Measurement method

Abstract

A method for determining shape measurements for a curved feature of an artificial object, the method comprising a positioning apparatus that moves the artificial object and a measuring device relatively in a first direction along a curved path to acquire a first set of data points along a surface of the curved feature, and the positioning apparatus that moves the artificial object and the measuring device relatively in a second direction opposite the first direction along another curved path to acquire a second set of data points along the surface of the curved feature, the method further comprising determining shape measurements for the artificial object using the first and second sets of data points.

Description

[Technical Field]

[0001] The present invention relates to a method for inspecting the features of a workpiece for a positioning device, and in particular to a method for evaluating the morphology of curved features (for example, a method for determining the roundness value for a circular feature). [Background technology]

[0002] For example, it is well known that the features of a workpiece are measured before, during, or after production to ensure that the features conform to desired tolerances and / or to determine the position and / or orientation of the features and / or workpiece. This typically involves measuring different points on the workpiece by moving the measuring device ("probe") and the workpiece relative to each other through the apparatus. While products and techniques are available for measuring workpieces on machined (or to be machined) machine tools, it is still common to measure those parts of components requiring very high tolerances on dedicated coordinate measuring machines (CMMs). In particular, the morphology of features, such as engine cylinder blocks and valve seats, needs to be precisely controlled, and their measurements are often performed on dedicated CMMs. An example of shape measurement is the "roundness" or "roundness" of a circular feature. As will be understood, and as defined by well-established geometric dimensions and tolerances (GD&T) systems, shape measurement is a specific measurement distinct from the position or size (e.g., diameter) of a surface.

[0003] Because CMMs are primarily designed to measure workpieces (whereas machine tools are primarily designed to machine workpieces), CMMs can traditionally offer a higher level of measurement accuracy and are less susceptible to error than machine tools. This is usually due to differences in the mechanical configuration of CMMs and machine tools. For example, the drive mechanism of a machine tool is often quite different from that of a CMM, as the drive system of a machine tool needs to be able to withstand large forces for the mechanical movements it is required to perform (unlike a CMM, which only needs to carry measuring devices that require small forces during operation).

[0004] However, the increasing demand for measuring one or more workpieces in their as-is state on a machine tool is due to the potential for significant time and cost savings.

[0005] However, the inventors of this application have found that it is difficult to accurately measure the shape / roundness of a curve on a machine tool. [Overview of the project]

[0006] The present invention relates to a technique for improving the shape measurement of curved features, such as the roundness or circularity of circular features, which can be of concrete value when such features are measured on a machine tool, and which often does not provide sufficiently accurate measurement data.

[0007] A first aspect of the present invention provides a method for determining the shape measurement of a curved feature of an artifact, the method comprising: i) a positioning device moving the artifact and a measuring device relative to each other along a curved path in a first direction to acquire a first set of data points along the surface of the curved feature; ii) the positioning device moving the artifact and a measuring device relative to each other along a curved path in a second direction opposite to the first direction to acquire a second set of data points along the surface of the curved feature; and iii) using the first and second sets of data points to determine the shape measurement of the artifact.

[0008] The inventors of the present invention have found that errors in measuring curved features can be caused by machine motion (specifically, by so-called "lost motion" of the machine). Although such measurement errors (caused by machine motion) are small (e.g., on the order of a few microns), they hindered the inventors from achieving the accurate shape measurements required. The inventors have found that the technology of the present invention reduces the impact of errors, enabling the acquisition of more accurate and reliable shape measurements.

[0009] Using the first and second sets of data points can include obtaining a representation of an intermediate surface located between the lines / surfaces represented by the first and second sets of data points (from the first and second sets of data points). As will be understood, the intermediate surface and the representation of the lines / surfaces represented by the first and second sets of data points can be conceptual. For example, the representation of an intermediate surface can include a third set of data points (present on the conceptual intermediate surface). That is, "using the first and second sets of data points" can include obtaining a third set of data points located on an equidistant (conceptual) surface between the lines / surfaces represented by the first and second sets of data points. Shape measurements can then be determined from the representation of the intermediate surface (e.g., from the third set of data points). As will be understood by those skilled in the art, the representation of an intermediate surface can include something on which the intermediate surface can be constructed, for example, the representation of an intermediate surface can include features of one or more splines (e.g., B-splines or non-uniform rational B-spline "NURBS" surfaces). Furthermore, as will be understood by those skilled in the art, the representation of the mid-surface can include, for example, a two-dimensional (2D) representation of a 2D curvature. This is possible even if the first and second sets of data points include a three-dimensional (3D) dataset. Furthermore, as will be understood by those skilled in the art, the representation of the mid-surface can include, for example, a three-dimensional (3D) representation of a 3D curvature.

[0010] The data points are related to positional information rather than non-positional information (e.g., temperature information), and are therefore referred to as “positional” data points (and thus the first and second sets of data points can be referred to as the first and second sets of positional data points). As will be understood, positional data points can be obtained by combining positional data / information about the positioning device (e.g., the relative position of the artifact and the measuring device) with data / information from the measuring device (e.g., stylus deflection data in the case of a contact probe). As will be described in more detail below, positional data / information about the positioning device (e.g., the relative position of the artifact and the measuring device) can be assumed or made known (e.g., via one or more positional encoders on the positioning device). Furthermore, as will be described in more detail below, the direction of measurement obtained by the measuring device (e.g., the direction of stylus deflection) can be assumed or made known.

[0011] Data points can be obtained by combining information about the relative positions of the measuring device and artifact with a vector from those positions to the surface being measured. The relative positions of the measuring device and artifact can be known or assumed (e.g., via one or more position encoders on a positioning device). The direction of the vector to the surface being measured can be known or estimated.

[0012] The present invention is found to be particularly useful when the curved feature is a curved surface of an artifact (for example, the curved surface of a cylinder or cone rather than the end of a cylinder / cone).

[0013] The present invention is found to be particularly useful when the measuring device is used to measure an artifact in a dimension that extends substantially along the radius of curvature of a curved path at a measurement point. Therefore, preferably, the first and second sets of data points include measurement data acquired / taken / 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, and particularly preferably within 15° of the radius of curvature of the curved path at the measurement point, e.g., within 10° of the radius of curvature of the curved path at the measurement point).

[0014] Shape measurements can include profile tolerances for curved features. Optionally, shape measurements can include roundness tolerances or "circularity" for circular features. Optionally, shape measurements can include cylindricity tolerances for cylinders. Shape measurements can include values ​​(shape values), for example, shape measurements can include roundness values, circularity values, or cylindricity values.

[0015] Preferably, the intermediate surface is equidistant between lines / surfaces represented by the first and second sets of data points. Determining the representation of the intermediate surface, which is equidistant between lines / surfaces represented by the first and second sets of data points, is understood to allow for more accurate shape measurements to be determined from there.

[0016] The relative motion in the first and second directions can be at least partially brought about by the motion of at least one movable axis of the positioning device, which is reversed between the motions in each of the first and second directions. The movable axis may be a linear axis. Thus, the relative motion in the first and second directions can be at least partially brought about by the motion of at least one linear motion axis of the positioning device. Specifically, the relative motion in the first and second directions can be at least partially brought about by the motion of at least one linear motion axis of the positioning device, which is reversed between the motions in each of the first and second directions. The relative motion in the first and second directions can be brought about by a combination of motions of at least two non-parallel and linear motion axes of the positioning device. Specifically, the relative motion in the first and second directions can be brought about by a combination of motions of at least one non-parallel and linear motion axes of the positioning device, which is reversed between the motions in each of the first and second directions. One source of measurement error that can have a significant adverse effect on shape measurement is caused by the reversal of motion along at least one linear axis (axis reversal error), which is a specific problem in machine tools. This has been found to be at least in part due to machine / axis backlash. For example, when a motor reverses the direction of control / power, a slight delay can occur before the axis that the motor actually controls begins to move. As will be understood, in order to help solve the backlash problem, the same operating axis of the positioning device should be used to produce relative motion in steps i) and ii). As will be understood, relative motion can include moving an artifact and holding the measuring device stationary, or moving the measuring device and holding the artifact stationary, or moving the artifact and the measuring device simultaneously.

[0017] Machine tools typically do not have position-measuring encoders to directly measure the position of their operable axes. Instead, the machine tool controller can monitor the position of the axis via encoders for the axis drive / motor (which does not necessarily provide an accurate representation of the axis's actual position). Even when a machine tool does have position-measuring encoders to directly measure the position of its operable axes, such position information may not be available for the process of determining shape measurements. Indeed, machine tools are often not designed to continuously report axis position information to a separate measurement process. Therefore, it is common practice in the field of machine tools to assume the axis position for the measurement process (e.g., based on commands to drive the machine tool rather than determined from encoders on the machine tool) and combine the assumed position information with data from probes to determine surface position information. Furthermore, measurement information about features may be determined solely from the output of the measuring device (e.g., a probe) (i.e., without combining it with reported or assumed axis position information). Therefore, particularly in the field of machine tools, the effect of axial reverse backlash on axial position is often not directly measured or known, and even if the measuring device attached to the machine tool is well calibrated, accurate, and error-free, errors can be introduced into the measurement data used to determine the shape of parts due to the structure and / or configuration of the machine tool itself.

[0018] Machine manufacturers have taken steps to address the backlash problem, for example, by servoizing the drive that attempts to compensate for the backlash. However, such a solution itself can further introduce measurement errors such as "axis reversal spikes" that are well-known in the field. Axis reversal is one source of error in shape measurement, while there are other sources / causes such as dynamic structural deformations caused by the acceleration of the machine. Such axis reversals and dynamic structural deformations can result in errors caused by so-called "lost motion" or non-converted operations of the machine.

[0019] The inventors of the present application have found that by using the technology of the present application, the influence of such errors on shape measurement can be reduced, and it is possible to achieve shape measurement with a level of accuracy exceeding the expected capabilities of machine tools.

[0020] Preferably, the relative angular direction of the measuring device and the artifact is the same for both the first and second measurements. That is, preferably, the relative angular direction of the measuring device and the artifact does not change between the first and second measurements. This avoids the need to consider the influence of the relative angular direction of the measuring device and the artifact on shape measurement. For example, in the case of a contact probe, if there is a probe tip offset error, changing the relative angular direction of the measuring device and the artifact can affect shape measurement. Ensuring that the relative angular direction of the measuring device and the artifact is the same for both the first and second measurements avoids the need to know / determine the probe tip offset error.

[0021] The positioning device can be a dedicated measuring device such as a coordinate measuring machine (CMM). The positioning device can include a machine tool. Specifically, the positioning device can include a machine tool on which the artifact has been previously machined or on which the artifact is to be machined.

[0022] The curved features can include arcuate features. The curved features can include circular features. For example, the curved features can include cylindrical bosses or bores. Thus, obtaining the first and / or second set of data points can include, for example, a positioning device that moves the artifact and the measurement device relative to each other along a partially or fully circular path. Measurement of the curved features can be constrained to a plane, but does not necessarily have to be. For example, the positioning device can move the artifact and the measurement device relative to each other along a spiral or helical path.

[0023] The relative movement of the artifact and the measurement device can include the movement of the artifact while the measurement device is stationary, or vice versa. The relative movement of the artifact and the measurement device can also include the movement of both the measurement device and the artifact.

[0024] Preferably, the first and second sets 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 artifact and the measurement device are moved relative to each other to obtain the set of data points can be pre-defined / pre-determined.

[0026] As will be appreciated, particularly when seeking a solution to the backlash problem, the same axis of movement of the positioning device should be used to effect the relative movement in steps i) and ii).

[0027] The curved paths in steps i) and ii) do not necessarily have to be identical. For example, the curved paths in steps i) and ii) can be configured such that the same target scan line (or “nominal measurement line”) is measured on the surface of the curved feature, but at different locations within the operating space of the positioning device. However, preferably, the curved path through which the artifact and measuring device are moved relative to each other to acquire a second set of data points is identical to the curved path through which the artifact and measuring device are moved relative to each other to acquire a first set of data points. Preferably, the first and second sets of data points can be acquired by the positioning device by moving the artifact and measuring device relative to each other along the same curved path, where different data sets are acquired by moving along the same curved path in different directions. Therefore, the curved paths in i) and ii) are preferably identical so that the first and second sets of data points are acquired along the same target scan line (or “nominal measurement line”) on the surface of the curved feature for steps i) and ii), the position of the target scan line in the operating space of the positioning device is the same as for steps i) and ii), and the outline / shape of the curved path moved relatively by the artifact and the measuring device is preferably the same.

[0028] Making the curved paths i) and ii) identical can help prevent any differences in the performance of the position-dependent positioning device (e.g., the magnitude of backlash) from affecting the first and second sets of data points acquired by the positioning device. This can also help ensure that, in the case of a contact probe, nominally identical probe forces are used during the acquisition of the first and second sets of data points.

[0029] Furthermore, it is preferable that the rotational position / orientation of the artifact with respect to the positioning device, and / or the rotational position / orientation of the measuring device with respect to the artifact, do not change between steps i) and ii). In other words, it is preferable that the physical arrangement and configuration of the artifact and the measuring device remain substantially the same for steps i) and ii).

[0030] Furthermore, it is preferable that the speeds at which the measuring device and the artifact move relative to each other along the curved path in steps i) and ii) are substantially the same.

[0031] Therefore, it is preferable that each aspect of step i) and ii) is substantially identical except for the direction in which the measuring device and artifact move relative to each other along the curved path.

[0032] Measuring devices may include contact or non-contact measuring devices. As is common in the field of measuring device apparatus, measuring devices may also be referred to as probes. Measuring devices (or "probes") suitable for use with the present invention generally include those referred to as scanning probes (also called "analog probes"). A scanning probe (or "analog probe") is a probe that can be scanned along and against a surface to obtain a set of along / measurement points on the surface, while continuously interacting with the surface (for example, while continuously in contact with the surface in the case of a contact probe). This is in contrast to touch-triggered probes, which require the probe to be advanced toward and retracted from the surface for each data point to be measured.

[0033] A scanning probe can provide one or more output signals that vary depending on the degree of interaction between the probe and the surface (i.e., in a non-binary manner, i.e., over a range). This allows the scanning probe's signals to continuously determine the position of the probe body and the surface as the probe is scanned along / against the surface.

[0034] A contact scanning probe typically includes a body that can be mounted on / mounted on a positioning device, and a stylus extending from the body. Typically, the contact tip (e.g., a "stylus ball") is provided at the free end of the stylus for contact with an artifact. The range (and optionally the direction) of stylus deflection is known by configuring the contact scanning probe to output one or more signals that vary in response to the amount the stylus deflects from its stationary position (and optionally the direction of the stylus deflection). This is in contrast to a so-called "contact trigger probe," which is configured to output a trigger signal when the stylus deflects by a threshold amount from its stationary position.

[0035] Therefore, the present method can include a positioning device that moves the artifact and the measuring device relative to each other along a curved path (in a first and a second direction), thereby continuously scanning the measuring device along / relative to the surface of a curved feature (by which a first and a second set of data points along the surface of the curved feature is obtained). Thus, in the case of a contact scanning probe, the first and second sets of data points can be obtained by moving the contact scanning probe while the contact scanning probe is in continuous contact with the surface of the curved feature.

[0036] Accordingly, the present invention describes a method for determining shape measurements for curved features of a workpiece using measurement data obtained by a measuring device attached to a positioning device having multiple linear axes, wherein the measurement data collection process by the measuring device involves relative movement of the measuring device and the workpiece along a predetermined curved path, brought about by the movement of at least one linear axis of the positioning device which is reversed while the measuring device and the workpiece are moved relative to each other along a predetermined curved path, and the method comprises: the positioning device moving the artifact and the measuring device relative to each other along a predetermined curved path in a first direction to obtain a first set of measurement data; the positioning device moving the artifact and the measuring device relative to each other along the same predetermined curved path in a second direction opposite to the first direction to obtain a second set of measurement data; and determining shape measurements for curved features from the first and second sets of measurement data.

[0037] As will be understood, the above method can be implemented by a computer. Therefore, any or all of the above steps of the method can be performed under the control of a computer / processor / processing device such as a computer controller, numerical controller (NC), e.g., a computer numerical controller (CNC). Suitable processing devices include, but are not limited to, a CPU (Central Processor Unit), FPGA (Field Programmable Gate Array), ASIC (Application-Specific Integrated Circuit), etc. As will be understood, steps i), ii) and / or iii) can be performed under the control of different computer / processor / processing devices. Specifically, for example, step iii) can be performed under the control of a different computer / processor / processing device than those used in steps i) and ii). For example, step iii) can be performed by a different computer / processor / processing device than those used in steps i) and ii), e.g., a physically remote computer / processor / processing device, and / or can be performed at a substantially different time than steps i) and ii).

[0038] The present invention also describes an apparatus configured to perform the above-described method. For example, the apparatus described herein comprises a positioning device (such as a machine tool) on which a measuring device and an artifact are positioned, wherein the positioning device is configured to move the artifact and the measuring device relative to each other along a curved path in a first direction and to obtain a first set of data points along the surface of the curved feature. The apparatus is further configured to move the artifact and the measuring device relative to each other along a curved path in a second direction opposite to the first direction and 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 artifact using the first and second sets of data points. For example, a computer / processor / processing device such as a PC can be configured to determine a shape measurement of an artifact using the first and second sets of data points.

[0039] Accordingly, the present invention also describes a computer / processor / processing device (e.g., a PC) including computer program code which takes a first set of data points acquired by a measuring device attached to a positioning device while the measuring device and the artifact are moving relative to each other along a path curved in a first direction by the positioning device, and takes a second set of data points acquired by the measuring device while the measuring device and the artifact are moving relative to each other along a path curved in a second direction, and is configured to determine a shape measurement of the artifact using the first and second data sets.

[0040] Accordingly, this application also describes a computer program code (for example, stored on a computer-readable medium) on which it is executed, which causes a processor device (for example, a controller and / or PC) to take a first set of data points acquired by a measuring device attached to a positioning device while the measuring device and the artifact are being moved relative to each other along a curved path in a first direction by the positioning device, and to take a second set of data points acquired by the measuring device while the measuring device and the artifact are being moved relative to each other along a curved path in a second direction, and to determine a shape measurement of the artifact using the first and second data sets. [Brief explanation of the drawing]

[0041] Embodiments of the present invention will be described herein by reference only to the following drawings.

[0042] [Figure 1] This is a schematic diagram showing an isometric view of a machine tool. [Figure 2] (a) is a figure showing the effect of axial reversal error on expected net probe deflection versus actual net probe deflection during measurement of a circular feature with an analog / scanning probe, and (b) is a figure showing the axial positions of the x and y axes required over time to move the table of a machine tool in a circular motion. [Figure 3] This figure shows the effect of axis reversal error on a dataset obtained for circular features. [Figure 4] This is a flowchart for a process according to an exemplary embodiment of the present invention. [Figure 5] This is a graph containing lines that represent the acquired dataset of circular features taken in opposite directions (e.g., clockwise and counterclockwise). [Figure 6] This figure shows a magnified view of the graph portion of Figure 5. [Modes for carrying out the invention]

[0043] Referring to Figure 1, a machine tool assembly 2 is illustrated, comprising a machine tool 4, a numerical controller 6 (NC) (e.g., a computer numerical controller or "CNC"), a PC 8, a transceiver 9, and an interface 10. The machine tool 4 includes a spindle 12 attached to a movable arm 17 via a motor 11 in the z-dimensional direction, and a movable table 14 that can be moved in the x and y dimensions via motors 13 and 15. When a workpiece 16 is used for work (e.g., machine), the workpiece 16 is attached to the movable table 14, and a tool (not shown) is attached to the spindle 12. The NC 6 controls the rotation of the spindle 12, the z-movement of the arm 17, and the x / y-movement of the movable table 14 to bring the tool and workpiece together and cause the tool to act on the workpiece. The NC 6 can be programmed along with the machining operation, for example, via the PC 8.

[0044] When used to inspect a workpiece 16, an inspection device such as a probe 20 is attached to the spindle 12. Again, the NC6 can control the z-movement of the arm 17 and the x / y-movement of the movable table 14, causing the probe 20 to interact with the workpiece 16 (for example, by contacting the workpiece if the probe is a contact probe) and measure the workpiece 16. The NC6 can be programmed along with the inspection operation, for example, via the PC8.

[0045] Although not shown in the embodiments described, the machine tool 4 may include one or more axes of rotation for rotating the table 14 (e.g., around the x-axis, y-axis, and / or z-axis). Additionally / alternatively, the machine tool 4 may be configured such that the spindle 12 is rotatable around other further axes of rotation (e.g., so that a probe mounted inside can rotate around the x-axis and / or y-axis). Additionally / alternatively, the machine tool 4 may be configured such that the spindle 12 is movable around the x-axis and / or y-axis, and / or the table is movable around the z-axis.

[0046] In this embodiment, the probe 20 is a contact probe comprising a body 22 attached to a spindle 12, a stylus 24 extending from the body 22, and a stylus tip 26 at the distal end of the stylus 24 of the body 22. In the embodiment described, the stylus 24 is capable of deflection relative to the body 22 (for example, when the stylus tip 26 is in contact with a surface), and such deflection can be detected by a sensor on the body 22. Specifically, in the embodiment described, the probe 20 is a scanning probe (also known to those skilled in the art as an analog probe) in which the range / amount / degree of stylus deflection from a stationary position can be sensed and reported by the probe 20 (in contrast to a touch-trigger probe that reports only when the stylus is deflected by, for example, a predetermined threshold amount). The probe 20 is capable of outputting distinct values ​​indicating the range of stylus deflection in different directions relative to the probe (e.g., distinct values ​​for the deflection range in the x-direction, distinct values ​​for the deflection range in the y-direction, and / or distinct values ​​for the deflection range in the z-direction, all measured relative to the probe), and / or outputting an aggregated deflection value. Scanning probes for machine tools are well known, such as the SPRINT® probe available from Renishaw plc. Other probes and other techniques may be used, as will be understood.

[0047] Features of the workpiece to be inspected can be measured by bringing the stylus tip 26 into contact with the features of the workpiece 16. Stylus deflection data from the probe 20 can be streamed instantaneously and continuously to the PC 8, for example, via a wired connection, or wirelessly via the receiver 9 and interface 10, as illustrated in this invention. The connection between the probe 20 and the receiver 9 can be, for example, optical or wireless. As will be understood, other techniques can be used to transfer the stylus deflection data. For example, the data can be transmitted at intervals (periodically or irregularly), or, for example, only when requested. In other exemplary embodiments, the stylus deflection data can be stored locally in memory in the probe 20 and later downloaded to the PC 8, for example, via a wired or wireless link.

[0048] Optionally, data from probe 20 can be combined with data relating to the relative positions of probe 20 and table 14 (and thus workpiece 16). For example, a process running on PC8 can be configured to combine data from probe 20 with position data of spindle 12 and table 14. Such position data of spindle 12 and table 14 can be assumed (based on commands to drive the machine tool, rather than being determined, for example, from an encoder on the machine tool) or obtained from a measuring device (not shown) that monitors the positions of spindle 12 and table 14 in any or all of the x, y, and z axes. Such a measuring device could be, for example, a resolver or encoder that monitors the rotational positions of some of the motors 11, 13, 15 that control the positions of spindle 12 and table 14 in the x, y, and z axes. When the process for determining the shape of a part uses data from a measuring device that monitors the positions of the spindle 12 and table 14, a clock / synchronization signal (e.g., issued by interface 10) can be used to simultaneously determine the position data and probe data of the spindle 12 and table 14 (e.g., as described in U.S. Patent No. 7970488).

[0049] Therefore, data points can be obtained by combining information about the (known or assumed) relative positions of the measuring device and the artifact with a vector from that position to the surface being measured. As described above, the probe 20 can output separate values ​​indicating the range of stylus deflection in directions different relative to the probe, for example, a value for the deflection range in the x direction, a separate value for the deflection range in the y direction, and / or a separate value for the deflection range in the z direction. In such cases, the direction of the vector from the known / assumed relative position in which the measuring device is held relative to the artifact, and the surface being measured, can be known. Also, as described above, the probe 20 can output aggregated deflection values, where the direction of the vector from the known / assumed relative position in which the measuring device is held relative to the artifact, and the surface being measured, can be assumed.

[0050] Using the example setup in Figure 1, when a cylindrical workpiece 16 is measured by a probe, the spindle 12 needs to be moved vertically (along the z-axis), and the table 14 needs to be moved laterally (along the x-axis and / or y-axis) so that the tip 26 of the probe comes into contact with the surface of the cylindrical workpiece 16. Next, the table 14 needs to be moved simultaneously (e.g., in a synchronous manner) along the x and y axes to move the workpiece 16 in a circle, for example, by scanning the probe tip 26 along the target scan line (or "nominal measurement line") 19. To achieve such circular motion, it is necessary to reverse the direction of movement of the table 14 along the x and y axes a number of times. Optionally, the z-axis can also be moved simultaneously to allow spiral scanning of the workpiece 16.

[0051] As illustrated in the graph of Figure 2(b), in order to drive the table 14 in a circular motion, it is required to command the x-axis and y-axis to move back and forth according to sinusoidal motion, and the movement of the x-axis and y-axis is phase-shifted by 90°. In Figure 2(b), the requested / commanded / instructed position of the table along the x-axis over time is illustrated by the dotted line 34, and the requested / commanded / instructed position of the table along the y-axis over time is illustrated by the dashed and double dotted line 36.

[0052] To aid in describing the present invention, the workpiece 16 is assumed to be a perfect cylinder, and the table 14 and workpiece 16 are assumed to move along the x and y axes according to instructions intended to drive the table 14 in a perfect circular motion centered on the center of the cylindrical workpiece 16. In this situation, the radial / total deflection of the probe is expected to be constant at all positions around the target scan line 19 on the workpiece 16. This constant deflection is schematically illustrated by the dashed line 30 in the graph of Figure 2(a), where "D" represents the deflection of the probe and θ represents the position relative to the circular feature.

[0053] However, in this situation, the deflection of the probe stylus 24 is found to tend to change as the table moves, allowing the probe to measure the circular surface of the workpiece 16, which is schematically illustrated by the solid jagged line 32 in the graph of Figure 2(a). As shown in Figures 2(a) and (b), such a change in probe deflection is found to coincide with the position / time at which the linear axis driving the workpiece 16 reverses direction. This can be caused, for example, at least in part, by motor backlash.

[0054] Specifically, when an axis motor reverses direction (for example, the motor for the x-axis), there is a slight delay before the motor actually starts moving the axis, due to the motor's configuration for "looseness" or "play." However, as clearly illustrated in Figure 2, at the point where the motor for one axis reverses direction (for example, the motor for the x-axis 13 in this example), the motor for the other axis (i.e., the motor for the y-axis 15 in this example) is still driving the other axis in a constant direction (and at a substantially constant peak speed). Thus, the combination of i) the delay before one axis actually starts moving after reversing, and ii) the fact that the other axis is still moving, results in the range of deflection of the probe stylus 24 changing as the probe moves around the target scan line 19.

[0055] Figure 3 schematically illustrates the effect of this change in probe deflection on the determined shape of a circular feature taken in a counterclockwise direction. In Figure 3, the dashed line 38 illustrates the actual circular shape of the feature, and the solid line illustrates the irregular circular feature that would be determined when the probe output is combined with the assumed position of the axis or with position information determined from the motor encoder.

[0056] Axis reversal errors are variable and therefore can be difficult to calibrate. For example, the axis reversal error for each axis may differ depending on the position and may be affected by environmental factors such as temperature. Nevertheless, the method of the present invention (which will be illustrated in more detail below) reduces the effects of axis reversal errors regardless of position and environmental factors.

[0057] The figures used herein assume perfectly circular features. Nevertheless, as will be understood, in practice, curved features of production parts have unknown (e.g., uncalibrated) outlines / shapes. Therefore, for example, a feature intended to be cylindrical / circular is unlikely to have a perfectly circular outline due to inaccuracies in the manufacturing process, and thus difficult to detect axial reversal errors. Nevertheless, the method of the present invention (an example thereof is described in more detail below) eliminates the need to identify it and essentially reduces the impact of axial reversal errors. Furthermore, the figures described herein greatly exaggerate errors that are likely to be actually experienced (and do not represent the scale of problems typically expected from a standard machine tool). Actual errors in measurements caused by axial reversal that have been actually experienced are so small (on the order of a few microns) that they are not visible and may be mixed with other sources of error. Nevertheless, the inventors have identified that errors caused by axial reversal and other sources of "lost motion" can have a significant impact on the shape measurement of curved features.

[0058] Figure 4 illustrates an exemplary process 50 for improving the accuracy of measuring the curvature of a workpiece 16 (specifically, for improving the accuracy of measuring the curvature of a workpiece 16) in accordance with the present invention. The method comprises, in step 52, controlling the machine tool 2 to cause the probe 20 to measure around the cylindrical surface of the workpiece 16 in a first direction along a curved path, which in this embodiment is measured along a circular target scan line 19. For example, according to the embodiment described above, the method may include controlling the x and y axes and moving the table 14 in a circular motion so that the probe 20 measures the cylindrical surface clockwise along the circular target scan line 19. As will be understood, each measurement taken along the circular target scan line is acquired substantially radially with respect to the circular target scan line.

[0059] Next, the method, in step 54, comprises controlling the machine tool 2 to cause the probe 20 to perform a second measurement around the cylindrical surface of the workpiece 16 along the same circular target scan line 19, in which case the x and y axes are controlled to move the table 14 and workpiece 15 along the circular target scan line 19, so that the probe 20 measures the cylindrical surface in a counterclockwise direction along the circular target scan line 19. Again, each measurement taken along the circular target scan line is acquired substantially radially with respect to the circular target scan line.

[0060] In the embodiments described, the artifact and the measuring device are moved relative to each other by a positioning device along the same curved path, so that the same target scan line is measured in both clockwise and counterclockwise directions, thereby ensuring that the position of the target scan line in the operating space of the machine tool is identical in both clockwise and counterclockwise directions. Furthermore, in the embodiments described, the relative orientation configuration of the measuring device and the artifact is identical for both clockwise and counterclockwise scanning / measuring operations (for example, the workpiece does not rotate between measurements in clockwise and counterclockwise scanning / measuring operations).

[0061] Next, the method comprises, in step 56, using the data obtained from the first and second measurements to determine the shape of the roundness of the cylindrical surface of the workpiece 16. Step 56 can be performed by a process executed on PC8, but as will be understood, this is not necessarily required. 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 are used to determine roundness will be described in more detail below.

[0062] The graph of FIG. 5 illustrates a first line 60 representing a first set of data points obtained along the target scan line 19 of the inner cylindrical surface of the workpiece 16 as the table is moved in the clockwise direction. FIG. 5 also illustrates a second line 62 representing a second set of data points obtained along the same target scan line 19 of the cylindrical surface of the workpiece 16, but with the table 14 being moved in the counterclockwise direction. As shown, each of these respective sets of data points includes the effects of axis reversal errors and results in a distorted representation of the cylindrical surface of the workpiece 16.

[0063] In accordance with an embodiment of the present invention, the data points obtained during clockwise and counterclockwise operations are used to obtain a set of third data points that exist on an intermediate surface between the surfaces represented by the first and second data sets. Next, the roundness can be determined from the set of third data points. For example, (x1 n , y1 n ) = clockwise data points, (x2 n , y2 n ) = counterclockwise data points, (x n , y n ) = set of third data points of the intermediate surface

[0064] Referring to FIG. 6, as shown, there may not be a 1:1 correspondence between the clockwise 60 and counterclockwise 62 data points. In accordance with one exemplary 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 counterclockwise data set), a point (x2 m , y2 m ) is selected from the other data set (e.g., the clockwise data set) that is closest to the same angle θ. Next, the (x n , y n ) data points are obtained as follows.

[0065]

number

[0066] Figure 6 illustrates three exemplary derived data points. The roundness is given over all n (x n ,y n It can be calculated using (x1 n ,y1 n ) and (x2 m ,y2 m The slight angle difference between ) and point (x n ,y n ) still exists on the desired surface and therefore does not substantially affect the shape / roundness value. The process described above finds derived data points for each of one of the datasets to be measured (e.g., a counterclockwise dataset) and for each measured data point, but this is not necessarily required. For example, the method of the present application may comprise deriving derived data points for a subset of points in one of the datasets to be measured (e.g., a counterclockwise dataset), for example, for each other point in one of the datasets to be measured.

[0067] Referring back to Figure 5, an intermediate surface representing a third set of data points is shown, with dotted and dashed lines 64 that are equidistant from the surfaces / lines 60 and 62 representing the first and second datasets. As shown in the figure, the intermediate line 64 is more circular than the first line 60 and the second line 62. Thus, the shape / roundness measurement determined from the third set of data points provides a roundness measurement that better represents the cylindrical surface of the workpiece 16 than the shape / roundness measurement determined from either the first or second dataset.

[0068] The process described above involves determining a third set of data points, but this is not necessarily required, as will be understood. For example, the method may include fitting a curve so that it is equidistant between the first and second sets of data points. In an exemplary embodiment, the curve may pass through each of the first and second sets of data points, and then the third curve may be equidistant between the first and second curves so that it can be fitted.

Claims

1. 1. A method for determining shape measurements for curved features of an artificial object, the method comprising: i) a positioning apparatus relatively moving the artefact and the measuring device along a curved path in a first direction to acquire a first set of data points along a surface of the curved feature; ii) the positioning apparatus relatively moving the artefact and the measuring device along a curved path in a second direction opposite to the first direction to acquire a second set of data points along the surface of the curved feature; iii) determining shape measurements for the artifact using the first and second sets of data points; method.

2. using the first and second sets of data points includes obtaining therefrom a representation of an intermediate surface that exists between the surfaces represented by the first and second sets of data points, and determining the shape measurement from the representation of the intermediate surface. The method of claim 1.

3. the intermediate surface is equidistant between the lines represented by the first and second sets of data points; The method of claim 2.

4. the representation of the intermediate surface includes a third set of data points; 4. The method of claim 2 or 3.

5. the relative movement along the curved path in the first and second directions is effected by movement of at least one linear axis of motion of the positioning device being reversed during movement along the curved path in each of the first and second directions; 5. The method according to any one of claims 1 to 4.

6. the first and second sets of data points are acquired at the same relative angular orientation of the measurement device and workpiece; 6. The method according to any one of claims 1 to 5.

7. the first and second sets of data points are acquired along substantially the same target scan line of the surface of the curved feature; 7. The method according to any one of claims 1 to 6.

8. the curved path along which the artefact and the measuring device move 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 measuring device move relative to each other to obtain the first set of data points; 8. The method according to any one of claims 1 to 7.

9. the positioning device comprises a machine tool on which the artefact has previously been machined or on which the artefact will be machined; 9. The method according to any one of claims 1 to 8.

10. the curved features include arcuate features; 10. The method according to any one of claims 1 to 9.

11. The curved features include circular features.

11. The method according to any one of claims 1 to 10.

12. The shape measurement includes a roundness measurement.

12. The method according to any one of claims 1 to 11.

13. the measurement device includes a contact scanning probe, and the first and second data sets are obtained by continuous contact scanning along a surface of the curved feature; 13. The method according to any one of claims 1 to 12.

14. the representation of the intermediate surface includes curvature; 14. The method according to any one of claims 1 to 13.

15. The computer implementation method is a measuring device and an artefact are moved relative to one another along a curved path in a first direction by a positioning device, and a first set of data points are acquired by the measuring device attached to the positioning device; the positioning apparatus moves the measuring device and the artefact relative to one another along a curved path in a second direction, resulting in a second set of data points acquired by the measuring device; and determining shape measurements for the artifact using the first and second sets of data points. method.

16. Computer program code comprising instructions configured to cause an apparatus to carry out the method of any one of claims 1 to 15.

17. 17. A computer readable medium containing the computer program code of claim 16.

18. configured to move a measuring device and an artefact relative to one another along a curved path in a first direction by a positioning apparatus, take a first set of data points acquired by the measuring device attached to the positioning apparatus, move the measuring device and the artefact relative to one another along a curved path in a second direction by the positioning apparatus, take a second set of data points acquired by the measuring device, and determine shape measurements for the artefact using the first and second data sets. Processing device.

19. an apparatus comprising a measuring device and a positioning device on which an artefact is placed; i) causing the positioning apparatus to relatively move the artefact and the measurement device along a curved path in a first direction to acquire a first set of data points along a surface of the curved feature; ii) causing the positioning apparatus to relatively move the artefact and the measurement device along another curved path in a second direction opposite to the first direction to acquire a second set of data points along the surface of the curved feature; iii) configured to determine shape measurements for the artifact using the first and second sets of data points; Device.

Images