchart illustrating the operation of adjusting the light intensity using the same optical distance meter. [Figure 3] (A) is a diagram illustrating the adjustment of light intensity using the same optical distance meter, and (B) is a diagram illustrating the adjustment of light intensity using a conventional optical distance meter. [Figure 4] (A) is a diagram illustrating the adjustment of light intensity using the same optical distance meter, and (B) is a diagram illustrating the adjustment of light intensity using a conventional optical distance meter. [Figure 5] This is a block diagram of an optical distance meter according to a second embodiment of the present invention. [Modes for carrying out the invention]

[0015] Preferred embodiments of the present invention will be described below with reference to the drawings, but the present invention is not limited thereto. In each embodiment, elements having the same mechanical configuration are given the same names and reference numerals, and redundant descriptions are omitted as appropriate.

[0016] (First Embodiment) Figure 1 is a block diagram of an optical distance meter 100 according to the first embodiment. The optical distance meter 100 is a phase difference type optical 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 distance meter 100 mainly comprises two light-emitting elements (i.e., a first light-emitting element 20 and a second light-emitting element 30), one light-receiving element 40, a calculation processing unit 60, a storage unit 70, and a light intensity adjustment circuit 80.

[0017] The first light-emitting element 20 and the second light-emitting element 30 are, for example, LEDs (Light-Emitting Diodes), and emit visible light or infrared light. The first light-emitting element 20 emits light modulated at a plurality of main modulation frequencies F1, F2. The second light-emitting element 30 emits light modulated at side modulation frequencies F1 + b·F1, F2 + b·F2 that are respectively close to each main modulation frequency. The number of main modulation frequencies is not limited to 2, and may be 3 or more as needed.

[0018] Here, the main modulation frequencies F1, F2 are in descending order of frequency. The frequency of F1 is preferably 10 MHz to 1 MHz, and the frequency of F2 is preferably 1 MHz or less. The reason will be described later. Also, the deviations |b|·F1, |b|·F2 between the main modulation frequencies F1, F2 and the side modulation frequencies F1 + b·F1, F2 + b·F2 are several tens of kHz to several kHz. That is, b is a value sufficiently small with respect to 1.

[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, a control arithmetic unit including at least one processor (e.g., CPU) and a memory (such as DRAM (Dinamic Random Access Memory)). The arithmetic processing unit 60 realizes its function by the processor reading a program for executing the function into the memory and executing it.

[0021] Alternatively, at least a part of the function of the arithmetic processing unit 60 may be configured hardware-wise using a CPLD (Complex Programmable Logic Device), FPGA (Field Programmable Gate Array), etc. Also, when using a CPLD or FPGA, circuits configured on these elements, etc., realize its function.

[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 using the signal amplitude and initial phase obtained by analyzing the light-receiving signal from the photodetector 40. The arithmetic processing unit 60 also includes a light intensity determination unit 61 and a light intensity adjustment instruction unit 62, which will be described later, as functional units.

[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 executing the functions of the light intensity determination unit 61 and the light intensity adjustment instruction unit 62, which adjust the amount of light emitted by the arithmetic processing unit 60, as well as the setting value for the appropriate light intensity.

[0024] The light intensity adjustment circuit 80 is connected to the first light-emitting element 20. The light intensity adjustment circuit 80 includes a digital potentiometer 81, which is a variable resistor means for changing the resistance applied to the first light-emitting element 20; a load resistor 82 that applies a constant resistance to the first light-emitting element 20; a digital switch 83 that can switch between conducting and non-conducting states; and a DC power supply 84 that supplies a DC voltage to the digital potentiometer 81 and the load resistor 82 via the digital switch 83.

[0025] The digital potentiometer 81 adjusts the amount of light emitted by the first light-emitting element 20 by changing the resistance applied to the first light-emitting element 20 under the control of the arithmetic processing unit 60. The resistance value applied to the first light-emitting element 20 is determined by the combined resistance value of the load resistor 82 and the digital potentiometer 81. When the first light-emitting element 20 receives a DC voltage, increasing the resistance value of the digital potentiometer 81 by the arithmetic processing unit 60 decreases the amount of light emitted, and decreasing the resistance value increases the amount of light emitted.

[0026] The following describes the configuration of the optical distance meter 100 and the details of its operation related to distance measurement. 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 the PLL (Phase-Locked Loop) units 9 and 10. The PLL units 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.

[0027] The frequency divider 2 divides the signal at the main modulation frequency F1 to generate a signal at the main modulation frequency F2. This signal at the main modulation frequency F2 and 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 by the main modulation frequencies F1 and F2.

[0028] 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 a signal with paramodulation frequency F2+b·F2. These signals with paramodulation frequencies F1+b·F1 and F2+b·F2 are input to drive circuit 8 via frequency superposition circuit 7. The second light-emitting element 30 is driven by drive circuit 8 and transmits light modulated with paramodulation frequencies F1+b·F1 and F2+b·F2 in synchronization with the transmission of light modulated with the main modulation frequencies F1 and F2 of the first light-emitting element 20.

[0029] The local signal oscillator 11 generates a local signal with local frequency F1+a·F1. This local signal with local frequency F1+a·F1 is divided by the frequency generation circuit 12 to generate a local signal with local frequency F2+a·F2. These local signals with local frequency F1+a·F1 and local signals with local frequency F2+a·F2 are input to the first frequency converter 44 and the second frequency converter 49, respectively, as will be described later. Here, the deviations |a|·F1 and |a|·F2 between the local frequencies F1+a·F1 and F2+a·F2 and their corresponding main modulation frequencies F1 and F2 are, for example, several hundred kHz to several tens of kHz.

[0030] Therefore, the local frequencies F1+a·F1 and F2+a·F2 are frequencies that are close to both the corresponding main modulation frequencies F1 and F2 and the paramodulation frequencies F1+b·F1 and F2+b·F2, respectively.

[0031] Light emitted from the first light-emitting element 20 travels through the distance measuring optical path 21, is reflected by the target reflector 22, and reaches the photodetector 40 via the distance measuring optical path 23. The distance measuring optical path 23 is provided with a distance measuring optical system 25 for focusing the distance measuring light.

[0032] Meanwhile, the light emitted from the second light-emitting element 30 reaches the photodetector 40 via the reference optical path 31. In the reference optical path 31, a density filter 32 for adjusting the light intensity is placed in front of the photodetector 40. The density filter 32 is a disc-shaped neutral density filter configured such that the filter density increases continuously, for example, so that the light intensity attenuation rate in the circumferential direction is 0 to 100%. The density filter 32 is fixed in the reference optical path so that the amount of light received (amplitude of the reference intermediate frequency signal) based on the reference light emitted from the second light-emitting element 30 becomes the appropriate light intensity for the A / D converters 48 and 53.

[0033] The density filter 32 is not limited to a disc-shaped neutral density filter configured as described above, but may also be a neutral density filter having an attenuation rate such that the amount of light received by the reference light becomes the appropriate amount of light for the A / D converter.

[0034] The light-receiving element 40 is connected to the amplifier 41, and the amplifier 41 is connected to the frequency converter group 43. 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 four frequencies F1, F1+b·F1, F2, and F2+b·F2. These signals are amplified by the amplifier 41. The amplified signals are then 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 F1 + a·F1 is input, and frequency multiplication is performed. The multiplied signal is output to bandpass filters 45 and 47. The bandpass filter 45 removes the high-frequency range, and the intermediate frequency signal of frequency a·F1 is input to the A / D converter 46. The bandpass filter 47 removes the high-frequency range, and the intermediate frequency signal of frequency (ab)·F1 is input to the A / D converter 48.

[0037] In the second frequency converter 49, a local signal of local frequency F2 + a·F2 is input, and frequency multiplication is performed. The multiplied signal is output to bandpass filter 50 and bandpass filter 52. The bandpass filter 50 removes the high frequency range, and the intermediate frequency signal of frequency a·F2 is input to A / D converter 51. The bandpass filter 52 removes the high frequency range, and the intermediate frequency signal of frequency (ab)·F2 is input to A / D converter 53.

[0038] Thus, the frequency converter group 43 includes the same number of frequency converters as the number of main modulation frequencies of the distance measuring light used to calculate 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 emitted light.

[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·F1 and a·F2 to calculate the distance measurement value of the optical path using the ranging light of frequencies F1 and F2. The arithmetic processing unit 60 also analyzes the signal amplitude and phase information of the intermediate frequency signals of frequencies (ab)·F1 and (ab)·F2 to calculate the reference optical path measurement value of the reference light of frequencies F1 and F2. Then, for each main modulation frequency, the arithmetic processing unit 60 calculates the distance value to the target reflector using the ranging light of frequencies F1 and F2 by subtracting the reference optical path measurement value from the distance measurement value of the optical path. The number of digits of the distance value is determined according to the resolution of the signals of frequencies F1 and F2.

[0041] Next, we will explain how to adjust the amount of emitted light using the light intensity adjustment circuit 80. The light intensity adjustment circuit 80 adjusts the amount of light emitted by the first light-emitting element 20 according to the instructions of the arithmetic processing unit 60. The light intensity determination unit 61 determines the received light intensity level based on the signal amplitude of the distance measurement intermediate frequency signal input to the A / D converter 46 or 51. The determination of the received light intensity level can be performed using an A / D converter that performs A / D conversion on a signal based on distance measurement light, which is connected to one of the frequency converters in any of the multiple frequency converter groups. Therefore, in this embodiment, it is sufficient to use either the A / D converter 46 or 51. The A / D converters 46, 48, 51, and 53 are provided with input ranges. If the amount of received distance measurement light is large and the distance measurement signal exceeds the upper input limit (upper light intensity limit) of the A / D converter, it enters the saturation region, and the signal does not become within the measurable range, so the distance measurement value is not calculated.

[0042] On the other hand, if the amount of light received by the distance measuring device is small and is less than the input lower limit (lower limit light intensity), the A / D converters 46, 48, 51, and 53 cannot properly decompose the amplitude, resulting in a count of 1 or less during signal conversion, which is then digitized to 0, making signal detection impossible. The light intensity determination unit 61 determines whether the signal amplitude of the distance measuring intermediate frequency signal is within the input range of the A / D converters 46 and 51, whether it is less than the lower limit, or whether it is greater than the upper limit. If it is within the input range, it also determines whether it is greater than or less than the appropriate light intensity.

[0043] The light intensity adjustment instruction unit 62 controls the drive of the drive circuit 4 by changing the resistance value of the digital potentiometer 81 according to the determination result of the light intensity determination unit 61, and adjusts the light emission intensity of the first light-emitting element 20 so that the light intensity level of the light-receiving element 40 becomes the appropriate light intensity. The appropriate light intensity is set as, for example, the upper limit of the input range of the A / D converters 46, 51, or a predetermined value slightly smaller than the upper limit, and is stored in the memory unit 70.

[0044] Specifically, if the amount of light received by the distance measuring device (amplitude of the distance measuring intermediate frequency signal) falls below the lower limit of the input range of the A / D converters 46 and 51, or if it falls below the appropriate light level even within the input range, the resistance value of the digital potentiometer 81 is reduced while checking the received light level to control the amount of emitted light. Conversely, if it exceeds the upper limit of the input range, or if it exceeds the appropriate light level even within the input range, the resistance value of the digital potentiometer 81 is increased while checking the received light level to control the amount of emitted light.

[0045] Figure 2 is a flowchart of the processing of the calculation processing unit 60 related to light intensity adjustment during distance measurement by the optical distance meter 100. When the distance measurement process starts, in step S01, the first light-emitting element 20 and the second light-emitting element 30 are made to emit light according to the instructions of the calculation processing unit 60.

[0046] Next, in step S02, the light intensity determination unit 61 detects the received light intensity level. Specifically, the light intensity determination unit 61 detects whether the signal amplitude of the distance measuring intermediate frequency signal is within the input range of the A / D converters 46, 51, whether it is smaller than the lower limit, or whether it is larger than the upper limit. If it is within the input range, it also determines whether it is greater or less than the appropriate light intensity.

[0047] Next, in step S03, the light intensity adjustment indicator unit 62 adjusts the emitted light intensity so that the received light intensity level becomes the appropriate light intensity.

[0048] Specifically, in step S02, if the received light intensity level exceeds the upper limit of the input range of the A / D converter (upper light intensity), or if the received light intensity level is within the input range of the A / D converter but is greater than the appropriate light intensity, the system is instructed to increase the resistance value of the digital potentiometer 81. On the other hand, if the received light intensity level falls below the lower limit of the input range (lower light intensity), or if the received light intensity level is within the input range of the A / D converter but is less than the appropriate light intensity, the system is instructed to decrease the resistance value of the digital potentiometer 81.

[0049] Next, in step S04, it is determined whether the received light intensity level has reached the appropriate level. If the appropriate level is reached in step S04 (Yes), the process proceeds to step S06, where distance measurement is performed with the light intensity adjusted to the appropriate level. The first light-emitting element 20 and the second light-emitting element 30 are illuminated, and the initial phases of the distance measurement intermediate frequency signal and the reference intermediate frequency signal based on the distance measurement light and reference light are analyzed from the received signal to calculate the distance to the target reflector 22 and terminate the process.

[0050] On the other hand, if the received light intensity level in step S04 is not at the appropriate level (No), then in step S05, it is determined whether the judgment in step S04 has been made N times. If it is not the Nth time (No), the process returns to step S03 and further adjusts the light intensity. If the received light intensity level is not at the appropriate level even after repeating the process N times (Yes in step S05), the process proceeds to step S07, where an adjustment error is output and the process ends. In step S07, the arithmetic processing unit 60 may notify the operator by displaying a warning on a display unit (not shown) or by emitting a warning sound from a speaker (not shown).

[0051] Figures 3 and 4 illustrate the difference between the light intensity adjustment in the optical distance meter 100 according to this embodiment and the light intensity adjustment in a conventional optical distance meter (the optical distance meter described in Patent Document 2). Figures 3(A) and 4(A) illustrate the light intensity adjustment in the optical distance meter 100, while Figures 3(B) and 4(B) illustrate the light intensity adjustment in a conventional optical distance meter. Each figure schematically shows the received light intensity spectrum of the reference light and the distance measuring light before and after light intensity adjustment.

[0052] In the optical distance meter described in Patent Document 2, the device comprises one light-emitting element and one light-receiving element. Light emitted from the light-emitting element is switched between a distance measuring optical path and a reference optical path by a switching shutter. When the distance measuring optical path is selected, the light is used as distance measuring light and irradiated onto a target reflector. The reflected light is received by the light-receiving element to generate a distance measuring signal. On the other hand, when the reference optical path is selected, the light is used as reference light, entering the light-receiving element via the reference optical path, and is received by the light-receiving element to generate a reference light reception signal.

[0053] In other words, the system is configured to emit both the distance measuring light and the reference light from the same light-emitting element. Therefore, as shown in the left diagram of Figure 3(B), if the reference light is at the appropriate intensity and the distance measuring light exceeds the upper limit intensity, adjusting the emitted light intensity to bring the distance measuring light intensity to the appropriate intensity will simultaneously reduce the intensity of the reference light, as shown in the right diagram of Figure 3(B). As shown in the right diagram, the received light intensity level of the reference light may fall below the lower limit intensity. In this case, the desired accuracy cannot be guaranteed, and distance measurement becomes impossible.

[0054] On the other hand, in the optical distance meter 100 according to this embodiment, the first light-emitting element 20 emits distance measuring light, and the second light-emitting element 30 emits reference light. Furthermore, the light intensity adjustment circuit 80 is configured to adjust only the amount of light emitted by the first light-emitting element 20, i.e., the distance measuring light. In addition, the reference light is always set to an appropriate light intensity. For this reason, as shown in the left figure of Figure 3(A), even when the amount of light emitted by the distance measuring light is adjusted to an appropriate light intensity when the received light intensity level of the distance measuring light exceeds the input range of the A / D converter, it is possible to adjust only the light intensity of the distance measuring light without affecting the light intensity of the reference light, as shown in the right figure of Figure 3(A). This is also true in the case of Figure 4, when the light intensity of the distance measuring light falls below the lower limit light intensity.

[0055] Thus, in the optical distance meter 100 according to this embodiment, the light intensity of the distance measuring light can be adjusted without changing the light intensity of the reference light. Therefore, it is possible to prevent situations where the light intensity of the reference light deviates from the input range due to fluctuations in the light intensity of the reference light, making distance measurement impossible. As a result, it becomes possible to effectively utilize the light intensity range of the distance measuring light.

[0056] Furthermore, in conventional optical distance meters, as described above, the light intensity of the reference light fluctuates in conjunction with the adjustment of the light intensity of the measuring light. Therefore, even if both the measuring light and the reference light are within the input range, a difference in the received light intensity levels of the measuring light and the reference light occurs. Generally, when the signal level is low, the signal-to-noise ratio (S / N ratio) decreases, and the detection accuracy of amplitude and phase decreases (values ​​fluctuate). Therefore, if there is a difference in the light intensity level of the measuring light and the light intensity level of the reference light, the measurement accuracy will correspond to the lower light intensity level. With optical distance meter 100, the measuring light can be adjusted without causing fluctuations in the reference light, and distance can be measured with both the measuring light and the reference light at the appropriate light intensity. As a result, it is possible to determine the distance value with an accuracy commensurate with the appropriate light intensity.

[0057] The optical distance meter 100 includes a first light-emitting element 20 that emits a distance-measuring light modulated at the main modulation frequency, and a second light-emitting element 30 that emits a reference light modulated at a paramodulation frequency close to the main modulation frequency. The two light-emitting elements synchronously emit light to the distance-measuring optical paths 21, 23 and the reference optical path 31, respectively, and the light is received by a single light-receiving element 40. As a result, there is no need to use a shutter to switch between the distance-measuring light and the reference light, and the initial phases of the intermediate frequency signals related to the distance-measuring optical paths 21, 23 and the reference optical path 31 can be determined simultaneously, allowing for faster distance measurement than a shutter-type optical distance meter. Furthermore, cost reduction is possible due to the elimination of the shutter.

[0058] Furthermore, conventional optical distance meters perform distance measurement using a measuring light and a reference light by switching the optical path. This can cause a temperature phase drift difference due to the time difference in measurement. For this reason, the light-emitting element was kept powered on during continuous distance measurement. However, with optical distance meter 100, simultaneous distance measurement using the measuring light path and the reference light path is possible, so it is not affected by temperature phase drift due to the time difference, and the power to the light-emitting element can be turned on and off for each distance measurement, resulting in power saving.

[0059] It is known that the higher the modulation frequency of the carrier signal, the smaller the variation in the measured distance and the higher the accuracy. On the other hand, it is known that for elements such as light-emitting elements and photodetectors, the higher the frequency of the applied signal, the larger the temperature phase drift inherent to the element. The optical distance meter 100 uses two light-emitting elements, a first light-emitting element 20 and a second light-emitting element 30, to emit the measuring light and the reference light. Between the first light-emitting element 20 and the second light-emitting element 30, a difference in temperature phase drift due to individual differences may occur depending on the applied carrier signal. For this reason, it is preferable to use a modulation frequency of 10 MHz or less, rather than the generally preferred modulation frequency of around 100 MHz. This is because, in the case of a modulation frequency of 10 MHz or less, the temperature phase drift occurring in the element is negligibly small.

[0060] Furthermore, it is preferable that the difference |a|·F1,|a|·F2 between the local frequencies F1+a·F1,F2+a·F2 and the corresponding main modulation frequencies F1,F2 is at least 10 times larger (|a|≧10|b|) than the difference |b|·F1,|b|·F2 between the paramodulation frequencies F1+b·F1,F2+b·F2 and the corresponding main modulation frequencies F1,F2. In this case, the frequencies a·F1,a·F2 of the intermediate frequency signals derived from the ranging light and the frequencies (ab)·F1,(ab)·F2 of the intermediate frequency signals derived from the reference light will be close in value. This is because the temperature phase drift occurring in the light-receiving electronic components such as the band filters 45, 47, 50, 52 can be approximated when the frequencies are close, so when subtracting the measured value of the reference light from the measured value of the ranging light, the effect of the temperature phase drift occurring in the light-receiving electronic components included in each can be canceled out.

[0061] (Second Embodiment) Figure 5 is a block diagram of the optical distance meter 200 according to the second embodiment. The optical distance meter 200 has a configuration similar to that of the optical distance meter 100 in general, but differs in that it has an optical intensity adjustment circuit 280 instead of the optical intensity adjustment circuit 80, and an arithmetic processing unit 260 instead of the arithmetic processing unit 60.

[0062] The light intensity adjustment circuit 280 includes a variable DC power supply 281 that supplies a variable DC voltage to the load resistor 283 via a digital switch 282. The light intensity adjustment instruction unit 262 of the arithmetic processing unit 260 adjusts the amount of light emitted by the first light-emitting element 20 by changing the voltage of the variable DC power supply 281.

[0063] In other words, by controlling the light intensity adjustment instruction unit 262, the amount of light emitted from the first light-emitting element 20 increases by increasing the voltage of the variable DC power supply 281, and decreases the amount of light emitted by decreasing the voltage.

[0064] Thus, even if the digital potentiometer 81 is replaced with a variable DC power supply 281 and the amount of emitted light is adjusted by varying the voltage rather than changing the resistance, it is possible to achieve the same effect as the optical distance meter 100 according to the first embodiment.

[0065] Although preferred embodiments of the present invention have been described above, these embodiments are merely examples and are not limited thereto. For example, in the light intensity adjustment circuit of the first embodiment, instead of a digital potentiometer which is a variable resistance means, a group of resistors each having a predetermined fixed resistance value from high to low resistance values ​​may be used, and the arithmetic processing unit may be configured to adjust the amount of emitted light by selecting a resistor using a selection signal that corresponds one-to-one with each resistor. [Explanation of symbols]

[0066] 20: First light-emitting element 21: Distance measurement optical path 22: Target reflector 23: Distance measurement optical path 30: Second light-emitting element 31: Reference optical path 40: Photodetector 46: A / D converter 51: A / D converter 60: Arithmetic Processing Unit 61:Light amount judgment section 62: Light intensity adjustment instruction section 80: Light amount adjustment circuit 81: Digital potentiometer 82: Load resistance 84:DC power supply 100:Lightwave distance meter 200:Lightwave distance meter 260: Arithmetic Processing Unit 262: Light intensity adjustment instruction section 280: Light amount adjustment circuit 281: Variable DC power supply 283: Load resistance

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 transmits light modulated at a paramodulation frequency slightly different from each of the aforementioned main modulation frequencies to the reference optical path in synchronization with the distance measuring light, as reference light. A photodetector that receives light emitted from the first light-emitting element and the second light-emitting element and outputs a distance measurement signal and a reference signal, A frequency converter connected to the light-receiving element, the same number as the plurality of main modulation frequencies, each of which frequency multiplies the distance measurement signal and the reference signal with a local signal having a frequency slightly different from both the corresponding main modulation frequency and paramodulation frequency to convert them into a distance measurement intermediate frequency signal and a reference intermediate frequency signal, respectively. Each frequency converter is connected to two band filters, one of which separates the distance measuring intermediate frequency signal and the other separates the reference intermediate frequency signal. An A / D converter that digitally converts the distance measurement intermediate frequency signal and the reference intermediate frequency signal, A calculation processing unit that calculates the distance to the target reflector based on the phase difference between the digitally converted intermediate frequency signal and the reference intermediate frequency signal, A light intensity adjustment circuit adjusts the amount of light emitted by the first light-emitting element based on the signal amplitude of the intermediate frequency signal used for distance measurement. A light wave distance meter characterized by having the following features.

2. The calculation processing unit detects the signal amplitude of the distance measuring intermediate frequency signal input to the A / D converter as the amount of light received by the light receiving element, The optical distance meter according to claim 1, characterized in that the calculation processing unit controls the light intensity adjustment circuit to adjust the amount of light emitted from the first light-emitting element so that the amount of light received falls within the input range of the A / D converter.

3. The light intensity adjustment circuit includes a digital potentiometer, which is a variable resistor, connected to the first light-emitting element and receiving a DC voltage supply. The optical distance meter according to claim 1 or 2, characterized in that the calculation processing unit adjusts the amount of light emitted by the first light-emitting element by changing the resistance value of the digital potentiometer.

4. The light intensity adjustment circuit includes a load resistor that applies a constant resistance to the first light-emitting element, and a variable DC power supply that supplies a variable DC voltage to the load resistor. The optical distance meter according to claim 1 or 2, characterized in that the calculation processing unit adjusts the amount of light emitted by the first light-emitting element by varying the voltage supplied by the variable DC power supply.

5. The optical distance meter according to any one of claims 1 to 4, characterized in that the plurality of main modulation frequencies are 10 MHz or less.

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