Patent Description:
Nowadays, optical range sensors are commonly used to contactlessly measure the distance to a measurement target. For example, a known optical range sensor includes an optical interferometric range sensor that generates, from light emitted from a wavelength swept light source, interference light based on reference light and measurement light and measures the distance to a measurement target based on the interference light.

<CIT> describes an optical measurement device that causes a returning beam component of a reference beam reflected from the end faces of multiple optical fibers to coherently interfere with a reflected component of a measurement beam reflected from a surface of a measurement target to obtain stable measurement results.

<CIT> discloses a system that includes a first object mounted relative to a second object, the first object being moveable with respect to the second object. The system includes a plurality of interferometers each configured to derive a first wavefront and a second wavefront from input radiation and to combine the first and second wavefronts to provide output radiation including information about an optical path length difference between the paths of the first and second wavefronts, each interferometer including a reflective element positioned in the path of the first wavefront, and at least one of the interferometer's reflective element is mounted on the first object. The system also includes a plurality of fiber waveguides and an electronic controller. Each fiber waveguide is configured to deliver the input radiation to a corresponding interferometer or deliver the output radiation from the corresponding interferometer to a corresponding detector. The electronic controller is configured to monitor a degree of freedom of the first object relative to the second object based on the information from at least one of the interferometers, wherein the degree of freedom is an absolute displacement between the first and second objects.

<CIT> discloses a fiber optic system including a plurality of optical sensors, each with an identification system. The fiber optic system includes a fiber extending a distance, a demodulator, and at least one coupler, optical sensor and corresponding identification system. The identification system is powered by light shunted from the fiber by the coupler to a modulating device. The modulating device modulates the light and transmits it to a power converting device, which transforms the light energy into electrical energy. The electrical energy powers a high temperature integrated circuit upon which is stored a digital identification of a respective optical sensor. The integrated circuit, upon being powered up, sends a modulated response back up to the surface through the modulating device. Alternatively, a passive identification system is described, where reflective devices are placed at pre-determined locations along the length of the fiber. As the optical beam passes through the reflective devices, identification information for a sensor is encoded onto the beam. An optical frequency domain reflectometer generates the optical beam and detects the encoded information.

<CIT> discloses systems, methods and applications for adjusting the imaging depth of a Fourier Domain optical coherence tomography system without impacting the axial resolution of the system are presented. One embodiment of the invention involves changing the sweep rate of a swept-source OCT system while maintaining the same data acquisition rate and spectral bandwidth of the source. Another embodiment involves changing the data acquisition rate of a SS-OCT system while maintaining the same sweep rate over the same spectral bandwidth. Several applications of variable imaging depth in the field of ophthalmic imaging are described.

For the optical measurement device described in <CIT>, a sensor head and measurement conditions are to be set as appropriate for the measurement distance to a measurement target. For each operation, a sensor head that can cover the measurement distance to the measurement target is to be attached, and measurement conditions are to be set manually as appropriate for the measurement distance to the measurement target.

One or more aspects of the present invention are directed to an optical interferometric range sensor that uses a measurement condition appropriate for a measurement distance to a measurement target.

An optical interferometric range sensor according to one aspect of the present invention includes a light source that emits light with a changing wavelength, an interferometer that receives the light emitted from the light source and generates interference light based on measurement light emitted from a sensor head to a measurement target and reflected from the measurement target and reference light traveling on an optical path at least partially different from an optical path of the measurement light, a light receiver that receives the interference light from the interferometer to convert the interference light to an electric signal, a processor that calculates a distance from the sensor head to the measurement target based on the electric signal resulting from conversion performed by the light receiver, an identifier that identifies the sensor head based on a beat signal generated by the interferometer, and a setter that sets a measurement condition corresponding to the sensor head identified by the identifier, whereby the beat signal results from a portion of the light emitted from the light source and received by the interferometer being reflected from a component of the interferometer including a reflective surface, the reflected light interfering with the reference light of the interferometer.

In this aspect, the identifier identifies the sensor head based on the beat signal generated by the interferometer, and the setter sets the measurement condition corresponding to the sensor head identified by the identifier. The structure allows setting of a measurement condition appropriate for the measurement distance to the measurement target, thus allowing appropriate measurement of the distance to the measurement target. The structure reduces the user work of, for example, identifying the type of sensor head and manually setting the corresponding measurement conditions in each measurement operation.

In the above aspect, the setter may adjust a sweep rate of the light emitted from the light source based on the sensor head identified by the identifier. The sweep rate represents a frequency sweep width per sweep time.

In this aspect, the setter adjusts the sweep rate of the light emitted from the light source based on the sensor head identified by the identifier, thus allowing appropriate detection of a signal peak based on the interference light received by the light receiver in the circuit bandwidth to be processed by the processor.

The optical interferometric range sensor according to the above aspect may further include a correction signal generator that generates a correction signal for sampling to convert the interference light received by the light receiver to the electric signal. The setter may adjust a degree by which a frequency of the correction signal is multiplied based on the sensor head identified by the identifier.

In this aspect, the setter adjusts, based on the sensor head identified by the identifier, the degree by which the frequency of the correction signal generated by the correction signal generator is multiplied, thus allowing appropriate sampling of the interference light received by the light receiver. The distance to the measurement target can thus be measured appropriately.

In the above aspect, the identifier may identify the sensor head based on at least one of a peak frequency of the beat signal or the number of peaks in the beat signal.

In this aspect, the identifier identifies the sensor head based on at least one of the peak frequency of the beat signal or the number of peaks in the beat signal, thus facilitating identification of the sensor head without causing the user to determine the type of sensor head.

In this aspect, the beat signal results from a portion of the light emitted from the light source and received by the interferometer being reflected from the component of the interferometer including the reflective surface, thus facilitating identification of the sensor head without causing the user to determine the type of sensor head.

In the above aspect, the reflective surface may be located inside the sensor head.

In this aspect, the reflective surface is located inside the sensor head. This reduces the user work including determination and preparation associated with the sensor head, other than attaching the sensor head, thus facilitating identification of the sensor head.

In the above aspect, the reflective surface may be located on an objective lens included in the sensor head.

In this aspect, the reflective surface is located on the objective lens included in the sensor head, thus eliminating preparation of, for example, a separate component and facilitating identification of the sensor head.

In the above aspect, the reflective surface may be located on a collimating lens included in the sensor head.

In this aspect, the reflective surface is located on the collimating lens included in the sensor head, thus eliminating preparation of, for example, a separate component and facilitating identification of the sensor head.

In the above aspect, the reflective surface may be located inside an optical fiber that guides the light emitted from the light source to the sensor head.

In this aspect, the reflective surface is located inside the optical fiber that guides light emitted from the light source to the sensor head, thus facilitating identification of the sensor head without a reflective surface in the sensor head.

The optical interferometric range sensor according to the above aspects of the present invention can set a measurement condition appropriate for the measurement distance to a measurement target.

Embodiments of the present invention will now be described specifically with reference to the accompanying drawings. The embodiments described below are merely illustrative for implementing the present invention and are not to be construed as limiting the invention. To facilitate understanding, the same reference numerals denote the same components in the drawings, and such components will not be described repeatedly.

An overview of a displacement sensor according to an embodiment of the present disclosure will be described first. <FIG> is a schematic external view of a displacement sensor <NUM> according to the embodiment of the present disclosure. As shown in <FIG>, the displacement sensor <NUM> includes a sensor head <NUM> and a controller <NUM> to measure the displacement of a measurement target T (distance to the measurement target T).

The sensor head <NUM> and the controller <NUM> are connected with an optical fiber <NUM>. An objective lens <NUM> is attached to the sensor head <NUM>. The controller <NUM> includes a display <NUM>, a setting unit <NUM>, an external interface (I/F) <NUM>, an optical fiber connector <NUM>, an external storage <NUM>, and an internal measurement processor <NUM>.

The sensor head <NUM> irradiates the measurement target T with light output from the controller <NUM> and receives reflected light from the measurement target T. The sensor head <NUM> includes a reference surface inside that reflects light output from the controller <NUM> and received through the optical fiber <NUM> and causes such light to interfere with the reflected light from the measurement target T described above.

The objective lens <NUM> attached to the sensor head <NUM> is detachable. The objective lens <NUM> is replaceable with another objective lens having a focal length appropriate for the distance between the sensor head <NUM> and the measurement target T, or is a variable-focus objective lens.

The sensor head <NUM>, the measurement target T, or both may be positioned to have the measurement target T appropriately being within the measurement area of the displacement sensor <NUM> by irradiating the measurement target T with guide light (visible light).

The optical fiber <NUM> is connected to the optical fiber connector <NUM> on the controller <NUM> and extends to connect the controller <NUM> and the sensor head <NUM>. The optical fiber <NUM> thus guides light emitted from the controller <NUM> to the sensor head <NUM> and returning light from the sensor head <NUM> to the controller <NUM>. The optical fiber <NUM> is detachable from the sensor head <NUM> and the controller <NUM>. The optical fiber may have any length, thickness, and characteristics.

The display <NUM> is, for example, a liquid crystal display or an organic electroluminescent (EL) display. The display <NUM> displays the setting values of the displacement sensor <NUM>, the amount of returning light received from the sensor head <NUM>, and measurement results measured by the displacement sensor <NUM>, such as the displacement of the measurement target T (distance to the measurement target T).

The setting unit <NUM> receives settings associated with measurement of the measurement target T through user operations performed on mechanical buttons or a touchscreen. All or some of the associated settings may be preset or set through an external connection device (not shown) connected to the external I/F <NUM>. The external connection device may be connected with a wire or wirelessly through a network.

The external I/F <NUM> includes, for example, Ethernet (registered trademark), Recommended Standard (RS) 232C, and analog output. The external I/F <NUM> may be connected to an external connection device to allow the associated settings to be input through the external connection device or to output, for example, measurement results measured by the displacement sensor <NUM> to the external connection device.

The controller <NUM> may import data stored in the external storage <NUM> to perform the settings associated with measurement of the measurement target T. The external storage <NUM> is, for example, an auxiliary storage, such as a universal serial bus (USB) memory, which prestores settings associated with measurement of the measurement target T.

The measurement processor <NUM> in the controller <NUM> includes, for example, a wavelength swept light source that emits light with continuously changing wavelengths, a light receiving element that receives returning light from the sensor head <NUM> to convert the received light to an electric signal, and a signal processing circuit that processes the electric signal. The measurement processor <NUM> performs various processes using, for example, a controller and a storage to calculate the displacement of the measurement target T (distance to the measurement target T) based on the returning light from the sensor head <NUM>. These processes will be described in detail later.

<FIG> is a flowchart showing measurement of the measurement target T with the displacement sensor <NUM> according to the embodiment of the present disclosure. As shown in <FIG>, this procedure includes steps S11 to S14.

In step S11, the sensor head <NUM> is positioned. For example, the sensor head <NUM> irradiates the measurement target T with guide light, which is used as a reference to position the sensor head <NUM> appropriately.

More specifically, the display <NUM> in the controller <NUM> may display the amount of returning light from the sensor head <NUM>. The user may refer to the amount of returning light to adjust, for example, the orientation of the sensor head <NUM> and the distance (position in height) from the measurement target T. Typically, when the sensor head <NUM> irradiates the measurement target T perpendicularly (at angles closer to <NUM> degrees), more light is reflected from the measurement target T, and more returning light is received from the sensor head <NUM>.

The objective lens <NUM> may be replaced with another objective lens having a focal length appropriate for the distance between the sensor head <NUM> and the measurement target T.

When settings appropriate for measuring the measurement target T are not possible (e.g., the amount of light appropriate for measurement is not received, or the focal length of the objective lens <NUM> is inappropriate), a message indicating an error or incomplete settings may be displayed on the display <NUM> or output to the external connection device for the user.

In step S12, various measurement conditions are set to measure the measurement target T. For example, the user sets unique calibration data (e.g., a function to correct linearity) for the sensor head <NUM> by operating the setting unit <NUM> in the controller <NUM>.

Various parameters may also be set. For example, the sampling time, the measurement area, and the threshold for determining whether the measurement result is normal or abnormal are set. The measurement cycle may also be set based on the characteristics of the measurement target T such as the reflectance and the material of the measurement target T. The measurement mode may be set based on the material of the measurement target T.

These measurement conditions and various parameters are set by operating the setting unit <NUM> in the controller <NUM>, but may also be set through an external connection device or by importing data from the external storage <NUM>.

In step S13, the sensor head <NUM> positioned in step S11 measures the measurement target T in accordance with the measurement conditions and various parameters set in step S12.

More specifically, in the measurement processor <NUM> included in the controller <NUM>, the wavelength swept light source emits light, the light receiving element receives returning light from the sensor head <NUM>, and the signal processing circuit performs, for example, frequency analysis, distance conversion, and peak detection to calculate the displacement of the measurement target T (distance to the measurement target T). The measurement will be specifically described in detail later.

In step S14, the results of the measurement performed in step S13 are output. For example, the displacement of the measurement target T (distance to the measurement target T) measured in step S13 and other information are displayed on the display <NUM> in the controller <NUM> or output to the external connection device.

The measurement result displayed or output may also include whether the displacement of the measurement target T (distance to the measurement target T) measured in step S13 is within a normal range or is abnormal based on the threshold set in step S12. The measurement conditions, the parameters, and the measurement mode set in step S12 may also be displayed or output together.

<FIG> is a functional block diagram of a sensor system <NUM> including the displacement sensor <NUM> according to the embodiment of the present disclosure. As shown in <FIG>, the sensor system <NUM> includes the displacement sensor <NUM>, a control device <NUM>, a sensor <NUM> for control signal input, and an external connection device <NUM>. The displacement sensor <NUM> is connected to the control device <NUM> and the external connection device <NUM> with, for example, a communication cable or an external connection cord (e.g., an external input line, an external output line, or a power line). The control device <NUM> and the sensor <NUM> for control signal input are connected with a signal line.

The displacement sensor <NUM> measures the displacement of the measurement target T (distance to the measurement target T) as described with reference to <FIG> and <FIG>. The displacement sensor <NUM> may then output the measurement results and other information to the control device <NUM> and the external connection device <NUM>.

The control device <NUM> is, for example, a programmable logic controller (PLC), which provides various instructions to the displacement sensor <NUM> measuring the measurement target T.

For example, the control device <NUM> may output a measurement time signal to the displacement sensor <NUM> based on an input signal from the sensor <NUM> for control signal input connected to the control device <NUM> or may output, for example, a zero reset command signal (a signal to set the current measurement value to zero) to the displacement sensor <NUM>.

The sensor <NUM> for control signal input outputs an on-signal or an off-signal to the control device <NUM> to indicate the time for the displacement sensor <NUM> to measure the measurement target T. For example, the sensor <NUM> for control signal input is installed near the production line carrying the measurement target T to output the on- or off-signal to the control device <NUM> upon detecting the measurement target T moved to a predetermined position.

The external connection device <NUM> is, for example, a personal computer (PC), which is operable by the user to perform various settings with the displacement sensor <NUM>.

In an example, a measurement mode, an operation mode, a measurement cycle, and the material of the measurement target T are set.

The measurement mode is selectively set to, for example, an internal synchronous measurement mode in which measurement starts periodically in the control device <NUM> or to an external synchronous measurement mode in which measurement starts in response to an input signal from outside the control device <NUM>.

The operation mode is selectively set to, for example, an in-operation mode in which the measurement target T is actually measured or to an adjustment mode in which the measurement conditions are set for measuring the measurement target T.

The measurement cycle for measuring the measurement target T is set based on the reflectance of the measurement target T. For any measurement target T with low reflectance, a longer measurement cycle may be set as appropriate to allow appropriate measurement of the measurement target T.

For example, a rough surface mode is selected for a measurement target T reflecting light containing a relatively large amount of diffuse reflection component. A specular surface mode is selected for a measurement target T reflecting light containing a relatively large amount of specular reflection component. A standard mode is selected for a measurement target T reflecting light containing about a half diffuse reflection component and a half specular reflection component.

Appropriate mode setting based on the reflectance and the material of the measurement target T allows more accurate measurement of the measurement target T.

<FIG> is a flowchart showing measurement of the measurement target T with the sensor system <NUM> including the displacement sensor <NUM> according to the embodiment of the present disclosure. As shown in <FIG>, this procedure is performed in the external synchronous measurement mode and includes steps S21 to S24.

In step S21, the sensor system <NUM> detects a measurement target T, which is an object to be measured. More specifically, the sensor <NUM> for control signal input detects the measurement target T moved to a predetermined position on the production line.

In step S22, the sensor system <NUM> instructs the displacement sensor <NUM> to measure the measurement target T detected in step S21. More specifically, the sensor <NUM> for control signal input outputs an on-signal or an off-signal to the control device <NUM> to indicate the time to measure the measurement target T detected in step S21. The control device <NUM> outputs a measurement time signal to the displacement sensor <NUM> in response to the on- or off-signal to instruct the displacement sensor <NUM> to measure the measurement target T.

In step S23, the displacement sensor <NUM> measures the measurement target T. More specifically, the displacement sensor <NUM> measures the measurement target T in response to the measurement instruction received in step S22.

In step S24, the sensor system <NUM> outputs measurement results obtained in step S23. More specifically, the displacement sensor <NUM> causes the display <NUM> to display the measurement results or outputs the results to, for example, the control device <NUM> or the external connection device <NUM> through the external I/F <NUM>.

Although the procedure described above with reference to <FIG> is performed in the external synchronous measurement mode in which the measurement target T is measured in response to the sensor <NUM> for control signal input detecting the measurement target T, the measurement may be performed with a procedure in any mode. For example, in the internal synchronous measurement mode, the processing in steps S21 and S22 may be replaced with generation of a measurement time signal in preset cycles to instruct the displacement sensor <NUM> to measure the measurement target T.

A basic measurement procedure of the measurement target T with the displacement sensor <NUM> according to the embodiment of the present disclosure will now be described. <FIG> is a diagram describing a basic measurement procedure of the measurement target T with the displacement sensor <NUM> according to the embodiment of the present disclosure. As shown in <FIG>, the displacement sensor <NUM> includes the sensor head <NUM> and the controller <NUM>. The sensor head <NUM> includes the objective lens <NUM> and multiple collimating lenses 22a to 22c. The controller <NUM> includes a wavelength swept light source <NUM>, an optical amplifier <NUM>, multiple isolators <NUM>, 53a, and 53b, multiple optical couplers <NUM> and 54a to 54e, an attenuator <NUM>, multiple light receiving elements (e.g., photodetectors, or PDs) 56a to 56c, multiple amplifier circuits 57a to 57c, multiple analog-to-digital (AD) converters 58a to 58c, a processor <NUM>, a balance detector <NUM>, and a correction signal generator <NUM>.

The wavelength swept light source <NUM> emits a laser beam with a swept wavelength. The wavelength swept light source <NUM> may be, for example, a vertical-cavity surface-emitting laser (VCSEL) modulated with an electric current. Such a wavelength swept light source <NUM> with a short cavity length is less likely to cause mode hopping and facilitates wavelength changes at low cost.

The optical amplifier <NUM> amplifies light emitted from the wavelength swept light source <NUM>. The optical amplifier <NUM> may be, for example, an erbium-doped fiber amplifier (EDFA) for light with <NUM>.

The isolator <NUM> is an optical element that transmits incident light in one direction. The isolator <NUM> may be located immediately downstream from the wavelength swept light source <NUM> to reduce returning light affecting the measurement as noise.

Light emitted from the wavelength swept light source <NUM> is amplified by the optical amplifier <NUM>, travels through the isolator <NUM>, and is split by the optical coupler <NUM> and incident on a main interferometer and a secondary interferometer. For example, the optical coupler <NUM> may split the light to be incident on the main interferometer and the secondary interferometer at a ratio of <NUM>:<NUM> to <NUM>:<NUM>.

The light split and incident on the main interferometer is further split by the first-stage optical coupler 54a into light toward the sensor head <NUM> and light toward the second-stage optical coupler 54b.

The light split toward the sensor head <NUM> by the first-stage optical coupler 54a travels across the sensor head <NUM> from the end of the optical fiber through the collimating lens 22a and the objective lens <NUM> to the measurement target T. The end (end face) of the optical fiber serves as the reference surface. The light reflected from the reference surface and the light reflected from the measurement target T interfere with each other to form interference light, which returns to the first-stage optical coupler 54a and is received by the light receiving element 56a for conversion to an electric signal.

The light split by the first-stage optical coupler 54a toward the second-stage optical coupler 54b enters the second-stage optical coupler 54b through the isolator 53a, and is further split by the second-stage optical coupler 54b into light toward the sensor head <NUM> and light toward the third-stage optical coupler 54c. The light directed by the optical coupler 54b to the sensor head <NUM> travels across the sensor head <NUM> from the end of the optical fiber through the collimating lens 22b and the objective lens <NUM> to the measurement target T in the same manner as in the first stage. The end (end face) of the optical fiber serves as the reference surface. The light reflected from the reference surface and the light reflected from the measurement target T interfere with each other to form interference light, which returns to the second-stage optical coupler 54b and is split by the optical coupler 54b into light toward the isolator 53a and light toward the light receiving element 56b. The light directed by the optical coupler 54b to the light receiving element 56b is received by the light receiving element 56b and converted to an electric signal. The isolator 53a transmits light from the optical coupler 54a in the preceding stage to the optical coupler 54b in the subsequent stage, but blocks light from the optical coupler 54b in the subsequent stage to the optical coupler 54a in the preceding stage. The light directed by the optical coupler 54b to the isolator 53a is thus blocked.

The light split toward the third-stage optical coupler 54c by the second-stage optical coupler 54b enters the third-stage optical coupler 54c through the isolator 53b, and is further split by the third-stage optical coupler 54c into light toward the sensor head <NUM> and light toward the attenuator <NUM>. The light directed by the optical coupler 54c to the sensor head <NUM> travels across the sensor head <NUM> from the end of the optical fiber through the collimating lens 22c and the objective lens <NUM> to the measurement target T in the same manner as in the first and second stages. The end (end face) of the optical fiber serves as the reference surface. The light reflected from the reference surface and the light reflected from the measurement target T interfere with each other to form interference light, which returns to the third-stage optical coupler 54c and is split by the optical coupler 54c into light toward the isolator 53b and light toward the light receiving element 56c. The light directed by the optical coupler 54c to the light receiving element 56c is received by the light receiving element 56c and converted to an electric signal. The isolator 53b transmits light from the optical coupler 54b in the preceding stage to the optical coupler 54c in the subsequent stage, but blocks light from the optical coupler 54c in the subsequent stage to the optical coupler 54b in the preceding stage. The light directed by the optical coupler 54c to the isolator 53b is thus blocked.

The light directed by the third-stage optical coupler 54c in the direction other than to the sensor head <NUM> is not used in the measurement of the measurement target T. Such light may thus be attenuated by the attenuator <NUM> such as a terminator to avoid returning back by reflection.

As described above, the main interferometer includes three stages of optical paths (three channels) each with an optical path length difference being twice (round trip) the distance from the end (end face) of the optical fiber connected to the sensor head <NUM> to the measurement target T, thus generating three beams of interference light corresponding to the respective optical path length differences.

The light receiving elements 56a to 56c receive interference light from the main interferometer as described above and generate electric signals corresponding to the amounts of received light.

The amplifier circuits 57a to 57c amplify the electric signals output from the respective light receiving elements 56a to 56c.

The AD converters 58a to 58c receive the electric signals amplified by the respective amplifier circuits 57a to 57c and convert the electric signals from analog signals to digital signals (AD conversion). The AD converters 58a to 58c perform AD conversion based on a correction signal from the correction signal generator <NUM> in the secondary interferometer.

The secondary interferometer obtains an interference signal in the secondary interferometer and generates the correction signal, referred to as a K clock, to correct nonlinearity in the swept wavelength of the wavelength swept light source <NUM>.

More specifically, the light split by the optical coupler <NUM> and incident on the secondary interferometer is further split by the optical coupler 54d. The optical paths for the resultant light beams can have different lengths with the use of, for example, optical fibers having different lengths extending between the optical coupler 54d and the optical coupler 54e, thus outputting interference light corresponding to the optical path length difference from the optical coupler 54e. The balance detector <NUM> receives the interference light from the optical coupler 54e and amplifies the optical signal while removing noise as the difference from the signal having a phase inverted from the phase of the interference light to convert the optical signal to an electric signal.

Each of the optical coupler 54d and the optical coupler 54e may split the light at a ratio of <NUM>:<NUM>.

The correction signal generator <NUM> determines, based on the electric signal from the balance detector <NUM>, the nonlinearity in the swept wavelength of the wavelength swept light source <NUM> and generates a K clock corresponding to the nonlinearity for output to the AD converters 58a to 58c.

The nonlinearity in the swept wavelength of the wavelength swept light source <NUM> indicates that the waves of the analog signals input into the AD converters 58a to 58c in the main interferometer occur at unequal intervals. The AD converters 58a to 58c perform AD conversion (sampling) by correcting the sampling time based on the K clock described above to cause the waves at equal intervals.

As described above, the K clock is a correction signal for sampling the analog signal in the main interferometer. The K clock is thus to be generated to have a higher frequency than the analog signal in the main interferometer. More specifically, the length difference between the optical paths extending between the optical coupler 54d and the optical coupler 54e in the secondary interferometer may be designed longer than the optical path length difference corresponding to the distance between the end (end face) of the optical fiber and the measurement target T in the main interferometer, or the frequency of the K clock may be multiplied (e.g., by eight times) to a higher frequency by the correction signal generator <NUM>.

The processor <NUM> obtains digital signals converted from analog signals by the AD converter 58a to 58c with the nonlinearity being corrected, and calculates the displacement of the measurement target T (distance to the measurement target T) based on the digital signals. More specifically, the processor <NUM> converts the digital signals to a frequency spectrum using a fast Fourier transform (FFT) and analyzes the resultant frequencies to calculate the distance. The processing performed by the processor <NUM> will be described in detail later.

The processor <NUM>, which is to perform high speed processing, is often implemented with an integrated circuit such as a field-programmable gate array (FPGA).

In the present embodiment, the main interferometer includes three stages of optical paths (multiple channels), with the sensor head <NUM> emitting measurement light along the light paths to the measurement target T. The interference light (returning light) obtained through each light path is used to measure, for example, the distance to the measurement target T. The main interferometer may include any number of channels other than three channels, such as one, two, or four or more channels.

<FIG> is a diagram describing another basic measurement procedure of the measurement target T with the displacement sensor <NUM> according to the embodiment of the present disclosure. As shown in <FIG>, the displacement sensor <NUM> includes the sensor head <NUM> and the controller <NUM>. The sensor head <NUM> includes the objective lens <NUM> and the multiple collimating lenses 22a to 22c. The controller <NUM> includes the wavelength swept light source <NUM>, the optical amplifier <NUM>, the multiple isolators <NUM>, 53a, and 53b, multiple optical couplers <NUM> and 54a to 54j, the attenuator <NUM>, the multiple light receiving elements (e.g., PDs) 56a to 56c, the multiple amplifier circuits 57a to 57c, the multiple AD converters 58a to 58c, the processor <NUM>, the balance detector <NUM>, and the correction signal generator <NUM>. The displacement sensor <NUM> shown in <FIG> differs from the displacement sensor <NUM> shown in <FIG> mainly in including the optical couplers 54f to 54j. The basic procedure of this structure will be described in detail by focusing on its differences from the structure shown in <FIG>.

Light emitted from the wavelength swept light source <NUM> is amplified by the optical amplifier <NUM>, travels through the isolator <NUM>, and is split by the optical coupler <NUM> into light toward the main interferometer and light toward the secondary interferometer. The light split toward the main interferometer is further split by the optical coupler 54f to serve as measurement light and reference light.

As described with reference to <FIG>, the measurement light is directed by the first-stage optical coupler 54a to travel through the collimating lens 22a and the objective lens <NUM> to the measurement target T and is reflected from the measurement target T. In the structure in <FIG>, the light reflected from the end (end face) of the optical fiber serving as the reference surface and the light reflected from the measurement target T interfere with each other to form interference light. In the structure in <FIG>, the reference surface to reflect light is eliminated. In other words, in the structure in <FIG>, no light is reflected from the reference surface unlike in <FIG>, and thus the measurement light reflected from the measurement target T returns to the first-stage optical coupler 54a.

Similarly, the light directed by the first-stage optical coupler 54a to the second-stage optical coupler 54b enters the second-stage optical coupler 54b, which directs a portion of light to travel through the collimating lens 22b and the objective lens <NUM> to the measurement target T. The light is then reflected from the measurement target T to return to the second-stage optical coupler 54b. The light directed by the second-stage optical coupler 54b to the third-stage optical coupler 54c enters the third-stage optical coupler 54c, which directs a portion of the light to travel through the collimating lens 22c and the objective lens <NUM> to the measurement target T. The light is then reflected from the measurement target T to return to the third-stage optical coupler 54c.

The reference light resulting from the split performed by the optical coupler 54f is further split by the optical coupler <NUM> to be incident on the optical couplers <NUM>, 54i, and 54j.

In the optical coupler <NUM>, the measurement light reflected from the measurement target T and output from the optical coupler 54a and the reference light output from the optical coupler <NUM> interfere with each other to form interference light, which is received by the light receiving element 56a and converted to an electric signal. In other words, the light split by the optical coupler 54f to serve as the measurement light and the reference light forms interference light corresponding to the length difference between the optical path for the measurement light (from the optical coupler 54f through the optical coupler 54a, the collimating lens 22a, and the objective lens <NUM> to the measurement target T and back to the optical coupler <NUM>) and the optical path for the reference light (from the optical coupler 54f through the optical coupler <NUM> to the optical coupler <NUM>). The interference light is received by the light receiving element 56a and converted to an electric signal.

Similarly, in the optical coupler 54i, interference light is formed to correspond to the length difference between the optical path for the measurement light (from the optical coupler 54f through the optical couplers 54a and 54b, the collimating lens 22b, and the objective lens <NUM> to the measurement target T and back to the optical coupler 54i) and the optical path for the reference light (from the optical coupler 54f through the optical coupler <NUM> to the optical coupler 54i). The interference light is received by the light receiving element 56b and converted to an electric signal.

In the optical coupler 54j, interference light is formed to correspond to the length difference between the optical path for the measurement light (from the optical coupler 54f through the optical couplers 54a, 54b, and 54c, the collimating lens 22c, and the objective lens <NUM> to the measurement target T, and back to the optical coupler 54j) and the optical path for the reference light (from the optical coupler 54f through the optical coupler <NUM> to the optical coupler 54j). The interference light is received by the light receiving element 56c and converted to an electric signal. The light receiving elements 56a to 56c may be, for example, balance PDs.

As described above, the main interferometer includes three stages of optical paths (three channels) to generate three beams of interference light each corresponding to the length difference between the optical path for the measurement light reflected from the measurement target T and input into the optical coupler <NUM>, 54i, or 54j and the optical path for the reference light input through the optical couplers 54f and <NUM> into the optical coupler <NUM>, 54i, or 54j.

The length difference between the optical path for the measurement light and the optical path for the reference light may be set to differ in each of the three channels with, for example, the optical path length differing between the optical coupler <NUM> and each of the optical couplers <NUM>, 54i, and 54j.

The interference light obtained through each of the optical paths (multiple channels) is used to measure, for example, the distance to the measurement target T.

The structure of the sensor head used in the displacement sensor <NUM> will now be described. <FIG> is a schematic perspective view of the sensor head <NUM>. <FIG> is a schematic view of the sensor head showing the internal structure.

As shown in <FIG>, the sensor head <NUM> includes a lens holder <NUM> holding the objective lens <NUM> and collimating lenses. For example, the size of the lens holder <NUM> is about <NUM> on one side surrounding the objective lens <NUM> and about <NUM> in length in the optical axis direction.

As shown in <FIG>, the lens holder <NUM> holds one objective lens <NUM> and three collimating lenses 22a to 22c. Light from the optical fiber is guided through an optical fiber array <NUM> to the three collimating lenses 22a to 22c. The light through the three collimating lenses 22a to 22c reaches the measurement target T through the objective lens <NUM>.

The optical fiber, the collimating lenses 22a to 22c, and the optical fiber array <NUM> are held by the lens holder <NUM> together with the objective lens <NUM>, thus together serving as the sensor head <NUM>.

The lens holder <NUM> in the sensor head <NUM> may be formed from a metal (e.g., A2017), which has high strength and is machinable with high precision.

<FIG> is a block diagram of the controller <NUM> showing signal processing in the controller <NUM>. As shown in <FIG>, the controller <NUM> includes multiple light receiving elements 71a to 71e, multiple amplifier circuits 72a to 72c, multiple AD converters 74a to 74c, a processor <NUM>, a differential amplifier circuit <NUM>, and a correction signal generator <NUM>.

As shown in <FIG>, the controller <NUM> splits, with the optical coupler <NUM>, light emitted from the wavelength swept light source <NUM> into light to be incident on the main interferometer and light to be incident on the secondary interferometer and processes a main interference signal obtained from the main interferometer and a secondary interference signal obtained from the secondary interferometer to calculate a distance value to the measurement target T.

The multiple light receiving elements 71a to 71c correspond to the light receiving elements 56a to 56c shown in <FIG>. The light receiving elements 71a to 71c receive main interference signals from the main interferometer to output the signals to the respective amplifier circuits 72a to 72c as current signals.

The amplifier circuits 72a to 72c convert the current signals to voltage signals (I-V conversion) and amplify the resultant signals.

The AD converters 74a to 74c correspond to the AD converters 58a to 58c shown in <FIG>. The AD converters 74a to 74c convert the voltage signals to digital signals (AD conversion) based on the K clock from the correction signal generator <NUM> (described later).

The processor <NUM> corresponds to the processor <NUM> shown in <FIG>. The processor <NUM> converts the digital signals from the AD converters 74a to 74c to a frequency spectrum using an FFT and analyzes the frequencies to calculate the distance value to the measurement target T.

The light receiving elements 71d and 71e and the differential amplifier circuit <NUM> correspond to the balance detector <NUM> shown in <FIG>. The light receiving elements 71d and 71e each receive interference light from the secondary interferometer. One of the light receiving elements 71d and 71e outputs an interference signal with the phase being inverted. The differential amplifier circuit <NUM> amplifies the interference light while removing noise as the difference between the two signals and converts the signal to a voltage signal.

The correction signal generator <NUM> corresponds to the correction signal generator <NUM> shown in <FIG>. The correction signal generator <NUM> binarizes the voltage signal with a comparator, generates a K clock, and outputs the K clock to the AD converters 74a to 74c. The K clock is to be generated with a higher frequency than the frequency of the analog signal in the main interferometer. The frequency of the K clock may by multiplied (e.g., by eight times) to a higher frequency by the correction signal generator <NUM>.

<FIG> is a flowchart showing a method for calculating the distance to the measurement target T with the processor <NUM> in the controller <NUM>. As shown in <FIG>, the method includes steps S31 to S34.

In step S31, the processor <NUM> converts the waveform signal (voltage versus time) to a frequency spectrum (voltage versus frequency) using the FFT below. <FIG> is a diagram of conversion of the waveform signal (voltage versus time) to a frequency spectrum (voltage versus frequency).

In the above formula, N is the number of data points.

In step S32, the processor <NUM> converts the frequency spectrum (voltage versus frequency) to a distance spectrum (voltage versus distance). <FIG> is a diagram of conversion of the frequency spectrum (voltage versus frequency) to a distance spectrum (voltage versus distance).

In step S33, the processor <NUM> calculates the distance value corresponding to a peak based on the distance spectrum (voltage versus distance). <FIG> is a diagram of peak detection based on distance spectra (voltage versus distance) and calculation of distance values corresponding to the respective peaks. In <FIG>, peaks are detected in three channels based on the respective spectra (voltage versus distance) to calculate the distance values corresponding to the respective peaks.

In step S34, the processor <NUM> calculates an average of the distance values calculated in step S33. More specifically, the processor <NUM> calculates an average of the distance values calculated from the respective peaks detected based on the spectra (voltage versus distance) in the three channels in step S33. The processor <NUM> outputs the average of the values as the distance to the measurement target T.

In step S34, the processor <NUM> may calculate an average of distance values with a signal-to-noise ratio (SNR) greater than or equal to a threshold selected from the distance values calculated in step S33. For example, among the peaks detected in all of the three channels based on the respective spectra (voltage versus distance), any distance value calculated based on a spectrum with a SNR less than the threshold is determined to be less reliable and is not used.

Specific embodiments of the present disclosure will now be described in detail focusing on distinctive components, functions, and characteristics. An optical interferometric range sensor described below corresponds to the displacement sensor <NUM> described with reference to <FIG>. All or some of the basic components, functions, and characteristics of the optical interferometric range sensor are the same as the components, functions, and characteristics of the displacement sensor <NUM> described with reference to <FIG>.

<FIG> is a schematic diagram of an optical interferometric range sensor <NUM> according to a first embodiment of the present invention. As shown in <FIG>, the optical interferometric range sensor <NUM> includes a wavelength swept light source <NUM>, a light splitter <NUM>, an interferometer <NUM>, a light receiver <NUM>, a processor <NUM>, an identifier <NUM>, and a setter <NUM>. The interferometer <NUM> includes a sensor head <NUM> with an objective lens <NUM>. The light receiver <NUM> includes a light receiving circuit <NUM> including light receiving elements and an AD converter <NUM>.

The wavelength swept light source <NUM>, the light splitter <NUM>, the light receiver <NUM>, and the processor <NUM> are included in a controller <NUM>. The identifier <NUM> and the setter <NUM> may be functional components implementable by the processor <NUM> or by, for example, a separate controller.

The wavelength swept light source <NUM> is directly connected to the light splitter <NUM> or indirectly connected to the light splitter <NUM> with other components (e.g., the optical amplifier <NUM>, the isolator <NUM>, and the optical coupler <NUM>). The wavelength swept light source <NUM> emits light with continuously changing wavelengths. In other words, the wavelength of the light emitted from the wavelength swept light source <NUM> changes continuously.

The light emitted from the wavelength swept light source <NUM> is transmitted to the interferometer <NUM> through the light splitter <NUM> and an optical fiber.

The light splitter <NUM> includes, for example, an optical coupler or a circulator to direct light emitted from the wavelength swept light source <NUM> to the interferometer <NUM> and the returning light from the interferometer <NUM> to the light receiver <NUM>. The light splitter <NUM> may include, for example, a <NUM> × <NUM> optical coupler. In this case, the light splitter <NUM> may include, for example, an attenuator to attenuate light spilt toward another path to reduce returning light to the optical coupler.

The interferometer <NUM> includes the sensor head <NUM> with the objective lens <NUM>. Light transmitted to the interferometer <NUM> is input into the sensor head <NUM> through the optical fiber. A portion of the light input into the sensor head <NUM> reaches the measurement target T through the objective lens <NUM> and is reflected from the measurement target T as measurement light. The measurement light reflected from the measurement target T is then collected by the objective lens <NUM> in the sensor head <NUM> to be input into the sensor head <NUM>. Another portion of the light input into the sensor head <NUM> is reflected from a reference surface at the end of the optical fiber as reference light. The measurement light and the reference light interfere with each other to form interference light corresponding to the length difference between the optical path for the measurement light and the optical path for the reference light. The interference light is output from the interferometer <NUM>.

The sensor head <NUM> may include a collimating lens between the end of the optical fiber and the objective lens <NUM> or may include the collimating lens without including the objective lens <NUM>.

The interference light output from the interferometer <NUM> is received by the light receiver <NUM> through the light splitter <NUM> and converted to an electric signal. More specifically, the light receiver <NUM> includes the light receiving circuit <NUM> including light receiving elements and the AD converter <NUM>. The light receiving circuit <NUM> includes, for example, light receiving elements that are PDs, which receive light output from the light splitter <NUM> and convert the light to electric signals each corresponding to the received amount of light. The AD converter <NUM> converts the electric signals from analog signals to digital signals.

The processor <NUM> calculates the distance from the sensor head <NUM> to the measurement target T based on the digital signals resulting from conversion performed by the light receiver <NUM>. For example, the processor <NUM> includes an integrated circuit such as an FPGA. The processor <NUM> converts each input digital signal to a frequency spectrum using an FFT and calculates the distance to the measurement target T based on the frequency spectra.

The distance from the sensor head <NUM> to the measurement target T is typically, but not limited to, the distance from the distal end of the sensor head <NUM> to the measurement target T. The processor <NUM> calculates this distance. For example, the processor <NUM> may calculate, as the distance from the sensor head <NUM> to the measurement target T, the distance from the end of the optical fiber connected to the sensor head <NUM> to the measurement target T, the distance from the objective lens <NUM> in the sensor head <NUM> to the measurement target T, or the distance from a reference position preset within the sensor head <NUM> to the measurement target T.

The measurement distance to the measurement target T, the type of sensor head <NUM>, and the circuit bandwidth in the processor <NUM> will now be described.

<FIG> each show the relationship between the measurement distance to the measurement target T, the type of sensor head <NUM>, and the circuit bandwidth in the processor <NUM> in one specific example. As shown in <FIG>, for a shorter measurement distance to the measurement target T, a short-range sensor head 131a with an appropriate objective lens 132a is used based on the focal length to the measurement target T. As shown in <FIG>, for a longer measurement distance to the measurement target T, a long-range sensor head 131b with an appropriate objective lens 132b is used based on the focal length to the measurement target T.

In the structure in <FIG>, the controller <NUM> detects, with the processor <NUM>, a signal peak of the interference light received by the light receiver <NUM>. The processor <NUM> calculates the distance from the sensor head 131a to the measurement target T based on the frequency at the signal peak. For the processor <NUM> to appropriately calculate the distance to the measurement target T, the measurement conditions are set with the processor <NUM> to allow the frequency at the signal peak based on the interference light received by the light receiver <NUM> to fall within the circuit bandwidth of the light receiver <NUM>. In other words, the measurement conditions are appropriately set to correspond to the short-range sensor head 131a.

In <FIG>, the measurement distance to the measurement target T is longer, and a long-range sensor head 131b is used. In this case, the processor <NUM> is expected to detect the frequency at a signal peak based on the interference light received by the light receiver <NUM> in a high frequency band. When the measurement conditions corresponding to the short-range sensor head 131a described above are set for the long-range sensor head 131b, the processor <NUM> cannot detect the frequency at the signal peak based on the interference light received by the light receiver <NUM> and may inappropriately calculate the distance from the sensor head 131b to the measurement target T. In other words, the measurement conditions corresponding to the long-range sensor head 131b are to be set with the processor <NUM> to allow the frequency at the signal peak based on the interference light received by the light receiver <NUM> to fall within the circuit bandwidth of the light receiver <NUM>.

More specifically, the type of sensor head <NUM> is identified as the short-range sensor head 131a or the long-range sensor head 131b to set measurement conditions corresponding to the identified type of sensor head.

The identifier <NUM> identifies the type of sensor head <NUM> based on a beat signal generated by the interferometer <NUM>.

<FIG> are each a schematic diagram of the sensor head <NUM> to be identified as a short-range sensor head 131a or a long-range sensor head 131b in one specific example. As shown in <FIG>, a portion of the light input into the sensor head 131a through the optical fiber reaches the measurement target T as measurement light and is reflected from the measurement target T. Another portion of the light input into the sensor head 131a is reflected from a reference surface 133a at the end of the optical fiber as reference light. The measurement light and the reference light form interference light, allowing detection of the distance from the sensor head 131a to the measurement target T as a signal peak.

Still another portion of the light input into the sensor head 131a through the optical fiber is reflected from a reflective surface 134a on the objective lens 132a. The reflected light and the reference light described above form a beat signal (interference light), allowing detection of a position Lp of the objective lens 132a (specifically, the reflective surface 134a) in the sensor head 131a (specifically, in the area from a distal end position LH of the sensor head 131a toward the reference surface 133a) as a signal peak.

The reflective surface 134a on the objective lens 132a may be on either surface of the objective lens 132a. The reflective surface 134a may be a partial reflective coating applied to the objective lens 132a, or may be a portion using Fresnel reflection with, for example, low reflectance (about <NUM>% or less).

Similarly, in the structure in <FIG>, the objective lens 132b in the sensor head 131b (specifically, in the area from a distal end position LH of the sensor head 131b toward a reference surface 133b) includes a reflective surface 134b to reflect a portion of the light input into the sensor head 131b through the optical fiber. The reflected light and the reference light described above form a beat signal (interference light), allowing detection of a position Lp of the objective lens 132b (specifically, the reflective surface 134b) in the sensor head 131b as a signal peak.

In the manner described above, a beat signal is generated by the interferometer <NUM> including the sensor head 131a or the sensor head 131b. The beat signal is received by the light receiver <NUM>, allowing detection of the signal peak as shown in <FIG>.

The identifier <NUM> can identify the type of sensor head <NUM> based on the peak of the beat signal, with, for example, the objective lens 132a and the objective lens 132b at different positions respectively in the short-range sensor head 131a and the long-range sensor head 131b.

The setter <NUM> sets measurement conditions corresponding to the type of sensor head <NUM> identified by the identifier <NUM>. Referring back to <FIG>, the measurement conditions set with the processor <NUM> include, for example, adjusting the sweep rate of the wavelength swept light source <NUM>. The measurement conditions are set to allow the high frequency at the signal peak based on the interference light received by the light receiver <NUM> to fall within the circuit bandwidth of the light receiver <NUM>.

The sweep rate herein refers to the frequency sweep width per sweep time, calculated with the sweep rate α = δf/T (where δf is the frequency sweep width, and T is the sweep time) based on the frequency sweep width and the sweep time in accordance with the frequency-modulated continuous-wave (FMCW) method. A coherent FMCW will be described in detail.

<FIG> is a diagram describing a coherent FMCW. As described above, the wavelength swept light source <NUM> emits light with the continuously changing wavelength (frequency). The measurement light reaching and reflected from the measurement target T and the reference light reflected from the reference surface at the end of the optical fiber have a difference in optical path lengths, thus forming interference light.

As shown in <FIG>, the light emitted from the wavelength swept light source <NUM> includes the measurement light delayed from the reference light by the optical path length difference to cause interference. The resultant light is received by the light receiver <NUM> as a beat signal (interference light) having a beat frequency that is the difference in frequency between the measurement light and the reference light. The beat frequency fb is calculated with fb = δf/T·2Ln/c, where δf is the frequency sweep width, T is the sweep time, L is the distance from the end (reference surface) of the optical fiber to the measurement target T, n is the refractive index in the optical path difference, and c is the speed of light.

Referring back to <FIG>, the measurement conditions set with the processor <NUM> are to include a lower sweep rate α (δf/T) to allow the frequency (beat frequency) at the signal peak based on the interference light received by the light receiver <NUM> to fall within the circuit bandwidth of the light receiver <NUM>.

A method for setting measurement conditions corresponding to the type of sensor head <NUM> will now be described.

<FIG> is a flowchart showing a measurement condition setting method M100 including identifying the type of sensor head <NUM> and setting a measurement condition corresponding to the identified type of sensor head <NUM>. As shown in <FIG>, the measurement condition setting method M100 includes steps S110 to S150. Each step is performed by the processor included in the optical interferometric range sensor <NUM>.

In step S110, the identifier <NUM> detects a signal peak based on the interference light received by the light receiver <NUM> within the area up to the distal end position LH of the sensor head.

In step S120, the identifier <NUM> calculates a distance Lp corresponding to the signal peak detected in step S110.

In step S130, the identifier <NUM> identifies the sensor head <NUM> as a sensor head 131a set as a short-range sensor or a sensor head 131b set as a long-range sensor based on the distance (position) Lp calculated in step S120.

In an example, for the short-range sensor head 131a, the objective lens 132a (reflective surface 134a) is pre-positioned to achieve Lp = L1. For the long-range sensor head 131b, the objective lens 132b (reflective surface 134b) is pre-positioned to achieve Lp = L2. The distance Lp calculated in step S120 is determined to be closer to (corresponds to) the distance L1 or the distance L2 with comparison of | Lp - L1 | and | Lp - L2 |.

When determining that the distance Lp corresponds to the distance L1 in step S130 (Yes in step S130), the identifier <NUM> identifies the short-range sensor head 131a as the sensor head <NUM> being used. In step S140, the setter <NUM> sets a measurement condition corresponding to the short-range sensor head 131a. In an example, the setter <NUM> sets a sweep rate α1 with the wavelength swept light source <NUM> as a measurement condition corresponding to the short-range sensor head 131a.

When determining that the distance Lp corresponds to the distance L2 in step S130 (No in step S130), the identifier <NUM> identifies the long-range sensor head 131b as the sensor head <NUM> being used. In step S150, the setter <NUM> sets a measurement condition corresponding to the long-range sensor head 131b. In an example, the setter <NUM> sets a sweep rate α2 with the wavelength swept light source <NUM> as a measurement condition corresponding to the long-range sensor head 131b.

The measurement condition setting method M100 may be performed upon setting measurement conditions for measurement of the measurement target T, or for example, before start of the measurement or upon each measurement of the measurement target T.

As described above, the optical interferometric range sensor <NUM> according to the first embodiment of the present invention includes the identifier <NUM> that identifies the sensor head <NUM> as a short-range sensor head or a long-range sensor head based on the beat signal generated by the interferometer <NUM>. The setter <NUM> then adjusts the sweep rate as a measurement condition corresponding to the type of sensor head <NUM> identified by the identifier <NUM>. This allows an appropriate measurement condition to be set for the measurement distance to the measurement target T, thus allowing appropriate measurement of the measurement distance to the measurement target T. This reduces the user work of, for example, identifying the type of sensor head <NUM> and manually setting the corresponding measurement conditions in each measurement operation.

Each of the sensor head 131a and the sensor head 131b described with reference to <FIG> includes a collimating lens between the end of the optical fiber and the objective lens 132a or 132b. This structure collimates light incident on the objective lens 132a or 132b, thus allowing more flexibility in the position of the reflective surface 134a or 134b on the objective lens 132a or 132b.

The short-range sensor head 131a and the long-range sensor head 131b may have any structures, other than the structures shown in <FIG>, that can identify the type of sensor head <NUM> based on the beat signal generated by the interferometer <NUM>. Such structures that allow identification of the type of sensor head <NUM> will now be described.

<FIG> are each a schematic diagram of a sensor head <NUM> including a collimating lens to be identified as a short-range sensor head 131a or a long-range sensor head 131b in one specific example. As shown in <FIG>, the short-range sensor head 131a includes a collimating lens 135a without an objective lens, and the long-range sensor head 131b includes a collimating lens 135b without an objective lens.

The collimating lens 135a includes a reflective surface 136a to reflect a portion of the light input into the sensor head 131a through the optical fiber. The collimating lens 135b includes a reflective surface 136b to reflect a portion of the light input into the sensor head 131b through the optical fiber.

The collimating lens 135a and the collimating lens 135b are at different positions in the respective short-range sensor head 131a and long-range sensor head 131b to allow the identifier <NUM> to identify the type of sensor head <NUM> based on the peak of the beat signal.

The sensor head 131a and the sensor head 131b described with reference to <FIG> each have a simple structure without an objective lens, thus facilitating detection of the peak of the beat signal.

<FIG> are each a schematic diagram of a sensor head <NUM> including an internal component including a reflective surface to be identified as a short-range sensor head 131a or a long-range sensor head 131b in one specific example. As shown in <FIG>, the short-range sensor head 131a includes a predetermined component 137a inside the housing, and the long-range sensor head 131b includes a predetermined component 137b inside the housing.

The predetermined component 137a reflects, within the housing of the short-range sensor head 131a, a portion of the light input into the short-range sensor head 131a through the optical fiber. The predetermined component 137b reflects, within the housing of the long-range sensor head 131b, a portion of the light input into the long-range sensor head 131b through the optical fiber.

The predetermined component 137a and the predetermined component 137b are at different positions in the respective short-range sensor head 131a and long-range sensor head 131b to allow the identifier <NUM> to identify the type of sensor head <NUM> based on the peak of the beat signal.

The sensor head 131a and the sensor head 131b described with reference to <FIG> each eliminate a reflective surface located on a collimating lens or an objective lens, facilitating detection of the peak of the beat signal with the predetermined component 137a or 137b positioned to reflect a portion of the light input into the sensor head 131a or 131b.

The predetermined components 137a and 137b may each be a portion of a component of the short-range sensor head 131a or the long-range sensor head 131b, or may be a separate component for generating the peak of the beat signal.

<FIG> is a schematic diagram of an optical fiber including a reflective surface to allow identification of the type of sensor head <NUM> in one specific example. As shown in <FIG>, a reflective surface <NUM> is located inside the optical fiber that guides light to the sensor head <NUM>, instead of inside the sensor head <NUM>. More specifically, the reflective surface <NUM> may be a partial reflective coating applied to a joint of optical fibers.

For example, an optical fiber connected to a short-range sensor head 131a and an optical fiber connected to a long-range sensor head 131b may include reflective surfaces <NUM> at different positions. This allows the identifier <NUM> to identify the type of sensor head <NUM> based on the peak of the beat signal.

For a Fizeau interferometer, the end (end face) of the optical fiber is used as the position at which the optical path difference is zero. The peak of the beat signal thus occurs, along the return path, in an area toward the distal end of the sensor head <NUM> from the position at which the optical path difference is zero.

<FIG> is a schematic diagram of an optical fiber including a reflective surface at an end (end face) adjacent to a controller <NUM> to allow identification of the type of sensor head <NUM> in one specific example. As shown in <FIG>, a reflective surface <NUM> is located at, instead of inside the sensor head <NUM>, the end (end face) of the optical fiber adjacent to the controller <NUM> that guides light to the sensor head <NUM>. More specifically, the reflective surface <NUM> may be a partial reflective coating applied to the end (end face) of the optical fiber adjacent to the controller <NUM>.

For a Fizeau interferometer, the end (end face) of the optical fiber is used as the position at which the optical path difference is zero. The peak of the beat signal thus occurs, along the return path, in an area toward the distal end of the sensor head <NUM> offset from the position at which the optical path difference is zero by the optical path length difference.

Using such peak occurrence, an optical fiber connected to a short-range sensor head 131a and an optical fiber connected to a long-range sensor head 131b may be designed to have different lengths (optical path lengths) to allow the identifier <NUM> to identify the type of sensor head <NUM> based on the peak of the beat signal.

As described above, the reflective surface is located inside the sensor head <NUM> including the collimating lens, the objective lens, and other components, or located inside or on an end face of the optical fiber to generate a beat signal in the interferometer <NUM>. A short-range sensor head 131a and a long-range sensor head 131b may be designed to allow detection of different peaks in the beat signals to allow the identifier <NUM> to identify the type of sensor head <NUM> based on the peak of the beat signal.

For the structure to detect different peaks in the beat signals, the structures described with reference to <FIG> and <FIG> may be combined. For example, for a short-range sensor head 131a being used, the predetermined component 137a is placed inside the sensor head 131a (<FIG>). For a long-range sensor head 131b being used, the reflective surface <NUM> is located inside the optical fiber (<FIG>).

<FIG> are each a schematic diagram of a sensor head <NUM> to be identified as a short-range sensor head 131a or a long-range sensor head 131b based on the number of peaks in the beat signal in one specific example. As shown in <FIG>, the short-range sensor head 131a and the long-range sensor head 131b have different structures. The type of sensor head <NUM> is identified based on the number of peaks, instead of the frequency (position and distance) at the peak of the beat signal generated by the interferometer <NUM>.

More specifically, as shown in <FIG>, the short-range sensor head 131a includes a collimating lens 135a and an objective lens 132a. A portion of the light input into the sensor head 131a through the optical fiber is reflected from a reflective surface 136a on the collimating lens 135a, and another portion of the light is reflected from a reflective surface 134a on the objective lens 132a. Each of the light reflected from the reflective surface 136a and the light reflected from the reflective surface 134a forms a beat signal (interference light) with the reference light reflected from a reference surface 133a at the end of the optical fiber. This allows detection of a position Lp <NUM> of the collimating lens 135a (specifically, the reflective surface 136a) and a position Lp2 of the objective lens 132a (specifically, the reflective surface 134a) as signal peaks within the sensor head 131a (specifically, in the area from the distal end position LH of the sensor head 131a toward the reference surface 133a).

As shown in <FIG>, the long-range sensor head 131b includes a collimating lens 135b without including an objective lens. A portion of the light input into the sensor head 131b through the optical fiber is reflected from a reflective surface 136b on the collimating lens 135b. The reflected light then forms a beat signal (interference light) with the reference light reflected from a reference surface 133b at the end of the optical fiber. This allows detection of a position Lp of the collimating lens 135b (specifically, the reflective surface 136b) as a signal peak within the sensor head 131b (specifically, in the area from the distal end position LH of the sensor head 131b toward the reference surface 133b).

As described above, the short-range sensor head 131a includes the collimating lens 135a and the objective lens 132a, and the long-range sensor head 131b includes the collimating lens 135b. The identifier <NUM> can identify the type of sensor head <NUM> based on the number of peaks detected in the beat signal.

<FIG> is a flowchart showing a measurement condition setting method M101 including identifying the type of sensor head <NUM> based on the number of peaks detected in the beat signal and setting a measurement condition corresponding to the identified type of sensor head <NUM>. As shown in <FIG>, the measurement condition setting method M101 includes steps S111, S131, S140, and S150. Each step is performed by the processor included in the optical interferometric range sensor <NUM>.

In step S111, the identifier <NUM> detects any signal peak based on the interference light received by the light receiver <NUM> within the area up to the distal end position LH of the sensor head. For example, the identifier <NUM> may detect any signal peak having a signal strength of a predetermined value or greater.

In step S131, the identifier <NUM> determines the number of signal peaks detected in step S111.

In an example, for the short-range sensor head 131a, the collimating lens 135a and the objective lens 132a are pre-positioned to cause two signal peaks (as in, for example, <FIG>). For the long-range sensor head 131b, the collimating lens 135b is pre-positioned to cause one signal peak (as in, for example, <FIG>).

The identifier <NUM> then determines whether the number of signal peaks detected in step S111 is two to identify the type of sensor head as a sensor head 131a set as a short-range sensor head or a sensor head 131b set as a long-range sensor head.

When determining that the number of signal peaks is two in step S131 (Yes in step S131), the identifier <NUM> identifies a short-range sensor head 131a as the sensor head <NUM> being used. In step S140, the setter <NUM> sets a measurement condition corresponding to the short-range sensor head 131a. In an example, the setter <NUM> sets a sweep rate α1 with the wavelength swept light source <NUM> as a measurement condition corresponding to the short-range sensor head 131a.

When determining that the number of signal peaks is not two in step S131 (No in step S131), the identifier <NUM> identifies a long-range sensor head 131b as the sensor head <NUM> being used. In step S150, the setter <NUM> sets a measurement condition corresponding to the long-range sensor head 131b. In an example, the setter <NUM> sets a sweep rate α2 with the wavelength swept light source <NUM> as a measurement condition corresponding to the long-range sensor head 131b.

In the present embodiment, the identifier <NUM> identifies a short-range sensor head 131a or a long-range sensor head 131b as the sensor head being used. In some embodiments, identifying the sensor head <NUM> includes identifying a single sensor head <NUM> that is switchable between a short-range head sensor and a long-range head sensor. For example, such a single sensor head <NUM> may be switchable between a short-range sensor head and a long-range sensor head by changing (placing, adding, or eliminating) the collimating lens, the objective lens, and predetermined components within the housing. In this case as well, the identifier <NUM> identifies the type of sensor head <NUM>.

In a second embodiment of the present invention described below, the setter sets another measurement condition in place of or in addition to the sweep rate described in the first embodiment. In the present embodiment, the same reference numerals in the drawings denote the same components of the optical interferometric range sensor <NUM> according to the first embodiment. Such components will not be described in detail, and components different from those in the first embodiment will be described.

<FIG> is a schematic diagram of an optical interferometric range sensor <NUM> according to the second embodiment of the present invention. As shown in <FIG>, the optical interferometric range sensor <NUM> includes a wavelength swept light source <NUM>, a light splitter <NUM>, an interferometer <NUM>, a light receiver <NUM>, a processor <NUM>, an identifier <NUM>, and a setter <NUM>. The interferometer <NUM> includes a sensor head <NUM> with an objective lens <NUM>. The light receiver <NUM> includes a light receiving circuit <NUM> including light receiving elements and an AD converter <NUM>. The optical interferometric range sensor <NUM> further includes a light splitter <NUM> (e.g., an optical coupler) that splits light emitted from the wavelength swept light source <NUM> to be incident on a main interferometer (interferometer <NUM>) and a secondary interferometer.

The secondary interferometer is used to correct nonlinearity in the swept wavelength of the wavelength swept light source <NUM>, as described with reference to <FIG>, <FIG>, and <FIG>. The correction signal generator <NUM> generates a correction signal, referred to as a K clock, and outputs the signal to the AD converter <NUM>. The AD converter <NUM> converts (samples) the interference light received from the interferometer <NUM> from an analog signal to a digital signal based on the correction signal, thus correcting the nonlinearity in the swept wavelength of the wavelength swept light source <NUM>.

The processor <NUM> calculates the distance from the sensor head <NUM> to the measurement target T based on the digital signal resulting from the AD conversion performed by the AD converter <NUM>. When the number of samples for AD conversion performed by the AD converter <NUM> is insufficient, the distance may be calculated inappropriately. In other words, the distance to the measurement target T is to be calculated using an appropriate number of samples obtained per cycle of the signal for AD conversion to be performed by the AD converter <NUM>.

As described in the first embodiment of the present invention, for shorter measurement distances to the measurement target T, the identifier <NUM> identifies a short-range sensor head 131a as the sensor head being used. For longer measurement distances to the measurement target T, the identifier <NUM> identifies a long-range sensor head 131b as the sensor head being used.

The setter <NUM> controls the correction signal generator <NUM> to adjust the degree by which the frequency of the correction signal is multiplied based on the type of sensor head <NUM> identified by the identifier <NUM>. For example, the setter <NUM> may set the factor by which the frequency of the correction signal is multiplied to four for the short-range sensor head 131a identified as the sensor head being used by the identifier <NUM>, or may set the factor to eight for the long-range sensor head 131b identified as the sensor head being used by the identifier <NUM>.

The frequency of the correction signal may be adjusted by any degree of multiplication, other than by the factor of four or eight, that allows an appropriate number of samples to be obtained from the signal resulting from AD conversion performed by the AD converter <NUM> to calculate the distance to the measurement target T appropriately.

The degree by which the frequency of the correction signal is multiplied may be adjusted based on the sweep rate of the wavelength swept light source <NUM> described in the first embodiment of the present invention. For example, the degree by which the frequency of the correction signal is multiplied may be adjusted based on any sweep rate of the wavelength swept light source <NUM> adjusted as a measurement condition that is set with the processor <NUM>. This allows the frequency of the signal peak based on the interference light received by the light receiver <NUM> to fall within the circuit bandwidth of the light receiver <NUM>.

<FIG> is a flowchart showing a measurement condition setting method M200 including identifying the type of sensor head <NUM> and setting measurement conditions corresponding to the identified type of sensor head <NUM>. As shown in <FIG>, the measurement condition setting method M200 includes steps S110 to S130, S240, and S250. Each step is performed by the processor included in the optical interferometric range sensor <NUM>.

Steps S110 to S130 are similar to those in the measurement condition setting method M100 in the first embodiment of the present invention.

In response to the identifier <NUM> identifying the short-range sensor head 131a as the sensor head <NUM> being used (Yes in step S130), the setter <NUM> sets measurement conditions corresponding to the short-range sensor head 131a in step S240. In an example, the setter <NUM> sets a sweep rate α1 with the wavelength swept light source <NUM> and a frequency multiplication M1 for the correction signal as measurement conditions corresponding to the short-range sensor head 131a.

In response to the identifier <NUM> identifying the long-range sensor head 131b as the sensor head <NUM> being used (No in step S130), the setter <NUM> sets measurement conditions corresponding to the long-range sensor head 131b in step S250. In an example, the setter <NUM> sets a sweep rate α2 with the wavelength swept light source <NUM> and a frequency multiplication M2 for the correction signal as measurement conditions corresponding to the long-range sensor head 131b.

As described above, the optical interferometric range sensor <NUM> according to the second embodiment of the present invention includes the identifier <NUM> that identifies the type of sensor head <NUM> as the short-range sensor head 131a or the long-range sensor head 131b based on the beat signal generated by the interferometer <NUM>. The setter <NUM> adjusts the degree by which the frequency of the correction signal is multiplied in place of or in addition to the sweep rate as a measurement condition corresponding to the type of sensor head <NUM> identified by the identifier <NUM>. This allows an appropriate measurement condition to be set for the measurement distance to the measurement target T, thus allowing appropriate measurement of the measurement distance to the measurement target T. This reduces the user work of, for example, identifying the type of sensor head <NUM> and manually setting the corresponding measurement conditions in each measurement operation.

In each of the embodiments described above, the optical interferometric range sensor <NUM> or <NUM> includes the interferometer <NUM> being a Fizeau interferometer that generates reference light using the end of the optical fiber as a reference surface. The interferometer is not limited to the Fizeau interferometer.

<FIG>, and <FIG> are diagrams of interferometers that generate interference light using measurement light and reference light in modifications.

<FIG>, the light splitter <NUM> splits light on an optical path into reference light that uses the end of the optical fiber as a reference surface and measurement light emitted from the sensor head and then reaching and reflected from the measurement target T. Interference light occurs based on the difference in optical path lengths between the reference light and the measurement light. This structure includes the same interferometer as the interferometer <NUM> (Fizeau interferometer) in each of the optical interferometric range sensors <NUM> and <NUM> according to the embodiments described above. The reference surface may reflect light due to the difference in refractive index between the optical fiber and air (Fresnel reflection). The end of the optical fiber may be coated with a reflective film, or may be coated with a non-reflective film and receive a reflective surface such as a lens surface separately.

In <FIG>, the light splitter <NUM> splits light into measurement light to be guided along a measurement optical path Lm to the measurement target T and reference light to be guided along a reference optical path Lr. The reference optical path Lr includes a reference surface at its end (Michelson interferometer). The reference surface may be an end of an optical fiber coated with a reflective film, or may be an end of an optical fiber coated with a non-reflective film and receiving, for example, a mirror separately. This structure generates interference light with the length difference that is set between the measurement optical path Lm and the reference optical path Lr.

In <FIG>, the light splitter <NUM> splits light into measurement light to be guided along a measurement optical path Lm to the measurement target T and reference light to be guided along a reference optical path Lr. A balance detector is located on the reference optical path Lr (Mach-Zehnder interferometer). This structure generates interference light with the length difference that is set between the measurement optical path Lm and the reference optical path Lr.

As described above, the interferometer is not limited to the Fizeau interferometer described in each embodiment, and may be, for example, a Michelson or Mach-Zehnder interferometer, or any other interferometer that can generate interference light by setting the optical path length difference between the measurement light and the reference light. These or other interferometers may be combined.

In each of the first and second embodiments of the present invention, the optical interferometric range sensor <NUM> or <NUM> is a single-channel sensor, but may be another sensor, for example, a multi-stage optical interferometric range sensor that splits light emitted from the wavelength swept light source <NUM> using multiple optical couplers. The structure according to one or more embodiments of the present invention is also applicable to multi-stage optical interferometric range sensors.

Claim 1:
An optical interferometric range sensor (<NUM>; <NUM>; <NUM>), comprising:
a light source (<NUM>; <NUM>) configured to emit light with a changing wavelength;
an interferometer (<NUM>) configured to receive the light emitted from the light source (<NUM>; <NUM>) and generate interference light based on measurement light and reference light, the measurement light being light emitted from a sensor head (<NUM>;
<NUM>, 131a, 131b) to a measurement target (T) and reflected from the measurement target (T), the reference light being light traveling on an optical path at least partially different from an optical path of the measurement light;
a light receiver (56a to 56c, 58a to 58c, 71a to 71c, 74a to 74c; <NUM>) configured to receive the interference light from the interferometer (<NUM>) to convert the interference light to an electric signal;
a processor (<NUM>, <NUM>; <NUM>) configured to calculate a distance from the sensor head (<NUM>; <NUM>, 131a, 131b) to the measurement target (T) based on the electric signal resulting from conversion performed by the light receiver (56a to 56c, 58a to 58c, 71a to 71c, 74a to 74c; <NUM>);
an identifier (<NUM>) configured to identify the sensor head (<NUM>; <NUM>, 131a, 131b) based on a beat signal generated by the interferometer (<NUM>); and a setter (<NUM>; <NUM>) configured to set a measurement condition corresponding to the sensor head (<NUM>; <NUM>, 131a, 131b) identified by the identifier (<NUM>),
wherein the beat signal results from a portion of the light emitted from the light source (<NUM>; <NUM>) and received by the interferometer (<NUM>) being reflected from a component (137a, 137b) of the interferometer (<NUM>) including a reflective surface, the reflected light interfering with the reference light of the interferometer.