Geometrical instruments – Distance measuring – Environmental isolation
Patent
1995-01-20
1996-09-17
Fulton, Christopher W.
Geometrical instruments
Distance measuring
Environmental isolation
33503, G01B 5008
Patent
active
055556337
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a scale assembly for use in monitoring movement of a component on a structure, and is particularly but not exclusively concerned with monitoring movement of a component on a linear guideway of a high accuracy measuring and checking machine.
For purposes of accuracy, it is necessary to construct coordinate measuring machines using either heavyweight materials, such as granite, which remain as stable as possible with variations in temperature, in order that guideways of the machine do not distort and thereby detract from the accuracy of measurement, or special designs of mountings for the guideways which can accommodate differential thermal expansion between the materials used. However, irrespective of the construction used, there remains a problem with scales which are mounted on the guideways for use in a non-contact linear encoder system. Some such scales have been made of a non-thermally stable material and, as the guideways require to be made of a material whose surface can be machined to a high accuracy, and therefore inevitably a non-thermally stable material, there still results a differential thermal expansion between the materials off the scale and the guideway. Other scales are expensive and have length limitations.
In DE-U-9205393, an arrangement for measuring the relative positions of two objects mounts a scanning unit on one of the objects, positioned to scan a graduated scale on the other object in order to obtain positional data. The design of the prior device attempts to make the scale predictable in terms of length change due to temperature change, in order to allow the user to be able to compensate for that change. The intention is to avoid the need for all the various components to be made from materials with the same coefficient of thermal expansion. Such a design is not, however, appropriate for use in a high accuracy measuring and checking machine, where it is desirable to maintain the scale in the same spatial position regardless of temperature changes.
According to the present invention there is provided a scale assembly for use in monitoring movement of a component on a guideway structure, the assembly comprising an elongate scale, a support for the scale, and means for fixedly mounting the support on the structure at one reference location and flexibly mounting the support on the structure at another location, characterised in that, in order to substantially prevent a change in the spatial position of the scale due thermal expansion of materials used, the support is formed of a thermally stable material, and means fixedly secures the scale to the support to substantially prevent relative movement therebetween, the positions of the fixed and flexible mountings for the support on the structure accommodating movement of the latter due to thermal expansion thereof, without changing the spatial position of the support and therefore the scale.
Preferably the scale is in the form of a metal tape which may be produced to have an indefinite longitudinal dimension and may be cut to a required length. The securing means may comprise an adhesive layer provided on a face of the tape opposed to a scaled face thereof, and a component for clamping each end of the tape to the support.
Preferably also the support is formed as an elongate strip, for example of unidirectional carbon fibre material. Alternatively the strip may be formed of INVAR or the like. The strip may be formed with an indefinite longitudinal dimension and may be cut to a desired length.
The flexible mounting means for the strip may include a plurality of blocks, each of which may be formed of two hinged parts, one of which parts is secured to the support and the other of which parts adapted to be secured to the structure. Each block may be formed of aluminum with an integrally formed flexible section defining the hinge.
Alternatively the flexible mounting means for the strip may comprise a sealant which can be applied in flowable state to extend between each side of the support and the structure over part
REFERENCES:
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patent: 4776098 (1988-10-01), Nelle
patent: 4912856 (1990-04-01), Ernst
patent: 5031331 (1991-07-01), Herzog et al.
patent: 5095632 (1992-03-01), Hassler, Jr. et al.
patent: 5182867 (1993-02-01), Nelle
patent: 5375338 (1994-12-01), Nelle
Fulton Christopher W.
LK Limited
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