Bearing for a measuring instrument, particularly a coordinate measuring instrument

A bearing (32) for a coordinate measuring instrument including a slide (36) which can be axially displaced in relation to a fixing device (34) and which can be supported opposite the fixing device by means of at least one air bearing (38, 40). In order to be able to utilize the bearing for all axes of the coordinate measuring instrument while simultaneously using outer surfaces of the slide as mounting surfaces for sensors, the invention provides that a force that works counter to the air bearing can be generated by a magnetic field (42, 44).

The invention pertains to a bearing for a measurement apparatus, especially a coordinate-based measurement apparatus, that comprises a movable element, which preferably runs axially relative to, and especially along, a mounting device or guiding device, whereby this element is vertical, or runs at an angle α to the vertical, with α<90°, especially in the range α±45°, whereby this movable element, such as a carriage or column, has a measurement sensor, and is capable of being supported relative to the mounting device by at least one air bearing, whereby the air bearing's force, which is transferred to the element, is capable of being compensated by a counter force. In addition, the invention pertains to a bearing for a coordinate-based measurement apparatus that comprises a column or center sleeve, which is movable along the Z-axis of the coordinate-based measurement apparatus, with at least one measurement sensor that is supported, in a sliding manner, by at least one air bearing relative to a guiding device.

The use of air bearings has found extensive acceptance for achieving highly accurate moving carriages, especially in the area of coordinate-based measurement apparatus. Advantages, especially in regard to low reverse play or low friction, are achieved via bearings of the type in question, whereby these advantages are reflected in high measurement accuracy. However, a disadvantageous feature of bearings of the type in question is that they necessitate a considerable force for the initial tension.

Bearings are known in which the movable elements, such as carriages or traveling tables, are supported by oppositely facing arrangements of air bearings. A disadvantageous feature of oppositely facing air bearings is that wrapping around of the mechanical components takes place out of necessity. As a result, the installation of accessory units or accessory elements, such as sensors, is impaired. Servicing activities are also made difficult.

Alternatively, use can also be made of gravity for producing the initial tensioning force. In this case, however, limitations arise in regard to the spatial arrangement of the bearings so that, as a consequence, utilization for all the axes of a coordinate-based measurement apparatus cannot be considered.

A bearing for a coordinate-based measurement apparatus can be taken from DE 34 41 426 A1. In this case, the pedestal of a movable traveling table is supported by means of air bearings. Each bearing component of the air bearing has a projection, which is constructed in the form of an elastic element, in order to delineate a hollow zone that is capable of having compressed air admitted into it. A coordinate-based measurement apparatus is known from DE 44 08 912 A1 in which a vertically movable measurement carriage is supported in a bearing arrangement via compressed air cylinders. An air bearing for a measurement head of a coordinate-based measurement machine can also be taken from WO 91/13315.

DE 28 29 715 A1 pertains to a tool machine or measurement machine in which carriages, which are movable in the X- and Y-directions, are supported via air bearings, whereby the forces that are produced by the air bearings are compensated by counteracting forces that are produced by magnets. The column of the known measurement machine is supported, in a sliding manner, exclusively via air bearings.

According to U.S. Pat. No. 4,985,651, a tool table is supported via air bearings, whereby the forces from them are capable of being compensated by magnets. In this case, the table is capable of being moved in the X-direction or in the Y-direction.

According to JP 02 00 10 50 271 A, magnetic forces, which act on a carrier, are compensated via an air bearing.

The problem that forms the basis of the present invention is to further develop a bearing of the type that was designated at the beginning, especially one intended for a coordinate-based measurement apparatus, in such a way that it is preferably usable for all the axes, though especially for the Z-axis, of a coordinate-based measurement apparatus without the necessity arising that the element, which is to be supported by the bearing, is surrounded to such an extent by mechanical components that a hindrance arises for accessory elements, or that servicing activities are made difficult.

In accordance with the invention, the problem is essentially solved, on the one hand, by way of the feature that the counteracting force, which compensates the force that is produced by the air bearing as a result of the admission of gas thereto and whereby there is at least one such air bearing, is capable of being produced by at least one magnet that is allocated to the mounting device. In particular, the problem is solved in the case of the bearing for a coordinate-based measurement apparatus by way of the feature that the force, which is produced by the air bearing, is capable of being compensated by a counteracting force that is produced by a magnet that is allocated to the guiding device.

In this way, and in accordance with a proposal that is to be emphasized, the feature is provided that the element or column has an area on the outside that is not covered by the mounting device or the guiding device, whereby this area is above the element's or column's movement pathway or is approximately in the movement range thereof, whereby the measurement sensor and/or at least one additional measurement sensor and/or one or more components of the measurement apparatus or coordinate-based measurement apparatus originate at this non-covered area. In particular, the element or column has at least three outer surfaces, which run along the movement pathway, of which one outer surface is an attachment surface for the sensor or the component of the measurement apparatus, whereby there is at least one such sensor or component.

In accordance with the invention, the initial tension that is required for the air bearing, which supports the axially movable element, is compensated by a tensile force, which acts between the mounting device and the axially movable element, whereby this tensile force is produced by a magnet, whereby use is made in particular of a permanent magnet and a paramagnetic or ferromagnetic material, which is allocated thereto, as the magnet system. In this way, the component parts that form the bearing in question act on the same side of the axially movable element, so that the remaining area of the element is freely accessible and can thus be used for e.g. the installation of further component parts.

If the mounting device or guiding device for the carriage or column has L-shaped geometry, then the feature is provided in accordance with a further proposal that the mounting device or guiding device has U-shaped geometry, and that an air bearing originates at least along an inner surface of one of the shanks of the U-shape, and by the feature that the element or column has the geometry of a rectangle, whereby three outer surfaces run along the inner surfaces of the shanks, and by the feature that the remaining outer surface, which runs parallel to the transverse shank of the mounting device or guiding device, serves for attaching at least one measurement sensor and/or one or more components of the measurement apparatus or coordinate-based measurement apparatus. In this way, the feature is provided in particular that at least one magnet for producing the counteracting force is allocated to the transverse shank of the mounting device or guiding device. In particular, a magnet extends on each of the two sides of the air bearing that originates at the transverse shank.

As a result of the magnetically produced initial tension, the initial tensile force within the bearing is essentially independent of the incorporation position of the bearing itself, so that usage is possible for all the axes of a coordinate-based measurement apparatus.

In particular, the feature is provided that a permanent magnet originates at the axially movable element, whereby a paramagnetic or ferromagnetic material, which originates at the mounting device, is allocated to this permanent magnet or contains it.

In a preferred, further design, the feature is proposed that the axially movable element has a permanent magnet with a planar surface, whereby this permanent magnet is, in particular, strip-shaped or parallelepiped-shaped, and runs along the element and parallel to the direction of movement thereof, whereby the mounting device's material, which has the shape of a batten or parallelepiped, extends along this planar outer surface originating with an outer surface that runs parallel to that of the permanent magnet. Naturally, the permanent magnet can also originate at the mounting device, and the ferromagnetic or paramagnetic material can originate at the movable element.

In addition, the feature is provided that the movable element is a carriage, which has a square or rectangular cross sectional shape, and that an air bearing runs between each of two mutually adjacent first outer surfaces of the carriage and the area of the mounting device that faces toward it, and that the additional bearing element is arranged between the air bearings, whereby the additional bearing element extends, in particular, along an edge that runs between the first outer surfaces. In order to achieve this, this can be beveled in order to attach the strip-shaped or parallelepiped-shaped permanent magnet or the paramagnetic or ferromagnetic material, which has a corresponding geometry, in the case where the magnet originates at the mounting device.

If the magnet originates at the movable element, such as the carriage, then a batten, especially one comprising steel that originates at the mounting device, is allocated to the former in order to form the magnet system in order to produce the tensile force that is required in order to compensate, to the desired extent, the force due to pressure that is produced by the air bearings.

A basic illustration of a bearing10in cross sectional form in accordance with the prior art can be taken fromFIG. 1, whereby this bearing is intended for the column12of a coordinate-based measurement apparatus. The column12is surrounded by a mounting device14and is supported relative to this by air bearings16,18,20,22.

The air bearings16,18,20,22are of conventional construction above which an air gap24,26,28,30is formed between the column12and the bearings16,18,20,22in order to be able to move the carriage in a floating manner, via the mounting device14, in the axial direction of the carriage12.

As the basic illustration ofFIG. 1additionally makes clear, appropriate air bearings16,18,20,22have to be arranged on opposite sides of the column12in order to the achieve the required initial tension since otherwise support would not be possible in a floating manner. This again means that the column12has to be surrounded—peripherally in this example of an embodiment—by the mounting device14with the consequence that components, such as the sensors of a coordinate-based measurement apparatus, are only capable of being attached to the column12to a limited extent. Difficulties at the time of servicing activities can also increase because of this mechanical wrapping around of the column12.

In contradiction to the prior art, a bearing32is proposed in accordance with the invention that exhibits the advantages of an air bearing, though without taking on the disadvantages thereof. Thus the feature is provided that the forces due to pressure, which are provided by the air bearing, are compensated to the required extent by a magnet system, using the tensile forces that are produced by it, in order to permit the guidance, in a floating manner, of a movable element, especially a carriage, traveling table or center sleeve, of a coordinate-based measurement apparatus, whereby the direction of movement is preferably along the Z-axis, or at an angle α relative to the Z-axis with α<90°, especially in the range α±45°. The direction of the movable element, such as the column, can optionally be displaced, i.e. it is adjustable.

The air bearing32, which is illustrated inFIG. 2in the form, likewise, of a purely basic structure, comprises, by way of example, a V-shaped or L-shaped mounting device34or guiding device along which an element36, in the form of a traveling table, a carriage, or column or center sleeve, is required to be axially movable. In general, the element36will be termed a carriage. The carriage36is supported by two air bearings38,40relative to the mounting device34. In order to bring about the required initial tension, a magnet system, which comprises a permanent magnet42and a paramagnetic or ferromagnetic material44that is allocated to it, is provided between the carriage36and the mounting device34.

The air bearings38,40act on adjacent outer surfaces46,48of the carriage36that has the cross sectional shape of a rectangle or parallelepiped. Accordingly, the mounting device34, with shanks50,52, extends along the surfaces46, in order to form the air bearings38,40. The initial tension, which is required for the air bearings38,40in order to admit air into them, is produced by the magnet system, which is formed from the permanent magnet42and the paramagnetic or ferromagnetic material44, in the form of a batten.

In this way, the permanent magnet42in this example of an embodiment originates at a beveled edge54, which runs between the mutually adjacent surfaces46,48, and has a planar outer surface56that runs parallel to a surface58, which is aligned therewith, of the batten44that comprises e.g. steel.

Naturally, the permanent magnet and the batten44can also be interchanged.

As mentioned, the required initial tension for the air bearings38,40is produced by the magnet system42,44, so that the required gap60,62is formed in the area of the air bearings38,40in order to be able to move the carriage36axially, and in a floating manner, along the mounting device34.

Consequently, the remaining outer surfaces64,66of the carriage36do not have air bearings, and are thus freely accessible on the basis of the teaching in accordance with the invention, and they can be used in an essentially unrestricted manner as installation surfaces for e.g. components, such as the sensors of a coordinate-based measurement apparatus.

As a result of the formation of the bearing32, the possibility also arises of appropriately constructing an X-axis, a Y-axis and/or a Z-axis of a coordinate-based measurement apparatus.

The teaching in accordance with the invention will explained, in particular, by means of FIG.3. Thus a coordinate-based measurement apparatus100is illustrated in the form of a purely basic structure, whereby this coordinate-based measurement apparatus has a base frame102in a known manner on which a measurement table108has been arranged. A traveling table104is movable in the Y-direction along the base frame102. In order to do this, columns or posts104,106are supported, in a sliding manner, on the base frame102. A cross arm110originates at the columns106,108, whereby a carriage112is movable along this cross arm, i.e. in the X-direction, whereby the carriage, for its part, accommodates a center sleeve or column114that is movable in the Z-direction. A measurement sensor116, such as the optical system of an image processing sensor, originates at the center sleeve or column114. The center sleeve, which as mentioned is also termed the column114, is accommodated by a guiding device, which is constructed in accordance with the invention correspondingly toFIG. 2, so that identical reference symbols are used for identical elements.

Since two outer surfaces64,66of the column114are freely accessible, measurement sensors, such as a laser distance sensor120or an opto-tactile or tactile probe118, can originate at these.

A compact structure arises as a result of the fact that the outer surfaces64,66of the center sleeve or column114are freely accessible, whereby the sensors118,120are movable in the Z-direction even inside a covering arrangement or housing122, i.e. they are protected.

A further form of design of a bearing in accordance with the invention is illustrated in FIG.4. Thus a guiding device122, which is cross sectionally U-shaped, accommodates a carriage, such as in particular the column124or center sleeve, that has rectangular cross sectional geometry. The guiding device122has a transverse shank126and lateral shanks128,130, which run vertically relative to it, whereby air bearings138,140,142originate at the inner surfaces132,134,136of these lateral shanks. In order to produce the required force counter to the repulsive force that is produced by admitting air into the air bearings138,140,142, i.e. in order to be able to guide the column124in a sliding manner, magnet arrangements144,146originate at the inner surface132of the transverse shank126on the two sides of the air bearing138. The attractive counteracting force that is necessary is produced by these. As a result of the construction in this way, the column124has a free outer surface148, which runs parallel to the transverse shank126, at which one or more measurement sensors and/or one or more components of the coordinate-based measurement apparatus or similar device originate or are attached.

The column124is moved along the Z-axis via a propulsion unit. In order to do this, a cable can originate at the head of the column124, whereby this cable is turned around in order to be capable of connection to a servomotor or step motor, as a result of which apparent shortening or lengthening of the cable takes place and this results in the column124being movable along the Z-axis. Other propulsion possibilities are also conceivable.