GB2153995A

GB2153995A - Coordinate measuring instrument

Info

Publication number: GB2153995A
Application number: GB08501708A
Authority: GB
Inventors: Hiroshi Hanaoka; Sadayuki Matsumiya
Original assignee: Mitutoyo Manufacturing Co Ltd
Current assignee: Mitutoyo Manufacturing Co Ltd
Priority date: 1984-02-02
Filing date: 1985-01-23
Publication date: 1985-08-29
Also published as: GB8501708D0; GB2153995B; JPS60161523A; DE3503116C2; DE3503116A1

Abstract

The instrument incorporates for each coordinate direction, relatively movable portions A and B and an optical type displacement detecting device 10 for measuring the magnitude of the movement between the portions. Each device 10 includes a main scale 11 having a grating or fringes 15 formed by reflecting portions 15A and non-reflecting portions 15B arranged alternately in a row, an index scale 12 having a grating or fringes 16 formed by light transmitting portions 16A and non-light transmitting portions 16B also arranged alternately in a row, a light emitting element 17 for illuminating the index scale 12 and the main scale 11, and a light receiving element which receives light reflected by the main scale through the index scale and emits an electric signal corresponding to the magnitude of a relative displacement between the scales. The main scale is provided with an optical disturbance preventing layer 22 which either absorbs light transmitted through the non- reflecting portions of the main scale to prevent the light being reflected to the index scale or makes the light which is transmitted through the non-reflecting portions of the main scale on to the index scale, of pre-determined properties. The layer 22 may be sandwiched between the main scale 11, which is of glass and another glass strip 21. <IMAGE>

Description

SPECIFICATION Measuring instrument This invention relates to measuring instruments, and is particularly concerned with optical type displacement detecting devices of such instruments.

Such measuring instruments have been widely used in the industrial field in a form in which a measuring element is supported in a manner such that it is movable in directions of three axes which perpendicularly intersect one another. In use, the measuring element is brought into contact with successive points on a portion of an object to be measured, and displacement values of the measuring element in the directions of the three axes are detected by displacement detecting devices provided to measure displacements in the directions of the three axes. In this way the dimensions and the shape of the portion of the object are measured.

In measuring instruments such as this, the performance of the displacement detecting devices as well as the mechanical construction of the instrument exert a great influence on the measuring accuracy, with the result that various detecting devices, such as an optical type and an elctromagnetic type, are used with the measuring instruments in accordance with the purposes for which the instrument is intended.When displacement detecting devices of the optical type are adopted, mainly due to spatial restrictions, long main scale is generally secured to a stationary part of the measuring instrument, a short index scale is mounted on a movable part, which is displaced with the measuring element, and a socalled reflection type optical displacement detecting device is generally arranged in such a manner that the device can move relative to both scales and is provided thereon with a light emitting element and a light receiving element on the side of the index scale.

In adopting reflecting type displacement detecting devices, heretofore there have been many cases where metallic main scales are used. However, in the case of metallic main scales, measurement with very high accuracy has been required from the measuring instruments, and, recently, the necessity has been voiced for still larger sized instruments. This presents the following disadvantages: (1) It is very difficult to finish parallel and smooth flat surfaces with high accuracies whereby optical gratings or fringes may be effected. Particularly difficulties arise when the main scale is long; (2) When the main scale is fixed by spot welding, distortions may be easily caused to the main scale.The distortions are most appreciable when the main scale is long; (3) When the main scale is mounted on a ceramic surface plate or the like, the main scale may suffer from a distortion with age because the metal and the ceramic material are greatiy different in their coefficients of thermal expansion; (4) The gratings or fringes are formed by an etching method, whereby side etches become large. The extent of the side etches becomes not negligible from the viewpoint of accuracy.

These disadvantages have been overcome by making the main scale of glass. However, in this case, the following new problems have arisen:- In reflection type devices, a change in the intensity or other properties of the reflected light obtained by the reflection of the grating or fringes of the main scale is detected by a light receiving element. However, when the main scale is made of glass, the light transmitted through non-reflecting portions of the grating or fringes has been reflected by a substance on the rear surface of the main scale (on the side remote from the index scale) for example, and this reflected light has been received by the light receiving element.The influence of the reflection of the light transmitted through the above-mentioned nonreflecting portions (which is called a disturbance) is greatly influenced by the condition of the rear surface of the main scale, that is the optical properties of the reflecting surface when the light transmitted through the nonreflecting portion is reflection. As a result, an S/N ratio fluctuates in various ways, thus giving an adverse influence to the measuring accuracy.

In co-ordinate measuring instruments. usually main scales of displacement detecting devices are mounted such that a main scale for detecting movements in the direction of the X-axis is mounted on a guide member made of stainless steel, another main scale for detecting movements in the direction of the Yaxis is mounted on a stone or ceramic surface plate, and a further main scale for detecting movements in the direction of the Z-axis is mounted on a spindle made of some other material. In consequence, when main scales made of glass are adopted, even if displacement detecting devices identical with one another are mounted in one and the same coordinate measuring instrument, the reflected light varies in its intensities and modes one from another in the different devices owing to the differences in optical properties of the parts on which the main scales are mounted.

It therefore becomes necessary for the light receiving elements to be individually adjusted.

There are some cases where the optical properties of the parts on which the main scales are mounted may change with age. In this case, even if the individual adjustments are made, lowered accuracies cannot be avoided over a period of time. Moreover, there are many cases where it is unclear what adjustments should be provided to allow for the optical properties of the parts on which the main scales are mounted. In this case it is impossible to make the adjustments.

The object of the present invention is to provide a measuring instrument which includes two or more optical type displacement detecting devices and which is of high measuring accuracy, and particularly, even when the instrument as a whole is very large in size, can still be of high measuring accuracy.

To this end, according to the present invention, the displacement detecting devices of such an instrument each includes: a main scale having a grating or fringes formed by reflecting portions and non-reflecting portions arranged alternately in a row; an index scale having a grating or fringes formed by light transmitting portions and nonlight transmitting portions arranged alternately in a row; the index scale being movable relative to the main scale in a direction along the main scale; a light emitting element provided on one side of the index scale, for illuminating the index scale and the main scale; and a light receiving element which receives light reflected by the main scale through the index scale and emits an electric signal corresponding to the magnitude of relative displacement between the scales; wherein the main scales of the detecting devices are made of a light transmitting material and are each provided with an optical disturbance preventing layer which absorbs light transmitted through the non-reflecting portions of the main scales to prevent the light from being reflected to the index scales or reflects only light of predetermined properties to and through the index scale.

When the light transmitted through the non-reflecting portions of the main scales is absorbed so that it does affect the light receiving element, individual adjustment of the receiving element can be dispensed with, or, even when the light transmitted through the non-reflecting portions is reflected and received by the light receiving element, because the light is made to be of pre-determined properties at all times, the adjustment of the light receiving element is made on a predetermined level at all times. Thus the fluctuation of the reflected light due to the differences in the optical properties between the parts on which the main scales are mounted, can be prevented from occurring.

Several examples of measuring instruments in accordance with the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a perspective view showing the general arrangement of one example of the instrument; Figure 2 is a perspective view to a much larger scale showing the internal construction of an optical type displacement detecting device in the instrument shown in Fig. 1; Figure 3 is a sectional view showing a main scale and an index scale in the optical type displacement detecting device shown in Fig.

2; and, Figures 4 to 6 are sectional views similar to Fig. 3, but showing the main scale and the index scale in other examples of optical type displacement detecting devices.

Description will hereunder be given of embodiments of the present invention with reference to the drawings.

Fig. 1 shows one embodiment of the measuring instrument according to the present invention. This measuring instrument is provided on the top face of a surface plate 1 as being a base thereof with a gate-shaped frame 2 movable back and forth (a direction of the Y-axis). along a horizontal beam 3 of the gateshaped frame 2 with a slider 4 movable in a lateral direction (a direction of the X-axis). and in a slider 4 with a probe shaft 6 having a measuring element 5 and movable in a vertical direction (a direction of the X-axis). In short, the measuring element 5 is provided in a manner to be movable in the directions of the three axes (three-dimensional directions).

Furthermore, there are respectively provided optical type displacement detecting devices 10 between the surface plate 1 and the gateshaped frame 2 as a Y-axis direction's position detecting device for detecting a displacement value of the gate-shaped frame 2 in the direction of the Y-axis, between the horizontal beam 3 and the slider 4 as an X-axis direction's position detecting device 10 for detecting a displacement value of the slider in the direction of the X-axis, and between the slider 4 and the probe shaft 6 as a Z-axis direction's position detecting device 10 for detecting a displacement value of the probe shaft 6 in the direction of the Z-axis. Displacement values of the measuring element 5 in the directions of the X, Y and Z axes are automatically detected by these detecting devices 10 and respectively displayed on an indicator or the like, not shown.

Fig. 2 shows the internal construction of the optical type displacement detecting device 10.

Referring to the drawing, both of a long main scale 11 and a short index scale 1 2 are made of glass. Both the main scale 11 and the index scale 12, which are spaced a very small distance apart from each other, are disposed in the longitudinal direction of the main scale 11 in a manner to be movable relative to each other. Furthermore, the main scale 11 is fixed to a stationary member A, while, the index scale 1 2 is solidly secured to a movable member B. Herein, the surface plate 1 is the stationary member A and the gate-shaped frame 2 is the movable member B in the relation between the surface plate 1 and the gate-shaped frame 2. Furthermore, the hori zontal beam 3 of the gate-shaped frame 2 is the stationary member A and the slider 4 is the movable member B in the relation between the horizontal beam 3 and the slider 4.

Additionally, the slider 4 is the stationary member A and the probe shaft 6 is the movable member B in the relation between the slider 4 and the probe shaft 6.

Fringes 1 5 are depositedly formed on the front surface of the main scale 11 (the surface of the main scale on the side of the index scale) in the longitudinal direction of the main scale 11. These grating fringes 1 5 are constituted by reflecting portions 1 5A and nonreflecting portions (namely, portions where the reflecting portions 1 SA are not deposited) 1 SB, both of which have widths equal to each other and line up alternately.

On the other hand, fringes 1 6 are also formed on the index scale 12. These fringes 1 6 are constituted by light transmitting portions 1 6A and non-light transmitting portions 1 6B, both of which have pitches equal to each other. Additionally, two fringes 1 6 of the index scale 1 2 are provided in a manner to be shifted by 1/4 pitch, so that the directional property of the index scale 1 2 can be discriminated.

A light emitting element 1 7 and a light receiving element 1 8 are provided at predetermined positions on the side of the index scale 1 2 in the slider 4. A light irradiated from the light emitting element 1 7 at a predetermined angle to the fringes 1 6 of the index scale 1 2 is transmitted through the light transmitting portions 1 6A of the fringes 16, thereafter, reflected by the main scale 11, transmitted through the light transmitting portions 1 6A of the fringes 1 6 of the index scale 1 2 again, and adapted to be received by the light receiving element 18.The light receiving element 1 8 is connected thereto with a signal processing device 1 9 and an indicator 20, whereby a displacement value (displacement in quantity of light) of the reflected light is processed by the signal processing device 19, and thereafter, displayed on the indicator 20 as a relative displacement value between the both scales 11 and 12.

A glass plate 21 other than the main scale 11 is disposed close to the rear surface of the main scale 11 (on the side opposite to the index scale 12), and the main scale 11 is mounted to the stationary member A through this glass plate 21.

A disturbance preventing layer 22 is provided on the side surface of the glass plate 21 on the side of the main scale 11. This disturbance preventing layer 22 is formed of a film of a light absorbing material or a film of a light reflecting material, which is de-posited onto the side surface of the glass plate 21.In the case of being formed of the light absorbing material, the glass plate 21 absorbs the light transmitted through the non-reflecting portions 1 SB of the main scale 11 so as not to generate reflected light on the side of the index scale 1 2. While, in the case of being formed of the light reflecting material, the film 22 makes the light transmitted through the non-reflecting portions 1 SB to be the predetermined (not influenced by the material and the like on the side of the stationary member A) reflected light which is reflected to the index scale 12.

According to this embodiment as described above, the following advantages can be offered.

Works with high accuracy can be easily performed, the change with age is low and no distortion is generated in mounting, so that, even when the measuring instrument is largesized, no distortion is generated in the main scales 11, high precision measurement can be performed.

Furthermore, the disturbance preventing layer 22 is disposed on the rear surface (on the side opposite to the index scale 12) of the main scale 11, so that the light transmitted through the non-reflecting portions 1 SB of the fringes 1 5 of the main scale 11 is not reflected to the index scale 12, or is made to be the predetermined (of a known property) reflected light and reflected to the index scale 1 2. Because of this, no noises are generated at the light receiving element 18, or a predetermined quantitative correction is performed at the signal processing device 19, so that an accurate (high precision) displacement detection can be performed at all times.In other words, the measured value is not influenced by the optical property of the stationary member A as being the portions. to which the main scale 11 is mounted.

Further, it is only enough to mount the main scale 11 to the stationary member A through the glass plate 21 provided thereon with the disturbance preventing layer 22, so that the mounting can be easily performed.

Additionally, in working, the fringes 15 of the main scale 11 may be provided on the stationary member A (on the side opposite to the index scale 12) (Refer to Fig. 4). In this case, such an advantage is offered that, when the stationary member A is deformed, the influence exerted on the main scale 11 by the expansion or shrinkage of the stationary member A due to the deformation is low. Furthermore, the disturbance preventing layer 22 may be directly depositedly formed on the rear surface of the main scale 11 (Refer to Fig. 5). Additionally, after the disturbance preventing layer 22 is directly formed on the stationary member A, the main scale 11 may be mounted (Refer to Fig. 6). In these cases, the influence exerted on the expansion or shrinkage of the pitches of the fringes of the main scale 11 by the glass plate 21 is low.

Further, the disturbance preventing layer 22 is made of the black coating film, the light absorbing material such as synthetic resin, or the light reflecting material having a low degree of reflection, such as a metallic deposited film. In the case of using the metallic deposited film, it is preferable that the main scale 11 is closely attached to the portions to be mounted in order to make the disturbance preventing layer 22 have a uniform thickness.

Furthermore, the displacement detecting devices mounted in the directions of the respective axes need not necessarily be identical with one another.

As has been described hereinabove, the present invention can provide a measuring instrument high in the measuring accuracy, and particularly, even when the intrument as a whole is enlarged in size, still high in the measuring accuracy.

Claims

1. A measuring instrument including two or more optical type displacement detecting devices each of which includes: a main scale having a grating or fringes formed by reflecting portions and non-reflecting portions arranged alternately in a row; an index scale having a grating or fringes formed by light transmitting portions and nonlight transmitting portions arranged alternately in a row; a light emitting element provided on one side of the index scale for illuminating the index scale and the main scale; and a light receiving element which receives light reflected by the main scale through the index scale and emits an electric signal corresponding to the magnitude of a relative displacement between the scales; wherein the main scales of the detecting devices are made of a light transmitting material and are each provided with an optical disturbance preventing layer which absorbs light transmitted through the non-reflecting portions of the main scales to pevent the light from being reflected to the index scales or reflects only light of pre-determined properties to and through the index scale.

2. A measuring instrument according to Claim 1, in which there are three of the optical type displacement detecting devices arranged perpendicularly to one another, for detecting displacements in the directions of three mutually perpendicular axes and the disturbance preventing layers of the three devices are made of materials identical with one another.

3. A measuring instrument according to Claim 1 or Claim 2, wherein the disturbance preventing layers are each made of a film of a light absorbing material.

4. A measuring instrument according to Claim 1 or Claim 2, wherein the disturbance preventing layers are each made of a film of a light reflecting material.

5. A measuring instrument according to any one of the preceding Claims, wherein a glass plate is disposed on the side of the main scale remote from the index scale and the disturbance preventing layer is provided on the face of the glass plate which is directed towards the main scale.

6. A measuring instrument according to any one of the preceding Claims, in which the main scales are each made of glass.

7. A measuring instrument comprising: a frame which is movable in the direction of a Y axis on a surface plate; a slider which is movable on the frame in the direction of an X axis; a measuring element which is movable on the slider and in the direction of a Z axis, three optical type displacement detecting devices provided on portions movable relative to one another in the directions of the X, Y and Z axes and arranged perpendicularly to one another, each of the optical type displacement detecting devices including a main scale having a grating or fringes formed by reflecting portions and non-reflecting portions arranged alternately in a row, an index scale having a grating or fringes formed by light transmitting portions and non-light transmitting portions arranged alternately in a row, the index scale being movable relative to the main scale in a direction along the main scale, a light emitting element provided on the side of the index scale, for illuminating the index scale and the main scale, and a light receiving element which receives light reflected by the main scale through the index scale and emits an electric signal corresponding to the magnitude of a relative displacement between the scales; wherein the main scales are made of a light transmitting material and are mounted through glass plates: an optical disturbance preventing layer is provided between each of the glass plates and the main scales, which is mounted through it, and each of the disturbance preventing layers is formed of a film of a light absorbing material or a light reflecting material, so that the disturbance preventing layer either absorbs light transmitted through the non-reflecting portions of the main scale to prevent the light being reflected to the index scale or reflects the light transmitted through the non-reflecting portions, to make it reflected light of pre-determined properties, to and through the index scale.

8. A measuring instrument according to Claim 1, in which the displacement detecting devices are substantially as described with reference to Figs. 2 and 3 or any one of Figs.

4 to 6 of the accompanying drawings.

9. A measuring instrument according to Claim 8, substantially as described with reference to Fig. 1 of the accompanying drawings.