Patent Publication Number: US-2019178618-A1

Title: Measurement method and apparatus

Description:
The present invention relates to method of measuring an object and associated apparatus. In particular, the present invention relates to a technique that comprises imparting identifiable motions to the scan path traversed by a scanning probe to introduce time stamps that allow an object to be measured by analysis of the probe data alone. 
     Machine tools for manufacturing workpieces are known. It is also known that a measurement probe can be mounted in the spindle of such a machine tool to allow certain features of a workpiece to be measured using the machine tool. Such measurements may be used to establish the location of the workpiece and/or dimensions of cutting tools prior to a machining process. Measurements may also be performed to inspect a machined workpiece to verify a cutting operation has been performed correctly. 
     One known way to measure a workpiece using a machine tool is to use a spindle mounted scanning probe that comprises a deflectable stylus and one or more transducers for measuring stylus deflection in a local (probe) coordinate system. The stylus deflection measurements acquired by such a scanning probe are typically termed “probe data” and the measured position of the scanning probe within the coordinate system of the machine tool is typically termed “machine position data”. In use, the scanning probe is moved along a certain scan path relative to the object. The machine position data generated as the scan path is traversed are combined with the corresponding probe data to establish the position of points on the surface of the object. 
     Various techniques have been devised previously for combining probe data with corresponding machine position data. For example, U.S. Pat. No. 7,970,488 describes how a timing (synchronisation) signal received by both the machine tool and scanning probe system can be used to ensure the probe data and machine position data are temporally aligned before they are combined. A technique for precisely measuring the system delay between probe and machine measurements has also been described in U.S. Pat. No. 7,866,056. The process of combining probe data and machine position data is, for a variety of reasons, not easy to implement in practice on most machine tool systems. For example, difficulties can arise even if the machine tool does include an external data link (e.g. a RS-232, file polling or software connection) for transferring information to an external processing system. In particular, such a data link can often be difficult to configure and typical machine tool data links are relatively slow which negatively impacts on cycle times. The provision of a slow machine tool data link is a particular problem when transferring the very large amounts of machine position data that are typically generated during a scanning process. 
     A number of methods have also been used previously which avoid the need for the machine position data altogether. U.S. Pat. No. 7,523,561 describes how probe data can be combined with assumed machine position data instead of using the actual machine position data output from the machine tool. It has been found that, in certain situations, these techniques can be susceptible to variations in the probe data sets that are collected for different measurements. For example, changes in the feed rate or the time at which the scanning probe is activated can introduce errors when trying to compare a series of measurements. 
     The present invention thus attempts to obviate at least some of the above mentioned disadvantages by providing a way of encoding timing and/or position information into the acquired probe data itself. 
     According to a first aspect, the present invention provides a method for measuring an object using a machine tool apparatus comprising a scanning probe, the method comprising driving the scanning probe along a scan path relative to the object whilst the scanning probe acquires probe data describing a series of positions on the surface of the object relative to the scanning probe, the scan path comprising at least a first scan path segment for producing probe data that can be analysed to measure the object, characterised in that the scan path is also arranged to impart a plurality of identifiable probe motions to the scanning probe that can be identified from the acquired probe data alone, each identifiable probe motion defining a time stamp. 
     The method of the present invention thus employs a scan path for a scanning probe that has been configured to include a segment or section that can be analysed to measure the object. The scan path also includes a plurality of identifiable probe motions which act as markers or time stamps. These time stamps allow, for example, the start and end or other points of the relevant section of the scan path to be identified from the probe data alone. In other words, each identifiable probe motion occurs at a known time during traversal of the scan path thereby acting as a timing stamp. This can also be thought of as identifying certain commanded or nominal positions within the scan path. 
     An advantage of including a plurality of such time stamps within the probe data itself is that it removes the need to also extract machine position data from the machine tool. The beginning and end of the first scan segment can, for example, be found from the probe data alone, even if the feed rate of the machine tool has been changed or if the probe starts acquiring probe data at a variable point in time before the scan segment is scanned. As explained below, the time stamps allow multiple sets of separately collected probe data (e.g. measurements of a plurality of nominally identical objects) to be compared to one another. It also allows the combination of probe data with machine position data not collected during that particular scanning procedure. For example, probe data could be combined with assumed (nominal) machine position data or machine position data previously acquired by moving the scanning probe around the same scan path. This enables the speed of measurements (e.g. for part set-up) to be increased thereby decreasing cycle times. 
     The identifiable probe motions may occur at any suitable point in the scan path. They may occur before the first scan path segment, after the first scan path segment or even during the first scan path segment. Conveniently, the scan path is arranged to impart identifiable probe motions before and after the first scan path segment. The plurality of identifiable probe motions may allow any two points of the first path segment to be identified. Advantageously, a start and an end of the first scan path segment may be identified. 
     The identifiable probe motions may be incorporated into the scan path in a variety of ways. The only requirement is that each of the plurality of identifiable probe motions results in the generation of probe data having a characteristic of some type that is different to that expected during traversal of the rest of the scan path. The plurality of identifiable probe motions may be similar or identical to each other, or the scan path may include different identifiable probe motions. Advantageously, at least one of the plurality of identifiable probe motions comprises a dwell period in which scanning probe motion relative to the object is halted. The scanning probe may thus be held substantially stationary relative to the object for a short duration of time. If a gradual variation in probe data is expected for the scan path, a dwell alone could be identified from the probe data. 
     At least one of the plurality of identifiable probe motions may comprise altering the distance between the scanning probe and the object. Advantageously, at least one of the plurality of identifiable probe motions may comprise reducing and then increasing the distance between the scanning probe and the object. For example, the identifiable probe motion could involve the scanning probe being stepped towards and then away from the object to impart an identifiable change or “dink” in the probe data. 
     If the scanning probe comprises a contact probe having a deflectable stylus for contacting the object, then at least one of the plurality of identifiable probe motions may thus comprise increasing and then reducing the stylus deflection. 
     The reversal in motion (i.e. the transition between moving the scanning probe towards and away from the surface to increase and then decrease the stylus deflection) provides a well-defined time stamp. Such a reversal in motion preferably occurs shortly after the stylus is brought into contact with the surface of the object or shortly before it is moved out of contact with the surface. The “dink” will then occur after or before the stylus deflection is below a minimum deflection level that indicates the stylus is not in contact with the surface. This helps such dinks to be identified. It is also possible for an identifiable probe motion to comprise increasing the stylus deflection to exceed the stylus deflection expected during traversal of the rest of the scan path. The increased stylus deflection can then be recognised from the probe data. The scan path may optionally cause the stylus to move off the surface prior to any such stylus deflection increase to allow it to be identified more readily. 
     The method may comprise the step of identifying the plurality of identifiable probe motions in the acquired probe data. For example, the position of a peak in the data that corresponds to the identifiable probe motion may be determined. Advantageously, the method comprises the step of using the identifiable probe motions to synchronise the acquired probe data with a separately collected data set. The separately collected data set may be any data set that matches the acquired probe data. 
     Advantageously, the separately collected data set comprises probe data separately acquired by a machine tool driving a scanning probe along the scan path. For example, the method may comprise comparing the acquired probe data with similar probe data that was previously collected when scanning an object using the same scan path. This probe data may have been collected when a different object (e.g. nominally identical to the object being measured) was placed on the same machine tool and scanned using the same scan path. Alternatively, the probe data may have been collected using the same scan path on a different machine tool. 
     Conveniently, the separately collected data comprises machine position data that describes the position of the scanning probe as it traverses the scan path. In other words, rather than waiting to extract machine position data from the machine tool it is possible to combine the probe data with previously acquired machine position data. Such machine position data could be machine position data previously collected from the machine tool when it traversed the same scan path. 
     Alternatively, it could be nominal, commanded or assumed machine position data that is generated using knowledge of the scan path with which the machine tool is programmed. The identifiable probe motions again allow synchronisation of the acquired probe data with the separately acquired machine position data. 
     The scanning probe preferably captures probe data at a predetermined capture rate. The scanning probe may also be arranged to output a continual stream of probe data. This stream of probe data may be passed from the scanning probe to an external processor (e.g. computer) via a probe interface. The collected probe data may comprise a discrete set of data points collected between the machine tool issuing instructions to the scanning probe to start collecting probe data and to stop collecting probe data. In other words, the probe data may comprise a discrete set of data points collected between start scanning (probe enable-on) and stop scanning (probe enable-off) instructions that are issued by the machine tool to the scanning probe. Appropriate start and stop signals may thus be issued to the scanning probe by the machine tool at suitable points in the scan path. 
     The method may be implemented using any scanning probe. The scanning probe may be a non-contact (e.g. optical, capacitive, inductive) scanning probe. The scanning probe may be a contact scanning probe. In particular, a contact scanning probe may be provided that has a deflectable stylus. The stylus may be deflectable relative to the housing of the scanning probe in any one of two mutually perpendicular directions or in any of three mutually perpendicular directions. At least one transducer may be provided within the scanning probe for measuring the amount of deflection of the stylus. The scanning probe may include a sensor that can only measure the magnitude (not direction) of stylus deflection; i.e. the scanning probe may comprise a multidirectional, single output scanning probe. For example, the scanning probe may comprise a TC76-Digilog or a TC64-Digilog scanning probe as manufactured by Blum Novotest GmbH, Germany or a model G25 probe sold by Marposs, Italy. The scanning probe may comprise sensors that can measure both the magnitude and direction of any stylus deflection. For example, the analogue measurement probe may generate three output signals that relate to the deflection of the stylus tip in three mutually orthogonal directions. The SPRINT (OSP-60) probing system manufactured by Renishaw plc, Wotton-Under-Edge, UK is an example of such a scanning probe. 
     It should be noted for completeness that scanning probes as described herein (which can sometimes also be called analogue probes) are different to so-called touch trigger probes. Touch trigger probes, which are sometimes termed digital or switching type probes, simply act as a switch. Deflection of the probe stylus from a rest position (e.g. when the stylus tip is moved into contact with the surface of an object) causes a trigger signal to be issued that is fed to a “SKIP” (or equivalent) input of the machine tool. The machine tool measures the position of the touch trigger probe in the machine coordinate system (x,y,z) at the instant the trigger signal is issued, thereby allowing (with suitable calibration) the position of a single point on the surface of the object to be measured. A touch trigger probe is thus repeatedly driven into, and out of, contact with the surface of an object to take point-by-point position measurements of an object. Touch trigger probes are thus different to scanning probes in that they do not allow the collection of probe data whilst being scanned along a path on the surface of a workpiece. The method of the present invention is applicable only to scanning (not touch trigger) measurements. 
     The first scan path segment may produce probe data that can provide a measure of any property of the object. For example, the first scan path segment may produce probe data that can be used for part set-up or for measuring a feature or features of the object. The first scan path segment conveniently produces probe data that is analysed to determine a dimension of the object. Advantageously, the first scan path segment produces probe data that is analysed to determine a location and/or orientation of the object. The location and/or orientation of the object is preferably determined prior to a cutting operation performed by the machine tool and the measurement may be used to set one or more cutting parameters. 
     The scan path may comprise only the first scan path segment. Advantageously, the scan path comprises a plurality of further scan path segments that each produce probe data that can be analysed to measure a property of the object. The scan path is preferably arranged to impart identifiable probe motions before and after each further scan path segment to allow a start and an end of each further scan path segment to be identified from the probe data alone. In other words, time stamps may be applied at the beginning and end of each of a plurality of scan path segments. 
     The method may be used to measure any object. Advantageously, the object comprises a component of a consumer electronics device. 
     According to a second aspect of the invention an apparatus is provided that comprises a machine tool and a scanning probe, the machine tool comprising a controller for moving the scanning probe, wherein the apparatus is configured to drive the scanning probe along a scan path relative to the object whilst the scanning probe acquires probe data describing a series of positions on the surface of the object relative to the scanning probe, the scan path comprising at least a first scan path segment for producing probe data that can be analysed to measure the object, characterised in that the scan path is also arranged to impart a plurality of identifiable probe motions to the scanning probe that can be identified from the acquired probe data alone, each identifiable probe motion defining a time stamp. 
     The scan path may be arranged to impart the identifiable probe motions before and after the first scan path segment to allow a start and an end of the first scan path segment to be identified from the probe data alone. The apparatus may also comprise any of the features outlined above for the analogous method. 
     Also described herein is a method for measuring an object using a machine tool comprising a scanning probe, the method comprising the step of driving the scanning probe along a scan path relative to the object, the scanning probe being arranged to acquire probe data which describes a series of positions on the surface of the object relative to the measurement probe as the scan path is traversed, wherein the scan path is configured to impart at least one identifiable probe motion that can be identified from the probe data alone, wherein each identifiable probe motion occurs at a known time during traversal of the scan path thereby acting as a time stamp such that analysis of the probe data alone can be used to determine a property of the object. The scan path may be selected so as to impart a plurality of identifiable probe motions (e.g. at the beginning and end of a region of the scan path) that can be identified from the probe data alone. The method may also comprise any one or more of the features described above. 
     A method is also described herein for measuring an object using a machine tool comprising a scanning probe. The method may comprise the step of driving the scanning probe along a scan path relative to the object. The scanning probe is preferably arranged to acquire probe data which describes a series of positions on the surface of the object relative to the measurement probe as the scan path is traversed. Advantageously, the scan path is configured to impart a plurality of identifiable probe motions that can be identified from the probe data alone. For example, the scan path may comprise at least a first scan path segment which may be analysed to measure the object. A first identifiable probe motion may be provided in the scan path. This first identifiable probe motion may be prior to the first scan path segment. A second identifiable probe motion may be provided in the scan path. This second identifiable probe motion may be after the first scan path segment. A start and end of the section of the scan path can then be identified from the probe data alone. The method may also comprise any one or more of the features described above. 
     A method is also described for measuring a workpiece using a machine tool apparatus comprising a scanning probe, the method comprising driving the scanning probe along a scan path relative to the workpiece whilst the scanning probe acquires probe data describing a series of positions on the surface of the workpiece relative to the scanning probe, the scan path producing probe data that can be analysed to measure the workpiece, wherein the scan path is arranged to impart a plurality of identifiable probe motions to the scanning probe that can be identified from the probe data alone. In this example, the workpiece may be a workpiece in a series of nominally identical workpieces. It should also be noted that the term workpiece in this context does not include a probe qualification artefact or the like. The method may also comprise any one or more of the features described above. 
    
    
     
       The invention will now be described, by way of example only, with reference to the accompanying drawings in which; 
         FIG. 1  illustrates a machine tool carrying a spindle mounted scanning probe, 
         FIG. 2  illustrates a scan path along which a scanning probe is driven by a machine tool, 
         FIG. 3  shows a scan path incorporating identifiable probe motions that define time stamps, 
         FIG. 4  shows the probe data collected between the points  80   a  and  80   b  of the scan shown in  FIG. 3 , 
         FIG. 5  shows two sets of probe data collected by moving a scanning probe along the same scan path at different feed rates, 
         FIG. 6  shows how the data of  FIG. 5  can be scaled for comparison purposes using the time stamps, and 
         FIG. 7  shows the difference between the probe data of  FIG. 6 . 
     
    
    
     Referring to  FIG. 1 , a machine tool is schematically illustrated having a spindle  2  holding a scanning probe  4 . 
     The machine tool comprises motors (not shown) for moving the spindle  2  relative to a workpiece  6  located on a workpiece holder  7  within the work area of the machine tool. The location of the spindle within the work area of the machine is accurately measured in a known manner using encoders or the like; such measurements provide spindle position data (herein termed “machine position data”) that is defined in the machine co-ordinate system (x,y,z). A computer numerical controller (CNC)  8  of the machine tool controls movement of the spindle  2  within the work area of the machine tool and also receives the machine position data describing spindle position (x,y,z). 
     The scanning probe  4  comprises a probe body or housing  10  that is attached to the spindle  2  of the machine tool using a standard releasable tool shank connection. The probe  4  also comprises a workpiece contacting stylus  12  that protrudes from the housing. A ruby stylus ball  14  is provided at the tip of the stylus  12  for contacting the associated workpiece  6 . The stylus tip can deflect relative to the probe housing  10  and a transducer system within the probe body  10  measures deflection of the stylus in a local or probe coordinate system (a,b,c). The stylus deflection data acquired by the scanning probe is herein termed “probe data”. The probe  4  also comprises a transmitter/receiver portion  16  that communicates with a corresponding receiver/transmitter portion of a remote probe interface  18 . In this manner, probe data (i.e. a,b,c data values) from the scanning probe  4  are transmitted over a wireless communications link to the interface  18 . A general purpose computer  20  is also provided to receive the probe data from the probe interface  18 . The scanning probe  4  and interface  18  of the present example may comprise a SPRINT measurement probe system as manufactured by Renishaw plc, Wotton-Under-Edge, Glos., UK. 
     In use, the CNC  8  runs a so-called part program that contains a series of command codes that cause the scanning probe to be moved or driven along a certain path in space. Such a driven path is often termed a tool path, although because a scanning probe rather than a cutting tool is being carried it can also be termed a scan path. 
     Probe data (i.e. a, b, c data values describing stylus deflection) and machine position data (i.e. x, y, z values describing the position of the scanning probe in the machine coordinate system) are acquired as the scanning probe driven along the scan path. Probe data is typically collected at a pre-set rate (e.g. a stylus deflection reading may be taken every millisecond). The CNC  8  can also be programmed to move around the scan path at a certain feed rate. The feed rate is typically a variable that can be adjusted by the user to control the speed at which the spindle is moved around in space according to the instructions of the part program. For example, feed rate can be defined using a parameter that is set in the part program (e.g. the command F1000 may be used to set the feed-rate for subsequent interpolated moves to 1000 mm/minute). Machine tools also tends to have a feed-rate override control that is used during program prove-out; this is typically a knob allowing an operator to slow down all moves to a percentage of their programmed value. 
       FIG. 2  shown an example of a prior art scan path  60  for measuring a rectangular object  62 . The scan path starts and ends at a point  64  and defines the motion of the probe body  66  around the object  62 . In this example, the scan path  60  is offset slightly from the surface of the object  62  so that the stylus (not shown) of the scanning probe remains in contact with the surface of the object as the scan path is traversed. The scanning probe is instructed to start collecting probe data at the point  64  and to stop collecting probe data when it has returned to that point. There will be further segments of the scan path to move the probe into and out of contact with the surface, but these are not shown in  FIG. 2 . 
     In prior art systems, probe data (e.g. a,b,c stylus deflection values) are collected by the scanning probe system whilst machine position data (e.g. x,y,z position values) are collected by the machine tool that is moving the scanning probe. Each piece of collected probe data is combined with machine position data acquired at the same point in time in order to derive a series of measured points on the surface of the object. These measured points are found in the machine coordinate system. As described in U.S. Pat. No. 7,970,488, the two data sets are synchronised to a common clock thereby allowing them to be combined. However, combining such sets of data is time consuming and might not be possible for certain types of machine tool. This is especially the case for part set-up applications, where the location and/or orientation of a workpiece (including a blank) within the machine tool coordinate system needs to be established as a quickly as possible so that machining operations can occur as quickly as possible. 
     As explained above, the technique described in U.S. Pat. No. 7,523,561 allows measurements to be performed by combining collected probe data with assumed machine position data. The assumed position data describes the commanded position of the measurement probe, rather than using actual probe data measured by the machine tool. Although such a technique can be used for many types of measurement, it has been found to be difficult to compare collected sets of probe data if aspects of the machine tool configuration are adjusted. For example, the rate at which probe data is collected is typically fixed. If the feed rate of the machine tool is adjusted but the probe data collection rate is unchanged, then the amount of probe data collected when traversing the same scan path will vary. There can also be different feed rates for different commanded moves within a part program, and only some of these commanded moves may be changed by altering the interpolated feed rate parameter. Accelerations and decelerations as the probe approaches the object may also introduce variations between data sets. This makes robust comparisons of data sets difficult to implement in practice. 
     The present inventors have thus devised a method in which time stamps are encoded in the probe data itself. In particular, the scan path that defines the motion of the scanning probe relative to the object being measured is arranged to include characteristic movements (e.g. “clinks”) that can be used as time stamps. These characteristic movements allow the start and end of certain segments of the scan path to be identified from the probe data alone. Embedding timing information in the probe data removes issues when comparing probe data sets that can arise from changes to the feed rate or the time at which the scanning probe starts outputting data. 
       FIG. 3  is an example of a scan path  70  for a rectangular object  62  that includes multiple time stamps. The scan path  70  starts and ends at a point  74 . The scan path  70  follows the same general path as the scan path  60  of  FIG. 2 , except that it includes multiple characteristic moves  76   a - 76   h  (collectively termed characteristic moves  76 ) that cause the scanning probe to be briefly moved inwardly towards the surface (i.e. increasing the magnitude of stylus deflection). The characteristic moves  76  are arranged to lie outside of the regions that are to be measured so that they do not impact on measurement accuracy. In this example, the characteristic moves  76  are located at the start and end of each side of the rectangular object  62 . 
     The scanning probe is thus driven around the scan path  70  from the start to the end point  74  whilst probe data is collected at a set rate. The scanning probe initially moves along the scan path in a straight line until it reaches the region where it makes the first characteristic move  76   a  (i.e. a move toward the surface and back out again). The scanning probe then continues to move along a straight line until reaching the second characteristic move  76   b . A first scan path segment  78  is thus provided which is preceded by a first timing stamp (i.e. characteristic move  76   a ) and followed by a second timing stamp (i.e. characteristic move  76   b ). After being driven around the corner of the object, a similar procedure is performed on the second, third and fourth faces of the object in turn. 
     Referring to  FIG. 4 , the magnitude (M) of stylus deflection as measured by the scanning probe is plotted as a function of time. In particular,  FIG. 4  shows the resultant deflection (i.e. the magnitude of the resultant of the measured a, b, c deflections) between the points  80   a  and  80   b  shown in  FIG. 3 . It can be seen that the first and second characteristic moves  76   a  and  76   b  result in first and second peaks  86   a  and  86   b  in the deflection data. These occur at times t 1  and t 2 . Knowing the times t 1  and t 2  allows the set of probe data  88  collected from the first scan path segment  78  to be determined. This probe data alone may be analysed to determine a workpiece offset or rotation. 
     The presence of the time stamps in the probe data has a number of advantages. In particular, as described below, it allows comparison of collected probe data with other data sets. 
       FIG. 5  shows the data  90  of  FIG. 4  plotted against a similar set of data  92  that was collected for a nominally identical rectangular object using the same scan path and scanning probe. The two sets of data were collected using slightly different feed rates which led to the variation in the positions of the first and second peaks  86   a  and  86   b  in the deflection data  90  and the first and second peaks  96   a  and  96   b  in the deflection data  92 . 
     Referring to  FIG. 6 , the timing stamps (i.e. the pair of peaks in the two data sets) allow a comparison of the data sets to be performed without also needing to know the machine position data associated with the probe data. For example, the data  92  can be scaled and offset to generate data  92 ′ in which the timing stamps are aligned with those of the other data  90 . 
     Referring to  FIG. 7 , a comparison of the two data sets can then be made by analysing the difference in probe data  88  collected from the first scan path segment  78 . This allows any variation in the dimension or position of the two objects along the first scan path segment  78  to be assessed. 
     It should be remembered that the above embodiments are examples of the present invention. Although the analysis of probe data from a single side of the object is described above, it should be note that the three other segments of the scan path could be analysed in the same way. The measurements from a scan path segment may be analysed alone, or variations in multiple segments may be analysed together (e.g. to establish an offset in the centre position of the object). The technique can also be applied to objects of different shape and to different scan paths. The skilled person would be aware of many variations and alternatives that would be possible in accordance with the present invention.