Linear Encoder

The present invention relates to a linear encoder for measuring a relative position of two objects displaceable relative to each other and including a scale for being secured on one of the objects and formed of a graduation carrying measuring standard and a measuring standard support, and a device for scanning the scale and securable on another of the objects, with the measuring standard support having two regions, which are spaced from each other in a direction of a longitudinal extent of the measuring standard for supporting the measuring standard.

German publication DE-05 25 05 587 discloses attachment of a measuring standard to its support by an elastic glue layer to prevent thermal tensions in the measuring standard. It was also suggested to use instead of a continuous elastic adhesive layer, spaced from each other resin pieces to which the measuring standard is secured and which should provide for compensation of thermal tension between the measuring standard and its support. Using of spaced resin pieces is disclosed in German patent No. 1,176,382, in German publication DE-OS 33 12 534, and in U.S. Pat. No. 4,569,137. However, when resin pieces are used for attachment of the measuring standard to the support, rather large residual forces remain which unacceptably deform the measuring standard. A drawback of this method consists in that after a temperature compensation, the measuring standard does not return to its initial position, but, as a result of friction forces, occupies a new undetermined position. As a result, the longlasting constrained forces cause deformations noticeable as some kind of ripples on the glass of the measuring standard, which lead to an erroneous measurement of a relative position of two objects measured in a longitudinal direction of the measuring standard.

German document DE-OS 34 20 734 discloses mounting of an encoder so that the encoder housing is fixedly attached to a machine at one of its end, with the opposite end being elastically secured with a deformable element.

European publication EP 0 264 801 discloses securing a scale or a ruler along a small region with rollable balls which provide for retaining the scale or the ruler in a fixed position with respect to its mounting. The costs of providing such a support of a measuring standard are very high.

It is further known to support a scale at two areas so that the scale has a minimal deflection. The exact position of these areas depends on the allowable deflection (e.g., dependent on whether the graduation is provided on the outer surface or in the neutral area of the scale). Also, a reduction of the total length (Bessel points), or a minimal deflection along the entire length at parallel end surfaces (Airy points), or a disappearing deflection in the middle can be optimized.

An object of the invention is a linear encoder in which the constraining forces acting on the measuring standard are minimized as much as possible, and thermally induced tensions between the measuring standard and its support are eliminated.

This and other objects of the invention, which will become apparent hereinafter, are achieved by providing a linear encoder in which one of the supporting regions for the measuring standard forms a stationary support area and another of the supporting regions forms an elastic support area, and the stationary and elastic support areas are formed as substantially linear support areas extending transverse to the longitudinal extent of the measuring standard, and in which the linear support areas comprises one of continuous support areas and interrupted support areas.

With the attachment of the measuring standard to its support according to the present invention, they are mechanically decoupled from each other at the greatest possible extent. The measuring standard can, with changing temperatures, move effortlessly relative to its support, so that no thermally induced deformations of the measuring standard occur.

The features and objects of the present invention will become more apparent, and the invention itself will be best understood from the following detailed description of the preferred embodiment when read with reference to the accompanying drawings, wherein:

FIG. 1 shows a partially cross-sectional view of a linear encoder according to the present invention;

FIG. 2 shows a plan view of a scale of the linear encoder shown in FIG. 1;

FIG. 3 shows a plan view of a measuring standard support of the linear encoder shown in FIG. 1;

FIG. 4 shows a cross-sectional view along line IV--IV in FIG. 3; and

FIG. 5 shows a plan view of a frame-shaped measuring standard support.

FIG. 1 shows a basic view of a linear encoder 1, which serves for measuring of a relative position of two objects, e.g., a bed and a slide of a machine. The first object, the slide, is designated in FIG. 1 with a reference numeral 2 and the second object, the bed, is designated with a reference numeral 3. A scanning device 4 for scanning a scale 5 is supported on the bed 3. The scanning device 4 is connected with an evaluation unit 4a. The scale 5 is secured to the slide 2.

The scale 5 is formed of a measuring standard 6 carrying a graduation 7, which is shown in FIG. 2. The measuring standard 6 is secured to a support 8. The support 8 has a web strap 9 and a recess 10. The web 9 and the recess 10 are spaced from each other a predetermined distance in a direction of a longitudinal extent of the measuring standard 6 and the support 8.

The web 9 and the recess 10 provide for securing of the measuring standard 6 on the support 8. The securing is effected by gluing the measuring standard 6 to the support 8.

The bonding compound 11 secures the measuring standard 6 on the support 8 at the web 9 in a predetermined position and with a predetermined width of the gluing area. As the bonding compound 11, an epoxy resin can be used.

The measuring standard 6 is also secured to the support 8 at the recess 10 with an elastic bonding compound 12, e.g., a silicone glue. The glue joint at the recess 10 is relatively thicker than that at the web g. The measuring standard 6 is elastically secured to the support 8 at the recess 10 due to the greater elasticity of the bonding compound 12.

The two glue joint form a substantially linear support areas 13 and 14 which support the measuring standard 6 in suitable regions. The distance a between the support areas 13 and 14 is optimally selected with regard to the length of the measuring standard in accordance with the Norms for support of bar-shaped bodies which are generally known from the linear measuring technique.

By the above-described attachment of the measuring standard 6 to the support 8, a predetermined thermal behavior for the combination of the measuring standard 6 and the support 8 is achieved.

For maintaining this predetermined thermal behavior of the combination measuring standard 6 and the support 8, which together form the scale 5, the support 8 has two regions 15 and 16, having a predetermined surface area, at which the support 8 is secured to the slide 2. The distance b of between the areas 15 and 16 in the direction of the longitudinal extent of the measuring standard 6 and the support 8 approximately corresponds to the distance a between the areas 13 and 14 at which the measuring standard 6 is secured to the support 8.

With regard to the expansion behavior, it is advantageous when one of the connection regions, e.g., region 15 forms a neutral point (or line), and this point, transverse to the longitudinal extent of the scale 5, forms together with the fixed area 13 a neutral line with respect to the thermal behavior. At this neutral point, the scale 5 is fixedly connected with slide 2 by a screw (not shown).

The surface area 16 is formed as a length compensating element 17 for compensating a thermally induced expansion. This compensating element 17 can be formed as a spring parallelogram incorporated into the support 8, as shown in FIGS. 2, 3 and 5.

Of course, the length compensating element can be formed in another way, e.g., by a spring-biased slot screw joint extending transverse to the longitudinal extent of the scale 5 in the vicinity of the glued area 12.

To minimize the danger of wrapping of the support 8 as a result of an uneven support surface, with which the support 8 is attached to the slide 2, the backside of the support 8 can be made thicker in the surface areas 15 and 16. The cross-section of the support 8 is not limited to a rectangular shape, but can also have a U-, L-, T-, I- and etc. shape.

In case the support 8 is formed as a frame 18, as shown in FIG. 5, and beneath the graduation 7 of the measuring standard 6, has a break-through 19, so called transmitting light scales can be used.

In this case, the measuring standard supporting surfaces should be interrupted transverse to the longitudinal extent of the support 8 by the break-through 19. In this case, surface areas 20, 21, 22 and 23, which can be seen in FIG. 5, should be respectively, substantially aligned on opposite sides of opposite longitudinal sides of the break-through 19.

To provide a scale 5 according to the present invention, advantageously, the support 8 is formed of a material having a coefficient of thermal expansion similar to that of a material from which the measuring standard 6 is formed. E.g., in case of the measuring standard being made of ZERODUR.TM., INVAR.TM., an alloy steel with a similarly small coefficient of thermal expansion, is used for forming the support 8.

Though the present invention was shown and described with reference to the preferred embodiments, various modifications thereof will be apparent to those skilled in the art and, therefore, it is not intended that the invention be limited to the disclosed embodiments or details thereof, and departure can be made therefrom within the spirit and scope of the appended claims.