Measuring the distance between a real object and a virtual object

Abstract

Method for measuring a virtual distance between a real measuring point (P) of a real object (2) and a virtual measuring point (P') of a virtual object (3), which corresponds to a further object not physically present, which is to be installed, for example, in an environment of the real object (2), by means of a measuring system (1) which has a measuring arm (5), wherein - a spatial position of the virtual measuring point (P') is specified with respect to three or more reference points of the real object (2); - for each of the three or more reference points, respective three-dimensional coordinates are specified in a measurement coordinate system of the measurement system (1); - three-dimensional coordinates of the real measuring point (P) in the measuring coordinate system are determined using the measuring arm (5); - the virtual distance is calculated by means of a control (10) of the measuring system (1) based on the spatial location of the virtual measuring point (P'), the coordinates of the real measuring point (P) and the respective coordinates of the three or more reference points.

Description

[0001] The present invention relates to a method for measuring a virtual distance between a real measuring point of a real object and a virtual measuring point of a virtual object. The invention further relates to a corresponding measuring system.

[0002] When developing complex products, such as motor vehicles, complex workspaces with a high component density in a limited space must be geometrically validated. In motor vehicles, such complex workspaces occur, for example, in the engine compartment and the underbody, particularly in the front end. Geometric validation includes, for example, installation and removal studies, identifying bottlenecks and potential component contact points, and comparing the design specifications with the actual installation positions. Deviations between the specifications and actual positions must be identified as precisely as possible. Similar investigations may also be necessary during operation, for example, during testing, endurance testing, or final end-customer use.

[0003] Especially during the development phase, it's conceivable that individual components, while fully designed and available as precise CAD data (Computer-Aided Design), don't yet exist as physical, real parts. This can be due to the fact that the manufacturing or prototype production of the component is not yet complete. Alternatively, there may be several variants or design studies of a component, of which only one is intended for actual production. In such situations, it would be desirable to be able to perform geometric validation, taking into account the not-yet-available physical component in relation to existing parts, in order to avoid delays in the product development process and the associated costs of multiple prototype productions.

[0004] Document EP 2 916 099 A1 describes a device with an articulated arm for measuring the geometric features of an object. A graphic display unit is attached to a movable end of the articulated arm, so that the information provided by the display unit is visible to an operator during the measurement. The display unit provides a virtual view that graphically represents a portion of the object to be measured, thus resembling the operator's natural view of the object. Within the virtual view, a specific point or area of ​​the object to be measured can be graphically marked. By marking specific points, the virtual view helps the operator determine where measurements should be taken. In other words, the system provides a work instruction.

[0005] Document EP 2 322 897 A1 describes a method for measuring workpieces to check their dimensional accuracy, using a robot arm with a stylus. The measurement is carried out by comparing measured coordinates with data on the workpiece topography, also known as virtual reality, which is obtained, for example, from CAD data.

[0006] Against this background, it is an object of the present invention to provide an improved concept for a measuring system with a measuring arm that makes it possible to measure a virtual distance between a virtual object and a real object.

[0007] This problem is solved by the respective subject matter of the independent claims. Advantageous further developments and preferred embodiments are the subject matter of the dependent claims.

[0008] The improved concept is based on the idea of ​​determining the coordinates of a real measuring point on a real object using the measuring arm and calculating the distance of the real measuring point from a virtual measuring point based on the coordinates of the real measuring point determined in this way and a spatial position of the virtual measuring point relative to several reference points.

[0009] According to the improved concept, a method for measuring a virtual distance between a real measuring point of a real object and a virtual measuring point of a virtual object is described, using a measuring system that includes a measuring arm. The spatial position of the virtual measuring point, particularly a three-dimensional position, is defined with respect to three or more reference points of the real object. For each of the three or more reference points, corresponding three-dimensional coordinates are defined in a measurement coordinate system of the measuring system. Using the measuring system, specifically the measuring arm and a control system, the three-dimensional coordinates of the real measuring point are determined within the measurement coordinate system, particularly by direct or indirect measurement.The virtual distance between the real and virtual measuring point is calculated by controlling the measuring system based on the spatial location of the virtual measuring point, the coordinates of the real measuring point in the measuring coordinate system, and the respective coordinates of the three or more reference points in the measuring coordinate system.

[0010] In other words, the virtual distance can be understood as the distance of the real measuring point from a point corresponding to the virtual measuring point, if another real object corresponding to the virtual object were arranged according to the given spatial position of the virtual measuring point with respect to the reference points in the vicinity of the real object.

[0011] The virtual object can, for example, be given in the form of a description through a digitally stored data set, such as a CAD data set.

[0012] The measuring arm includes, in particular, a manually controllable or movable measuring head, for example, an optical measuring head, but preferably a tactile measuring head, which is connected to a stationary end via an articulated arm. The articulated arm comprises one or more rigid arm segments which are connected to the stationary end on the one hand and the measuring head on the other by means of appropriate joint connections. A sensor system of the measuring arm can determine the current position, in particular the rotational position or angular position, of the movable parts of the articulated arm, so that the position and orientation, i.e., the pose, of the measuring head relative to the stationary end as a reference point can always be tracked. For this purpose, rotary encoders, also referred to as rotary encoders, can be provided in appropriate joint connections.

[0013] The measuring head contains a measuring element; in the case of an optical measuring head, this is an active optical sensor, while in the case of a tactile measuring head, it is, for example, a tactile probe. The movable end of the measuring arm, with the measuring head and, in particular, the measuring element, can then be manually guided to a point to be measured. The measurement then corresponds to the determination of the corresponding three-dimensional coordinates of the point to be measured in a measurement coordinate system, which is defined by the fixed position of the stationary end. Specifically, the measurement coordinate system is therefore a coordinate system rigidly connected to the stationary end of the measuring arm. By means of registration, i.e., measurement of the reference points, the measurement coordinate system of the measuring arm is synchronized with the model coordinate system. In other words, the stationary end remains at rest within the measurement coordinate system.This allows the coordinates of the points to be measured to be determined with high accuracy. Tolerances can, for example, range from 10 µm to 100 µm.

[0014] The three or more reference points correspond in particular to points on the real object, especially on a surface of the real object, or in the vicinity of the reference object, in which case the position of the corresponding reference point relative to the surface of the real object is known. By specifying the corresponding three-dimensional coordinates for the three or more reference points in the measurement coordinate system, the corresponding spatial position and thus the three-dimensional coordinates in the measurement coordinate system can be specified for every point in real space whose spatial position relative to the three or more reference points is known. Accordingly, by specifying the spatial position of the virtual measurement point relative to the three or more reference points, the position of the virtual measurement point in the measurement coordinate system can also be derived.For example, in addition to the location of the virtual measurement point of the virtual object, the spatial location of every other point on the virtual object with respect to the three or more reference points can also be specified. In this case, the spatial location of each point of the virtual object in the measurement coordinate system can therefore be determined.

[0015] The position of a point of the virtual object, in particular the position of the virtual measurement point, can be given, for example, by coordinates in a reference coordinate system defined by three or more reference points. It should be noted that by specifying three-dimensional coordinates from exactly three reference points, a three-dimensional coordinate system is uniquely defined. Any additional reference points are therefore redundant. If P1, P2, and P3 are each designated as the three-dimensional position vectors of the three reference points, for example, in the measurement coordinate system, then the three vectors (P1-P2), (P1-P3), and (P1-P2) × (P1-P3) span the corresponding reference coordinate system. The position of the virtual measurement point, or of the other points of the virtual object, can then be given, for example, by three-dimensional coordinates in the reference coordinate system.

[0016] The fact that the coordinates of the real measuring point are determined in the measuring coordinate system by means of the measuring arm can be understood, in particular, as meaning that the coordinates of the real measuring point are measured directly or indirectly using the measuring arm. A direct measurement can be understood as a measurement in which the measuring element is moved directly to the real measuring point. In an indirect measurement, the measuring element can, for example, be moved to one or more other points of the object, especially the real object, and their coordinates measured. Since the position of the real measuring point with respect to the coordinates thus measured is predetermined or known, the coordinates of the real measuring point can be deduced.The measuring element can be used, for example, to trace the contour of a real object, such as the inner surface of a bore or similar, provided the actual measuring point is known to be located on a rotational axis of the bore or similar. By additionally measuring the surface in which the bore is located, the actual measuring point can be uniquely defined by its distance from this surface.

[0017] The control system can include one or more control units and / or computing units, which may, for example, be arranged spatially separated from one another. For instance, a control unit for acquiring the corresponding sensor signals from the sensor system can be located on the measuring arm or within a housing of the measuring arm. The calculations required to determine the virtual distance can be performed, for example, by a computer system, which can also be part of the control system and can also be referred to as a computing unit, such as a powerful CAD computer system.

[0018] The coordinates of the three or more reference points are also determined using the measuring arm. These can be measured directly using the measuring arm or indirectly, as described above.

[0019] The improved concept allows the exact position of the virtual measuring point relative to the real measuring point to be determined, thus enabling the virtual distance to be measured with high accuracy.

[0020] In other words, the virtual distance can be determined, for example, by moving the measuring arm to the real measuring point and then to the virtual measuring point in a CAD geometry. The measurement is based on the spatial positioning of the measuring system in model coordinates or vehicle coordinates, which is achieved by surveying or registering at least three reference points on the physical model. This synchronizes the model coordinate system with the measurement coordinate system.

[0021] According to at least one embodiment of the improved concept, for a first reference point of the three or more reference points, the respective three-dimensional coordinates of a plurality of reference measurement points of the real object in the measurement coordinate system are measured, in particular directly, by means of the measuring arm. The coordinates of the first reference point, which can also be referred to as model coordinates, are calculated by the controller based on the respective coordinates of the plurality of reference measurement points, which can also be referred to as measurement coordinates.

[0022] Unlike the first reference point, the reference measurement points are not fixed. Rather, only a rule is given according to which the reference measurement points must be selected. In particular, the geometric position of the reference measurement points relative to the first reference point is specified. This allows the coordinates of the first reference point to be measured indirectly by directly measuring the reference measurement points.

[0023] This is particularly advantageous when the first reference point is not located directly on a surface of the real object, but is instead defined, for example, by the center point of a hole or similar feature. This allows the position of the first reference point to be determined with high accuracy. Holes or slots are preferably used for tactile measurement.

[0024] According to at least one embodiment, the coordinates of a second reference point of the three or more reference points, as well as the coordinates of a third reference point of the three or more reference points, are determined accordingly based on respective further reference measurement points, as described for the first reference point.

[0025] According to at least one embodiment, the plurality of reference measurement points lies on a surface of the real object, wherein the surface has a predetermined geometric shape and the coordinates of the first reference point are calculated based on a predetermined position of the first reference point with respect to the surface.

[0026] According to at least one embodiment, part of the surface corresponds to the inside of a circular cylinder shell and the first reference point lies on an axis of rotation of the circular cylinder shell.

[0027] In other words, the reference measurement points can be located on the inside of a cylindrical bore or other approximately cylindrical or circular recess or cavity, with the first reference point at the center of the bore or the like. In other words, the bore center can serve as the reference point. The reference measurement points can also be located on or within a slot or the like.

[0028] For example, additional reference measurement points can be located on another part of the surface, wherein the axial position of the additional reference measurement points with respect to the axis of rotation is equal to the axial position of the first reference point with respect to the axis of rotation. In corresponding embodiments, the first reference point can, for example, be located on a surface of the bore or the slot.

[0029] According to the improved concept, a measuring system for measuring the distance between a real measuring point of a real object and a virtual measuring point of a virtual object is also specified. The measuring system includes a storage unit that stores the spatial position of the virtual measuring point relative to three or more reference points of the real object and stores the respective three-dimensional coordinates for each of the three or more reference points in a measurement coordinate system of the measuring system. The measuring system includes a measuring arm with a measuring element and a sensor system configured to generate sensor signals depending on the pose of the measuring element in the measurement coordinate system.The measuring system has a control unit that is set up to determine three-dimensional coordinates of the real measuring point in the measuring coordinate system, depending on the sensor signals, and to calculate the virtual distance based on the spatial location of the virtual measuring point, the coordinates of the real measuring point and the respective coordinates of the three or more reference points.

[0030] The sensor system includes, in particular, two or more sensors, such as rotary encoders, with each sensor capable of generating a corresponding sensor signal. These sensor signals can be interpreted, for example, as time-dependent signals. Depending on the position of the measuring element or the measuring head with the measuring element, the sensor signals may change. To determine the three-dimensional coordinates of the actual measurement point, the sensor signals are evaluated at a time when the measuring element is located at the actual measurement point or, in the case of indirect measurement, at corresponding auxiliary measurement points.

[0031] According to at least one embodiment of the measuring system, it includes a camera mounted on the measuring arm in such a way that the camera's pose changes when the position of the measuring element changes. The control system is configured to superimpose a representation of the virtual object onto the camera image, depending on the sensor signals.

[0032] In particular, the camera is rigidly connected to the measuring head. The camera's field of view therefore lies, for example, within the field of view of a user of the measuring system when the user is guiding the measuring element as intended. The representation of the virtual object can thus be superimposed on the camera image with correct positioning and, due to the mechanical registration and tracking, with high accuracy.

[0033] By overlaying the virtual object's representation with the camera image based on sensor signals, the virtual object's position and orientation relative to the real-world object can be precisely determined. This results in a highly precise augmented reality (AR) or mixed reality (MR) system.

[0034] This allows the user to orient themselves and assess geometric distances simply by visually inspecting the overlay image. Measuring the distances between the virtual and real objects, and especially the virtual distance between the real and virtual measuring points, can therefore be performed particularly efficiently.

[0035] Based on the sensor signals, the control system can precisely determine the pose of the measuring element and, accordingly, via the rigid connection of the camera, the pose of the camera or the pose of the camera's field of view.

[0036] According to at least one embodiment, the measuring arm is designed as a tactile measuring arm and has a tactile measuring head that includes the measuring element. The measuring element can, for example, include a stylus ball or a probe tip.

[0037] Tactile measuring arms offer the particular advantage of being relatively small while maintaining high measurement accuracy, especially compared to laser-based optical measuring heads. Consequently, tactile measuring arms make it possible to reach difficult-to-access real measurement points on the actual object, particularly in complex and confined workspaces.

[0038] According to at least one embodiment, the measuring arm has at least one rigid arm segment which connects the measuring head to a reference point of the measuring system by means of at least two articulated connections.

[0039] The reference point is located, in particular, at the stationary end, for example, a housing or the like. The reference point can, for example, define the origin of the measurement coordinate system.

[0040] For example, the measuring arm can have two rigid arm segments and a stationary base component located at or corresponding to the stationary end. The base component is connected to a first arm segment via at least one joint, and the first arm segment is connected to the second arm segment via at least two joints. The second arm segment is connected to the measuring head via at least one third joint.

[0041] Preferably, the base component and the first arm segment are connected via two first joint connections, the first arm segment and the second arm segment are connected via two second joint connections, and the second arm segment and the measuring head are connected via two third joint connections. The two first joint connections allow movements, in particular rotational movements, of the first arm segment relative to the base component in various directions. The same applies analogously to the two second joint connections and the two third joint connections.

[0042] In such embodiments, the measuring arm has at least six joints to enable the movement of the measuring head in space according to six degrees of freedom. In particular, the measuring element can be moved to any point in three-dimensional space, provided that this point is within a corresponding reach of the measuring arm, and rotated according to three spatial angles.

[0043] According to at least one embodiment, the sensor system has a corresponding rotary encoder for each joint connection of the at least two joint connections, which is arranged and configured in such a way that it can generate an associated sensor signal depending on a rotational position of the respective joint connection.

[0044] Further embodiments of the measuring system according to the improved concept follow directly from the various configurations of the method according to the improved concept, and vice versa. In particular, a measuring system according to the improved concept is configured to carry out a method according to the improved concept. The invention also includes combinations of the features of the described embodiments.

[0045] The following describes exemplary embodiments of the invention. This is illustrated by: Fig. a schematic representation of an exemplary embodiment of a measuring system according to the improved concept.

[0046] The embodiments described below are preferred embodiments of the invention. In these embodiments, the described components each represent individual features of the invention that can be considered independently of one another. Each of these features further develops the invention independently and can therefore be considered part of the invention individually or in a combination other than that shown. Furthermore, the described embodiments can also be supplemented by other features of the invention already described.

[0047] Figure 1 shows an exemplary embodiment of a measuring system 1 for measuring a virtual distance between a real measuring point P of a real object 2 and a virtual measuring point P' of a virtual object 3.

[0048] The real object 2 could, for example, be a component of the front end of a motor vehicle. The virtual object 3 does not exist physically. Rather, a CAD drawing or other digital representation of the virtual object 3 is stored on a storage unit 4 of the measuring system 1. For example, the virtual object could correspond to a windshield washer fluid reservoir or any other object that is to be installed in the vicinity of the real object 2.

[0049] Using the measuring system 1, a method for measuring a virtual distance between the real measuring point P and the virtual measuring point P' can be carried out without the virtual object 3 needing to be physically present. In this way, geometric validation of the installation of the virtual object 3 with respect to the real object 2 can be carried out at an early stage in the development phase of the product, in particular of the motor vehicle.

[0050] The measuring system 1 has a computing unit 10, which is connected to or contains the storage unit 4. The measuring system 1 also has a tactile measuring arm 5, which is arranged in the vicinity of the real object 2. In particular, the tactile measuring arm 5 has a base component 9, for example, a housing, a base, or a stand, which is fixed in space, i.e., it has a fixed position and orientation with respect to the real object 2. The position and orientation of the base component 9 thus define, for example, a measurement coordinate system for the measuring system 1, and in particular for the measuring arm 5. The measuring arm 5 has a measuring head 14, in particular a tactile measuring head, with a measuring element 6, which is movably connected to the base component 9, so that the pose of the measuring element 6 in the measurement coordinate system can always be determined. The measuring element can, for example, be designed as a measuring tip or a measuring sphere.

[0051] The measuring arm 5, for example, has two rigid arm segments 14a, 14b that connect the measuring head 14, and thus the measuring element 6, to the base component 9. The measuring arm 5 also has, for example, two first joint connections 7a that connect the first arm segment 14a to the base component 9, two second joint connections 7b that connect the first arm segment 14a to the second arm segment 14b, and two third joint connections 7c that connect the second arm segment 14b to the measuring head 14. The measuring arm therefore has six joint connections 7a, 7b, 7c, arranged such that the measuring head 14 can be moved or rotated according to the six spatial degrees of freedom. For each of the joint connections 7a, 7b, 7c, the measuring arm 5 contains a corresponding sensor unit 8a, 8b, 8c.The sensor units 8a, 8b, 8c include, for example, a rotary encoder for each of the joint connections 7a, 7b, 7c, in order to output the corresponding rotational position to the processing unit 10 based on a sensor signal from the respective rotary encoder. Based on the sensor signals, the processing unit 10 can thus determine the current pose of the measuring element 6 in the measuring coordinate system at any time and at any location.

[0052] The real object 2, for example, has three measuring points 12a, 12b, 12c, which can be defined, for instance, by bores or slots in a surface of the real object 2. The centers of the bores, for example, at the level of the surface into which the bores are placed, can be defined as reference points. The storage unit 4 stores a spatial location for the virtual object 3, i.e., in particular, a position and orientation of the virtual object 3 with respect to each of the three reference points. In other words, the storage unit stores a target installation position of the virtual object 3. This also predefines, in particular, the spatial location of the virtual measuring point P' with respect to the three reference points.

[0053] The measuring arm 5 can be tactilely aligned with the model coordinates via the three reference points. For this purpose, a user can, for example, guide the measuring element 6 directly to the positions of the reference points, and the processing unit 10 can determine the corresponding three-dimensional coordinates of the reference points in the measuring coordinate system directly from the measurement. Alternatively, the measuring head 14, in particular the measuring element 6, can be equipped with a precisely manufactured adapter so that the measuring element 6 can be inserted into the corresponding measuring points 12a, 12b, 12c, e.g., bores, thus ensuring a defined position of the measuring element 6 relative to the reference points.

[0054] In alternative embodiments, the calibration is performed indirectly by scanning the measuring points 12a, 12b, 12c, so that the processing unit 10 can itself calculate the respective coordinates of the reference points from the coordinates of corresponding reference measuring points at the measuring points 12a, 12b, 12c. For this purpose, the user can, for example, perform a corresponding coordinate measurement at three points on the inner radius of the bores, for example, at three points on the surface of the real object 2. By specifying, for example, a geometric shape of the surface and the measuring points 12a, 12b, 12c, i.e., in particular an inner radius of the bores, the position of the overall model can be unambiguously determined if the three-dimensional coordinates of the reference points are stored in the system.

[0055] Once the three-dimensional coordinates of the reference points in the measurement coordinate system have been determined, the measuring arm 5 can effectively be used to perform measurements in a reference coordinate system defined by the reference points. In other words, by measuring the coordinates of the real object 2 in the measurement coordinate system, an exact relationship to the virtual object 3 can be established. The user can then guide the measuring element 6 to the real measurement point P, and the processing unit 10 can accordingly determine the coordinates of the real measurement point P in the measurement coordinate system. Since the spatial position of the virtual measurement point P' relative to the reference points is also known, the processing unit can calculate the distance of the virtual measurement point P' from the real measurement point P based on the coordinates of the real measurement point P determined by measurement.

[0056] In various configurations, the measuring system 1 includes a camera 13, for example, an industrial camera. The camera 13 is mounted on the measuring head 14, in particular rigidly connected to the measuring head 14, such that the field of view of the camera 13 approximately corresponds to the field of view of a user operating the measuring head 14. Furthermore, the measuring system 1 includes a display unit 11, in particular a monitor or a display, on which a superimposed image can be shown. The superimposed image corresponds to a superimposition of a representation of the virtual object 3 with a camera image generated by the camera 13, which represents a Fig. of the real object 2. Thus, the virtual object 3 can be correctly positioned in the overlay image with respect to the Fig. of the real object 2 will be displayed.

[0057] The processing unit 10 can also superimpose the representation of the virtual object 3 with the camera image based on the sensor signals, i.e., based on the current pose of the measuring head 14 and thus of the camera 13. The fixed reference determined by the measurement using the reference points allows for a very precise overlay of the virtual object 3 onto the superimposed image.

[0058] According to the improved concept, as explained in particular by the figure, a way is given to measure a virtual distance between the virtual measuring point of a virtual object and a real measuring point of a real object with high accuracy.

[0059] In certain embodiments, the coordinates of a real measuring point on a real object are determined using the measuring arm, effectively calibrating the measuring arm to the model or vehicle coordinates. Distances between a real measuring point of the real object or model and a virtual measuring point can then be determined. These distances are based on the measured coordinates of the real measuring points or reference points of the real object and the spatial position of the virtual measuring point within the virtual CAD geometry, which is part of the overall model. The calibration or registration is performed using at least three reference points of the overall model and thus forms the basis for calculating the distance between physical and virtual components.

[0060] In various configurations, a high-precision visualization system, such as an augmented reality or mixed reality system, is used as a measurement system. This system can project precise CAD data—the virtual component—into the complex real-world construction space and measure the distances between the projected components and their physical surroundings. The visualization system can be understood as a measuring, visualization, and working tool. In particular, it can simplify and accelerate decision-making processes.

[0061] By using a measuring arm, especially a tactile measuring arm, and the associated high-precision mechanical tracking, a high degree of measurement accuracy can be achieved. For example, the measuring system or the measuring arm can be calibrated tactilely using reference points, allowing highly precise CAD component coordinates to be projected onto the physical model and the distances to be determined accordingly.

[0062] In certain configurations, a superimposed image—that is, a camera image enhanced with digital information—can be transmitted to a large monitor or display screen, making it suitable for large groups. This allows several people to work on the model simultaneously. Alternatively, the superimposed image can be streamed to multiple AR / VR headsets, thus also achieving group-based functionality.

[0063] In other words, a measuring instrument, namely the measuring arm, is combined with appropriate camera technology in various configurations. For example, a tactile measuring arm can be combined with an industrial camera. The tactile measuring arm offers very high precision and correspondingly low tolerances. The industrial camera, with its robustness and numerous adjustment options, is also particularly well-suited for use in an augmented reality system. Reference symbol list 1 measuring system 2 real object 2' Figure 3 virtual object 4 storage units 5 measuring arm 6 measuring element 7a, 7b, 7c Joint connections 8a, 8b, 8c Sensor units 9 Basic component 10 computing unit 11 Display unit 12a, 12b, 12c measuring points 13 Camera 14 Measuring head 14a, 14b arm segments P real measuring point P' virtual measuring point

Claims

[1] Method for measuring a virtual distance between a real measuring point (P) of a real object (2) and a virtual measuring point (P') of a virtual object (3), which corresponds to a further object not physically present, which is to be installed, for example, in an environment of the real object (2), using a measuring system (1) which has a measuring arm (5), wherein - a spatial position of the virtual measuring point (P') is specified with respect to three or more reference points of the real object (2); - for each of the three or more reference points, respective three-dimensional coordinates are specified in a measurement coordinate system of the measurement system (1); - three-dimensional coordinates of the real measuring point (P) in the measuring coordinate system are determined using the measuring arm (5); - the virtual distance is calculated by means of a control (10) of the measuring system (1) based on the spatial location of the virtual measuring point (P'), the coordinates of the real measuring point (P) and the respective coordinates of the three or more reference points. [2] Method according to claim 1, characterized by , that the respective coordinates of the three or more reference points are measured using the measuring arm (5). [3] Method according to claim 1, characterized by , that - for a first reference point of the three or more reference points, the respective three-dimensional coordinates of a plurality of reference measurement points of the real object (2) are measured in the measurement coordinate system using the measuring arm (5); and - the coordinates of the first reference point are calculated using the control (10) based on the respective coordinates of the multitude of reference measurement points. [4] Method according to claim 3, characterized by , that - the multitude of reference measurement points lie on a surface of the real object (2), wherein the surface has a predefined geometric shape; and - the coordinates of the first reference point are calculated using the control (10) based on a given position of the first reference point with respect to the surface. [5] Method according to claim 4, characterized by , that part of the surface corresponds to the inside of a circular cylinder shell and the first reference point lies on an axis of rotation of the circular cylinder shell. [6] Measuring system (1) for measuring a virtual distance between a real measuring point (P) of a real object (2) and a virtual measuring point (P') of a virtual object (3), which corresponds to a further object not physically present, which is to be installed, for example, in an environment of the real object (2), comprising the measuring system (1) - a storage unit (4) which stores a spatial position of the virtual measuring point (P') with respect to three or more reference points of the real object (2) and stores for each of the three or more reference points respective three-dimensional coordinates in a measuring coordinate system of the measuring system (1); - a measuring arm (5) with a measuring element (6) and a sensor system (8a, 8b, 8c) which is configured to generate sensor signals depending on a pose of the measuring element (6) in the measuring coordinate system; - a controller (10) that is set up to, - to determine three-dimensional coordinates of the real measurement point (P) in the measurement coordinate system based on the sensor signals; and - to calculate the virtual distance based on the spatial location of the virtual measuring point (P'), the coordinates of the real measuring point (P) and the respective coordinates of the three or more reference points. [7] Measuring system (1) according to claim 6, characterized by , that - the measuring system (1) includes a camera (13) which is mounted on the measuring arm (5) in such a way that when the pose of the measuring element (6) changes, the pose of the camera (13) changes accordingly; and - the control (10) is set up to overlay a representation of the virtual object (3) on a camera image from the camera (10) depending on the sensor signals. [8] Measuring system (1) according to one of claims 6 or 7 characterized by , that the measuring arm (5) is designed as a tactile measuring arm (5) and has a tactile measuring head (14) which includes the measuring element (6). [9] Measuring system (1) according to claim 8, characterized by , that the measuring arm (5) has at least one rigid arm segment (14a, 14b) which connects the measuring head (14) to a reference point of the measuring system (1) by means of at least two joint connections (7a, 7b, 7c). [10] Measuring system (1) according to claim 9, characterized by , that the sensor system (8a, 8b, 8c) has a rotary encoder (8a, 8b, 8c) for each of the at least two joint connections (7a, 7b, 7c), which is arranged and configured to generate a corresponding sensor signal depending on a rotational position of the respective joint connection (7a, 7b, 7c).

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