JP2861804B2 - Length measuring device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to an optical length measuring device that detects a change in interference fringes caused by the relative movement of two gratings and measures the amount of relative movement of each grating.

[0002]

2. Description of the Related Art For example, a length measuring device using an optical grating is known as a device for reading a moving amount of a processing table of a machine tool or the like with high accuracy.

FIGS. 2 to 4 are views for explaining the principle of such an optical length measuring device. As shown in FIG. 2, the main scale 10 is an elongated rectangular plate-shaped glass substrate. On its surface, a fine grid 11 is formed in which metal films are arranged at the same pitch in a direction perpendicular to the longitudinal direction. The index scale 12 is a rectangular plate-shaped glass substrate smaller than the main scale 10.
On its surface, a lattice 13 made of a metal film and having the same pitch as the lattice 11 of the main scale 10 is formed. When the index scale 12 is inclined to face the main scale 10 in parallel, a bright and dark stripe pattern is formed in a direction substantially orthogonal to the direction of the grid 11. This is generally called Moiré fringes. When the index scale 12 is moved along the longitudinal direction of the main scale 10 in FIG. 2, the moiré fringes move in the vertical direction in the drawing orthogonal to the moving direction of the index scale 12.

[0004] As shown in FIG. 3 in an enlarged manner, moire fringes generally occur when two lattices overlap, and the locus of the intersection of the two lattices becomes a dark portion of the stripe pattern. Assuming that the lattice spacing is P, the inclination angle of the two lattices is θ, and the spacing between the stripes is W, the following equation holds. W = (Pcos (θ / 2)) / sinθ (1)

In equation (1), when θ is sufficiently small, the following equation is established. W = P / θ (θ: radian) (2)

As can be seen from equation (2), the interval W between moire fringes is obtained by optically multiplying the lattice interval P by 1 / θ. This means that when the grating moves by P in the longitudinal direction of the main scale, the moire fringes move by W in the vertical direction perpendicular thereto. By reading the enlarged change in W, the amount of movement of the grating can be known.

FIG. 4 shows an example of the structure of a length measuring device for reading the movement of moire fringes. On the opposite side of the index scale 12 with respect to the main scale 10, there is a light source 14. Further, a detection unit 15 is provided on the opposite side of the main scale 10 with respect to the index scale 12. The light source 14, the index scale 12, and the detection unit 15 share the common base 1.
6 and moves relatively to the main scale 10.

The detecting section 15 has two photoelectric conversion elements for measuring the amount of movement. The two photoelectric conversion elements are arranged along a direction orthogonal to the longitudinal direction of the main scale 10, and detect the bright and dark portions of the moire fringes shown in FIGS. 2 and 3, respectively. The detection part is si
An n-wave signal is output, but the signals output from the two photoelectric conversion elements are 90 degrees out of phase with each other. The signal of one photoelectric conversion element is called an A phase (sine wave), and the signal of the other photoelectric conversion element is called a B phase (cos wave). If both signals are obtained, the relative movement direction of both scales can be detected together with the amount of movement of the moire fringes.

According to the above-described conventional optical length measuring device, since the mounting position of the length measuring device is a processing table of a machine tool or the like, the thickness (in the short side direction of the main scale) is larger than the thickness of the processing table. Thickness) cannot be increased,
It is desired to be formed as thin as possible. Also,
If a plurality of detectors are provided, a stable detection waveform can be obtained. However, in a conventional length measuring device, a grid of a main scale is provided to extend in a short side direction, and an index is provided at an acute angle to the grid. A scale grid is provided. Therefore, the moiré fringes are formed so as to move substantially in the short side direction of the main scale.

[0010]

Therefore, according to the conventional optical length measuring device, if a plurality of detecting portions are provided, the thickness of the scale is inevitably increased. However, there is a problem that it is difficult to secure a mounting space. In order to secure this space, usually, two
Since the light and dark of the moire fringe is detected by one detection unit composed of a set of photoelectric conversion elements, the detection waveform is greatly affected by a lattice defect of the scale or dust attached to the scale, and the accuracy of the length measurement decreases. There was a problem of doing.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical length measuring device whose measurement accuracy is not easily affected by the attachment of lattice defects or dust on the scale.

[0012]

According to a first aspect of the present invention, there is provided a length measuring apparatus comprising: a main scale having a fine grid over at least the entire length to be measured; and a grid of the main scale having a shorter length than the main scale. An index scale having a fine grid positioned at an angle with respect to a detector, and a detector for outputting two signals having a phase difference of 90 degrees by detecting light and dark fringes generated by the relative movement of the main scale and the index scale. In the length measuring device having, the light and dark Moire fringes substantially perpendicular to the longitudinal direction of the main scale and arranged along the longitudinal direction, the moving direction of the index scale by the relative movement of the main scale and the index scale To move in parallel with the main scale and index scale. The grid of the main scale is tilted at an acute angle close to parallel, and the grid of the index scale is tilted at a predetermined angle with respect to the grid of the main scale, and a plurality of detectors are arranged in parallel along the moving direction. It is characterized in that a plurality of sets of signals having the same phase are output.

[0013]

The moire fringes move along the relative movement direction of the main scale and the index scale. The plurality of detectors arranged in the same direction detect a plurality of sets of detection signals including the A-phase signal and the B-phase signal at a plurality of detection positions.

[0014]

1A, 1B and 1C show an embodiment of the present invention.
This will be described with reference to FIG. The length measuring device of the present embodiment is different from the conventional one in the lattices formed on the main scale and the index scale, and the number and arrangement of the detection units. The other structure, for example, the mechanical structure shown in FIG. 4 is substantially the same as the conventional one.

As shown in FIG. 1A, the main scale 1 is an elongated rectangular plate-shaped glass substrate. On its surface, a grating 2 intersecting the longitudinal direction at an acute angle θ has a pitch P
It is formed with. The interval W between the adjacent gratings 2 and 2 reaching one side parallel to the longitudinal direction of the main scale 1 is naturally larger than the pitch P of the grating 2 and W = P / sin θ.

As shown in FIG. 1B, the index scale 3 is a rectangular plate-shaped glass substrate smaller than the main scale 1. On its surface, a grating 4 having the same pitch P as the grating 2 of the main scale 1 is formed.

The main scale 1 and the index scale 3 are faced in parallel at a predetermined interval so that the longitudinal direction of the main scale 1 and the grid 4 of the index scale 3 are parallel. As shown in FIG. 1C, bright and dark stripes (moiré stripes) extend in a direction perpendicular to the direction of the grid 4 of the index scale 3 or the longitudinal direction of the main scale 1.

In FIG. 1, when the index scale 3 is moved in the longitudinal direction of the main scale 1 (that is, in the direction parallel to the grid 4 of the index scale 3), the moire fringes are parallel to the moving direction of the index scale 3. Move in the middle left and right directions.

[0019]

The longitudinal direction of the main scale 1, the direction of the grating 4 of the index scale 3, or both scales 1,
A plurality of detectors 5 are arranged along the direction of relative movement 3. The detection unit of this embodiment includes a detection unit 5a including two photoelectric conversion elements A and B that output an A-phase signal and a B-phase signal whose phases are different from each other by 90 degrees, and outputs an inverted A-phase signal and an inverted B-phase signal. The detection unit 5b includes two photoelectric conversion elements a and b for outputting. As shown in FIG. 1C, in this embodiment, three detection units 5a and five detection units 5b are alternately arranged in a row so as to correspond to the light and dark of the moire fringes.

If the main scale 1 and the index scale 3 move relatively, the brightness of the moire fringes moves along the moving direction. The plurality of detectors 5 arranged along the same direction generate a detection signal including an A-phase signal and a B-phase signal, and a detection signal including an inverted A-phase signal and an inverted B-phase signal.
Detection is performed at each of a plurality of detection positions.

Even if the main scale 1 and the index scale 3 are partially damaged or have dust attached thereto, the length measuring apparatus of the present embodiment processes these plural detection signals to process both scales. Relative movement direction and the movement amount thereof can be stably detected with high accuracy.

In the above embodiment, the index scale 3
The movement direction coincides with that of the lattice 4, but this need not always be the case. The grid 2 of the main scale 1 is inclined at an acute angle with respect to the relative movement direction of the main scale 1 and the index scale 3, and the main scale 1 and the index scale 3 cause light and dark along the longitudinal direction of the main scale 1. Moire fringes in which the fringes are arranged may be formed. When the scales 1 and 3 are configured as described above, the plurality of detection units 5 (5a and 5b) can be arranged along the relative movement direction of the scales 1 and 3, so that a stable detection signal is obtained as described above. Therefore, many detectors can be arranged without increasing the size of the entire apparatus.

[0024]

According to the length measuring apparatus of the present invention, in an optical length measuring apparatus utilizing moiré fringes, the moiré fringes are moved in the same direction as the relative movement direction of both scales, and the detecting unit is moved in the same direction. , The number of detectors can be increased without increasing the overall size of the apparatus (especially, the dimension in the short side direction of the main scale). There is an effect that it can be reduced.

[Brief description of the drawings]

1A is a main scale 1 according to an embodiment of the present invention, FIG. 1B is an index scale 3 according to the embodiment, and FIG. 1C is a moire fringe obtained in the embodiment and an arrangement of a detection unit for the moire fringe. It is a figure showing an example.

FIG. 2 is a front view and a side view showing a schematic structure of a conventional length measuring device.

FIG. 3 is a diagram showing moire fringes obtained in a conventional length measuring device.

FIG. 4 is a side view showing a more specific configuration of a conventional length measuring device.

[Explanation of symbols]

 Reference Signs List 1 main scale 2 main scale grid 3 index scale 4 index scale grid 5, 5a, 5b detection unit

Claims (1)

(57) [Claims] 1. A main scale having a fine grid over at least the entire length to be measured, and an index scale having a fine grid shorter in length than the main scale and inclined with respect to the grid of the main scale. A detection unit for detecting light and dark fringes generated by relative movement of the main scale and the index scale and outputting two signals having phases different from each other by 90 degrees. The main scale and the index scale are moved relative to each other so that the light and dark moiré fringes arranged orthogonally and along the same longitudinal direction move parallel to the moving direction of the index scale by the relative movement of the main scale and the index scale. The main-scale grid has an acute angle close to Grid of the index scale with inclined Te is inclined at a predetermined angle to the grating of the main scale, that the detection unit along the direction of movement of a plurality of sets output signals of a plurality juxtaposed in the same phase Characteristic length measuring device.