GB2284057A

GB2284057A - Centring device for a deflectably mounted mechanical probe

Info

Publication number: GB2284057A
Application number: GB9418715A
Authority: GB
Inventors: Matthias Lindner
Original assignee: Leitz Messtecknik GmbH
Current assignee: Hexagon Metrology GmbH
Priority date: 1993-09-17
Filing date: 1994-09-16
Publication date: 1995-05-24
Anticipated expiration: 2014-09-16
Also published as: DE4331655C2; GB9418715D0; DE4331655A1; DE4331655C3; GB2284057B

Description

1 2284057 Centring device for a deflectably mounted mechanical probe The

invention relates to a centring device for a deflectably mounted mechanical probe for determining the coordinates of measurement points on a workpiece. Probes of this type are deflectably mounted because, as the probe approaches the measurement point of the workpiece, it is subjected to mechanical forces which for example cause the probe ball of the probe to move away from its set position. If the probe is to be used subsequently for a new, connected measurement, it must be certain that the probe ball has re-adopted its initial Position after being detached from the workpiece. In practice, this cannot be checked.

For this reason, in order to ensure that the probe re-adopts its initial position after each measurement, the probe is deflectably mounted in the probe head, for example is suspended on a leaf spring parallelogram, so that when it approaches the measurement point the probe can move away and the spring parallelogram guides the probe back to its initial position after it is detached from the workpiece. Precise guidance of the probe back to its initial position is not, however, successful with 2 a simple leaf spring parallelogram. For this reason, in accordance with the prior art there is additionally provided for the probe a centring device which, in particular, ensures proper and hysteresis-free centring of the probe in its initial position. A centring device of this type acts, for example in accordance with DE-PS 23 56 030, with balls which are under spring pressure on a bending rod connected to the probe. In order to ensure proper and hysteresis-free centring in the basic position, with this construction all the contact points provided must bear precisely between the ball of the entrainer and the balls of the stops and the pressure parts at the same time. This is only possible with high production costs in relation to the precision of the balls and the planar pressure parts. In use, this quality is necessarily reduced by wear, for example as a result of an impact action or for example by the friction of the ball of the entrainer against the pressure parts.

A further development of a centring device of this type is disclosed by DE-OS 40 27 136. In this centring device, the restoring means, which act in opposite directions, are constructed such that for a restoring means a stop which may also be in multiple parts is provided, in which case the restoring means bearing against the stop in the rest state exerts twice as great 3 a restoring force, at least close to the rest state, as the second opposing restoring means. Because the restoring means bearing against the stop does not act on the entrainer when the entrainer is deflected in opposition to the restoring force of the opposing restoring means, only the normal restoring force acts on the ball of the entrainer. If the entrainer is deflected in a different direction, it is true that twice the restoring force acts on the ball of the entrainer, but the normal restoring force of the opposing restoring means counteracts this restoring force, so that effectively only the difference between the two restoring forces is active. As a result of this, the same size of restoring force is achieved in both directions. The restoring means provided are either spring-loaded plungers or plungers acted upon by means of a fluid.

This construction permits at most the maximum force occurring during deflection to be varied by varying the system pressure, but does not permit the linear range of the characteristic curve around the zero point to be changed. If the probing system is to be matched to the optimum to various measurement tasks, various centring devices have to be used for these various measurement 4 tasks, and in this case a particular centring device which has the force/distance characteristic curve which 4 is optimum for the measurement task has to be used for each particular measurement task.

Moreover, these mechanical centring devices which are part of the prior art are sensitive to the oscillations of the probe which are excited for example if the measurement head is moved to and fro between different measurement positions. These oscillations are dependent on the acceleration of the probe. The oscillations create a considerable disruption to the measurement as the probe approaches the workpiece, with the result that the probe has to approach the workpiece without substantial acceleration, which is undesirable from a process engineering point of view.

It is the object of the invention to specify a centring device for a mechanical probe in which the restoring means act on the-probe without contact and the above-mentioned disadvantages do not occur.

Claims

This object is achieved by the characterizing feature of Claims 1 or 2. Because magnetic forces now guide the probe back into its initial position without contact, there is no longer any mechanical friction, which has an unfavourable influence on the guiding back. Furthermore, there is primarily no requirement for energy supply for guiding the probe back into its

1 initial position. Moreover, a centring device of this type uses only simple and inexpensive parts.

In the case of the centring device according to the invention, only two magnets or one magnet and a ferromagnetic body which are arranged opposite one another in such a way that the body or the second magnet is mounted laterally deflectably with respect to the first magnet are provided for centring in a coordinate direction. Here, the air gap between the magnets or te magnet and the ferromagnetic body at least approximately maintains its constant thickness. As a result of this, the magnets or the magnet and the ferromagnetic body constantly attempt to bring their magnetic main field lines back into congruence. In this manner, a precise guiding back into the zero position of the probe connected to the magnet system is achieved.

As has been shown, a desired force/distance characteristic curve for the measuring probe head is also obtained with a magnet centring of this type only if the geometry of the mutually opposing poles of magnet and counterpole and their spacing correspondingly from one another are matched in particular to its linearity.

If a spring parallelogram suspension for the probe is provided, preferably a leaf spring suspension, then the 6 centring device in the embodiment of Fig. 3, caused by the magnetic attractive force, exerts a pull on the leaf springs. This tensile force allows the spring parallelogram, in the event of an external pressure load, to kink later than usual, which means that the loading capacity by external forces and moments becomes greater than was hitherto the case.

In a further embodiment of the invention, one of the magnets or the counter-pole can be borne by two spring parallelograms turned through 90 with respect to one another, so that the probe is restored in two coordinate directions at the same time. The mutually opposing magnets or the magnet with the ferromagnetic counter-pole opposite are advantageously constructed to be centrally symmetrical in this embodiment. Thus, in principle, a single magnet centring comprising pole and counter- pole is sufficient for guiding the probe back in two coordinate directions.

In a further embodiment of the invention, the magnet can be constructed as an electromagnet, and the opposing ferromagnetic body as the counter- pole can then be mounted to be movable in the axial direction in opposition to the pressure of a spring in such a way that the force of the spring tends to push the ferromagnetic body away from the electromagnet.

7 Depending on the intensity of the current flow through the winding of the electromagnet, the probe is guided back into its initial position when the spring forces an air spacing between the poles, or indeed the counterpole is attracted in opposition to the action of the spring by the electromagnet to such an extent that it touches the opposing pole of the electromagnet. The contact then, so to speak, firmly clamps the probe in its position.

The magnet centring in this embodiment provides an extremely simple clamping option for the probe, for example if the probe moves rapidly from one measurement position to the next.

Embodiments of the invention are illustrated in the drawing, in which in particular:

Fig. 1 shows a sketch of the principle; Fig. 2 shows the force/distance characteristic curve; Fig. 3 shows a first embodiment; Fig. 4 shows a second embodiment; and Figs. 5a, 5b show a further embodiment.

8 In accordance with Fig. 1, a permanent magnet (1) with a north pole (N) and a south pole (S) is provided. Opposite the north pole (N), leaving an air gap (37), is a ferromagnetic body (2) with its face (2a) as the counter-pole. The body (2) bears the probe (3) (not illustrated in Fig. 1) and is movable in the direction of the arrow (8) with the aid of a transverse guide (15). The field lines running between the_north pole (N) and the body (2) have the effect that in the event of a movement of the body (2) in the direction of the arrow (8), that is to say out of its basic position, the body (2) is always drawn back into its illustrated initial position.

The force/distance characteristic curve (F = force, s = distance) of this construction is illustrated in Fig. 2. In respect of its upward gradient in the region of the zero point (0), it largely has the usual shape of the force/distance characteristic curve of a centring device operating with a spring parallelogram for guiding the probe back in accordance with the prior art.

Fig. 3 shows an embodiment for suspending the probe (3). The probe (3) is secured to a body (4) which is 6 secured with the aid of leaf springs (5 and 6) to a fixed part (7) of the centring device.

A 9 For moving towards the measurement point on the workpiece in accordance with coordinates, the part (7) is displaced in accordance with coordinates in the direction of the arrow (16). When the probe ball (3a) of the probe (3) meets the workpiece to be measured (not illustrated), the ball (3a) moves for example in the direction of the arrow (9) because of the spring parallelogram suspension of the probe (3). -At the same time, the body (4) is displaced parallel to the body (1) with the aid of the leaf springs (5, 6). If the probe head is then moved into a position in which there is no longer any contact between the ball (3a) and the workpiece, the probe (3) should re-adopt its initial position (zero position).

In accordance with Fig. 3, the body (2) serving as a counter-Pole to the permanent magnet (1) is connected to the body (4) bearing the probe (3) with the aid of connection pieces (10), for guiding the probe (3) back into its initial position. This means that the probe (3) can move with the body (4) in the direction of the arrow (8). So that the body (2) and thus the probe (3) are moved into their precise initial position, the north pole (N) Qf the permanent magnet (1) is placed opposite the counterpole of the body (2). The permanent magnet (1) is for this purpose fixedly connected to the part (7) of the centring device by way of connection pieces (12).

The mutually opposing faces (13 and 14) of the permanent magnet (1) and the body (2) are constructed as planar faces lying parallel to one another. They have the air gap (37) between them. The force lines of the magnetic field which run between the north pole (N) and the body (2) have the effect that the body (2) is always moved precisely back into the initial position after a lateral deflection and thus so is the probe (3) connected to the body (2).

The probe in accordance with Fig. 3 is only deflectable in one coordinate direction by the suspension on the leaf springs (5, 6).

Fig. 4 shows an arrangement for the deflection of the probe in two coordinate directions (X and Y) lying perpendicular to one another. Once again, the body (4) is secured to the part (7) borne by the probe head by way of leaf springs (5, 6), in a manner deflectable in the X direction in the plane of the drawing. However, the part (4) does not now bear the probe (3) directly, but the probe (3) is also movable in the Y direction, that is to say perpendicular to the plane of the drawing, by way of a further spring parallelogram (20, 11 21) turned through 90' with respect to the spring parallelogram (5, 6). The leaf springs (20, 21) are secured on the one hand to the body (4) and on the other hand to the body (22) which now bears the probe (3). The z body (22) is thus deflectable in a plane perpendicular to the plane of the drawing.

In order to guide the probe (3) back into its initial position again once it has approached a workpiece (not illustrated), the body (22) bears the permanent magnet (1). Opposite the north pole (N) of the permanent magnet (1) is the body (2) with its face (2a). The body (2) is secured to the body (7) in this construction. This securing is not mandatory. It can also be carried out in reverse, that is to say in such a way that the body (2) and the permanent magnet (1) are exchanged from one another, or indeed that two similar permanent magnets are placed opposite one another. The same also applies to the arrangement in accordance with Fig. 3.

In the construction in accordance with Fig. 4, the probe (3) is moved back into its initial position by the attractive force between the body (2) and the north pole (N) of the magnet (1) without a further magnet arrangement being needed. Advantageously, in this application the body (2) and the permanent magnet (1) are constructed to be centrally symmetrical with respect 12 to the axis (A-A) in order to obtain a rotationally symmetrical magnetic field between the magnet (1) and the body (2).

Figs. 5a and 5b show a further embodiment. The permanent magnet (1) is replaced by an electromagnet (30). A current can flow through a winding (32) of the electromagnet (30) by way of the lines (31). The body (2) with its face (2a) serving as the counter-pole is arranged in a guide (33) of a body (35). The body (35) bears the probe (3). A spring (34) is supported on the one hand against an inwardly angled part (33a) of the guide (33) and on the other hand in the guide (33) by means of a plate (36) of the body (2). The spring (34) has the effect that the faces (2a) of the body (2) and the face (38a) of the core (38) of the electromagnet (30) are opposite one another with an air spacing.

With this construction, it is possible to eliminate the air gap (37) between the core (38) and the body (2) which is necessary for guiding the probe back into its initial position by allowing a sufficiently intense current to flow through the winding (32) around the core (38) of the electromagnet (30) by way of the line (31). In this case, the electromagnet (30) draws the body (2) into the position (2b) (Fig. 5b), with the result that the faces (40 and 41) of the electromagnet (30) and the 13 body (2) touch. If such a contact takes place, the body (2) can no longer be displaced with respect to the electromagnet (30). In other words, it is firmly clamped and thus so is the probe (3), which is advantageous for rapidly approaching a new measurement point.

If a weak current is sent through the winding (32) of the electromagnet (30), the spring (34) holds the body (2) at an air spacing (37) with respect to the electromagnet (30), with the result that the latter displays the effect of the permanent magnet (1) in accordance with Figs. 3 and 4.

If no current at all is sent through the winding (32) of the electromagnet (30), there is no automatic guiding of the probe back into the initial position.

4 1 14 Claims 1. A centring device for a deflectably mounted mechanical probe comprising a fixedly arranged first magnet and a second magnet or a ferromagnetic body connected to the probe in such a way that a pole of the first magnet and the body as the counter-pole are opposite one another with an air gap therebetween.

2. A centring device for a deflectably mounted mechanical probe wherein the probe has a fixedly arranged first magnet and the centring device has a second magnet or a ferromagnetic body connected thereto in such a way that a pole of the first magnet and the second magnet or body as the counter-pole are opposite one another with an air gap therebetween.

3. A centring device according to Claim 1 or 2, wherein the second magnet or the ferromagnetic body is mounted laterally deflectably with respect to the first magnet in.such a way that the air gap has at least an substantially constant width.

4. A centring device according to Claim 1 to 3, wherein the probe.is connected to the fixed part of the centring device by means of at least one spring parallelogram.

5. A centring device according to Claim 1 to 4, wherein the probe is connected to the fixed part of the centring device by means of two spring parallelograms, one spring parallelogram being turned through 90 with respect to the other spring parallelogram.

6. A centring device according to any one of Claim 1 to 5, wherein the magnets or the first magnet and the ferromagnetic body are constructed to be rotationally symmetrical.

7. A centring device as claimed in any one of the preceding claims, wherein the first magnet is constructed as an electromagnet.

8. A centring device according to Claim 7, wherein in that the ferromagnetic body opposite the electromagnet acting as the coUnter-pole is movable in opposition to the pressure of a spring until the mutually opposing poles of the electromagnet and the ferromagnetic body touch.

9. A centring device according to Claim 7, wherein the ferromagnetic body opposite the electromagnet is arranged in a guide.

16

10. A centring device according to any one of the preceding claims, wherein the opposed poles of the magnets or the magnet and the ferromagnetic body are constructed as planar surfaces at least substantially parallel to one another.

11. A centring device according to any one of the preceding claims, wherein a linear guide is provided for the moving magnet or counter-pole.

12. A centring device according to Claim 10, wherein the linear guide is constructed as a sliding, air or rolling bearing.

13. A centring device for a deflectably mounted mechanical plobe substantially as herein described with reference to any one of the embodiments shown in the accompany drawings.