GB2039048A

GB2039048A - Three coordinate measuring head

Info

Publication number: GB2039048A
Application number: GB7944201A
Authority: GB
Original assignee: Carl Zeiss Jena GmbH
Current assignee: Jenoptik AG
Priority date: 1978-12-27
Filing date: 1979-12-21
Publication date: 1980-07-30
Also published as: IT1192794B; GB2039048B; DD141197A1; FR2445513A1; DE2938080C2; DE2938080A1; JPS5590810A; JPH0127364B2; IT7969470A0

Abstract

The head comprises a housing 1 accommodating a carrier 5 supporting at least one measuring unit or sensor 7 mounted in an intermediate member 2. The member 2 is supported in the housing 1 through diaphragm springs 3 and 4 which resist torsion but allow z movement, and the carrier 5 is supported in the member 2 so as to be deflectable in an x-y plane by means of a torsionally rigid elastic spring bar guide system comprising four upright resilient bars 6 providing a reproducible zero-position. A stabiliser giving torsional rigidity to the spring bar guide system, comprises pairs 8, 9 and 10, 11 of spring bars arranged at right-angles to one another and respectively secured to the carrier 5 and the intermediate member 2 and both secured in an angle piece 12 fast with the member 2. Three detectors 13, 14 and 15 electrically measure values in the x, y, z directions. A safety mounting 16, 17 which may be unseated by shock is provided, and a motor driven device 18 weight balances on the z coordinate. <IMAGE>

Description

SPECIFICATION Measuring head for sensing three-dimensional workpieces This invention relates to a sensing head for sensing three-dimensional workpieces, particularly for use in coordinate measuring devices.

The German Patent Specification 2 242 355 describes an electronic multicoordinate sensor with a sensing head for sensing thre-dimensional workpieces, which comprises, arranged in a housing, a leaf spring straight guide system which is free of backlash and friction, and rigid against torsion, and which forms a three-dimensional coordinate system.

Furthermore, signal generators are provided which indicate direction and position. Said signal detectors produce for each of the coordinate directions electrical signals which correspond to the movement of deviation of the mount carrying the signal generator units when sensing a workpiece.

The device further comprises means to exert a defined measuring force in all guided directions.

The origin of the coordinate system of the straight guide systems and the zero point position of the signal generator are aligned by precision locking mechanism, which are fine adjusted through servo motors and which are operative in the guiding direction of the respective straight guide.

The movements of the straight guide systems are damped to eliminate oscillations.

Furthermore, a weight balance is provided for the sensing units and the straight guide systems.

The German Democratic Republic Patent Sl:'ecification 92567 discloses a three-coordinate sensing head in which a mount for supporting a plurality of sensing units is seated on gimbals in a housing and which is provided with means to produce a measuring force and to effectzerpositioning.

Each coordinate direction is provided with a measuring value detector for detecting the deviation of the mount when sampling workpieces. The measuring unit is inclined towards the workpiece. The known threecoordinate sensing devices can be divided into two groups: trailing sensing devices and devices which are inclined towards the workpiece.

The leading sensing devices are inclined to the workpiece before the sensing starts and an indication signal is triggered for detecting the zero-position of the measuring systems. In the case of trailing sensing devices the measuring mount is maintained in the zero-position and when leaving the same due to workpiece sensing a signal is delivered.

It is a common disadvantage of said sensing devices that measuring errors result when sensing slanted workpiece faces. Said errors are not to be neglected when even a low measuring force is exerted in the event of precision measurements.

In the known devices the measuring unit mount has, due to the seating, preferred directions of movement in mutually perpendicular coordinates which define the measuring forces.

When the measuring unit mounts which support the measuring units are located. that is, when the workpiece is sensed in directions departing from the preferred directions (for example, when sampling slanting faces) a higher measuring force results, depending on the size of the departure, which causes additional deformations in the entire device, which are not detected by the conventional calibration through cube gauges or spheric gauges, and, hence, are introduced as measuring errors into the measuring result.

A further disadvantage, of all the above described arrangements, lies in the immense technological expenditure, which, for example, is necessary to obtain a high reproducibility of the sensing system and for varying the measuring force.

Thus, for example, expensive arresting means for the coordinate directions x, y and z and complicated measuring force generators for +#x, ly and Iz are necessary.

Similar is the condition with respect to a sensing device for workpiece testing as disclosed in the German Published Specification 2356030, in which the measuring unit is arranged in a multicoordinate leaf spring parallelogram seating. The seating of a measuring unit by means of a diaghragm spring in the housing of a measuring head is disclosed and illustrated in the German Patent Specification 2 440 692.

The movement transfer from the measuring unit to the signal generator is carried out by a common connecting piece attached to the measuring unit A signal generator is provided for each coordinate direction x, y, z.

When sensing the face of a workpiece an angular movement of the sensing unit is effected resulting from the variation of the distance between the point of sensing and the point of rotation of the measuring unit.

Furthermore, different reset and measuring forces occur in the different deflection planes of the measuring unit which are not reproducible at all times.

It is an object of the invention to obviate the above disadvantage and to provide a coordinate measuring head which substantially eliminates measuring errors. It is a further object of the invention to provide a reliable coordinate sensing head of simple construction which eliminates measuring errors due to measuring forces when sensing slanting workpieces.

Accordingly, the present invention consists in a coordinate measuring head for sensing threedimensional workpieces, comprising a housing, a mount arranged in said housing for supporting at least one measuring sensor and mounted for movements in three coordinate directions, measuring value detectors for each coordinate direction for producing, when senSing workpieces, electric signals corresponding to the movement or deflection of the mount, and an adjustable weight balancing means for balancing the mount, arranged in a perpendicular coordinate direction in the housing, characterised in that in the housing an intermediate member is mounted torsionally fast in diaphragm springs for displacement in a z coordinate, in which intermediate member the sensor mount is deflectable in an Fy plane and is supported torsionally fast in an elastic spring bar guide system which has such a spring rate (c) that said mount in rest position is maintained in a reproducible zero-position, that a stabiliser producing the torsional rigidity of the spring bar guide system is provided in the intermediate member, and in that in the housing three measuring value detectors in the Cartesian coordinate directions (x, y, z) are arranged, which are preferably connected to a computer, the output signals of which are so combined that so called signal spheres are established when definite radii are used about the zero-position of the measuring sensor mount in dependence on the measuring force selected for testing the workpieces, where

and Xl, Y1, Z are the typical deflections of the measuring unit mount from the zero-position at the respective measuring force used, and in that a safety device is provided for control at overload of the measuring head.

Advantageously the stabiliser is constituted by a torsionally rigid spring bar system arranged in an wy plane and is connected to the intermediate member and to the measuring unit mount, the spring bar system comprising two pairs of spring bars arranged preferably at right-angles to each other and interconnected by means of an angular element.

Preferably, photoelectric, inductive or capacitive measuring value detectors are provided in the measuring head.

Conveniently, liquid damping means operating in three dimensions and connected to the measuring unit mount is provided to obviate oscillations.

It is a further advantage with respect to the coordinate measuring head when the guide systems have the same rigidity in any deflection direction of the measuring unit mount. Hence, a definite force produces in each f these directions the same deflection of the measuring unit mount and vice versa. There are no longer any preferential directions. Hence the measuring force is independent of the direction for sensing the workpiece.

Measuring errors due to undefined measuring forces when sensing slanting workpiece surfaces are obviated.

A Afurther advantage consists in the simple construction of the sensing head, which does not require any longer measuring force generators for a reproducible measuring force and expensive arrest systems for realising the zero-position of the measuring unit mount.

In order that the invention may be more readily understood, reference is made to the accompanying drawings which illustrate diagrammatically and by way of example one embodiment thereof, and in which~ Fig. 1 is a longitudinal section through a coordinate sensing head, and Fig. 2 is a cross-section on the line A-A of Fig.

1.

The coordinate sensing head shown in Fig. 1 comprises a housing 1 in which an intermediate member 2 is seated in diaphragm springs 3 and 4 rigid against torsions about a z-axis, and displaceable along the z-coordinate.

In the intermediate member 2, a measuring unit mount 5 displaceable in the x-y plane is guided in a spring bar guiding system rigid against torsion, elastical and preferably constituted of four resilient bars 6. The spring stiffness c of the spring bar guiding system is so defined that the measuring unit mount 5 in its rest position, that is in a nondeflected state, will always be maintained in a reproducible zero-position and, after deflection from the same, always will return thereto.

One or a plurality of identical or different measuring units (sensors) 7, which sense the workpiece (not shown) to be tested, can be inserted in the measuring unit mount 5. The four resilient bars 6 permit a relative movement in the x-y plane when external forces P act on the sensors 7 and hence the measuring unit mount 5, whereas the diaphragm springs 3 and 4 permit a relative movement between the housing 1 and the intermediate member 2. Thus a three-dimensional displacement of the measuring unit mount 5 is ensured.

The entire seating system of the measuring unit mount 5 is so constructed that the spring stiffness of the seating system is the same in all directions.

Compared to the known solutions there are no preferred directions.

To ensure torsional rididity of the spring bar guide system about the z-coordinate an intermediate element is arranged in the stabiliser (Fig. 2) which comprises spring bar system 8, 9; 10, 11 rigid against torsion and arranged in the x-y plane and which is connected both to the intermediate member 2 and to the measuring unit mount 5.

The spring bar system comprises two pairs of spring bars 8, 9 and 10, 1 which pairs are arranged at right-angles to each other and are connected to an angular member 12.

The stabiliser permits movements of the measuring unit mount 5 in all coordinate directions, however, no rotation about the zcoordinate.

As Fig. 1 shows, three photoelectric, inductive or capacitive measuring value generators 13, 14, 15 are arranged along a Cartesian coordinate system x, y, z in the housing 1 and in the intermediate member 2, respectively.

The generators are preferably connected to a computer, not shown, via intermediate components and deliver electric output signals which correspond to the departure of the measuring unit mount 5 from the zero-position.

One measuring value generator is provided for each coordinate direction.

The output signals from the measuring value generators 13, 14, 15 are, after processing in the computer, combined in such a manner that depending on the measuring force P, used to sense the workpieces, selected predetermined radii

establish so-called signal spheres, about the zeroposition, where c is the spring stiffness and x,, y, and zl the departure of the measuring unit mount 5 from the zero-position typical for the respectively used measuring force.

When sensing a workpiece face which is randomly arranged in the space, the sensor 7 which is operative and with it the mount 5 is deflected from the zero-position towards the surface normal.

When the square sum x12 + y12 + z12 yields a given signal sphere radius R,2 according to a selected measuring force P, the computer feeds a sensing signal to the measuring system of the measuring machine.

Owing to the fact that the guiding system for the measuring unit mount 5, which is constituted by the diaphragm springs 3 and 4 of the spring bar guide system and the stabiliser, has the same rigidity in any direction of departure, a measuring force P produces the same departure R in each direction. P =czar is valid.

The measuring force is independent of the sensing directions and the inclination of the workpiece surface.

Due to its spring stiffness the elastic guide system only permits limited movements in the coordinate directions x, y, z, which are, however, so dimensioned that a reliable measuring of workpieces is feasible. When, however, a critical force is obtained in a sensing procedure, for example, in the event of collision, an arrestable safety system 16 and 17 is triggered and releases the sensing head to be additionally deflected so protecting it from destruction. The mechanical safety system 16 and 17 is constructed as an arresting system in several coordinates.

To obviate possible oscillations of the measuring units 7 in the mount 5 when sensing the workpiece, a three-dimensional liquid damping (not shown) is provided, which is connected to the measuring unit mount 5.

As shown in Fig. 1 a balancing device 18 is provided in the z-coordinate for compensating the measuring unit weights at differing loads of the measuring unit mount 5.

The balancing device 18 is preferably provided with a motor driven screwthreaded spindle 19 -which is connected via a spring 20 to the intermediate member 2 seated in diaphragm springs 3 and 4.

Claims

1. Coordinate measuring head for sensing three-dimensional workpieces, comprising a housing, a mount arranged in said housing for supporting at least one measuring sensor and mounted for movements in three coordinate directions, measuring value detectors for each coordinate direction for producing, when sensing workpieces, electric signals corresponding to the movement or deflection of the mount, and an adjustable weight balancing means for balancing the mount, arranged in a perpendicular coordinate direction in the housing, characterised in that in the housing an intermediate member is seated mounted torsionally fast in diaphragm springs for displacement in a z-coordinate, in which intermediate member the sensor mount is deflectable in and x-y plane and is supported torsionally fast in an elastic spring bar guide system which has such a spring rate (c) that said mount in rest position is maintained in a reproducible zero-position, that a stabiliser producing the torsional rigidity of the spring bar guide system is provided in the intermediate member, and in that in the housing three measuring value detectors in the Cartesian coordinate directions (x, y, z) are arranged, which are preferably connected to a computer, the output signals of which are so combined that so called signal spheres are established when definite radii are used about the zero-position of the measuring sensor mount in dependence on the measuring force selected for testing the workpieces,

are the typical deflections of the measuringunit mount from the zero-position at the respective measuring force used, and in that a safety device is provided for control at overload of the measuring head.

2. Coordinate measuring head as claimed in claim 1, wherein the stabiliser is constituted by a torsionally rigid spring bar system arranged in an Fy plane and is connected to the intermediate member and to the measuring unit mount, the spring bar system comprising two pairs of spring bars arrange preferably at right-angles to each other and interconnected by means of an angular element.

3. Coordinate measuring head as claimed in claim 1 or 2, wherein photoelectric, inductive or capacitive measuring value generators are provided.

4. Coordinate measuring head claimed in claims 1,2 and 3, wherein liquid damping means operating in three dimensions and connected to the measuring unit mount is provided to obviate oscillations.

5. Coordinate measuring head, substantially as herein described with reference to and as shown in the accompanying drawings.