Optical Interferometric Distance Sensor

Abstract

To provide a light interference distance measuring sensor that can appropriately measure distance even when a rough surface and a mirror surface are included in an object to be measured.SOLUTION: A light interference distance measuring sensor 100 comprises: a wavelength sweep light source 110; an interferometer 120 that includes a branching unit 121 that branches light projected from the wavelength sweep light source, and for the rays of branched light, generates rays of interference light based on measurement light with which an object to be measured is irradiated and reflected on the object to be measured, and reference light traveling through an optical path at least partially different from the measurement light; a light receiving unit 130 that receives the rays of interference light; and a processing unit 140 that, according to a mirror surface mode or a rough surface mode, associates detected peaks and spots with each other to calculate the distance to the object to be measured. The rays of branched light are set to have optical path length differences different from each other. In the mirror surface mode, the processing unit takes a distance calculated based on the peak corresponding to the spot in which the optical path length difference becomes shortest as a result of measurement of the distance to the object to be measured.SELECTED DRAWING: Figure 10

Description

[Technical Field]

[0001] The present invention relates to an optical interferometric distance measuring sensor. [Background technology]

[0002] In recent years, optical distance measuring sensors that measure the distance to a measurement object without contact have become widespread. For example, an optical interferometric distance measuring sensor is known that generates interference light based on a reference light and a measurement light from light projected from a wavelength swept light source and measures the distance to the measurement object based on the interference light.

[0003] Furthermore, some conventional optical interferometric distance measuring sensors are known that are configured to irradiate a measurement object with a plurality of beams and measure the measurement object with high accuracy.

[0004] The optical measuring device described in Patent Document 1 obtains stable measurement results by coherently interfering the return light beam components of the reference beam reflected by multiple optical fiber end faces with the reflected components of the measurement beam reflected by the surface of the object to be measured. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 2686124 Summary of the Invention [Problem to be solved by the invention]

[0006] However, with the optical interference unit disclosed in Patent Document 1, for example, if the measurement object includes a rough surface and a specular surface, even if the rough surface can be measured properly, there is a problem in that the specular surface cannot be measured accurately because multiple reflections cause disturbance light.

[0007] SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide an optical interferometric distance measuring sensor that can appropriately measure distances even when the measurement object includes a rough surface and a mirror surface. [Means for solving the problem]

[0008] An optical interferometric distance measuring sensor according to one aspect of the present invention includes a light source that projects light while continuously changing the wavelength, and a branching section that branches the light projected from the light source so that the light projected from the light source is irradiated onto multiple spots on a measurement object.The sensor is equipped with an interferometer that generates interference lights for each of the branched lights corresponding to the multiple spots based on measurement light that is irradiated onto the measurement object and reflected by the measurement object, and reference light that follows an optical path that is at least partially different from that of the measurement light, a light receiving section that receives each interference light from the interferometer, and a processing section that detects peaks among the received interference lights and matches the detected peaks with the spots depending on either a mirror surface mode or a rough surface mode in which the measurement object is measured, to calculate the distance to the measurement object.The optical path length difference between the measurement light and the reference light is set to be different for each of the branched lights corresponding to the multiple spots, and in the mirror surface mode, the processing section determines the distance to the measurement object as the distance measured.

[0009] According to this aspect, the interferometer generates interference light beams for each of the beams branched corresponding to the plurality of spots based on the measurement light beam irradiated onto the measurement object and reflected by the measurement object and the reference light beam that follows at least a partial optical path different from that of the measurement light beam. The light receiving unit receives each interference light beam from the interferometer. The processing unit detects peaks in each interference light beam and calculates the distance to the measurement object by associating the detected peaks with the spots according to either the mirrored surface mode or the rough surface mode in which the measurement object is measured. Since the optical path length difference between the measurement light beam and the reference light beam is set to be different for each of the beams branched corresponding to the plurality of spots, each peak can be properly detected. Furthermore, in the mirrored surface mode, the distance calculated based on the peak corresponding to the spot with the shortest optical path length difference is used as the distance measurement result to the measurement object. As a result, the distance to the measurement object can be properly measured even if the measurement object includes both a rough surface and a mirrored surface.

[0010] In the above aspect, in the case of the rough surface mode, the processing unit may take the distance calculated based on the detected multiple peaks as the distance measurement result.

[0011] According to this aspect, in the rough surface mode, the processing unit calculates the distance to the measurement object based on the detected multiple peaks, and therefore, the distance to the measurement object can be measured with higher accuracy.

[0012] In the above aspect, the interferometer may have one objective lens, and the measurement light of each of the beams branched by the branching unit corresponding to the plurality of spots may be irradiated onto the measurement object via the single objective lens.

[0013] According to this aspect, the measurement light of each of the beams branched by the branching unit corresponding to the multiple spots is irradiated onto the measurement object through a single objective lens, so that the spot diameter can be made small, and even if the measurement object is small, the distance to the measurement object can be measured accurately and appropriately. Furthermore, since there is only one objective lens, the optical axis can be easily adjusted, which also leads to cost reduction.

[0014] In the above aspect, the spot having the shortest optical path length difference among the optical path length differences corresponding to the plurality of spots may be positioned on the optical axis of one objective lens.

[0015] According to this aspect, since the spot having the shortest optical path length difference is arranged on the optical axis of one objective lens, in the mirror mode, it is possible to more appropriately detect the peak corresponding to the spot having the shortest optical path length difference. Furthermore, since the distance calculated based on the detected peak is used as the distance measurement result to the measurement object, it is possible to appropriately measure the distance to the measurement object even if the measurement object includes both a rough surface and a mirror surface.

[0016] In the above aspect, the interferometer may generate each interference light based on a first reflected light of the measurement light that is irradiated onto the measurement object and reflected by the measurement object, and a second reflected light of the reference light that is reflected by the reference surface.

[0017] According to this aspect, interference light is generated based on a first reflected light of the measurement light irradiated onto the measurement object and reflected by the measurement object, and a second reflected light of the reference light reflected by the reference surface. The optical path length difference between the measurement light and the reference light is set to be different for each of the beams branched to correspond to the multiple spots, thereby enabling appropriate detection of each peak. Furthermore, in the mirrored surface mode, the distance calculated based on the peak corresponding to the spot with the shortest optical path length difference among the detected peaks is used as the distance measurement result to the measurement object. As a result, the distance to the measurement object can be appropriately measured even if the measurement object includes both a rough surface and a mirrored surface.

[0018] In the above aspect, for the optical fibers that transmit the respective light beams branched to correspond to the multiple spots, the tip positions of the optical fibers that serve as reference surfaces may be positioned at different positions in the optical axis direction.

[0019] According to this aspect, the tip positions of the optical fibers arranged in each optical path are arranged with a shift in the optical axis direction, so that the optical path length differences in each optical path can be set to be different, allowing each peak to be detected more appropriately.

[0020] In the above embodiment, the difference ΔL in the optical path length difference between the light beams branched corresponding to the plurality of spots is at least the distance resolution δL expressed by the following formula: FWHM It may be larger than δL FWHM =c / nδf (c: speed of light, n: refractive index in the optical path difference, δf: frequency sweep width)

[0021] According to this aspect, the difference ΔL in the optical path length difference between the optical paths is the distance resolution δL FWHM Since the peaks are set to be larger than , it is possible to reduce overlapping of multiple peaks among the interference lights received by the light receiving section, and to more appropriately detect each peak.

[0022] In the above aspect, the user may be able to select either the mirror surface mode or the rough surface mode.

[0023] According to this aspect, the user can select either the specular mode or the rough mode, and therefore an appropriate mode can be selected depending on the material, condition, etc. of the measurement object. As a result, even if the measurement object includes both a rough surface and a specular surface, the distance to the measurement object can be appropriately measured depending on the material, condition, etc. of the measurement object.

[0024] In the above aspect, the processing unit may use the specular mode when the number of peaks detected from each of the received interference lights exceeds the number of spots.

[0025] According to this aspect, when a peak is detected in each of the received interference lights exceeding the number of spots, the mirror mode is used, so that an appropriate mode can be automatically selected and used even if the mode is not selected by the user. [Effects of the Invention]

[0026] According to the present invention, it is possible to provide an optical interferometric distance measuring sensor that can appropriately measure distances even when the measurement object includes a rough surface and a mirror surface. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a schematic view showing an outline of a displacement sensor 10 according to the present disclosure. [Figure 2] 1 is a flowchart showing a procedure for measuring a measurement object T by a displacement sensor 10 according to the present disclosure. [Figure 3] 1 is a functional block diagram showing an overview of a sensor system 1 in which a displacement sensor 10 according to the present disclosure is used. [Figure 4] 1 is a flowchart showing a procedure for measuring a measurement object T by a sensor system 1 using a displacement sensor 10 according to the present disclosure. [Figure 5A] 1 is a diagram for explaining the principle of measurement of a measurement object T by a displacement sensor 10 according to the present disclosure. [Figure 5B] 10A and 10B are diagrams for explaining another principle by which the measurement object T is measured by the displacement sensor 10 according to the present disclosure. [Figure 6A] FIG. 2 is a perspective view showing a schematic configuration of a sensor head 20. [Figure 6B] 2 is a perspective view showing a schematic configuration of a collimator lens holder arranged inside the sensor head 20. FIG. [Figure 6C] 2 is a cross-sectional view showing the internal structure of the sensor head 20. FIG. [Figure 7] FIG. 2 is a block diagram for explaining signal processing in a controller 30. [Figure 8] 10 is a flowchart showing a method for calculating a distance to a measurement object T, which is executed by a processing unit 59 in the controller 30. [Figure 9A]FIG. 1 is a diagram showing how a waveform signal (voltage vs. time) is frequency-converted into a spectrum (voltage vs. frequency). [Figure 9B] FIG. 10 is a diagram showing how a spectrum (voltage vs. frequency) is distance-transformed into a spectrum (voltage vs. distance). [Figure 9C] FIG. 10 is a diagram showing how values (distance value, SNR) corresponding to peaks are calculated based on a spectrum (voltage vs. distance). [Figure 10] 1 is a schematic diagram showing the general configuration of an optical interferometric distance measuring sensor 100 according to an embodiment of the present invention. [Figure 11] 10 is a diagram showing an example of a signal waveform obtained by frequency-converting returned light received by the light-receiving section 130 using FFT. FIG. [Figure 12] 10 is a diagram schematically showing an example of a signal waveform obtained by frequency conversion using FFT for return light received by the light receiving section 130 when the surface of the measurement object T is a mirror surface. FIG. [Figure 13A] 10 is a flowchart showing a method for calculating a distance to a measurement object T, which is executed by a processing unit 140. [Figure 13B] 10 is a flowchart showing a method for automatically determining the specular mode or the rough surface mode and calculating the distance to the measurement object T, which is executed by the processing unit 140. [Figure 14] 10A and 10B are diagrams showing variations in which various objective lenses are arranged relative to a plurality of collimating lenses. [Figure 15] FIG. 10 is a diagram showing a variation in which a collimator lens is not provided, but multiple objective lenses are provided. [Figure 16] FIG. 10 is a diagram showing a variation in which no objective lens is arranged, but multiple collimator lenses are arranged. [Figure 17] 10A and 10B are diagrams showing variations of an interferometer that generates interference light using measurement light and reference light. DETAILED DESCRIPTION OF THE INVENTION

[0028] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the embodiment described below is merely a specific example for carrying out the present invention and is not intended to limit the scope of the present invention. Furthermore, to facilitate understanding of the description, the same components in each drawing will be designated by the same reference numerals wherever possible, and duplicate descriptions may be omitted.

[0029] [Displacement sensor overview] First, an overview of the displacement sensor according to the present disclosure will be described. 1 is a schematic external view showing an overview of a displacement sensor 10 according to the present disclosure. As shown in Fig. 1, the displacement sensor 10 includes a sensor head 20 and a controller 30, and measures the displacement of a measurement object T (the distance to the measurement object T).

[0030] The sensor head 20 and the controller 30 are connected by an optical fiber 40, and an objective lens 21 is attached to the sensor head 20. The controller 30 also includes a display unit 31, a setting unit 32, an external interface (I / F) unit 33, an optical fiber connection unit 34, and an external storage unit 35, and further includes a measurement processing unit 36 inside.

[0031] The sensor head 20 irradiates the measurement object T with light output from the controller 30 and receives the light reflected from the measurement object T. The sensor head 20 has an internal reference surface that reflects the light output from the controller 30 and received via the optical fiber 40 and causes it to interfere with the light reflected from the measurement object T described above.

[0032] The sensor head 20 is provided with an objective lens 21, which is detachable. The objective lens 21 can be replaced with an objective lens having an appropriate focal length depending on the distance between the sensor head 20 and the measurement target T, or a variable-focus objective lens may be used.

[0033] Furthermore, when installing the sensor head 20, guide light (visible light) may be irradiated onto the measurement object T, and the sensor head 20 and / or the measurement object T may be installed so that the measurement object T is appropriately positioned within the measurement area of the displacement sensor 10.

[0034] The optical fiber 40 is connected to and extends from an optical fiber connection unit 34 arranged in the controller 30, connecting the controller 30 and the sensor head 20. As a result, the optical fiber 40 is configured to guide light emitted from the controller 30 to the sensor head 20, and further guide return light from the sensor head 20 to the controller 30. The optical fiber 40 is detachable from the sensor head 20 and the controller 30, and various optical fibers having different lengths, thicknesses, characteristics, etc. can be used.

[0035] The display unit 31 in the controller 30 is configured with, for example, a liquid crystal display or an organic EL display, etc. The display unit 31 displays the set value of the displacement sensor 10, the amount of received return light from the sensor head 20, and measurement results such as the displacement of the measurement object T measured by the displacement sensor 10 (the distance to the measurement object T).

[0036] The setting unit 32 in the controller 30 performs settings necessary for measuring the measurement target T, for example, by a user operating a mechanical button, a touch panel, or the like. All or part of these necessary settings may be set in advance, or may be set from an externally connected device (not shown) connected to the external I / F unit 33. The externally connected device may be connected via a network in a wired or wireless manner.

[0037] Here, the external I / F unit 33 is configured with, for example, Ethernet (registered trademark), RS232C, analog output, etc. The external I / F unit 33 may be connected to another connected device so that necessary settings can be made from the external connected device, or the measurement results measured by the displacement sensor 10, etc., may be output to the external connected device.

[0038] Furthermore, the controller 30 may perform settings necessary for measuring the measurement object T by importing data stored in the external storage unit 35. The external storage unit 35 is, for example, an auxiliary storage device such as a USB (Universal Serial Bus) memory, and stores settings and the like necessary for measuring the measurement object T in advance.

[0039] The measurement processing unit 36 in the controller 30 includes, for example, a wavelength swept light source that emits light while continuously changing the wavelength, a light receiving element that receives the returned light from the sensor head 20 and converts it into an electrical signal, and a signal processing circuit that processes the electrical signal. The measurement processing unit 36 performs various processes using a control unit, a memory unit, etc., based on the returned light from the sensor head 20 so that the displacement of the measurement object T (the distance to the measurement object T) is ultimately calculated. These processes will be described in detail later.

[0040] 2 is a flowchart showing a procedure for measuring the measurement target T by the displacement sensor 10 according to the present disclosure. As shown in FIG. 2, the procedure includes steps S11 to S14.

[0041] In step S11, the sensor head 20 is installed. For example, guide light is irradiated from the sensor head 20 onto the measurement target T, and the sensor head 20 is installed in an appropriate position based on the guide light.

[0042] Specifically, the amount of received light returning from the sensor head 20 is displayed on the display unit 31 of the controller 30, and the user may adjust the orientation of the sensor head 20 and the distance (height position) from the measurement object T while checking the amount of received light. Basically, if the light from the sensor head 20 can be irradiated perpendicularly to the measurement object T (at an angle closer to perpendicular), the amount of reflected light from the measurement object T will be large and the amount of received light returning from the sensor head 20 will also be large.

[0043] Furthermore, depending on the distance between the sensor head 20 and the object T to be measured, the objective lens 21 may be replaced with one having an appropriate focal length.

[0044] Furthermore, if appropriate settings cannot be made when measuring the measurement object T (for example, the amount of light received required for measurement cannot be obtained, or the focal length of the objective lens 21 is inappropriate), an error or incomplete settings may be displayed on the display unit 31 or output to an externally connected device to notify the user.

[0045] In step S12, various measurement conditions are set when measuring the measurement object T. For example, the user sets the inherent calibration data (such as a function for correcting linearity) of the sensor head 20 by operating the setting unit 32 in the controller 30.

[0046] Various parameters may also be set. For example, a sampling time, a measurement range, and a threshold for determining whether a measurement result is normal or abnormal may be set. Furthermore, a measurement cycle may be set according to the characteristics of the measurement object T, such as the reflectance and material of the measurement object T, and a measurement mode may be set according to the material of the measurement object T.

[0047] These measurement conditions and various parameters are set by operating the setting unit 32 in the controller 30, but they may also be set from an externally connected device or by importing data from the external memory unit 35.

[0048] In step S13, the sensor head 20 installed in step S11 measures the measurement target T in accordance with the measurement conditions and various parameters set in step S12.

[0049] Specifically, in the measurement processing unit 36 of the controller 30, light is projected from the wavelength swept light source, and the light returning from the sensor head 20 is received by a light receiving element, and frequency analysis, distance conversion, peak detection, etc. are performed by a signal processing circuit to calculate the displacement of the measurement object T (the distance to the measurement object T). Specific details of the measurement processing will be described later.

[0050] In step S14, the measurement results measured in step S13 are output. For example, the displacement of the measurement object T (distance to the measurement object T) measured in step S13 is displayed on the display unit 31 of the controller 30 or output to an externally connected device.

[0051] In addition, whether the displacement of the measurement object T (distance to the measurement object T) measured in step S13 is within a normal range or abnormal based on the threshold set in step S12 may also be displayed or output as a measurement result. Furthermore, the measurement conditions, various parameters, measurement mode, etc. set in step S12 may also be displayed or output.

[0052] [System overview including displacement sensors] Fig. 3 is a functional block diagram showing an overview of a sensor system 1 that uses a displacement sensor 10 according to the present invention. As shown in Fig. 3, the sensor system 1 includes the displacement sensor 10, a control device 11, a control signal input sensor 12, and an external connection device 13. The displacement sensor 10 is connected to the control device 11 and the external connection device 13 by, for example, a communication cable or an external connection cord (including, for example, an external input line, an external output line, a power line, etc.), and the control device 11 and the control signal input sensor 12 are connected by a signal line.

[0053] 1 and 2, the displacement sensor 10 measures the displacement of the measurement object T (the distance to the measurement object T). The displacement sensor 10 may then output the measurement results and the like to the control device 11 and the externally connected device 13.

[0054] The control device 11 is, for example, a PLC (Programmable Logic Controller), and gives various instructions to the displacement sensor 10 when the displacement sensor 10 measures the measurement target T.

[0055] For example, the control device 11 may output a measurement timing signal to the displacement sensor 10 based on an input signal from a control signal input sensor 12 connected to the control device 11, or may output a zero reset command signal (a signal for setting the current measurement value to 0) or the like to the displacement sensor 10.

[0056] The control signal input sensor 12 outputs an on / off signal to the control device 11, which indicates the timing at which the displacement sensor 10 measures the measurement object T. For example, the control signal input sensor 12 may be installed near a production line on which the measurement object T moves, and may output an on / off signal to the control device 11 upon detecting that the measurement object T has moved to a predetermined position.

[0057] The externally connected device 13 is, for example, a PC (Personal Computer), and various settings can be made to the displacement sensor 10 by the user operating it.

[0058] As specific examples, the measurement mode, operation mode, measurement period, material of the measurement object T, etc. are set.

[0059] The measurement mode can be set to either an "internal synchronous measurement mode" in which measurement is started periodically within the control device 11, or an "external synchronous measurement mode" in which measurement is started in response to an input signal from outside the control device 11, or the like.

[0060] As the setting of the operation mode, an "operation mode" in which the measurement object T is actually measured, or an "adjustment mode" in which measurement conditions for measuring the measurement object T are set, etc., can be selected.

[0061] The measurement period is the period for measuring the measurement object T, and can be set according to the reflectance of the measurement object T. However, even if the reflectance of the measurement object T is low, the measurement object T can be measured appropriately by lengthening the measurement period and setting the measurement period appropriately.

[0062] For the measurement object T, a "rough surface mode" suitable for when diffuse reflection is a relatively large component of the reflected light, a "specular surface mode" suitable for when specular reflection is a relatively large component of the reflected light, or a "standard mode" that is intermediate between these two is selected.

[0063] In this way, by making appropriate settings depending on the reflectance and material of the measurement object T, the measurement object T can be measured with higher accuracy.

[0064] 4 is a flowchart showing a procedure for measuring a measurement target T by the sensor system 1 that uses the displacement sensor 10 according to the present disclosure. As shown in FIG. 4, this procedure is a procedure for the external synchronization measurement mode described above, and includes steps S21 to S24.

[0065] In step S21, the sensor system 1 detects a measurement target T, which is an object to be measured. Specifically, the control signal input sensor 12 detects that the measurement target T has moved to a predetermined position on the production line.

[0066] In step S22, the sensor system 1 issues a measurement instruction to measure the measurement object T detected in step S21 using the displacement sensor 10. Specifically, the control signal input sensor 12 outputs an on / off signal to the control device 11 to instruct the timing of measuring the measurement object T detected in step S21, and the control device 11 outputs a measurement timing signal to the displacement sensor 10 based on the on / off signal to instruct the displacement sensor 10 to measure the measurement object T.

[0067] In step S23, the measurement object T is measured by the displacement sensor 10. Specifically, the displacement sensor 10 measures the measurement object T based on the measurement instruction received in step S22.

[0068] In step S24, the sensor system 1 outputs the measurement result obtained in step S23. Specifically, the displacement sensor 10 displays the result of the measurement process on the display unit 31, or outputs the result to the control device 11 or the externally connected device 13 via the external I / F unit 33.

[0069] 4, the procedure for the external synchronization measurement mode in which the measurement object T is measured by detecting the measurement object T with the control signal input sensor 12 has been described, but the present invention is not limited to this. For example, in the internal synchronization measurement mode, instead of steps S21 and S22, a measurement timing signal is generated based on a preset cycle to instruct the displacement sensor 10 to measure the measurement object T.

[0070] Next, the principle of measurement of the measurement target T by the displacement sensor 10 according to the present disclosure will be described. 5A is a diagram illustrating the principle of measurement of a measurement object T by a displacement sensor 10 according to the present disclosure. As shown in FIG. 5A, the displacement sensor 10 includes a sensor head 20 and a controller 30. The sensor head 20 includes an objective lens 21 and a plurality of collimating lenses 22a to 22c, and the controller 30 includes a wavelength swept light source 51, an optical amplifier 52, a plurality of isolators 53 and 53a to 53b, a plurality of optical couplers 54 and 54a to 54e, an attenuator 55, a plurality of light receiving elements (e.g., photodetectors (PD)) 56a to 56c, a multiplexing circuit 57, an analog-to-digital (AD) conversion unit (e.g., an analog-to-digital converter) 58, a processing unit (e.g., a processor) 59, a balanced detector 60, and a correction signal generation unit 61.

[0071] The wavelength swept light source 51 emits a laser beam with a swept wavelength. For example, if a method of modulating a VCSEL (Vertical Cavity Surface Emitting Laser) with current is applied as the wavelength swept light source 51, mode hopping is unlikely to occur due to a short cavity length, wavelength change is easy, and it can be realized at low cost.

[0072] The optical amplifier 52 amplifies the light emitted from the wavelength swept light source 51. The optical amplifier 52 may be, for example, an erbium-doped fiber amplifier (EDFA), and may be, for example, an optical amplifier dedicated to 1550 nm.

[0073] The isolator 53 is an optical element that transmits incident light in one direction, and may be disposed immediately after the wavelength swept light source 51 to prevent the influence of noise caused by returning light.

[0074] In this way, the light emitted from the wavelength swept light source 51 is amplified by the optical amplifier 52, passes through the isolator 53, and is branched to the main interferometer and the sub interferometer by the optical coupler 54. For example, the optical coupler 54 may be configured to branch the light to the main interferometer and the sub interferometer at a ratio of 90:10 to 99:1.

[0075] The light branched to the main interferometer is further branched by the first-stage optical coupler 54a into the direction of the measurement object T and the direction of the second-stage optical coupler 54b.

[0076] The light branched by the first-stage optical coupler 54a toward the measurement object T passes from the tip of the optical fiber through the collimator lens 22a and the objective lens 21 in the sensor head 20 and is irradiated onto the measurement object T. Then, the tip (end face) of the optical fiber serves as a reference surface, and the light reflected by the reference surface interferes with the light reflected by the measurement object T, generating interference light that returns to the first-stage optical coupler 54a and is then received by the light-receiving element 56a and converted into an electrical signal.

[0077] The light branched by the first-stage optical coupler 54a toward the second-stage optical coupler 54b travels through the isolator 53a to the second-stage optical coupler 54b, where it is further branched toward the sensor head 20. As in the first stage, the light branched toward the sensor head 20 passes from the tip of the optical fiber in the sensor head 20 through the collimator lens 22b and the objective lens 21 and is irradiated onto the measurement object T. The tip (end face) of the optical fiber serves as a reference surface, and light reflected from the reference surface interferes with light reflected from the measurement object T, generating interference light. This light returns to the second-stage optical coupler 54b, and is branched by the optical coupler 54b toward the isolator 53a and the light receiving element 56b. The light branched toward the light receiving element 56b is received by the light receiving element 56b and converted into an electrical signal. On the other hand, the isolator 53a transmits light from the upstream optical coupler 54a to the downstream optical coupler 54b and blocks light from the downstream optical coupler 54b to the upstream optical coupler 54a, so that the light branched toward the isolator 53a is blocked.

[0078] The light branched by the second-stage optical coupler 54b toward the third-stage optical coupler 54c travels through the isolator 53b to the third-stage optical coupler 54c, where it is further branched toward the sensor head 20. As in the first and second stages, the light branched toward the sensor head 20 passes from the tip of the optical fiber through the collimator lens 22c and the objective lens 21 in the sensor head 20 and is then irradiated onto the measurement object T. The tip (end face) of the optical fiber serves as a reference surface, and light reflected from the reference surface interferes with light reflected from the measurement object T, generating interference light. The interference light returns to the third-stage optical coupler 54c, which then branches toward the isolator 53b and the light receiving element 56c by the optical coupler 54c. The light branched toward the light receiving element 56c is received by the light receiving element 56c and converted into an electrical signal. On the other hand, the isolator 53b transmits light from the upstream optical coupler 54b to the downstream optical coupler 54c and blocks light from the downstream optical coupler 54c to the upstream optical coupler 54b, so that the light branched toward the isolator 53b is blocked.

[0079] In addition, since the light branched in a direction other than the sensor head 20 by the third-stage optical coupler 54c is not used to measure the measurement object T, it is preferable to attenuate it by an attenuator 55 such as a terminator to prevent it from being reflected back.

[0080] In this way, the main interferometer has three optical paths (three channels), each with an optical path length difference that is twice the distance (round trip) from the tip (end face) of the optical fiber of the sensor head 20 to the object to be measured T, and generates three interference lights corresponding to the optical path length difference.

[0081] As described above, the light receiving elements 56a to 56c receive the interference light from the main interferometer and generate electrical signals according to the power of the received light.

[0082] The multiplexing circuit 57 multiplexes the electrical signals output from the light receiving elements 56a to 56c.

[0083] The AD conversion unit 58 receives the electrical signal from the multiplexing circuit 57 and converts the electrical signal from an analog signal to a digital signal (AD conversion). Here, the AD conversion unit 58 performs AD conversion based on the correction signal from the correction signal generation unit 61 in the sub-interferometer.

[0084] In order to correct the nonlinearity of the wavelength when the wavelength swept light source 51 is swept, the secondary interferometer acquires an interference signal and generates a correction signal called a K clock.

[0085] Specifically, the light branched to the sub-interferometer by optical coupler 54 is further branched by optical coupler 54d. Here, the optical paths of the branched light are configured to have an optical path length difference, for example, by using optical fibers of different lengths between optical coupler 54d and optical coupler 54e, and interference light corresponding to the optical path length difference is output from optical coupler 54e. Then, balance detector 60 receives the interference light from optical coupler 54e and amplifies the optical signal and converts it into an electrical signal while removing noise by taking the difference with a signal of the opposite phase.

[0086] The optical coupler 54d and the optical coupler 54e may both branch light at a ratio of 50:50.

[0087] The correction signal generator 61 determines the nonlinearity of the wavelength during the sweep of the wavelength swept light source 51 based on the electrical signal from the balance detector 60, generates a K clock according to the nonlinearity, and outputs it to the AD converter 58.

[0088] Due to the nonlinearity of the wavelength when the wavelength swept light source 51 is swept, the waves of the analog signal input to the AD converter 58 in the main interferometer are not spaced at equal intervals. The AD converter 58 performs AD conversion (sampling) by correcting the sampling time based on the K clock described above so that the waves are spaced at equal intervals.

[0089] As described above, the K clock is a correction signal used to sample the analog signal of the main interferometer, and therefore needs to be generated at a higher frequency than the analog signal of the main interferometer. Specifically, the optical path length difference between the optical couplers 54d and 54e in the sub interferometer may be made longer than the optical path length difference between the tip (end face) of the optical fiber in the main interferometer and the measurement object T, or the frequency may be multiplied (e.g., eight times) by the correction signal generator 61 to increase the frequency.

[0090] The processing unit 59 acquires the digital signal that has been AD converted while the nonlinearity has been corrected by the AD conversion unit 58, and calculates the displacement of the measurement object T (the distance to the measurement object T) based on the digital signal. Specifically, the processing unit 59 converts the frequency of the digital signal using a fast Fourier transform (FFT), and calculates the distance by analyzing the frequency converted digital signal. Detailed processing in the processing unit 59 will be described later.

[0091] Since the processing unit 59 is required to perform high-speed processing, it is often realized by an integrated circuit such as an FPGA (field-programmable gate array).

[0092] Moreover, here, the multiplexing circuit 57 is arranged before the AD conversion unit 58, but it may be arranged after the AD conversion unit 58. The outputs from the plurality of light receiving elements 56a to 56c are each AD converted, and then multiplexed by the multiplexing circuit 57.

[0093] Also, here, three optical paths are provided in the main interferometer, and the sensor head 20 irradiates measurement light from each optical path onto the measurement object T, and based on the interference light (return light) obtained from each, the distance to the measurement object T and the like are measured (multi-channel). The number of channels in the main interferometer is not limited to three, and may be one, two, or four or more.

[0094] 5B is a diagram illustrating another principle by which the measurement target T is measured by the displacement sensor 10 according to the present disclosure. As shown in FIG. 5B, the displacement sensor 10 includes a sensor head 20 and a controller 30. The sensor head 20 includes an objective lens 21 and a plurality of collimating lenses 22a to 22c, and the controller 30 includes a wavelength swept light source 51, an optical amplifier 52, a plurality of isolators 53 and 53a to 53b, a plurality of optical couplers 54 and 54a to 54j, an attenuator 55, a plurality of light receiving elements (e.g., photodetectors (PD)) 56a to 56c, a multiplexing circuit 57, an analog-to-digital (AD) conversion unit (e.g., an analog-to-digital converter) 58, a processing unit (e.g., a processor) 59, a balanced detector 60, and a correction signal generation unit 61. The displacement sensor 10 shown in FIG. 5B differs from the configuration of the displacement sensor 10 shown in FIG. 5A mainly in that it includes optical couplers 54f to 54j. The principle of this different configuration will be described in detail below in comparison with FIG. 5A.

[0095] The light emitted from the wavelength swept light source 51 is amplified by an optical amplifier 52, passes through an isolator 53, and is branched by an optical coupler 54 to the main interferometer side and the sub-interferometer side. The light branched to the main interferometer side is further branched by an optical coupler 54f into measurement light and reference light.

[0096] As described in FIG. 5A, the measurement light is passed through the collimator lens 22a and the objective lens 21 by the first-stage optical coupler 54a, irradiated onto the measurement object T, and reflected by the measurement object T. Here, in FIG. 5A, the tip (end face) of the optical fiber is used as a reference surface, and light reflected from the reference surface interferes with light reflected by the measurement object T to generate interference light, but in FIG. 5B, no reference surface is provided for reflecting light. That is, in FIG. 5B, no light is reflected by the reference surface as in FIG. 5A, and therefore the measurement light reflected by the measurement object T returns to the first-stage optical coupler 54a.

[0097] Similarly, light branched from the first-stage optical coupler 54a toward the second-stage optical coupler 54b passes through the collimator lens 22b and the objective lens 21 by the second-stage optical coupler 54b, is irradiated onto the measurement object T, is reflected by the measurement object T, and returns to the second-stage optical coupler 54b. Light branched from the second-stage optical coupler 54b toward the third-stage optical coupler 54c passes through the collimator lens 22c and the objective lens 21 by the third-stage optical coupler 54c, is irradiated onto the measurement object T, is reflected by the measurement object T, and returns to the third-stage optical coupler 54c.

[0098] On the other hand, the reference light split by the optical coupler 54f is further split by the optical coupler 54g to optical couplers 54h, 54i, and 54j.

[0099] In the optical coupler 54h, the measurement light output from the optical coupler 54a and reflected by the measurement object T interferes with the reference light output from the optical coupler 54g, generating interference light that is received by the light-receiving element 56a and converted into an electrical signal. In other words, the measurement light and the reference light are split by the optical coupler 54f, and interference light is generated according to the difference in optical path length between the optical path of the measurement light (the optical path from the optical coupler 54f, via the optical coupler 54a, the collimator lens 22a, and the objective lens 21, reflected by the measurement object T, and reaching the optical coupler 54h) and the optical path of the reference light (the optical path from the optical coupler 54f, via the optical coupler 54g, to reach the optical coupler 54h). The interference light is received by the light-receiving element 56a and converted into an electrical signal.

[0100] Similarly, in the optical coupler 54i, interference light is generated according to the difference in optical path length between the optical path of the measurement light (the optical path from the optical coupler 54f, through the optical couplers 54a and 54b, the collimator lens 22b, and the objective lens 21, reflected by the measurement object T, and reaching the optical coupler 54i) and the optical path of the reference light (the optical path from the optical coupler 54f, through the optical coupler 54g, and reaching the optical coupler 54i), and the interference light is received by the photodetector 56b and converted into an electrical signal.

[0101] In the optical coupler 54j, interference light is generated according to the difference in optical path length between the optical path of the measurement light (the optical path from the optical coupler 54f, through the optical couplers 54a, 54b, and 54c, the collimator lens 22c, and the objective lens 21, reflected by the measurement target T, and reaching the optical coupler 54j) and the optical path of the reference light (the optical path from the optical coupler 54f, through the optical coupler 54g, and reaching the optical coupler 54j), and the interference light is received by the light-receiving element 56c and converted into an electrical signal. Note that the light-receiving elements 56a to 56c may be, for example, balanced photodetectors.

[0102] In this way, the main interferometer has three optical paths (three channels) and generates three interference lights according to the optical path length difference between the measurement light reflected by the measurement object T and input to the optical couplers 54h, 54i, and 54j, and the reference light input to the optical couplers 54h, 54i, and 54j, respectively, via the optical couplers 54f and 54g.

[0103] The optical path length difference between the measurement light and the reference light may be set to be different for each of the three channels, for example, the optical path lengths of the optical coupler 54g and the optical couplers 54h, 54i, and 54j may be set to be different.

[0104] Then, based on the interference light obtained from each, the distance to the measurement object T and the like are measured (multi-channel).

[0105] [Sensor head structure] Here, the structure of the sensor head used in the displacement sensor 10 will be described. FIG. 6A is a perspective view showing the schematic configuration of the sensor head 20, FIG. 6B is a perspective view showing the schematic configuration of a collimator lens holder arranged inside the sensor head 20, and FIG. 6C is a cross-sectional view showing the internal structure of the sensor head.

[0106] 6A, in the sensor head 20, the objective lens 21 and the collimator lens are stored in the objective lens holder 23. For example, the size of the objective lens holder 23 is such that the length of one side surrounding the objective lens 21 is about 10 mm, and the length in the optical axis direction is about 22 mm.

[0107] 6B, the collimating lens unit 24 is configured by fixing the collimating lens 22 to a collimating lens holder using an adhesive. The collimating lens unit 24 is configured so that the spot diameter can be adjusted by inserting an optical fiber into the holder. For example, the size of the collimating lens 22 is about 2 mm in diameter.

[0108] 6C, three collimating lenses 22a to 22c are held by collimating lens holders to form collimating lens units 24a to 24c, and three optical fibers are inserted into the collimating lens units 24a to 24c so as to correspond to the three collimating lenses 22a to 22c, respectively. Alternatively, each of the three optical fibers may be held by a collimating lens holder.

[0109] These optical fibers and collimator lens units 24a to 24c, together with the objective lens 21, are held by an objective lens holder 23 to form the sensor head 20.

[0110] Here, as shown in FIG. 6C, the three collimator lens units are arranged with offsets from one another in order to form different optical path length differences at positions in the optical axis direction of the sensor head 20.

[0111] Furthermore, the objective lens holder 23 and the collimator lens units 24a to 24c that constitute the sensor head 20 may be made of a metal material (for example, A2017) that is strong and can be processed with high precision.

[0112] Fig. 7 is a block diagram for explaining signal processing in the controller 30. As shown in Fig. 7, the controller 30 includes a plurality of light receiving elements 71a to 71e, a plurality of amplifier circuits 72a to 72c, a multiplexing circuit 73, an AD conversion unit 74, a processing unit 75, a differential amplifier circuit 76, and a correction signal generation unit 77.

[0113] As shown in Figure 5A, the controller 30 splits the light emitted from the wavelength swept light source 51 into a main interferometer and a sub interferometer using an optical coupler 54, and calculates the distance to the measurement object T by processing the main interference signal and the sub interferometer signal obtained from each.

[0114] The plurality of light receiving elements 71a to 71c correspond to the light receiving elements 56a to 56c shown in FIG. 5A, and receive the main interference signal from the main interferometer and output it as a current signal to the amplifier circuits 72a to 72c, respectively.

[0115] The plurality of amplifier circuits 72a to 72c convert the current signals into voltage signals (IV conversion) and amplify them.

[0116] The multiplexing circuit 73 multiplexes the voltage signals output from the amplifier circuits 72a to 72c and outputs the result to the AD conversion unit 74 as one voltage signal.

[0117] AD conversion section 74 corresponds to AD conversion section 58 shown in FIG. 5A, and converts the voltage signal into a digital signal (AD conversion) based on K clocks from correction signal generation section 77, which will be described later.

[0118] Processing unit 75 corresponds to processing unit 59 shown in FIG. 5A, converts the digital signal from AD conversion unit 74 into a frequency using FFT, analyzes it, and calculates the distance value to measurement object T.

[0119] The multiple light receiving elements 71d to 71e and the differential amplifier circuit 76 correspond to the balanced detector 60 shown in Figure 5A, and each receive the interference light in the sub-interferometer, one of which outputs an interference signal with an inverted phase, and by taking the difference between the two signals, noise is removed, while the interference signal is amplified and converted into a voltage signal.

[0120] 5A, the correction signal generating unit 77 binarizes the voltage signal using a comparator, generates a K clock, and outputs it to the AD converting unit 74. Since the K clock needs to be generated at a higher frequency than the analog signal of the main interferometer, the correction signal generating unit 77 may multiply the frequency (for example, by 8 times) to increase the frequency.

[0121] 7, the multiplexing circuit 73 is arranged before the AD conversion unit 74, but may be arranged after the AD conversion unit 74. The outputs of the plurality of light receiving elements 71a to 71c and the plurality of amplifier circuits 72a to 72c are each AD converted, and then multiplexed by the multiplexing circuit 73.

[0122] 8 is a flowchart showing a method for calculating the distance to the measurement object T, which is executed by the processing unit 59 in the controller 30. As shown in FIG. 8, the method includes steps S31 to S35.

[0123] In step S31, processing unit 59 performs frequency conversion on the waveform signal (voltage vs. time) into a spectrum (voltage vs. frequency) using the following FFT: Fig. 9A is a diagram showing how the waveform signal (voltage vs. time) is frequency converted into a spectrum (voltage vs. frequency).

number

[0124] In step S32, the processing unit 59 performs distance conversion on the spectrum (voltage vs. frequency) to convert it into a spectrum (voltage vs. distance). Fig. 9B is a diagram showing how the spectrum (voltage vs. frequency) is converted into a spectrum (voltage vs. distance).

[0125] In step S33, processing unit 59 calculates values (distance value, SNR) corresponding to the peak based on the spectrum (voltage vs. distance). Fig. 9C is a diagram showing how values (distance value, SNR) corresponding to the peak are calculated based on the spectrum (voltage vs. distance).

[0126] (1) Calculate the peak value of the voltage. Specifically, for the voltage shown in FIG. 9C, the pair of the distance value and the voltage value at the distance where the differential value of the voltage changes from positive to negative (D x ,V x ) and sort the sets in descending order of voltage value. (D1,V1),(D2,V2),(D3,V3),...,(D n ,V n )

[0127] (2) Combinations exceeding the number of multi-heads are excluded. For example, as shown in FIG. 5A, the displacement sensor 10 has three optical paths in the main interferometer, and the sensor head 20 irradiates the measurement object T with measurement light from each optical path, and receives the interference light (return light) obtained from each (number of multi-heads = 3). If there are four or more peaks, peaks exceeding three are due to noise and can be excluded from the calculation. When the number of multi-heads is 3, the results are (D1, V1), (D2, V2), and (D3, V3).

[0128] (3) Sort by distance. For example, sorting by distance from shortest to longest would be (D3, V3), (D1, V1), (D2, V2).

[0129] (4) Obtain the peak-to-peak voltage. Specifically, D 31 Voltage V 31 and obtain the intermediate distance D between D1 and D2. 12 Voltage V 12 Then, the average voltage Vn = (V 31 +V 12 ) / 2.

[0130] (5) Calculate the SNR of each signal. Specifically, SN1 = V1 / V n , SN2=V2 / V n , SN3=V3 / V n This becomes:

[0131] In this way, values (distance value, SNR)=(D1, SN1), (D2, SN2), (D3, SN3) corresponding to the peaks are calculated based on the spectrum (voltage vs. distance).

[0132] 8, in step S34, the processing unit 59 corrects the distance values among the values (distance values, SNRs) corresponding to the peaks calculated in step S33. Specifically, as shown in Fig. 6C, the three collimator lens units 24a to 24c (collimator lenses 22a to 22c and the optical fibers) are arranged with a deviation from each other in the optical axis direction of the sensor head 20, and therefore the processing unit 59 corrects the distance values D1, D2, and D3 corresponding to the peaks, respectively, in accordance with the deviation amounts (for example, h1, h2, h3, etc.).

[0133] As a result, the values corresponding to the peaks (corrected distance value, SNR)=(D1+h1, SN1), (D2+h2, SN2), (D3+h3, SN3).

[0134] In step S35, processing unit 59 averages the distance values among the values (corrected distance values, SNR) corresponding to the peaks calculated in step S34. Specifically, processing unit 59 preferably averages the corrected distance values having an SNR equal to or greater than a threshold among the values (corrected distance values, SNR) corresponding to the peaks, and outputs the averaged calculation result as the distance to measurement object T.

[0135] Next, the present disclosure will be described in detail as a specific embodiment, focusing on its more characteristic configurations, functions, and properties. Note that the optical interferometric distance measuring sensor shown below corresponds to the displacement sensor 10 described with reference to Figures 1 to 9, and all or part of the basic configurations, functions, and properties included in the optical interferometric distance measuring sensor are common to the configurations, functions, and properties included in the displacement sensor 10 described with reference to Figures 1 to 9.

[0136] <One embodiment> [Configuration of optical interferometric ranging sensor] 10 is a schematic diagram showing the general configuration of an optical interferometric distance measuring sensor 100 according to an embodiment of the present invention. As shown in Fig. 10, the optical interferometric distance measuring sensor 100 includes a wavelength swept light source 110, an interferometer 120, a light receiving unit 130, and a processing unit 140. The interferometer 120 includes a branching unit 121 and a sensor head 122, the light receiving unit 130 includes a light receiving element 131 and an AD conversion unit 132, and the processing unit 140 includes a frequency analysis unit 141, a mode determination unit 142, and a distance calculation unit 143.

[0137] Furthermore, the branching unit 121 branches the input light into a plurality of optical paths, and each of the optical paths is guided via an optical fiber to the sensor head 122. In the sensor head 122, for example, as shown in FIGS. 6A to 6C, collimating lenses 123a to 123c are arranged in the plurality of optical paths, respectively, and an objective lens 124 is further attached to or included therein.

[0138] The wavelength swept light source 110 is connected to the branching unit 121 and emits light while continuously changing the wavelength.

[0139] The branching unit 121 branches and outputs the light projected and input from the wavelength swept light source 110 into optical paths A to C so as to irradiate a plurality of spots (three spots in this case) on the measurement object T. The branching unit 121 may be, for example, an optical coupler or the like.

[0140] The light branched into optical path A passes through the optical fiber, collimator lens 123a and objective lens 124 as measurement light, and is irradiated onto the measurement object T, where it is reflected by the measurement object T. Then, the reflected light (first reflected light) reflected by the measurement object T returns to the branching section 121 from the tip of the optical fiber via the objective lens 124 and collimator lens 123a.

[0141] The light branched into optical path A is irradiated as measurement light onto the measurement object T via the optical fiber, but a part of it is reflected by a reference surface as reference light. Here, the tip of the optical fiber serves as the reference surface, and the reflected light (second reflected light) reflected by the reference surface returns to the branching unit 121 via the optical fiber.

[0142] At this time, with regard to the light output from branching unit 121 to the optical fiber of optical path A, the measurement light is irradiated onto the measurement object T and returns to branching unit 121 via the optical fiber as first reflected light, and the reference light is reflected by a reference surface, which is the tip of the optical fiber, and returns to branching unit 121 via the optical fiber as second reflected light. As a result, interference light is generated according to the optical path length difference between the measurement light and the reference light. In other words, the round-trip distance from the tip of the optical fiber of optical path A to the measurement object T is the optical path length difference, and interferometer 120 generates interference light based on the first reflected light and the second reflected light, and uses this as return light to branching unit 121. Note that the optical path lengths of the measurement light and the reference light may both be values obtained by multiplying the spatial length of the optical path by the refractive index.

[0143] Similarly, the light branched to optical path B passes through the optical fiber, collimator lens 123b and objective lens 124 as measurement light, is irradiated onto the measurement object T, and is reflected by the measurement object T. Then, the reflected light (first reflected light) reflected by the measurement object T returns to the branching unit 121 from the tip of the optical fiber via the objective lens 124 and collimator lens 123b. Also, a part of the light branched to optical path B is reflected as reference light by a reference surface that is the tip of the optical fiber, and the reflected light (second reflected light) reflected by the reference surface returns to the branching unit 121 via the optical fiber.

[0144] At this time, interference light is generated in accordance with the optical path length difference between the measurement light and the reference light for the light output from the branching unit 121 to the optical fiber of the optical path B. That is, the round trip distance from the tip of the optical fiber of the optical path B to the measurement target T is the optical path length difference, and the interferometer 120 generates interference light based on the first reflected light and the second reflected light, and uses it as return light to the branching unit 121.

[0145] Similarly, the light branched to optical path C passes through the optical fiber, collimator lens 123c and objective lens 124 as measurement light, is irradiated onto the measurement object T, and is reflected by the measurement object T. Then, the reflected light (first reflected light) reflected by the measurement object T returns to the branching unit 121 from the tip of the optical fiber via the objective lens 124 and collimator lens 123c. Also, a part of the light branched to optical path C is reflected as reference light by a reference surface that is the tip of the optical fiber, and the reflected light (second reflected light) reflected by the reference surface returns to the branching unit 121 via the optical fiber.

[0146] At this time, interference light is generated in accordance with the optical path length difference between the measurement light and the reference light for the light output from the branching unit 121 to the optical fiber of the optical path C. That is, the round trip distance from the tip of the optical fiber of the optical path C to the measurement target T is the optical path length difference, and the interferometer 120 generates interference light based on the first reflected light and the second reflected light, and uses it as return light to the branching unit 121.

[0147] In this way, the light projected and input from the wavelength swept light source 110 is branched by the branching unit 121, and in each of the branched optical paths A to C, interference light is generated based on the optical path length difference between the measurement light that irradiates each spot on the measurement object T and the reference light that is reflected by the reference surface that is the tip of the optical fiber in each of the optical paths A to C, and is output to the light receiving unit 130 by the interferometer 120 as return light.

[0148] The optical path length differences between the measurement light and the reference light are set to be different for each of the three spots (corresponding to the optical paths A to C).

[0149] The light receiving unit 130 receives the return light (each interference light) from the interferometer 120. In the light receiving unit 130, a light receiving element 131 is, for example, a photodetector, which receives the return light output from the interferometer 120 and converts it into an electrical signal. Then, the AD conversion unit 132 converts the electrical signal from an analog signal to a digital signal.

[0150] Here, the light receiving unit 130 is configured to receive, as a single light receiving unit, an optical signal containing each interference light corresponding to each of the three spots (corresponding to optical paths A to C) as return light from the interferometer 120, rather than a configuration in which each interference light is received by a separate light receiving unit, thereby achieving low cost with a simple configuration.

[0151] The processing unit 140 calculates the distance to the measurement object T based on the return light received by the light receiving unit 130. Specifically, the processing unit 140 detects a peak in the return light received by the light receiving unit 130, and calculates the distance to the measurement object T by associating the detected peak with the above-mentioned spot (corresponding to the optical paths A to C). Furthermore, for example, the processing unit 140 may be a processor realized by an integrated circuit such as an FPGA, and may frequency-convert an input digital signal using FFT, and calculate the distance to the measurement object T based on the frequency conversion.

[0152] More specifically, for example, in the processing unit 140, the frequency analysis unit 141 performs frequency conversion on the digital signal from the AD conversion unit 132 of the light receiving unit 130 using FFT, as in step S31 shown in FIG.

[0153] Fig. 11 is a diagram schematically illustrating an example of a signal waveform obtained by frequency conversion using FFT for the return light received by the light receiving unit 130. As shown in Fig. 11, peaks corresponding to three spots (corresponding to optical paths A to C) appear in the return light received by the light receiving unit 130.

[0154] Typically, an optical interferometer having multiple heads (here, the number of multi-heads is three) calculates the distance to the measurement object T based on peaks corresponding to each of the three detected spots (corresponding to optical paths A to C). As described above, the optical paths A to C are set so that the optical path length difference between the measurement light and the reference light is different for each of the light beams branched to correspond to the three spots. Specifically, since the tip positions of the optical fibers arranged in the optical paths A to C are shifted in the optical axis direction, the processing unit 140 corrects the distance values corresponding to the frequencies Fa to Fc according to the amount of shift, and further calculates the distance to the measurement object T by averaging the distance values corrected according to the amount of shift. Note that the tip positions of the optical fibers may be shifted in the optical axis direction, for example, as shown in FIG. 6C , by inserting the collimating lens units into which the optical fibers are inserted.

[0155] In order to prevent the peaks corresponding to the three spots (corresponding to optical paths A to C) from overlapping and to enable proper detection, the difference ΔL in the optical path length difference between the measurement light and the reference light in each of optical paths A to C is set to the distance resolution ΔL FWHM Here, the distance resolution δL FWHM =c / nδf (c: speed of light, n: refractive index in the optical path difference, δf: frequency sweep width), and by increasing the frequency sweep width δf, the distance resolution δL FWHM can be reduced, the half-width of the peak waveform can be reduced, and the peak can be made to appear more clearly.

[0156] Incidentally, when the surface of the measurement target T is a specular surface, the measurement light irradiated on the specular surface and its reflected light (first reflected light) are multiple-reflected to become disturbance light, which is included in the return light received by the light receiving unit 130. Then, a peak due to the multiple reflections may appear in the signal waveform frequency-converted by the frequency analysis unit 141 using FFT.

[0157] FIG. 12 is a diagram schematically illustrating an example of a signal waveform obtained by frequency conversion using FFT for the return light received by the light-receiving unit 130 when the surface of the measurement object T is a mirror surface. As shown in FIG. 12, because the surface of the measurement object T is a mirror surface, the peak based on the spot corresponding to optical path B appears large, while the peaks based on the spots corresponding to the other optical paths are smaller. Furthermore, multiple peaks due to the influence of multiple reflection signals appear. Optical path B corresponds to the spot among optical paths A to C where the optical path length difference between the measurement light and the first reflected light and the reference light and the second reflected light is shortest, and is also located on the optical axis of the objective lens 124. The multiple reflection signal occurs when, even if the surface of the measurement object T is a mirror surface, specularly reflected light is guided to an unexpected position or direction, further reflected, and received by the light-receiving unit 130. This signal appears at a higher frequency than the signals corresponding to each spot.

[0158] Here, the peaks based on the spots corresponding to optical paths A and C and the multiple peaks due to the influence of the multiple reflection signals are displayed so that they can be distinguished separately, but in reality, it is not possible to predict how the multiple peaks due to the influence of the multiple reflection signals will appear, and these may be mixed together, causing the peaks based on the spots corresponding to optical paths A and C to be buried.

[0159] Therefore, in the processing unit 140, the mode determination unit 142 determines whether to use the mirror mode or the rough surface mode for measuring the measurement object T, depending on whether the surface to be measured of the measurement object T is a mirror surface or a rough surface.

[0160] When the mode determination unit 142 determines that the mode is the rough surface mode, the distance calculation unit 143 calculates the distance to the measurement object T based on the peaks corresponding to each of the three detected spots (corresponding to optical paths A to C), as described using Figure 11.

[0161] On the other hand, when the mode determination unit 142 determines that the mode is the specular mode, as described with reference to FIG. 12 , since there are peaks due to multiple reflections in addition to the peaks corresponding to the three spots (corresponding to optical paths A to C), the distance calculation unit 143 calculates the distance to the measurement object T based on the peak corresponding to the spot corresponding to optical path B, which has the shortest optical path length difference. Specifically, among optical paths A to C, optical path B is arranged such that the tip position of the optical fiber for optical path B is closest to the measurement object T and is also arranged on the optical axis of the objective lens 124 so that the optical path length difference between the measurement light and the reference light is shortest. In other words, the peak corresponding to the spot corresponding to optical path B is set so that it can be reliably and appropriately detected as the lowest frequency peak. As a result, the distance value corresponding to the frequency Fb corresponding to the peak can be used as the distance to the measurement object T.

[0162] [Method for calculating the distance to the measurement target T depending on the mode] Next, a method for calculating the distance to the measurement target T depending on the mode will be specifically described in detail.

[0163] 13A is a flowchart showing a method for calculating the distance to the measurement object T, which is executed by the processing unit 140. As shown in FIG. 13A, the method includes steps S101 to S111.

[0164] In step S101, frequency analysis section 141 in processing section 140 performs frequency conversion on the waveform signal from light receiving section 130 using FFT, for example, as in step S31 shown in FIG.

[0165] In step S102, the processing unit 140 removes unnecessary peaks from the signal waveform that has been frequency converted in step S101. For example, the processing unit 140 removes unnecessary peaks that appear on the low-frequency side (near-distance side) due to the influence of lens reflection or the like.

[0166] In step S103, the processing unit 140 determines peaks by threshold processing in the signal waveform frequency-converted in step S101. For example, the processing unit 140 determines the noise level in the signal waveform frequency-converted in step S101 and then detects signals having a signal strength equal to or greater than a predetermined threshold Th1 as peaks.

[0167] The threshold value Th1 may be preset or may be dynamically changed. For example, the noise between peaks may be estimated, the SNR for each peak may be calculated, and peaks exceeding a predetermined threshold value Th1 (e.g., SNR>9) may be detected.

[0168] In step S104, mode determination unit 142 in processing unit 140 determines the mirrored mode or the rough surface mode based on the user's selection. The mirrored mode or the rough surface mode is configured to be selectable by the user, and for example, the mirrored mode or the rough surface mode may be selected in advance by the user operating setting unit 32 in controller 30. Alternatively, the mirrored mode or the rough surface mode may be selected by the user inputting the material of measurement object T or by automatically determining the material.

[0169] In step S104, if the mode determination unit 142 determines that the mode is the rough surface mode, the process proceeds to step S105, and if the mode determination unit 142 determines that the mode is the mirror surface mode, the process proceeds to step S108.

[0170] In step S105, the processing unit 140 detects three peaks that appear in the signal waveform that has been frequency converted using FFT, as described with reference to FIG.

[0171] In step S106, processing unit 140 determines which of optical paths A to C each of the three peaks detected in step S105 corresponds to. As described above, the optical path length differences between the measurement light and the reference light are set to be different for each of the three spots (corresponding to optical paths A to C), so it is possible to determine which of optical paths A to C each of the detected peaks corresponds to.

[0172] In step S107, since the tip positions of the optical fibers arranged in the optical paths A to C are arranged with a deviation in the optical axis direction, the processing unit 140 corrects the distance values corresponding to the frequencies Fa to Fc in accordance with the deviation amount.

[0173] On the other hand, in step S108, the processing unit 140 detects the lowest frequency peak among the peaks appearing in the signal waveform frequency-converted using FFT, as described with reference to Fig. 12. As described above, the lowest frequency peak is a peak based on the spot corresponding to optical path B, and optical path B is arranged such that the tip position of the optical fiber is closest to the measurement target T among optical paths A to C so that the optical path length difference between the measurement light and the reference light is shortest, and furthermore, is arranged on the optical axis of the objective lens 124.

[0174] In step S109, distance calculation unit 143 in processing unit 140 converts the frequency into distance, for example, as in step S32 shown in FIG.

[0175] In step S110, in the case of the rough surface mode, the distance calculation unit 143 averages and obtains the three distance values corrected in step S107 according to the amount of deviation of the tip position of the optical fiber, and in the case of the mirror surface mode, obtains the distance value calculated from the peak based on the spot corresponding to the optical path B detected in step S108.

[0176] In step S111, the distance calculation unit 143 outputs the distance value acquired in step S110 as the distance measurement result to the measurement object T.

[0177] In this way, the processing unit 140 calculates the distance to the measurement object T according to the mirror surface mode or rough surface mode selected by the user, and appropriately calculates the distance to the measurement object T even if the measurement object T includes both a rough surface and a mirror surface.

[0178] Note that even if the user does not select the mirror mode or the rough mode, the mirror mode or the rough mode may be automatically determined depending on the number of peaks detected in the signal waveform frequency-converted in step S101.

[0179] 13B is a flowchart showing a method for automatically determining the mirror surface mode or the rough surface mode and calculating the distance to the measurement target T, which is executed by the processing unit 140. As shown in FIG. 13B, this method is different from the method shown in FIG. 13A in step S204, but otherwise is the same process.

[0180] In step S204, processing unit 140 determines the number N of signals having a signal intensity equal to or greater than predetermined threshold Th1 detected as peaks in step S103. Specifically, in a multi-head (here, the number of multi-heads=3), three peaks based on spots corresponding to optical paths A to C are usually detected as shown in Fig. 11, and in this case, mode determination unit 142 in processing unit 140 may determine the mode as rough surface mode.

[0181] Furthermore, peaks may disappear due to noise caused by the surface shape of the measurement object T or the surrounding environment, and taking this into consideration, the mode determination unit 142 in the processing unit 140 determines the mode to be rough surface mode when the number of detected peaks N is equal to or less than 3. In other words, when the number of detected peaks is equal to or less than the number of multi-heads, the mode determination unit 142 in the processing unit 140 may determine the mode to be rough surface mode.

[0182] If a peak disappears, for example, if the number of detected peaks N=0, it may be determined that the distance to the measurement target T cannot be calculated and an error may be generated, or the previously calculated distance value may be output. Similarly, if the number of detected peaks N=1 or 2 (number of detected peaks ≠ number of multi-heads), it may be determined that a peak has disappeared and an error may be generated.

[0183] 12, when multiple peaks appear due to the influence of the multiple reflection signal and the number of detected peaks N is greater than 3, the mode determination unit 142 in the processing unit 140 determines the mode to be the mirror mode. That is, when the number of detected peaks N is greater than the number of multi-heads, it is estimated that the surface of the measurement object T is a mirror surface and that the measurement light irradiated on the mirror surface and its reflected light (first reflected light), etc. are multiple-reflected, so the mode determination unit 142 in the processing unit 140 may determine the mode to be the mirror mode.

[0184] In this way, even if the user does not select the mirror surface mode or the rough surface mode, the mirror surface mode or the rough surface mode is automatically determined based on the number of peaks detected in the signal waveform frequency converted in step S101, and the distance to the measurement object T is appropriately calculated even if the measurement object T includes both a rough surface and a mirror surface.

[0185] 13A and 13B, processing unit 140 performs distance conversion on the frequency in step S109 immediately after step S107 or step S108, and performs processing such as averaging the distance values in subsequent steps, but the distance conversion in step S109 does not have to be performed immediately after step S107 or step S108. Processing unit 140 may perform distance conversion on the frequency immediately after step S101, as in the distance conversion (step S32) shown in Fig. 8, for example, or may perform the conversion at another timing.

[0186] As described above, in the optical interferometer distance measuring sensor 100 according to one embodiment of the present invention, the interferometer 120 generates interference light for each of the light beams split into three spots based on the measurement light irradiated onto the measurement object T and reflected by the measurement object T and the reference light that follows at least a partial optical path different from that of the measurement light, and outputs the interference light as return light. The light receiving unit 130 receives the return light from the interferometer 120, and the processing unit 140 detects peaks in the return light and calculates the distance to the measurement object T by associating the detected peaks with the spots in either the mirrored surface mode or the rough surface mode used to measure the measurement object T. Here, the optical path length difference between the measurement light and the reference light is set to be different for each of the light beams split into three spots, allowing each peak to be appropriately detected. Furthermore, in the mirrored surface mode, the distance to the measurement object T is calculated based on the peak corresponding to the spot (optical path B) with the shortest optical path length difference among the detected peaks. That is, even if the measurement object T includes a rough surface and a specular surface, the distance to the measurement object T can be measured appropriately.

[0187] For the user, there is no need to use different sensor heads for mirrored and rough surfaces, and a single sensor head can be used to properly measure the distance to the measurement object T. Even if the measurement object T includes a mirrored surface, the problem of multiple reflections acting as disturbance light and preventing accurate measurements can be avoided.

[0188] In this embodiment, the branching unit 121 is configured to branch the light from the wavelength swept light source 110 into three optical paths A to C and irradiate the measurement light onto three spots on the measurement object T, but this is not limited to this, and for example, the number of branched optical paths and spots may be two or four or more.

[0189] [Modifications of Collimating Lens and Objective Lens] In the above-described embodiment, the optical interferometric ranging sensor 100 has one objective lens 124 for multiple collimating lenses (e.g., three), but the configuration of the collimating lenses and objective lenses is not limited to this.

[0190] FIG. 14 shows variations in which various objective lenses are arranged relative to multiple collimating lenses. In FIG. 14(a), collimating lenses 123a to 123c are arranged in each of the optical paths formed by three optical fibers A to C, and one objective lens 124 is arranged. This is the configuration of the sensor head of the optical interferometric distance measuring sensor 100 according to the embodiment described above, and it is possible to reduce the spot diameter and accurately measure the distance to the measurement object T even if the measurement object T is very small. Furthermore, since there is only one objective lens 124, optical axis adjustment is easy, which also leads to cost reduction. Furthermore, the size of the sensor head can be reduced.

[0191] In Fig. 14(b), collimator lenses 123a to 123c and objective lenses 124a to 124c are arranged in the optical paths formed by the three optical fibers A to C, respectively. With this configuration, although optical axis adjustment is required for each and the spot diameter becomes large, it is possible to flexibly arrange each optical path. In addition, it is possible to reduce the size of the sensor head.

[0192] 14(c), collimator lenses 123a to 123c are arranged in the optical paths formed by the three optical fibers A to C, respectively, and one objective lens 224 is arranged. Compared to the configuration in FIG. 14(b), this configuration has only one objective lens 224, which makes it easier to adjust the optical axis and also leads to cost reduction.

[0193] Fig. 15 shows a variation in which multiple objective lenses are arranged without a collimating lens. In Fig. 15(a), objective lenses 124a to 124c are arranged in the optical paths formed by three optical fibers A to C, respectively. With this configuration, although the spot diameter becomes larger, since no collimating lens is arranged, the optical axis can be easily adjusted, which also leads to cost reduction. In addition, the size of the sensor head can be reduced.

[0194] In FIG. 15(b), objective lenses 124a to 124c are arranged in the optical path formed by the three optical fibers A to C, respectively. Here, the positions and angles of the optical path formed by the three optical fibers A to C and the objective lenses 124a to 124c are adjusted so that the spot diameter becomes small. With this configuration, the spot diameter can be made small, and even if the measurement object T is very small, the distance to the measurement object T can be measured accurately and appropriately. Furthermore, since a collimating lens is not arranged, costs can be reduced.

[0195] Fig. 16 shows a variation in which no objective lens is provided, but multiple collimating lenses are provided. In Fig. 16, collimating lenses 123a to 123c are provided in the optical paths formed by three optical fibers A to C, respectively. With this configuration, although the spot diameter becomes larger, since no objective lens is provided, optical axis adjustment is easy, leading to cost reduction. In addition, the size of the sensor head can be reduced.

[0196] As described above, various sensor heads are available depending on the configuration of the collimator lens and objective lens, and an appropriate sensor head may be used depending on the size, shape, and material of the measurement object T, the required measurement accuracy, and the like.

[0197] [Modified interferometer] In the above-described embodiment, the optical interferometer distance measuring sensor 100 uses a Fizeau interferometer that generates interference light by using the tip (end face) of each optical fiber as a reference surface (reference light and its reflected light) in the optical paths A to C branched by the branching section 121, but the interferometer is not limited to this.

[0198] FIG. 17 shows variations of an interferometer that generates interference light using measurement light and reference light. In FIG. 17(a), in optical paths A to C branched by branching unit 121, the tips (end faces) of the optical fibers are used as reference surfaces, and the tips of the optical fibers are positioned offset in the optical axis direction so that the optical path length difference is different. This is the configuration of interferometer 120 of optical interferometer distance measuring sensor 100 according to the present embodiment described above (Fizeau interferometer), and the reference surface may be configured to reflect light due to the difference in refractive index between the optical fiber and air (Fresnel reflection). Alternatively, the tips of the optical fibers may be coated with a reflective film, or the tips of the optical fibers may be anti-reflective coated and a separate reflective surface such as a lens surface may be disposed thereon.

[0199] In FIG. 17(b), optical paths A to C branched by branching unit 121 are formed with measurement optical paths Lm1 to Lm3 that guide measurement light to measurement object T and reference optical paths Lr1 to Lr3 that guide reference light, and a reference surface is disposed at the end of each reference optical path Lr1 to Lr3 (Michelson interferometer). The reference surface may be formed by coating a reflective film on the tip of an optical fiber, or by applying an anti-reflective coating to the tip of an optical fiber and disposing a separate reflective surface such as a lens surface. In this configuration, the optical path lengths of the measurement optical paths Lm1 to Lm3 are made the same, and optical path length differences are provided among the reference optical paths Lr1 to Lr3, so that the optical path length differences are different among the optical paths A to C. Since the optical path lengths of the measurement optical paths Lm1 to Lm3 can be made the same, the optical design of the sensor head can be simplified.

[0200] 17(c), optical paths A to C branched by branching unit 121 are formed into measurement optical paths Lm1 to Lm3 that guide measurement light to measurement object T and reference optical paths Lr1 to Lr3 that guide reference light, and balance detectors are arranged in the reference optical paths Lr1 to Lr3 (Mach-Zehnder interferometers). In this configuration, the optical path lengths of the measurement optical paths Lm1 to Lm3 are made the same, and optical path length differences are provided in the reference optical paths Lr1 to Lr3, so that the optical path length differences are different among the optical paths A to C. Because the optical path lengths of the measurement optical paths Lm1 to Lm3 can be made the same, the optical design of the sensor head can be simplified.

[0201] As described above, the interferometer is not limited to the Fizeau interferometer described in this embodiment, but may be, for example, a Michelson interferometer or a Mach-Zehnder interferometer. Any interferometer may be applied as long as it is possible to generate interference light by setting the optical path length difference between the measurement light and the reference light, or a combination of these or other configurations may be applied.

[0202] The optical interferometric distance measuring sensor described in this embodiment is used in a displacement sensor that measures the distance to a measurement object T, a rangefinder, a LIDAR, and the like.

[0203] The above-described embodiments are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The elements of the embodiments, as well as their arrangement, materials, conditions, shapes, sizes, etc., are not limited to those illustrated and can be modified as appropriate. Furthermore, configurations shown in different embodiments can be partially substituted or combined with each other.

[0204] [Appendix] a light source (110) that emits light while continuously changing the wavelength; an interferometer (120) including a branching unit (121) that branches the light projected from the light source so as to irradiate a plurality of spots on a measurement object (T), and that generates interference light for each of the branched lights corresponding to the plurality of spots based on measurement light that is irradiated onto the measurement object and reflected by the measurement object, and reference light that follows an optical path that is at least partially different from that of the measurement light; a light receiving unit (130) that receives each interference light from the interferometer; a processing unit (140) that detects peaks in the received interference light, and calculates a distance to the measurement object by associating the detected peaks with the spots in accordance with either a mirror surface mode or a rough surface mode for measuring the measurement object, The optical path length difference between the measurement light and the reference light is set to be different for each of the beams branched corresponding to the plurality of spots, In the case of the specular mode, the processing unit determines, as a distance measurement result to the measurement object, a distance calculated based on a peak corresponding to a spot with the shortest optical path length difference among the detected peaks. An optical interferometric ranging sensor (100). [Explanation of symbols]

[0205] 1...sensor system, 10...displacement sensor, 11...control device, 12...sensor for control signal input, 13...external connection device, 20...sensor head, 21...objective lens, 22, 22a to 22c...collimating lens, 23...objective lens holder, 24, 24a to 24c...collimating lens unit, 30...controller, 31...display unit, 32...setting unit, 33...external interface (I / F) unit, 34...optical fiber connection unit, 35...external memory unit, 36...measurement processing unit, 40...optical fiber, 51...wavelength swept light source, 52...optical amplifier, 53, 53a to 53b...isolator, 54, 54a to 54j...optical coupler, 55...attenuator, 56a to 56c...light receiving element, 57...multiplexing circuit, 58...AD conversion unit , 59...processing unit, 60...balanced detector, 61...correction signal generation unit, 71a to 71e...light receiving element, 72a to 72c...amplification circuit, 73...multiplexing circuit, 74...AD conversion unit, 75...processing unit, 76...differential amplification circuit, 77...correction signal generation unit, 100...optical interferometric distance measuring sensor, 110...wavelength swept light source, 120...interferometer, 121...branching unit, 122...sensor head, 123a to 123c...collimating lens, 124, 124a to 124c, 224...objective lens, 130...light receiving unit, 131...light receiving element, 132...AD conversion unit, 140...processing unit, 141...frequency analysis unit, 142...mode determination unit, 143...distance calculation unit, T...measurement object, Lm1 to Lm3...measurement light path, Lr1 to Lr3...reference light path

Claims

1. a light source that projects light while continuously changing the wavelength; an interferometer including a branching unit that branches the light projected from the light source so as to irradiate a plurality of spots on a measurement object, and for each of the lights branched corresponding to the plurality of spots, generates interference light based on measurement light that is irradiated onto the measurement object and reflected by the measurement object, and reference light that follows an optical path at least partially different from that of the measurement light; a light receiving unit that receives each interference light from the interferometer; a processing unit that detects peaks of the received interference light beams, and calculates a distance to the measurement object by associating the detected peaks with the spots in accordance with either a mirror surface mode or a rough surface mode for measuring the measurement object, The optical path length difference between the measurement light and the reference light is set to be different for each of the beams branched corresponding to the plurality of spots, In the mirror mode, the processing unit determines, as a distance measurement result to the measurement object, a distance calculated based on a peak corresponding to a spot with the shortest optical path length difference among the detected peaks. Optical interferometric ranging sensor.

2. the processing unit, in the case of the rough surface mode, determines the distance calculated based on the detected plurality of peaks as the distance measurement result. The optical interferometric distance measuring sensor according to claim 1 .

3. the interferometer has one objective lens; the measurement light beams of the respective beams branched by the branching unit corresponding to the plurality of spots are irradiated onto the measurement object via the one objective lens; 3. The optical interferometric distance measuring sensor according to claim 1.

4. a spot having the shortest optical path length difference among the optical path length differences corresponding to the plurality of spots is disposed on the optical axis of the one objective lens; The optical interferometric distance measuring sensor according to claim 3 .

5. the interferometer generates interference light beams based on a first reflected light beam of the measurement light beam that is irradiated onto the measurement object and reflected by the measurement object, and a second reflected light beam of the reference light beam that is reflected by a reference surface; The optical interferometric distance measuring sensor according to claim 1 .

6. With respect to optical fibers that transmit the respective light beams branched corresponding to the plurality of spots, the tip positions of the optical fibers that serve as the reference surface are arranged so as to be shifted in the optical axis direction. The optical interferometric distance measuring sensor according to claim 5 .

7. The difference ΔL in the optical path length difference between the light beams branched corresponding to the plurality of spots is at least the distance resolution δL expressed by the following formula: FWHM greater than The optical interferometric distance measuring sensor according to claim 1 . δL FWHM =c / nδf (c: speed of light, n: refractive index in the optical path difference, δf: frequency sweep width)

8. The specular mode or the rough mode can be selected by a user. The optical interferometric distance measuring sensor according to any one of claims 1 to 7.

9. the processing unit uses the specular mode when detecting peaks in the received interference light beams that exceed the number of the plurality of spots. The optical interferometric distance measuring sensor according to any one of claims 1 to 7.

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