Length measuring arrangement

A length measuring arrangement for measuring the relative position of two objects, the length measuring arrangement including a housing made of a non-magnetic material and longitudinally extending in a measuring direction. The length measuring arrangement including a scale in the housing, the scale having a measuring graduation, a first guide face and a second guide face which extends perpendicular to the first guide face. A mounting piece attachable to one of the two objects rigidly in the measuring direction and resiliently transversely to the measuring direction. The length measuring arrangement including a scanning carriage that scans the measuring graduation, wherein the scanning carriage is coupled to the mounting piece rigidly in the measuring direction and resiliently transversely to the measuring direction, and the scanning carriage is linearly guided in the measuring direction on the first and second guide faces and is pressed against the first and second guide faces. A pressure force is generated by a magnetic force between at least one first element of the scanning carriage and a second element provided on the housing.

RELATED APPLICATIONS

Applicant claims, under 35 U.S.C. § 119, the benefit of priority of the filing date of Jul. 19, 2007 of a German patent application, copy attached, Serial Number 10 2007 033 574.3, filed on the aforementioned date, the entire contents of which is incorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention relates to a length measuring arrangement

2. Background Information

Such length measuring arrangements, such as for example described in DE 28 10 341 C2, are employed for measuring lengths as well as distances, and are in particular used in processing machinery for measuring the relative movement of a tool in relation of a workpiece to be processed, in coordinate-measuring machinery, and increasingly also in the semiconductor industry.

Here, a scale with a measuring graduation, which is protected from environmental effects, is used as the measurement representation and is placed into a housing, which extends linearly in the measuring direction. For simple and cost-effective manufacture, the housing is an extruded profile made of a non-magnetic material, in particular aluminum. In the course of position measurement, the measuring graduation, which is incrementally or absolutely coded, is scanned by a scanning carriage. For this purpose, the scanning carriage is linearly conducted along two guide faces extending perpendicularly in relation to each other and is pressed against them. This guidance has proven itself, because it is assured that a constant scanning distance between the scanning carriage, in particular the scanning plate, and the scale is maintained over the entire measurement length, which guarantees the good quality of the scanning signal. The guidance of the scanning carriage is decoupled from the guidance of the object to be measured in that a mounting piece with a coupling is provided between the object to be measured and the scanning carriage, which coupling couples the scanning carriage rigidly to the mounting piece in the measuring direction and resiliently transversely to it. Pressing the scanning carriage against the two guide faces of the scale, which extend perpendicularly in relation to each other, i.e. the generation of the pressing force, takes place in the prior art by a spring arm (DE 23 49 944 A1), or by several pressure springs, wherein each of the guide feces is provided its own spring (DE 28 10 341 C2), whose pressure force is oriented toward the respective guide face. As a rule, these springs, which are oriented perpendicularly to each other, are arranged between the scanning carriage and the mounting piece, this leads to the distance between the scanning carriage and the mounting piece having an influence on the pressure forces. It is therefore necessary to select the installation tolerances to be particularly small.

The demands made on length measuring arrangements are continuously increasing, higher resolution, as well as greater accuracy and reproducibility of the position measurement is continuously being demanded. Along with this, a compact mechanical structure should be provided, and the length measuring arrangement should be produced in a cost-effective manner.

OBJECTS AND SUMMARY OF THE INVENTION

The present invention is based in part on the object of creating a length measuring arrangement having high measuring accuracy, which can be produced in a cost-effective manner.

In accordance with the present invention, this object is attained by a length measuring arrangement for measuring the relative position of two objects, the length measuring arrangement including a housing made of a non-magnetic material and longitudinally extending in a measuring direction. The length measuring arrangement including a scale in the housing, the scale having a measuring graduation, a first guide face and a second guide face which extends perpendicular to the first guide face. A mounting piece attachable to one of the two objects rigidly in the measuring direction. The length measuring arrangement including a scanning carriage that scans the measuring graduation, wherein the scanning carriage is coupled to the mounting piece rigidly in the measuring direction and resiliently transversely to the measuring direction, and the scanning carriage is linearly guided in the measuring direction on the first and second guide faces and is pressed against the first and second guide faces. A pressure force is generated by a magnetic force between at least one first element of the scanning carriage and a second element provided on the housing.

These requirements demand an encapsulated length measuring arrangement with a protectively housed scale. High resolution requires a continuously shorter scanning distance, which is constant over the entire measurement distance. This is achieved by guiding the scanning carriage on the scale, wherein, for undisturbed precise guidance, the scanning carriage is coupled to the mounting piece by a coupling which is rigid only in one direction. In all other directions this coupling makes possible a movement of the mounting piece without causing a reaction to the precise guidance and movement of the scanning carriage in the measuring direction. For a precise straight-ahead guidance, the scanning carriage is guided on two guide faces of the scale, which extend perpendicularly to each other, and is pressed against them, wherein the pressing force is provided by the magnetic force between at least one first element of the scanning carriage and a second element provided on the non-magnetic housing.

A compact construction of a length measuring arrangement is made possible by the present invention, and a high degree of measuring accuracy and a reproducible position measurement can also be achieved.

With the length measuring arrangement embodied in accordance with the present invention, the pressure force with which the scanning carriage is urged against the scale is independent from its installation. The distance between the scanning carriage and the mounting piece does not affect the pressing force.

The guidance of the scanning carriage on the scale guarantees an exact parallel guidance with respect to the measuring graduation, because the surface of the scale is precisely aligned with the course of the measuring graduation.

Further details and advantages of the present invention will be explained by the following description of exemplary embodiments of the position measuring arrangement in accordance with the present invention in connection with the drawing figures.

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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be presented by an example of an optical length measuring arrangement, with which it is intended to measure the relative position of two objects1and2, which can be displaced relative to each other in the measuring direction X. In the course of this a transparent scale20, in particular made of glass, is scanned by a scanning carriage10, which can be moved relative to the scale20in the measuring direction X. The scale20has a measuring graduation21, which is scanned in incident light by the scanning carriage. For this purpose, the scanning carriage10has an illumination unit11, which emits a beam of light, which passes through the scale20and at the end impinges on light-sensitive scanning sensors12on the scanning carriage10. In the process, the beam of light is modulated as a function of the position of the measuring graduation21on the scale20.

The scale20is arranged inside a housing22, which in turn is fastened on the object2to be measured, for example the machine bed of a machine tool. In this case the scale20is connected in a known manner with the housing22, for example by gluing or clamping. The housing22has a slit extending in its longitudinal direction in the measuring direction X which is closed by seeing lips which are inclined in a roof-shape and through which a mounting piece13extends with a sword-shaped center piece. The mounting piece13has a mounting area131, by which it can be attached to the object1, for example a carriage of the machine tool, which can be displaced relative to the machine bed2.

A coupling14is arranged between the scanning carriage10and the mounting piece13, which couples the scanning carriage10, rigidly in the measuring direction X and resiliently transversely to it, to the mounting piece13. Because of this measure, erroneous alignments of the mounting piece13are not transmitted to the scanning carriage10. The coupling14is only schematically represented. In a manner known per se, it is for example designed as a ball coupling in accordance with EP 0 733 882 B1, in which a ball made of a ferromagnetic material is clamped and maintained by a magnetic force between a face of the mounting piece13extending perpendicularly in relation to the measuring direction and a face of the scanning carriage10extending perpendicularly in relation to the measuring direction.

The scanning carriage10is guided along the scale20for exact parallel guidance on it. To this end, the scanning carriage10is supported via guide elements151to155on two faces201,202of the scale20, which are aligned perpendicularly to each other. One of these faces is the surface201supporting the measuring graduation21, and the other a narrow side202of the scale20extending perpendicularly to it The guide elements can be sliding elements, but in particular rollers or rolls151to155, seated on ball bearings.

The scanning carriage10is urged against the faces201,202of the scale20by the cooperation between magnetic elements161and261, as well as162and262. This pressing force is provided by a magnetic force between at least one first element161,162of the scanning carriage10and at least one second element261,262provided on the housing22. The term magnetic material defines a material which is ferromagnetic or is permanently magnetic. The first element in particular is a permanent magnet161,162, and the second element261,262is an element made of ferromagnetic material and attached to the housing22. The permanent magnets are in particular made of NdFeB.

The housing22is made of a non-magnetic material, it is an extruded profile made of aluminum in particular. This has the advantage that it is possible to form various functional elements on the profile22without additional efforts, for example supports or receptacles for the scale20, as well as receivers for the sealing lips for attachment to the object2. This possibility is utilized by the present invention in that grooves231,232are formed in the housing22for drawing in tapes261,262. The tapes261,262are made of ferromagnetic steel.

The scanning carriage10is supported on the face201by guide elements, in particular several rollers151,152,153seated in ball bearings. The pressure force Fz of the scanning carriage10on the face201is oriented perpendicularly to this face201and is generated by the mutual action of the magnet161fixed in the scanning carriage10and the steel tape261pushed into the groove231. The steel tape261extends parallel with the face201in the measuring direction X and is placed opposite the magnet161, namely at a distance of approximately 0.8 mm. The pressure force Fz is approximately 2.3 N.

Perpendicularly to this, the scanning carriage10is supported on the face202by the guide elements, in particular by several rollers154and155seated in ball bearings. The pressure force Fy of the scanning carriage10on this face202is oriented perpendicularly to this face202and is generated by the mutual action of the magnet162fixed in the scanning carriage10and the steel tape262pushed into the groove232. The steel tape262extends parallel with the face202in the measuring direction X and is placed opposite the magnet162, namely at a distance of approximately 0.8 mm. The pressure force Fy is approximately 1.8 N.

The two rollers154and155are arranged symmetrically with respect to the magnet162, and the two rollers151and153are arranged symmetrically with respect to the magnet161, wherein the roller152is arranged in the X-position of the magnet151, which corresponds to the sectional position represented inFIG. 1. The spatial arrangement of the rollers151to155, and the pressure forces Fy and Fz generated by the magnetic force, are laid out in such a way that the forces acting on the individual rollers151to155are approximately equal, in the example approximately 0.8 N. The pressure forces Fy, Fz generated by the magnetic force are oriented and dimensioned in such a way that the force Fr resulting therefrom extends at least approximately through the center of gravity S of the scanning carriage10.

The size and strength of the magnets161,162, as well as the distance from the corresponding steel tape261,262has been selected to be such, that the magnetic force changes as little as possible, even in case of a change of the distance. Thus, the distance is placed into as flat as possible a range of the magnetic force-distance characteristic.

Holding the magnets161,162takes advantageously place in a pocket-shaped receptacle17of the scanning carriage10. As represented in the sectional representation inFIG. 1by the example of the magnet162, the receptacle is embodied in such a way that the magnet162can be pushed into the receptacle17and the magnet162snaps into it. In the inserted state the magnet162is held by a protrusion181of a resilient tongue18. Thus, the magnet162is snapped into the receptacle.17.

In the extensively explained example, the first element of the scanning carriage10is at least one permanent magnet161,162, namely the permanent magnet161for the pressing force Fx, and the permanent magnet162for the pressing force Fy. The second element includes two steel tapes261and262drawn into the grooves231,232. Alternatively to this the steel tapes261,262can also be glued to the insides of the housing22. A further variation would be to replace the steel tapes by magnetic tapes, i.e. tape-shaped permanent magnets, which are drawn into grooves of the housing or are glued to the housing and, together with permanent magnets, or ferromagnetic elements, of the scanning carriage generate the required magnetic force.

The present invention is not restricted to an optical scanning principle. Scanning of the scale can also be capacitive, magnetic or inductive, for which the measuring graduation and the scanning sensors must be appropriately embodied.

The foregoing description is provided to illustrate the invention, and is not to be construed as a limitation. Numerous additions, substitutions and other changes can be made to the invention without departing from its scope as set forth in the appended claims.