JP2006003290A - Laser interference type displacement measuring device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To adjust a detection signal system easily and highly accurately, even when measuring a fine displacement based on an interference signal of laser light. <P>SOLUTION: In this laser interference type displacement measuring device, an incident laser beam is divided so that one is reference light L2 wherein the optical path length is not changed and that the other is measuring light L1 wherein the optical path length is changed according to the distance to the surface by being reflected by the surface of a measuring body 5, and the displacement of the measuring body surface is measured based on a sine wave signal acquired from interference light between the measuring light L1 and the reference light L2. In the device, a PZT (piezoelectric element) 49 for advancing or retreating in the same direction a reflecting mirror 46 for reflecting the measuring light on a standard position in the measuring body direction is installed as an optical path length changing means for changing forcibly the optical path length of the measuring light L1. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a laser interferometric displacement measuring apparatus, and more particularly, a laser interferometric flatness measuring apparatus that uses a laser interference to detect a minute displacement below the wavelength of the surface of the object to be measured in a non-contact manner, or a substantially parallel relationship. The present invention relates to a laser interference type displacement measuring apparatus suitable for being applied to a laser interference type gap measuring apparatus or the like that measures a minute parallelism between two measured object surfaces in a non-contact manner.

  Conventionally, measurement light whose optical path length changes according to the distance to the surface by splitting the incident laser beam and using one as reference light whose optical path length does not change and reflecting the other on the surface of the object to be measured And laser interference for measuring the displacement (distance) to the surface of the object to be measured based on a sine wave signal obtained by photoelectric conversion of the interference light when the measurement light and the reference light are combined. A type displacement measuring device is used (for example, see Patent Document 1).

In such a displacement measuring apparatus, when the object to be measured is scanned in the direction of the optical path of the measuring light, a sufficiently large displacement can be obtained. Therefore, as shown in FIG. Sine wave signals V ₁ and V ₂ having a phase difference of 90 ° are output. That is, when the displacement is sufficiently large, the interference signal changes periodically, so that a cos component and a sin component of one cycle or more are obtained as an output signal.

  By the way, if the reflectance of the object to be measured is different each time, the detection system needs to be adjusted for each measurement. As shown in FIG. When a wave signal is output, the amplitude: A, the DC offset: D, and the phase difference can be known from this signal, so that the correction of the signal is easy, and the adjustment of the detection system is also easy.

  However, a displacement measuring device having a similar function is scanned along the surface of the object to be measured to measure flatness (flatness) (for example, see FIG. 1 described later). When only a minute change can be expected in the displacement, a 90 ° phase difference sine wave signal is not repeatedly detected in a short cycle unlike the detection signal in the case of a linear scale or the like that measures a sufficient displacement. Since it is difficult to correct the distortion of the detection signal using the previous data (for example, the signal before one cycle), the adjustment work is complicated.

  That is, although the change in the interference signal can be detected as shown in FIG. 5B, the signal output when the surface of the object to be measured is scanned and measured under the same adjustment conditions as in FIG. If the displacement is large, a signal such as that shown in FIG. 5A can be obtained if the displacement is returned midway. The waveform is as if waveform distortion or DC offset has occurred. . Moreover, this signal seems to detect the interference signal periodically, but this is a phenomenon that the displacement returns in the middle, and does not detect periodic interference fringes.

  Therefore, if the adjustment is performed using this signal, incorrect adjustment will be performed, and correct measurement cannot be performed. In order to adjust the signal, it is necessary to give enough displacement to accurately detect the period of the interference fringes. However, the better the flatness and installation state of the measured object, the more difficult it is to detect the interference fringes periodically. Will be difficult.

  As described above, when scanning and measuring the flatness of the surface of the object to be measured by a displacement measuring device using laser interference, the distortion of the detection signal is corrected using the previous data, and the accurate internal It was impossible to obtain an interpolated signal.

  For this reason, conventionally, the detection system has a repeatability by forcibly changing the measurement distance (displacement) by, for example, tapping the measured object in order to obtain a signal necessary for the adjustment when setting the measuring device. The detection signal system is adjusted by trial and error by generating a signal and repeating the generation of this signal.

JP 2003-202203 A

  However, as described above, it is possible to expect a change of the displacement almost close to the stationary state to acquire a repeatable signal by forcibly changing the measurement distance by hitting the measurement object in the stationary state as described above. Even though this is an unavoidable task for the adjustment, there is a problem that the adjustment operation of the measuring apparatus performed in this way takes time and lacks accuracy.

  The present invention has been made to solve the above-described conventional problems, and can easily and accurately adjust the measurement apparatus even when measuring a minute displacement such as flatness based on an interference signal of a laser beam. It is an object of the present invention to provide a laser interference type displacement measuring apparatus capable of performing the above.

  The present invention divides an incident laser beam, uses one as reference light whose optical path length does not change, and reflects the other at the surface of the object to be measured, thereby changing the optical path length according to the distance to the surface. In a laser interference displacement measuring apparatus that measures the displacement of the surface of the object to be measured based on a sine wave signal obtained from interference light between the measurement light and the reference light, an optical path of the measurement light By providing an optical path length changing means for forcibly changing the length, the above-mentioned problem is solved.

  In the invention, it is preferable that the optical path length changing means is a driving means for moving a reflecting mirror that reflects the measurement light in the direction of the measured object at a reference position in the same direction. The driving means is a piezoelectric element fixed to the reflecting mirror.

  According to the present invention, it is possible to generate a sine wave signal necessary for adjusting or correcting the detection signal by the sensor by the optical path length changing means while the measured object is stationary. It is possible to correct the distortion of the detection signal system even in a state where there is almost no change. As a result, even in measurements where there is almost no change in displacement or only a signal with a gradual change can be obtained, the detection signal can be corrected, so that errors can be minimized even if the detection signal is interpolated. Thus, the measurement accuracy of the displacement measuring device can be improved.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  First, an outline of a laser interference type flatness measuring apparatus having the basic structure shown in FIG. 1 will be described.

  The linearly polarized laser beam output from the laser light source 1 is guided to the sensor body 4 via the condenser lens 2 and the polarization plane preserving optical fiber 3. In this sensor body 4, the laser beam emitted from the rear end of the optical fiber 3 is converted into parallel light by the collimating lens 41, and then bent in a direction orthogonal to the prism mirror 42, and (1/2) λ plate After the ratio of the measurement light L1 and the reference light L2 is adjusted in 43, the light is guided to a measurement PBS (polarization beam splitter) 44, where the measurement light (for example, S polarization) L1 and the reference light (for example, P polarization). ) Divided into L2. The divided reference light L 2 passes through the PBS 44 and is guided to the four-phase signal detector 48.

  On the other hand, the divided measurement light L1 is reflected by the PBS 44 in the orthogonal direction, passes through the (1/4) λ plate 45, and is applied to a reflecting mirror (mirror in the figure) 46 whose reference position is set. The light is reflected in the opposite direction, passes through the (1/4) λ plate 45 again, and returns to the PBS 44 as P-polarized light.

  Next, the measurement light L1 passes through the PBS 44, passes through another (1/4) λ plate 47, and is irradiated on the measured object 5, and is reflected by this surface and again (1/4) λ plate. 47 and returns to PBS 44 as S-polarized light.

  Thereafter, the measurement light L 1 is reflected in the orthogonal direction by the PBS 44, and simultaneously combined with the reference light L 2 and travels in the same direction, and is guided to the four-phase signal detector 48. As a result, the detector 48 converts the four-phase interference signal generated by the interference between the measurement light L 1 and the reference light L 2 into an electrical signal, and the electrical signal is converted into a signal processing circuit 7 via the cable 6. The distance (displacement) from the sensor body 4 to the surface of the measured object 5 is detected based on a 90 ° phase difference sine wave signal input to the processing circuit 7 and converted into an electrical signal by the processing circuit 7. It has become.

Here, the reason why the four-phase signals of ± sin θ and ± cos θ are detected using the four-phase signal detector 48 is to cancel the DC component. That is, the amplitude A _n Now, when the DC component is _{D n,} the signals of four phases: _{V n} is expressed by the following equation.

V ₁ = + A ₁ sin θ + D ₁
V ₂ = −A ₂ sin θ + D ₂
V ₃ = + A ₃ cos θ + D ₃
V ₄ = −A ₄ cos θ + D ₄

Assuming that A ₁ = A ₂ = A ₃ = A ₄ and D ₁ = D ₂ = D ₃ = D _{4 in} each equation, V ₁ −V ₂ = 2A sin θ, and the DC component can be removed.

  Since it is only necessary to finally obtain sin and cos signals, two-phase detection may be used instead of four-phase detection, but this detector 48 is employed because the DC component can be easily removed.

  In the displacement measuring apparatus for measuring the distance to the surface of the measured object 5 based on the principle as described above, the sensor main body 4 is relatively moved in the direction of the arrow along the surface of the measured object 5 to thereby measure the measured object. When the flatness of the surface of 5 is measured by scanning, the better the flatness, the change of the signal detected by the detector 48 is very slight and may not change at all. Therefore, as described above, it is virtually impossible to adjust the 90 ° phase difference sine wave signal based on the electrical signal detected during the scanning measurement.

  Therefore, as described above, at present, a method of performing a persevering adjustment operation is adopted in which the signal is adjusted by forcibly generating displacement by hitting the measured object 5 before scanning measurement.

  FIG. 2 shows an outline of the laser interference type minute displacement measuring apparatus of the present embodiment in which such inconvenience is improved. Here, the same members as those in FIG. 1 are denoted by the same reference numerals, and detailed description thereof is omitted.

  The displacement measuring apparatus according to this embodiment is different in that a mechanism for forcibly changing the optical path length of the measuring light L1 and generating a detection signal having a 90 ° phase difference of one wavelength or more is added to the inside of the sensor body 4. The basic principle is substantially the same as the apparatus of FIG.

  Specifically, a PZT (piezoelectric element) 49 is attached as a driving means to a convex portion formed on the top of the casing constituting the sensor main body 4, and the measurement object L 1 is measured on the PZT 49 at the reference position. The reflecting mirror 46 that reflects in the direction is fixed, and a predetermined voltage is applied to the PZT 49 by the PZT drive circuit 9 via the cable 8 to forcibly advance and retract the PZT 49 in the traveling direction of the measuring light L1 by about several μm. The optical path length of the measuring light L1 can be changed by one wavelength or more.

  As a result, it is possible to reliably generate a sine wave signal having a phase difference of 90 ° for several cycles as shown in FIG. As in the case of measuring a linear displacement that is longer than the wavelength length by using, the amplitude of the signal detected by the signal processing circuit 7, the DC offset, and the phase are adjusted to correct the distortion of the detected signal. Thus, the detection signal system can be adjusted.

  Further, if the voltage application to the PZT 49 is canceled to make no voltage, the reflecting mirror 46 naturally returns to the reference position, so that the measurement is not affected at all.

  Therefore, according to the present embodiment, it is possible to adjust the detection signal by changing the optical path length by several μm by linearly changing the voltage applied to the PZT 49 at the time of signal adjustment before the start of measurement, and then performing scanning. By measuring, flatness can be measured with high accuracy.

  As described above, according to the present invention, since the measurement apparatus can be calibrated with the measurement object stationary, the present invention is not limited to the laser interference type flat surface measurement apparatus, but the applicant of the present invention. Can be effectively applied to the laser interference type gap measuring device already proposed by Japanese Patent Application No. 2003-143843, and furthermore, since the reflection state of the surface changes depending on the object to be measured, calibration is performed in addition to the measurement. By applying it to any measuring device that needs to be performed, it is possible to significantly reduce the measuring time.

Schematic diagram showing the basic principle of the laser interference type displacement measuring apparatus according to the present invention. The schematic diagram which shows the characteristic of the laser interference type displacement measuring apparatus which concerns on this invention Diagram showing examples of output signals output from the displacement measuring device

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Laser light source 2 ... Condensing lens 3 ... Optical fiber 4 ... Sensor main body 5 ... To-be-measured object 6, 8 ... Cable 7 ... Signal processing circuit 9 ... PZT drive circuit 41 ... Collimating lens 42 ... Prism mirror 43 ... (1 / 2) Lambda plate 44 ... PBS (polarization beam splitter)
45, 47 (1/4) λ plate 46 ... Reflector 48 ... Detector 49 ... PZT (piezoelectric element)

Claims (3)

While splitting the incident laser beam, one is used as a reference light whose optical path length does not change, and the other is reflected from the surface of the object to be measured, thereby obtaining a measuring light whose optical path length changes according to the distance to the surface. ,
In a laser interference displacement measuring apparatus that measures the displacement of the surface of the object to be measured based on a sine wave signal acquired from the interference light between the measurement light and the reference light,
A laser interference type displacement measuring apparatus comprising an optical path length changing means for forcibly changing the optical path length of the measuring light.   2. The laser interference displacement according to claim 1, wherein the optical path length changing unit is a driving unit that moves a reflecting mirror that reflects the measurement light in a direction toward the measurement object at a reference position in the same direction. 3. measuring device.   3. The laser interference displacement measuring apparatus according to claim 2, wherein the driving means is a piezoelectric element fixed to the reflecting mirror.

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