1. Field of the Invention
This invention relates to linear scale type displacement measuring devices, and more particularly, to improvements in a linear scale type displacement measuring device suitable for use in a linear scale type displacement measuring device provided with a comparatively short main scale, wherein the measuring device comprises a frame member connected to one of two bodies to be measured, between which a relative displacement is to be measured, a main scale held on the frame member and formed of a material different in coefficient of thermal expansion from the frame member and an index scale connected to the other of the two bodies and movable along the main scale, whereby a relative displacement between the two bodies is measured from a relative movement between the main scale and the index scale.
2. Description of the Prior Art
In general, there has heretofore been known a displacement measuring device for measuring length and the like of bodies, wherein, when a movement value between two bodies relatively movable to each other, such as a movement value of a measuring element relative to the main body of measuring device, a movement value of a slider relative to a column and the like, is to be measured, a frame member holding a main scale is provided on one hand and a detector including an index scale is affixed on the other hand, whereby a relative movement between the frame member and the detector is read by an optical method or an electromagnetic method, for example.
The above-described displacement measuring device, particularly, the linear scale type displacement measuring device provided with a transmission type displacement detector has been disadvantageous in that, since the frame member complicated in its configuration is formed of an aluminum extruded section in order to attain sealing property, non-combustibility, light weight and the like of the detecting portion, and the main scale held on the frame is made of glass, the frame member and the main scale are varied in value of thermal expansion from each other, when temperature is varied, and the main scale is deformed to cause lowered measuring accuracy, or the main scale is broken down in an extreme case.
In consequence, as shown in FIG. 1 for example, such a practice has heretofore been adopted that a main scale 10 is directly bondedly fixed to a frame member 14 by use of an elastic bonding agent 16 or the like in a state where two surfaces perpendicularly intersecting each other of the main scale 10 are urged against guide surfaces 14A and 14B of the frame member 14 by a rubber rods 12, to thereby elastically holding the main scale 10.
In FIG. 1, designated at 20 is a slider movable in the longitudinal direction of the main scale 10 in a manner to hold a predetermined positional relationship with the main scale 10 through the agency of slidable blocks 22 slidable on the surfaces of the main scale 10, 24 an index scale provided with vertical fringe graduations similar to those of the main scale 10 and affixed to a surface of the slider 20, which is opposed to a graduated surface 10A of the main scale 10, and 26 and 28 a light emitting element and a light receiving element both being affixed to the slider 20 in a state of interposing therebetween the main scale 10 and the index scale 24.
However, owing the manufacturing method, in an alumunum extruded section forming the frame member 14, there are irregularly generated a bend or bends, torsion, undulation and the like of length of about 0.03 mm to 300 mm for example, i.e. about 30 times of the graduation resolving-power of 1 micrometer for example. It is practically impossible to correct it and finish it with high accuracy as high as the main scale 10. Furthermore, in the use conditions, thermal deformations tend to occur depending on a change in temperature. In consequence, heretofore, there have been such problems that, even if the main scale 10 is finished at very high accuracy in order to attain a graduation resolving-power of about 1 micrometer for example, the main scale 10 is deformed, following a bend and the like of the frame member 14, when the main scale 10 is being bondedly fixed to the frame member 14, whereby the graduations are enlarged or contracted, thus resulting in considerably lowered measuring accuracy.
Now, when presumption is made on the effect of a bend of the main scale 10 on the measuring accuracy in the case where the main scale 10 is bent under the influence of the bend of the frame member 14, if graduations 10B are formed symmetrically with a neutral shaft A of the main scale 10 as shown in FIG. 2, then, when the bend on the main scale 10 is generated in the dirction of its height as indicated by arrows B, no error will be generated. However, since the graduated surface 10A where graduations 10B are formed normally does not conform to a neutral surface C of the main scale 10, when the bend on the main scale is generated in the direction of thickness of the main scale 10 as shown in FIG. 3, an error .epsilon. therebetween will be approximately represented by the follwing equation, if a radius of the bend is R, a virtual angle of a unit section, by which the error is evaluated, is .DELTA..theta., the length obtained before the bend is generated, i.e. the length of the neutral surface C is L, and the length of the graduated surface 10A is Lo. EQU .epsilon.=Lo-L (1) EQU Lo.div.(R+t/2) (2) EQU L.div.R.DELTA..theta. (3)
where t is the thickness of the main scale 10.
Now, a value of the largest deformation .delta. will be approximately represented by the following equation. ##EQU1##
In consequence, the aforesaid error .epsilon. will be given by the following equation by use of this .delta.. ##EQU2##
In consequence, when the length L of the error evaluating unit section of the main scale 10 is 300 mm, the thickness t is 5 mm and the largest deformation .delta. is 0.06 mm, the error .epsilon. becomes about 4 micrometers. Hence, in the case of a main scale having the total length of 900 mm, an error of 12 micrometers is generated, which will lead to a fatal defect with the linear scale type displacement measuring device, of which a measuring accuracy of 1 micrometer or less is required.
To obviate the above-described problem, it is conceivable that, in bondedly fixing the main scale 10 to the frame member 14, a test indicator, electric micrometer and the like is used, the electric micrometer or the like is brought into contact with the two surfaces of the main scale 10 for example, the accuracy is measured in the longitudinal direction, and a shim or shims are inserted between the main scale 10 and the frame member 14 in accordance with the irregularities formed on the main scale 10, whereby the positioning of the main scale 10 is carried out. However, this is disadvantageous in that the operation is very low in the efficiency, and moreover, it is considerably difficult in accurate positioning of the main scale 10.