NON-CONTACT DISPLACEMENT SENSOR

Abstract

Non-contact displacement sensor, comprising: a light source that emits light; a liquid lens in which a refractive index changes periodically in response to an input drive signal; a beam splitter which splits light emitted from the light source and passing through the liquid lens into a measurement light and a reference light; a measurement-side objective lens which emits the measurement light split by the beam splitter to a measurement object; a reference-side objective lens onto which the reference light, which is split by the beam splitter, is incident; an optical system for the reference light, which includes a first reference surface and a second reference surface, each having a mutually determined length of an optical path measured from the reference-side objective lens, and in which the reference light passing through the reference-side objective lens is incident on each of the first reference surface and the second reference surface; a photodetector that receives the measurement light reflected by the measurement object and the reference light that has traveled through the reference light optical system and outputs a photodetection signal; a signal processor that works as: a focus time calculation device which calculates based on the photodetection signal: a measurement-side focal point time at which the measuring light is focused on the surface of the measuring object; a first reference-side focal point time at which the reference light is focused on the first reference surface, and a second reference-side focal point time at which the reference light is focused on the second reference surface; a feature calculation device that calculates the features of the refractive index of the liquid lens based on the first reference-side focal point time, the second reference-side focal point time, and an optical path length difference, which is a difference between an optical path length extending from the reference-side objective lens to the first reference surface and the optical path length extending from the reference-side objective lens to the second reference surface; and a position calculation device which calculates a position of the measurement object based on the characteristics of the refractive index and a phase of the measurement-side focal point time relative to a period of the drive signal.

Description

REFERENCE TO RELATED APPLICATIONSThe present application claims priority to Japanese Application No. 2018-148386 filed on Aug. 7, 2018, the disclosure of which is expressly incorporated herein by reference in its entirety.BACKGROUND OF THE INVENTION1. Field of the InventionThe present invention relates to a non-contact displacement sensor.2. Description of the Prior ArtUS 2018 / 0 180 773 A1 describes a lens system including a tubular vibrating element that vibrates due to an input driving signal; a housing that accommodates the vibrating element; a liquid that fills the housing and dips in the vibrating element; and a pressure reducing element installed in a gap between the vibrating element and the housing farther outward than the vibrating element. As the pressure-reducing element, a foamed body made of fluororubber having many closed cells is used.DE 10 2017 207 176 A1 describes a system comprising an imaging system for providing an automatically focused image, which imaging system comprises a high speed periodically modulated variable focal length (VFL) lens, a VFL lens controller, a VFL projected light source, a focus determining section, an exposure time adjusting circuit and a strobe exposure timing controller. The focus determining section includes an optical detector that inputs reflected VFL-projected light projected by and reflected from a workpiece through the VFL lens and provides a focus error signal. The exposure time adjustment circuit provides, based on the focus deviation signal, an exposure time adjustment signal indicating a time when the focus Z height of the imaging system approximately coincides with the Z height of the workpiece surface. The strobe exposure timing controller uses the exposure time adjustment signal to adjust the image exposure time such that the Z height of the imaging system coincides with the Z height of the workpiece surface at the adjusted image exposure time.Conventionally, as a non-contact displacement sensor that measures a displacement of a surface of a measured object / measurable object / object to be measured, a laser displacement sensor, a chromatic point sensor, and the like are available. In such a non-contact displacement sensor, a distance to the surface of the measurement object is found by detecting reflected light from the measurement object while changing a focus position of a measurement light.For example, a laser displacement sensor uses a confocal method or the like, and changes a focus position of measurement light by driving an objective lens along an optical axis. Based on information on a position of the objective lens on the optical axis, when measurement light reflected by a surface of the measurement object is detected, the sensor finds the distance to the surface of the measurement object (see, for example, Japanese Patent Application Publication JP H11-23 219 A). On the other hand, a chromatic point sensor uses the white confocal method and changes a focal point position for each wavelength by scattering a white light source using axial chromatic aberration. Then, by analyzing an intensity profile for each wavelength, wavelength light focused on the surface of the measurement object is detected, and the distance to the surface of the measurement object is found based on the wavelength light (for example, see Japanese Patent Application Publication JP 2009-122 105 A).In recent years, variable focal length lenses using a liquid lens system in which the refractive index periodically changes (hereinafter also referred to simply as a "lens system") have been developed (for example, refer to the description of U.S. Patent Application Publication US 2010 / 0 177 376 A1). The lens system is formed by dipping in a transparent liquid a hollow cylindrical oscillating member formed of a piezoelectric material. In the lens system, when an AC voltage is applied to an inner circumferential surface and an outer circumferential surface of the oscillating member, the oscillating member expands and contracts in a thickness direction and oscillates the liquid inside the oscillating member. By adjusting a frequency of the applied voltage according to the natural frequency of the liquid, a standing wave of concentric circles is formed in the liquid, and concentric regions having different refractive indices are formed, which are centered on a central axis line of the oscillating member. Therefore, in the lens system, when light passes along the central axis line of the oscillating member, the light moves along a path which spreads or converges in accordance with the refractive index of each concentric circular region.The above-described lens system and an objective lens for focusing the light (for example, an ordinary convex lens or a group of lenses) are arranged on the same optical axis to configure a variable focal length lens. When parallel light impinges on the ordinary objective lens, light passing through the lens comes to a focus at a focus position which is at a certain focal length. In contrast, when parallel light impinges on the lens system which is disposed coaxially with the objective lens, the light is either diffused or converged by the lens system, and the light passing through the objective lens comes to a focus at a position which is offset either farther away or closer than the original focus position (state without the lens system). Accordingly, in the variable focal length lens, a drive signal (AC voltage of a frequency generating a standing wave in the inside liquid) input to the lens system is applied, and by increasing or decreasing the amplitude of the drive signal, the focal position of the variable focal length lens can be controlled as desired within a set range (a predetermined variable range by which the lens system can increase or decrease the focal position with the focal length of the objective lens as a reference).The following circumstances exist in the conventional non-contact displacement sensor mentioned above. The laser displacement sensor requires a lens driving mechanism that drives the objective lens and a scale for measuring a driving amount of the lens driving mechanism, which can complicate the configuration of the laser displacement sensor. On the other hand, while the chromatic point sensor does not require a lens drive mechanism or scale, there is an increased amount of data processing to analyze the intensity profile for each wavelength.In order to solve the problems with the conventional non-contact displacement sensors, the present inventors studied using the variable focal length lens described above in a non-contact displacement sensor. However, when the variable focal length lens is influenced by the lapse of time, fluctuations in temperature, and the like, characteristics such as a variable range of a focal length may change. Therefore, in a non-contact displacement sensor using a variable focal length lens, there is a possibility that measurement accuracy is reduced due to changes in characteristics of the variable focal length lens.SUMMARY OF THE INVENTIONThe present invention provides a non-contact displacement sensor in which configuration and machining can be simplified and in which measurement accuracy is improved.This object is solved according to the invention by the features defined in the independent claims. Particular embodiments of the invention are the subject of the dependent claims.A non-contact displacement sensor according to an aspect is provided with: a light source that emits light; a liquid lens device in which a refractive index periodically changes in response to an input drive signal; a beam splitter that splits light emitted from the light source and passing through the liquid lens device into a measurement light and a reference light; a measurement-side objective lens that emits the measurement light split by the beam splitter to a measurement object; a reference-side objective lens on which the reference light split by the beam splitter impinges; an optical path portion for the reference light, which includes a first reference portion and a second reference portion each having an alternately determined length of an optical path measured by the reference-side objective lens and in which the reference light passing through the reference-side objective lens is incident on each of the first reference portion and the second reference portion; a photodetector which receives the measurement light reflected by the measurement object and the reference light having passed through the portion of the optical path for the reference light and outputs a photodetection signal; a focus-time calculating means that calculates, based on the photodetection signal, a measurement-side focus time at which the measurement light is focused on the surface of the measurement object, a first reference-side focus time at which the reference light is focused on the first reference portion, and a second reference-side focus time at which the reference light is focused on the second reference portion; a feature calculating means that calculates the refractive index characteristics of the liquid lens means based on the first reference-side focus time, the second reference-side focus time, and an optical path length difference that is a difference between an optical path length that passes from the reference-side objective lens to the first reference portion and the optical path length that passes from the reference-side objective lens to the second reference portion; and a position calculation means that calculates a position of the measurement object based on the characteristics of the refractive index and a phase of the measurement-side focal point timing relative to a period of the drive signal.In such a configuration, the liquid lens device includes the lens system mentioned above, and the refractive index periodically changes in response to the input driving signal. The liquid lens device, together with the measurement-side objective lens, constitutes a measurement-side variable focal length lens. The focus position of the measurement-side variable focal length lens periodically changes in response to the drive signal input to the liquid lens device. Therefore, of the light emitted from the light source and passing through the liquid lens device, the measurement light split by the beam splitter passes through the measurement-side objective lens and is emitted to the measurement object while changing a condensing position in an optical axis direction. The measurement light emitted to the measurement object focuses on the surface of the measurement object at a timing based on the position of the surface of the measurement object in accordance with the periodic change in the focal point position formed by the measurement-side variable focal length lens.Also, the liquid lens device constitutes, together with the reference-side objective lens, a reference-side variable focal length lens. The focus position formed by the reference-side variable focal length lens periodically changes in response to the drive signal input to the liquid lens device. Therefore, of the light emitted from the light source and passing through the liquid lens device, the reference light split by the beam splitter passes through the reference-side objective lens and enters the portion of the optical path for the reference light while changing the condensing position in the optical axis direction. The reference light entering the portion of the optical path for the reference light focuses on each of the first reference portion and the second reference portion at a predetermined timing in accordance with the periodic change of the focal point position formed by the reference-side variable focal length lens.The photodetector receives the measurement light reflected by the measurement object as well as the reference light that has moved over the portion of the optical path for the reference light, and outputs a photodetection signal. Based on the photodetection signal, the focus time calculator calculates the measurement-side focus time at which the measurement light is focused on the surface of the measurement object, the first reference-side focus time at which the reference light is focused on the first reference portion, and the second reference-side focus time at which the reference light is focused on the second reference portion. A method for finding each of the measurement-side focal point time point and the reference-side focal point time point may employ various focal point detection methods such as a confocal method, a double pinhole method, an astigmatic method, and a knife edge method. For example, when a confocal method is employed to find the measurement-side focal point time, the liquid lens device, the measurement-side objective lens, and the photodetector constitute an optical system in which the photodetection signal forms a peak when the focal position formed by the measurement-side variable focal length lens coincides with the surface of the measurement object. Accordingly, the focus time point calculating means can calculate a peak time of the photodetection signal caused by the measurement light as the measurement-side focus time point. The same applies to a method for finding the first reference-side focal point time and the second reference-side focal point time as well.In this example, the optical path length between the reference-side objective lens and the first reference portion and the optical path length between the reference-side objective lens and the second reference portion are different from each other. Therefore, a time difference may arise between the first reference-side focal point time and the second reference-side focal point time based on the difference of the optical path length between the optical path length that extends from the reference-side objective lens to the first reference portion and the optical path length that extends from the reference-side objective lens to the second reference portion. This time difference changes in response to changes in the refractive index characteristics of the liquid lens device. Accordingly, the feature calculation means may calculate the features of the refractive index of the liquid lens means based on the first reference-side focal point time and the second reference-side focal point time.Moreover, the phase of the measurement-side focal point timing relative to the period of the drive signal corresponds to the position of the surface of the measurement object on the optical axis passing through the measurement-side objective lens. The position calculation means calculates the position of the measurement object based on the features of the refractive index and the phase of the measurement-side focal point timing relative to the period of the drive signal. In other words, in calculating the position of the measurement object corresponding to the phase of the measurement-side focal point timing, an accurate position of the measurement object can be calculated by performing correction in accordance with the features of the refractive index.As stated in the foregoing, in the present invention, the measurement-side variable focal length lens is configured by the liquid lens device and the measurement-side objective lens. Therefore, the present invention does not require the use of a lens driving mechanism and a scale which are required structures in a conventional laser displacement sensor. In addition, the position of the measurement object can be calculated using the photodetection signals from the measurement light and the reference light, and therefore, processing of a large amount of data performed in a conventional chromatic point sensor is not necessary. Moreover, in the present invention, the liquid lens device constitutes the reference-side variable focal length lens together with the reference-side objective lens, and the refractive index characteristics of the liquid lens device can be calculated using the photodetection signal caused by the reference light. Therefore, in calculating the position of the measurement object by performing correction in accordance with a change in the characteristics of the refractive index, the influence of the lapse of time or environmental changes such as temperature can be reduced (improved robustness). Accordingly, according to the present invention, there is provided a non-contact displacement sensor in which the configuration and processing can be simplified and in which measurement accuracy is improved.Preferably, a benchmark signal output means that outputs a benchmark signal synchronized with the drive signal is further provided; the feature calculation means calculates the features of the refractive index of the liquid lens means based on a delay time of the first reference-side focal point time relative to the benchmark signal and a delay time of the second reference-side focal point time relative to the benchmark signal; and the position calculation means calculates the phase of the measurement-side focal point time based on a delay time of the measurement-side focal point time relative to the benchmark signal. In particular, the characteristics of the refractive index of the liquid lens device and the phase of the measurement-side focal point timing can be easily found even without performing complex calculation.Preferably, the portion of the optical path for the reference light is provided with a partially reflecting mirror having, as the first reference portion, a first reference surface reflecting a portion of the reference light; and a reflecting mirror having, as the second reference portion, a second reference surface reflecting the reference light passing through the first reference surface. Particularly, by using the partially reflecting mirror and the reflecting mirror, the optical path length between the first reference surface and the second reference surface (i.e., the difference in optical path length used by the feature calculation means) is easily and accurately defined.Preferably, the portion of the reference light optical path is provided with a reference light optical beam splitter that splits the reference light passing through the reference-side objective lens; a first optical fiber having, as the first reference portion, a first reference end surface where a first reference light split by the reference light optical beam splitter is incident; and a second optical fiber having, as the second reference portion, a second reference end surface where a second reference light split by the reference light optical beam splitter is incident. In particular, by using the first optical fiber and the second optical fiber, optical components such as mirrors can be omitted, and therefore, costs can be reduced.Specifically, the light source may include a measurement light source that emits the measurement light and a reference light source that emits the reference light, and the photodetector may include a measurement light photodetector for the measurement light that receives the measurement light reflected by the measurement object and outputs the photodetection signal caused by the measurement light, and a reference light photodetector for reference light that receives the reference light that passes through the optical path portion for the reference light and that outputs the photodetection signal caused by the reference light. In particular, the photodetection signal caused by the measurement light and the photodetection signal caused by the reference light can be easily distinguished, and thus calculation in the signal processor is simplified.According to the present invention, there is provided a non-contact displacement sensor in which configuration and processing can be simplified and in which measurement accuracy is improved.BRIEF DESCRIPTION OF THE DRAWINGSThe present invention will be further described in the following detailed description with reference to the stated plurality of drawings, given by way of non-limiting examples of exemplary embodiments of the present invention, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein: FIG. 1 is a schematic view illustrating a non-contact displacement sensor according to a first embodiment of the present invention; FIG. 2 is a schematic view illustrating a configuration of a liquid lens device according to the first embodiment; FIGS. 3A to 3C are schematic views illustrating oscillation states of the liquid lens device according to the first embodiment; FIGS. 4A to 4E are schematic views illustrating focus positions of the liquid lens device according to the first embodiment; FIG. 5 is a block diagram schematically illustrating a lens controller and a controller according to the first embodiment; FIG. 6 is a block diagram schematically illustrating a signal processor according to the first embodiment; FIG. 7 provides graphs illustrating a drive signal, a benchmark signal, a measurement-side focal point position, and a photodetection signal of the measurement system according to the first embodiment; FIG. 8 provides graphs illustrating a drive signal, a benchmark signal, a reference-side focus position, and a photodetection signal of the reference system according to the first embodiment; FIG. 9 provides graphs describing changes in refractive index characteristics of the liquid lens device; FIG. 10 illustrates a calibration table according to the first embodiment; FIG. 11 is a schematic view illustrating a non-contact displacement sensor according to a second embodiment of the present invention; FIG. 12 is a schematic view illustrating a non-contact displacement sensor according to a third embodiment of the present invention; FIG. 13 is a schematic view illustrating a non-contact displacement sensor according to a fourth embodiment of the present invention; FIG. 14 is a schematic view illustrating a non-contact displacement sensor according to a fifth embodiment of the present invention; and FIG. 15 provides graphs illustrating a drive signal, a benchmark signal, focus positions, and a photodetection signal according to the fifth embodiment.DETAILED DESCRIPTION OF THE INVENTIONThe details shown herein are merely exemplary and for purposes of illustrative discussion of the embodiments of the present invention and are presented in the case of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the present invention. In this regard, no attempt is made to show structural details of the present invention in greater detail than is required for the fundamental understanding of the present invention, and the description taken with the drawings will make it apparent to those skilled in the art how the modes of the present invention may be embodied in practice.Hereinafter, various embodiments of the present invention will be described with reference to the drawings. It should be understood that even though embodiments are described separately, individual features thereof may be combined into additional embodiments.First Embodiment(Non-contact displacement sensor)As illustrated in FIG. 1, a non-contact displacement sensor 1 is configured to include a liquid lens device 2 in which a refractive index periodically changes, and the non-contact displacement sensor 1 measures a position change of a surface of a measurement object W that is arranged intersecting an optical axis OA 1 passing through the liquid lens device 2. In addition, the non-contact displacement sensor 1 is configured to measure the position change of the surface of the measurement object W while measuring characteristics of the liquid lens device 2, and prevents the influence of changes in the characteristics of the liquid lens device 2 on measurement results for the measurement object W.Specifically, the non-contact displacement sensor 1 includes a measurement system 3 that measures the change in position of the measurement object W. The measurement system 3 is provided with a measurement light source 31 that emits measurement light, an optical system (a collimator lens 32 and an optical guide 4) that forms an optical path for the measurement light, a measurement-side objective lens 33 that is configured as a measurement-side variable focal length lens 101 together with the liquid lens device 2, and a measurement light photodetector 34 that receives the measurement light reflected by the measurement object W. In addition, the non-contact displacement sensor 1 includes a reference system 5 that measures characteristics of the liquid lens device 2. The reference system 5 is provided with a reference light source 51 emitting a reference light, an optical system (a collimator lens 52, an optical guide 6, and beam splitters 53 and 54) forming an optical path for the reference light, a reference-side objective lens 55 configuring a variable focal length reference-side lens 102 together with the liquid lens device 2, a reference light optical path portion (reference light optical system) 7 where the reference light passing through the reference-side objective lens 55 is incident, and a reference light photodetector 56 receiving the reference light arriving via the reference light optical path portion 7. The measurement light source 31 and the reference light source 51 are equivalent to light sources according to the present disclosure. The measurement light photodetector 34 and the reference light photodetector 56 are equivalent to photodetectors according to the present disclosure.Moreover, the non-contact displacement sensor 1 is provided with a lens controller 8 that controls operation of the liquid lens device and a controller 9 that operates the lens controller 8. The controller 9 imports and processes a photodetection signal Sm and Sr and calculates a position of the surface of the measurement object W on the optical axis OA 1 during measurement of the characteristics of the liquid lens device 2.The liquid lens device 2 is constructed with a system of liquid lens on an inner side thereof, and a refractive index changes in response to an input drive signal Cf. The drive signal Cf is a sinusoidal alternating current signal of a frequency that generates a standing wave in the liquid lens device 2. A focus position Pf 1 formed by the measurement-side variable focal length lens 101 (which is a constitutional unit of the liquid lens device 2 and the measurement-side objective lens 33) while being based on a focus position of the measurement-side objective lens 33 can be changed as desired by changing the refractive index of the liquid lens device 2. Meanwhile, a focal point position Pf 2 formed by the reference-side variable focal length lens 102 (which is a constituent of the liquid lens device 2 and the reference-side objective lens 55) while being based on a focal point position of the reference-side objective lens 55 can be changed as desired by changing the refractive index of the liquid lens device 2.(Variable Focal Length Lens)The liquid lens device 2 and the measurement-side variable focal length lens 101 configured to include the liquid lens device 2 will now be described with reference to FIG. 2. In FIG. 2, the liquid lens device 2 includes a hollow cylindrical shell 21, and a hollow oscillating member 22 is installed in an interior of the shell 21. The oscillating member 22 is supported by spacers 29 made of elastomer and disposed between an outer circumferential surface 23 of the oscillating member 22 and an inner circumferential surface 24 of the casing 21. The oscillating member 22 is a member where a piezoelectric material is formed into a hollow cylindrical shape. The oscillating member 22 oscillates in a thickness direction due to an AC voltage of the drive signal Cf applied between the outer circumferential surface 23 and the inner circumferential surface 24. The inside of the jacket 21 is filled with a highly transparent liquid 25, the entire oscillating member 22 is immersed in the liquid 25, and an inner side of the hollow cylindrical oscillating member 22 is filled with the liquid 25. The AC voltage of the drive signal Cf is set to a frequency that generates a standing wave in the liquid 25 on the inner side of the oscillating member 22.As shown in FIGS. 3A to 3C, in the liquid lens device 2, when the oscillating member 22 is oscillated, a standing wave arises in the inner liquid 25 and concentric circular regions in which the refractive index changes arise (see FIGS. 3A and 3B ). At this point, a relationship between a distance from a line of a central axis of the liquid lens device (radius) and the refractive index of the liquid 25 is shown by a refractive index distribution R illustrated in FIG. 3C.In FIGS. 4A to 4E, since the drive signal Cf is a sinusoidal alternating current signal, bands in the refractive index distribution R of the liquid 25 in the liquid lens device 2 also change in accordance with the drive signal Cf. Also, the refractive index of the concentric circular regions formed in the liquid 25 changes sinusoidally, and accordingly the focus position Pf 1 fluctuates sinusoidally. In FIGS. 4A to 4E, a distance D from the focal point position of the measurement-side objective lens 33 to the focal point position Pf 1 is shown. In the state illustrated in FIG. 4A, an amplitude of the refractive index distribution R is the largest, the liquid lens device causes passing light to converge, and the focal point position Pf 1 is closest to the measurement-side objective lens 33. In the state illustrated in FIG. 4C, the amplitude of the refractive index distribution R is the largest at the opposite pole from that of FIG. 4A, the liquid lens device 2 causes light passing therethrough to spread, and the focal point position Pf 1 is the farthest from the measurement-side objective lens 33. The state shown in FIG. 4E returns to the state shown in FIG. 4A again, and similar fluctuations are repeated thereafter.In this way, in the measurement-side variable focal length lens 101, the drive signal Cf is a sinusoidal alternating current signal, and the focal point position Pf 1 fluctuates sinusoidally as in a fluctuation waveform Mf 1 in FIGS. 4A to 4E.There may also be cases where, in the measurement-side variable focal length lens 101, a principal point of the measurement-side variable focal length lens 101 fluctuates, whereby the focal point position Pf 1 changes while the focal length (distance from the principal point of the measurement-side variable focal length lens 101 to the focal point position Pf 1) remains constant. The measurement-side variable focal length lens 101 is described above, but a similar description is also applicable to the reference-side variable focal length lens 102.(Measurement system)Referring again to FIG. 1, various configurations of the measurement system 3 in the non-contact type displacement sensor 1 will be described. The measurement light source 31 is, for example, a laser light source and emits measurement light. The optical guide 4 includes a fiber splitter 41 and optical fibers 42 to 44. the fiber splitter 41 has an optical path where a first end portion of each of the optical fibers 42 to 44 is connected, and is configured to guide the light incident from the optical fiber 42 to the optical fiber 43 and guide the light incident from the optical fiber 43 to the optical fiber 44.A second end portion of the optical fiber 42 is connected to the measurement light source 31. Therefore, the measurement light emitted from the measurement light source 31 passes through the optical fiber 42, the fiber splitter 41, and the optical fiber 43, and is emitted from an end surface 43 eof the second end portion of the optical fiber 43. That is, the end face 43 eof the optical fiber 43 performs operations as a point light source of measurement light. In addition, the second end portion of the optical fiber 44 is connected to the measurement light photodetector 34. Therefore, the measurement light incident on the end face 43 eof the optical fiber 43 passes through the optical fiber 43, the fiber splitter 41, and the optical fiber 44, and is incident on the measurement light photodetector 34. In this example, the end face 43 eof the optical fiber 43 is disposed at a focal point on a rear side of the collimator lens 32. In other words, the end face 43 eof the optical fiber 43 is disposed at a position that creates a conjugate relationship relative to the focal point position Pf 1 formed by the measurement-side variable focal length lens 101 on the optical axis OA 1.The collimator lens 32 is positioned on the optical axis OA 1, converts the measurement light emitted from the end surface 43 eof the optical fiber 43 into a parallel light, and causes the light to be incident on the liquid lens device 2. Also, the collimator lens 32 collects the measurement light reflected by the measurement object W and in turn passes through the liquid lens device 2.The measurement-side objective lens 33 is configured by a known convex lens or lens group, is positioned between the liquid lens device 2 and the measurement object W on the optical axis OA 1, and constitutes, together with the liquid lens device 2, the measurement-side variable focal length lens 101 described above. The measurement object W is disposed within a variable range VR 1 of the focal point position Pf 1 formed by the measurement-side variable focal length lens 101. The measurement-side objective lens 33 is not made integral with other structures, but is instead configured to be interchangeable with separate measurement-side objective lenses 33 having different powers of amplification.The measurement light photodetector 34 may be, for example, a photomultiplier tube or a photodiode, and is connected to the second end portion of the optical fiber 44. The measurement light photodetector 34 receives the measurement light incident via the optical fiber 44 and outputs a photodetection signal Sm in accordance with the intensity of the received light.In the above-mentioned measurement system 3, the measurement light emitted from the end face 43 eof the optical fiber 43 is collimated along the optical axis OA 1 by the collimator lens 32, after which the light is emitted to the measurement object W via the measurement-side variable focal length lens 101. The measurement light reflected by the measurement object W is collected by the collimator lens 32 after passing through the measurement-side variable focal length lens 101 again. In this example, the focal point position Pf 1 formed by the measurement-side variable focal length lens 101 periodically changes on the optical axis OA 1. Therefore, only when the focal point position Pf 1 coincides with the surface of the measurement object W, the measurement light reflected by the surface forms a point at the focal point on the rear side of the collimator lens 32 and impinges on the end face 43 eof the optical fiber 43. In other words, the photodetection signal Sm output from the measurement light photodetector 34 shows a peak when the focal point position Pf 1 coincides with the surface of the measurement object W.(Reference System)Referring to FIG. 1, various configurations of the reference system 5 in the non-contact displacement sensor 1 will now be described. The reference light source 51 is, for example, a laser light source and emits reference light having a different wavelength from the measurement light. The optical conductor 6 includes a fiber splitter 61 and optical fibers 62 to 64. the fiber splitter 61 has an optical path where a first end portion of each of the optical fibers 62 to 64 is connected, and is configured to guide the light incident from the optical fiber 62 to the optical fiber 63 and guide the light incident from the optical fiber 63 to the optical fiber 64.A second end portion of the optical fiber 62 is connected to the reference light source 51. Therefore, the reference light emitted from the reference light source 51 passes through the optical fiber 62, the fiber splitter 61, and the optical fiber 63 and is emitted from an end surface 63 eof the second end portion of the optical fiber 63. That is, the end face 63e of the optical fiber 63 performs operations as a point light source of reference light. In addition, the second end portion of the optical fiber 64 is connected to the reference light photodetector 56. Therefore, the measurement light incident on the end face 63 eof the optical fiber 63 passes through the optical fiber 63, the fiber splitter 61, and the optical fiber 64, and is incident on the reference light photodetector 56. In this example, the end face 63 eof the optical fiber 63 is positioned at a focal point on a rear side of the collimator lens 52 on an optical axis OA 2. In other words, the end face 63 eof the optical fiber 63 is disposed at a position that creates a conjugate relationship with respect to the focal point position Pf 2 formed by the reference-side variable focal length lens 102.The collimator lens 52 is positioned on the optical axis OA 2, converts the measurement light emitted from the end surface 63 eof the optical fiber 63 into parallel light, and causes light to be incident on the liquid lens device 2 via the beam splitter 53. Also, the collimator lens 52 collects the reference light which passes through the reference light optical path portion 7 and the liquid lens device 2 again.The beam splitters 53 and 54 may be, for example, beam splitters or dichroic mirrors and act to allow measurement light to pass through and also reflect reference light. The beam splitters 53 and 54 are arranged so as to have the liquid lens device 2 therebetween on the optical axis OA1, and the splitters form optical axes OA2 and OA3 which are divided from the optical axis OA1 before and after the liquid lens device 2, respectively.The reference-side objective lens 55 is configured by a known convex lens or lens group, is positioned between the liquid lens device 2 and the reference light optical path portion 7 on the optical axis OA 3, and constitutes the reference-side variable focal length lens 102 described above together with the liquid lens device.The reference light optical path portion 7 is provided with a partially reflecting mirror 71 and a reflecting mirror 72 on the optical axis OA3, each mirror having an alternately determined optical path length measured by the reference side objective lens 55. The partially reflecting mirror 71 is disposed on a side closer to the reference-side objective lens 55, and includes a first reference surface 71 s(first reference portion) that reflects a portion of the reference light and also allows another portion thereof to pass. The reflecting mirror 72 is disposed on a side farther from the reference-side objective lens 55, and includes a second reference surface 72 s(second reference portion) that reflects the reference light passing through the partially reflecting mirror 71. A known value is defined for an optical path length between the first reference surface 71 sand the second reference surface 72 s(i.e., for a difference L of the optical path length which is the difference between the optical path length extending from the reference-side objective lens 55 to the first reference surface 71 sand the optical path length extending from the reference-side objective lens 55 to the second reference surface 72 s). The position of each of the first reference surface 71 sand the second reference surface 72 smay be set as desired as long as the position is within a variable range VR 2 of the focal point position Pf 2 formed by the reference-side variable focal length lens 102.The reference light photodetector 56 may be, for example, a photomultiplier tube or a photodiode, and is connected to the second end portion of the optical fiber 64. The reference light photodetector 56 receives the reference light incident via the optical fiber 64, and outputs a photodetection signal Sr in accordance with the intensity of the received light.In the above-mentioned reference system 5, the reference light emitted from the end face 63 eof the optical fiber 63 is collimated along the optical axis OA 2 by the collimator lens 52, after which the light is deflected toward the object side of the optical axis OA 1 direction by the beam splitter 53. Then, the reference light passes through the liquid lens device 2 and is bent in the optical axis OA 3 direction by the beam splitter 54, after which the light enters the optical path portion 7 of the reference light while being collected by the reference-side objective lens 55. The reference light entering the optical path portion 7 of the reference light is split into a first reference light reflected by the first reference surface 71 sand a second reference light passing through the first reference surface 71 s, and then is reflected by the second reference surface 72 s. The first and second reference lights reflected by the first reference surface 71 sor the second reference surface 72 strace the reverse path, and are then collected by the collimator lens 52. In this example, the focus position Pf 2 formed by the reference-side variable focal length lens 102 periodically changes in the optical axis OA 3 direction. Therefore, only when the focal point position Pf 2 coincides with the reference surface of either the first reference surface 71 sof the second reference surface 72 s, the first or second reference light reflected by the corresponding reference surface forms a point at the focal point on the rear side of the collimator lens 52 and impinges on the end surface 63 eof the optical fiber 63. Accordingly, the reference light impinging on the reference light photodetector 56 is maximized when the focal point position Pf 2 coincides with either the first reference surface 71 sof the second reference surface 72 s. In other words, the photodetection signal Sr output from the reference light photodetector 56 exhibits a peak when the focal point position Pf2 coincides with either the first reference surface 71s or the second reference surface 72s.(Lens Controller)As illustrated in FIG. 5, the lens controller 8 is configured as a controller that controls the operation of the liquid lens device 2 and includes a drive signal output device 81 that outputs the sinusoidal drive signal Cf to the liquid lens device 2. In addition, the lens controller 8 includes a benchmark signal output device 82 that outputs, to the controller 9, a pulse-like benchmark signal Sc synchronized with the period of the drive signal Cf. The timing of the output of the benchmark signal Sc relative to the period of the drive signal Cf may be defined as desired, however, in the present embodiment, the benchmark signal Sc rises once each time the drive signal Cf crosses the level 0 twice (in FIG. 7, for example, at timings where the fluctuation waveform Mf 1 of the focal point position Pf 1 reaches a positive peak).(Control Device)The controller 9 is constituted by a personal computer or the like including, for example, a central processing unit (CPU) and a memory. The controller 9 achieves an expected functionality by executing predetermined software, and includes a lens setting section 91 defining settings of the lens controller 8, and a signal processor 92 processing various input signals. Also, the controller 9 includes a memory 93 configured by a memory and the like.The lens setting section 91 defines, for example, settings for the frequency, amplitude, and maximum drive voltage of the drive signal Cf output by the lens controller 8. In the liquid lens device 2, a number of changes in resonance may be changed according to a change in ambient temperature, for example. Therefore, the lens setting section 91 performs processes to change the frequency of the drive signal Cf in real time by feedback control and stabilize the liquid lens device 2.In the signal processor 92, the photodetection signal Sm is input from the measurement light photodetector 34, the photodetection signal Sr is input from the reference light photodetector 56, and the benchmark signal Sc is input from the lens controller 8. The signal processor 92 calculates a position Zcalc of the measurement object by performing processing based on the input photodetection signals Sm and Sr. Therefore, as illustrated in FIG. 6, the signal processor 92 performs operations as a focus timing calculator 921, a delay time calculator 922, a feature calculator 923, and a position calculator 924.A calibration table 94 prepared in advance using a target or the like is stored in the memory 93. As discussed in detail below, the calibration table 94 associates the position Zcalc of the measurement object, which is a calculated value obtained by signal processing (calculated value relating to a position Pw of the measurement object W on the optical axis OA 1), with a position Z of the measurement object, serving as a display value for measurement results.(Photodetection signal)As illustrated in FIGS. 7 and 8, when the sinusoidal drive signal Cf is input to the liquid lens device 2, the device is synchronized with the period of the drive signal Cf and the benchmark signal Sc is output in one pulse. Moreover, when the sinusoidal drive signal Cf is input to the liquid lens device 2, by periodically changing the refractive index of the liquid lens device 2, the focal point positions Pf 1 and Pf 2 formed by the variable focal length lenses 101 and 102, respectively, are periodically changed in synchronization with the drive signal Cf, respectively.FIG. 7 illustrates an example of the position Pw of the measurement object W on the optical axis OA 1 for a measurement object W that is disposed within the variable range of the focus position Pf 1. The photodetection signal Sm shows a peak when the focal point position Pf1 coincides with the position Pw of the measurement object W, and shows two peaks per cycle of the drive signal Cf. In this example, a time amount from the time when the benchmark signal Sc rises at each peak time of the first and second peaks of the photodetection signal Sm (measurement-side focal point time Tm) is expressed as a delay time Δtm 1 and Δtm 2. In the present embodiment, processing is performed using a value of the delay times Δtm 1 and Δtm 2 (for example, Δtm 1), but in the modifications described below, both delay times Δtm 1 and Δtm 2 may also be used.FIG. 8 illustrates an example of positions Pr 1 and Pr 2 of the first reference surface 71 sand the second reference surface 72 s, respectively, which are located within the variable range of the focal point position Pf 2. The photodetection signal Sr shows a peak when the focal point position Pf2 coincides with either of the positions Pr1 and Pr2 of the first reference surface 71s and the second reference surface 72s, respectively, and shows four peaks per cycle of the drive signal Cf. In this example, a time amount from the time when the benchmark signal Sc rises at each peak time of the first to fourth peaks of the photodetection signal Sr is expressed as a delay time Δtm 1, Δtm 2, Δtm 3, and Δtm 4. The first to fourth peak times of the photodetection signal Sr are respectively equivalent to a reference-side focus time point Tr where the focus position Pf 2 coincides with the first reference surface 71 s(first reference-side focus time point). In addition, the second and third peak times of the photodetection signal Sr are respectively equivalent to a reference-side focus time Tr where the focus position Pf 2 coincides with the second reference surface 72 s(second reference-side focus time).(Refractive Index Characteristics of Liquid Lens Device)The liquid lens device 2 is influenced by the lapse of time, changes in temperature, and the like, and accordingly, the refractive index characteristics of the liquid lens device 2 change, and the variable ranges VR 1 and VR 2 of the focal point positions Pf 1 and Pf 2 of the variable focal length lenses 101 and 102 change, for example. In such a case, the fluctuation waveforms Mf 1 and Mf 2 of the focal point positions Pf 1 and Pf 2 each change from an ideal waveform calculated from the drive signal Cf.In FIG. 9, with reference to the focal point position Pf 2, an ideal fluctuation waveform Mf 2I calculated from the drive signal Cf is illustrated with a dashed line, and an actual fluctuation waveform Mf 2 subsequent to the refractive index characteristics of the liquid lens device 2 that change due to a temperature change or the like is illustrated with a solid line. As illustrated in FIG. 9, the actual fluctuation waveform Mf 2 has a phase delay Φd and a deviation B relative to the ideal fluctuation waveform Mf 2I. The deviation B is equivalent to an amount of offset of a central position Pc of the variable range VR 2 of the focal position Pf 2.In the present embodiment, the refractive index characteristics of the liquid lens device 2 are calculated as an amplitude A, the deviation B, and the phase delay Φd of the fluctuation waveform Mf 2 of the focal position Pf 2. In this example, when the delay times Δtr 1, Δtr 2, Δtr 3, and Δtr 4 of the photodetection signal Sr, the difference L of the optical path length between the first reference surface 71 sand the second reference surface 72 srelative to the reference-side objective lens 55, and a frequency v of the drive signal Cf are used, the following equations (1) to (4) express the amplitude A, the deviation B, and the phase delay Φd of the fluctuation waveform Mf 2.According to the above equations (1) to (4), the amplitude A, the deviation B, and the phase delay Φd of the fluctuation waveform Mf 2, which are unknown values, are respectively expressed by the following equations (5), (6), and (7) (or (8)).(Signal Processor)Next, signal processing by the signal processor 92 during measurement operations of the non-contact displacement sensor 1 will be described. After initiating the measurement operations of the non-contact displacement sensor 1, the signal processor 92 obtains the benchmark signal Sc and the photodetection signals Sm and Sr illustrated in FIGS. 7 and 8.First, in the signal processor 92, the focus time calculator 921 calculates the peak time of the photodetection signal Sm as the measurement-side focus time Tm. Similarly, the focus time calculator 921 calculates the peak time of the photodetection signal Sr as the reference-side focus time Tr.Next, the delay time calculator 922 calculates the delay times Δtm 1 and Δtm 2 which are the amount of time from the time point at which the benchmark signal Sc rises to the measurement-side focal point time Tm, and the delay times Δtr 1, Δtr 2, Δtr 3, and Δtr 4 which are the amount of time from the time point when the benchmark signal Sc rises to the reference-side focal point time Tr. The delay time calculator 922 may also use a clock signal or the like.Then, the feature calculator 923 uses the calculated delay times Δtr 1, Δtr 2, Δtr 3, and Δtr 4 for the reference-side focus time Tr to calculate the amplitude A, the deviation B, and the phase delay Φd of the fluctuation waveform Mf 2 as refractive index characteristics of the liquid lens device 2 based on the above equations (5), (6), and (7) (or (8)).In addition, the position calculator 924 uses the calculated delay time Δtm 1 of the measurement-side focal point time Tm to calculate a phase Φmp 1 of the measurement-side focal point time Tm relative to the period of the drive signal Cf based on the following equation (9).Thereafter, the position calculator 924 uses the calculated phase Φmp 1 of the measurement-side focal point time Tm and the phase delay Φd to find a phase Φm 1 of the measurement-side focal point time Tm which is corrected by the refractive index characteristics of the liquid lens device 2 based on the following equation (10).Also, the position calculator 924 uses the amplitude A and the deviation B of the fluctuation waveform Mf 2, a focal length Fr of the reference-side objective lens 55, and a focal length Fm of the measurement-side objective lens 33 to calculate the position Zcalc of the measurement object based on the following equation (11).Then, referring to the calibration table 94, as illustrated in FIG. 10, the position calculator 924 finds the position Z of the measurement object corresponding to the position Zcalc of the measurement object calculated by the above equation (11). Specifically, the position calculator 924 calculates the position Z of the measurement object corresponding to the position Zcalc of the measurement object using a linear square method or the like, based on two positions Zk of the measurement object having the position Zcalc of the measurement object therebetween and positions Zcalc, k of the measurement object corresponding to the positions Zk of the measurement object.With the above-mentioned signal processing of the signal processor 92, the non-contact displacement sensor 1 can calculate the position Z of the measurement object, which is a value showing the position Pw of the measurement object W on the optical axis OA 1. The signal processor 92 may perform the above-mentioned processing for each fixed amount of time, and the obtained position Z of the measurement object may be sequentially stored in the memory 93.(Calibration Table Preparation Method)A method for preparing the calibration table 94 will now be described. Before performing the displacement measurement described above, the calibration table 94 is prepared using the non-contact displacement sensor 1. First, a target is arranged instead of the measurement object W, and the target is again positioned at a plurality of positions Zk (k=1 to n) in the optical axis direction with a high degree of accuracy using an interferometer or the like. In a state where the target is positioned, the positions Zcalc,kof the measurement object are performed by performing the measurement described above with the non-contact displacement sensor 1. Then, the positions Zk of the measurement object (value representing the positions Zk of the target positioned with a high degree of accuracy) and the positions Zcalc,k of the measurement object (calculated value) are associated with each other and stored in the memory 93. The calibration table 94 illustrated in FIG. 10 is prepared in the above manner.The calibration table 94 is preferably prepared for each of a plurality of kinds of magnification power prepared as the measurement-side objective lens 33. During measurement, upon switching between the magnification factors of the measurement-side objective lens 33, the embodiment is configured to switch between the calibration tables 94 to which reference is made.(Effects of the First Embodiment)In the present embodiment, the measurement-side variable focal length lens 101 is configured by the liquid lens device 2 and the measurement-side objective lens 33. Therefore, the present embodiment does not require use of a lens driving mechanism and a scale, which are required structures in a conventional laser displacement sensor. In addition, the position Zcalc of the measurement object can be calculated using the photodetection signal Sm, and therefore, the processing of a large amount of data performed in a conventional chromatic point sensor is not necessary. Moreover, in the present embodiment, the liquid lens device 2 configuring the measurement-side variable focal length lens 101 also constitutes, together with the reference-side objective lens 55, the reference-side variable focal length lens 102, and the refractive index characteristics of the liquid lens device 2 are calculated using the photodetection signal Sr for the reference light. Therefore, by performing correction in accordance with a change in the refractive index characteristics, changes in the measurement results due to the lapse of time or environmental changes such as the temperature can be prevented (improved robustness). Accordingly, the present embodiment provides a non-contact displacement sensor 1 in which the configuration and the processing can be simplified and in which measurement accuracy is improved.The non-contact displacement sensor 1 according to the present embodiment is further provided with the benchmark signal output means 82 that outputs the benchmark signal Sc synchronized with the drive signal Cf. The feature calculator 923 calculates the refractive index characteristics of the liquid lens device 2 based on the delay times Δtr 1 to Δtr 4 of the reference-side focal point time Tr relative to the benchmark signal Sc. The position calculator 924 calculates the phase Φm 1 of the measurement-side focal point time Tm relative to the period of the drive signal Cf based on the delay time Δtm 1 of the measurement-side focal point time Tm relative to the benchmark signal Sc. Therefore, the refractive index characteristics of the liquid lens device 2 and the phase Φm 1 of the measurement-side focal point time Tm can be easily found even without performing complex calculation.In the present embodiment, the reference light optical path portion 7 is provided with the partially reflecting mirror 71 having the first reference surface 71 s(first reference portion) reflecting a portion of the reference light and the reflecting mirror 72 having the second reference surface 72 s(second reference portion) reflecting the reference light passing through the first reference surface 71 s. In this configuration, by using the partially reflecting mirror 71 and the reflecting mirror 72, the difference L of the optical path length between the first reference surface 71 sand the second reference surface 72 scan be easily set.As light sources according to the present invention, the non-contact displacement sensor 1 according to the present embodiment is provided with the measurement light source 31 emitting measurement light and the reference light source 51 emitting reference light, and as photodetectors according to the present invention, the non-contact displacement sensor 1 is provided with the measurement light photodetector 34 and the reference light photodetector 56. Therefore, the photodetection signals Sm and Sr can be easily discriminated, and such a calculation in the signal processor 92 is simplified.The non-contact displacement sensor 1 according to the present embodiment is capable of switching between magnification powers of the measurement-side objective lens 33, which is difficult in the conventional technology. Specifically, in the conventional technology, a laser displacement sensor has an objective lens embedded in a lens driving mechanism, and a chromatic point sensor has an objective lens modularized together with a specific lens group that diffuses white light using axial chromatic aberration. Therefore, in the laser displacement sensor and the chromatic point sensor, it is difficult to exchange only the objective lens with a separate one having different magnifying power, and a separate device must be prepared to perform measurements in a different measurement range and with different resolution. In contrast, in the non-contact displacement sensor 1 according to the present embodiment, there is no need to integrate the measurement-side objective lens 33 with other structures as in the related art. Therefore, a configuration capable of switching the measurement-side objective lens 33 to a separate measurement-side objective lens 33 having different magnification power can be easily provided.In the present embodiment, a confocal optical system is configured in both the measurement system 3 and the reference system 5 to detect the measurement-side focal point time Tm and the reference-side focal point time Tr. Therefore, compared with cases using other focus detection methods, since the present embodiment is less prone to be affected by the inclination of the surface of the measurement object W, measurement accuracy for surface characteristics such as roughness, etc., measurement accuracy can be further improved. Also, by using the optical fibers 43 and 63, the light sources (measurement light source 31 and reference light source 51) and the photodetectors (measurement light photodetector 34 and reference light photodetector 56), which are heat sources, can be arranged away from a portion serving as a measurement head, and thermal effects on the measurement can be reduced. Further, the end face 43 eof the optical fiber 43 and the end face 63 eof the optical fiber 63 each play both roles as a point light source and as a pinhole for detection in the confocal optical system, and therefore the number of adjustment steps during manufacturing can be significantly reduced.(Modifications of the First Embodiment)In the first embodiment described above, the phase Φm 1 of the measurement-side focal point time Tm is performed by performing the calculations of Equations (9) and (10) given above using the delay time Δtm 1 of the measurement-side focal point time Tm, but the present invention is not limited thereto. For example, a phase Φm 2 of the measurement-side focal point time Tm may also be calculated by performing calculations similar to Equations (9) and (10) given above by using the delay time Δtm 2 of the measurement-side focal point time Tm.Alternatively, the equations (9) and (10) according to the first embodiment may be omitted, and the phase Φm 1 of the measurement-side focal point time Tm may also be calculated by performing calculation according to an equation (12) given below by using the delay times Δtm 1 and Δtm 2 of the measurement-side focal point time Tm. In equation (12), v is the frequency of the drive signal Cf.According to this method, the calculation step in the first embodiment can be partially omitted, and therefore measurement due to shortening of the calculation time can be expedited. Also, in the equation (12), "Δtm2 - Δtm1" is equivalent to the time difference between the two measurement-side focal points Tm that appear during one cycle of the drive signal Cf. Therefore, instead of measuring the delay times Δtm 1 and Δtm 2 of the measurement-side focus time Tm, the time difference can be directly measured.In the first embodiment, for the delay times Δtm 1 and Δtm 2 of the measurement-side focus time Tm relative to the benchmark signal Sc, measurement starts from the time when the benchmark signal Sc rises, but measurement may also start from the time when the benchmark signal Sc falls. The same can also apply to the delay times Δtr 1 to Δtr 4 of the reference-side focus time Tr.In the first embodiment, the amplitude A, the deviation B, and the phase delay Φd of the fluctuation waveform Mf 2 of the focus position Pf 2 are calculated based on the delay times Δtr 1 to Δtr 4 of the reference-side focus time Tr, but the present invention is not limited thereto, and these values can also be found by calculation or the like based on the sine wave shown by the drive signal Cf. Similarly, the phase φm 1 of the measurement-side focal point time Tm is measured based on the delay times Δtm 1 and Δtm 2 of the measurement-side focal point time Tm, but the present invention is not limited thereto, and this value may also be found by calculation or the like based on the sine wave shown by the drive signal Cf.Second EmbodimentA non-contact displacement sensor 1A according to a second embodiment will be described with reference to FIG. 11. In the second embodiment, identical reference numerals are used for structures similar to those in the first embodiment, and detailed description thereof will be omitted. As illustrated in FIG. 11, in the non-contact displacement sensor 1A, a pinhole-type confocal optical system is constructed in each of the measurement system 3 and the reference system 5. Specifically, the non-contact displacement sensor 1A is provided with a beam splitter 45 and pinhole members 46 and 47 instead of the optical conductor 4 according to the first embodiment. Also, the non-contact displacement sensor 1A is provided with a beam splitter 65 and pinhole members 66 and 67 instead of the optical conductor 6 according to the first embodiment.In the measurement system 3, the beam splitter 45 is configured to bend the measurement light emitted from the measurement light source 31 toward the collimator lens 32 and also allow light incident from the collimator lens 32 side to pass through to the measurement light photodetector 34 side. Pinhole element 46 is situated between beam splitter 45 and measurement light source 31. Since the measurement light source 31 emits the measurement light via the pinhole of the pinhole member 46, the pinhole serves as a point light source. The pinhole 47 is disposed between the beam splitter 45 and the measurement light photodetector 34, and has a pinhole disposed at the focal point on the rear side of the collimator lens 32. The measurement light focused on the surface of the measurement object W and reflected thereby passes through the pinhole of the pinhole member 47 and then is incident on the measurement light photodetector 34.In the reference system 5, the beam splitter 65 is configured to bend the measurement light emitted from the reference light source 51 toward the collimator lens 52 and allow light incident from the collimator lens 52 side to pass through to the reference light photodetector 56 side. Pinhole element 66 is situated between beam splitter 65 and reference light source 51. Since the reference light source 51 emits the reference light via the pinhole of the pinhole member 66, the pinhole serves as a point light source. The pinhole 67 is disposed between the beam splitter 65 and the reference light photodetector 56, and has a pinhole disposed at the focal point on the rear side of the collimator lens 52. The reference light focused on and reflected by the surface of the measurement object W passes through the pinhole of the pinhole member 67 and then impinges on the reference light photodetector 56.According to such a non-contact displacement sensor 1A, similar to the first embodiment, the configuration and the processing can be simplified, and measurement accuracy can be improved. Also, according to the non-contact displacement sensor 1A, the optical fibers 42 to 44 and 62 to 64 of the first embodiment are omitted, and therefore there is no need to provide a space for laying these optical fibers, and the non-contact displacement sensor 1A can be made more compact as a whole.Third EmbodimentA non-contact displacement sensor 1B according to a third embodiment will be described with reference to FIG. 12. In the third embodiment, identical reference numerals are used for structures similar to those in the first embodiment, and detailed description thereof will be omitted.As illustrated in FIG. 12, instead of the reference light optical path portion 7 according to the first embodiment, the non-contact displacement sensor 1B is provided with a reference light optical path portion 7B having a different configuration from that of the reference light optical path portion 7. Specifically, the reference light optical path portion 7B is provided with a reference light optical path splitter 73 that splits the reference light, an optical fiber 74 (first optical fiber) that is disposed further along in a direction traversed by a first reference light split by reference light optical path splitters 73, and an optical fiber 75 (second optical fiber) that is disposed further along in a direction traversed by a second reference light split by reference light optical path splitters 73.The splitter 73 of the reference light optical path is a beam splitter such as a half mirror. The reference light passing through the reference-side objective lens 55 is split into the first reference light passing through the reference light optical path splitter 73 and the second reference light reflected by the reference light optical path splitter 73. In other words, the reference light optical path splitter 73 is disposed on the optical axis OA 3 and constitutes an optical axis OA 4 branching from the optical axis OA 3.A first reference end surface 74 e(first reference portion), which is an end surface of a first end portion of the optical fiber 74, is disposed on the optical axis OA 3. On the other hand, a second reference end surface 75 e(second reference portion) which is an end surface of a first end portion of the optical fiber 75 is disposed on the optical axis OA 4. The position of each of the first reference end surface 74 eand the second reference end surface 75 emay be set as desired as long as the position is within a variable range of the focus position Pf 2 formed by the reference-side variable focal length lens 102. In the present embodiment, a known value is defined for the difference L of the optical path length which is the difference between the optical path length extending from the reference-side objective lens 55 to the first reference surface 74 eand the optical path length extending from the reference-side objective lens 55 to the second reference surface 75 e. The optical path length difference L is a length obtained by taking the difference between a distance La on the optical axis OA 3 from the reference light optical path splitter 73 to the first reference end surface 74 eof the optical fiber 74 and a distance Lb on the optical axis OA 4 from the reference light optical path splitter 73 to the second reference end surface 75 eof the optical fiber 75.Also, the reference light optical path portion 7B is further provided with an optical fiber 76 having the first end portion connected to the reference light photodetector 56, and a fiber splitter 77 to which the second end portion of each of the optical fibers 74 to 76 is connected. The fiber splitter 77 is configured such that the light incident from each of the optical fibers 74 and 75 is guided to the optical fiber 76.The reference light incident on the reference light optical path portion 7B is divided into the first reference light passing through the reference light optical path splitter 73 and the second reference light reflected by the reference light optical path splitter 73. After splitting, the first reference light moves toward the first reference end surface 74 eof the optical fiber 74, and the second reference light moves toward the second reference end surface 75 eof the optical fiber 75. Therefore, when the focal point position Pf 2 coincides with the first reference end surface 74 e, the first reference light that has been collected is incident on the first reference end surface 74 eand is incident on the reference light photodetector 56 via the optical fibers 74 and 76. In contrast, when the focal point position Pf 2 coincides with the second reference end surface 75 e, the second reference light that has been collected is incident on the second reference end surface 75 eand is incident on the reference light photodetector 56 via the optical fibers 75 and 76. Accordingly, the reference light incident on the reference light photodetector 56 is maximized when the focal point position Pf 2 coincides with either the first reference end surface 74 eor the second reference end surface 75 e. That is, the photodetection signal Sm output from the reference light photodetector 56 shows a peak when the focus position Pf2 coincides with either the first reference end face 74e or the second reference end face 75e.Also, instead of the optical fiber 6 according to the first embodiment, the non-contact displacement sensor 1B is provided with an optical fiber 6B having a different configuration from that of the optical fiber 6. Specifically, the optical guide 6B includes an optical fiber 68, a first end portion of which is connected to the reference light source 51. The reference light emitted from the reference light source 51 passes through the optical fiber 68, and is emitted from an end surface 68 eof a second end portion of the optical fiber 68. That is, the end face 68 eof the optical fiber 68 performs operations as a point light source for reference light. The end face 68 eof the optical fiber 68 may be disposed at any desired position on the optical axis OA 2, because the end face 68 edoes not receive any reference light.According to such a non-contact displacement sensor 1B, similar to the first embodiment, the configuration and the processing can be simplified, and measurement accuracy can be improved. Also, according to the non-contact displacement sensor 1B, the partially reflecting mirror 71 and the reflecting mirror 72 according to the first embodiment can be omitted, and therefore, costs can be lowered by reducing the number of optical components.Modifications of the Third EmbodimentIn place of the optical fibers 74 and 75, the reference light optical path portion 7B according to the third embodiment may be provided with a pinhole disposed on each of the optical axes OA 3 and OA 4 and a reference light photodetector disposed on the rear side of each of the pinhole members. In such a case, the pinhole of each pinhole element is respectively equivalent to the first reference section and the second reference section according to the present invention. In addition, the optical conductors 4 and 6B may be interchanged for a configuration using a pinhole similar to the second embodiment.In the third embodiment, when the beam splitter 54 splits the light emitted from the measurement light source 31 into a measurement light and a reference light, the reference light source 51, the optical guide 6B, the collimator lens 52, and the beam splitter 53 may also be omitted.Fourth EmbodimentA non-contact displacement sensor 1C according to a fourth embodiment will be described with reference to FIG. 13. In the fourth embodiment, identical reference numerals are used for structures similar to those in the first embodiment, and detailed description thereof will be omitted. As illustrated in FIG. 13, in the non-contact displacement sensor 1C, the collimator lenses 32 and 52 in the measurement system 3 and the reference system 5 are respectively omitted, and a finite correction optical system is configured. In the measurement system 3 configured in this manner, the measurement light reflected by the measurement object W is imaged by the measurement-side objective lens 33. Moreover, the end face 43 eof the optical fiber 43 is not positioned at the focal point on the rear side of the collimator lens 32 as in the first embodiment, but is instead disposed on the rear side of a focal point on the rear side of the measurement-side objective lens 33 at a position separated by a distance obtained by multiplying the imaging magnification power by the focal distance of the measurement-side objective lens 33. On the other hand, in the reference system 5, the reference light reflected by one of the first reference surface 71 sand the second reference surface 72 sis imaged by the reference-side objective lens 55. Also, the end face 63 eof the optical fiber 63 is not disposed at the focus on the rear side of the collimator lens 52 as in the first embodiment, but is instead disposed at the rear side of a focus on the rear side of the reference-side objective lens 55 at a position separated by a distance obtained by multiplying the imaging magnification power by the focus distance of the reference-side objective lens 55.According to such a non-contact displacement sensor 1C, similar to the first embodiment, configuration and processing can be simplified, and measurement accuracy can be improved. In addition, in the non-contact displacement sensor 1C, cost reduction can be achieved by omitting the collimator lenses 32 and 52 of the first embodiment.Modifications of the Fourth EmbodimentIn the fourth embodiment, the finite correction optical system is configured for each of the measurement system 3 and the reference system 5, but a finite correction optical system may be configured only in one, while an finite correction optical system (including the collimator lens 32 or 52) may be configured in the other as in the first embodiment.Fifth EmbodimentA non-contact displacement sensor 1D according to a fifth embodiment will be described with reference to FIG. 14. In the fifth embodiment, identical reference numerals are used for structures similar to those in the first embodiment, and detailed description thereof will be omitted. As illustrated in FIG. 14, in the non-contact displacement sensor 1D, a light source 35 and a photodetector 36 are shared by the measurement system 3 and the reference system 5. Specifically, in the non-contact displacement sensor 1D, the reference light source 51, the collimator lens 52, the beam splitter 53, the reference light photodetector 56, and the optical guide 6 of the first embodiment are omitted. Also, the measurement light source 31 and the measurement light photodetector 34 according to the first embodiment are configured as the light source 35 and the photodetector 36 shared by the measurement system 3 and the reference system 5.In such a configuration, the light emitted from the light source 35 passes through the optical conductor 4 and is emitted from the end face 43 eof the optical fiber 43. The light is collimated by the collimator lens 32, after which the light passes through the liquid lens device 2 and is divided into a measurement light and a reference light by the beam splitter 54. Similarly to the first embodiment, the measurement light passing through the beam splitter 54 is emitted to the measurement object W, and after moving along the reverse path, the measurement light is collected by the collimator lens 32. In this example, the measurement light reflected by the measurement object W when the focal point position Pf 1 coincides with the surface of the measurement object W is incident on the end surface 43 eof the optical fiber 43 and incident on the photodetector 36 via the optical guide 4. Then, the reference light reflected by the first reference surface 71 sor the second reference surface 72 straces the reverse path, and is then collected by the collimator lens 32. In this example, when the focal point position Pf 2 coincides with the reference surface of either the first reference surface 71 sof the second reference surface 72 s, the reference light reflected by the corresponding reference surface falls on the end surface 43 eof the optical fiber 43 and impinges on the photodetector 36 via the optical conductor 4.As illustrated in FIG. 15, the photodetection signal Smr is a signal in which the photodetection signals Sm and Sr according to the first embodiment are mixed together. Based on the photodetection signal Smr output from the photodetector 36, the focus timing calculator 921 finds the peak timing of the photodetection signal Smr as a focus timing Tmr. Based on the calculated focal point time Tmr and the benchmark signal Sc, the delay time calculator 922 calculates delay times Δt1 to Δt6 of the focal point time Tmr. The delay time calculator 922 may also use a clock signal or the like.In this example, the plurality of focus timings Tmr that exist within one cycle of the drive signal Cf include both the measurement-side focus timing Tm and the reference-side focus timing Tr according to the first embodiment. That is, the delay times Δt 1 to Δt 6 correspond to each of the delay times Δtm 1, Δtm 2, Δtr 1, Δtr 2, Δtr 3, and Δtr 4 according to the first embodiment.Therefore, in the fifth embodiment, the first reference surface 71 sand the second reference surface 72 sare disposed close to boundaries on both sides of the variable range VR 2 of the focal point position Pf 2 formed by the reference-side variable focal length lens 102. Accordingly, each peak in the photodetection signal Smr caused by the measurement light (measurement-side focal point time) is output to occur between peaks in the photodetection signal Smr caused by the reference light (reference-side focal point time). Thus, based on the order of the focus time Tmr during a single cycle of the drive signal Cf, the feature calculator 923 and the position calculator 924 handle the delay times Δtm 1, Δtm 2, Δtr 1, Δtr 2, Δtr 3 and Δtr 4 according to the first embodiment by substituting the corresponding delay times Δt 1 to Δt 6. The correspondence relationship between the delay times Δtm 1, Δtm 2, Δtr 1, Δtr 2, Δtr 3, and Δtr 4 according to the first embodiment and the delay times Δt 1 to Δt 6 illustrated in FIG. 15 is as noted below, but may be modified in accordance with the phase of the benchmark signal Sc.According to such a non-contact displacement sensor 1D, similar to the first embodiment, the configuration and the processing can be simplified, and measurement accuracy can be improved. Also, according to the non-contact displacement sensor 1D, the light source 35 and the photodetector 36 are shared in the measurement system 3 and the reference system 5, and therefore significant cost reductions can be achieved.ModificationsThe present invention is not limited to the various embodiments described above, and includes modifications and improvements within a range capable of achieving the advantages of the present invention. For example, the configurations in each of the above-described embodiments may be combined with the configurations of another embodiment.In each of the above-described embodiments, the drive signal Cf and the fluctuation waveforms Mf 1 and Mf 2 are sine waves. However, the drive signal Cf and the fluctuation waveforms Mf 1 and Mf 2 may instead be triangular waves, saw-tooth waves, rectangular waves, or some other waveform. The specific configuration of the liquid lens device 2 may be appropriately modified. Instead of having a hollow cylindrical shape, the casing 21 and the oscillating member 22 may have a hollow hexagonal shape, for example. The dimensions of these components and the properties of the liquid 25 can also be selected appropriately for the application.In each of the above-described embodiments, the lens controller 8 includes the benchmark signal output device 82, but the controller 9 may also include a benchmark signal output device. Alternatively, the benchmark signal output means may be constructed separately from the lens controller 8 and the controller 9. Moreover, the lens controller 8 and the controller 9 may be configured as an integrated control device.Also, in each of the above-described embodiments, the non-contact displacement sensors 1 and 1A to 1D find the different focal points Tm, Tr, and Tmr by configuring an optical system using a confocal method in each of the measurement system 3 and the reference system 5, but the present invention is not limited thereto. Specifically, the non-contact displacement sensors 1 and 1A to 1D can find the different focus timings Tm, Tr, and Tmr by using other focus detection methods such as a double pinhole method, an astigmatic method, and a knife edge method. For example, when the non-contact displacement sensors 1 and 1A to 1D construct an optical system using a double pinhole format in the measurement system 3, a photodetector is provided before and after a condensing position forming a conjugate relationship with the focal point position Pf 1, respectively, and calculation is performed based on the photodetection signal output from each photodetector, whereby the measurement-side focal point time Tm can be found. The same applies to the reference system 5 in a case where the reference-side focal point time Tr is found. In the confocal method, peak positions of the photodetection signals Sm, Sr and Smr must be detected to find the respective focal points Tm, Tr and Tmr. The calculation for this detection is complicated, and compared with the confocal method, the calculation required to find the different focal points Tm, Tr and Tmr is simple in the double pinhole method, the astigmatic method and the knife edge method. Therefore, by applying these methods, the calculation time can be reduced and measurement can be expedited.It is noted that the foregoing examples have been provided for the purpose of illustration only and are not to be construed as limiting the present invention at all. While the present invention has been described with reference to exemplary embodiments, it is to be understood that the words used herein are words of description and illustration rather than words of limitation. Changes may be made within the scope of the appended claims without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular structures, materials and embodiments, the present invention is not intended to be limited to the details disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses as are within the scope of the appended claims.The present invention is not limited to the above-described embodiments, and various alterations and modifications may be possible without departing from the scope of the present invention.

Claims

A non-contact displacement sensor comprising: a light source emitting light; a liquid lens in which a refractive index periodically changes in response to an input drive signal; a beam splitter splitting light emitted from the light source and passing through the liquid lens into a measurement light and a reference light; a measurement-side objective lens emitting the measurement light split by the beam splitter to a measurement object; a reference-side objective lens on which the reference light split by the beam splitter impinges; an optical system for the reference light, which includes a first reference surface and a second reference surface each having an alternately determined length of an optical path measured by the reference-side objective lens, and in which the reference light passing through the reference-side objective lens impinges on each of the first reference surface and the second reference surface; a photodetector that receives the measurement light reflected by the measurement object and receives the reference light having passed through the optical system for the reference light, and outputs a photodetection signal; a signal processor that functions as: focus timing calculation means that calculates, based on the photodetection signal: a measurement-side focus timing at which the measurement light is focused on the surface of the measurement object; a first reference-side focal point time point at which the reference light is focused on the first reference surface and a second reference-side focal point time point at which the reference light is focused on the second reference surface; a feature calculator calculating the features of the refractive index of the liquid lens based on the first reference-side focal point time point, the second reference-side focal point time point, and an optical path length difference, which is a difference between an optical path length that extends from the reference-side objective lens to the first reference surface and the optical path length that extends from the reference-side objective lens to the second reference surface; and a position calculator calculating a position of the measurement object based on the features of the refractive index and a phase of the measurement-side focal point time relative to a period of the drive signal.The non-contact displacement sensor according to claim 1, further comprising: a lens controller that functions as a benchmark signal output device that outputs a benchmark signal synchronized with the drive signal, wherein: the feature calculation device calculates the features of the refractive index of the liquid lens based on a delay time of the first reference-side focal point time relative to the benchmark signal and a delay time of the second reference-side focal point time relative to the benchmark signal, and the position calculation device calculates the phase of the measurement-side focal point time based on a delay time of the measurement-side focal point time relative to the benchmark signal.The non-contact displacement sensor according to claim 1 or 2, wherein the optical system of the reference light comprises: a partially reflecting mirror including the first reference surface that reflects a portion of the reference light; and a reflecting mirror including the second reference surface that reflects the reference light that passes through the first reference surface.The non-contact displacement sensor according to claim 1 or 2, wherein the reference light optical system comprises: a reference light optical splitter that splits the reference light passing through the reference-side objective lens; a first optical fiber that includes, as the first reference surface, a first reference end surface where a first reference light split by the reference light optical splitter impinges; and a second optical fiber that includes, as the second reference surface, a second reference end surface where a second reference light split by the reference light optical splitter impinges.The non-contact displacement sensor according to any one of claims 1 to 4, wherein: the light source comprises: a measurement light source that emits the measurement light; and a reference light source that emits the reference light, and the photodetector comprises: a measurement light photodetector that receives the measurement light reflected by the measurement object and outputs the photodetection signal triggered by the measurement light; and a reference light photodetector that receives the reference light passing through the optical system for the reference light and that outputs the photodetection signal caused by the reference light.

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