GB2042189A - Displacement sensor - Google Patents

Abstract

A sensor for multi-axial path measurement has a casing 4, a sensor shaft 1 which is resiliently suspended in the casing 4 via a plate 5 by means of a resilient strip 3, and a sensing ball 2 which is carried by the shaft 1. Three path-measuring transformers 6; 7 are attached to the plate 5 and operate in directions axially parallel with one another substantially in the plane of the resilient suspension 3. The transformers 6; 7 are connected to an electronic computer. Further resilient elements may be provided for adjusting the measuring force and position of the sensor. <IMAGE>

Description

SPECIFICATION Senses for multi-axial path measurement Field of use of the invention The invention relates to a senser for multi-axial path measurement in uses for length-measuring techniques, preferably for coordinate-measuring machines and for the open and closed loop control, for instance, of multi-axialy controlled measuring and milling machines, the contour to be measured being sensed by a senser shaft which can be deflected measurably in space in one or more directions and at whose end a sensing ball is disposed.

Characteristics of the known technical methods Sensers are known for multi-axial path measurement, using a sensing ball. Such sensers can be constructed controllablywith one, two or three degrees of freedom, or simultaneously with a deflection in the direction of the coordinate axes (usually x, y, z). To measure the deflection, corresponding to the number of degrees of freedom, electronic, pneumatic or opto-electronic path-measuring transformers are so disposed in the senser casing and connected to the movable senser, that the senser deflection is transmitted along an axis even only to one path-measuring transformer. For this purpose the path-measuring transformers are disposed orthogonally.According to German Auslegeschrift 2440692 the senser shaft is mounted cardanfashion in a diaphragm spring, so that it can be satisfactorily moved in all directions. However, this mobility is heavily limited by the proposed joint attachment of the path-measuring transformers to the senser shaft. The measuring range is small and the free movement (stroke) substantially nil. Accord ing to German Auslegeschrift 2242355 the senser is guided orthogonally by means of three crossed parallel spring guides, a path-measuring transfor mer being associated with each direction. Mobility is satisfactory, but also strictly related to direction.

A number of sensing head constructions are also known which have pre-deflecting elements, to produce reliable bearing ofthe measuring ball even in the spatial zero position. However, since the measur ing process makes it difficult to adjust the spatial zero point of the senser deflection, clamping ele ments are also provided which can block one or usually two degrees of freedom at choice (e.g., East German Patent 92 567). However, one disadvantage then is that in the sensing of inclined surfaces, the measuring force, which is adjusted at an inclination, results in transverse deformation of the senser, leading to considerable measuring errors.

Moreover, prior art senser constructions with resilient joints or guides and resilient re-setting elements to the spatial zero point require very satisfactoryworkpiece attachment, since if the de flection accidentally increases, the measuring force rises very considerably, resulting in displacements or deformations of the workpiece or senser, and causing errors.

Another disadvantage of the known universal three-coordinate sensers is their large mass and their large overall volume, due to the guides disposed in the x-y-z direction, the measured value transformers and clamping elements, and the elements for pre-deflection and zero re-setting.

These are the disadvantages which off-set the advantage of obtaining three senser output signals which agree with the axial directions, so that the measuring and sensing coordinates can be evaluated in a very simple manner.

Object of the invention The object of the invention is to provide a senser for multi-axial path measurement which has a high degree of utility.

Statement of the essential nature of the invention The invention solves the problem of rapidly and very accurately measuring paths in x and y and also in x, y and z coordinates, using a senser which is resiliently suspended and bears at its end a sensing ball, without the senser having to be aligned in the x ory direction when incorporated in the measuring machine or machine tool.

To this end the invention provides a senser for multi-axial path measurement, having: a casing; a senser shaft, which is resiliently suspended in the casing and bears a sensing ball at its end; and path measuring transformers, which are attached between the senser shaft and the casing, characterised in that at least two path-measuring transformers are used, all of which operate in directions axially parallel with one another, substantially in the plane of the resilient suspension, and are connected to an electronic computer, and if necessary in addition to the resilient suspension, further resilient elements are used which adjust measuring force and position.

Preferably the path measuring transformers are inductive and disposed in pairs, off-set by 90 , or in threes off-set by 120 , at the same radial distance from the centre of the resilient suspension.

In a special construction, exclusively for measurements in the x-y direction, the iron core and coil of the inductive path measuring transformer are so bent and disposed that their axes lie substantially on a circle around the centre of the resilient suspension.

For very accurate measurements of very small paths with large free movement (stroke) in the plane of the resilient suspension, three resilient rods are attached, off-set by 1200, radially to the senser shaft and are each borne by their ends with a spring pre-stressing on a support; the pre-stressing is achieved, for instance, by a pre-stressed spring which is disposed above each cutting edge and the resilient rod end.

Preferably, also for very accurate measurement, attached to the senser shaft, in its prolongation, is a resilient rod, which bears at its end a cylindrical member with end-face bore and edge, and attached to the casing is a matching abutment, which is constructed similarly to the cylindrical member and is disposed coaxially and laterally inverted in relation thereto and is guided in parallel springs parallel with the resilient rod axis and is pre-stressed by an end-face spring, a ball being disposed between the cylindrical member and the matching abutment.

The senser can be attached to the measuring machine or machine tool, without having to be aligned in the x or y direction. The values measured in the incorporated position are converted by the computer to x, y and z coordinates. The correctional factors can immediately be fed to the computer, after a brief calibrating operation. The x-y-z components of the sensing ball displacement thus computed are superimposed in known manner additively on the axial coordinates of the measuring machine or are used for the open and closed loop control of automatic measuring machines and measuring tools.

The path measuring transformers disposed in parallel can be attached to a senser shaft resiliently suspended in known manner. However, to increase accuracy, additional or specially constructed resilient elements are conveniently used which adjust position, measuring force and free movement (stroke).

The advantages of the senser according to the invention are small dimensions, low mass, large free movement, limited measuring force, high sensitivity in the vicinity of the spatial zero point and a negligible inclination to oscillation. As a whole measurements can be performed very rapidly and accurately, combined with immediate electronic evaluation.

Exemplary embodiments In the drawings: Figure 1 shows a very simply constructed senser according to the invention for the measurement of paths in the x, y and z directions.

Figure 2 shows a senser according to the invention for the highly accurate measurement of paths in the x, y and z directions, and Figure 3 shows a senser according to the invention for the highly accurate measurement of paths in the x andy directions.

All the constructions have a senser shaft 1 and sensing ball 2.

In a first, very simple variant for measuring and control purposes of a minor nature, a plate 5 is attached to the senser shaft. The senser is resiliently mounted on the plate with a resilient strip 3 in a casing 4. Three inductive path-measuring transformers are disposed, off-set by 120 , in the plane of the spring. Their ferrite cores 6 are connected to the plates, and their coils 7 to the casing. The coils are also connected to an electronic computer. With a sufficiently large radial distance of the path measuring transformers from the centre of the resilient bearings andior a long enough senser shaft, the normal clearance between the ferrite core and coil of commercially available transmitters permits an angular mobility which is adequate for measuring purposes.With 3 measured values, in accordance with the given programme and after an uncomplicated calibration operation, the computer can immediately compute the measuring results in x, y and z coordinates.

If it is intended that the senser should be used only for measurements in the x or y. direction, 2 path measuring transformers are adequate, which in that case are conveniently disposed off-set by about 90 in relation to one another. To increase angular mobility, more particularly for a larger free movement (stroke), inductive path measuring transformers might also be used whose ferrite cores and coils are arcuate in construction.

In another constructions, for highly accurate measurements in the x, y and z directions, again a plate 5 is attached to the senser shaft 1, the plate being resiliently mounted in resilient strips 3. Three measured value transformers 6, 7 are disposed on the plate 5 parallel with one another and with the senser shaft axis. Three resilient rods 8 are attached radially in the plane of the plate. The ends of the resilient rods bear against cutting edges 16, against which they are forced by pre-stressed springs 17. In this case the resilient strips provide security against twisting. With lateral limitations of the resilient rods and two cutting edges, they might be completely eliminated. In the case of very small paths, all the resilient rod ends are forced firmly onto the cutting edges. The measuring force is proportional to the deflection.With relatively large deflections (e.g. free movement) the pre-stressing is overcome; the resilient rod end lifts off. The force continues to increase only slightly.

In a third construction the senser shaft is resiliently suspended from the casing by means of three springs 3. The springs are off-set by 120 in relation to one another and are disposed radially in one plane. The iron cores 6 of inductive path measuring transformers are attached parallel with one another and with the senser shaft axis on three arms of equal length. The coils 7 are disposed in the casing 4. A resilient rod 8 is attached to the senser shaft in its prolongation. The resilient rod 8 bears at its end a cylindrical member 9 with an end-face bore 12 and a collar 15. Disposed in the axial direction and laterally inverted in relation to the cylindrical member 9 is an identically constructed matching abutment 11. Disposed between these two elements is a ball 14, which lies in bores 12.The matching abutment 11 is attached to the casing by two parallel springs 10 and is guided axially parallel. The members are prestressed by spring 13. In the zone of very small deflections, the preferred measuring zone, the ball 14 remains lying in the bores 12. The measuring force is mainly determined by the resilient stiffness of the resilient rods 8 and is proportional to the deflection. If this zone is exceeded, the ball 14 rolls out of the bores, at the furthest as far as edge 15. The springs are so adjusted to one another that the measuring force does not substantially change in this zone (more particularly the free movement zone). However, in principle measurement can be carried out even in this zone. The diameters of the collar, balls and bores are so adjusted to one another that the zero position is automatically set up after deflection by the resilient re-setting forces. The senser is constructed for highly accurate measurements in the x and y direction. However, deflections in the z direction can also be measured and processed via the electronic computer connected to the coils 7.

Claims (9)

1. Asenserformulti-axial path measurement, having: a casing; a senser shaft, which is resiliently suspended in the casing and bears a sensing ball at its end; and path measuring transformers, which are attached between the senser shaft and the casing, characterised in that at least two path-measuring transformers are used, all of which operate in directions axially parallel with one another, substantially in the plane of the resilient suspension, and are connected to an electronic computer, and if necessary in addition to the resilient suspension, further resilient elements are used which adjust measuring force and position.

2. A senser according to claim 1, characterised in that two path measuring transformers, off-set by 90 , are disposed at the same radial distance from the centre of the resilient suspension.

3. A senser according to claim 1, characterised in that thee path measuring transformers, off-set by 120 , are disposed at the same radial distance from the centre of the resilient suspension.

4. A senser according to claim 1, characterised in that inductive path measuring transformers are used.

5. A senser according to claims 1 and 4, characterised in that the iron core and coil of the inductive path measuring transformer are so bent and disposed that their axes lie substantially on a circle around the centre of the resilient suspension.

6. A senser according to claim 1, characterised in that in the plane of the resilient suspension, three resilient rods are attached, off-set by 120 , radially to the senser shaft and are each borne by their ends with spring pre-stressing on a support.

7. A senser according to claims 1 and 6, characterised in that a pre-stressed helical spring is disposed above each cutting edge and resilient rod end.

8. A senser according to claim 1, characterised in that attached to the senser shaft, in its prolongation, is a resilient rod (8), which bears at its end a cylindrical member (9) with end-face bore (12) and edge (15), and attached to the casing is a matching abutment (11), which is constructed similarly to the cylindrical member and is disposed coaxially and laterally inverted in relation thereto and is guided in parallel springs (10) parallel with the resilient rod axis and is pre-stressed by an end-face spring (13), a ball (14) being disposed between the cylindrical member and the matching abutment.

9. A senser for multi-axial path measurement substantially as herein described with reference to the accompanying drawings.