Patent Description:
Coordinate measuring devices, such as stationary coordinate measuring machines (CMM) or portable articulated arm coordinate measuring machines (AACMM) or laser-based coordinate measuring devices including laser trackers, laser scanners and total stations are used in a wide variety of applications in quality management and quality assurance. Conventionally, highly precise CMM need to be very stable in order to withstand inertial distortions that may arise due to its own operating weight and - especially since fast measurements are also desirable - its movements. Conventional CMM are thus very heavy devices that are complicated to move and cannot be installed everywhere, e.g. due to weight-loading restrictions.

It would thus be desirable to provide a light-weight CMM that still allows highly-precise measurements.

It is therefore an object of the present invention to provide an improved CMM which is less heavy than conventional CMM.

It is a further object of the present invention to provide such a CMM that allows determining spatial coordinates with high precision.

It is a further object of the present invention to provide such a CMM that may have light-weight and flexible structural components.

It is a further object of the present invention to provide such a CMM having a distortion-compensation functionality.

It is a further object of the present invention to provide such a CMM, wherein the CMM can be portable, handheld and/or battery-operated.

At least one of these objects is achieved by the CMM of claim <NUM>, the method of claim <NUM> and/or the dependent claims of the present invention.

A first aspect of the invention pertains to a CMM for determining at least one spatial coordinate of a measurement point on an object, the CMM comprising a structure movably connecting a probe head to a base, the structure comprising a plurality of rotary joints and a plurality of elongate components, the components comprising a plurality of links. At least one of the rotary joints movably connects two of the components with each other, comprises a driving unit comprising a motor to actuate the connected components relative to another, and comprises a measuring unit comprising one or more sensors to determine at least one angle between the connected components and to generate angular data. The CMM comprises a control unit configured to control the motor of each driving unit for driving the probe head relative to the base for approaching the measurement point, to receive the angular data, and to determine the at least one spatial coordinate of the measurement point based on the angular data. The control unit has access to distortion information about distortions occurring in the components and/or joints under a multitude of different distortion-influencing conditions of the structure.

According to some embodiments of the CMM, these conditions comprise at least a current pose of the structure that is defined by the angles between the components, the distortion information comprising pose distortion information for a multitude of different poses of the structure. In these embodiments, the control unit is configured.

According to some embodiments of the CMM, the conditions comprise at least one or more current accelerations of the structure that are a consequence of a motorized movement of the components, wherein the distortion information comprises acceleration distortion information for a multitude of different movements of the structure. In these embodiments, the control unit is configured.

According to some embodiments of the CMM, the conditions comprise at least a current temperature distribution in the structure, wherein the distortion information comprises thermal distortion information for a multitude of different temperature distributions in the structure. In these embodiments, the control unit is configured.

For instance, one or more temperature sensors may be provided at each link or at each component, the temperature data being generated by these temperature sensors.

According to some embodiments of the CMM, the amount of pose distortion is at least partly a consequence of gravity, wherein the distortion information comprises the pose distortion information for a multitude of different poses of the structure under the influence of a multitude of different gravitational values. In these embodiments, the control unit is configured.

For instance, the control unit may be configured to receive position data related to a location of the CMM and to determine the current location of the CMM based on the position data. The location in particular may be a geographic location, e.g. comprising longitude and latitude values as well as a height.

In some embodiments of the CMM, the distortion information relates to distortions occurring in each of the links.

In some embodiments of the CMM, the elongate components further comprise the base and/or the probe head.

According to some embodiments of the CMM,.

Optionally, the first material or material composition has a lower overall density than the second material or material composition.

The first material composition for instance may comprise aluminium or other light metals, light metal alloys, ceramics, plastics and/or carbon-fibre-reinforced polymers. The second material or material composition for instance may comprise high-alloy steel or a comparable material, e.g. coated low-alloy steel. The second material should provide a uniform coefficient of thermal expansion (CTE) along the whole metrology chain (which includes bearings and sensor interfaces) and a high Young modulus for high stiffness with low form factor.

In some embodiments of the CMM, the measuring unit and the driving unit of each rotary joint are thermally decoupled from each other and/or are provided in separate housings.

In some embodiments of the CMM, at least one rotary joint comprises a measuring unit that includes at least two rotary encoders as angular sensors, each rotary encoder being configured to determine a relative pose between a first link and a second link with at least three degrees of freedom.

In some embodiments of the CMM, at least one measuring unit is configured to determine relative poses between two links in at least five degrees of freedom.

In some embodiments of the CMM, the distortion information is provided as part of a digital model of the CMM.

A second aspect of the invention pertains to a computer-implemented method for controlling a CMM - e.g. the coordinate measuring machine according to the first aspect of the invention - to determine at least one spatial coordinate of a measurement point on an object to be measured, wherein the CMM comprises a structure movably connecting a probe head to a base, the structure comprising a plurality of rotary joints and a plurality of elongate components, the components comprising a plurality of links, each rotary joint movably connecting two of the components with each other, comprising a driving unit with a motor to actuate the connected components relative to another, and a measuring unit with one or more angular sensors to measure at least one angle between the connected components and to generate angular data.

The method, which may be performed by a control unit of the CMM, comprises controlling the motors for driving the probe head relative to the base for approaching the measurement point, and receiving the angular data.

According to some embodiments of the method, the conditions comprise at least a current pose of the structure that is defined by the angles between the links, the distortion information comprising pose distortion information for a multitude of different poses of the structure. In these embodiments, the method comprises determining a current pose distortion based on the pose distortion information and on the angular data. Determining the current overall distortion of the structure is then based at least on the current pose distortion.

According to some embodiments of the method, the conditions comprise at least current accelerations of the structure that are a consequence of a motorized movement of the components, wherein the distortion information comprises acceleration distortion information for a multitude of different movements of the structure. In these embodiments, the method comprises determining a current movement or acceleration of the structure, particularly based on the angular data, and determining a current acceleration distortion based on the acceleration distortion information and on the current movement. Determining the current overall distortion of the structure is then based at least on the current acceleration distortion.

According to some embodiments of the method, the conditions comprise at least a current temperature distribution in the structure, wherein the distortion information comprises thermal distortion information for a multitude of different temperature distributions in the structure. In these embodiments, the method comprises receiving temperature data, determining a current temperature distribution of the structure based on the temperature data, and determining a current thermal distortion based on the thermal distortion information and on the current temperature distribution. Determining the current overall distortion of the structure is then based at least on the current thermal distribution.

According to some embodiments of the method, the amount of pose distortion is at least partly a consequence of gravity, wherein the distortion information comprises the pose distortion information for a multitude of different poses of the structure under the influence of a multitude of different gravitational values. In these embodiments, the method comprises receiving position data related to a location of the coordinate measuring machine, determining a current location of the coordinate measuring machine based on the position data, and determining a gravitational value for the current location. Determining the current pose distortion is then also based on the gravitational value for the current location.

A third aspect of the invention pertains to a computer program product comprising program code which is stored on a machine-readable medium, or being embodied by an electromagnetic wave comprising a program code segment, and having computer-executable instructions for performing the method according to the second aspect of the invention, particularly when executed in a control unit of a CMM according to the first aspect of the invention.

The invention in the following will be described in detail by referring to exemplary embodiments that are accompanied by figures, in which:.

<FIG> shows an exemplary embodiment of a CMM <NUM> according to the invention. The CMM <NUM> is configured for determining spatial coordinates of measurement points on an object <NUM>. It comprises a structure movably connecting a probe head <NUM> to a base <NUM>.

In the shown example, the structure of the CMM <NUM> comprises three rotary joints 20a, 20b, 20c and three links 10a, 10b, 10c. The rotary joints 20a-c movably connect the links 10a-c with each other and with the base <NUM>. A first rotary joint 20a provides movability of a first link 10a relative to the base <NUM> about the two axes of rotation R1 and R2. A second rotary joint 20b provides movability of a second link 10b relative to the first link 10a about the axis of rotation R3. A third rotary joint 20c provides movability of a third link 10c relative to the second link 10b about the two axes of rotation R4 and R5.

Each of the joints 20a-c comprises an actuator for moving the connected components relative to another, and a measuring unit with sensors for determining one or more angles between the connected components. A control unit <NUM> of the CMM <NUM> is configured to receive angular data related to the measured angles from the measuring units, to control the actuators for driving the probe head <NUM> relative to the base <NUM> for approaching the measurement point on the object <NUM>, and to determine spatial coordinates of the measurement point based on the angular data.

The CMM <NUM> is built light-weight. To save weight, the structure may be built so flexible that that its different distortions due to different poses of the structure alone lead to significant deviations of the probe head <NUM> from its assumed position so that the measured coordinates may deviate from the real coordinates in such a way that, conventionally, the CMM <NUM> could not be used for highly-precise measurements. Especially the links 10a-c may have a material composition and/or be constructed in a manner that are not considered stable enough, i.e. too flexible, for conventional measurement with a CMM. For instance, the links 10a-c may be made from light metals, particularly aluminium, light metal alloys, ceramics, plastics and/or carbon-fibre-reinforced polymers.

<FIG> illustrates different distortions of the structure of the CMM of <FIG> due to different poses of its arm-like structure. In a first pose 50a of the structure (elements shown with dashed lines) the probe head is at a first position 51a. For a movement <NUM> of the probe head to a second position 51b, the structure assumes a second pose 50b, by changing the angles at each of the rotary joints 20a-c. This new pose 50b leads to a different load borne by each of the links. Here, this is illustrated for the first link 10a only. The new pose 50b leads to a load 60b to be borne by the first link 10a that his higher than the load 60a in the first pose 50a. This effects a distortion <NUM> in the first link 10a that differs from the previous distortion during the first pose 50a. Due to these distortions, the real positions 51a, 51b of the probe head differs from the assumed positions 52a, 52b. Also, the difference between the real position 51a and the assumed position 52a in the first pose 50a is smaller than the difference between the real position 51b and the assumed position 52b in the second pose 50b. To overcome this lack of precision due to inertial distortions <NUM>, according to the invention the CMM <NUM> comprises a functionality to compensate these distortions.

<FIG> show two exemplary embodiments of a joint <NUM> of a CMM according to the invention. As shown here, a joint <NUM> movably connects two of the links 10a, 10b. Additionally (not shown here), one of the joints may movably connect one of the links 10a, 10b with a base of the CMM, and another one of the joints may movably connect another one of the links 10a, 10b with a probe head of the CMM. In both embodiments of <FIG>, the joint <NUM> comprises a driving unit <NUM> having a motor to actuate the two links 10a, 10b that are connected by the joint <NUM> relative to one another. Also, in both embodiments, the joint <NUM> comprises a measuring unit <NUM> having one or more angular sensors to determine at least one angle between the two links 10a, 10b.

In the example of <FIG>, the measuring unit <NUM> and the driving unit <NUM> are provided together, for instance together in a single housing. This allows for a compact setup.

In the example of <FIG>, the measuring unit <NUM> and the driving unit <NUM> are provided thermally decoupled from each other, for instance in separate housings. This reduces the heat transfer between the two components, particularly from the motor of the driving unit <NUM> to the sensors of the measuring unit <NUM> and the structural elements of the joints. At least one heating element may be is provided at the joint <NUM> to active control the temperature at the joint, particularly at the measuring unit <NUM>.

<FIG> schematically shows a system <NUM> for distortion compensation. In the shown example, the system <NUM> comprises the control unit <NUM> of the CMM of <FIG>. The control unit <NUM> is configured to receive angular data related to the measured angles from the measuring units <NUM> of each rotary joint 20a-c and to control the actuators <NUM> of each rotary joint 20a-c for driving the probe head relative to the base for approaching a measurement point on an object to be measured. Based on the received angular data, spatial coordinates of the probe head (and, thus, the measurement point) can be derived. However, due to distortions occurring in the structure - for instance dependent on a current pose of the structure -, the derived spatial coordinates may differ from the real coordinates in a relevant manner, i.e. to an extent that it negatively influences the measuring precision and provides incorrect values.

Thus, according to the invention, the control unit <NUM> may have access to distortion information, i.e. information about distortions (or information that enables evaluation of distortions) occurring in the structure under a multitude of different distortion-influencing conditions. This information may be pre-determined and provided in a data base <NUM>. Using this information, the control unit <NUM> can reconstruct these distortions and then compute the correct coordinates of the probe head and the measurement point. Optionally, further sensors may be connected and provide their data to the control unit <NUM>, e.g. temperature sensors <NUM> and/or a GNSS sensor <NUM>.

The distortion-influencing conditions comprise at least a current pose of the structure, i.e. the combination of measured angles between the components. In every pose of the structure, each of the links bears a different load and is thus subject to a different distortion (unless the CMM <NUM> would be used in zero-gravity environments). Thus, distortion information for a multitude of different poses of the structure need to be provided and accessible by the control unit <NUM>. The control unit <NUM> then determines a current pose of the structure based on the angular data received from the measuring units <NUM>, accesses the distortion information for the current pose from the data base <NUM>, determines a current overall distortion based on the distortion information and on the current pose, and determines the spatial coordinate of the measurement point based on the based on the angular data and on the determined distortion of the structure.

The distortion information may be generated using an archetype of the machine, assuming that each machine that is built with the same components will have the same distortion. Alternatively, the distortion information may be generated for each CMM individually. For generating the distortion information, the structure is brought sequentially into a multitude of different poses, e.g. hundreds or thousands of poses, wherein for each pose a distortion is determined. This may comprise a target-performance comparison, for instance determining a deviation of the measured coordinates from actual coordinates of a measurement point and setting the difference between these values as the overall distortion. To each of the multitude of poses the overall distortion is assigned.

For instance, each of the multitude of poses, for which an overall distortion is determined, may differ from the next poses by increments of <NUM>° about at least one of the axes of rotation R1-R5. For values in-between the increments, the overall distortion may be computed, e.g. by extrapolation. Values for extrapolated overall distortion then may be provided in the data base. Alternatively, the control unit may be configured to perform the extrapolation.

Additionally, the overall distortion may comprise accelerational distortions that are proportional to the second derivative of the pose. Accelerational distortions are a result of movements of the structure, i.e. for assuming a certain position in order to measure a certain measurement point. It is possible to wait at the assumed position before measuring the points - and thus to avoid accelerational distortions to influence the measurement of the CMM. However, it would be advantageous to increase the measurements by not having to wait. Therefore, the distortion information preferably also comprises information relating to accelerational distortions occurring from a multitude of different movements, including an extent of the distortion and a duration of the distortion, e.g. as an increase and decrease of the distortion, in response to the movement.

Distortion generally also depends on a temperature of the structural elements. Therefore, optionally, the control unit <NUM> may be configured to consider thermal distortions when calculating an overall distortion. The system <NUM> or the CMM may comprise one or more temperature sensors <NUM> that generate temperature data which is received by the control unit <NUM>. Optionally, especially if there are more than one temperature sensors, the control unit <NUM> knows where each of the sensors <NUM> is positioned, so that an actual temperature distribution within the structure of the CMM may be derived. The control unit <NUM> accesses the distortion information for the current temperature or temperature distribution from the data base <NUM>, and determines the current overall distortion based also on this distortion information and on the current temperature. To further improve the thermal behavior of the CMM, preferably all structural parts in the measurement chain of the machine (i.e. structural parts of at least all links 10a-c and all joints 20a-c) have the same or basically the same coefficient of thermal expansion (CTE) along the metrology chain (e.g., all joints have the same axial CTE, whereas the radial CTE is less important). Preferably, the CTE is within +-<NUM>% of variation along the structure (+-3PPM at 10PPM of absolute CTE value of steel), particularly within +-<NUM>%. To achieve the uniform CTE, the same isotropic material may be used for all structural parts. Alternatively, different materials having the same CTE might be used, for instance a stainless steel and a carbon-based composite that is designed to have the same CTE as the used stainless steel.

Moreover, gravity is not the same everywhere on earth. For instance, gravity generally increases towards the poles and decreases with height ("normal gravity"). Thus, the effect of the poses on the distortion may vary to a degree that negatively effects the accuracy of the compensation. For instance, depending on the pose, the distortion of a CMM located in Alaska may be somewhat different than that of the same CMM located in Singapore. Also, gravity anomalies can be found all over the planet; for instance, the composition of the ground at the location of the CMM may influence the value of gravity so that it significantly deviates from normal gravity. For extremely precise applications also tidal effects might be considered by calculating the current positions of celestial bodies, particularly Moon and Sun, e.g. based on the location of the CMM and the exact time and date.

The local value of gravity may be inquired by a user of the CMM and provided to the control unit as a user input, so that the control unit can use it for calculating the overall distortion. Alternatively, a location of the CMM may be received as user input or detected by a sensor of the system or CMM, e.g. a GNSS sensor <NUM>. The normal gravity for that location may then be calculated. Also, a local value of gravity may be accessed by the control unit, e.g. from its internal data base <NUM> or via the Internet, e.g. from a data base provided online by the manufacturer of the CMM.

<FIG> illustrates the use of a digital model of the CMM for deriving the position of the probe head and a calibration of the digital model using a reference system. Encoders and other sensors of the CMM allow observations by the CMM, which includes actually measured angles and optionally also other values. Applying a digital model of the CMM may lead to conclusions of the CMM based on its observations, i.e. what we actually want to know, such as the position of the probe head. The digital model comprises CMM parameters, i.e. quantitative descriptions of the CMM, e.g. including the lengths of the links, and a simulation software that provides qualitative descriptions of the CMM, i.e. including the kinematics. The distortion information of the structure may be provided as part of the digital model, particularly as part of the CMM parameters.

The simulation of the CMM can be performed with varying level of details depending on the required performance. For instance, the following options can be used:.

Calibration of the CMM to improve the conclusions of the CMM may involve a reference system including an independent metrological device, such as a laser tracker measuring the position of the probe head. The conclusions of the reference system, i.e. about the probe head's position, are the same as those of the CMM but are considered to be correct. Thus, if there is a deviation between the conclusions of the CMM and the reference system regarding a probe head position, the conclusions of the CMM are considered to be incorrect. A calibration software tunes the CMM parameters until the conclusions match, i.e. until the position of the probe head is determined correctly by the CMM. This tuning may include the distortion information.

<FIG> and <FIG> show two flow charts illustrating two exemplary embodiments of a method <NUM> according to the invention for controlling the CMM to determine spatial coordinates of measurement points on an object to be measured. The method for instance may be performed by the control unit of the CMM. The method <NUM> comprises controlling <NUM> motors of rotary joints of the CMM for driving the probe head relative to the base in order to approach a measurement point with the probe head. The method further comprises receiving <NUM> angular data from sensors of the rotary joints. The angular data may be received <NUM> continuously, e.g. during movements of the actuators. The received angular data allows determining a current pose of the structure, the pose being defined by the angles.

In the embodiment shown in <FIG>, the method then continues with receiving <NUM> pose distortion information, which is provided for a multitude of different poses of the structure, e.g. in a data base of the control unit, and with determining <NUM> a current pose distortion based on the received angular data (or on a determined pose) and the accessed pose distortion information. Optionally (not shown here), distortion information about distortions occurring in the components under a multitude of different distortion-influencing conditions (or circumstances) may be accessed, the current pose of the structure being only one of these conditions (or circumstances).

Based at least on the current pose distortion - and optionally distortions from other distortion-influencing conditions - a current overall distortion of the structure can be determined <NUM>. Finally, spatial coordinates of the measurement point can be determined <NUM> based on the angular data and on the current overall distortion of the structure.

The method <NUM> may then continue with controlling <NUM> the actuators to approach the next measurement point, repeating the shown steps until spatial coordinates of all measurement points have been determined <NUM>.

In the embodiment shown in <FIG>, the method <NUM> comprises the same steps as in the embodiment of <FIG>. Additionally, further distortion-influencing conditions are considered for determining the overall distortion.

These further distortion-influencing conditions comprise a current acceleration of the structure, for instance due to the movements of the structure to approach a measurement point. Controlling <NUM> these movements may be based on continuously received <NUM> angular data. If the distortion due to acceleration is not considered during these movements, the probe head is at the wrong position until the acceleration is stopped and the acceleration distortion returns to "normal". Thus, in order to prevent providing wrong coordinates, it is necessary to stop the movement for some time at every measurement point, thereby slowing down the measuring process. The method <NUM> of <FIG> thus comprises determining <NUM> a current acceleration of the structure. Preferably, this includes determining current accelerations for each link of the structure separately. The acceleration may be derived based on the change in the angular data. Alternatively, acceleration sensors may be provided on the structure. Then, acceleration distortion data for the determined acceleration(s) is received <NUM>, e.g. by accessing a database. Based on the acceleration distortion data, a current acceleration distortion is determined <NUM>, and the current overall distortion is determined <NUM> also based on the current acceleration distortion.

The further distortion-influencing conditions also comprise a temperature distribution in the structure. Depending on the used materials, temperature changes may lead to relevant distortions in the structure. The method <NUM> of <FIG> thus comprises receiving <NUM> temperature data from one or more temperature sensors and determining <NUM>, based thereon, a current temperature distribution. If the positions of the sensors on the structural elements are known, based on the known pose a 3D distribution of the sensors can be derived. Based on the known 3D distribution of the sensors and on the temperature values each sensor provides, the temperature distribution can be deduced, e.g. including interpolation between sensor positions. Then, thermal distortion data for the determined temperature distribution is received <NUM>, e.g. by accessing a database. Based on the thermal distortion data, a current thermal distortion is determined <NUM>, and the current overall distortion is determined <NUM> also based on the current thermal distortion.

Claim 1:
Coordinate measuring machine (<NUM>) for determining at least one spatial coordinate of a measurement point on an object (<NUM>), the coordinate measuring machine (<NUM>) comprising a structure movably connecting a probe head (<NUM>) to a base (<NUM>), the structure comprising a plurality of rotary joints (20a-c) and a plurality of elongate components, the components comprising a plurality of links (10a-c), wherein at least one rotary joint
- movably connects two of the components with each other,
- comprises a driving unit (<NUM>) comprising a motor to actuate the connected components relative to another, and
- comprises a measuring unit (<NUM>) comprising one or more sensors to determine at least one angle between the connected components and to generate angular data,
wherein the coordinate measuring machine (<NUM>) comprises a control unit (<NUM>) configured
- to control (<NUM>) the motor of each driving unit (<NUM>) for driving the probe head (<NUM>) relative to the base (<NUM>) for approaching the measurement point,
- to receive (<NUM>) the angular data, and
- to determine the at least one spatial coordinate of the measurement point based on the angular data,
characterized in that
the control unit (<NUM>) has access to distortion information about distortions occurring in the components and/or joints under a multitude of different distortion-influencing conditions of the structure, wherein the conditions comprise at least a current pose (50a-b) of the structure that is defined by the angles between the components, the distortion information comprising pose distortion information for a multitude of different poses (50a-b) of the structure, the control unit (<NUM>) being configured
- to receive (<NUM>) pose distortion information for the current pose (50a-b),
- to determine (<NUM>) a current pose distortion based on the pose distortion information for the current pose (50a-b), the pose distortion being a distortion of the structure due to the pose (50a-b),
- to determine (<NUM>) a current overall distortion of the structure based at least on the current pose distortion, and
- to determine (<NUM>) the at least one spatial coordinate also based on the current overall distortion of the structure.