JP2010038674A - Measuring apparatus of absolute position - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive measuring apparatus capable of precisely measuring an absolute position. <P>SOLUTION: This measuring apparatus 100 includes a first light source 1 for radiating a first light beam, a second light source 2 for radiating a second light beam whose coherency is lower than that of the first light beam, a first beam splitter 3 for superposing the second light beam to the first light beam, a second beam splitter 4 for splitting radiation light radiated from the first beam splitter into measurement light and reference light, an interference measuring device 8 for detecting an interference signal by synthesizing the reference light with reflection light of the measurement light, and a signal processing device 20 for processing a signal output from the interference measuring device. Wavelengths of the first light beam and the second light beam are set to be the same. The signal processing device 20 includes signal intensity measuring sections 14-1, 14-2 for measuring an intensity of an interference signal obtained from the second light source 2 and an origin determining section 16 that determines that a maximum point of the intensity of the interference signal obtained from the second light source 2 is an origin point. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

    The present invention relates to an absolute position measurement apparatus, and more particularly to a measurement apparatus that measures the absolute position of an object to be measured using two light sources.

    Conventionally, a laser interferometer that measures the absolute position of an object to be measured using two light sources having different coherence and wavelengths has been proposed. In such a laser interferometer, the coherence that is closest to the point where the intensity of the interference signal from the light source with low coherence among the two light sources is maximized or the phase of the interference signal from both light sources coincides. The specific point of the interference signal phase by the light source is determined as the origin of the object to be measured.

    For example, Patent Document 1 discloses that an origin (measurement origin) is determined from an interference output peak (amplitude peak) of low coherence light and an interference output of high coherence laser light. . Patent Document 2 discloses that the origin is detected from the phase difference between the interference output of the low coherence light and the interference output of the high coherence laser light and the intensity information of the interference output of the low coherence light. Is disclosed.

JP 2007-33317 A JP 2007-33318 A

    However, the characteristics of the optical element provided in the interferometer change depending on the wavelength, that is, the optical element has wavelength dependency. For this reason, when two light sources having different wavelengths are used, it is difficult to keep the interference signals from both light sources in an optimum state due to the wavelength dependence of the optical element.

    For example, a glass material used for an interferometer has a wavelength dependency in refractive index. Therefore, when the glass material is optimally arranged for one light source, the other light source is deviated from the optimum point. This results in non-uniform interference signals. Similar problems occur with various parts constituting the interferometer, such as wave plates and gratings. This places restrictions on the characteristics of the interferometer.

    In addition, the position where the intensity of the interference signal from the light source with low coherence becomes maximum, or the position where the phases of the interference signals from both light sources match, fluctuates with the change in the light source wavelength, and the origin position is accurately determined. There is no problem.

  The absolute position measuring device according to one aspect of the present invention includes a first light source that emits a first light beam, a second light source that emits a second light beam having lower coherence than the first light beam, A first beam splitter for superimposing the second light beam on the first light beam, a second beam splitter for branching the emitted light from the first beam splitter into measurement light and reference light, and the reference An interference measurement device for detecting an interference signal by combining light and reflected light of the measurement light, and a signal processing device for processing a signal output from the interference measurement device, wherein the first light beam and the first light beam The wavelengths of the two light beams are set to be the same, and the signal processing device includes a signal intensity measuring unit that measures the intensity of the interference signal obtained from the second light source, and the interference obtained from the second light source. Origin that determines the maximum point of signal strength as the origin Tough, with a.

  Other objects and features of the present invention are illustrated in the following examples.

    According to the present invention, an inexpensive and highly accurate absolute position measuring device can be provided.

    Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In each figure, the same members are denoted by the same reference numerals, and redundant description is omitted.

    First, the overall configuration of the measurement apparatus 100 in the present embodiment will be described. FIG. 1 is an overall configuration diagram of a measuring apparatus 100 according to the present embodiment.

    The measuring apparatus 100 is an absolute position measuring apparatus. Specifically, the measuring apparatus 100 is a laser interferometer capable of measuring the origin of an object to be measured using two light sources including a first light source 1 and a second light source 2. is there. In the measuring apparatus 100, the wavelengths of the first light beam emitted from the first light source 1 and the second light beam emitted from the second light source 2 are set to be the same.

    When the wavelengths of the first light beam and the second light beam are equal to each other as in the present embodiment, the characteristics of each optical element in the measuring device are equivalent to the light from both light sources. For this reason, in the measuring apparatus 100 of a present Example, optimal component arrangement | positioning is attained with respect to the light inject | emitted from both light sources.

    The first light source 1 is a laser diode (LD) and emits a first light beam having high coherence. The second light source 2 is a light emitting diode (LED), and emits a second light beam having low coherence. That is, the second light beam emitted from the second light source 2 is less coherent than the first light beam emitted from the first light source 1. Specifically, the coherence distance of the second light beam is set to be 1/10 or less, more preferably 1/100 or less, of the coherence distance of the first light beam.

    In the present embodiment, as will be described later, the first light source 1 is always lit and the second light source 2 repeats blinking intermittently.

    Light rays emitted from the first light source 1 and the second light source 2 are combined by a non-polarized beam splitter 3 (first beam splitter) having a reflectance of 50%. That is, the non-polarized beam splitter 3 combines the transmitted light of the light beam from the first light source 1 and the reflected light of the light beam from the second light source 2. In this way, the non-polarized beam splitter 3 superimposes the second light beam on the first light beam. The light beam synthesized by the non-polarized beam splitter 3 is guided to the polarized beam splitter 4.

    The first light source 1 and the second light source 2 are arranged so that both light beams incident on the polarized beam splitter 4 have a deflection angle of 45 °. Further, the non-polarized beam splitter 3 has the same light beam (in the vertical direction) in the downward direction in FIG. 1 in addition to the light beam emitted in the right direction (in the direction of the polarized beam splitter 4) in FIG. The optical axis) is emitted. Light rays emitted downward can be treated with a suitable absorber. Alternatively, it can be used as a light source of another interferometer.

    The polarized beam splitter 4 (second beam splitter) transmits half of the incident light beam and reflects the other half. The light beam (measurement light) that has passed through the polarized beam splitter 4 passes through the quarter-wave plate 5-1 to become circularly polarized light, and is reflected by the movable target mirror 6. Similarly, the light beam (reference light) reflected by the polarized beam splitter 4 becomes circularly polarized light by passing through the quarter-wave plate 5-2 and is reflected by the fixed reference mirror 7. Thus, the polarized beam splitter 4 branches the emitted light from the non-polarized beam splitter 3 into measurement light and reference light.

    The circularly polarized light reflected by the target mirror 6 passes through the quarter wavelength plate 5-1 again and returns to linearly polarized light. Similarly, the circularly polarized light reflected by the reference mirror 7 passes again through the quarter wavelength plate 5-2 and returns to linearly polarized light. At this time, the direction of the circularly polarized light of the reflected light reflected by the target mirror 6 and the reference mirror 7 is reversed. For this reason, the deflection angle is rotated by 90 ° from the original direction. Therefore, the relationship between reflection and transmission in the polarized beam splitter 4 is reversed, and all the light beams are guided to the interference measuring device 8.

    The interference measuring device 8 outputs a cosine signal C and a sine signal S corresponding to the phase difference between the reflected light from the target mirror 6 and the reflected light from the reference mirror 7. These cosine signal C and sine signal S are led to the signal processing device 20 provided in the measuring device 100 of the present embodiment.

    In the measurement apparatus 100 of the present embodiment, when the difference in the thickness of the glass material included in the optical path that reflects the target mirror 6 inside the interferometer and the optical path that reflects the reference mirror 7 is large, the nonlinearity of the refractive index of the glass material causes The origin moves when the wavelength fluctuates. This size is, for example, about 10 nm for a wavelength variation of 1 nm for a glass material thickness difference of 100 μm and about 200 nm for a wavelength difference of 20 nm.

    When such a movement of the origin cannot be allowed, it is necessary to make the wavelength difference between the two lights smaller. For this purpose, methods such as obtaining a light source having a uniform wavelength, selecting the obtained light source, and using a light source having a wavelength close to each other can be employed.

    However, if the specification range with respect to the wavelength is narrowed, the non-defective product rate on the light source manufacturing side is reduced, and the cost is increased. In addition, when selecting and combining the obtained light sources, it becomes difficult to obtain an appropriate combination if the requirement for the difference in wavelength is tightened. For this reason, it is not realistic to require a specification that is much less than, for example, 5 nm as the wavelength difference of light.

    Thus, in the present embodiment, the wavelengths of the first light beam emitted from the first light source 1 and the second light beam emitted from the second light source 2 are generally around 10 nm, respectively. Has an error. For this reason, even when the first light beam and the second light beam are designed to have the same wavelength, an error of about 20 nm at the maximum occurs in the wavelength of both light beams.

    Therefore, in the present embodiment, the wavelengths of the first light beam and the second light beam are set to be the same, not only that the wavelengths of both lights are exactly the same, but also the wavelengths of both lights. Is defined as substantially the same including the case where the error is 20 nm or less. However, in this embodiment, the error in wavelength of the first light beam and the second light beam is preferably 10 nm or less, more preferably 5 nm or less.

    As in this embodiment, when the origin is detected using the first light beam and the second light beam having the same wavelength, the point where the phase difference of the interference signals obtained from both light sources matches is used as the origin. Conventionally generally used methods cannot be adopted. However, the origin can be detected by using a light source with low coherence as one of the two light sources and detecting the point where the intensity of the interference signal of this light source is maximum.

    When two light sources set to the same wavelength are used, the interference signals of these two light sources cannot be separated by optical means such as a dichroic mirror, and both interference signals are separated electronically. There is a need to.

    For this reason, in the present embodiment, the second light source 2 having lower coherence than the first light source 1 is blinked. While the second light source 2 is turned off, the phase measured using the interference signal obtained from the first light source 1 having high coherence is regressed with respect to time. By using this regression constant (regression coefficient), the interference signal from the first light source 1 is estimated while both light sources are turned on, and this is subtracted from the signal on which the interference signals from both light sources are superimposed, thereby obtaining the second. The interference signal of the light source 2 is measured. Such control is executed by the signal processing device 20 of the measurement device 100.

    Next, the signal processing apparatus 20 of the present embodiment will be described. The signal processing device 20 processes the signal output from the interference measurement device 8. FIG. 2 is a block diagram of the signal processing device 20 in the present embodiment.

    Inside the signal processing device 20, first, analog signals of both the cosine signal C and the sine signal S input from the interference measuring device 8 are digitally converted using analog / digital converters 9-1 and 9-2, respectively. Convert to signal. Each digital signal output from the analog / digital converters 9-1 and 9-2 indicates a value (superimposed signal) in which the interference signal from the first light source 1 and the interference signal from the second light source 2 are superimposed.

    Therefore, when the interference signal from the second light source 2 is superimposed, the predictors 10-1 and 10-2 are used based on the phase of the interference signal from the first light source 2 updated inside. An interference signal from the current first light source 1 is predicted. The interference signals predicted by the predictors 10-1 and 10-2 from the output values (superimposed signals) from the analog / digital converters 9-1 and 9-2 using the subtractors 31-1 and 31-2. Is deducted.

    Specifically, the multiplexers 36-1 and 36-2 select the signals of the predictors 10-1 and 10-2 when the interference signal from the second light source 2 is superimposed, and the subtracter 31-1. , 31-2. On the other hand, when only the interference signal from the first light source 1 is input, the multiplexers 36-1 and 36-2 select the zero signal and output it to the subtracters 31-1 and 31-2.

    The value of the superimposed signal of the interference signal by the first light source 1 and the second light source 2 is calculated using the predicted value of the interference signal by the first light source 1 calculated by the predictors 10-1 and 10-2. Subtraction is performed by the subtracters 31-1 and 31-2.

    In FIG. 2, reference symbol “*” represents a selection signal indicating whether or not an interference signal from the second light source 1 is superimposed. Based on this selection signal, the operation of each unit is controlled.

    By such arithmetic processing, the signal processing device 20 can obtain the interference signal from the second light source 2 during the period in which the interference signal from the second light source 2 is superimposed. In addition, during the period in which only the interference signal from the first light source 1 is input, the signal processing device 20 can obtain the interference signal from the first light source 1.

    These interference signals are input to the phase calculator 11. The phase calculator 11 obtains the phase of the interference signal based on the interference signal from the first light source 1 and the interference signal from the second light source 2. The phase calculator 11 calculates the phase by performing, for example, an arctangent calculation. However, the phase calculator 11 is not limited to this, and the phase may be calculated by other methods.

    When only the interference signal from the first light source 1 is input, the phase of the interference signal from the first light source 1 held in the internal register 12 is updated. Specifically, the subtracter 32 subtracts the value held in the internal register 12 from the output value of the phase calculator 11. The subtracted value is added to the value held in the internal register 12. The internal register 12 has a bit length larger than an area (bit length) required for storing the phase. In this embodiment, the upper bits in the internal register 12 are set to indicate the wave number of the interference signal from the first light source 1.

    Based on the phase of the interference signal from the first light source 1 calculated by the phase calculator 11, the regression calculator 13 calculates a regression coefficient for this phase measurement time. The regression coefficient is calculated by multiplying the phase of the interference signal by the first light source 1 that is a measurement value by an exponential weighting coefficient corresponding to the elapsed time from the measurement time. Although the regression calculator 13 is comprised by a Kalman filter, for example, it is not limited to this.

    The regression coefficients calculated by the regression calculator 13 are input to the predictors 10-1 and 10-2. As described above, the predictors 10-1 and 10-2 calculate the current predicted value of the interference signal from the first light source 1 when the interference signal from the second light source 2 is input.

    When the interference signal from the second light source 2 is input, the change speed (regression coefficient) output from the regression calculator 13 is added to the contents of the internal register 12 by the adder 34 via the multiplexer 37. . In this way, the phase and wave number information of the interference signal from the first light source 1 is updated.

    When only the interference signal from the first light source 1 is input, the value obtained by subtracting the value obtained by subtracting the contents of the internal register 12 from the output signal of the phase calculator 11 to the bit length of the internal register 12 is stored in the internal register 12. Add to content. In this way, information related to the phase and wave number of the interference signal from the first light source 1 is updated.

    The interference signal from the second light source 2 is squared using the squarers 14-1 and 14-2. The values squared by the squarers 14-1 and 14-2 are added by the adder 33 to be converted into the intensity of the interference signal from the second light source 2. That is, the squarers 14-1 and 14-2 function as a signal intensity measuring unit that measures the intensity of the interference signal obtained from the second light source 2.

    The intensity of the interference signal from the second light source 2 is input to the peak detector 15. The peak detector 15 detects a point (peak position) where the intensity of the interference signal from the second light source 2 is maximum. The peak detector 15 transfers the upper part of the internal register 12 at that time, that is, the part indicating the wave number of the interference signal from the first light source 1 to the bias register 16. The bias register 16 functions as an origin determination unit that determines the maximum point of the intensity of the interference signal obtained from the second light source 2 as the origin.

    When the peak detector 15 detects the peak position of the interference signal from the second light source 2, the value stored in the bias register 16 is subtracted from the value in the internal register 12 by the subtractor 35. In this way, an absolute position signal having the origin at the peak position of the interference signal from the second light source 2 can be obtained.

    As described above, the absolute position measuring apparatus according to the present embodiment can accurately measure the origin of an object to be measured with a simple hardware configuration. For this reason, according to the present embodiment, it is possible to provide a highly accurate absolute position measuring apparatus at low cost.

In addition, the signal processing device (logical operation part) in this embodiment can be easily implemented as an LSI, but the logical operation part that does not require speed can be softened using an MPU (microprocessor) or DSP (digital signal processor). It can be realized by hardware. If the signal processing apparatus of the present embodiment is realized by software, the measurement apparatus can be configured at a lower cost.
The embodiment of the present invention has been specifically described above. However, the present invention is not limited to the matters described as the above-described embodiments, and can be appropriately changed without departing from the technical idea of the present invention.

It is a whole block diagram of the measuring device in a present Example. It is a block diagram which shows the signal processing apparatus of the measuring device in a present Example.

Explanation of symbols

1: First light source 2: Second light source 3: Non-polarized beam splitter 4: Polarized beam splitter 5-1, 5-2: 1/4 wavelength plate 6: Target mirror 7: Reference mirror 8: Interference measuring devices 9-1, 9-2: analog / digital converters 10-1, 10-2: predictor 11: phase calculator 12: internal register 13: regression calculator 14-1, 14-2: squarer 15: Peak detection means 16: Bias register 20: Signal processing devices 31-1, 31-2, 32, 35: Subtractors 33, 34: Adders 36-1, 36-2, 37: Multiplexer 100: Measuring device

Claims (2)

A first light source (1) for emitting a first light beam;
A second light source (2) that emits a second light beam that is less coherent than the first light beam;
A first beam splitter (3) for superimposing the second light beam on the first light beam;
A second beam splitter (4) for branching the emitted light from the first beam splitter into measurement light and reference light;
An interference measurement device (8) for detecting an interference signal by combining the reference light and the reflected light of the measurement light;
A signal processing device (20) for processing a signal output from the interference measurement device,
The wavelengths of the first light beam and the second light beam are set to be the same,
The signal processing device (20) includes a signal intensity measuring unit (14-1, 14-2) for measuring the intensity of an interference signal obtained from the second light source, and an interference signal obtained from the second light source. An absolute position measuring device, comprising: an origin determining unit (16) for determining the maximum point of the intensity as an origin. The absolute position measuring apparatus according to claim 1, wherein an error in wavelength between the first light beam and the second light beam is 20 nm or less.