Patent Publication Number: US-11650050-B2

Title: Method of calibrating a surface sensing device, corresponding calibrating program for a control computer and corresponding calibration kit

Description:
FIELD OF THE INVENTION 
     The present invention relates to a surface sensing device for use in position determining apparatus or machine such as, for example, a co-ordinate-measuring machine (CMM), a scanning machine, a machine tool or an inspection/measurement robot. More particularly the invention relates to the calibration of such a surface sensing device. 
     DESCRIPTION OF PRIOR ART 
     Such a position determining apparatus or machine may be used for measuring a workpiece, and typically comprises a member which is movable in three coordinate directions X,Y,Z relative to a table on which the workpiece is supported. Movement of the member in each of the directions X,Y,Z is measured by transducers on the machine. A surface sensing device or probe provided on the movable member produces a signal indicating the relationship between it and the workpiece surface to be measured. The position of the surface may thus be determined. In an alternative machine, for example some types of machine tool, the table moves in X and Y and the movable member moves in Z. 
     Some types of contact probe, e.g. touch trigger probes and contact scanning probes, act in three dimensions. Typically they can approach a workpiece surface in any direction within the X,Y,Z coordinate system of the machine&#39;s transducers, and they have a deflectable stylus with a workpiece-contacting tip. Prior to making measurements with such probes, it is usual to perform a kind of calibration known as “datuming”. This involves touching the stylus tip against a datum surface (such as a datum sphere), in order to determine a repeatable location of the stylus tip in the X,Y,Z coordinate measurement system of the machine. 
     In a flexible measurement system, in order to access different workpiece surfaces, the probe may be mounted on a CMM via a motorised articulating head, by which it is rotatable about two mutually orthogonal axes of rotation. The machine and the head may operate under computer control. 
     Probes (or surface sensing devices) may be exchangeable. When not in use, they are stored in respective storage ports of a storage rack. When required for measurement, the articulating head can automatically pick up a probe from a storage port of the rack, and then dock it back in the storage port afterwards, all under computer control. The present applicants Renishaw plc sell such a system under the trade mark Autochange. To enable alignment of the articulating head with respect to the rack, the Autochange rack is fitted with a fixed touch trigger probe. When setting up the rack, surfaces of the head are touched against the rack&#39;s probe. This establishes a spatial relationship between the head and the ports of the rack, to ensure their alignment. 
     U.S. Pat. No. 8,006,399 (Wallace et al) and U.S. Pat. No. 8,468,672 (Wallace) describe a surface sensing device. In order to access different workpiece surfaces, it is mounted on a CMM via an articulating head, by which it is rotatable about two mutually perpendicular axes of rotation. The surface sensing device may comprise a surface finish probe which senses the surface unidirectionally, or another type of unidirectional (single axis) probe, for example an optical probe such as a laser spot or laser line probe. To enable such a unidirectional probe to address differently-oriented surfaces, it has a third axis of rotation, enabling it to be rotated manually. The third axis of rotation may be aligned with a generally longitudinal axis of the surface sensing device, which may sense the surface in a direction transverse to or offset from the third axis. Alternatively, the surface sensing device may be arranged at an angle to the third axis of rotation. 
     However, this third axis of rotation only enables surfaces to be addressed in certain possible orientations. There is also a problem of datuming or calibrating the geometry of such a surface sensing device. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of calibrating a surface sensing device for use in position determining apparatus, the surface sensing device comprising one or more elongate members mounted to the position determining apparatus, and a surface sensing element mounted to the apparatus via the one or more elongate members, the method comprising probing at least one of the elongate members with a separate probe to determine its orientation. 
     One of the one or more elongate members may be connected to a pivotable joint whereby it can be set at a desired angle. 
     The separate probe may probe one or more side locations of the elongate member or members. The elongate member may be probed a plurality of times by the same probe. 
     There may be first and second elongate members, and a pivotable joint connecting the first and second elongate members whereby the second member can be set at a desired angle relative to the first member, the first elongate member being connected to the position determining apparatus, and the surface sensing element being carried by the second elongate member. 
     Preferably, the probing comprises probing at least the second elongate member with the separate probe to determine its orientation. The method may also comprise probing the first elongate member with a separate probe to determine its orientation, and then determining the angle of the second member relative to the first member. 
     Suitably, the or each elongate member is probed at at least two positions spaced along its length. Preferably the or each elongate member is probed at at least six positions. This gives a more accurate indication of its orientation and location in space. 
     In a preferred embodiment, the surface sensing device is one which senses unidirectionally, e.g. a surface finish or surface roughness probe. The surface sensing element may be a stylus with a tip, mounted to deflect in a normal direction when the stylus tip is dragged across a surface, and connected to a transducer to measure the deflection of the stylus thereby to determine surface finish or surface roughness. 
     The method may include determining the normal direction of the stylus tip and/or the direction in which the tip is dragged along the surface. This may be determined from the orientation(s) of the one or more elongate elements. 
     The method may also include determining an offset which describes the location of the stylus tip relative to the position determining apparatus. This may be calculated from the orientation(s) and the length(s) of the one or more elongate members. Alternatively, it may be determined by probing the surface sensing device in the vicinity of the stylus tip with a separate probe. 
     The surface sensing device may include a rotary joint configured to rotate the elongate member connected to the position determining apparatus, generally about the longitudinal axis of the elongate member. The method may include determining the orientation of the axis of rotation (which may not coincide exactly with the orientation of the elongate member). The rotary joint may have a motor for rotating the elongate member. 
     The surface sensing device may be mounted to the position determining apparatus via an articulating head, configured to rotate the surface sensing device about two mutually orthogonal axes. The articulating head may include motors for rotating the surface sensing device about the mutually orthogonal axes. 
     The invention also provides a program for a computer control of a position determining apparatus, configured to perform any of the above methods. 
     A further aspect of the invention provides a kit for use with a position determining apparatus, including a surface sensing device comprising one or more elongate members for mounting to the position determining apparatus, a surface sensing element for mounting to the apparatus via the one or more elongate members, and a program configured to perform any of the above methods. 
     Programs discussed herein may be provided on any suitable machine readable medium, including memory discs, memory sticks, memory cards or a local or remote server from which the program can be downloaded. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will now be described by way of example, with reference to the accompanying drawings, wherein: 
         FIG.  1    is an isometric view of a surface sensing device mounted on an articulating probe head; 
         FIG.  2    shows a section through axes A and B of  FIG.  1   ; 
         FIG.  3    is a schematic diagram of the geometry of the surface sensing device; and 
         FIGS.  4 - 7    show the surface sensing device and articulating probe head mounted on a coordinate measuring machine (CMM). 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG.  1    shows an articulating probe head  7 , which supports a surface sensing device  4  for rotation about two mutually orthogonal axes of rotation A, B.  FIG.  2    shows a section through the articulating head  7  and the surface sensing device  4  in a plane defined by the axes A, B. 
     The articulating probe head  7  comprises first and second housing members  1  and  2  respectively. The first housing member  1  is adapted for attachment to a position determining apparatus, for example to a movable arm  26  of a CMM as seen in  FIG.  4   . The CMM moves the arm  26  in three linear dimensions X,Y,Z. As shown in  FIG.  2   , the housing member  1  houses a motor M 1  for effecting angular displacement of a first shaft  20  about the first axis A. Attached to the first shaft  20  is the second housing member  2 , which houses a motor M 2  for effecting angular displacement of a second shaft  22  about the second axis B. The surface sensing device  4  is attached to the second shaft  22  for rotation therewith. The CMM is driven in the X,Y,Z directions under the control of a program in a computer control  3 , which also controls the movements of the motors M 1 , M 2  about the axes A, B. 
     The surface sensing device  4  includes an elongate probe holder  8  which extends generally along an axis C, transverse to and intersecting the axis B. This is attached to the articulating head  7  via a housing  9 . The housing  9  contains a motor M 3  which rotates the probe holder  8  about the axis C, again controlled by the program in the computer control  3 . 
     To enable the exchange of surface sensing devices of different types or configurations, the housing  9  has a kinematic coupling joint  6  of a known type, by which the holder  8  of the surface sensing device  4  is attachable to and detachable from the housing  9 . When not in use, surface sensing devices may be stored in ports  34  of a storage rack  30 , as illustrated at  4 A in  FIG.  4   . Their exchange is carried out automatically by movements of the CMM arm  26  and the articulating head  7 , under the control of the program in the computer control  3 . When the surface sensing device  4  is re-attached, the kinematic joint  6  ensures that it is repeatably located with respect to the housing  9 , so that the calibration described below need not be repeated each time the surface sensing device is exchanged. The kinematic joint  6  includes magnets (not shown) which hold the surface sensing device in place when in use. 
     The surface sensing device  4  includes an elongate sensing module  10 , containing a surface finish or surface roughness sensor. The module  10  is pivotably attached to the holder  8  by a knuckle joint  12 . This enables the module to be oriented manually to a desired set angle with respect to the holder  8 , prior to a measurement task. The knuckle joint  12  then 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 module  10  may be further altered under program control during a measurement task, by rotation about the axis C by the motor M 3 . 
     The surface finish or roughness sensor contained in the module  10  may 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 stylus  5 , 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 module  10 , relative to a skid  14  ( FIG.  3   ). In use, the skid and the stylus tip are dragged across a surface, by X,Y,Z motions of the CMM arm  26 , or by rotations of the articulating head  7 , under the control of a program in the computer control  3 . The stylus  5  is connected to a transducer in the module  10 , 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 computer  3 . 
     The holder  8  and sensing module  10  can be provided in numerous different configurations to suit varied measurement tasks. For example, they can be provided in different lengths, or the broken lines  10 A in  FIG.  3    show that the outer end of the module  10  may 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 module  10  includes a kinematic coupling joint  16 , which provides “overtravel” in a known manner. This permits the outer end of the module  10  to 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 joint  16  locates 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 joint  16  can be simpler than the kinematic joint  6 . 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 device  4  is calibrated or datumed. The calibration is performed after the elongate sensing module  10  has been manually set to a desired angle at the knuckle joint  12 , which can be set approximately using a protractor. The calibration is performed under program control from the control  3 . It determines several aspects of the geometry, as seen schematically in  FIG.  3   :
         The direction and origin of the axis of rotation C in relation to the CMM&#39;s coordinate system X,Y,Z. Swash and runout of the axis C may also be determined.   The axis  40  of the elongate holder  8 . This may for example be determined with respect to the axis C. (It will be appreciated that the axis  40  will not coincide precisely with the axis C because of manufacturing tolerances. In  FIG.  3    this has been exaggerated for purposes of illustration.)   The axis  42  of the elongate sensing module  10 , giving the point of intersection K of the axes  40 , 42  at the knuckle joint  12 , and a more accurate measurement of the angle θ k  between these axes.   A vector TN (tip normal) describing the direction of deflection of the tip  5  (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 tip  5  is dragged along the surface being measured.  FIG.  3    shows this as being in the longitudinal direction of the sensing module  10 , 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 tip  5  relative to the origin C 0  of 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 in  FIGS.  4 - 7   , a separate touch trigger probe  32  is provided, having a deflectable stylus  36 . The probe  32  is fixed with respect to the CMM or other position determining apparatus. Suitably it may be fixed on the storage rack  30 . To link the measurements of the surface sensing device  4  to the CMM&#39;s coordinate measurement system, the touch trigger probe  32  should 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 arm  26 , as is known to those skilled in this field. 
     During the calibration of the surface sensing device  4 , the holder  8  and module  10  of the surface sensing device (previously set at the desired angle of the knuckle joint  12 ) are brought into contact with the deflectable stylus  36  of the touch trigger probe  32  in various positions and orientations, under the control of a calibration program in the control computer  3 , as shown for example in  FIGS.  5 ,  6  and  7   . This contact deflects the stylus  36  and causes the touch trigger probe to issue a trigger signal to the computer control  3 . 
     Preferably, these contacts against the separate touch trigger probe  30  are performed by first orienting the surface sensing device  4  in a desired direction, by rotating the articulating head  7  about 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 control  3  freezes the readings of the CMM&#39;s X,Y,Z position transducers, which indicates the X,Y,Z coordinates of the point on the holder  8  or module  10  which has made contact. 
     The following measurements all take place with the A and B axes of the articulating head  7  kept in a single orientation. 
     Referring more particularly to  FIG.  3   , the C axis is driven to a chosen orientation, e.g. such that the elongate holder  8  and sensing module  10  lie approximately in the Y-Z plane. Then the CMM is driven on its X,Y,Z axes to bring the elongate holder  8  into contact with the touch trigger probe  32  at points  44 A,  44 B. Suitably these points are on the sides of the holder  8 . Readings of the X,Y,Z coordinates are taken of each point. Points should be taken at at least two positions  44 A,  44 B spaced along the length of the elongate holder. From this the direction of the axis  40  is determined. It is preferable to take at least 6 points in total, since this enables the precise determination of the cylinder defining the holder  8 , and hence the direction of its axis  40 . 
     In order to determine the direction of the axis of rotation C and its origin C 0 , the motor M 3  is 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 points  44 A,  44 B is repeated at each orientation. This gives the direction of the axis  40  at 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 axis  40  of the elongate holder  8  with respect to the C axis. The swash and runout of the C axis may also be calculated. 
     If there is confidence that the axis  40  coincides with the C axis of rotation (within a desired tolerance) then it may not be necessary to repeat this determination of the axis  40  at 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 M 3  of the C axis in one of the above orientations (e.g. the last one measured) points  46 A,  46 B are taken on the elongate sensing module  10 , in the same way as above (suitably on the sides of the module). Like the points  44 A,  44 B, these points should preferably be at two spaced positions along the length of the module  10 , and preferably at least six points are taken in total in order to determine precisely the cylinder defining the module  10 . This gives the direction of the axis  42  of the sensing module  10 . Knowing the axes  40  and  42  in the chosen orientation, it is straightforward to calculate their point of intersection K at the knuckle joint  12 , and an accurate value of the angle θ k  between these axes. 
     However, although we prefer six points  46 A,  46 B at two spaced positions, it may be possible to use less. For example, if the length L 1  to the point K is known sufficiently accurately with respect to the C axis origin C 0 , then it may be possible to determine the axis  42  from that in conjunction with e.g. three points  46 B at one position along the module  10 . In this case, the determination is based on the fact that the points  46 B and the point K are spaced along the length of the module  10 . 
     Furthermore, from the nominal design lengths L 1 , L 2  of the elongate holder  8  and module  10 , together with the point K and the knuckle angle θ k , it is now also straightforward to calculate an approximate value for the tip offset TO of the tip  5  relative to the C axis origin C 0 . From the knuckle angle θ k  and the nominal lengths L 1 , L 2 , values can be calculated for the tip normal TN and drag vector DV. If it is desired to scan the sensing module  10  sidewise 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 lines  10 A) 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 tip  5  to 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 holder  8  or sensing module  10  fouling 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 tip  5  precisely 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 points  48  may be taken on the nose of the sensing module  10 , in the vicinity of the tip  5 . Two such points  48  are shown in  FIG.  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 tip  5  can 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 axes  40 ,  42  and/or the position K and angle θ k  of the knuckle joint  12 . 
     As mentioned, all the above calibration measurements have been carried out with the A and B axes of the articulating head  7  kept 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 in  FIGS.  5 ,  6  and  7   . This may be the case, for example, if the elongate holder  8  and/or the elongate sensing module  10  of the surface sensing device is subject to droop caused by gravity when moved from the vertical position of  FIG.  5    to the horizontal position of  FIG.  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 module  10  which 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).