Measurement method

Abstract

A method of determining a shape measurement of a curved feature of an artifact. The method includes positioning an apparatus to relatively move the artifact and a measurement device along a curved path in a first direction to obtain a first set of data points along a surface of the curved feature, and positioning the apparatus to relatively move the artifact and the measurement device along the curved path in a second direction opposite the first direction to obtain a second set of data points along the surface of the curved feature. The method further includes determining the shape measurement of the artifact using the first set of data points and the second set of data points.

Description

[0001] The present invention relates to a method for inspecting a feature of a workpiece on a positioning apparatus, in particular a method for assessing the shape of a curved feature (e.g. a method for determining a roundness value of a circular feature).

[0002] It is known to measure features of a workpiece before, during or after production, for example to confirm that the feature meets desired tolerances, and / or to determine the position and / or orientation of the feature and / or workpiece. This typically involves moving a measuring device ("probe") and the workpiece relative to each other via an apparatus, so that measurements can be made of different points on the workpiece. While a variety of products and techniques are available for measuring workpieces (that have been machined or are to be machined) on a machine tool, it is still common to measure those parts of components on a dedicated coordinate measuring machine (CMM) where the tolerance requirements are very high. In particular, the shape of features such as engine block cylinders and valve seats needs to be tightly controlled, so measurements of these features are often performed on a dedicated CMM. One example of a shape measurement is the "roundness" or "circularity" of a circular feature. As will be understood, and as defined by the system of Geometric Dimensioning and Tolerancing (GD&T), a shape measurement is a particular measurement that is different from the position or size (e.g. diameter) of a surface.

[0003] Since CMMs are primarily used for measuring workpieces (whereas machine tools are primarily used for machining workpieces), CMMs are traditionally able to provide a higher level of measurement accuracy, and are less affected by errors than machine tools. This is often due to differences in the mechanical configuration of CMMs and machine tools. For example, the drive mechanisms of machine tools are typically very different from those of CMMs, since the drive systems of machine tools need to be able to withstand large forces due to the machining operations that the machine tool needs to perform (which is different from CMMs, which only need to carry a measuring device that does not need to withstand large forces during operation).

[0004] However, there is an increasing desire to perform more and more workpiece measurements on machine tools in situ, since this can save a significant amount of time and cost.

[0005] However, the inventors have found that accurate shape / roundness measurements of curved surfaces are difficult to achieve, particularly on machine tool apparatuses.

[0006] The present invention relates to a technique for improving the shape measurement of curved features (such as the circularity or roundness of circular features), which can be particularly valuable when such features are measured on a machine tool that typically cannot provide sufficiently accurate measurement data.

[0007] According to a first aspect of the application, there is provided a method of determining a shape measurement of a curved feature of an artefact, the method comprising: i) positioning apparatus to cause relative movement of the artefact and a measurement device along a curved path in a first direction to obtain a first set of data points along a surface of the curved feature; ii) the positioning apparatus causing relative movement of the artefact and the measurement device along a curved path in a second direction opposite to the first direction to obtain a second set of data points along the surface of the curved feature; iii) using the first set of data points and the second set of data points to determine a shape measurement of the artefact.

[0008] The inventors have found that errors in measurements of curved features can be caused by motion of the machine (in particular due to so-called “lost motion” of the machine). Although such (motion of the machine caused) measurement errors are small (e.g. on the order of a few microns), they hinder the inventors from achieving the required accurate shape measurements. The inventors have found that the techniques of the present application reduce the effect of errors and enable more accurate / reliable shape measurements to be obtained.

[0009] Using the first set of data points and the second set of data points can comprise obtaining (from the first set of data points and the second set of data points) a representation of an intermediate surface located between the line / surface represented by the first set of data points and the line / surface represented by the second set of data points. As will be appreciated, the representation of the intermediate surface, as well as the line / surface represented by the first set of data points and the second set of data points, can be conceptual. For example, the representation of the intermediate surface can comprise a third set of data points (which lie on the conceptual intermediate surface). In other words, “using the first set of data points and the second set of data points” can comprise obtaining a third set of data points which lie on a (conceptual) surface which is equidistant between the line / surface represented by the first set of data points and the line / surface represented by the second set of data points. The shape measurement can then be determined from the representation of the intermediate surface (e.g. from the third set of data points). As will be appreciated by the skilled person, the representation of the intermediate surface can comprise something which enables the intermediate surface to be constructed; for example, the representation of the intermediate surface can comprise a function, such as one or more splines (e.g. B-splines or non-uniform rational B-spline “NURBS” surfaces). As will also be appreciated by the skilled person, the representation of the intermediate surface can comprise a 2-dimensional (2D) representation, such as a 2D curve. This can be the case even if the first set of data points and the second set of data points comprise 3-dimensional (3D) data sets. As will also be appreciated by the skilled person, the representation of the intermediate surface can comprise a 3-dimensional (3D) representation, such as a 3D curve.

[0010] The data points can be referred to as "positional" data points (so the first and second data point sets can be referred to as first and second positional data point sets) in that they relate to positional information rather than non-positional information (e.g. temperature information). As will be appreciated, the positional data points can be obtained by combining positional data / information relating to the positioning apparatus (such as the relative positions of the artefact and the measuring device) with data / information from the measuring device (e.g. stylus deflection data (in the case of a contact probe being employed)). As explained in more detail below, the positional data / information relating to the positioning apparatus (e.g. the relative positions of the artefact and the measuring device) can be assumed or the positional data / information can be known (e.g. via one or more position encoders on the positioning apparatus). Furthermore, as explained in more detail below, the direction of measurement (e.g. the direction of stylus deflection) obtained by the measuring device can be assumed or the direction of measurement can be known.

[0011] The data points can be obtained by combining information relating to the relative positions of the measuring device and the artefact with a vector from that position to the surface being measured. The relative positions of the measuring device and the artefact can be known (e.g. via one or more position encoders on the positioning apparatus) or can be assumed. The direction of the vector to the surface being measured can be known or can be assumed.

[0012] It has been found that the present application is particularly beneficial when the curved feature is a curved surface of the artefact (e.g. a curved surface of a cylinder or cone, rather than an end of the cylinder / cone).

[0013] It has been found that the present application is particularly beneficial when the measuring device is used to make measurements of the artefact in a dimension extending substantially along a radius of curvature of the curved path at the measurement point. Thus, preferably, the first and second data point sets comprise measurement data obtained / acquired / measured substantially radially with respect to the curvature of the curved path at the measurement point (e.g. within at least 45° of the radius of curvature of the curved path at the measurement point, more preferably within at least 30° of the radius of curvature of the curved path at the measurement point, particularly preferably within 15° of the radius of curvature of the curved path at the measurement point, for example within 10° of the radius of curvature of the curved path at the measurement point).

[0014] The shape measurement values can comprise profile tolerances of the curved feature. Alternatively, the shape measurement values comprise roundness tolerances or "roundness" of the circular feature. Alternatively, the shape measurement values comprise cylindricity tolerances of the cylinder. The shape measurement values can comprise values (shape values) and, thus, for example, the shape measurement values can comprise roundness values, roundness values or cylindricity values.

[0015] Preferably, the intermediate surface is equidistant between the line / surface represented by the first set of data points and the line / surface represented by the second set of data points. Determining a representation of an intermediate surface equidistant between the line / surface represented by the first set of data points and the line / surface represented by the second set of data points should be understood as enabling a more accurate shape measurement to be determined from the representation of the intermediate surface.

[0016] The relative movement in the first direction and the second direction can be at least partially achieved by movement of at least one moveable axis of the positioning apparatus, the movement of which is reversed during said movement in each of the first direction and the second direction. The moveable axis can be a linear axis. Thus, the relative movement in the first direction and the second direction can be at least partially achieved by movement of at least one linearly moveable axis of the positioning apparatus. In particular, the relative movement in the first direction and the second direction can be at least partially achieved by movement of at least one linearly moveable axis of the positioning apparatus, the movement of which is reversed during said movement in each of the first direction and the second direction. The relative movement in the first direction and the second direction can be achieved by combined movement of at least two non-parallel linearly moveable axes of the positioning apparatus. In particular, the relative movement in the first direction and the second direction can be achieved by combined movement of at least two non-parallel linearly moveable axes of the positioning apparatus, the movement of at least one of which is reversed during said movement in each of the first direction and the second direction. One source of measurement error which can have a significant detrimental effect on the shape measurement is due to movement along the at least one linear axis which is being reversed (axis reversal error) and this is a particular problem on machine tools. It has been found that this can be at least partially due to machine / axis backlash. For example, when a motor reverses the direction of its control / power, there can be a small delay before the motor controlled axis actually starts to move. As will be appreciated, to help address the backlash problem, the relative movement in steps i) and ii) should be achieved using the same motion axis / axes of the positioning apparatus. As will be appreciated, said relative movement can comprise moving the artefact and leaving the measurement device stationary, or can comprise moving the measurement device and leaving the artefact stationary, or can comprise moving both the artefact and the measurement device.

[0017] Machine tools typically do not have position measurement encoders for directly measuring the position of their moveable axes; instead, the machine tool controller can monitor the position of the machine tool's axes via encoders on the axes' drives / motors (which do not necessarily provide an accurate representation of the axes' actual positions). Even if a machine tool does have position measurement encoders for directly measuring the position of its moveable axes, such position information can not be available for use in the process of determining shape measurements. Indeed, machine tools are not typically designed to continuously report axis position information to a separate measurement process. It is therefore known in the machine tool art to assume the position of an axis (e.g. based on instructions for driving the machine tool rather than determined from encoders on the machine tool) for a measurement process and to combine the assumed position information with data from a probe to determine surface position information. It is also possible that metrology information about a feature can be determined from the output of a measurement device (e.g. a probe) alone (i.e. without combining that output with reported or assumed axis position information). It is therefore particularly common (especially in the machine tool art) that the effect of axis backlash on axis position will not be directly measured / known, and so errors can be introduced into measurement data used to determine the shape of a part due to the structure and / or configuration of the machine tool itself, no matter how well calibrated, accurate and error-free the measurement device mounted on the machine tool is.

[0018] Machine manufacturers have taken steps to try to deal with the backlash problem, for example by servo drives to try to compensate for backlash, but such a solution can itself introduce additional measurement errors, for example "axis reversal spikes" as known in the art. While axis reversal is one source of error for shape measurements, there are other sources / causes, such as dynamic structural distortion caused by acceleration of the machine. Such axis reversal and dynamic structural distortion can result in so-called "lost motion" or errors caused by untransformed motion of the machine.

[0019] The inventors have found that, by using the techniques of the present invention, the effect of such errors on shape measurements can be reduced, and shape measurements can be made with a level of accuracy that exceeds the expected capabilities of the machine tool.

[0020] Preferably, the relative angular orientation of the measurement device and the artefact is the same for both the first measurement and the second measurement. In other words, preferably the relative angular orientation of the measurement device and the artefact is unchanged between the first measurement and the second measurement. This avoids the need to take into account the effect of the relative angular orientation of the measurement device and the artefact on the shape measurement. For example, in the case of a contact probe, if there is a stylus tip offset error, then changing the relative angular orientation of the measurement device and the artefact can have an effect on the shape measurement. Ensuring that the relative angular orientation of the measurement device and the artefact is the same for both the first measurement and the second measurement avoids the need to know / determine the stylus tip offset error.

[0021] The positioning apparatus can be a dedicated measuring apparatus, such as a coordinate measuring machine (CMM). The positioning apparatus can comprise a machine tool. In particular, the positioning apparatus can comprise a machine tool on which the artefact has previously been machined or on which the artefact is to be machined.

[0022] The curved feature can comprise an arcuate feature. The curved feature can comprise a circular feature. For example, the curved feature can comprise a cylindrical boss or bore. Thus, obtaining the first set of data points and / or the second set of data points can comprise the positioning apparatus moving the artefact and the measuring device relative to each other along a circular path (e.g. along a partially circular or fully circular path). The measurement of the curved feature can be constrained to a plane, but this need not necessarily be the case. For example, the positioning apparatus can move the artefact and the measuring device relative to each other along a helical or spiral path.

[0023] The relative movement of the artefact and the measuring device can comprise movement of the artefact while the measuring device is held stationary, or vice versa. The relative movement of the artefact and the measuring device can also comprise movement of both the measuring device and the artefact.

[0024] Preferably, the first set of data points and the second set of data points are obtained along substantially the same target scan line (or "nominal measurement line") on the surface of the curved feature.

[0025] The curved path along which the artefact and the measuring device are moved relative to each other to obtain the set of data points can be predefined / determined in advance.

[0026] As mentioned above, especially when seeking to address the issue of backlash, the relative movement in steps i) and ii) should be achieved using the same motion axis / axes of the positioning apparatus.

[0027] The curved paths for step i) and step ii) need not be identical. For example, the curved paths for step i) and step ii) can be configured such that the same target scan line (or "nominal measurement line") is measured at different locations on the surface of the curved feature, but within the operating volume of the positioning apparatus. Preferably, however, the curved path along which the artefact and the measurement device are moved relative to each other to obtain the second set of data points is the same as the curved path along which the artefact and the measurement device are moved relative to each other to obtain the first set of data points. Preferably, the first set of data points and the second set of data points can be obtained by the positioning apparatus moving the artefact and the measurement device relative to each other along the same curved path, but wherein the different sets of data are obtained by movement in different directions along the same curved path. It can therefore be preferred that the curved paths for i) and ii) are the same, such that for step i) and step ii) the first set of data points and the second set of data points are obtained along the same target scan line (or "nominal measurement line") on the surface of the curved feature, the location of the target scan line within the operating volume of the positioning apparatus is the same for step i) and step ii), and the profile / shape of the curved path along which the artefact and the measurement device are moved relative to each other will be the same.

[0028] Making the curved paths for i) and ii) the same can help to prevent any position-dependent performance differences of the positioning apparatus (e.g. the size of a play) from affecting the first set of data points and the second set of data points obtained by the positioning apparatus. This can also help to ensure that, in the case of a contact probe is employed, the same nominal probing force is used during the obtaining of the first set of data points and the second set of data points.

[0029] Still further, it can be preferred that the rotational position / orientation of the artefact relative to the positioning apparatus, and / or the rotational position / orientation of the measurement device relative to the artefact, is unchanged between step i) and step ii). In other words, therefore, it can be preferred that the physical arrangement and configuration of the artefact and the measurement device remains substantially the same for step i) and step ii).

[0030] Still further, it can be preferred that the speed at which the measurement device and the artefact are moved relative to each other along the curved path in step i) and ii) is substantially the same.

[0031] Therefore, it can be preferred that each aspect of step i) and step ii) is substantially the same, except for the direction in which the measurement device and the artefact are moved relative to each other along the curved path.

[0032] The measurement device can comprise a contact or non-contact measurement device. As is common in the field of measurement apparatuses, the measurement device can also be referred to as a probe. Measurement devices (or "probes") suitable for use in the present application include devices commonly referred to as scanning probes (also referred to as "analogue probes"). A scanning probe (or "analogue probe") is a probe which can be caused to scan along / relative to a surface whilst continuously interacting with the surface (e.g. continuously contacting the surface (in the case of a contact probe) whilst) to obtain a sequence / set of measurement points along the surface. This is in contrast to a touch-triggered probe which requires the probe to be advanced towards and retracted from the surface for each data point to be measured.

[0033] A scanning probe can provide one or more output signals which vary (i.e. in a non-binary fashion, in other words over a range) in dependence on the relative extent of the interaction of the probe and the surface. Thus, when the probe is caused to scan along / relative to the surface, the signal(s) of the scanning probe can enable the relative position of the probe body and the surface to be determined continuously.

[0034] A contact scanning probe typically comprises a body which is mounted / mountable on a positioning apparatus, and a stylus which extends from the body. Typically, a contact tip (e.g. a "dome") is provided at the free end of the stylus for contacting the artefact. The contact scanning probe is configured to output one or more signals which vary in dependence on the amount by which the stylus has been deflected from its rest position (and optionally, in dependence on the direction of the stylus deflection), such that the extent of the stylus deflection (and optionally, the direction of the stylus deflection) is known. This is in contrast to a so-called "touch-triggered probe" which is configured to output a trigger signal when the stylus has been deflected from the rest position by a threshold amount.

[0035] Thus, the method can comprise the positioning apparatus causing the relative movement of the artefact and the measurement device along the curved path (in the first direction, and then in the second direction) to continuously scan the measurement device along / relative to the surface of the curved feature (and thereby obtain the first set of data points along the surface of the curved feature, and then the second set of data points). Thus, in the case of a contact scanning probe, the first set of data points and the second set of data points can be obtained by moving the contact scanning probe whilst the contact scanning probe is in continuous contact with the surface of the curved feature.

[0036] Accordingly, the present document describes a method of determining a shape measurement of a curved feature of a workpiece using measurement data obtained by a measurement device mounted on a positioning apparatus comprising a plurality of linear axes, wherein the process of collecting the measurement data by the measurement device involves relative movement of the measurement device and the workpiece along a predefined curved path, the relative movement being achieved by movement of at least one linear axis of the positioning apparatus, the at least one linear axis being reversed during the relative movement of the measurement device and the workpiece along the predefined curved path, the method comprising: the positioning apparatus moving the artefact and the measurement device relative to each other in a first direction along the predefined curved path and obtaining a first measurement data set; the positioning apparatus moving the artefact and the measurement device relative to each other in a second direction opposite to the first direction along the same predefined curved path and obtaining a second measurement data set; determining the shape measurement of the curved feature from the first and second measurement data sets.

[0037] As will be appreciated, the above-described methods can all be computer-implemented. Thus, any or all of the above-described steps of the method can be performed under the control of a computer / processor / processing device, such as, for example, a computer controller, a numerical controller (NC) (e.g. a computer numerical controller (CNC)), etc. Suitable processing devices include, but are not limited to, CPUs (central processing units), FPGAs (field-programmable gate arrays), ASICs (application-specific integrated circuits), etc. As will be appreciated, step i), step ii) and / or step iii) can be performed under the control of different computers / processors / processing devices. In particular, for example, step iii) can be performed under the control of a computer / processor / processing device that is different from the computer / processor / processing device used in steps i) and ii). For example, step iii) can be performed by a computer / processor / processing device that is different from and, for example, physically remote from the computer / processor / processing device used in steps i) and ii), and / or can be performed at substantially different times from steps i) and ii).

[0038] The present document also describes an apparatus configured to perform the above method(s). For example, described herein is an apparatus comprising a positioning apparatus (such as a machine tool), a measurement device and an artefact positioned on the positioning apparatus, and the apparatus is configured such that the positioning apparatus relatively moves the artefact and the measurement device along a curved path in a first direction to obtain a first set of data points along a surface of the curved feature. The apparatus is further configured such that the positioning apparatus relatively moves the artefact and the measurement device along the curved path in a second direction opposite to the first direction to obtain a second set of data points along the surface of the curved feature. The apparatus is also configured to determine a shape measurement of the artefact using the first set of data points and the second set of data points. For example, a computer / processor / processing device (such as a PC) can be configured to determine a shape measurement of the artefact using the first set of data points and the second set of data points.

[0039] Accordingly, the present document also describes a computer / processor / processing device (such as a PC) comprising computer program code configured to: obtain a first set of data points obtained by a measurement device mounted on a positioning apparatus and an artefact relatively moving to each other along a curved path in a first direction by the positioning apparatus; and obtain a second set of data points obtained by the measurement device relatively moving to the artefact along the curved path in a second direction by the positioning apparatus; and determine a shape measurement of the artefact using the first set of data and the second set of data.

[0040] Accordingly, the present document also describes a computer program code (such as a computer program code stored on a computer readable medium) configured to cause a processor device (such as a controller and / or a PC) executing the computer program code to: obtain a first set of data points obtained by a measurement device mounted on a positioning apparatus and an artefact relatively moving to each other along a curved path in a first direction by the positioning apparatus; and obtain a second set of data points obtained by the measurement device relatively moving to the artefact along the curved path in a second direction by the positioning apparatus; and determine a shape measurement of the artefact using the first set of data and the second set of data.

[0041] Embodiments of the present application will now be described, by way of example only, with reference to the following drawings in which:

[0042] Figure 1 An isometric view of a machine tool apparatus is schematically illustrated;

[0043] Figure 2(a) demonstrates a comparison of the effect of axis reversal error on the expected net probe deflection versus the actual net probe deflection during the measurement of a circular feature by a simulated / scanning probe;

[0044] Figure 2 (b) demonstrates the required axial position of the x-axis and y-axis over time for circularly moving the table of a machine tool apparatus;

[0045] Figure 3 The effect of axis reversal error on the data set obtained for a circular feature is demonstrated;

[0046] Figure 4 A flowchart of a process according to an example embodiment of the application is shown;

[0047] Figure 5 is a plot comprising lines representing obtained data sets for a circular feature acquired in opposite (e.g., clockwise and counter-clockwise) directions; and

[0048] Figure 6 demonstrates Figure 5 a close-up view of a portion of the plot of

[0049] Referring to Figure 1 , a machine tool apparatus 2 is shown, which comprises a machine tool 4, a numerical controller 6 (NC) (e.g., a computer numerical controller or "CNC"), a PC 8, a transmitter / receiver 9 and an interface 10. The machine tool 4 comprises a spindle 12 attached to an arm 17 which is movable in the z-dimension via a motor 11, and a movable table 14 which is movable in the x-dimension and y-dimension via motors 13, 15. When used for machining (e.g., machining) a workpiece 16, the workpiece 16 is mounted on the movable table 14 and a tool (not shown) is mounted in the spindle 12. The NC 6 controls the rotation of the spindle 12, the z-movement of the arm 17 and the x / y motion of the movable table 14 to bring the tool and workpiece together so that the tool machines the workpiece. The NC 6 can be programmed, e.g., via the PC 8, to perform a machining operation.

[0050] When used for inspecting a workpiece 16, an inspection device (such as a probe 20) is mounted in the spindle 12. Again, the NC 6 can control the z-movement of the arm 17 and the x / y motion of the movable table 14 to cause the probe 20 to interact with the workpiece 16 (e.g., if the probe is a contact probe, the probe interacts with the workpiece by contacting the workpiece) to measure the workpiece 16. The NC 6 can be programmed, e.g., via the PC 8, to perform an inspection operation.

[0051] Although not shown in the described embodiments, the machine tool 4 can comprise one or more rotary axes for rotating the worktable 14 (e.g. about the x, y and / or z axes). Additionally / alternatively, the machine tool 4 can be configured such that the spindle 12 is rotatable about other additional rotary axes (e.g. such that a probe mounted in the spindle is rotatable about the x and / or y axes). Additionally / alternatively, the machine tool 4 can be configured such that the spindle 12 is movable in the x and / or y axes, and / or the worktable can be movable in the z axis.

[0052] In this embodiment, the probe 20 is a contact probe, comprising a body 22 mounted to the spindle 12, a stylus 24 extending from the body 22, and a stylus tip 26 at an end of the stylus 24 distal from the body 22. In the described embodiment, the stylus 24 can deflect relative to the body 22 (e.g. when the stylus tip 26 touches a surface), and such deflection can be detected by a sensor in the body 22. In particular, in the described embodiment, the probe 20 is a scanning probe (also known in the art as an analogue probe), in which the probe 20 can sense and report the degree / amount / extent of deflection of the stylus from a rest position (in contrast, a touch-triggered probe only reports when the stylus has deflected, e.g. by a predetermined threshold amount). The probe 20 can output separate values indicative of the degree of deflection of the stylus in different directions relative to the probe (e.g. a value relating to the degree of deflection in the x direction, a separate value relating to the degree of deflection in the y direction, and / or a separate value relating to the degree of deflection in the z direction - all measured relative to the probe) and / or can output a composite deflection value. Scanning probes for machine tools are known; for example the SPRINT TM probe available from Renishaw pic. As will be appreciated, other probes and other techniques can be used.

[0053] A feature of the workpiece can be measured by bringing the stylus tip 26 into contact with the feature of the workpiece 16 to be inspected. The stylus deflection data from the probe 20 can be streamed instantaneously and continuously to the PC 8, e.g. via a wired connection or, as in the present example, wirelessly via the receiver 9 and interface 10. The connection between the probe 20 and the receiver 9 can be optical or radio, for example. As will be appreciated, other techniques can be used to communicate the stylus deflection data. For example, the data can be transmitted at (regular or irregular) time intervals or only when requested, for example. In other example embodiments, the stylus deflection data can be stored locally in a memory in the probe 20 and downloaded to the PC 8 at a later time, e.g. via a wired or wireless link.

[0054] Optionally, data from the probe 20 can be combined with machine position data; for example, data relating to the relative positions of the probe 20 and the worktable 14 (and hence the workpiece 16). For example, the program running on the PC 8 can be configured to combine data from the probe 20 with position data for the spindles 12 and worktable 14. Such position data for the spindles 12 and worktable 14 can be assumed (for example, based on instructions for driving the machine rather than being determined from encoders on the machine) or can be obtained from measurement devices (not shown) monitoring the positions of the spindles 12 and worktable 14 in any or all of the x, y and z axes. Such measurement devices can be, for example, resolvers or encoders monitoring the rotational positions of parts of the motors 11, 13, 15 controlling the positions of the spindles 12 and worktable 14 in the x, y and z axes. Where the process for determining the shape of the artefact uses data from measurement devices monitoring the positions of the spindles 12 and worktable 14, then a clocking / synchronisation signal (for example, issued by the interface 10) can be used to aid the simultaneous determination of the spindle and worktable position data and the probe data (for example, as described in US 7970488).

[0055] Thus, a data point can be obtained by combining information about the relative position of the measurement device and the artefact (known or assumed) with a vector from that position to the surface being measured. As mentioned above, the probe 20 can output separate values indicative of the degree of deflection of the stylus in different directions, for example, a value relating to the degree of deflection in the x direction, a separate value relating to the degree of deflection in the y direction and / or a separate value relating to the degree of deflection in the z direction. In this case, the direction of the vector from the known / assumed relative position of the measurement device to the artefact being held and the surface being measured can be known. As also mentioned above, the probe 20 can output a composite deflection value, in which case the direction of the vector from the known / assumed relative position of the measurement device to the artefact being held and the surface being measured can be assumed.

[0056] In the case of the exemplary set-up of Figure 1 With the exemplary set-up of, if a cylindrical workpiece 16 is to be measured by the probe, the spindle 12 will need to be moved vertically (along the z axis) and the worktable 14 will need to be moved laterally (along the x and / or y axes) to bring the tip 26 of the probe into contact with the surface of the cylindrical workpiece 16. The worktable 14 will then need to be moved simultaneously (for example, in a synchronised manner) along the x and y axes to move the workpiece 16 in a circular motion, for example, to cause the probe tip 26 to scan along a target scan line (or "nominal measurement line") 19. To achieve this circular motion, the direction of movement of the worktable 14 along the x and y axes will need to be reversed a number of times. Optionally, the z axis can also be moved simultaneously to obtain a helical scan of the workpiece 16.

[0057] As Figure 2 The plot of (b) shows that, in order to drive the worktable 14 in a circle, it can be necessary to command each of the x- and y-axes to move back and forth according to a sinusoidal motion, with the motions of the x- and y-axes being phase shifted by 90°. In Figure 2 In (b), the worktable's time-varying demanded / commanded / instructed position along the x-axis is illustrated by the dashed line 34, and the worktable's time-varying demanded / commanded / instructed position along the y-axis is illustrated by the double-dotted line 36.

[0058] To help illustrate the invention, it will be assumed that the workpiece 16 is a perfect cylinder, and it will be assumed that the worktable 14 and workpiece 16 are moved along the x- and y-axes according to instructions intended to drive the worktable 14 in a perfect circle centered on the center of the cylindrical workpiece 16. In this case, it would be expected that the probe's radial deflection / total deflection would be constant at all locations around the target scan line 19 on the workpiece 16. This constant deflection is schematically illustrated by Figure 2 the straight dashed line 30 in the plot of (a), where "D" denotes the probe's deflection, and Θ denotes the position with respect to the circular feature.

[0059] However, it has been found that, in this case, the deflection of the probe's stylus 24 tends to vary as the worktable is moved in order for the probe to measure the circular surface of the workpiece 16; as schematically illustrated by Figure 2 the jagged solid line 32 in the plot of (a). As Figure 2 (a) and Figure 2 (b) illustrate, this variation in the probe deflection has been found to coincide with the position / time at which the linear axis driving the workpiece 16 reverses direction. This can be caused, at least in part, by motor lash, for example.

[0060] In particular, when the motor for one axis (e.g., the motor for the x-axis) reverses, there is a small delay before the motor actually starts moving the axis it is configured to drive due to "slack" or "lash" in the motor. However, as Figure 2 clearly illustrated in (b), at the point at which the motor for one of the axes (e.g., the motor 13 for the x-axis in this example) reverses, the motor for the other axis (i.e., the motor 15 for the y-axis in this example) is still driving the other axis in a constant direction (and at a substantially constant peak rate).

[0061] Thus, due to the combined effects of i) the delay of one axis after it reverses and before it actually starts moving, and ii) the other axis still being in motion, the degree of deflection of the probe's stylus 24 will vary as the probe moves around the target scan line 19.

[0062] Figure 3 The effect of this variation in probe deflection, taken in the anti-clockwise direction, on the determined shape of the circular feature is illustrated schematically. In Figure 3 In this embodiment, the dashed line 38 illustrates the actual circular profile of the feature, and the solid line illustrates the distorted circular feature that would be determined if the output of the probe was combined with the assumed position of the axis or combined with position information determined from the encoder of the motor.

[0063] Axis reversal errors can be difficult to calibrate because they can vary. For example, the axis reversal error of each axis can differ from position, and can also be affected by environmental factors such as temperature. However, the methods of the present invention (examples of which are described in more detail below) will reduce the effect of axis reversal errors regardless of position and environmental factors.

[0064] The illustrations used herein employ perfect circular features. However, as will be appreciated, in practice, the curved features of a production part have an unknown (e.g. uncalibrated) profile / shape. Thus, for example, a feature intended to be cylindrical / circular is unlikely to have a perfect circular profile due to inaccuracies in the manufacturing process, and so the axis reversal error can be difficult to notice. However, the methods of the present invention (examples of which are described in more detail below) will inherently reduce the effect of axis reversal errors without the need for identification of the axis reversal error. Furthermore, the illustrations used herein greatly exaggerate the errors that can be experienced in real life (and do not represent the severity of problems typically expected in a typical machine tool installation). The actual errors in the measured values caused by axis reversal in real life can be small (on the order of a few microns) so as to be invisible to the eye, and so mixed with other sources of error. However, the inventors have recognised that the errors caused by axis reversal and other sources of “lost motion” can have a significant effect on the shape measurements of curved features.

[0065] Figure 4 An exemplary process 50 for improving the accuracy of shape measurements of curved surfaces, in particular for improving the accuracy of shape measurements of curved surfaces of a workpiece 16, in accordance with the present invention is illustrated. The method comprises: at step 52, controlling the machine tool installation 2 to cause the probe 20 to measure around the cylindrical surface of the workpiece 16 in a first direction along a curved path; in this embodiment, the curved path is a circular target scan line 19. For example, consistent with the above-described embodiment, the method can comprise controlling the x-axis and y-axis to cause the worktable 14 to move in a circular motion such that the probe 20 measures the cylindrical surface in a clockwise direction along the circular target scan line 19. As will be appreciated, each measurement taken along the circular target scan line is taken substantially radially with respect to the circular target scan line.

[0066] The method then comprises, at step 54, controlling the machine tool apparatus 2 to cause the probe 20 to perform a second measurement along the same circular target scan line 19 around the cylindrical surface of the workpiece 16, but in this case the x-axis and y-axis are controlled to cause the worktable 14 and workpiece 16 to move in opposite directions along the circular target scan line 19 so that the probe 20 measures the cylindrical surface in a counter-clockwise direction along the circular target scan line 19. Again, each measurement made along the circular target scan line is obtained substantially radially with respect to the circular target scan line.

[0067] In the described embodiment, the artefact and the measurement device are moved relative to each other along the same curved path by the positioning apparatus, so that the same target scan line is measured for both the clockwise and counter-clockwise directions, and so that the position of the target scan line within the operating volume of the machine tool apparatus is the same for both the clockwise and counter-clockwise directions. Furthermore, in the described embodiment, the relative orientation of the measurement device and the artefact is configured to be the same for both the clockwise and counter-clockwise scan / measurement operations (e.g. the workpiece is not rotated between the clockwise and counter-clockwise scan / measurement operations).

[0068] The method then comprises, at step 56, using the data obtained from the first and second measurements to determine the shape, in particular the roundness, of the cylindrical surface of the workpiece 16. Step 56 can be performed by the program running on the PC 8, but as will be appreciated, this need not necessarily be the case. For example, the measurement data can be transferred to another processor device (not shown) and step 56 can be performed by that other processor device. How the data from the first and second measurements is used to determine the roundness is explained in more detail below.

[0069] Figure 5 The graph of Figure 6 shows a first line 60 representing the first set of data points obtained along the target scan line 19 of the inner cylindrical surface of the workpiece 16, taken with the worktable moving in a clockwise direction. Figure 5 A second line 62 is also shown, representing a second set of data points obtained along the same target scan line 19 on the cylindrical surface of the workpiece 16 as the first measurement, but taken with the worktable 14 moving in a counter-clockwise direction. As shown, each of these sets of data points contains the effects of the axis reversal error, and results in a distorted representation of the cylindrical surface of the workpiece 16.

[0070] According to one embodiment of the present application, the data points obtained during the clockwise and counter-clockwise movements are used to obtain a third set of data points which lie on an intermediate surface between the surface represented by the first data set and the surface represented by the second data set, and the roundness can then be determined from the third set of data points. For example,

[0071] (x1 n ,y1 n ) = clockwise data points

[0072] (x2 n ,y2 n ) = counter-clockwise data points

[0073] (x n ,y n ) = third data point set on intermediate surface

[0074] Referring to Figure 6 As shown, there can not be a 1 : 1 correspondence between the clockwise data points 60 and the counter-clockwise data points 62. According to one example embodiment, the (x n ,y n ) data points can be calculated as follows. For each point (x1 n ,y1 n ) in one of the data sets (e.g., the counter-clockwise data set), the point (x2 m ,y2 m ) closest to the same angle Θ is selected from the other data set (e.g., the clockwise data set). The (x n ,y n ) data points are then obtained as follows:

[0075]

[0076] Figure 6 Three example derived data points are shown. The (x n ,y n ) can then be used to calculate the roundness over all n. The small difference in angle between (x1 n ,y1 n ) and (x2 m ,y2 m ) will not substantially affect the shape / roundness value, as the (x n ,y n ) points are still on the desired surface. The above process finds a derived data point for each measured data point and all measured data points in one of the measured data sets (e.g., the counter-clockwise data set), however, this need not be the case. For example, the method can include deriving a derived data point for a subset of the points in one of the measured data sets (e.g., the counter-clockwise data set); e.g., for every other point in one of the measured data sets.

[0077] Referring back to Figure 5representing the third data set, and is equidistant from the surfaces / lines 60, 62 representing the first and second data sets. As can be seen, the intermediate line 64 is more rounded than the first and second lines 60, 62. Thus, a shape / roundness measurement determined from the third data set of points will provide a more representative measure of the roundness of the cylindrical surface of the workpiece 16 than a shape / roundness measurement determined from either of the first and second data sets.

[0078] The above process involves determining a third set of data points, but it will be appreciated that this need not necessarily be the case. For example, the method can comprise fitting a curve such that the curve is equidistant between the first and second sets of data points. In an example embodiment, a curve can be made to pass through each of the first and second sets of data points, and a third curve can then be fitted such that the third curve is equidistant between the first and second curves.

Claims

1. A method for determining shape measurements of the bending characteristics of an artifact, the method comprising: i) The machine tool moves the artifact and the measuring device relative to each other along a predefined bending path in a first direction to obtain a first set of data points along the surface of the bending feature; ii) The machine tool moves the artifact and the measuring device relative to each other along a predefined curved path in a second direction opposite to the first direction to obtain a second set of data points along the surface of the curved feature; as well as iii) Use the first data point set and the second data point set to determine the shape measurement values ​​of the artifact; In steps i) and ii), the relative movement along the predefined curved path in the first and second directions is achieved by the combined movement of at least two non-parallel linear movement axes of the machine tool, wherein during the movement in each of the first and second directions, the movement of at least one of the at least two non-parallel linear movement axes is reversed.

2. The method as described in claim 1, wherein, Using the first data point set and the second data point set includes obtaining a representation of an intermediate surface located between the surface represented by the first data point set and the surface represented by the second data point set, and determining the shape measurement value based on the representation of the intermediate surface.

3. The method as described in claim 2, wherein, The intermediate surface is equidistant between the lines represented by the first set of data points and the lines represented by the second set of data points.

4. The method as described in claim 2 or 3, wherein, The representation of the intermediate surface includes a third set of data points.

5. The method of claim 1, wherein, The first and second data point sets comprise measurement data obtained substantially radially relative to the curvature of the predefined curved path at the measurement point.

6. The method of claim 1, wherein, The first data point set and the second data point set were obtained under the same relative angular orientation of the measuring device and the artifact.

7. The method of claim 1, wherein, The first data point set and the second data point set were obtained along substantially the same target scan line on the surface of the curved feature.

8. The method of claim 1, wherein, The predefined curved path along which the artifact and the measuring device are moved relative to each other to obtain the second set of data points is the same as the predefined curved path along which the artifact and the measuring device are moved relative to each other to obtain the first set of data points.

9. The method of claim 1, wherein, The machine tool includes a machine tool on which the artifact has been previously machined or on which the artifact is to be machined.

10. The method of claim 1, wherein, The bending feature includes an arc-shaped feature.

11. The method of claim 1, wherein, The bending feature includes a circular feature.

12. The method of claim 1, wherein, The shape measurements include roundness measurements.

13. The method of claim 1, wherein, The measuring device includes a contact scanning probe, wherein the first data point set and the second data point set are obtained by continuously contacting and scanning the surface of the curved feature with the scanning probe.

14. The method of claim 2 or 3, wherein, The representation of the intermediate surface includes curves.

15. A computer-implemented method, comprising: A first set of data points is acquired by the measuring device as the measuring device and the artifact move relative to each other in a first direction along a predefined curved path via the machine tool. A second set of data points is acquired by the measuring device as the measuring device and the artifact move relative to each other in a predefined curved path in a second direction via the machine tool. as well as The shape measurements of the artifact are determined using the first data point set and the second data point set; The relative movement along the predefined curved path in the first and second directions is achieved by the combined movement of at least two non-parallel linear movement axes of the machine tool, wherein during the movement in each of the first and second directions, the movement of at least one of the at least two non-parallel linear movement axes is reversed.

16. A computer program product comprising instructions configured to cause a processor device executing the instructions to control a machine tool on which a measuring device is mounted to perform steps i) and ii) of the method as claimed in any one of claims 1 to 14, and to be configured to cause the processor device executing the instructions or another processor device to perform step iii) of the method as claimed in any one of claims 1 to 14.

17. A computer-readable medium comprising the computer program product as described in claim 16.

18. A processing apparatus configured to: acquire a first set of data points, the first set of data points being obtained by the measuring device as a measuring device mounted on a machine tool and an artifact move relative to each other along a predefined bending path in a first direction via the machine tool; and acquire a second set of data points, the second set of data points being obtained by the measuring device as the measuring device and the artifact move relative to each other along a predefined bending path in a second direction via the machine tool; and use the first set of data points and the second set of data points to determine shape measurements of the artifact, wherein... The relative movement along the predefined curved path in the first and second directions is achieved by the combined movement of at least two non-parallel linear movement axes of the machine tool, wherein during the movement in each of the first and second directions, the movement of at least one of the at least two non-parallel linear movement axes is reversed.

19. An apparatus comprising a machine tool having a measuring device positioned thereon and an artifact, wherein, The device is configured to: i) The machine tool moves the artifact and the measuring device relative to each other along a predefined bending path in a first direction to obtain a first set of data points along the surface of the bending feature of the artifact, wherein the relative movement along the predefined bending path in the first direction is achieved by the combined movement of at least two non-parallel linear movement axes of the machine tool, and during the movement in the first direction, the movement of at least one of the at least two non-parallel linear movement axes is reversed. ii) The machine tool moves the artifact and the measuring device relative to each other along a predefined bending path in a second direction opposite to the first direction to obtain a second set of data points along the surface of the bending feature of the artifact, wherein the relative movement along the predefined bending path in the second direction is achieved by the combined movement of at least two non-parallel linear movement axes of the machine tool, during which the movement of at least one of the at least two non-parallel linear movement axes is reversed; and iii) Use the first data point set and the second data point set to determine the shape measurement values ​​of the artifact.

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