Abstract:
A method of scanning is disclosed comprising, providing a scanning system ( 10 ) having a sample holder ( 14 ) and a relatively movable scanning device ( 18 ), performing a scan of at least a part of an object ( 22 ) located on the sample holder ( 14 ), establishing orientation of the sample holder and interpreting data from a scan using the orientation whereby, the orientation is established using data from the scan of the surface of the sample holder. The orientation may be established by defining a plane ( 56   b ) of the sample holder, which may be limited by boundaries ( 76   a,    76   b ).

Description:
This invention relates to a method of scanning. 
   BACKGROUND 
   When a rotatable sample holder is used in conjunction with a relatively movable scanning device, it is known to scan around the holder in order to establish or confirm the radius of the holder and the co-ordinates of the origin of the holder. This information is used to interpret data produced during the scanning of an object located on the sample holder. 
   A problem with this method is that it assumes that the sample holder surfaces are square and that the axis of the circumferential surface of the sample holder remains co-linear with respect to its rotational axis along its length i.e. that the surface of the sample holder on which a sample is located is perpendicular to the rotational axis. Additionally, in order for this assumption to be treated as valid, the equipment must be manufactured to tight tolerances which increases the cost of the equipment. 
   An alternative scanning system uses Cartesian scanning in which case the sample holder is stationary during a scan. Traditionally it is assumed that the centre line of the sample holder is square to the axes of the scanning system however, for certain applications where it is important that the longitudinal axis of a sample is known this assumption may be invalid leading to errors. 
   SUMMARY 
   Accordingly, according to one aspect the invention provides a method of scanning comprising the steps of:
         providing a scanning system having a sample holder and a relatively movable scanning device;   performing a scan of at least a part of an object located on the sample holder;   establishing orientation of the sample holder; and   interpreting data from the scan using the orientation of the sample holder characterised in that, the orientation is established using data from the scan of the object.       

   Preferably, the sample holder rotates about a rotational axis and the orientation of the sample holder is established with respect to the rotational axis. Alternatively, the sample holder is stationary during a scan. Establishing the orientation of the sample holder provides a datum for the sample scan. 
   Preferably, the orientation is established by defining a plane of the sample holder. 
   According to a second aspect, the invention provides a method of scanning comprising the steps of:
         providing a scanning system having a sample holder and a relatively movable scanning device;   scanning a datum;   scanning a sample; and   interpreting data from the sample scan using data from the datum scan   characterised in that the scanning system automatically carries out the datum and sample scans.       

   Once the scanning system recognises that a sample is located on the sample holder, the process is automated so does not require operator intervention. The system may recognise location of a sample due to an operator informing it, for example by pushing a button, or, by sensing that a sample is located. Such sensing includes a contact being correctly made between the sample holder and scanning system, the weight of a sample being present or the breaking of a light beam within the sample envelope of the scanning system. 
   According to a third aspect, the invention provides a method of scanning comprising the steps of:
         providing a scanning system having a sample holder and a relatively movable scanning device;   scanning a datum;   scanning a sample; and   interpreting data from the sample scan using data from the datum scan   characterised in that both the datum and sample scans are carried out effectively as one scan.       

   The advantage of this is that the system does not require operator involvement. The datum and sample scans can be carried out as a single scan i.e. with continuous motion, or at the completion of one of the scans, the probe may pause before starting the next one. This enables the data from the two scans to be separated without an operator having to determine where the split occurs so reduces the chance of errors occurring. 
   The two scans (or parts of a single scan) can be carried out in either order. 
   The probe does not require re-positioning between the two scans when a pause is included. 

   
     BRIEF DESCRIPTION OF DRAWING 
     The invention will now be described by way of example, with reference to the accompanying drawings, of which: 
       FIG. 1  shows schematically a scanning system; 
       FIG. 2  shows schematically an alternative scanning system; 
       FIG. 3  shows schematically the effect of an orientation of a sample holder with respect to the rotatable axis; 
       FIG. 4  shows a preferred method of scanning; 
       FIG. 5  is a flow diagram showing different steps according to an embodiment of the invention; and 
       FIG. 6  is a flow diagram showing optimisation of the orientation plane. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a scanning system  10  having a base  12  which supports a sample holder  14  and a back portion  16 . A scanning device  18 , which in this case is a probe having a scanning tip  20 , is supported by the back portion  16 . A sample  22  is located on the sample holder  14  for scanning. The sample holder  14  is moveable along a vertical or Z-axis and is rotatable about an axis  24  which is substantially parallel to the vertical motion. Thus, the sample holder  14  moves along a helical path. The probe tip  20  is moveable along an axis A which is disposed at 45° to the axis of rotation  24  of the sample holder  14 . 
   In an alternative arrangement, the sample holder  14  is rotatable and the scanning device moves in the vertical or Z direction. 
     FIG. 2  shows an alternative scanning system having a c-shaped frame  200  onto which a sample mount  210  is placed. At the distal end of the sample mount  210  is an opening  212  designed to receive a sample holder  220 . The sample holder  220  is secured to the sample mount  210 , for the purposes of scanning a sample  250 , using a screw  230 . A probe  260  extends down from the frame  200  towards a sample  250  via adjustable struts  270 . 
   In this example, the sample mount  210  is fixed and the probe  260  moves around the sample. One way to scan a sample is to carry out a series of radial line scans vertically along the sample. 
   The Cartesian scanning system described with respect to  FIG. 2  may alternatively be used to conduct a spiral scan of a sample. 
     FIG. 3  shows the effect of a misaligned or non co-linear sample holder  54   b . Instead of lying co-linearly with respect to the rotatable or vertical axis  64  as shown by dotted lines designated  54   a  and  52   a , a misaligned sample holder  54   b  is at an angle, the swash angle S, with respect to the rotational or vertical axis  64 . This in turn means that a scan of a sample  52   b  located on the misaligned sample holder  54   b  will obtain distorted results as, traditionally when interpreting the scan data, it is assumed that the sample holder is co-linear with respect to the rotational or vertical axis  64 . 
   To remove this source of error, the orientation of the plane of the upper surface  56   b  of the sample holder  54   b  is established. Advantageously, according to the invention, this is achieved by extracting data which meets certain requirements from the data set of a scan of an object. This requires the scan of an object to include probing of at least a portion of the surface of the sample holder on which the sample is located. 
   In the simplest case, where the sample holder is stationary or merely rotates, this can be achieved by extracting three angularly spaced apart measurements of the upper surface  56   b  of the sample holder. 
   Preferably, three equidistanced angularly spaced apart measurements are taken. These three points define the plane of the upper surface  56   b  and can be used to interpret or correct data relating to the sample to reflect the real plane of the upper surface  56   b . It is preferred that the plane orientation is obtained by using a plurality of data points as this reduces the effect of surface defects. 
   Referring now to  FIG. 1 , when the sample holder  14  is assigned a vertical movement as well as a rotational movement, more than three points are required as the helical or spiral path through which the sample holder moves will mask the actual plane of the upper surface  26   b . In this situation, a number of points are taken encompassing at least two-thirds of a rotation of the sample holder. Two-thirds of a revolution is the minimum angular rotation required to define a plane accurately. 
   In the embodiment where the sample holder does not rotate, it is also preferred that data is extracted for at least 240° (or two thirds) around the surface of the sample holder. 
   The size of the upper surface of the sample holder is a further factor which determines the size of the orientation data set. If the upper surface is small, it is preferred that a larger number of data points are taken as this then reduces the error introduced by any surface defects. 
   Referring now to  FIG. 4 , it is important that the probe tip is located properly on the upper surface  56  when data for the orientation information is being extracted so that any edge effects are not included. For example, if the sample holder  54  is provided with a chamfer  74  and the sample  52  has a rounded edge  72  where it meets the sample holder  54 . If either the chamfer  74  or rounded edge  72  are included when the plane of the upper surface is being calculated, errors would be introduced. Thus, in a preferred embodiment, inner and outer radial boundaries are set which define a region from which the orientation data must be taken. 
   The boundaries are also established by extracting data from the sample scan. Data from one revolution around the circumference of the sample holder is used to establish the radius and co-ordinates of the origin of the sample holder. A minimum of at least three, but preferably four or more measurements are extracted. 
   A known defect in the surface, for example, the chamfer  74 , is subtracted from the radius of the sample holder  54  to create the outer boundary  76   b . The radius of the sample  52  plus rounded edge  72  is added to the origin of the sample holder  54  to create the inner boundary  76   a . A safety factor, for example, to account for non-central placement of the sample may also be added to or subtracted from the boundaries. The probe tip thus has a defined region  76  in which the orientation information must be collected in order to be considered viable. 
   As the orientation information is extracted from a scan of a sample, there is a chance of interference if, for example, the sample includes an overhang  78 . In this situation, the probe tip may encounter the sample overhang  78  instead of the upper surface of the sample holder  54 . To alleviate this problem, the orientation information may be further constricted by a z boundary which defines limits for the extraction of data in the vertical direction. 
   For the scanning system of  FIG. 2 , the z boundary does not need to be established for a particular sample holder and mount combination as their height is known and does not change appreciably from one scan to the next so, this information is used to set a z boundary, if required. 
   For the scanning system of  FIG. 1 , where a sample holder may move vertically it is required to set a z boundary for each scan. As the probe scans  40  perpendicularly to the rotational or vertical axis  64  (see  FIG. 3 ), it is assumed that the probe tip will first encounter the upper surface  56  of the sample holder  54   b  at the lowest point on the plane surface. This point of first encounter is used to define z boundaries for the orientation scan. As a minimum of between two-thirds and about three-quarters (see  FIG. 5 ) of a revolution is desired to define the plane of the upper surface, the upper vertical distance is defined as a minimum of the height change experienced during one revolution. The lower vertical distance is preferably defined not as zero, but as minus about half a revolution to account for circumstances where the swash angle S is small when the assumption that the first encounter is at the lowest point may not be valid. Again additional safety factors may be added to these limits. 
   When a z boundary has been defined, it has the effect of limiting the swash angle S that can be detected. Note, a system has a maximum swash angle with which it can function and this provides a maximum value that the z boundary cannot exceed. This can be advantageous as, the larger the swash angle S, the more eccentric the movement of the sample which can introduce additional errors into a scan. For example, when a touch probe is used, there are limits to the amount of motion the probe tip can undergo along its axis ( FIG. 1 , axis A). When the probe nears the limits of this motion, accuracy is reduced. 
     FIG. 5  shows a flow diagram  100  which details the steps involved in one embodiment of determining whether or not to accept the orientation information. 
   In this example, the probe automatically conducts a single scan of the upper surface and a sample. A single revolution consists of 1820 data points being taken by the probe tip. 
   Firstly, the scan  110  is conducted. Next, it is determined whether 240° around the surface of the sample holder has been conducted  120 . If not, then the orientation scan is rejected  122 . If the answer is yes, then the orientation information is accepted  124 . Two-thirds (or 240°) around the surface is the minimum angular range required in order that a plane can be determined accurately. In this example, two-thirds around the surface means that 1220 data points have been collected. 
   In a next, optional step, the surface is divided into quadrants  130  and each quadrant is checked against a minimum number of data points  140 . If any quadrant does not have the required minimum, the orientation information is rejected  142 . If all the quadrants meet this requirement, we proceed  144  to the next step. The requirement that data is collected in each quadrant provides more accuracy and consistency than the requirement that two-thirds of a revolution is completed. This is because two-thirds of a revolution can include either three or four quadrants. As the inclusion of a minimum data set in each of four quadrants is statistically more accurate than the two-thirds of a revolution requirement, this is a preferred feature. In this example, a minimum data set for each quadrant is slightly less than half a whole quadrant data set i.e. 200 data points. 
   If this final, optional requirement is met, a best fit for the orientation of the plane is determined  150  using known mathematical techniques. 
   Additionally or alternatively, to ensure that any sample overhang is avoided in the calculation of the orientation of the plane of the upper surface, a maximum data set for each quadrant may be set. In this example, the number of data points per quadrant in a single revolution i.e. 455 data points is used as this limit. Again, the orientation information may be subject to x,y boundaries detailed above. 
   If the orientation information has been rejected, then a further scan is made whose data may be used instead of or as an addition to the data used in the rejected plane. Thus the orientation information may comprise data from a number of discrete scans. Furthermore, the orientation information may comprise data points from anywhere within the x,y boundary (if used) including from more than one revolution. 
   As an alternative to conducting a single scan, the datum and sample scans can be effectively carried out as one scan with a pause in between enabling automatic separation of the orientation data from the sample data. The sequence followed when deciding whether or not to accept the orientation information is the same. 
     FIG. 6  is a flow diagram  300  showing a preferred embodiment where the orientation data is optimised. Once the orientation data has been extracted from scan data and an initial plane orientation calculated  310 , it is analysed  320  and measurement co-ordinates which are significantly different from the averaged plane are omitted. 
   The data is analysed  320  firstly to see is any of the data points lie outside the tolerance range  330  which is, for example, 15 μM. If no data is outside the tolerance range  330  then the initial calculation of the orientation of the plane is accepted  332 . If there is data which exceeds the tolerance  334  then data points which are beyond a certain range of the initial calculated plane are omitted  340 . This range can be the tolerance i.e. in this example 15 μM, or as is preferred less than this to ensure that the tolerance requirements are met, for example 10 μM. The orientation of the plane is recalculated  350 . 
   As a safety check, it is preferred that the remaining data set which is used to recalculate the plane orientation comprises at least 80% of the extracted data  360  for the acceptance of the recalculated plane  370 , otherwise the data is rejected and a further scan must be carried out  380 . 
   Although in the examples given, the scanning device used has been a touch probe, the invention is not limited to such devices and non-contact probes such as optical scanning devices are also suitable for use with the invention. 
   The method according to the invention is suitable for use in any circumstances where it is required that the relationship of scanned data with respect to a plane of axis be known. Examples include the medical field and in particular the dental industry and the production of replacement teeth for use as an abutment or part of a bridge, where the scanned data is used to produce a replica tooth which must fit orientation wise with adjacent teeth in the bridge and mouth.