The present application claims priority to Application No. 10 2017 213 258.2, filed in the Federal Republic of Germany on Aug. 1, 2017, which is expressly incorporated herein in its entirety by reference thereto.
The present invention relates to a device for interferometric distance measurement, which may, for example, be suitable for determining absolute distances between two objects movable relative to each other.
Certain multiple-wavelength methods for interferometrically determining absolute distances between two objects movable relative to each other are conventional. In this context, one or more beat phases, which allow an unequivocal, absolute position determination over a larger distance range, are determined from the subtraction of interference phases of different wavelengths. In this instance, corresponding devices may also be configured in a cascaded manner, and starting from a plurality of different wavelengths, the generation of a plurality of beat phases may be provided. Regarding such devices, reference is made, for example, to U.S. Pat. No. 6,496,266, which describes a system operable using three different wavelengths, a plurality of beat phases and/or synthetic wavelengths being derived from the three wavelengths, and from this, information regarding absolute position being generated.
In the system described in U.S. Pat. No. 6,496,266, a total of three laser light sources in the form of stabilized He—Ne lasers are provided for generating the required plurality of wavelengths, which constitutes a considerable expenditure for equipment. In addition, a complex calibration of the plurality of light sources is necessary, in order to ensure that the emitted beams of light of all of the light sources traverse the identical paths in the measuring and reference arm of the interferometer. Furthermore, the three separate laser light sources require control of each individual light source, in order to ensure that the wavelengths of the individual lasers are stable over time and have a specific value.
Example embodiments of the present invention provide a device for absolute interferometric distance measurement, in which expenditures on the side of the utilized light source may be reduced.
According to an example embodiment of the present invention, a device for interferometric distance measurement includes a multiple-wavelength light source, which supplies a beam of light having at least three different wavelengths and is arranged as a fiber laser, which includes at least three different Bragg gratings, whose grating constants are matched to the wavelengths generated. In addition, an interferometer unit is provided, which splits up the light beam into a measuring light beam and a reference light beam. The measuring light beam propagates in a measuring arm, in the direction of a measuring reflector, and there, it is reflected back, and the reference light beam propagates in a reference arm in the direction of a stationary reference reflector, and there, it is reflected back. The measuring and reference light beams reflected back by the measuring and reference reflectors are superimposed in an interfering manner to form an interference light beam. The interference light beam is split up via a detection unit such that, in each instance, a plurality of phase-shifted, partial interference signals result per wavelength. In addition, a signal processing unit is provided, which is configured to determine an absolute position information item regarding the measuring reflector, from the partial interference signals.
It is possible for the multiple wavelength light source to include at least the following components: a pump light source; at least three Bragg gratings, which are integrated into one or more laser-active fibers, each of the Bragg gratings having a phase shift of magnitude n; and coupling optics, through which the pump radiation emitted by the pump light source may be coupled into the at least one laser-active fiber.
In this connection, the at least three Bragg gratings may be positioned in the at least one laser-active fiber so as to overlap completely along the fiber extension direction, so that the phase shifts of all of the Bragg gratings are at the same location.
It is also possible for the at least three Bragg gratings to be positioned so as to be displaced with respect to each other along the fiber extension direction, by particular offset distances, so that the phase shifts of all of the Bragg gratings along the fiber extension direction are likewise displaced with respect to each other by the offset distances.
In this connection, in the case of three Bragg gratings and: a) with offset distances between 0% and 50% of the effective grating length of a Bragg grating, the laser-active fiber may include first grating sections having Bragg gratings that possess a grating constant, second grating sections having two overlapping Bragg gratings that possess different grating constants, as well as third grating sections having three overlapping Bragg gratings that possess different grating constants; or b) with offset distances between 50% and 100% of the effective grating length of a Bragg grating, the laser-active fiber may include first grating sections having Bragg gratings of one grating constant and second grating sections having two overlapping Bragg gratings that possess different grating constants; or c) with offset distances of 100% of the effective grating length of a Bragg grating, the laser-active fiber may include exclusively grating sections having Bragg gratings of one grating constant.
The at least one laser-active fiber may take the form of an erbium-doped, single-mode glass fiber, which emits laser radiation having a specific polarization direction.
The at least one laser-active fiber may be mounted in tension between two fixing points, in a fiber holding device, and the at least three Bragg gratings may be arranged in the region between the two fixing points in the fiber holding device.
It is possible for a control unit to act upon the multiple wavelength light source via one or more control elements, in order to generate laser radiation having specific wavelengths; and an electrical signal, which is derived from an optical signal of only one of the different wavelengths, acting as an input signal of the control unit.
In this connection, the control elements may include at least one of the following devices: a) a piezoelectric actuator unit for exerting a specific mechanical tension on the at least one laser-active fiber; b) a tempering unit for setting a specific temperature of the at least one laser-active fiber; and c) a current source for setting a specific pump current of a pumping light source for the at least one laser-active fiber.
The multiple wavelength light source may be configured to emit radiation having a first wavelength (λ1) and two further wavelengths (λ2, λ3), the following applying to the two further wavelengths (λ2, λ3):
and λ1, λ2, λ3 represent the emitted wavelengths of the multiple wavelength light source, and
The interferometer unit may include a beam splitter unit, the measuring reflector movable along at least one measuring direction, the stationary reference reflector, as well as a beam combiner unit. In this case, the light beam is spit up into a measuring and a reference light beam by the beam splitter unit, and the measuring and reference light beams are superimposed by the beam combiner unit to form the interference light beam.
In this connection, it is possible for the beam splitter unit and the beam combiner unit to be formed together in a beam splitter cube.
In addition, the detection unit may include at least one splitter element, at least one polarization element, as well as a downstream detector array made up of at least nine optoelectronic detector elements. The interference light beam is split up into at least three groups of interference light beams as a function of wavelength, using the at least one splitter element and the at least one polarization element. Each of the at least three groups of interference light beams includes at least three phase-shifted, partial interference light beams.
The detection unit may include two splitter elements, a splitting into a plurality of phase-shifted interference light beams taking place via a splitter element, and a wavelength-dependent splitting into a plurality of partial interference light beams taking place via the other splitter element.
The signal processing unit may be configured to: (a) determine one phase value per wavelength from the phase-shifted, electrical, partial interference signals of the different wavelengths; (b) form a plurality of differential phases from the phase values, the differential phases each being assigned to different synthetic wavelengths; and (c) determine a high-resolution, absolute position information item regarding the measuring reflector, from a rough position signal obtained via an additional rough position measurement, as well as from the differential phases.
In accordance with example embodiments of the present invention, expenditures relating to the light source, for generating the plurality of wavelengths, may be reduced considerably. Only a single light source, which supplies all of the necessary wavelengths for the absolute interferometric position determination, is provided in place of a plurality of individual light sources.
In addition, only the stabilization of a single wavelength is necessary, which considerably reduces the necessary control expenditure for the light source.
Furthermore, in comparison with the case of a plurality of separate light sources, the need for expensive adjustment of a plurality of light beams of different wavelengths to form a single collinear beam of light is eliminated.
Moreover, the multiple wavelength light source provided ensures an exceedingly low line width and, consequently, a large coherence length. The consequence of this for the position measurement is low background noise of the position measurement values and, consequently, increased measurement accuracy.
Further features and aspects of example embodiments of the present invention are described in further detail below with referenced to the appended Figures.
The two objects may be, for example, machine parts, which are movable relative to each other, and whose absolute spacing L is determinable with the aid of the interferometric distance measurement device. The information, which relates to absolute distance L and is generated with the aid of the interferometric distance measurement device, may be processed further by a superordinate machine control system.
In addition, it is also possible to use the interferometric distance measurement device in laser trackers or laser tracers. In this case, the absolute distance between the stationary components of interferometer unit 30 and the measuring reflector 33 movable in space is determined. Such systems may be used in connection with many different measuring and/or calibration tasks. Furthermore, there are additional and alternative uses for the interferometric distance measurement device.
Before the individual components of the interferometric distance measurement device, as well as a suitable method for operating the same, are described below in detail, in light of the Figures, the general configuration and functioning principle of the corresponding device will be explained first.
Multiple wavelength light source 10 provided in the interferometric distance measurement device emits a beam of light S having at least three different wavelengths λ1, λ2, λ3, which each have a small spectral line width. In this connection, multiple wavelength light source 10 takes the form of a fiber laser, which includes at least three different Bragg gratings, whose grating constants are matched to generated wavelengths λ1, λ2, λ3. For further details of multiple wavelength light source 10, reference is made to the subsequent description of
Light beam S supplied by multiple wavelength light source 10 arrives at interferometer unit 30, where light beam S is split up into a measuring light beam M and a reference light beam R with the aid of beam splitter unit 31, which is implemented as a polarizing beam splitter. After the splitting, measuring light beam M propagates in a measuring arm, in the direction of a measuring reflector 33 movable at least along measuring direction x, and there, it is reflected back in the direction of incidence. After the splitting, reference light beam R propagates in a reference arm, in the direction of a stationary reference reflector 34, and there, it is reflected back in the direction of incidence, as well. In the illustrated example embodiment, both measuring reflector 33 and reference reflector 34 take the form of retroreflecting triple mirrors. Measuring and reference light beams M, R reflected back by measuring and reference reflectors 33, 34 then arrive at the beam combiner unit 31, which is arranged as a polarizing beam splitter, and there, they are superimposed in an interfering manner to form an interference light beam IF. In the exemplary embodiment illustrated
With regard to interferometer unit 30, it should be understood that a Michelson interferometer, as illustrated in
The interference light beam IF generated with the aid of interferometer unit 30 then propagates in the direction of detection unit 40. With the aid of detection unit 40, interference light beam IF is split up and processed further in such a manner, that on the output side, in each instance, a plurality of electrical, phase-shifted, partial interference signals Sλ1_90, Sλ1_210, Sλ1_330, Sλ2_90, Sλ2_210, Sλ2_330, Sλ3_90, Sλ3_210, Sλ3_330 are produced, that is, for each wavelength, three partial interference signals Sλ1_90, Sλ1_210, Sλ1_330, Sλ2_90, Sλ2_210, Sλ2_330, Sλ3_90, Sλ3_210, Sλ3_330 shifted in phase by 120°. Then, in the present example including three wavelengths λ1, λ2, λ3, a total of nine partial interference signals Sλ1_90, Sλ1_210, Sλ1_330, Sλ2_90, Sλ2_210, Sλ2_330, Sλ3_90, Sλ3_210, Sλ3_330 are present at the output of detection unit 40, which are subsequently processed further for position measurement. With regard to a possible arrangement of detection unit 40, reference is made to the subsequent description of
The further processing of partial interference signals Sλ1_90, Sλ1_210, Sλ1_330, Sλ2_90, Sλ2_210, Sλ2_330, Sλ3_90, Sλ3_210, Sλ3_330 takes place subsequently in signal processing unit 50, which is schematically illustrated in
The multiple wavelength light source 10 used in the interferometric distance measurement device is described with reference to
As mentioned above, multiple wavelength light source 10 is configured as a fiber laser, and to be more precise, in the form of a so-called DFB fiber laser (DFB—distributed feedback). According to
In the present example, laser-active fiber 13 takes the form of an erbium-doped, single-mode glass fiber, which emits laser radiation having a specific polarization direction after suitable excitation. Therefore, in the present exemplary embodiment of multiple wavelength light source 10, erbium is used as a laser-active medium. This laser medium allows very narrow-band laser radiation to be produced at the three wavelengths λ1, λ2, λ3, which means that a large coherence length in the range of several kilometers may be ensured. The large coherence length is especially favorable, in particular, in the case of use for interferometric distance measurement, since the noise of the measured position values generated may be minimized over it. Further advantages of the multiple wavelength light source 10 configured as a fiber laser include its ability to be manufactured easily and its robustness.
At least three Bragg gratings, which are used, in each instance, for the specific wavelengths, in order to form the laser resonant cavity necessary for operation of the laser, are integrated and/or inscribed into laser-active fiber 13 and/or into the erbium-doped fiber core. In
Grating constants d1, d2, d3 of the three Bragg gratings are matched to the three wavelengths λ1, λ2, λ3 to be generated in light beam S, that is, the three Bragg gratings have different grating constants d1, d2, d3. For the selection of wavelengths λ1, λ2, λ3 suitably matched to each other, in addition to the following remarks, reference is also made to the description of
The three wavelengths λ1, λ2, λ3 may be selected as follows:
In this connection, the relationship between grating constants di (i=1 . . . 3) of the three Bragg gratings and respectively corresponding wavelengths λi (i=1 . . . 3) is derived according to:
λi=2·ni·di (Equ. 1)
where λi represents the emitted wavelength, ni represents the index of refraction of the laser-active fiber at wavelength λi, di represents the grating constant of the Bragg grating, and i=1, 2, 3.
Given a refractive index n1=n2=n3=1.45 of laser-active, erbium-doped fiber 13, then, for the wavelengths λ1=1560 nm, λ2=1547.11 nm, λ3=1534.32 nm, for example, the following grating constants of the corresponding Bragg gratings result:
As illustrated in
Fixing points, between which laser-active fiber 13 is mounted in tension in fiber holding device 14, are indicated by reference numerals 17, 18 in each of
In addition, in each instance, a piezoelectric actuator unit 15, as well as a tempering unit 16, are represented adjacent to the laser-active region of fiber 13 in
As mentioned above, in the present exemplary embodiment, three Bragg gratings, whose respective constructions are matched to the three wavelengths λ1, λ2, λ3 to be generated, are integrated and/or inscribed in laser-active fiber 13, i.e., in its fiber core. Specifically, in this connection, grating constants d1, d2, d3 of the three Bragg gratings are selected suitably. Furthermore, as likewise mentioned above, one phase shift of magnitude π is to be provided per designated Bragg grating, e.g., situated centrally or centrically in the Bragg grating.
In principle, there are several options with regard to the positioning in the laser-active fiber, of the three Bragg gratings provided in the present exemplary embodiment. Corresponding variants are explained below in view of
A first variant of a possible configuration of the three Bragg gratings in the laser-active fiber is illustrated in
In this connection, in the variant illustrated in
In contrast to the variant illustrated in
In the second variant of the possible positioning of the three Bragg gratings 113.1_λ1, 113.1_λ2, 113.1_λ3 in laser-active fiber 113 illustrated in
In the example illustrated in
Therefore, different first, second and third grating sections 113.1a, 113.1b, 113.1c are produced in the grating in fiber 113 resulting from the superpositioning, as illustrated in the upper part of
A further variant of the possible positioning of the three Bragg gratings 213.1_λ1, 213.1_λ2, 213.1_λ3 in laser-active fiber 213 is illustrated in
A fourth variant for possible positioning of the three Bragg gratings 313.1_λ1, 313.1_λ2, 313.1_λ3 in laser-active fiber 313 is illustrated in
In view of the positioning of laser-active fiber 13 in fiber holding device 14, in each of the variants explained above, it should be ensured that the region of fiber 13, in which all of the Bragg gratings are situated, is positioned between fixing points 17, 18.
A suitable control for multiple wavelength light source 10 of the interferometric distance measurement device, by which the three desired wavelengths λ1, λ2, λ3 may be provided on the output side, is explained below with reference to the schematic illustration provided in
In this connection, a portion of the multiple wavelength light source 10 illustrated in
As explained above, a portion of the light beam emitted by the laser-active fiber is coupled out by coupling-out element 20, e.g., taking the form of a fiber splitter, at a coupling-out ratio of 99:1 or 90:10. After the coupling-out, the three actual wavelengths λ1, λ2, λ3 produced are initially present in beam region A, as illustrated in
In the case of a variation of wavelength λ2, the shape of absorption line AL is covered virtually by a narrow laser needle LN. If the resulting signal at downstream photodetector 23 is plotted versus the wavelength, then one is measuring the shape of the absorption line. If one is at the middle of an edge of the absorption line, then a change in wavelength produces an increase or a decrease of the output signal of absorption cell 21 at downstream photodetector 23.
Thus, the radiation passing through absorption cell 21 represents a measure of the difference between actual wavelength λ2 and necessary, desired wavelength λ2. The corresponding optical control signal is subsequently supplied to a photodetector 23, which converts the optical control signal to an electrical control signal in the form of a current signal, which is subsequently fed to control unit 24. In addition, control unit 24 is supplied a reference signal, which is generated by a photoelectric cell 26, which radiation coupled out of beam region B by a coupling-out element 25 reaches. In this manner, fluctuations in the light intensity may be corrected in control unit 24.
Control unit 24 includes, for example, a PID controller and generates the necessary control variable at its output, in order to act upon one or more of the provided control elements and, in this manner, to set desired wavelengths λ1, λ2, λ3. As mentioned above, current source 11.1 for the pumping light source, piezoelectric actuator unit 15, as well as tempering unit 16, are provided in the multiple wavelength light source as control elements, upon which control unit 24 acts. At the same time, all of the wavelengths λ1, λ2, λ3 of the laser radiation emitted by the fiber laser may be changed selectively via the specific action upon each of these control elements. For example, a 1% strain of the fiber with the aid of piezoelectric actuator unit 15 causes a 1% change in each of all three wavelengths λ1, λ2, λ3, etc.
In this context, the different control elements are used in order to correct different time constants. In this manner, for instance, very rapid wavelength fluctuations in the range of more than 10 kHz may be corrected with the aid of current source 11.1 for the pumping light source. Piezoelectric actuator unit 15 is used to correct wavelength fluctuations in the range between 1 Hz and 10 KHz, and tempering unit 16 is used to correct very slow wavelength fluctuations.
Therefore, the multiple wavelength light source may be adjusted to intended, desired wavelength λ2 in the manner delineated above. In this context, at the same time, it is provided that adjustment to the further, necessary wavelengths λ1, λ2, λ3 may also be made on the basis of the configuration of the laser-active fiber explained above and the simultaneous action of the control elements on all of the Bragg gratings. Consequently, it is possible to adjust all three wavelengths λ1, λ2, λ3 in a precise manner. An electrical signal, which is derived from an optical signal of only one of the three wavelengths λ1, λ2, λ3, acts as an input signal of control unit 24. Therefore, markedly simplified control of the multiple wavelength light source in the interferometric distance measurement device is achieved in comparison with a light source having three individual lasers and the three control units thereby required.
In connection with the description of the multiple wavelength light source of the interferometric distance measurement device, reference is made to
In
The configuration of a detection unit 40, which may be used in the interferometric distance measurement device, is explained with reference to
As illustrated in
As an alternative to the variant illustrated in
In addition, it is possible for detection unit 40 to include only a single splitter element, which, in this case, is formed as a two-dimensional grating in the shape of a cross-grating. Over it, the at least 3 wavelengths are separated in a first splitter direction, via a very fine grating, which has, e.g., a grating period of less than 2 μm. In a second splitting direction, the at least three partial interference light beams are then split up by a coarse grating having, e.g., a grating period greater than 10 μm, before these then pass through the polarization elements, in order to produce the nine partial interference signals Sλ1_90, Sλ1_210, Sλ1_330, Sλ2_90, Sλ2_210, Sλ2_330, Sλ3_90, Sλ3_210, Sλ3_330 in this manner.
Furthermore, as an alternative to the depicted variant of the detection unit, integrated fiber optic wavelength splitting may be accomplished with the aid of so-called WDM demultiplexers. In this connection, interference light beam IF is initially split up into three spatially separated interference light beams by a suitable splitter device. The interference light beams then pass through a polarization element, which includes three linear polarization filters having, in each instance, polarization directions rotated by 60° relative to each other. These cause the three interference light beams separated by the splitter element to be converted into three interference light beams phase-shifted, in each instance, by 120°. Subsequently, these are each then coupled into an optical fiber, using lenses, e.g., implemented as a diffractive lens array including two offset lenses and one normal diffractive lens. That is, the three interference light beams are guided in three separate optical fibers, which are each connected to so-called wavelength division multiplexers that assume the splitting-up into the three wavelengths. Thus, three optical fibers lead out of each wavelength division multiplexer, the optical fibers guiding the light to nine detector elements of the detector array.
In general, interference light beam IF is split up in detection unit 40, into at least three groups of interference light beams IF90, IF210, IF330 as a function of wavelength, using the at least one splitter element and the at least one polarization element. Each of the at least three groups of interference light beams IF90, IF210, IF330 includes, in each instance, at least three phase-shifted, partial interference light beams.
The further processing of partial interference signals Sλ1_90, Sλ1_210, Sλ1_330, Sλ2_90, Sλ2_210, Sλ2_330, Sλ3_90, Sλ3_210, Sλ3_330 and the determination of an absolute position information item regarding the measuring reflector takes place with the aid of signal processing unit 50, which is schematically illustrated in
Prior to the description of the evaluation method, it is explained how, in the present example, different wavelengths λ1, λ2, λ3, which are emitted by the multiple wavelength light source of the interferometric distance measurement device according to the present invention, may be selected.
Thus, a first wavelength λ1, which corresponds to the highest incremental resolution of the position measurement, is initially set. The two further wavelengths λ2, λ3 are then selected according to the two following conditions 2a, 2b:
In this connection, variables CAF1 and CAF2 from the two equations 2a, 2b are determined as follows:
where λ1, λ2, λ3 represent the emitted wavelengths of the multiple wavelength light source.
Variables CAF1, CAF2 are may be selected to be in the range between 10 and 200.
Variables Λ1, Λ2, Λ3 from equations 3a, 3b are also referred to below as first synthetic wavelength Λ1, second synthetic wavelength Λ2, and third synthetic wavelength Λ3, these variables being determined as follows:
Consequently, third synthetic wavelength Λ3 results as a beat from first and second synthetic wavelengths Λ1, Λ2. For example, a first wavelength λ1=1.560 μm having a signal period SPλ1=0.78 μm may be selected. With variables CAF1=CAF2=120, then, for first and third synthetic wavelengths Λ1, Λ3, signal periods SPΛ1≈93.6 μm and SPΛ3≈11.232 mm are obtained. For example, for a Michelson interferometer having a retroreflector, 2·SPΛi=Λi, and 2·SPλ1=λ1 generally apply, with i=1 . . . 3.
In the evaluation method in signal processing unit 50, after a rough position determination of the movable measuring reflector, the cascaded or stepwise determination of absolute distance L between the movable measuring reflector and the stationary interferometer components is carried out with the aid of first wavelength λ1, as well as first and third synthetic wavelengths Λ1 and Λ3. The corresponding procedure is explained below.
In signal processing unit 50, the partial interference signals Sλ1_90, Sλ1_210, Sλ1_330, Sλ2_90, Sλ2_210, Sλ2_330, Sλ3_90, Sλ3_210, Sλ3_330 generated by the detection unit are initially amplified by amplifiers 51.1, 51.2, 51.3 and digitized by analog-to-digital converters 52.1, 52.2, 52.3. For each wavelength λ1, λ2, λ3, a phase value Φλ1, Φλ2, Φλ3 is then calculated by phase computation units 53.1 to 53.3. Consequently, the differential phases ΔΦ12, ΔΦ23, and ΔΦ belonging to the different synthetic wavelengths Λ1, Λ2, Λ3 are ascertained from phase values Φλ1, Φλ2, Φλ3 with the aid of differential phase computation units 54.1, 54.2, 54.3, in the manner specified below.
Thus, for the first synthetic wavelength λ1, corresponding differential phase ΔΦ12 is determined by differential phase computation unit 54.1 as follows:
ΔΦ12=Φλ1−Φλ2 (equ. 5a)
For the second synthetic wavelength Λ2, differential phase ΔΦ23 is determined with the aid of differential phase computation unit 54.2 according to:
ΔΦ23=Φλ2−Φλ3 (equ. 5b)
From the two differential phases ΔΦ12, ΔΦ23 ascertained in such a manner, differential phase ΔΦ of third synthetic wavelength Λ3 is determined by differential phase computation unit 54.3 as follows:
ΔΦ=ΔΦ12−ΔΦ23 (equ. 5c)
The two differential phases ΔΦ12 and ΔΦ of first and third synthetic wavelengths Λ1, Λ3 ascertained in the manner explained above are transferred to a position determination unit 55, just as phase value Φλ1 of wavelength λ1.
As mentioned above, a rough absolute position determination of the measuring reflector is to be undertaken initially to determine the absolute position. This may be accomplished, for example, using a transit time measurement between the measuring reflector and the stationary components of the interferometer unit. For such a transit time measurement, light pulses are transmitted to the movable measuring reflector, and the photons STOF reflected from there are converted to current pulses by a photoelectric cell 56. A time-to-digital converter unit 57 positioned downstream from photoelectric cell 56 allows the times of the current pulses and, consequently, the transit time to be determined accurately. In this context, with regard to the rough position determination, an accuracy in the range of several mm, which is theoretically possible by transit time measurement, is sufficient. The rough position signal TOF generated in this manner is supplied to position determination unit 55, as well.
On the basis of supplied signals TOF, ΔΦ23, ΔΦ, and Φλ1, the determination of absolute distance L is performed in position determination unit 55 in cascaded form in the manner explained below.
In this connection, in a first step, the absolute position, i.e., absolute distance LTOF of the measuring reflector is determined, using the implemented rough position measurement and the rough position signal TOF thereby generated, with an accuracy that is less than half of signal period SPΛ3 of third synthetic wavelength Λ3, and therefore, in the present example, with an accuracy of less than 5 mm.
In the next step, absolute distance LΛ3 of the measuring reflector is determined, using differential phase ΔΦ of third synthetic wavelength Λ3. This is accomplished with an accuracy, which is less than half of signal period SPΛ2 of the next smaller signal period SPΛ1 of first synthetic wavelength Λ1, e.g., less than 45 μm in the present example. In this context, absolute distance LΛ3 is ascertained as follows:
In this connection, variable N is determined, using the value for LTOF ascertained in the previous step, according to:
In equation 6b, rounding to a whole number is performed, using rounding function Round.
In the following step, absolute distance LΛ1 of the measuring reflector is then determined with the aid of differential phase ΔΦ12 of first synthetic wavelength Λ1. This is accomplished with an accuracy, which is less than half of signal period SPλ1 of first wavelength λ1, thus, e.g., less than 390 nm in the present example. In this context, absolute distance LΛ1 is ascertained as follows:
In this context, variable M is obtained as follows, using the value for LΛ3 ascertained in the previous step according to the following equation:
In the final step, using phase value Φλ1, absolute distance L is then determined with the highest available accuracy, namely, with the resolution of first wavelength λ1. In this context, absolute distance L is determined as follows:
In this context, variable n is obtained as follows, using the value for LΛ1 ascertained in the previous step according to equation 6a:
The absolute distance L ascertained in position determination unit 55 in this manner may then be transferred by signal processing unit 50 to subsequent electronics, for further processing.
The foregoing description should not be considered limiting, since there are modifications that may be made without departing from the spirit and scope hereof.
For example, with the aid of the multiple wavelength light source, it is possible to generate more than three different wavelengths, in order to use these in a suitable absolute interferometric method for position determination.
In the multiple wavelength light source, in place of integrating the Bragg gratings in a single laser-active fiber, a plurality of fibers may also be situated parallelly to each other in the fiber holding device, and only one Bragg grating may be integrated into each fiber.
As an alternative to the transit time measurement described above for the rough absolute position determination, other measuring methods may also be used. For example, in the case of use of the interferometric distance measurement device in a machine tool, the position measuring devices present in the machine tool may be used for a rough position determination.
In addition, instead of erbium, the laser-active fiber may also be doped with other dopants, such as ytterbium, thulium, a combination of erbium and ytterbium, etc. Furthermore, the laser-active fiber may also take the form of a non-polarization-maintaining fiber, as well as a polarizing fiber that only carries one polarization.
It is also possible to generate four partial interference signals, each phase-shifted by 90°, in place of three partial interference signals, each phase-shifted by 120°.
In addition to the above-mentioned use in measuring and/or calibrating tasks, the interferometric distance measurement device may also be used for imaging measurement of surfaces. In this context, the specific surface acts as a measuring reflector and may also be diffusive. Accordingly, in the case of such an application, the measuring reflector is positioned immovably. Through point-for-point sampling of the surface, i.e., of the measuring reflector, and determination of the absolute distance to each point of the surface, the respective surface topography may be recorded in this manner.
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