JPS6023282B2 - Relative displacement measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for measuring the displacement of a first member relative to a second member in a manner corresponding to a linear motion or rotational motion with one degree of freedom in one plane, and more specifically, , metrological grid (
This type of device utilizes metrolo calcining.

Various devices of this type are used in fields such as machine tool control.

Conventional devices of this type include a pair of gratings of the same or nearly the same frequency, located at fixed positions relative to each of the first and second members whose relative displacements are to be measured. One grating of the pair of gratings is transmissive and the other grating is either transmissive or reflective. The pair of gratings is illuminated by a suitable light source. Additionally, means are provided for responding to changes in light transmitted or reflected by the combination of gratings that vary in response to relative movement of the two gratings. This secondary device has many problems in design and use.

In particular, it is necessary to illuminate the grating system with a high degree of collimation, and the spacing between the gratings has to be maintained very precisely, and sometimes the spacing between the gratings has to be very small.
These problems are especially difficult to avoid when using fine grids. In general, a coarser grid is used than the grid that directly provides the resolution required for measurement, and the Tachibana method is used to obtain the required precision or resolution results, but this interpolation itself is a source of error. There is. The present invention has been made in view of the above points, and its purpose is to exploit the optical imaging properties of gratings for spatially periodic optical objects (i.e. emitting uncollimated light). Based on this idea, we provide a relative displacement measuring device that can solve at least part of the above-mentioned problems and has a desired disassembly even without using an interpolation method and is relatively simple and easy to manufacture. It's about doing.

The relative displacement measuring device of the present invention according to the above concept is, in principle, a method for measuring the relative displacement of a first member with respect to a second member in a manner corresponding to movement with one degree of freedom within one plane. an apparatus, each of which is provided in a fixed position with respect to the first and second members at a substantially uniform spacing from each other, and having a line spatially periodically in the direction of movement; irradiation means for illuminating the second grating with collimated light, and a light detection means, the first and second gratings and the irradiation means comprising: ,
a real image of a spatially periodic optical object defined by a second grating, spatially periodic in the direction of motion and substantially uniformly spaced from the first grating; when the relative movement occurs between the first and second members, the real image moves relative to the second member in accordance with the movement with one degree of freedom in one plane; and the photodetector means has a spatially periodic structure in the direction of movement provided in a fixed position relative to the first member, and the photodetector means is arranged to detect the real image. the structure is configured to receive the light forming the image, and when the relative movement occurs between the first and second members, the structure interacts with the image and the output of the light detection means is periodically It consists of a relative displacement measuring device which is built up in a variable manner.

Among the present inventions based on the above-mentioned principles, the first invention uses a grating that can practically ignore the diffraction of light and forms a real image of the shadow of an optical object. Apparatus for measuring the relative displacement of a first member with respect to a second member in a manner corresponding to a movement with one degree of freedom of are provided at fixed positions in the direction of movement, respectively at a frequency f.
, and f2 (just f, > f2), the first and second gratings have spatially periodic lines, and f2/L=v/(
uniformly spaced apart from the first grating by a distance v defined by u+v) and spatially periodic in said direction with a frequency b3 defined by f3/f,=u/(u+v); The first grating should produce a real image of the shadow of the second grating as an optical object, which moves in the manner described above with respect to the second member when a movement occurs in the manner described between the first and second members. irradiation means for illuminating the second grating with uncollimated light of an incomparably higher frequency than Z, f, and f2; a structure which is spatially periodic at substantially the same frequency and is disposed in a fixed position relative to the second member at a distance of substantially v from the first grating Z so as to produce a change in the structure; Ori,
and photodetector means responsive to said image.

Furthermore, among the present inventions based on the above-mentioned principles, according to a second invention using a grating that forms a real diffraction image of two optical objects, the above-mentioned object is a mode corresponding to movement with one degree of freedom within one plane. a counterfeit device for measuring the relative displacement of a first member with respect to a second member, the counterfeit counterfeit being provided at a fixed position with respect to the first and second members, respectively, spaced uniformly apart by two distances u; first and second gratings having spatially periodic lines at frequencies f and f2 (>f2) in the direction of movement, respectively;
F3 is uniformly spaced apart from the first grid by a distance V defined by 3/f, = 2V/(u + V).
It is spatially periodic in the above direction at a frequency F3 defined by /f,=sha/(u 1 V), and when the movement in the above manner occurs between the first and second members, the second member On the other hand, in order to produce a real diffraction image of the second grating as an optical object moving in the above manner by the first grating, the frequencies are comparable to f and f2, and the pitch of the first grating is w. In this case, a relative movement occurs between the illumination means that illuminates the second grating with uncollimated light whose maximum wavelength ^m is defined by u and /2^m, and the first and second members. a structure that is spatially periodic at a frequency of substantially F3 and is located in a fixed position relative to the second member at a distance of substantially V from the first grating so as to produce a periodic change in the output power; and photodetector means responsive to said image.

As used herein, the term ``light'' includes visible light as well as ultraviolet and infrared light.

In the relative displacement measuring device of the present invention, the relative displacement between the first member and the second member causes a larger relative displacement between the image and the second member.

0 Generally, the first grating, the second grating constituting the optical object, and the periodic structure of the photodetector means are all positioned in a plane substantially parallel to the given plane. will be placed.

In the device of the second aspect of the present invention, for example, the first grating comprises a reflection grating, and the second grating constituting the optical object and the periodic structure of the photodetector means are substantially In one preferred embodiment, the spatial frequencies of the first grating, the second grating constituting the optical object, and the periodic structure are coplanar (i.e., u=V). are substantially identical (i.e., f,=et=F3).

In this device, since (u + V)/u = 2, the relative displacement between the image of frequency F3 and the second member is twice the relative displacement between the first member and the second member. Therefore, a relative displacement between the first member and the second member corresponding to one period of the grating results in a periodic output change of two periods in the photodetector means. The present invention will now be described in detail with reference to the accompanying drawings.

0 The basic principle of the present invention is illustrated in the conceptually simplest form of FIG.

In the optical system shown in FIG. 1, light from a lamp 1 is focused by a lens 2 and three linear transmission gratings 3, 4, 5
and reaches the photovoltaic cell 6. The grating 3 as the second grating and the grating 4 as the first grating are provided in parallel planes separated by a distance u from each other so that their respective lines are parallel. The first grating 4 has a spatial frequency f. , and the second grating 3 has spatial frequencies et al. The grating 3 defines a spatially periodic optical zero object that diffusely illuminates the grating 4. In the above, the irradiation means for illuminating the second grating 3 with uncollimated light consists of the lamp 1 and the lens 2. First, when considering the imaging characteristics of the grating 4, the frequency of uncollimated light from the lamp 1 and lens 2 is incomparably larger than the spatial frequencies f2, f of the gratings 3 and 4, and the diffraction effect due to the grating 4 is Assume that the condition is negligible.

That is, a case corresponding to the first invention of the present invention will be considered. In this case, the light propagates linearly, and a real image of the shadow of the grating 4 is formed in a plane parallel to the grating 4 at a distance v from the grating 4. Here, the distance v is determined by the following equation '1}. et/f. iV/(u+V){1, This shadow image has a spatial frequency given by the following equation (2).

mo/f. =u/(u+V) ■By Equation '11, [2], f3=f, -f2, so f,>f
It is 2.

If the grating 4 is displaced with respect to the grating 3 by an amount d parallel to the planes of the gratings 3, 4 and perpendicular to the lines of the gratings 3, 4, the image will be displaced by an amount D in parallel.

Here, the magnitude D of the displacement of the image is determined by the following equation {31. D=d(10v/u)'3' Therefore, a grating 5 with a spatial frequency of f3 is formed, and this grating 5
is fixed to the grid 3 at a distance v from the grid 4 parallel to the grids 3 and 4 so that the lines of the grids 3 and 4 are parallel to the lines of the grid 5, then the grids 3 and 4 become As the image formed by the grating 4 interacts with the grating 5, the intensity of the light reaching the photovoltaic cell 6 changes periodically as it is moved 2 relatively parallel to the grating plane and perpendicular to the lines of the grating.

Furthermore, since the grating 5 is stationary with respect to the grating 3, the magnitude of the relative movement can be determined from the output of the photovoltaic cell 6.
3 and above, the photodetector means consists of a grating 5 as a spatially periodic structure and a photovoltaic cell 6. It can be seen from equation m that no image is formed if the spatial frequencies of gratings 3 and 4 are the same. Also, if f is twice as large as , then v is u
It is clear that W is equal to 3, and in this case, from equation (2), it can be seen that W is equal to f2. u=V. In the case of an apparatus in which the optical system is similar to f2, it is better to change the optical system shown in FIG. As a result, the optical system is reduced from a structure with three gratings to a structure with only two gratings, and the two functions of one of these gratings 3, 7 (grating 3 in FIG. and the function of grid 5). Thus, for example, the grating 3 in FIG. 2 not only acts as a second grating constituting a spatially periodic optical object, but also as a spatially periodic structure of the photodetector means that interacts with the image. work. Details of the specific optical system with this change will be described later. If the light incident on the grating 3 is perfectly collimated, no image is formed.

Furthermore, in the case of partially collimated incident light, no good image is formed for a distance u of approximately (N-1/2) L,f2. Here, N is an integer and I is the average wavelength of the light used. Generally, as the distance v increases, the assumption that light travels in a straight line becomes less and less valid, and the contrast of the image decreases. Next, lamp 1
When the frequency of the light from the lens 2 is comparable to the spatial frequency of the gratings 3 and 4, and the grating 4 acts as a diffraction grating, consider an input m2.

That is, a case corresponding to the second invention of the present invention will be considered. In this case, an interference image is formed. This image is formed at a distance V from the grating 4, where V is determined by the following equation '4}. et/f. =2V/(n10V)
(2) This image has a spatial frequency F3 determined by the following equation '5).

F3/f,=Fox/(u+V) ■In addition,
Formula {4), {5, F3= is -f2, so 2,>f
It is 2. An equation of the same form as equation {3} is applied to the interference image formed by the diffraction grating, and by providing a fixed grating 5 that fills the equation {41, side with respect to the grating 3, the light intensity reaching the photovoltaic cell 6 becomes periodic. A relative displacement measuring device is obtained that changes as follows.

Equation 'small' indicates that if gratings 3 and 4 have the same spatial frequency, an interference image is formed at V=u, and equation (2) indicates that in this case the interference image is in the same space as gratings 3 and 4. It shows that it has a frequency.

In this case as well, by replacing the grating 4 with a reflection grating, the first
It may be convenient to modify the optical system in the figure and again use a single grating that performs the function of gratings 3, 5 in FIG.
If the incident light on grating 3 is perfectly collimated,
No interference image is formed.

It is therefore important that the light incident on the grating 3 is at least partially diffuse. In reality, it is difficult to completely diffuse or diffuse irradiation of the grating 3, and partially collimated light works well for distances u that are close to (N-1/2)/^f,. It must be noted that no interference image is formed. Furthermore, if the distance u is smaller than the input force W2/2 m, the light will not be sufficiently diffracted, and the contrast of the formed interference image will be so low as to be useless. where ^m is the maximum wavelength of the light used and w is the grating 4
This is the pitch of In the optical systems corresponding to the respective examples of the first and second inventions above, the gratings 3, 4, and 5 are all provided so that their lines are parallel, thus causing periodic changes in light intensity. The interaction of the image with the grating 5 causes a "shuttering" effect
can be considered as

Of course, other methods are also possible. Connect the lines of grid 4 to grids 3 and 5
may be slightly distorted relative to the lines, and when the image interacts with the grating 5, moiré fringes are generated.
This can be detected by multiple photocells across a single Moiré fringe. The grating 5 may have a spatial frequency slightly different from the spatial frequency of the image formed by the grating 4, in which case fringes known as bagna lines are produced and detected in a similar way to moiré bran. obtain. The above explanation also applies to the case of radial gratings used for measuring rotational displacements, where the radicals f, , ra, w indicate the relevant parameters at the mean radius of the grating system.

However, in this case, compared to a linear grating, the image contrast deteriorates to the extent that the pitch changes over the available optical aperture of the system. FIGS. 2 and 3 show two variations of the optical system shown in FIG. 1, in which a reflection grating 7 is used in place of the transmission grating 4. FIGS.

In the optical system shown in Fig. 2, light from lamp 1 is transmitted through
The light is focused by a half mirror 8, passes through a transmission index grating 3, and enters a reflection scale grating 7.
The reflected light from the grating 7 passes through the grating 3 again, passes through the half mirror 8, and enters the photovoltaic cell 6. In the optical system shown in FIG. 3, light from a lamp 1 is reflected by a mirror 9, condensed by a lens 2, passes through an index grating 3, and then enters a reflective scale grating 7.

The reflected light from the grating 7 passes through the grating 3 again and reaches the photovoltaic cell 6 via the lens 10 and the mirror 11. optical elements 1, 9,
2, 3, 10, 11, 6 constitute a permanently assembled discussion head 12. In the relative displacement measuring device consisting of the compact head 12 and the reflection grating 7, the compact head 12
The relative lateral movement between the grid 7 and the grating 7 is measured. In both of these optical systems, the spatial frequencies of the gratings 3 and 7 and their mutual spacing are of course expressed by the equations {1' and
, or is selected to satisfy equations [4' and '51. In a variant of the device shown in FIGS. 2 and 3, the light detection means are advantageously constituted by a combination of a grating 3 and a photovoltaic cell 6, and if necessary a transmission grating and a spatially periodic light beam. It is possible to replace it with a single periodic structure that combines the functions of the detector.

This structure consists of rows of photosensitive elements each cooperating with a line of the grating 7 to receive the light reflected by the grating 7. Said structure is described, for example, in GB 123102y. In these variations, the apparatus used to illuminate the transmission grating may of course be similar to the diamond plate used to illuminate the grating 3 in FIG. Another variant, applicable in principle to devices that image using either transmission or reflection gratings, is to use the grating 3, which is illuminated by a separate light source, as a light emitting device that constitutes a spatially periodic optical object. The idea is to replace it with a device that incorporates an array of elements. In the case of imaging using a reflection grating, the row of light emitting elements may be part of a spatially periodic structure in which light emitting elements and photosensitive elements are arranged alternately. Next, a specific example of the present invention shown in FIGS. 4 and 5 will be described. This counterfeit is the member 1 as the second member.
4 and a linear reflective scale grating 15 fixedly mounted on the machined surface 16 of the backing material 17 as a first member. Member 14
is movable relative to the member 17 parallel to the plane of the grating 15 and perpendicular to the grating lines. The action of the guide screw 19 causes the section 140 to slide in the groove 18 formed in the member 17, with the result that the reading head 13 is moved relative to the grid 15, and the degree and direction of the relative movement between the sections 14, 17 is determined. Measurable. This relative motion may be made to match the motion of the component of the machine tool that is the object of control. Figure 5 is the 4th
2 is a perspective view of a portion of the read head 13 in the figure, in which a linear transmission index grating 20 is placed opposite the grating 15 on the read head 13 such that the spacing between the gratings 15, 20 is uniform; FIG.
is installed on.

Four identical units 21 are fixed to the back side of the grid 20 with a suitable adhesive. Each unit 21 has a solid state light emitter 22 and a solid state photodetector 23 shielded with synthetic resin. Further, a conducting wire is provided for supplying power to the light emitter 22 and for taking out an output signal from the photodetector 23. Light emitted from the light source 22 passes through the grating 20 and is reflected by the grating 15 so that an image is formed on the surface of the grating 20. Lattice 2
The reflected light that has passed through 0 reaches the photodetector 23. Each photodetector 23 basically receives the light emitted from the light source 22 in the unit 21 to which each detector 23 belongs, so the members 14, 17
When a relative movement occurs between them, the output of each photodetector 23 changes periodically. The grating 20 is provided in the reading head 13 with its lines slightly oblique to the lines of the grating 15, so that the image formed by the grating 15 is aligned with the grating 20.
When interacting with 0, this image produces moiré fringes. Four photodetectors 23 span a single moire fringe, and the member 1
Four photodetectors 2 caused by the relative movement between 4 and 17
The unit 21 is installed on the grating 20 so that the phase of the periodic change in the detection output of No. 3 changes sequentially by 90 degrees.

FIG. 6 illustrates one method of using the output of photodetector 23 to determine the direction and magnitude of relative motion between members 14 and 17.

In the circuit of FIG. 6, the output of the photodetector 23 is amplified by a matched amplifier 24, and the amplified outputs of the first and third photodetectors 23 are subtracted by a differential circuit 25 in phase order. The output of circuit 25 enters a Schmitt trigger 26 for square waves which generates signal A. Second and fourth photodetector 2
The amplified output of No. 3 is subtracted by a differential circuit 27, and the output of the circuit 27 is converted into a rectangular wave by a Schmitt trigger 28 which outputs a signal B. The changes in the magnitudes of the signals A and 8 that occur as a result of the relative movement between the materials 14 and 17 have a phase relationship that is shifted by 90 degrees. Signals A and B are applied to a pair of JK flip-flops 29, 30'. Signal A is applied to clock input 31 of flip-flop 29 and reset input 32 of flip-flop 30, and signal B is applied to flip-flop 2.
9 and a clock input 34 of flip-flop 30. Each flip-flop 29.3
The 0 J and K inputs are connected to a logic 1 maintained terminal 35. The Q output of flip-flop 29 goes into the UP input 36 of bidirectional counter 37, and the Q output of flip-flop 30 goes into the DOWN input 38 of counter 37.The output of counter 37 is connected to a suitable character and numeric display device. 3
Displayed by 9. Flip-flop 29 using the above method
and 3 signals A and B, there is only one flip for one direction of relative movement between members 14, 17.

The chip may provide an output to counter 37. This is because while the other flip-flop receives a signal at its clock input, a signal appears at its reset input, thus suppressing changes in its Q output. Which of the flip-flops 29, 30 provides an output depends on the direction of the phase difference between the signals A and B, which is determined by the direction of relative motion between the backings 14 and 17. The number of pulses sent to the counter 37 by a suitable flip-flop is of course proportional to the degree of relative movement. In the apparatus shown in FIGS. 4 and 5, the spatial frequencies of gratings 15 and 20 and their spacing are of course determined by the equation {1
} and ■, or formula '4} and ■. For example, when using a gallium arsenide infrared light emitting diode with a peak wavelength of 940 nanometers as the light source 22 and using an N1P-N silicon phototransistor as the photodetector 23, in the case of an interference image (corresponding to the second invention) In the case of an image (an example corresponding to the first invention), gratings 15 and 20 typically have a spatial frequency of 100 lines/rib and are spaced apart by 2 arcs. A suitable arrangement is such that the spatial frequencies of the gratings 15 and 20 are 100 lines/arc and 50 lines/arc, and the interval between the gratings 15 and 20 is increased by 2. As described above, the relative displacement measuring device of the present invention is provided with an irradiation means that illuminates the second grating with uncollimated light, so that the second grating defines a spatially periodic optical object. The second grating and the illumination means are arranged in such a way that the second grating emits diffuse light as an optical object, and in the direction of the relative movement of the first and second members of said optical object. Because the first and second gratings are configured such that the first grating forms a real image that is spatially periodic and substantially uniformly spaced from the first grating, Diffuse light from the second grating, acting as a periodic optical object, can be imaged as a real image by the first grating.

In the relative displacement measuring device of the present invention, since the second grating functions as an optical object that emits diffuse light and the first grating functions as an imaging means, the relative displacement of the first member with respect to the second member is The displacement may be converted into a magnified displacement of the image relative to the second member.

Moreover, in the relative displacement measuring device of the present invention, the light detection means has a spatially periodic structure in the movement direction provided at a fixed position with respect to the second member, and forms a real image. Being configured to receive light, it is able to output said magnified mound displacement in the form of a periodically varying output, allowing for highly accurate relative displacement measurements.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is an explanatory diagram showing the basic principle of the present invention, FIGS. 2 and 3 are explanatory diagrams showing modifications of the arrangement in FIG. 1, respectively.
FIG. 4 is a schematic perspective view of the device according to the present invention, and FIG.
FIG. 6 is a schematic perspective view of a part of the device shown in the figure, and FIG. 6 is a circuit diagram schematically showing an electric circuit used in the device shown in FIG. 1, 22... Light source, 2, 10... Lens, 3, 4, 5, 7, 15, 20... Grid, 6
, 23...photodetector, 8,9,11...mirror, 12,13...ta discussion head. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Mountain boat

Claims (1)

[Scope of Claims] 1. A device for measuring the relative displacement of a first member with respect to a second member in a manner corresponding to movement with one degree of freedom in one plane, the device comprising: uniformly measuring a distance u; spaced apart from each other in a fixed position relative to the first and second members, each having a frequency f_1 and f_2 (with f_1) in the direction of movement.
> f_2) with lines spatially periodic and uniformly spaced apart from the first grating by a distance v defined by f_2/f_2=V/(u+v). However, the frequency f_3 defined by f_3/f_1=u/(u+v) is spatially periodic in the above direction, and when the movement in the above manner occurs between the first and second members, the second In order to produce, by the first grating, a real image of the shadow of the second grating as an optical object, moving in said manner relative to the member, an uncollimated lens of a comparably higher frequency than f_1 and f_2 is used. illumination means for illuminating the second grating with light, the illumination means being spatially periodic at a frequency of substantially f_3 so as to produce a periodic variation in the output when relative motion occurs between the first and second members; a relative displacement measuring device having a structure mounted in a fixed position relative to the second member substantially v away from the first grating, and photodetector means responsive to said image. 2. A device for measuring the relative displacement of a first member with respect to a second member in a manner corresponding to movement with one degree of freedom in one plane, wherein each of the first members is uniformly separated by a distance u. and a second member, the frequency f_1 and f_2 (only 2f_1) are respectively provided in the direction of movement.
>f2) with spatially periodic lines and uniformly spaced apart from the first grating by a distance V defined by f_2/f_1=2V/(u+V). However, the frequency F_3 defined by F_3/f_1=2u/(u+V) is spatially periodic in the direction, and when the movement in the above manner occurs between the first and second members, the second f_
1 and f_2, and when the pitch of the first grating is w, the maximum wavelength λm is u≧w^2/
illumination means for illuminating the second grating with uncollimated light defined by 2λm; a structure spatially periodic in frequency and located in a fixed position relative to the second member at a distance of substantially V from the first grating; and photodetection means responsive to said image. Relative displacement measuring device.