Method and computer program product for filtering a measurement data set that can be used to specify and / or verify internal features of a workpiece - Patents.com

Abstract

To provide a method for filtering a measurement dataset usable for specifying and / or verifying the internal feature of a workpiece.SOLUTION: A method includes: providing a measurement dataset including a plurality of measurement points of an internal feature (S1); providing an auxiliary feature representing an ideal estimate value for the internal feature of a workpiece (S2); mirroring each measurement point of the measurement dataset on a boundary element of the auxiliary feature, thereby generating a first modified dataset (S3); determining a convex hull of first modified measurement points and projecting the first modified measurement points on to the determined convex hull, thereby generating a second modified dataset (S4); and mirroring each second modified measurement point on the boundary element of the auxiliary feature, thereby generating a filtered measurement dataset including a plurality of filtered measurement points (S5).SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for filtering a measurement data set that can be used to specify and / or verify internal features of a workpiece.Furthermore, the present invention relates to a corresponding computer program product. [Background technology]

[0002] In metrology, a key task is to verify that a workpiece is manufactured according to the designer's specifications. The specifications are usually specified by CAD (Computer Aided Design) models or drawings and are intended to describe functional needs. Many specifications, as well as orientation or position tolerances, are specified relative to reference features on the workpiece. These reference features are called "datums".

[0003] As an example, a functional need of a workpiece is to assemble the workpiece with its counterpart. The workpiece may include a plane with a hole and the workpiece counterpart may include a plane with a corresponding bolt. The plane of the counterpart with the bolt must fit into the corresponding hole and plane of the workpiece. Thus, a functional requirement of the specification may be to characterize the plane of the workpiece as orthogonal to the hole. In this case, the hole (which is geometrically cylindrical) serves as the datum.

[0004] To verify such specifications, the datum must be measured by a measuring device and the obtained measurement points must be further processed. According to the current ISO 5459 standard (i.e. ISO 5459:2011), the obtained measurement points shall be used to determine the best fit geometric shape of the datum (e.g. best fit cylinder). However, since any measurement method involves the possibility that some measurement points are outliers and do not reflect the actual surface, Annex A of the ISO 5459:2011 standard requires that filtering of the obtained measurement points be performed before determining or calculating the fitted cylinder based on the measurements. However, currently the ISO 5459:2011 standard does not explicitly specify which filter should be used.

[0005] For external or exterior features (such as bolts on a workpiece), it is known to filter the obtained measurement points by determining a convex hull of the measurement points and obtaining filtered measurement points by projecting the measurement points onto the determined convex hull. This procedure meets the following conditions necessary for filtering measurement data of external features: Outliers to the interior of features must be eliminated since their impact on the datum association process must be reduced. The outermost points must be maintained since these are the relevant points for the calculation of datum associations. The filter must behave similarly to a morphological filter, i.e. it must smooth out deviations.

[0006] Therefore, this known convex hull based filtering method qualifies as a candidate for the default filtering process for exterior features and is proposed in the new GPS ISO 5459 standard as the default for pre-processing datum calculations (associations).

[0007] However, the requirements and / or conditions for filtering measurement points of interior or internal features (such as holes in a workpiece) are different than those described above for exterior features. Specifically, a method for filtering measurement data of an internal feature must meet the following conditions to be a candidate for the default filtering process: Outliers to the outside of the feature must be eliminated since their impact on the datum association process must be reduced. The innermost points must be maintained since these are the relevant points for the calculation of datum associations. The filter must behave similarly to a morphological filter, i.e. it must smooth out deviations.

[0008] Considering these different conditions and / or requirements, the convex hull based filtering methods proposed for the outer features cannot be directly applied to the inner features, and therefore, for the inner features, the use of completely different other types of filters is currently being discussed, in particular morphological filters (e.g., a closing ball with some finite radius). Summary of the Invention [Problem to be solved by the invention]

[0009] It is therefore an object of the present invention to unify the filtering methods for both internal and external features. In particular, it is an object of the present invention to provide a convex hull based filtering method for internal features of a workpiece. This object is achieved by the subject matter of the independent claims. Preferred embodiments are defined in the dependent claims. [Means for solving the problem]

[0010] According to one aspect of the invention, a method is provided for filtering a measurement data set that can be used to specify and / or verify internal features of a workpiece, the method comprising the steps of: providing a measurement data set including a plurality of measurement points of the internal feature; providing assist features representing idealized estimates of internal features of the workpiece; generating a first modified data set comprising a plurality of first modified measurement points by mirroring each measurement point of the measurement data set on a boundary element of the auxiliary feature; determining a convex hull of the first modified measurement points and generating a second modified data set including a plurality of second modified measurement points by projecting the first modified measurement points onto the determined convex hull; Mirroring each second modified measurement point on a boundary element of the assist feature to generate a filtered measurement data set including a plurality of filtered measurement points.

[0011] The measurement data set may be experimentally obtained by a metrology device, in particular, the measurement data set is obtained for use in specifying and / or verifying internal features of an object or workpiece.

[0012] Within the present invention, the term "feature" specifically refers to or means a geometric shape. An "internal feature" of a workpiece (also referred to herein as an "inner feature") encompasses a void, an opening, and / or a hollow space of the workpiece. Specifically, an internal feature is the geometry of a cavity (such as a hole or a hole) of the workpiece. An internal feature or cavity of a workpiece may be a datum or reference feature of the workpiece.

[0013] "Providing a measurement dataset" may include obtaining, determining, and / or capturing measurement data. The measurement dataset of an internal feature includes a plurality of measurement points of the internal feature. In particular, the measurement points include or are candidates for measured points of the internal feature. The measured points may be surface or boundary points of the internal feature of the workpiece, i.e. points of the surface or boundary of the internal feature. Depending on the internal feature, the coordinates may include or be 2D and / or 3D coordinates. That is, the coordinates may relate to a two-dimensional and / or a three-dimensional coordinate system. The coordinate system may be predefined by the measurement device used to obtain the measurement data.

[0014] The measuring device used to acquire the measurement points may be a coordinate measuring device, which may for example be based on contact, optical and / or computed tomography measurements.

[0015] "Providing an auxiliary feature" may include determining and / or calculating the auxiliary feature. An "auxiliary feature" may be a specified (i.e., predetermined or determinable) ideal feature or geometric shape. In particular, an "auxiliary feature" may be an element (that can be mathematically described and / or defined) of an ideal feature or geometric shape. An auxiliary feature is, for example, a circle, a sphere, a cylinder, or a cone. These features have a radial parameter in common. In particular, the auxiliary feature is a least squares geometric element (e.g., least squares circle, least squares sphere, least squares cylinder, least squares cone), also called a "Gaussian element" or "Gaussian feature". In other words, "providing an auxiliary feature" may include providing auxiliary (ideal) geometric data of an internal feature of the workpiece. The auxiliary feature and / or the auxiliary geometric data represent an ideal estimate of the internal feature of the workpiece. An "ideal estimate" in particular means an ideal model having a perfect form.

[0016] Each measurement point of the measurement data set is mirrored on a boundary element of the assist feature. The boundary element of the n-dimensional assist feature can generally be a hyperplane that forms the boundary of the assist feature in the n-dimensional Euclidean space. In particular, the boundary element of the assist feature can be a boundary surface (also called a reflection surface), a boundary line (also called a reflection line), or a boundary point (also called a reflection point) of the assist feature, but the boundary point is not a hyperplane and can be derived from a hyperplane. For example, if the assist feature is a circle, the boundary element can be a circular line of the circle, if the assist feature is a sphere, the boundary element can be a surface (or curved surface area) of the sphere, if the assist feature is a cylinder, the auxiliary element can be the curved surface area of ​​the cylinder, and if the assist feature is a cone, the auxiliary element can be the curved surface area of ​​the cone.

[0017] A first modified data set (i.e. a set of modified measurement points {P'}) is generated by mirroring each measurement point of the measurement data set on a boundary element of the assist feature. Because the first modified data set includes mirrored measurement data, it may also be referred to as a mirrored measurement data set.

[0018] Based on the first modified data set, the convex hull is determined or calculated. In other words, the convex hull is determined based on the mirrored measurement points. Determining the convex hull means building a non-ambiguous and efficient representation of the required convex shape for a finite set of points (here the first modified data set). This is preferably performed by computational means, i.e. by a microprocessor and / or a computer. In particular, any suitable standard convex hull algorithm can be used within the present invention. For example, in the case of 2 and 3 dimensions, any one of the following well-known convex hull algorithms can be used: "incremental brute force algorithm" (also known as "gift wrapping" and in the case of 2 dimensions also known as "Jarvis march"), "Quickhull", "Divide and conquer" and "Chan's algorithm". Furthermore, for at least two dimensions, any one of the following well-known convex hull algorithms can be used: "Monotone chain" (also known as "Andrew's algorithm"), and "ultimate planar convex hull algorithm." These and other algorithms for determining or calculating the convex hull are well known in the art and will not be described further here. It should be noted that the above list is not exhaustive and any other suitable convex hull algorithm can also be used within the present invention.

[0019] The points of the first modified data set are then projected onto the determined convex hull. Specifically, each point of the first modified data set is projected onto the determined convex hull. By projecting the points of the first modified data set onto the determined convex hull, a second modified data set (i.e., a set of second modified measurement points {P''} projected onto the convex hull) is generated. These points {P''} may be referred to as projected points, and the second modified data set may be referred to as a projected data set.

[0020] Each point of the second modified data set (i.e. each second modified measurement point) is then mirrored onto a boundary element of the auxiliary feature, thereby generating a filtered measurement data set (also referred to as the third modified data set). The filtered measurement data set is a set of filtered measurement points {P'''}, also referred to as the third modified measurement points.

[0021] Specifically, the first mirroring step transforms measured data of the internal features of the workpiece to represent corresponding virtual external features of the workpiece. Specifically, the first corrected data set represents the corresponding virtual external features of the workpiece. The second mirroring step reverses this transformation (i.e., the first mirroring). In other words, the second mirroring step transforms or mirrors back the data representing the virtual external features into data representing the actual internal features of the workpiece.

[0022] In particular, the mirroring process (i.e. both the first and second mirroring steps) involves a characteristic measurement point transformation such that the outermost points become the innermost points and vice versa. For this purpose, an auxiliary feature is used as a "mirror". For example, if the auxiliary feature is a cylinder (e.g. a Gaussian cylinder), the points inside and / or outside this cylinder are moved to the outside / inside of the cylinder along a direction orthogonal to the cylinder's surface and / or axis. In particular, the moved points are moved such that they are at the same distance to the cylinder's surface as they were before the movement.

[0023] The method of the present invention allows the determination of the convex hull to be used not only to filter measurement data of external features of a workpiece, but also to filter measurement data of internal features of the workpiece. The present invention thus provides a convex hull based filtering method that can be used for one or more internal features of a workpiece, thus providing the advantage of unifying filtering methods for both internal and external features of a workpiece.

[0024] In a preferred embodiment, providing the measurement data set comprises capturing the measurement data (and / or the measurement points comprised in the measurement data) by means of a coordinate measuring device or coordinate measuring machine. The coordinate measuring device may be based on a contact measurement, e.g. a touch-trigger measurement. Additionally or alternatively, the coordinate measuring device may be based on an optical measurement. Additionally or alternatively, the coordinate measuring device may be based on a computed tomography measurement. In particular, the coordinate measuring device may be a contact coordinate measuring machine, an optical measuring device, a computed tomography device, or a combination thereof.

[0025] In another preferred embodiment, the assist feature is a predetermined ideal feature that is defined based on design data of the internal feature and / or design data of the workpiece. In other words, the assist feature may represent an ideal internal feature of the workpiece. For example, the design data may include or be CAD data of the internal feature of the workpiece.

[0026] In another preferred embodiment, the auxiliary feature is a Gaussian feature determined on the basis of a measurement data set. A "Gaussian feature" (also called a geometric compensation element according to Gauss) is a geometric element determined or calculated by the well-known least squares method on the basis of measurement data. According to this method, the sum of the squared distances of all measurement points to an ideal geometric shape (e.g. a circle, a sphere, a cylinder or a cone) is minimized. The ideal geometric shape may be a specified ideal geometric shape. That is, the ideal geometric shape may be predetermined or determinable.

[0027] In yet another preferred embodiment, the auxiliary feature is a circle, a sphere, a cylinder, or a cone, although it will be appreciated that the auxiliary feature may generally be any geometric element (particularly 2D or 3D) that can be mathematically described or defined.

[0028] In yet another preferred embodiment, the step of mirroring each measurement point of the measurement data on a boundary element of the auxiliary feature (first mirroring step) comprises: - defining for each measurement point a corresponding first reflection point on a boundary element of the assist feature; determining, for each measurement point, a corresponding first distance between the measurement point and a corresponding first reflection point; and generating a first corrected data set by determining, for each measurement point, a mirrored measurement point, the mirrored measurement point being obtained by moving the measurement point across the corresponding first reflected point by twice the determined corresponding first distance.

[0029] A corresponding first reflection point is defined in particular for the first mirroring step, the mirrored measurement point corresponding to the first modified measurement point.

[0030] For each measurement point, A corresponding first reflection point is positioned on a boundary element of the auxiliary feature; a virtual line between the measurement point and the corresponding first reflection point is perpendicular to a tangent or tangent plane of the assist feature at the corresponding first reflection point; A corresponding first reflection point can be defined such that:

[0031] Whether the imaginary line between the measurement point and the corresponding first reflection point is orthogonal to the tangent or to the tangent plane depends on the dimension of the assist feature: if the assist feature is two-dimensional, the imaginary line between the measurement point and the corresponding first reflection point is orthogonal to the tangent, and if the assist feature is three-dimensional, the imaginary line between the measurement point and the corresponding first reflection point is orthogonal to the tangent.

[0032] In yet another preferred embodiment, the step of projecting the points of the first modified data set onto the determined convex hull is performed in a direction orthogonal to the convex hull. In other words, the projection is performed such that each point of the first modified data set (i.e. each first modified measurement point) moves along a line orthogonal to the convex hull until the convex hull is reached. Alternatively or additionally, the projecting of the points of the first modified data set onto the determined convex hull is performed in a radial direction from the center or axis of the auxiliary feature. In other words, the projection is performed such that each point of the first modified data set (i.e. each first modified measurement point) moves from the center (particularly if the auxiliary feature is a circle or sphere) or axis (particularly if the auxiliary feature is a cylinder or a cone) of the auxiliary feature in a radial direction of the auxiliary feature until the convex hull is reached.

[0033] In yet another preferred embodiment, the step of mirroring each second corrected measurement point of the second corrected data set on a boundary element of the auxiliary feature comprises: defining for each second modified measurement point a corresponding second reflection point on a boundary element of the assist feature; determining, for each second corrected measurement point, a corresponding second distance between the second corrected measurement point and a corresponding second reflection point; and generating a filtered measurement data set by determining, for each second modified measurement point, a re-mirrored point, the re-mirrored point being obtained by moving the second modified measurement point across the corresponding second reflected point by twice the determined corresponding second distance.

[0034] A corresponding second reflection point is defined specifically for the second mirroring step. The corresponding second reflection point may correspond to the corresponding first reflection point. The re-mirrored point corresponds to the filtered measurement point.

[0035] In yet another preferred embodiment, for each second corrected measurement point: A corresponding second reflection point is positioned on a boundary element of the auxiliary feature; an imaginary line between the second modified measurement point and the corresponding second reflection point is perpendicular to a tangent or tangent plane of the assist feature at the corresponding second reflection point; A corresponding second reflection point is defined as follows:

[0036] Whether the imaginary line between the measurement point and the corresponding second reflection point is orthogonal to the tangent or to the tangent plane depends on the dimension of the auxiliary feature: if the auxiliary feature is two-dimensional, the imaginary line between the second modified measurement point and the corresponding second reflection point is orthogonal to the tangent, and if the auxiliary feature is three-dimensional, the imaginary line between the second modified measurement point and the corresponding second reflection point is orthogonal to the tangent.

[0037] In yet another preferred embodiment, the method further comprises specifying and / or verifying internal features of the workpiece based on the filtered measurement data set and / or the filtered measurement points.

[0038] In yet another preferred embodiment, specifying and / or verifying the internal feature of the workpiece comprises determining a least squares geometric element (Gaussian element) based on the filtered measurement data set. The determined least squares geometric element may be a measured specification of the internal feature. In particular, the determined least squares geometric element may be compared with a predefined ideal internal feature and / or design data of the internal feature and / or design data of the workpiece. For example, the design data may include or be CAD data of the internal feature and / or the workpiece. The above-mentioned comparison step allows for performing a verification of the internal feature. In particular, the above-mentioned comparison step allows for verifying whether the investigated or measured internal feature of the workpiece is within a predetermined tolerance.

[0039] The method according to the present invention relates in particular to a computer-implemented method. Thus, a computer with a processor, memory, and display can be provided to perform the method according to the present invention. More specifically, a computer or computer system can be provided that includes a processor that can execute or execute machine-readable instructions that perform some or all of the methods, functions, and other processes described herein. Commands and data from the processor are communicated over a communication bus. The computer or computer system can also include a main memory, such as a random access memory (RAM), where machine-readable instructions and data for the processor can reside during execution, and a secondary data storage device, which can be non-volatile and stores the machine-readable instructions and data. The memory and data storage devices are examples of computer-readable media.

[0040] According to another aspect of the invention, there is provided a computer program product comprising computer readable instructions which, when loaded into a computer memory and executed by a computer, cause the computer to carry out a method according to the invention. In particular, the computer program product may relate to a program stored on a computer readable medium. Alternatively, the computer program product may relate to a computer readable medium storing a program comprising computer readable instructions or corresponding program code.

[0041] In the above-mentioned independent aspects, and in particular in preferred embodiments in this respect, the above-mentioned or below-mentioned explanations regarding the embodiments of the first aspect also apply. In particular, in the above-mentioned or below-mentioned explanations regarding the embodiments of each of the other aspects of the invention, and in particular in preferred embodiments in this respect, also apply.

[0042] Hereinafter, individual embodiments for solving the problem will be described as an example with reference to the drawings. In this case, each described embodiment has some features that are not absolutely necessary to implement the claimed subject matter but provide desired properties in a particular application. In this regard, it is intended that an embodiment that does not have all the features of the embodiments described below is also considered to be disclosed as being under the described technical teaching. Furthermore, in order to avoid unnecessary repetition, certain features will be mentioned only in relation to each of the embodiments described below. Therefore, it is noted that each embodiment is intended to be considered jointly as well as individually. Based on this joint consideration, it will be recognized by those skilled in the art that each embodiment can be modified by including each feature or features of other embodiments. It is noted that a systematic combination of each feature or features described with respect to each embodiment and other embodiments is desirable and appropriate, and is intended to be taken into consideration and to be considered as encompassed by the description. [Brief description of the drawings]

[0043] The foregoing and other objects, features, and advantages of the present invention will become more apparent upon reading the following description of the preferred embodiments and the accompanying drawings. Other features and advantages of the subject matter described herein will become apparent from the description and drawings, and from the claims. Even if the embodiments are described separately, it will be understood that their independent features and functionality can be combined without violating additional embodiments. The present disclosure is described by way of example only and is not limited by the accompanying drawings.

[0044] Preferred embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, in which:

[0045] [Figure 1a] 1 shows a schematic diagram of an exemplary workpiece having an internal feature and a workpiece counterpart having an external feature that corresponds to the internal feature of the workpiece. [Figure 1b] FIG. 1 shows a schematic diagram illustrating filtering applied to a nominally flat surface of an outer feature according to the ISO 5459:2011 standard. [Figure 1c] FIG. 1 shows a schematic diagram illustrating filtering applied to a nominally cylindrical surface of an internal feature according to the ISO 5459:2011 standard. [Diagram 2] 1 shows a flow diagram of a method according to a preferred embodiment of the present invention; [Figure 3a] 1 shows a schematic diagram illustrating a method according to the invention by way of a first example. [Figure 3b] 2 shows a schematic diagram illustrating the method according to the invention by means of a second example. [Figure 4] FIG. 3 shows an enlarged view of the schematic diagram of FIG. 3a to further explain the details of the method according to the preferred embodiment. [Diagram 5] 4 shows measured points and filtered points of an internal feature of a workpiece according to a first example. [Figure 6] 13 shows measured points and filtered points of an internal feature of a workpiece according to a second example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0046] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following detailed description relates to exemplary embodiments of the present invention. Other embodiments of the present invention are possible within the scope of the present invention as defined by the appended claims. The same reference numbers are used throughout the drawings to refer to the same or similar elements.

[0047] 1a is a schematic diagram of an exemplary workpiece 100 having an internal or inner feature 10 (a hole in the illustrated example) and a counterpart 200 of the workpiece 100. The counterpart 200 has an external or outer feature 20 (a bolt in the illustrated example) that corresponds to the internal feature 10 of the workpiece 100. The functional need of the workpiece 100 is to assemble the workpiece 100 with its counterpart 200. The flat surface 2 of the counterpart 200 with the bolt 20 must fit into the corresponding hole 10 and flat surface 1 of the workpiece 100.

[0048] A functional requirement of the specification is to characterize a plane 1 of a workpiece 100 as orthogonal to a hole 20 of a counterpart 200. In this case, the hole 10, which is geometrically cylindrical, serves as the datum.

[0049] To verify such a specification, the first step may be to determine a datum. Thus, the workpiece 100 may be placed on a coordinate measuring machine (not shown in FIG. 1 ) to capture measurement points from inside the hole 10 (e.g. by contacting it from the inside with a probe). These measurement points shall geometrically represent the hole 10 as a cylinder. According to the current ISO 5459:2011 standard, the measurement points are used to determine the best fit cylinder of perfect form with the largest diameter that fits into the hole 10. This means that all measurement points must be outside the cylinder. The calculated cylinder (fitted cylinder) serves as the datum.

[0050] To capture the measurement points, various methods can be used using a coordinate measuring machine (CMM), such as scanning, touch trigger, etc. Alternatively or additionally, optical measuring machines and / or machines based on computed tomography (X-ray) can also be used. Any method carries the possibility that some measurement points are outliers and do not reflect the real surface. Therefore, Annex A of the ISO5459:2011 standard states that a filtering step must be performed before calculating the fitted cylinder. However, it does not explicitly specify which filter should be used.

[0051] Figure 1b is a schematic diagram of what filtering according to the ISO 5459:2011 standard for an external feature 20 should typically look like. In the illustrated example, which corresponds to Figure A.3 of Annex A of the ISO 5459:2011 standard, the external feature has a nominally flat surface. The continuous black line 3 in Figure 1b represents the actual integral feature, and the dashed line 4 represents the filtered feature.

[0052] Figure 1c is a schematic diagram of what filtering according to the ISO 5459:2011 standard for an internal feature 10 should typically look like. In the illustrated example, which corresponds to Figure A.3 of Annex A of the ISO 5459:2011 standard, the internal feature 10 has a nominally cylindrical surface. As in Figure 1b, the continuous black line 3 in Figure 1c represents the actual shell feature, and the dashed line 4 represents the filtered feature.

[0053] As already mentioned, for external features, such as the bolt 20 in FIG. 1a, it is known to filter the measurement points by determining the convex hull of the measurement points. More specifically, the overall process for filtering external features can be described as follows: for the measurement points {P}, find the convex hull and project the points onto this convex hull to obtain points {P*} representing the filtered points. However, for internal features, such as the hole 10 in the workpiece 100 shown in FIG. 1a, no convex hull-based filter with the same conditions is known so far. The present invention fills this gap and provides a filtering method that is applicable to internal features of a workpiece and is a convex hull-based filtering method. Thus, the present invention allows for a unified filtering method for internal and external features.

[0054] FIG. 2 shows a flow diagram of a method for filtering a measurement data set that can be used to specify and / or verify an internal feature 10 of a workpiece 100 according to a preferred embodiment of the present invention. In step S1, a measurement data set is provided that includes a plurality of measurement points of the internal feature. In step S2, an auxiliary feature is provided that represents an idealized estimate of the internal feature of the workpiece. In step S3, a first modified data set is generated that includes a plurality of first modified measurement points by mirroring each measurement point of the measurement data set on a boundary element of the auxiliary feature. In step S4, a convex hull of the first modified measurement points is determined, and a second modified data set is generated that includes a plurality of second modified measurement points by projecting the first modified measurement points on the determined convex hull. And in step S5, a filtered measurement data set is generated that includes a plurality of filtered measurement points by mirroring each second modified measurement point on a boundary element of the auxiliary feature.

[0055] Specifically, a measurement data set is provided and a filtered measurement data set is generated as output. The filtered measurement data set is the result of a smoothing process. In this smoothing process, an auxiliary feature (i.e. an auxiliary geometric element), for example a least squares cylinder, is provided (specifically calculated). The distances of the measurement points to this auxiliary feature are the values ​​to be smoothed. After smoothing these distance values, the original measurement points are moved from their original distance (to the auxiliary geometric element) to the smoothed distance values. This results in a set of filtered points. In general, all points of the measurement data have an influence on the calculation of the fitted feature (e.g. a fitted cylinder). Specifically, in a least squares based association, measurement points with a large distance to the auxiliary feature have a large influence on the result. Filtering is performed to avoid irrelevant measurement points (so-called "outliers") influencing the result. For example, such filtering has a similar effect to mechanically removing peaks on a bolt by shifting its gauge (inner diameter). Thus, the filtering method provided by the present invention can use cheap, simple and fast measurements combined with algorithmic calculations to replace mechanical testing (which is expensive, cumbersome and time consuming).

[0056] Specifically, to reduce the measured points of an inner feature of a workpiece to an outer feature, the measured points are mirrored on the reference feature, a convex hull filter is applied to the outer feature, and the result is mirrored back. This preserves the properties of the convex hull on the outer feature on the inner feature (e.g., preserves the high points). It is well known that the convex hull cannot be applied directly to the inner feature. However, the present invention allows the convex hull to be applied to the inner feature as well. Thus, the inner and outer features can be filtered with the same filter type.

[0057] In Figures 3a and 3b, the principle of the invention is illustrated in two different examples. More specifically, Figure 3a illustrates the inner feature filtering process for a measurement point having two peaks inside the inner feature 10 of the workpiece to be measured. And Figure 3b illustrates the inner feature filtering process for a measurement point having two peaks outside the inner feature 10 to be measured. In both examples, the inner feature 10 has a circular shape and thus the assist feature 30 is a circle. In Figures 3a and 3b, the continuous circular line represents a part of the assist feature 30, in particular a part of the boundary element of the assist feature 30. It should therefore be noted that in Figures 3a and 3b, only a part of the inner feature 10 and the assist feature 30 is illustrated or indicated. The points with reference P are measurement points obtained from a coordinate measuring device, the points with reference P' are mirrored measurement points, the points with reference P'' are points projected onto the convex hull H, and the points with reference P'' are mirrored points of the convex hull H. Thus, the measurement points P form a measurement data set, the mirrored measurement points P' form a first modified data set, the projected points P'' form a second modified data set and the points P'''' (also called filtered measurement points) form a third modified data set (also called filtered data set). In Figures 3a, 3b and 4, the measurement points P are represented by black circles (disks) and their corresponding continuous contours are represented by dashed lines, the mirrored measurement points P' are represented by black squares (boxes) and their corresponding continuous contours are represented by dashed and dotted lines, the projected points P'' are represented by white squares and their corresponding continuous contours are represented by dashed and dotted lines and the points P''' are represented by white circles and their corresponding continuous contours are represented by dotted lines.

[0058] As already mentioned with reference to FIG. 2, a measurement data set is provided, which comprises a number of measurement points P of the internal feature 10, in particular measured by means of a coordinate measuring device. In particular, the measurement points P are coordinates of the internal feature 10. An auxiliary feature 30 is then provided, which represents an idealized estimate of the internal feature 10 of the workpiece 100. Each measurement point P of the measurement data set is then mirrored on a boundary element of the auxiliary feature 30, which is a circle in the example shown in FIGS. 3a and 3b. This first mirroring step generates a first modified data set, which comprises a number of first modified measurement points P'. On the basis of these first modified measurement points P', a convex hull H is determined or calculated by means of a convex hull algorithm, and the first modified measurement points P' are projected onto this determined convex hull, thereby generating a second modified data set, which comprises a number of second modified measurement points P''. To this end, each second modified measurement point P'' is located on the determined convex hull H. Each second modified measurement point P'' is then mirrored on a boundary element of the auxiliary feature 30. This second mirroring step generates a filtered measurement data set comprising a plurality of filtered measurement points P'''. As can be seen from Figures 3a and 3b, the filtered measurement data set with filtered measurement points P''' is a smoothed data set based on the measurement data set with measurement points P.

[0059] Both examples shown in Figures 3a and 3b illustrate the behaviour of an inner feature filter that satisfies the following properties required of a filter for an inner feature: Outliers to the outside of the feature are eliminated because their impact on the datum association process must be reduced. The innermost points are kept as they are the relevant points for the calculation of datum associations. The filter behaves similarly to a morphological filter, i.e. it smooths out deviations, which is necessary to be a candidate for the default filtering process.

[0060] It should be noted that the diagrams in Figures 3a and 3b serve only as an example and do not show the exact locations of the points that are actually calculated.

[0061] Figure 4 shows an expanded view of the diagram of Figure 3a to further explain the details of the method according to the preferred embodiment. Specifically, according to the preferred embodiment, the following steps define a convex hull based method for internal features: All points P of the measurement data set {P} are mirrored on the assist feature to obtain a first corrected data set {P'}. For the first rectified data set {P'}, the second rectified data set {P''} is obtained by finding the convex hull and projecting the points P' of the first rectified data set {P'} onto this convex hull. This projection can be performed orthogonal to the convex hull or radially from the center / axis of the assist feature. All points P'' of the second corrected data set {P''} are mirrored in the auxiliary feature to obtain a third corrected data set {P'''}. The points P'' of the third corrected data set {P''} are filtered points.

[0062] The assist feature 30 may be a predetermined ideal feature that is defined based on design data of the internal feature 10 and / or design data of the workpiece 100. Alternatively, the assist feature 30 may be a Gaussian feature that is determined based on the measurement points P.

[0063] For each measurement point P, a corresponding first reflection point R1 on a boundary element of the auxiliary feature 30 is defined. The corresponding first reflection point R1 is located on the boundary element of the auxiliary feature 30. Furthermore, the corresponding first reflection point R1 is defined such that an imaginary line between the measurement point P and the corresponding first reflection point R1 is perpendicular to a tangent or tangent plane T of the auxiliary feature 30 at the corresponding first reflection point R1. Also, for each measurement point P, a corresponding first distance d1 between the measurement point P and the corresponding first reflection point R1 is defined.

[0064] A first corrected data set is generated by determining a mirrored measurement point for each measurement point P. To obtain the mirrored measurement point, the measurement point P is moved across the corresponding first reflection point R1 by twice the corresponding determined first distance d1.

[0065] For each second modified measurement point P'', a corresponding second reflection point R2 on a boundary element of the auxiliary feature 30 is defined. Similar to the first reflection point R1, the corresponding second reflection point R2 is positioned on a boundary element of the auxiliary feature 30. Furthermore, the corresponding second reflection point R2 is defined such that an imaginary line between the second modified measurement point P'' and the corresponding second reflection point R2 is perpendicular to a tangent or tangent plane T of the auxiliary feature 30 at the corresponding second reflection point R2. Also, for each second modified measurement point P'', a corresponding second distance d2 between the second modified measurement point P'' and the corresponding second reflection point R2 is determined.

[0066] A filtered measurement data set is then generated by determining a re-mirrored point for each second modified measurement point P″. To obtain the re-mirrored point, the second modified measurement point P″ is moved across the corresponding second reflection point R2 by twice the determined corresponding second distance d2.

[0067] In FIG. 4, P / P''' means that the measurement point P and the filtered measurement point P''' overlap. Correspondingly, P' / P'' means that the first modified measurement point P' and the second modified measurement point P'' overlap. Furthermore, R1 / R2 means that the first reflection point R1 and the second reflection point R2 overlap. In other words, R1 / R2 means that the second reflection point R2 corresponds to the first reflection point R1. Also, d1 / d2 means that the first distance d1 and the second distance d2 are equal. In other words, d1 / d2 means that the second distance d2 corresponds to the first distance d1. It should be noted that such a configuration, i.e. the overlapping configuration of points P and P'' and points P' and P'', applies only to points based on the innermost measurement points.

[0068] 5 and 6 show exemplary results from the implementation of the convex hull based filtering method for internal features according to the present invention. More specifically, FIG. 5 shows the measurement points P (represented by black dots) and filtered points P'''' (represented by white circles) of an internal feature of a workpiece according to a first example. The outer shape of the internal feature corresponds to the example disclosed in Figure A.3 of Annex A of the ISO5459:2011 standard (see FIG. 1c). Also, FIG. 6 shows the measurement points P (represented by black dots) and filtered points P'''' (represented by white circles) of an internal feature of a workpiece according to a second example. The outer shape of the internal feature is a circle with random deviations. Specifically, based on the measurement points P of a circular feature, the filter mirrors all measurement points P radially on a reference circle, applies a convex hull to these points, projects the points onto the convex hull, and finally mirrors the points back. The results obtained using the method according to the invention comply with figure A.3 of Annex A of the ISO5459:2011 standard (see figure 1c), which shows what a filtered profile looks like. [Explanation of symbols]

[0069] 1 surface 2 surface 3. Real shell shape 4 Filtered Elements 10 Internal features (internal features) 20 External feature (external feature) 30 Auxiliary features 100 work pieces 200 Workpiece Counterpart d1 First distance d2 Second distance H convex hull P is a point in the measurement data set (measurement point) P' Point of the first correction data set (first correction measurement point) P'' Point of the second corrected measurement data set (second corrected measurement point) P''' Points of the filtered data set (filtered measurement points) R1 First reflection point R2 Second reflection point T tangent / tangent plane

Claims

1. 1. A method for filtering a measurement dataset that can be used to specify and / or verify an internal feature (10) of a workpiece (100), comprising: providing a measurement data set comprising a plurality of measurement points (P) of said internal feature (10); providing an auxiliary feature (30) representing an idealized estimate of the internal feature (10) of the workpiece (100); generating a first modified data set comprising a plurality of first modified measurement points (P') by mirroring each measurement point (P) of the measurement data set on a boundary element of the auxiliary feature; determining a convex hull (H) of the first modified measurement points (P') and generating a second modified data set comprising a plurality of second modified measurement points (P'') by projecting the first modified measurement points (P') onto the determined convex hull (H); generating a filtered measurement data set comprising a plurality of filtered measurement points (P''') by mirroring each second modified measurement point (P'') on the boundary element of the auxiliary feature (30); The method includes:

2. The method of claim 1 , wherein providing the measurement data set comprises capturing the measurement data with a coordinate measuring device.

3. the auxiliary feature (30) is a predetermined ideal feature defined on the basis of design data of the internal feature (10) and / or design data of the workpiece (100), or The method of claim 1 or 2, wherein the auxiliary feature (30) is a Gaussian feature determined based on the measurement data set.

4. The method according to any one of claims 1 to 3, wherein the auxiliary feature (30) is a circle, a sphere, a cylinder, or a cone.

5. Mirroring each measurement point (P) of the measurement data on a boundary element of the auxiliary feature (30) comprises: - defining for each measurement point (P) a corresponding first reflection point (R1) on said boundary element of said auxiliary feature (30); determining, for each measurement point (P), a corresponding first distance (d1) between said measurement point (P) and said corresponding first reflection point (R1); 5. The method according to claim 1, further comprising: generating the first corrected data set by determining, for each measurement point (P), a mirrored measurement point, the mirrored measurement point being obtained by moving the measurement point (P) across the corresponding first reflection point (R1) by twice the determined corresponding first distance (d1).

6. For each measurement point (P), the corresponding first reflection point is said corresponding first reflection point (R1) being located on said boundary element of said auxiliary feature (30); an imaginary line between said measurement point (P) and said corresponding first reflection point (R1) is perpendicular to a tangent or tangent plane (T) of said auxiliary feature (30) at said corresponding first reflection point (R1); The method of claim 5 , wherein the method is defined as follows:

7. Projecting the first modified measurement points (P') of the first modified data set onto the determined convex hull (H) comprises: Orthogonal to the convex hull (H), and / or Radially from the centre or axis of said auxiliary feature (30), 7. The method according to claim 1 , wherein the method is performed in a step S100.

8. Mirroring each second modified measurement point (P'') of the second modified data set on the boundary element of the auxiliary feature (30) comprises: - defining for each second modified measurement point (P'') a corresponding second reflection point (R2) on said boundary element of said auxiliary feature (30); determining, for each second modified measurement point (P''), a corresponding second distance (d2) between said second modified measurement point (P'') and said corresponding second reflection point (R2); 8. The method according to claim 1, further comprising: generating the filtered measurement data set by determining for each second modified measurement point (P'') a re-mirrored point, the re-mirrored point being obtained by moving the second modified measurement point (P'') across the corresponding second reflection point (R2) by twice the determined corresponding second distance (d2).

9. The method according to claim 8, wherein the corresponding second reflection point (R2) corresponds to the corresponding first reflection point (R1) according to claim 6.

10. For a second corrected measurement point (P'') of the second correction data set, the corresponding second reflection point (R2) is said corresponding second reflection point (R2) being located on said boundary element of said auxiliary feature (30); an imaginary line between said second modified measurement point (P'') and said corresponding second reflection point (R2) is perpendicular to a tangent or tangent plane of said auxiliary feature (30) at said corresponding second reflection point (R2); The method of claim 8 , wherein:

11. The method of any one of claims 1 to 10, further comprising specifying and / or verifying the internal features (10) of the workpiece (100) based on the filtered measurement data set.

12. The method of claim 11 , wherein specifying and / or verifying the internal features (10) of the workpiece (100) comprises determining a least-squares geometric element based on the filtered measurement data set.

13. 13. A computer program product comprising computer readable instructions which, when loaded into a memory of a computer and executed by the computer, cause the computer to perform the method of any one of claims 1 to 12.

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