JPS639877A - Three-dimensional measuring method - Google Patents

Abstract

PURPOSE:To accurately take measurement in a short time by finding the optical path difference between reference light and reflected light by using a two-dimensional photodetector. CONSTITUTION:Coherent light is split into two and while one light component reflected by a corner cube 4 is passed through a reference optical path, the other light component is projected on a body 5; and scattered light from the body 5 is image-formed by a lens 2 and interferes with the reference light from the corner cube 4 on the two-dimensional photodetector 7. Further, frequency modulation is imposed on a laser 1 by a laser driving device 9 controlled by a computing element 11. Then, the interference pattern between the reference light from the corner cube 4 and the reflected light from the body 5 has beats in proportion to the optical path difference and the photodetection signal of the photodetector 7 is sampled at frequency more than twice the frequency of the beats to store at least three sampling images in an image memory 11, the phase of a sampling point is found from the stored data by an arithmetic unit 12 to calculate the frequency of the beat signal by using the phase information, thereby finding the optical path difference between the distance to the body 5 and the reference optical path length.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a three-dimensional measuring device, which sometimes measures the distance to a measured object using the phase difference of light between a reference optical path and an optical path to the measured object. This invention relates to a three-dimensional measurement device using an optical homodyne measurement method.

(Prior Art) Conventionally, methods using light and methods using image processing using a computer have been proposed as means for measuring three-dimensional objects. A common method using light is to project a grating and use moiré fringes generated between the deformed grating on the object plane and the reference grating. This method measures the shape of an object by projecting a grating image onto the object surface, superimposing the image of the grating deformed by the shape of the object on the reference grating, and analyzing the moiré fringes that occur as the difference frequency. This is the way to do it. For more details, see the article ``Moir6 Topography'' published in the magazine [Applied Optics, Volume 9, 1970, pages 1467-1472].
mentioned in phy month. However, with this method, it is difficult to quantitatively determine the three-dimensional shape, and it is not suitable for high-precision measurement. Therefore, as a high-precision measurement method using light, a focal position detection mechanism of an optical disc is used. There is an optical homodyne measurement method that detects the optical path difference from a reference optical path as a phase difference of light. There are several methods for detecting the focal position of an optical disc. For example, a detection mechanism using the astigmatism method focuses light on the object plane by moving an objective lens, and then converges the reflected light. A cylindrical lens is placed in the optical path. When the distance between the objective lens and the object is the standard length, the beam shape changes by changing the distance at which the 4-split photodetector is placed at the position where the beam shape becomes a perfect circle, and this is detected. can measure relative distance. For more information, see the magazine [IEEE Transactions on Conspicuous Electronics (IEEE Trans.
tions on Consumer Electr.
onics).

22, 1976, pages 304-308.
eadout of Video disc) J. However, this method has a limited measurement range and is not versatile. On the other hand, the optical homodyne measurement method irradiates an object with light and causes the reflected light that returns to interfere with the light on the reference optical path. At this time, a beat signal is generated in the interference light by modulating the phase of the reference light or the wavelength of the light, and the distance is measured from the frequency of the beat signal. For more information on how to modulate the wavelength of light, see, for example, the magazine Applied Optics, Vol. 20, 19.
1981, the paper "Absolute Distance Measurement Using a Tunable Wavelength Interferometer" on pages 400~
ceMeasurements by Var
iable WavelengthInterfe
rometryl J. This method is
It is a highly accurate and versatile measurement method that can measure distances of tens of meters or more with an accuracy of several pm or less.

(Problems to be Solved by the Invention) The above-mentioned conventional means for measuring three-dimensional objects is based on the fact that in the optical homodyne measurement method, - measurements are performed at one point on the object to be measured. However, in order to obtain the three-dimensional shape of an object, the beam must be scanned two-dimensionally, which has the disadvantage of requiring a large amount of time for measurement.Also, methods based on image processing, although versatile, lack measurement accuracy. However, it has the disadvantage that the processing time is low and the processing time is long.

(Means for Solving the Problems) The three-dimensional measurement method of the present invention divides coherent light into two parts, uses one as a reference optical path with a predetermined optical path length, and the other as an object beam that irradiates an object. A two-dimensional photodetector is used to receive a beat signal generated by interference between the wavefront of the optical path and the reflected light from the object as images at regular sampling intervals, and at least three of the sampled images are stored. Find the phase of the sampling point using the data,
This three-dimensional measurement method is characterized in that the frequency of the beat signal is calculated using the phase information to determine the optical path length difference between the distance to the object and the reference optical path length.

(Operation and Principle of the Invention) FIG. 2 is a conceptual diagram showing the principle of the invention, and FIG. 3 is a graph showing temporal changes in the oscillation frequency of laser light.

An interferometer is configured as shown in FIG.
The beat frequency of the interference between the reflected light from the distance measuring object 105 and the reflected light from the reference optical path reflected by the mirror 104 is measured to determine the optical path difference between the object 105 and the reference optical path.

The frequency-modulated laser beam is expressed by the following formula: % where Ao is the amplitude, vo is the center frequency, and ΔV(t) is the amount of frequency deviation. The instantaneous frequency at this time is V(t)=
Vo+ΔV(t)...
...(2). Here, if the frequency of the laser beam is modulated in the form of a triangular wave as shown in Figure 3, the half period time T
The amount of frequency change at =1/2f is constant. Let this amount be δ. On the other hand, the time difference between the signal lights is based on the optical path length up to the object plane being C2 and the reference optical path length being C1. however,
C is the speed of light. At this time, the phase difference between the two optical paths on the surface of the photodetector 107 is. Therefore, the amount of phase change in the interference fringe intensity of the two light beams in half-cycle time is ΔΦ−foTΔtp(t)dt = 2rE6r
..." (5). Therefore, by measuring the phase shift ΔΦ of the beat component ', r is determined,
c2 is obtained.

In this method, the beam must be focused on the object plane, and it is not possible to obtain three-dimensional information by irradiating light with a spread. Therefore, by scanning the laser beam two-dimensionally,
One possible method is to obtain dimensional information. However, in order to obtain a high number of decomposition points, it takes a lot of time and is not practical. Therefore, a method of arranging lasers in a one-dimensional array and scanning them is effective.

At this time, if lasers with the same oscillation wavelength are used, the lights irradiated onto the object will interfere with each other, making it impossible to measure the beat frequency. The amount of wavelength change when the frequency is changed by δ is expressed as. Therefore, it is effective to form an image of the object on a two-dimensional photodetector and measure the shape of the object all at once. At this time, measuring the beat frequency at each resolution point of the two-dimensional photodetector requires a large amount of time. Therefore, the three-dimensional shape is measured by sampling the output of the photodetector at regular intervals, storing it in an image memory, and performing calculations between the plurality of sampled images. FIG. 4 shows the time change of the beat signal at a certain point on the photodetector. Here, the wavelength scanning width of the laser and the length of the reference optical path are determined so that the beat frequency at the longest distance in the range to be measured is 1/2 or less of the sampling frequency of the photodetector output. Next, this beat signal is converted to 0 and stored in the image memory. Letting the amplitude value of the beat signal at time t be A(t), A(t) is expressed by the following equation.

A(t) = a, 5in (2nft + b)
-"(7) Time 'O+tlt"In-b
The waveform of the beat signal can be found from the amplitude value at - and the frequency can be obtained. However, shame is the measurement start time tlT
+ is the sampling interval and T is the time when mT has passed since shame. A(o), Act) stored in image memory
, A(--t), A(rQ) sequentially,
'Ott1m'fn-1. Find the phase at tTn,
You can calculate the frequency of the beat signal.

As described above, in the method of the present invention, highly accurate three-dimensional measurement can be performed without two-dimensional scanning of a light source as in conventional methods.

(Example) Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram of one embodiment of an apparatus used to implement the method of the present invention. The device of this embodiment has a laser 1,
Lens 2, bead splitter 3, corner cube 4, lens 6, photodetector 7, shutter 8, laser drive device 9
, an image input device 10, an image memory 11, and an arithmetic device 12.

Light emitted from the laser 1 is collimated by a lens 2 and enters a beam splitter 3. One of the lights separated by the beam splitter 3 is reflected by the corner cube 4 and becomes a reference optical path.

The other light illuminates the object 5, and the scattered light from the object 5 is imaged by a lens 6, and interferes with the reference light from the corner cube 4 on a two-dimensional photodetector 7, such as a COD. The laser 1 is frequency-modulated as shown in FIG. 3 by a laser driving device 9 such as a function generator with a GP-IB interface, which is controlled by an arithmetic unit 11 such as a personal computer with a GP-IB interface. be done. corner cube 4
The interference pattern of the reference light of the glass and the reflected light of the object 5 is as follows:
Waviness occurs in proportion to the optical path difference. The light reception signal of the photodetector is sampled at a frequency that is more than twice the frequency of this undulation, and data are obtained at, for example, measurement start time to, time tx after sampling interval T has elapsed from to, measurement end time tw, and tm-1. is stored in the image memory. At this time, when a charge accumulation type two-dimensional photodetector such as a COD is used, since the received light signal becomes the average value of the sampling time, the shutter 8 is used to limit the time for the light to enter the photodetector. If the phase at time t is φ(1), φ
(1) is expressed by the following formula.

φ(t)=sin −1(A(t))
...Song (8) Therefore, φ(0) and φ(T) are calculated using the arithmetic unit 12 from the values of the amplitudes A(0) and A(T) at times to and t1. At this time, the value of φ(0) can be uniquely determined from the information of two points whose phase difference is □ or less. Similarly, φ(m
The value of T) can be obtained, and the frequency f of the beat signal can be calculated from equations (9) and (8) from the values of φ(0) and φ(mT). For example, a laser with a wavelength of 1.3 pm is scanned at a width of 10 people, and a 10-bit image memory is used to scan a laser at 33 Hz.
When sampling is performed for 3 seconds at 500 x 500 points, a range of 170 mm can be measured with an accuracy of 7 x 10-5.

(Effects of the Invention) As explained above, the present invention makes it possible to measure the distance to the object to be measured two-dimensionally in parallel by determining the optical path difference between the reference light and the reflected light using a secondary photodetector. Furthermore, by processing the light reception signal of the photodetector, three-dimensional information can be obtained without two-dimensional scanning of the light source.

[Brief explanation of the drawing]

Fig. 1 is a configuration diagram of an embodiment of the present invention, Fig. 2 is a conceptual diagram showing the principle of the invention, Fig. 3 is a graph showing temporal changes in the oscillation frequency of laser light, and Fig. 4 is a graph showing the temporal change in the oscillation frequency of laser light. FIG. 3 is a diagram showing the time change of a beat signal at a certain point of the detector r. 1... Laser, 2... Lens, 3...
・Beam splitter, 4... Corner cube, 5
...Object, 6...Lens, 7...Two-dimensional photodetector, 8...Shutter, 9...Laser drive device, 10...Image input device, 11...Image memory, 12... Arithmetic device, 101...
・Laser, 103...Beam splitter, 104・
...Mirror, Figure 1 Figure 2 Figure 104 Mirror Figure 3

Claims (1)

[Claims] Coherent light is divided into two parts, one is used as a reference optical path with a fixed optical path length, and the other is used as an object beam that irradiates an object, and the beat signal generated by interfering the wavefront of the reference optical path and the reflected light from the object is generated. A two-dimensional photodetector is used to receive light as images at regular sampling intervals, at least three of the sampling images are stored, the phase of the sampling point is determined using the stored data, and the phase information is used to determine the phase of the sampling point. and calculating the frequency of the beat signal to determine the optical path length difference between the distance to the object and the reference optical path length.
Dimensional measurement method.