Coordinate measuring machine with adjustable focus lens

Abstract

The invention relates to a coordinate measuring machine with an adjustable focus lens. Coordinate measuring machine (1) for measuring a plurality of measurement points on a surface, comprising a base (2), a probe head (4) having an optical probe (41), a machine structure (3) for connecting the probe head (4) to the base (2), at least one drive mechanism for providing mobility of the probe head (4) relative to the base (2), at least one coding unit, an optical overview sensor unit (50) arranged at the machine structure (3) and a control and processing unit at least adapted to control the drive mechanism. The optical probe (41) comprises an optical detection unit, a variable focal length lens (42) capable of modifying its focal length, and an actuation unit constructed and arranged with respect to the variable focal length lens (42) such that the actuation unit provides a setting and change of the focal length in accordance with a control signal.

Description

Technical Field

[0001] The present invention relates to a coordinate measuring machine (CMM) comprising an optical probe having an adjustable focus lens (liquid lens) for focusing. The focus can be adjusted based on predetermined surface information derived on the CMM side. Background Art

[0002] A coordinate measuring machine (CMM) is a machine configured to measure the 3D coordinates of certain points of a workpiece, in particular the entire surface topography. In the sense of the present invention, a surface can refer to the 2D exterior of a workpiece, but can equally refer to a surface area, i.e. an area of ​​finite thickness. For reasons of brevity and clarity, from here on, a surface is used as a 2D surface, so that specific features of measuring a surface area can be applied.

[0003] CMMs are important in various industries, for example in production measurement, quality control or reverse engineering. They can be used, for example, to determine the deviation of the geometry of a manufactured product from the design model, for example in order to determine whether the deviation is within manufacturing tolerances. Such measurements are usually performed automatically or semi-automatically based on a computer-generated or operator-selected measurement path, wherein such a measurement path is provided relative to the design model.

[0004] Another application of CMM that is becoming more and more prominent is the reverse engineering of objects. For such applications, there is no design model, but the operator commands the 3D movement of the probe by manual manipulation commands using, for example, a joystick / joystick. Alternatively, the operator can directly manipulate the handheld sensor.

[0005] Typically, a CMM has a main structure, a detection system, and a data collection and data processing system.

[0006] The main structure usually includes a set of actuators responsible for positioning the detection system. A widespread example of a CMM is a 3-axis system. For example, DE4325347 discloses such a CMM system. Here, the main structure includes a base with a measuring platform and a movable frame. The workpiece can be positioned or mounted on the measuring platform. The movable frame is mounted on the base so that it can move along a first axis, and the frame includes an arm that is mounted so that it can move along a second axis perpendicular to the first axis. The detection system includes a probe that is mounted on the arm so that the probe can move along a third axis perpendicular to the first axis and the second axis. This structure enables the probe to be manipulated in all three-dimensional directions, thereby being able to measure the relevant 3D coordinates of the object. Current 3-axis systems also typically include components, such as stacked turntables, to provide 5 degrees of freedom regarding the relative posture of the probe and the workpiece.

[0007] Another typical embodiment of CMM is the so-called articulated arm coordinate measuring machine (AACMM). AACMM includes a base to be fixed and an arm including a plurality of arm segments connected by joints. The joint provides mobility for the movable end of the arm relative to the base, and wherein a probe may be attached. The probe is usually manually guided by an operator. Due to its design principle, this system is not as accurate as the above-mentioned 3-axis or 5-axis system, on the other hand, it provides higher flexibility. For example, EP2916099B1 discloses this AACMM instrument.

[0008] The detection system of a CMM can be based on contact or non-contact technology. In the first case, a mechanical probe, usually implemented as a stylus, achieves direct mechanical contact with the workpiece and the probe is guided through a given measurement path, while the coordinates of the end point of the stylus are derived from sensor readings about the state of the CMM (e.g. encoder readings). Non-contact technology can be based on projecting a main measuring beam onto the work object and recording auxiliary beams emanating from the surface area of ​​the object. One advantage of non-contact technology is that it is less likely to damage the work object due to the lack of mechanical contact. In addition, non-contact methods allow parallel acquisition of extended areas, unlike stylus-based methods, in which the coordinates of only a single point are recorded.

[0009] In order to obtain high-quality data, CMM measurements can be performed under steady-state measurement conditions. For a contact probe, this can be provided, for example, by keeping the contact and / or friction forces between the stylus and the work object within a certain range. In contrast to a contact probe, this simple feedback method cannot be used to automatically guide a non-contact probe. This problem is particularly severe in the case of manual measurement of unknown work objects, because in this case it is not even possible to provide a sufficiently good approximation path based on a digital model or previous measurement data.

[0010] It is usually quite complex and time-consuming to derive a digital model of the workpiece to be measured and the corresponding measurement path. In addition, in order to measure the workpiece according to the measurement path, it is necessary to detect the orientation of the workpiece and align the digital model according to the current orientation. This leads to a more complex measurement process. Summary of the invention

[0011] It is therefore an object of the present invention to provide a coordinate measuring machine capable of alleviating the above-mentioned drawbacks.

[0012] Another object of the present invention is to provide a coordinate measuring machine which provides a more efficient measurement workflow, in particular by utilizing a pre-measurement step.

[0013] The invention is based on the idea of ​​providing an optical probe of a CMM with a sensor and a fast focusing (e.g. liquid) lens to acquire surface information in an image-based manner. Such a variable focal length lens provides for acquiring surface information in a very efficient manner, particularly when moving the probe over the surface (on the fly). The focal length of the optical system can be adjusted depending on the distance between the probe and a particular measurement point or area. Such adjustment of the focal length can be provided relatively quickly when moving from one measurement point to the next. Preferably, the adjustment can be provided in a continuous manner during movement of the probe along the measurement path.

[0014] In order to adjust the focus, height information about a specific distance to a specific measuring point is required. The CMM is configured to provide corresponding height information of the measuring point and / or the surface to be measured. The CMM comprises an additional optical overview sensor unit (in addition to the optical probe) to derive height information, in particular to derive topographic data related to the surface.

[0015] Such an optical overview sensor unit is arranged at the machine structure, for example at a frame or an arm of the CMM, and is configured to acquire surface information.The optical overview sensor unit may be realized as a camera, a plurality of cameras, a triangulation sensor, a range-finding camera or the like.

[0016] Surface information, such as height information or a height map, can be derived by capturing at least one image, preferably a plurality of images, relating to the surface to be measured.The surface information can then be derived by image processing.

[0017] For example, the optical overview sensor unit comprises a camera, and the camera is moved over the surface by moving the machine structure. Accurate information about the position of the camera relative to the base (in particular relative to the surface) can be determined by utilizing sensors or encoders of the CMM. The camera is controlled to acquire a set of images, and accurate position information is collected and assigned to each image in the set of images. Surface information can be derived by applying photogrammetry and processing the image information together with the specified position information. Such photogrammetry can include any processing method capable of reconstructing 3D geometry from a set of 2D images, for example inverse rendering methods and AI-based reconstructions such as Neural Radiance Fields (NeRF) can also be used. In this way, topographic data about the surface can be calculated.

[0018] The present invention therefore relates to a coordinate measuring machine for measuring a plurality of measuring points on the surface of an object. The coordinate measuring machine comprises a base, a probe with an optical probe and a machine structure with structural components for connecting the probe to the base, the machine structure being in particular a portal (bridge) or Scara type machine structure. The CMM can also be implemented as a horizontal arm CMM or a mobile platform bridge type CMM (e.g. Optiv and PMM-C, and rack CMM). A Scara type machine is an industrial robot. The acronym SCARA stands for Selective Compliance Assembly Robot Arm or Selective Compliance Articulated Robot Arm.

[0019] The base can be provided as a measuring platform, for example made of granite, or as a physical reference point, such as the base or setup location of a CMM.

[0020] The probe may be arranged as an element for carrying a measurement probe, such as a tactile or optical probe, or a tool. The probe may also be implemented to provide rotation of the measurement probe about one or two axes. In the case where the CMM is constructed as a portal (3-axis) CMM, the probe is preferably mounted to a z-rod which provides mobility of the probe in the z direction.

[0021] The optical probe may preferably be arranged at the probe head in order to provide, with the optical probe, a measurement of the distance to the object or structure (1D data), of the lateral dimensions of the object or structure (2D data) or both (3D data) at the same time. Thereby, the optical probe may be moved relative to the base. The optical probe may be implemented as a camera, a triangulation sensor unit, a confocal measurement unit, a white light measurement unit or any other sensor unit capable of acquiring, for example, 1D, 2D or 3D data of a specific measurement point.

[0022] The optical probe may include a camera comprising a telecentric lens with adjustable lens elements, an image sensor with control electronics and illumination for the object. The illumination may be an external ring light or a coaxial top light coupled into the telecentric lens via a beam splitter. Typically, optical probes have a lateral object resolution of a few microns, while for comparison, the resolution of an optical overview sensor may be 100 times smaller.

[0023] As mentioned above, the machine structure may include specific elements that can move relative to each other to provide positioning of the probe within the measurement volume of the CMM. Relative mobility should be understood as relative linear or relative rotational movement, but should not be limited to a specific design of the CMM, but should cover CMMs and robots known in the prior art.

[0024] The coordinate measuring machine further comprises at least one drive mechanism for providing mobility of the machine structure and the probe relative to the base. The drive mechanism may be configured as a motor, a stepper motor, an actuator, etc. In particular, the coordinate measuring machine may comprise at least three drive devices arranged such that the probe can be moved in three directions, for example in the directions of the x-axis, the y-axis and the z-axis.

[0025] The coordinate measuring machine further comprises at least one encoding unit configured to provide position data about the position of the machine structure and / or the probe relative to the base. In particular, the coordinate measuring machine comprises a set of encoders which provide the relative position of the machine part on a specific coordinate axis. The at least one encoding unit may be provided by a linear encoder, a rotary encoder or a sensor unit capable of measuring the distance between two points.

[0026] The coordinate measuring machine comprises an optical overview sensor unit which is arranged at the machine structure and is configured to acquire surface information, in particular in the form of image data.The optical overview sensor unit provides a field of view which is significantly larger than the field of view of the optical probe.

[0027] Furthermore, a control and processing unit is provided at least for controlling the drive mechanism.

[0028] The optical probe comprises an optical detection unit, in particular a sensor, a variable focus lens capable of modifying its focal length, and an actuation unit constructed and arranged relative to the variable focus lens such that it provides for setting and changing the focal length in dependence on a control signal.

[0029] An actuation unit is to be understood as an element which provides the setting of the focal length. The term "actuation unit" is not limited to mechanical units but may also cover electronically based solutions. Furthermore, the actuation unit does not have to be provided as a separate element but may be integrated into the variable focal length lens. For example, an electrowetting liquid lens, a liquid crystal spatial light modulator or a MEMS mirror array may comprise a specific type of actuation unit.

[0030] The control and processing unit comprises a measuring function configured to derive auxiliary distance information and configured to derive effective surface information of the measuring point. The auxiliary distance information is derived by: controlling the optical overview sensor unit so as to acquire surface information relating to at least one measuring point of the plurality of measuring points; and determining the auxiliary distance information relating to the at least one measuring point based on the surface information. The surface information may comprise image data or distance data, such as a point cloud providing distance information for a plurality of surface points.

[0031] A control signal is derived based on the auxiliary distance information.

[0032] The effective surface information is derived by providing an adjusted measuring state of the optical probe by applying a control signal to the actuation unit and thereby providing a distance-dependent focal length. The effective surface information of at least one measuring point is determined by means of the optical probe in the adjusted measuring state.

[0033] In particular, effective surface information can be provided as an image or a set of images of the surface. This method of measuring an object is often used in so-called vision machines. An object can be scanned with an optical probe and a series of images can be acquired to represent the surface of the object.

[0034] In particular, the effective surface information may provide distance information.

[0035] The optical probe can be a 2D camera with a small field of view. Such a (micro) camera can have a very small depth of field and can utilize an adjustable lens to keep the surface to be measured in focus.

[0036] In contrast, an optical overview sensor unit may have a large field of view and a large depth of field.

[0037] In particular, the measuring function may be configured such that the auxiliary surface information is provided by means of the optical overview sensor, wherein the auxiliary surface information may preferably comprise image data relating to the surface. In particular, the active surface information provides a much greater accuracy than the auxiliary surface information.

[0038] In other words, auxiliary distance information can be acquired by means of an optical overview sensor unit arranged at the machine structure to provide (rough) surface data in order to allow the focus of the variable focus lens to be controlled according to the surface data. Thus, precise surface data can be derived with the optical probe by a corresponding adjustment of its focus (in particular based on the mainly available auxiliary distance data).

[0039] The invention can therefore provide that the focal depth of an optical probe can be achieved with the aid of the accuracy of the auxiliary distance information, so that for example a sharp 2D image of an object can be acquired with the probe. This is advantageous for "vision machines" or similar applications, such as detecting drill hole diameters or component dimensions. This also involves measuring 2D objects on flat surfaces (metal sheets, circuit boards, etc.).

[0040] In one embodiment, the measuring function may be configured such that deriving the auxiliary distance information comprises controlling the optical overview sensor unit to acquire second surface information associated with a second measuring point of the plurality of measuring points. The second auxiliary distance information associated with the second measuring point may be determined based on the second surface information.

[0041] In one embodiment, the measurement function can be configured such that deriving effective surface information includes: deriving a second control signal based on second auxiliary distance information; providing a second adjusted measurement state of the optical probe by applying the second control signal to the actuator unit and thereby providing a distance-dependent focal length; and determining second effective surface information of a second measurement point with the aid of the optical probe in the second adjusted measurement state.

[0042] According to one embodiment, the measurement function may be configured to carry out or execute the derivation of the auxiliary distance information before deriving the effective surface information.

[0043] In one embodiment, the measuring function may be configured to: arrange the optical overview sensor unit at a first position relative to the base by means of the drive mechanism; and derive the auxiliary distance information based on the surface information acquired at the first position.

[0044] According to one embodiment, the measuring function can be configured as follows: the optical overview sensor unit is continuously set at at least a first position and a second position relative to the base by means of a driving mechanism; auxiliary distance information is derived in the following manner: surface information related to the at least one measuring point among the multiple measuring points is acquired at least at the first position and the second position; and the auxiliary distance information related to the at least one measuring point is determined based on the surface information acquired at least at the first position and the second position.

[0045] In other words, the auxiliary distance information can be derived by different methods but using the same capture device (optical overview sensor unit). The auxiliary distance information can be determined by capturing one image of the surface, in particular by utilizing the projection of a light pattern and capturing an image of the illuminated surface. Thereby, the auxiliary distance information can be derived as distance data based on triangulation. The light pattern can be a single laser line, multiple laser lines, random dots, a laser speckle pattern, a random pattern or a stripe projection (multiple patterns in sequence). Alternatively or additionally, the auxiliary distance information can be determined by capturing multiple images from different poses and processing the images using photogrammetry. Thereby, a point cloud representing the topography of the captured surface can be derived. Furthermore, a time-of-flight (TOF) camera or a LIDAR scanner can be provided as the optical overview sensor unit.

[0046] In one embodiment, the measuring function can be configured to derive position data about at least a first position and a second position of the optical overview sensor unit relative to the base by means of the encoding unit. The corresponding position data can be assigned to corresponding surface information of the at least first position and the second position of the optical sensor, and the auxiliary distance information can be determined by processing the position data and the surface information.

[0047] In other words, the measurement function can be implemented to utilize the encoder output of the CMM in order to determine the corresponding position (and pose) of the optical overview sensor unit when acquiring surface information. This enables highly accurate photogrammetric image processing to be provided by using a single camera at different positions of the optical overview sensor unit and accurate CMM position data. For example, a stereo basis for photogrammetric image processing can be derived based on the CMM position data.

[0048] In particular, the optical overview sensor unit may comprise at least one camera, in particular two cameras, and provide the auxiliary distance information as a height map by means of photogrammetric processing.

[0049] In one embodiment, the base may comprise a set of reference points with known positions, in particular provided by threads (eg threaded inserts or threaded holes) or by screw holes of the measuring platform (base).

[0050] At least one of the reference points may be overlaid with the acquired surface information and the position of the at least one reference point processed for determining the auxiliary distance information.Alternatively or additionally, the position of the at least one reference point may be processed for calibrating the optical overview sensor unit.

[0051] Since the captured images can be referenced relative to the coordinate system of the CMM by means of the overlaid reference points, this provides a stable and accurate measurement of the surface.

[0052] In particular, extrinsic parameters of the optical overview sensor unit may be calibrated or updated by processing the position of the at least one reference point.Extrinsic parameters describe the external orientation of the optical overview sensor unit (camera), ie the position and orientation of the optical overview sensor unit in the coordinate system of the CMM.

[0053] In one embodiment, at least one of the reference points may be overlaid with the acquired surface information and the position of the at least one reference point may be processed for calibrating the optical overview sensor unit and / or updating the calibration of the optical overview sensor unit. In particular, the position of the at least one reference point is processed with each measurement, e.g. with each derivation of the auxiliary distance information. Thereby, drifts due to temperature changes or due to other external influences may be compensated, in particular continuously.

[0054] The position of at least one reference point or a plurality of reference points in the set of reference points on the base (measuring platform) may be known in advance and provided in a database.Alternatively or additionally, the position may be derived (measured) by a coordinate measuring machine, for example by using a tactile probe.

[0055] In one embodiment, the control and processing unit may include an auxiliary measurement function, which is configured to provide auxiliary distance information by: moving the optical overview sensor unit above the surface; acquiring surface information related to the surface at multiple positions provided by moving the optical overview sensor unit; and determining auxiliary distance information related to the surface based on the surface information, wherein the auxiliary distance information includes specific distance information for each of the multiple measurement points.

[0056] In particular, the auxiliary measurement function may be a sub-function of the measurement function and be implemented by the measurement function.

[0057] The movement of the optical overview sensor unit can be achieved by continuously moving the optical overview sensor unit, and acquiring the surface information can be provided by capturing images at specific positions or at defined intervals during the movement of the optical overview sensor unit, respectively. The position at which the image is captured can be determined by a CMM sensor (e.g. an encoder).

[0058] In one embodiment, the control and processing unit may comprise an active measurement function configured to provide active surface information.The active surface information may be provided by moving the optical probe over the surface according to a measurement path, wherein the optical probe successively approaches a first measurement point and a second measurement point.

[0059] A first control signal associated with the first measurement point and a second control signal associated with the second measurement point are received or derived.

[0060] During approaching the first measuring point, a first adjusted measuring state of the optical probe is provided by applying a first control signal to the actuation unit, and effective surface information of the first measuring point is determined by means of the optical probe in the first adjusted measuring state.

[0061] During approaching the second measuring point, a second adjusted measuring state of the optical probe is provided by applying a second control signal to the actuation unit, and effective surface information of the second measuring point is determined by means of the optical probe in the second adjusted measuring state.

[0062] The first control signal and / or the second control signal may be received from a database or may be derived based on measurements by means of the optical overview sensor unit before performing the active measuring function, for example by performing an auxiliary measuring function.

[0063] By means of the above method, for each measuring point a control signal can be provided, which control signal allows the focal length of the liquid lens to be adjusted when it is desired to measure the corresponding measuring point by means of the optical probe. Thus, the required measurement parameters can be provided for each specific measuring distance (measuring point). The measuring distance can be understood in particular as the distance between the surface of the workpiece to be measured (measuring point) and the optical probe.

[0064] Since suitable measurement parameters can be provided for each measuring point (within a defined measuring range in the z direction), additional movement of the optical probe in the z direction, for example in order to provide a suitable distance to measure the measuring point, can be avoided. This leads to a higher measuring speed.

[0065] According to an embodiment, the active measurement function can be configured to continuously move the optical probe over the surface according to the measurement path, and in the process of moving the optical probe, in particular during the movement of the optical probe, provide the application of the first control signal and / or the second control signal and the determination of the effective surface information of the first measurement point and / or the second measurement point. Thus, it is possible to perform on-the-fly measurement, which results in faster measurement and less measurement time.

[0066] The invention is not limited to obtaining a specific type of surface information. The evaluation may be performed by a computing unit physically separated from the CMM, in particular a remote computing unit. Such a computing unit comprises a controller in the sense of the invention.

[0067] In one embodiment, to obtain valid surface data, an optical probe with a variable focal length (e.g., through an adjustable lens) can be controlled to acquire a 2D image of an object. The variable focal length is controlled so that the surface of the object is in focus. The camera captures an image that provides accurate 2D information about the features of the object.

[0068] The height data (e.g. 3D coordinates) of an object acquired with the optical overview sensor unit may have limited accuracy. However, as long as the uncertainty of the height information of the object is lower than the focal depth of the optical probe, the image taken with the optical probe is clear.

[0069] However, since the typically used lens systems for vision machines (here with variable focal length lenses) are implemented as telecentric lens systems, accurate 3D data of the object cannot be derived from the sharp 2D images. Images taken at different lateral positions have the same viewing angle and a 3D reconstruction similar to that of an optical overview sensor (endocentric lens) is not possible. Alternatively, in case an endocentric lens is used for an optical probe, the risk of lower accuracy of the lateral object dimensions is much higher due to the larger variation of the optical scale with distance.

[0070] In one embodiment, to derive distance information using an optical probe with a telecentric lens system, a (eg periodic) variation of the focal length may be applied. The variation may be provided by varying the control signal, respectively. The focus variation is preferably controlled to be periodic (oscillating).

[0071] For example, the focus variation can be a periodic variation of a tunable lens in the form of a sine or sawtooth function, or it can be a non-periodic but well-defined function.

[0072] In addition to the controlled variation of the focal length, the lateral movement of the optical probe (via the CMM movement) and the image acquisition can be controlled in such a way that the field of view (FOV) of several consecutive images overlaps. The combination of focus variation and overlapping images leads to the fact that features of the surface to be measured can be imaged in multiple acquisitions with different focus settings.

[0073] Based on the above, the clearest image can be selected for further image processing and determination of 2D feature coordinates.

[0074] Furthermore, the sharpness of the image can be determined as a function of the z distance, for example by contrast evaluation. This allows the distance of features in the z direction (1D coordinate) to be calculated with greater accuracy. The z distance can be calculated by interpolation.

[0075] In particular, the focus shift can be selected to be greater than the depth of the contrast range. From this, a contrast maximum can be determined. In order to calculate the contrast maximum, a minimum of three images are required per object point or feature.

[0076] Therefore, in one embodiment, the optical probe may include a telecentric lens system, and the control and processing unit may include a distance measurement function configured to derive distance data for at least one of the plurality of measurement points. The distance data is derived by: moving the optical probe over the surface according to the auxiliary distance information; changing the focal length during the movement of the optical probe by providing a control signal as a modified control function; and capturing a set of at least three images as effective surface information during the movement of the optical probe, wherein changing the focal length and capturing the images are adapted to each other such that each image covers the at least one of the plurality of measurement points and each image provides a different focus level for the at least one of the plurality of measurement points.

[0077] A range image is determined from the set of at least three images by image processing using a contrast criterion and the distance is derived based on the focal length applied for capturing the range image, in particular by interpolating the focal length for capturing the set of at least three images.

[0078] According to an alternative embodiment of how to obtain distance information with the optical probe, the optical axis of the optical probe may be tilted relative to the surface to be measured, ie the optical axis is not perpendicular to the surface, in particular not oriented according to the surface normal. In particular, the optical probe is tilted.

[0079] The tilt can be chosen in such a way that a feature at the object can be imaged while moving the optical probe relative to the surface in multiple consecutive acquisitions with different focus / defocus. The sharpest image of a particular feature can be selected for further image processing and determination of the 2D feature coordinates. The sharpness can be determined as a function of the z distance, which allows calculation (e.g. by interpolation) of the exact distance of the feature in the z direction (1D coordinate).

[0080] Here, the focal length of the adjustable lens can also be oscillating, or the focal length of the adjustable lens can be allowed to follow an assumed surface topography. This assumed topography can be determined as auxiliary surface data (auxiliary distance information) by measuring with the optical overview sensor unit. Preferably, the tilt is selected so that the height difference of the focus relative to the surface on the field of view is sufficient for a significant contrast change.

[0081] Thus, in one embodiment, the active measurement function may be configured to provide a tilt of the optical axis of the optical probe relative to a normal associated with a first measurement point and relative to a normal associated with a second measurement point. The tilt may be provided by a machine structure and a drive mechanism. In particular, the orientation of the optical probe is controlled to be tilted relative to the surface normal.

[0082] At least one tilted image can be captured as effective surface information for each measurement point. Thus, common features of the surface can be covered by each tilted image, wherein each tilted image provides a different definition for the common features (due to different defocus caused by tilt).

[0083] An optimal definition value for the common feature may be determined based on the oblique image. The distance information may be derived based on a control signal applied for capturing the oblique image and based on the optimal definition value.

[0084] The invention also relates to a controller for a coordinate measuring machine. The coordinate measuring machine comprises a base including a set of reference points with known positions, in particular provided by threads of a measuring platform, a probe head with an optical probe, and a machine structure having structural parts for connecting the probe head to the base, in particular a portal or Scara type machine structure.

[0085] The coordinate measuring machine further comprises: at least one drive mechanism for providing mobility of the machine structure and the probe relative to the base; at least one encoding unit configured to provide position data about the position of the machine structure and / or the probe relative to the base; and an optical overview sensor unit arranged at the machine structure and configured to acquire surface information, the optical overview sensor unit providing a field of view that is significantly larger than the field of view of the optical probe.

[0086] The controller is configured to at least control the drive mechanism.

[0087] The controller is further configured to capture at least one reference image covering at least one of the reference points and at least a portion of a calibration target disposed on the base. External parameters of the optical overview sensor unit relative to the calibration target are calculated by processing the at least one reference image. A sensor image position of the at least one of the reference points is derived based on the at least one reference image, the sensor image position being provided as coordinates in a camera coordinate system of the optical overview sensor unit.

[0088] A reference position of at least one of the reference points is received or derived, the reference position being provided as a coordinate in a CMM coordinate system of a coordinate measuring machine. The camera coordinate system and the CMM coordinate system are referenced by processing the sensor position and the reference position.

[0089] The invention also relates to a method for calibrating an optical overview sensor unit of a coordinate measuring machine as described above. The method comprises: providing a calibration target; taking a set of images of the calibration target from different poses by means of the optical overview sensor unit; and deriving extrinsic parameters of the optical overview sensor unit by processing the set of images.

[0090] Furthermore, the calibration target is provided on a base of a coordinate measuring machine, wherein the base is provided as a measuring platform with a set of reference points having known positions, in particular the reference points are provided by threads of the measuring platform, and at least one reference image covering at least a part of the calibration target and at least one of the reference points is captured.

[0091] External parameters of the optical overview sensor unit relative to the calibration target are calculated by processing the at least one reference image, and a sensor image position of the at least one reference point is derived based on the at least one reference image, the sensor position being provided as coordinates in a camera coordinate system of the optical overview sensor unit.

[0092] A reference position of at least one of the reference points is received or derived (e.g. by measuring in advance), the reference position being provided as a coordinate in a CMM coordinate system of a coordinate measuring machine, and the camera coordinate system and the CMM coordinate system are referenced by processing the sensor image position and the reference position.

[0093] The invention also relates to a computer program product which, when executed by a computing unit and / or a controller, enables automatic execution and control of the above-mentioned steps. BRIEF DESCRIPTION OF THE DRAWINGS

[0094] By way of example only, specific embodiments of the present invention will be described more fully hereinafter with reference to the accompanying drawings, in which:

[0095] Figure 1 A schematic diagram of an embodiment of a CMM according to the invention is shown as a three-axis machine.

[0096] Figure 2 A schematic diagram of an embodiment of a CMM according to the present invention is shown.

[0097] Figure 3 A measuring path comprising a plurality of measuring locations and a measuring process according to the invention are described.

[0098] Figure 4 An embodiment of a CMM according to the invention is shown.

[0099] Figure 5 An exemplary embodiment of a measuring method for determining a distance to an object is shown. DETAILED DESCRIPTION

[0100] Figure 1 A schematic diagram of an embodiment of a CMM 1 according to the present invention as a three-axis CMM is shown. The CMM 1 includes a measurement platform 2 supporting a workpiece 20. The measurement platform 2 provides a base for the CMM. Although not shown in the illustrated embodiment, the measurement platform 2 may include elements that help position the workpiece 20. The measurement platform 2 may also include other elements that clamp or otherwise fix the workpiece 20, particularly by a vacuum chuck. These other elements may be temporarily mounted to the measurement platform 2.

[0101] The CMM comprises a machine structure 3, which is implemented as a movable frame mounted on a measuring platform 2, so that it can be moved along a first axis (in the embodiment depicted here, the x-axis) by a first set of motors. The machine structure 3 comprises an element spanning the measuring platform 2, the so-called bridge 31. Furthermore, the CMM comprises a carriage 32 movably mounted along the bridge 31. Provision is made for the mounting of the carriage 32 so that it can be moved along a second axis perpendicular to the first axis (in the embodiment depicted here, the y-axis).

[0102] The carriage carries another element 33 which is arranged and configured to move along a third axis, in the embodiment depicted here the z-axis. This element may also be referred to as a (z-) push rod or rod.

[0103] Furthermore, the CMM comprises a drive mechanism for providing movability of the machine structure 3 and the probe 4 relative to the base 2. The drive mechanism may comprise specific drive units providing relative motion of specific parts of the machine structure 3, such as a motor providing motion of the entire machine structure 3 (bridge 31) relative to the base 2, a motor providing motion of the carriage 32 relative to the bridge 31 and a motor providing motion of the z-thrust rod 33 relative to the carriage 32.

[0104] The CMM also includes at least one encoding unit configured to provide position data about the position of the machine structure and / or the probe relative to the base. The encoding unit may include several encoders that provide information about the position of the bridge 31 relative to the base 2, the position of the carriage 32 relative to the bridge 31, and the position of the z-thruster relative to the carriage 32.

[0105] The probe 4 is mounted at the end of the z-push rod 33. The probe 4 includes an optical probe 41. The optical probe 41 is configured to acquire data related to the surface of the workpiece 20. Therefore, the optical probe 41 includes a sensor (optical detection unit) to detect an optical signal such as light reflected at the surface of the workpiece 20 or measurement light.

[0106] In one embodiment, the CMM may include additional joints and / or actuators to provide additional degrees of freedom for moving the probe 4. For example, a rotation joint may be provided to provide rotation of the probe 4 and / or optical probe 41.

[0107] In one embodiment, the optical probe 41 may include a light emitting unit for illuminating the workpiece 20 and / or for projecting a measuring light (e.g., a point, a line, or a pattern) toward the workpiece 20. The sensor is configured to detect reflections of the light emitted by the light emitting unit. Thus, for example, surface measurement based on triangulation may be available. As a result, height information related to the surface of the workpiece 20 may be derived.

[0108] In one embodiment, the optical probe 41 may be provided by a triangulation sensor having a light emitting unit (eg a laser, a laser diode, an LED or an OLED) and an optical detection unit (eg a CMOS or a CCD).

[0109] The optical probe 41 comprises: a variable focal length lens 42, which is capable of modifying its focal length; and an actuation unit, which is constructed and arranged relative to the variable focal length lens so that the actuation unit provides for setting and changing the focal length according to a control signal. The variable focal length lens 42 is used to manipulate the focal length of the optical system of the optical probe 41 in a way to provide for acquiring optical information with an adjusted focal length according to the current distance of the optical probe 41 relative to the surface or relative to a specific measurement point at the surface.

[0110] Since the surface of the workpiece 20 to be measured is usually not a plane but includes protrusions and / or depressions, the distances of different measuring points are different. In other words, the workpiece 20 provides a varying height on its surface. This means that when the probe is moved relative to the workpiece 20, the focal length can be adjusted respectively to provide and maintain adjustment and appropriate measurement conditions. In this way, the relative movement of the optical probe 41 and / or the probe 4 in the z direction can be avoided or significantly reduced. This leads to faster and more efficient measurement of the workpiece 20.

[0111] In order to provide suitable measurement conditions, information about the height of the surface of the workpiece 20 , such as a height map or a topography distribution, must be obtained.

[0112] The CMM 1 comprises an optical overview sensor unit 50. The optical overview sensor unit 50 is arranged at the machine structure and is configured to acquire corresponding auxiliary distance information. The optical overview sensor unit 50 provides a field of view that is significantly larger than the field of view of the optical probe 41 .

[0113] The control and processing unit of the CMM comprises a measuring function configured to derive auxiliary distance information. The auxiliary distance information is derived by controlling the optical overview sensor unit 50 to acquire surface information related to at least one point of the plurality of measurement points. Based on the surface information, the auxiliary distance information related to the at least one measurement point is determined.

[0114] according to Figure 1 In an embodiment of the present invention, the optical overview sensor unit 50 is provided as a camera and auxiliary distance information can be derived by moving the machine structure relative to the measuring platform 2 in the x-direction and capturing a plurality of images with the camera 50. The images can be processed photogrammetrically. The machine position can be derived for each captured image. Combined processing of the image data and the position data allows to derive height information about the surface of the object 20. Such height information can include a height map and is considered as auxiliary distance information.

[0115] The auxiliary distance information is used as input for controlling the focal length of the variable focal length lens 42. The CMM 1 comprises an actuation unit which is arranged and constructed relative to the variable focal length lens, in particular in the optical probe 41, such that the actuation unit can provide setting and changing of the focal length according to the control signal. The control signal is derived based on said auxiliary distance information.

[0116] For example, the variable focus lens 42 is configured as a liquid lens, and the actuation unit is used to manipulate the curvature of the liquid lens, for example, by pressing the liquid lens or by pulling the edge of the liquid lens.

[0117] Alternatively, the variable focus lens 42 may be provided by an electrowetting device, by a device applying piezoelectrically driven modifications to an optical surface, by a tunable acoustic gradient lens, or by a device including a MEMS mirror.

[0118] The control signal may be provided in dependence on the position of the optical probe 41 or on the position of the probe head 4 (eg in dependence on the CMM position measured by a position encoder).

[0119] The control signal can be applied and varied in a continuous manner, for example during movement of the probe 4 in the xy plane over the object to be measured.

[0120] Thus, an adjusted measurement state of the optical probe 41 can be provided by applying a control signal to the actuation unit. The applied control signal is preferably related to a measurement point at the object 20 which is to be measured and relative to which the optical probe 41 is to be positioned. Thereby, a distance-dependent focal length can be provided.

[0121] In other words, the probe 4 can be moved relative to the object 20 and the control signal is adjusted according to the position of the probe 4 in the xy plane. Thereby, the adjustment of the z position of the probe 4 can be avoided and suitable measuring conditions can be provided by setting the focal length according to the previously determined auxiliary distance to the corresponding measuring point.

[0122] When the focal length is set according to the auxiliary distance, effective surface information of at least one measuring point can be determined by the optical probe in the adjusted measuring state. In particular, the effective surface information provides a much greater accuracy than the auxiliary distance information. The effective surface information can include an image, a specific distance to the measuring point, an intensity value related to a signal detected by the sensor, position information related to an incident position of the measuring light on the sensor or an image position in an image captured by the optical probe.

[0123] As an example, the measurement of the workpiece 20 using the CMM 1 can be performed according to the following scheme. First, an ambiguous object 20 is provided on the measurement platform 2. In the next step, the machine structure 3 is moved on the measurement platform 2 in the x direction while capturing a set of images with the overview sensor unit 50. For each image, the position of the sensor unit 50 is determined by the CMM internal position sensor (e.g., a linear encoder). In particular, the corresponding pose of the sensor unit 50 at each position is derived. Each image is combined with a corresponding position.

[0124] The images and the corresponding positions are processed using photogrammetry, such as stereophotogrammetry. The corresponding stereo baseline can be derived by using the position of the sensor unit 50, at which the image to be processed was captured. The processing can be used to derive the specific position and height of the specific measurement point to be measured. Alternatively or additionally, the processing can be performed to derive a current height map of the measurement platform 2 or at least the object 20. The corresponding auxiliary distance information can therefore be derived as a point cloud.

[0125] Information about, for example, the height distribution over the entire measuring platform 2 is used to control the movement of the probe head 4 and the focus of the variable focus lens 42 .

[0126] Now, in the next (precision) measurement step, the probe 4 is moved over the measurement platform according to the measurement path. The measurement path can be derived based on the auxiliary distance information. The measurement path is preferably determined so that no movement or at least only a small movement of the probe 4 in the z direction becomes necessary. The corresponding change in the distance between the optical probe 41 and the surface of the measurement platform 2 and / or the object 20 along the measurement path is adjusted at least mainly by the variable focal length lens 42 to continuously provide suitable measurement conditions, for example by adjusting the focal length according to the changing distance to the measurement point.

[0127] In this way, a set of accurate surface details or distance values ​​for a plurality of measurement points can be collected during movement of the probe 4, i.e. without having to pause the movement and perform the measurement when the probe is stationary. In other words: accurate surface or coordinate measurements can be provided on the fly.

[0128] As a result, effective surface information of the measuring points is determined by the optical probe 41. The effective surface information can be provided as specific image data or distance values ​​for each measuring point, as coordinates of the measuring points, or as image data, distance values ​​or coordinates about the entire measuring platform 2 or at least the object 20, in particular in the form of a point cloud.

[0129] Figure 2Another typical embodiment of a CMM 10 according to the present invention is shown. The CMM 10 is implemented as an articulated arm coordinate measuring machine (AACMM), here a SCARA type machine. The CMM 10 shown is placed next to a conveyor system 200, wherein the workpiece 20 travels towards the next station. The CMM 10 includes arm segments 35 and joints 36, in particular motorized joints, to provide relative lateral movement of the probe 4 relative to the workpiece 20. The joints 36 can provide more rotational degrees of freedom. The joints 36 of these arm segments 35 provide a machine structure for moving the probe 4.

[0130] exist Figure 2 In the embodiment shown, the distance between the probe 4 and the workpiece 20 is set by a distance adjustment arm section 33 (z-push rod), which is configured to set the distance. The CMM 10 should be understood as a specific embodiment of an AACMM. Of course, there are many additional but different embodiments known to those skilled in the art. It should be understood that the present invention also covers these alternatives.

[0131] The probe head 4 in the embodiment shown is mounted on a distance adjustment arm section 33. The probe head 4 carries an optical probe 41 comprising a variable focus lens 42 to provide accurate distance measurement over a defined measurement range (in the z direction) by varying its focal length.

[0132] The CMM 10 also includes an optical overview sensor unit having two cameras 51 and 52. The first camera 51 is arranged on a movable arm segment. Therefore, the position of the first camera 51 can be derived by a coding or position detection element of the CMM 10. The second camera 52 is mounted on the base of the CMM 10 and cannot be moved relative to the CMM base. Both cameras 51 and 52 are oriented so that their fields of view cover a common part of the conveyor system 200 on the right side of the base of the CMM 10.

[0133] The optical overview sensor unit is used to obtain surface data related to the conveyor system 200 and the workpiece 20 based on the images captured by the two cameras using stereo photogrammetric processing of the image data. The stereo baseline and relative pose of each of the cameras 51 and 52 can be derived based on measuring the position of the first camera 51.

[0134] Due to the movement of the workpiece 20, surface information can be determined with different viewing angles, which allows deriving a point cloud representing the (almost) entire workpiece 20. The movement of the probe 4 and the setting and changing of the focus of the optical probe can be controlled separately based on the surface information collected with the optical sensor unit.

[0135] The control of the movement of the probe head 4 and the setting and changing of the focal length of the optical probe 41 can be applied on the fly while the workpiece 20 is moved on the conveyor system 200. It is not necessary to stop the conveyor system 200 at any point in this measurement process, i.e. neither for capturing images with the cameras 51 and 52 nor for accurately measuring or capturing one or more measurement points at the workpiece 20 by the optical probe 41.

[0136] Figure 3 The motion of an optical probe 41 of a CMM according to the invention for accurately measuring a surface 21 of an object is shown. A plurality of measurement points 22 to be measured are shown. The measurement points 22 are related to the course and / or shape of the surface 21 and can be understood to provide a measurement path.

[0137] In contrast to measurement methods known from the prior art, here the distance between optical probe 41 and surface 21 is not adjusted according to the course of surface 21 to remain essentially constant, but rather varies, wherein the measurement distance to be applied is provided by changing the focal length of the variable focus lens of optical probe 41.

[0138] For example, to measure the measurement point 22a, the distance between the optical probe 41 and the surface 21 corresponds to the distance d1, and the variable focal length lens is controlled to provide a focal length corresponding to d1. When approaching or reaching the measurement point 22b, the focal length is adjusted according to the distance d2 between the optical probe 41 and the surface 21 at the measurement point 22b, while the optical probe 41 moves at a constant level, i.e. the z coordinate of the optical probe 41 (relative to the CMM or the base of the CMM) remains unchanged. Thus, by (only) changing the focal length, an appropriate focus of the measurement light can be provided for each measurement point 22.

[0139] The coarse coordinates (auxiliary distance information), in particular the coarse height information, of each measurement point 22 are provided to the CMM to appropriately control the variable focus lens for precise measurement. Such information is derived before performing precise measurement. The coarse coordinates can be derived based on the method described above, i.e. with the aid of an optical overview sensor unit arranged at the machine structure and configured to acquire surface information.

[0140] Figure 4 Shown according to Figure 1 The CMM is a coordinate measuring machine 1. Figure 1 Compared with the CMM of FIG. 1 , the CMM here includes a plurality of marks 61 on the measuring platform. The marks 61 may be provided by drilled holes in the platform or may be provided by drawings.

[0141] The position of each marker 61 may be accurately known in the CMM coordinate system. The position of the marker 61 may be determined by measuring using a tactile or optical sensor mounted to the CMM probe.

[0142] The markings 61 and their positions may be used to calibrate an optical overview sensor unit 50 (eg comprising a camera or at least two cameras) arranged at the CMM.

[0143] For calibration, images of the calibration target 62 are captured from different poses relative to the calibration target 62. The images are captured by the optical overview sensor unit 50, wherein the optical overview sensor unit 50 may be arranged at different positions, for example by providing the optical overview sensor unit 50 externally relative to the target, or by arranging the calibration target 62 on the base 2 (as shown in the figure) and moving the frame 3. Based on the image information provided by the images, intrinsic parameters of the optical overview sensor unit are calculated, for example by bundle adjustment.

[0144] In a next step, the optical overview sensor unit 50 captures images covering at least a portion of the base 2 and at least a portion of the calibration target 62. These images are used to derive extrinsic parameters of the camera 50 relative to the calibration target 62. Furthermore, the position of the marking 61 (relative to the calibration target 62) covered by the image is determined in the image based on image processing.

[0145] Since the position of the mark 61 is exactly known in the CMM coordinate system, in a next step the coordinate systems of the optical overview sensor unit 50 and of the CMM 1 are referenced (aligned) relative to each other based on the mark coordinates (positions).

[0146] Alternatively or additionally, the markings 61 and their positions may be used for accurate measurement of an object arranged at the CMM base 2. To this end, methods similar to aerial photogrammetry may be applied on a smaller scale.

[0147] The optical overview sensor unit 50 can be moved along a precise trajectory using the CMM machine structure 3 and encoder readings. Images can be taken at regular intervals during the movement. A 3D map of the base 2 and a 3D map of the objects on the base 2 can be generated based on the images.

[0148] The markers 61 in the image can be used as some kind of "ground control points". This also allows online calibration of the camera extrinsics. As a result, relative measurements between the base 2 and the object to be measured are provided.

[0149] With this approach, the absolute accuracy of the measurement can be improved since camera drift, in particular drift with respect to camera extrinsic parameters (angle and position), can be compensated by processing the position of the markers, and since the platform plane (and therefore the position of the markers) has essentially zero systematic error.

[0150] Figure 5 The movement of an optical probe 41 of a CMM according to the invention for accurately measuring a surface 21 of an object is shown. Furthermore, the change in the focal length of a variable focal length lens of the probe is shown.

[0151] The optical probe 41 is moved in the x-direction over the surface 21 of the object at a constant z-level (i.e. the z-position of the optical probe 41 does not change). However, the focal length of the variable focus lens changes when moving the probe 41. The change in focal length is provided by applying a defined signal function to an actuator of the probe 41.

[0152] The defined signal function may be in the form of a sinusoidal function as shown here, but may also be any other well-defined function. In particular, the signal may provide a periodic variation (eg by a sawtooth function) or an aperiodic variation.

[0153] As shown here, the focal length oscillates around a focal length corresponding to the distance d3 between the optical probe 41 and the surface 21. The focal length follows a sine function 44, which can be defined as following the variation of the distance d3 to the surface 21, such that the variation of the focal length relative to the distance to the surface is substantially the same in both z directions (positive and negative). Alternatively, as shown here, the focus variation remains constant when the probe is moved, wherein the variation is preferably arranged such that the distance d3 to the surface 21 is between a maximum and a minimum of the varied focal length.

[0154] Focus wobbling is applied to determine, for example, the distance to the measuring point 22. The FOVs of several consecutive images overlap, for example 45 - 47. The combination of focus wobbling and overlapping images provides that the measuring point 22 is imaged in multiple acquisitions with different focus settings (indicated by 45, 46 and 47).

[0155] The clearest image can be selected for further image processing and determination of, for example, the 2D coordinates (x and y coordinates) of the measuring point 22. Furthermore, the sharpness can be determined as a function of the z distance, for example by contrast evaluation. This allows the distance of the measuring point 22 in the z direction (one-dimensional z coordinate) to be calculated (interpolated) with high accuracy.

[0156] Although the present invention has been described above in part with reference to some specific embodiments, it must be understood that various modifications and combinations of different features of the embodiments are possible. All these modifications are within the scope of the appended claims.

Claims

1. A coordinate measuring machine (1, 10) for measuring a plurality of measuring points (22, 22a, 22b) on a surface (21), the coordinate measuring machine (1, 10) comprising: ■ Base (2); ■ a probe head (4) having an optical probe (41); ■ a machine structure (3) having structural components (31, 32, 33, 35, 36) for connecting the probe (4) to the base (2); ■ at least one drive mechanism for providing mobility of the machine structure (3) and the probe (4) relative to the base (2); ■ at least one encoding unit configured to provide position data about the position of the machine structure (3) and / or the probe head (4) relative to the base (2); ■ an optical overview sensor unit (50) arranged at the machine structure (3) and configured to acquire surface information, the optical overview sensor unit (50) providing a field of view that is significantly larger than the field of view of the optical probe (41); as well as ■ a control and processing unit adapted at least to control the drive mechanism, It is characterized in that ■The optical probe (41) comprises: □ Optical detection units, especially sensors; □ A variable focal length lens (42) capable of modifying its focal length; an actuation unit constructed and arranged relative to the variable focal length lens (42) such that it provides for setting and changing the focal length in dependence on a control signal, ■ The control and processing unit comprises a measuring function configured to derive auxiliary distance information and effective surface information of a measuring point (22, 22a, 22b), wherein the auxiliary distance information is derived in the following manner: □ controlling the optical overview sensor unit (50) so as to acquire surface information associated with at least one measuring point of the plurality of measuring points (22, 22a, 22b); □ determining the auxiliary distance information associated with the at least one measuring point based on the surface information, and ■Wherein, the effective surface information is derived in the following manner: □ deriving a control signal based on the auxiliary distance information; □ providing an adjusted measuring state of the optical probe (41) by applying the control signal to the actuation unit and thereby providing a distance-dependent focal length; and □ The effective surface information of the at least one measuring point is determined by means of the optical probe (41) in the adjusted measuring state.

2. The coordinate measuring machine (1, 10) according to claim 1, wherein: The measurement functionality is configured such that deriving the auxiliary distance information comprises: ■ controlling the optical overview sensor unit (50) so as to acquire second surface information associated with a second measurement point of the plurality of measurement points (22, 22a, 22b); and ■ Determining second auxiliary distance information associated with the second measuring point based on the second surface information.

3. The coordinate measuring machine (1, 10) according to claim 2, wherein: The measurement functionality is configured such that deriving the effective surface information comprises: ■ deriving a second control signal based on the second auxiliary distance information; ■ providing a second adjusted measuring state of the optical probe (41) by applying the second control signal to the actuation unit and thereby providing a distance-dependent focal length; and ■ Determining second effective surface information of the second measuring point by means of the optical probe (41) in the second adjusted measuring state.

4. A coordinate measuring machine (1, 10) according to any one of the preceding claims, wherein: The measurement function is configured to perform the derivation of the auxiliary distance information before deriving the effective surface information.

5. A coordinate measuring machine (1, 10) according to any one of the preceding claims, wherein: The measuring function is configured to: arrange the optical overview sensor unit (50) at a first position relative to the base (2) by means of the drive mechanism; and derive the auxiliary distance information based on the surface information acquired at the first position.

6. The coordinate measuring machine (1, 10) according to any one of claims 1 to 4, wherein: The measuring function is configured as follows: ■ continuously arranging the optical overview sensor unit (50) in at least a first position and a second position relative to the base (2) by means of the drive mechanism; ■ The auxiliary distance information is derived in the following way: □ Acquiring surface information related to at least one of the plurality of measurement points (22, 22a, 22b) at least at the first position and the second position; and □ Determining the auxiliary distance information associated with the at least one measuring point based on the surface information acquired at least at the first position and the second position.

7. The coordinate measuring machine (1, 10) according to claim 6, wherein: The measuring function is configured as follows: ■ deriving position data about at least the first position and the second position of the optical overview sensor unit (50) relative to the base (2) by means of the encoding unit; ■ assigning corresponding position data to corresponding surface information of at least the first position and the second position of the optical overview sensor unit (50); and ■ determining the auxiliary distance information by processing the position data and the surface information, ■ In particular, among them: □ The optical overview sensor unit (50) comprises at least one camera, in particular two cameras; and □ The auxiliary distance information is provided as a height map by photogrammetric processing.

8. A coordinate measuring machine (1, 10) according to any one of the preceding claims, wherein: ■ the base (2) comprises a set of reference points (61) with known positions, in particular, the reference points are provided by the threads of the measuring platform; and ■ at least one of the reference points (61) is covered by the acquired surface information and the position of the at least one reference point (62) is processed for determining the auxiliary distance information and / or for calibrating the optical overview sensor unit (50), In particular, external parameters of the optical overview sensor unit (50) are calibrated or updated.

9. A coordinate measuring machine (1, 10) according to any one of the preceding claims, wherein: The control and processing unit comprises an auxiliary measurement function configured to provide the auxiliary distance information by: ■ moving the optical overview sensor unit (50) over the surface (21); ■ acquiring surface information related to the surface (21) at a plurality of positions provided by moving the optical overview sensor unit (50); and ■ determining auxiliary distance information related to the surface (21) based on the surface information, wherein the auxiliary distance information comprises specific distance information for each of the plurality of measurement points (22, 22a, 22b), In particular, the auxiliary measurement function is a subfunction of the measurement function and is implemented by the measurement function.

10. A coordinate measuring machine (1, 10) according to any one of the preceding claims, wherein: The control and processing unit comprises an effective measurement function configured to provide the effective surface information by: ■ moving the optical probe (41) over the surface according to a measurement path, wherein the optical probe (41) successively approaches a first measurement point and a second measurement point; ■ receiving or deriving a first control signal associated with the first measurement point and a second control signal associated with the second measurement point; ■When approaching the first measuring point, □ providing a first adjusted measuring state of the optical probe (41) by applying the first control signal to the actuation unit; and □ determining the effective surface information of the first measuring point by means of the optical probe (41) in the first adjusted measuring state, and ■When approaching the second measuring point, □ providing a second adjusted measuring state of the optical probe (41) by applying the second control signal to the actuation unit; and □ Determining the effective surface information of the second measuring point by means of the optical probe (41) in the second adjusted measuring state.

11. The coordinate measuring machine (1, 10) according to claim 10, wherein: ■The effective measurement function is constructed as follows: □ continuously moving the optical probe (41) over the surface (21) according to the measurement path; and □ during the movement of the optical probe (41), in particular during the movement of the optical probe (41), providing the application of the first control signal and / or the second control signal and the determination of the effective surface information of the first measuring point and / or the second measuring point, and / or ■The effective measurement function is constructed as follows: □ providing an inclination of the optical axis of the optical probe (41) relative to a normal associated with the first measurement point and relative to a normal associated with the second measurement point; □ Capture at least one oblique image as effective surface information for each measurement point and cover common features with the oblique images, each oblique image providing a different definition for the common features; □ Determining the optimal definition value of the common feature based on the oblique image; □ Deriving distance information based on a control signal applied for capturing said oblique image and based on said optimal resolution value.

12. A coordinate measuring machine (1, 10) according to any one of the preceding claims, wherein: The optical probe (41) comprises a telecentric lens system, and the control and processing unit comprises a distance measurement function configured to derive distance data of at least one of the plurality of measurement points (22, 22a, 22b) by: ■ moving the optical probe (41) over the surface according to the auxiliary distance information; ■ varying the focal length during movement of the optical probe (41) by providing the control signal as a modified control function; ■ capturing a set of at least three images as the effective surface information during the movement of the optical probe (41); wherein changing the focal length and capturing the images are adapted to each other such that each image covers at least one of the plurality of measurement points (22, 22a, 22b) and each image provides a different focus level for the at least one of the plurality of measurement points (22, 22a, 22b); ■ determining a range image from the set of at least three images by using image processing with a contrast criterion; and ■ The distance is derived based on the focal length applied for capturing the range image, in particular by interpolating the focal length applied for capturing the set of at least three images.

13. A controller for a coordinate measuring machine (1, 10), the coordinate measuring machine (1, 10) comprising: ■ a base (2) having a set of reference points (61) with known positions, in particular provided by the threads of the measuring platform; ■ a probe head (4) having an optical probe (41); ■ a machine structure (3) having structural components (31, 32, 33, 35, 36) for connecting the probe (4) to the base (2); ■ at least one drive mechanism for providing mobility of the machine structure (3) and the probe (4) relative to the base (2); ■ at least one encoding unit configured to provide position data about the position of the machine structure (3) and / or the probe head (4) relative to the base (2); as well as an optical overview sensor unit (50) arranged at the machine structure (3) and configured to acquire surface information, the optical overview sensor unit (50) providing a field of view that is significantly larger than the field of view of the optical probe (41), The controller is at least configured to control the driving mechanism. It is characterized in that The controller is further configured to control: ■ capturing at least one reference image by means of the optical overview sensor unit (50), the reference image covering at least one of the reference points (61) and at least a part of a calibration target (62) arranged on the base (2), ■ calculating extrinsic parameters of the optical overview sensor unit (50) relative to the calibration target (62) by processing the at least one reference image; ■ deriving a sensor image position of at least one of the reference points (61) based on the at least one reference image, the sensor image position being provided as coordinates in a camera coordinate system of the optical overview sensor unit (50); ■ receiving or deriving a reference position of said at least one of said reference points (61), said reference position being provided as a coordinate in a CMM coordinate system of said coordinate measuring machine (1, 10); and ■ Referencing the camera coordinate system and the CMM coordinate system by processing the sensor image positions and the reference positions.

14. A method for calibrating an optical overview sensor unit (50) of a coordinate measuring machine (1, 10) according to any one of claims 1 to 12, wherein: The method comprises: ■Providing a calibration target (62); ■ taking a set of images of the calibration target (62) from different poses by means of the optical overview sensor unit (50); ■ deriving extrinsic parameters of the optical overview sensor unit (50) by processing the set of images; ■ providing the calibration target (62) on the base (2) of the coordinate measuring machine (1, 10), wherein the base (2) is provided as a measuring platform with a set of reference points (61) having known positions, in particular the reference points being provided by threads of the measuring platform; ■ capturing at least one reference image covering at least a portion of the calibration target (62) and at least one of the reference points (61); ■ calculating extrinsic parameters of the optical overview sensor unit (50) relative to the calibration target (62) by processing the at least one reference image; ■ deriving a sensor image position of at least one of the reference points (61) based on the at least one reference image, the sensor image position being provided as coordinates in a camera coordinate system of the optical overview sensor unit (50); ■ receiving or deriving a reference position of said at least one of said reference points (61), said reference position being provided as a coordinate in a CMM coordinate system of said coordinate measuring machine (1, 10); and ■ Referencing the camera coordinate system and the CMM coordinate system by processing the sensor image positions and the reference positions.

15. A computer program product which, when executed by a computing unit and / or a controller, causes the steps of the method according to claim 14 to be automatically performed and controlled.

Images