MEASUREMENT METHOD AND APPARATUS

Abstract

A method is described for measuring an object using a machine tool and a scanning probe. The scanning probe is driven 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 includes at least a first scan path segment for producing probe data that can be analysed to measure the object. 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 is used to define a time stamp. This allows the probe data to be tied to commanded or nominal positions around the scan path.

Claims

1. 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, wherein 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.

2. A method according to claim 1, wherein the scan path is arranged to impart identifiable probe motions before and after the first scan path segment.

3. A method according to claim 1, wherein the plurality of identifiable probe motions allow a start and an end of the first scan path segment to be identified.

4. A method according to claim 1, wherein at least one of the plurality of identifiable probe motions comprises reducing and then increasing the distance between the scanning probe and the object.

5. A method according to claim 4, wherein the scanning probe comprises a contact probe having a deflectable stylus for contacting the object and at least one of the plurality of identifiable probe motions comprises increasing and then decreasing the stylus deflection.

6. A method according to claim 1, wherein 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.

7. A method according to claim 1, comprising the step of using the identifiable probe motions to synchronise the acquired probe data with a separately collected data set.

8. A method according to claim 7, wherein the separately collected data set comprises probe data separately acquired by a machine tool driving a scanning probe along the scan path.

9. A method according to claim 7, wherein the separately collected data comprises machine position data that describes the position of the scanning probe as it traverses the scan path.

10. A method according to claim 1, wherein the scanning probe captures probe data at a predetermined capture rate and the feed rate of the machine tool apparatus can be varied by a machine tool operator.

11. A method according to claim 1, wherein the scanning probe comprises a contact probe having a deflectable stylus for contacting the object.

12. A method according to claim 1, wherein the first scan path segment produces probe data that is analysed to determine a dimension of the object, a location of the object and/or an orientation of the object.

13. A method according to claim 1, wherein 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, wherein the scan path is 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.

14. A method according to claim 1, wherein the object comprises a component of a consumer electronics device.

15. An apparatus comprising 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, wherein 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.

Description

[0030] The invention will now be described, by way of example only, with reference to the accompanying drawings in which;

[0031] FIG. 1 illustrates a machine tool carrying a spindle mounted scanning probe,

[0032] FIG. 2 illustrates a scan path along which a scanning probe is driven by a machine tool,

[0033] FIG. 3 shows a scan path incorporating identifiable probe motions that define time stamps,

[0034] FIG. 4 shows the probe data collected between the points 80a and 80b of the scan shown in FIG. 3,

[0035] FIG. 5 shows two sets of probe data collected by moving a scanning probe along the same scan path at different feed rates,

[0036] FIG. 6 shows how the data of FIG. 5 can be scaled for comparison purposes using the time stamps, and

[0037] FIG. 7 shows the difference between the probe data of FIG. 6.

[0038] Referring to FIG. 1, a machine tool is schematically illustrated having a spindle 2 holding a scanning probe 4.

[0039] 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).

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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 76a-76h (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.

[0048] 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 76a (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 76b. A first scan path segment 78 is thus provided which is preceded by a first timing stamp (i.e. characteristic move 76a) and followed by a second timing stamp (i.e. characteristic move 76b). 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.

[0049] 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 80a and 80b shown in FIG. 3. It can be seen that the first and second characteristic moves 76a and 76b result in first and second peaks 86a and 86b in the deflection data. These occur at times t1 and t2. Knowing the times t1 and t2 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.

[0050] 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.

[0051] 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 86a and 86b in the deflection data 90 and the first and second peaks 96a and 96b in the deflection data 92.

[0052] 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.

[0053] 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.

[0054] 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.