Patent ID: 12259234

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG.1shows an articulating probe head7, which supports a surface sensing device4for rotation about two mutually orthogonal axes of rotation A, B.FIG.2shows a section through the articulating head7and the surface sensing device4in a plane defined by the axes A, B.

The articulating probe head7comprises first and second housing members1and2respectively. The first housing member1is adapted for attachment to a position determining apparatus, for example to a movable arm26of a CMM as seen inFIG.4. The CMM moves the arm26in three linear dimensions X,Y,Z. As shown inFIG.2, the housing member1houses a motor M1for effecting angular displacement of a first shaft20about the first axis A. Attached to the first shaft20is the second housing member2, which houses a motor M2for effecting angular displacement of a second shaft22about the second axis B. The surface sensing device4is attached to the second shaft22for rotation therewith. The CMM is driven in the X,Y,Z directions under the control of a program in a computer control3, which also controls the movements of the motors M1, M2about the axes A, B.

The surface sensing device4includes an elongate probe holder8which extends generally along an axis C, transverse to and intersecting the axis B. This is attached to the articulating head7via a housing9. The housing9contains a motor M3which rotates the probe holder8about the axis C, again controlled by the program in the computer control3.

To enable the exchange of surface sensing devices of different types or configurations, the housing9has a kinematic coupling joint6of a known type, by which the holder8of the surface sensing device4is attachable to and detachable from the housing9. When not in use, surface sensing devices may be stored in ports34of a storage rack30, as illustrated at4A inFIG.4. Their exchange is carried out automatically by movements of the CMM arm26and the articulating head7, under the control of the program in the computer control3. When the surface sensing device4is re-attached, the kinematic joint6ensures that it is repeatably located with respect to the housing9, so that the calibration described below need not be repeated each time the surface sensing device is exchanged. The kinematic joint6includes magnets (not shown) which hold the surface sensing device in place when in use.

The surface sensing device4includes an elongate sensing module10, containing a surface finish or surface roughness sensor. The module10is pivotably attached to the holder8by a knuckle joint12. This enables the module to be oriented manually to a desired set angle with respect to the holder8, prior to a measurement task. The knuckle joint12then holds the module set at this angle by friction, or the joint may have a tightening screw. In order to address different surfaces of a workpiece, the orientation of the module10may be further altered under program control during a measurement task, by rotation about the axis C by the motor M3.

The surface finish or roughness sensor contained in the module10may be of a known type, e.g. as described in the above U.S. Pat. Nos. 8,006,399 and 8,468,672. Typically it comprises a surface sensing element in the form of a needle or stylus5, having a surface-sensing tip which is small compared to the surface irregularities that are to be measured. This is deflectable transversely to the elongate sensing module10, relative to a skid14(FIG.3). In use, the skid and the stylus tip are dragged across a surface, by X,Y,Z motions of the CMM arm26, or by rotations of the articulating head7, under the control of a program in the computer control3. The stylus5is connected to a transducer in the module10, to measure the resulting deflections of the stylus normal to the surface, thereby indicating its surface finish or surface roughness. The results are sent back and processed in the computer3.

The holder8and sensing module10can be provided in numerous different configurations to suit varied measurement tasks. For example, they can be provided in different lengths, or the broken lines10A inFIG.3show that the outer end of the module10may be angled in order to better address a surface to be measured. Different types of skid may also be provided to suit different types of measurement.

The sensing module10includes a kinematic coupling joint16, which provides “overtravel” in a known manner. This permits the outer end of the module10to deflect against the action of a spring (not shown), to protect it from damage if it accidentally travels too far towards the workpiece and crashes into it. The kinematic joint16locates the outer end of the module repeatably so that it returns to the same position after it has been removed out of contact with the workpiece. Since it is not required to be detachable or exchangeable, the joint16can be simpler than the kinematic joint6. It may comprise a spring flexure which normally holds the outer end of the module against a stop.

Prior to use, the geometry of the surface sensing device4is calibrated or datumed. The calibration is performed after the elongate sensing module10has been manually set to a desired angle at the knuckle joint12, which can be set approximately using a protractor. The calibration is performed under program control from the control3. It determines several aspects of the geometry, as seen schematically inFIG.3:The direction and origin of the axis of rotation C in relation to the CMM's coordinate system X,Y,Z. Swash and runout of the axis C may also be determined.The axis40of the elongate holder8. This may for example be determined with respect to the axis C. (It will be appreciated that the axis40will not coincide precisely with the axis C because of manufacturing tolerances. InFIG.3this has been exaggerated for purposes of illustration.)The axis42of the elongate sensing module10, giving the point of intersection K of the axes40,42at the knuckle joint12, and a more accurate measurement of the angle θkbetween these axes.A vector TN (tip normal) describing the direction of deflection of the tip5(which during subsequent measurements should be aligned with the normal to the surface being measured).A vector DV (drag vector) describing the direction along which the tip5is dragged along the surface being measured.FIG.3shows this as being in the longitudinal direction of the sensing module10, but if sideways scanning is intended then a drag vector may be calculated laterally to the sensing module (in addition or alternatively to the longitudinal vector.)The offset TO (tip offset) of the tip5relative to the origin C0of the axis C.

The calibration may determine all of the individual geometrical aspects listed above, or any of them individually or in any combination, to suit the measurement task which is to be performed.

This calibration will now be described in more detail.

As shown inFIGS.4-7, a separate touch trigger probe32is provided, having a deflectable stylus36. The probe32is fixed with respect to the CMM or other position determining apparatus. Suitably it may be fixed on the storage rack30. To link the measurements of the surface sensing device4to the CMM's coordinate measurement system, the touch trigger probe32should desirably have been datumed in the CMM coordinate system. This may be done by probing the tip of its stylus with a reference probe held on the movable arm26, as is known to those skilled in this field.

During the calibration of the surface sensing device4, the holder8and module10of the surface sensing device (previously set at the desired angle of the knuckle joint12) are brought into contact with the deflectable stylus36of the touch trigger probe32in various positions and orientations, under the control of a calibration program in the control computer3, as shown for example inFIGS.5,6and7. This contact deflects the stylus36and causes the touch trigger probe to issue a trigger signal to the computer control3.

Preferably, these contacts against the separate touch trigger probe30are performed by first orienting the surface sensing device4in a desired direction, by rotating the articulating head7about its A and B axes. Then the surface sensing device is moved in the linear directions X,Y,Z by driving the CMM on its X,Y,Z axes. On receipt of each trigger signal, the control3freezes the readings of the CMM's X,Y,Z position transducers, which indicates the X,Y,Z coordinates of the point on the holder8or module10which has made contact.

The following measurements all take place with the A and B axes of the articulating head7kept in a single orientation.

Referring more particularly toFIG.3, the C axis is driven to a chosen orientation, e.g. such that the elongate holder8and sensing module10lie approximately in the Y-Z plane. Then the CMM is driven on its X,Y,Z axes to bring the elongate holder8into contact with the touch trigger probe32at points44A,44B. Suitably these points are on the sides of the holder8. Readings of the X,Y,Z coordinates are taken of each point. Points should be taken at at least two positions44A,44B spaced along the length of the elongate holder. From this the direction of the axis40is determined. It is preferable to take at least 6 points in total, since this enables the precise determination of the cylinder defining the holder8, and hence the direction of its axis40.

In order to determine the direction of the axis of rotation C and its origin C0, the motor M3is now operated to rotate the surface sensing device to two or more further orientations about the C axis. For example, the two further orientations may be at 120° from the Y-Z plane. The above procedure to take readings at points44A,44B is repeated at each orientation. This gives the direction of the axis40at each of three orientations about the C axis, which enables the calculation of the direction and origin of the C axis of rotation, and of the angle of the axis40of the elongate holder8with respect to the C axis. The swash and runout of the C axis may also be calculated.

If there is confidence that the axis40coincides with the C axis of rotation (within a desired tolerance) then it may not be necessary to repeat this determination of the axis40at two or more further orientations.

Conveniently, the B axis rotation offset (a zero position of the B axis) may now be set mathematically such that the C axis of rotation (as determined above) lies in the plane defined by the A and B axes when the B axis is in the zero position. This simplifies future measurements.

Next, with the motor M3of the C axis in one of the above orientations (e.g. the last one measured) points46A,46B are taken on the elongate sensing module10, in the same way as above (suitably on the sides of the module). Like the points44A,44B, these points should preferably be at two spaced positions along the length of the module10, and preferably at least six points are taken in total in order to determine precisely the cylinder defining the module10. This gives the direction of the axis42of the sensing module10. Knowing the axes40and42in the chosen orientation, it is straightforward to calculate their point of intersection K at the knuckle joint12, and an accurate value of the angle θkbetween these axes.

However, although we prefer six points46A,46B at two spaced positions, it may be possible to use less. For example, if the length L1to the point K is known sufficiently accurately with respect to the C axis origin C0, then it may be possible to determine the axis42from that in conjunction with e.g. three points46B at one position along the module10. In this case, the determination is based on the fact that the points46B and the point K are spaced along the length of the module10.

Furthermore, from the nominal design lengths L1, L2of the elongate holder8and module10, together with the point K and the knuckle angle θk, it is now also straightforward to calculate an approximate value for the tip offset TO of the tip5relative to the C axis origin C0. From the knuckle angle θkand the nominal lengths L1, L2, values can be calculated for the tip normal TN and drag vector DV. If it is desired to scan the sensing module10sidewise across the surface, then an appropriate drag vector at 90° to the vector DV may be calculated (as well as or instead of the longitudinal vector DV). Of course, if the sensing module has an alternative geometric configuration (such as shown by the broken lines10A) then the nominal values of the angle(s) and length(s) of this alternative geometry should be taken into account.

During subsequent measurements, the above calibration values enable the tip5to be brought into contact with a desired location on a surface of a workpiece of complex shape and measurements to be made, all automatically under program control, without parts of the holder8or sensing module10fouling or crashing against other surface of the workpiece. The program can be written on the basis of CAD data of the workpiece, without the necessity to manually position the surface sensing device using a joystick control. This is useful, for example, in constricted locations such as a tight groove, or where the workpiece has a blind internal space where it is not possible to see the surface sensing device in order to position it manually with a joystick.

It may be desirable to obtain a more accurate value for the tip offset TO, in order to position the tip5precisely at a point on the surface which is to be measured, and to determine the surface topography more precisely. In this case, three or more further points48may be taken on the nose of the sensing module10, in the vicinity of the tip5. Two such points48are shown inFIG.3—another is hidden since it should be on a different surface of the nose. These points locate the position and orientation of the nose precisely, and the position of the tip5can be determined from a knowledge of the nominal design geometry of the tip relative to the nose of the sensing module.

It is not essential for the invention to obtain all of the above calibrations. For example, it may be decided that only some are important, for example the tip normal TN, the drag vector DV and/or the tip offset TO, which affect the accuracy of subsequent measurements, not just the ability to position the surface sensing device without fouling or crashing it. In other cases where there is a greater risk of fouling and crashing, it may be more important to determine the geometry of the surface sensing device by accurately determining the axes40,42and/or the position K and angle θkof the knuckle joint12.

As mentioned, all the above calibration measurements have been carried out with the A and B axes of the articulating head7kept in a single orientation. However, it may be desirable to repeat them at other orientations of the A and B axes, for example as suggested inFIGS.5,6and7. This may be the case, for example, if the elongate holder8and/or the elongate sensing module10of the surface sensing device is subject to droop caused by gravity when moved from the vertical position ofFIG.5to the horizontal position ofFIG.6. Calibration at each position enables the droop to be calibrated out.

It will be appreciated that the geometric calibration described above is separate from any calibration of the transducer in the sensing module10which may be carried out.

The above preferred embodiments of the invention have related to surface finish or surface roughness probes of the contact type, having a stylus which is dragged across the surface to be measured. However, the invention may be used with other surface sensing devices, both contact and non-contact types. For example, it can be used with non-contact optical surface finish and surface roughness probes. It can also be used with touch trigger and contact scanning probes. Other non-contact probes include, for example, optical, capacitance and inductance probes. Optical probes include laser spot and laser line probes.

The invention is particularly useful for single axis probes such as optical probes and surface finish or surface roughness probes. This is because for these types of probes especially, rotation about the longitudinal axis (axis C in the above embodiments) greatly increases the number of surfaces the probe can access. Rotation about this axis is also particularly useful for laser line probes as it is possible to rotate the line about the axis of the surface sensing device (the third axis as mentioned above).