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EP-4741757-A1 - CONTROL LOOP PARAMETERIZATION FOR A CMM

EP4741757A1EP 4741757 A1EP4741757 A1EP 4741757A1EP-4741757-A1

Abstract

The invention pertains to a computer-implemented method (100) of setting control parameters for a CMM, the CMM comprising a probe having a sensor to determine at least one coordinate of one or more measurement points on the surface of an object, one or more actuators, and a control unit configured to control the actuators according to pre-defined control parameters to move the probe to a target position, the method comprising controlling, by the control unit, the actuators to excite (120) one or more portions of the CMM; receiving (130) a sensor feedback signal comprising sensor values captured by the sensor while the portions of the CMM are being excited, the sensor values being indicative of an acceleration of the probe; determining (140) a behaviour of the CMM based on the received sensor feedback signal; and computing (150) adapted control parameters for the control unit based on the determined behaviour.

Inventors

  • IMHÄUSER, Kai
  • SUN, Rainer
  • ISELI, CLAUDIO

Assignees

  • Hexagon Innovation Hub GmbH

Dates

Publication Date
20260513
Application Date
20241108

Claims (15)

  1. Computer-implemented method (100) of setting control parameters for a coordinate measuring machine (1), the coordinate measuring machine comprising - a probe having a sensor to determine at least one coordinate of one or more measurement points on the surface of an object, - one or more actuators, and - a control unit configured to control the actuators according to pre-defined control parameters to move the probe to a target position, the method (100) comprising: - controlling, by the control unit, the actuators to excite (120) one or more portions of the coordinate measuring machine (1), wherein exciting a portion of the coordinate measuring machine (1) comprises oscillating or vibrating the portion at one or more known frequencies; - receiving (130) a sensor feedback signal comprising sensor values captured by the sensor while the one or more portions of the coordinate measuring machine (1) are being excited, the sensor values being indicative of an acceleration of the probe while the one or more portions of the coordinate measuring machine (1) are being excited; - determining (140) a behaviour of the coordinate measuring machine (1) based on the received sensor feedback signal; and - computing (150) adapted control parameters for the control unit based on the determined behaviour.
  2. Method (100) according to claim 1, comprising controlling the actuators - to move (110) the probe to a first target position, wherein the actuators are controlled to excite (120) the one or more portions of the coordinate measuring machine (1) while the probe is at the first target position, particularly wherein the first target position is a scanning position, the scanning position allowing the probe to determine at least one coordinate of the at least one measurement point; and/or - to subsequently move (110) the probe to a plurality of target positions, wherein the steps of controlling the actuators to excite (120) the one or more portions of the coordinate measuring machine (1), and of receiving (130) the sensor feedback signal are repeated at each of the plurality of target positions, particularly wherein the plurality of target positions or a subset of the plurality of target positions are scanning positions, each scanning position allowing the probe to determine at least one coordinate of the at least one measurement point.
  3. Method (100) according to claim 1 or claim 2, comprising controlling the actuators to move (110) the probe along a path, wherein - the actuators are controlled to continuously excite (120) the one or more portions of the coordinate measuring machine (1) while the probe moves along the path; and/or - the steps of controlling the actuators to excite (120) the one or more portions of the coordinate measuring machine (1), and of receiving (130) the sensor feedback signal are repeated at each of a plurality of positions along the path, particularly wherein the path is a scanning path, the scanning path allowing the probe to determine at least one coordinate of a multitude of measurement points on the object.
  4. Method (100) according to any one of the preceding claims, wherein the steps of controlling the actuators to excite (120) the one or more portions of the coordinate measuring machine (1), and of receiving (130) the feedback sensor values are repeated - with a plurality of setups of the coordinate measuring machine (1), particularly wherein each setup deviates at least by a configuration of the probe, particularly at least a length of a stylus of the probe; and/or - with a plurality of setups of the object, particularly wherein each setup deviates at least by a pose or shape of the object.
  5. Method (100) according to claim 3 or claim 4, further comprising generating a set of response models based on a multitude of sensor feedback signals, wherein at least one of determining (140) the behaviour of the coordinate measuring machine (1) and computing (150) the adapted control parameters is also based on the set of response models, particularly wherein the response models are frequency response models or time response models.
  6. Method (100) according to any one of the preceding claims, wherein the coordinate measuring machine (1) is one of a multitude of same coordinate measuring machines, wherein adjusting the control parameters comprises adjusting the control parameters for each of the multitude of same coordinate measuring machines.
  7. Method (100) according to any one of the preceding claims, wherein - the sensor is a force sensor configured to determine a force applied to the probe while the probe touches the object; and - the sensor feedback signal comprises sensor values captured by the force sensor while the at least one portion of the coordinate measuring machine (1) is being excited, particularly wherein the method comprises controlling the actuators to move (110) the probe to a scanning position, in which the probe touches a measurement point on the surface of the object, wherein - the actuators are controlled to excite (120) the one or more portions of the coordinate measuring machine (1) while the probe touches the measurement point; and - the sensor feedback signal comprises sensor values captured by the force sensor while the at least one portion of the coordinate measuring machine (1) is being excited.
  8. Method (100) according to claim 7, wherein the probe does not touch an object while the at least one portion of the coordinate measuring machine (1) is being excited.
  9. Method (100) according to any one of claims 1 to 6, wherein - the sensor is a distance sensor configured to determine a distance between the scan probe and a surface of the object; - the method comprises controlling the actuators to move (110) the probe to a scanning position, the scanning position allowing the distance sensor to determine a distance between the scan probe and one or more measurement points on the surface of the object; - the actuators are controlled to excite (120) the one or more portions of the coordinate measuring machine (1) while the distance sensor continuously determines the distance the between the scan probe and the one or more measurement points; and - the sensor feedback signal comprises sensor values captured by the distance sensor while the at least one portion of the coordinate measuring machine (1) is being excited.
  10. Method (100) according to any one of the preceding claims, wherein the coordinate measuring machine comprises one or more encoders configured to provide encoder signals indicating a nominal position of the probe, wherein - the method further comprises receiving encoder signals from the one or more encoders while the at least one portion of the coordinate measuring machine (1) is being excited; and - determining (140) the behaviour of the coordinate measuring machine (1) is also based on the received encoder signals.
  11. Method (100) according to any one of the preceding claims, wherein the control parameters comprise feedback and feedforward control loop parameters, and computing (150) the adapted control parameters comprises generating optimized feedback and feedforward control loop parameters.
  12. Method (100) according to any one of the preceding claims, wherein - the steps of determining (140) the behaviour and of computing (150) the adapted control parameters are performed by the control unit, particularly in real time; or - the sensor values are acquired by the control unit and provided to an external computing unit, particularly together with information about constraints and/or a setup of the coordinate measuring machine, and the steps of determining (140) the behaviour and of computing (150) the adapted control parameters are performed by the external computing unit.
  13. Method (100) according to any one of the preceding claims, wherein the coordinate measuring machine (1) - has a closed-loop scanning mode for measuring unknown objects, wherein computing (150) the adapted control parameters comprises computing (150) adapted control parameters for the closed-loop scanning mode; and/or - has an observer-based open-loop scanning mode for measuring known objects, wherein computing (150) the adapted control parameters comprises computing (150) adapted control parameters for the open-loop scanning mode.
  14. Coordinate measuring machine (1) comprising - a probe having a sensor to determine at least one coordinate of one or more measurement points on the surface of an object, - one or more actuators, and - a control unit configured to control the actuators according to pre-defined control parameters to move the probe to a target position, characterized in that the coordinate measuring machine (1), particularly the control unit, is configured to perform the method (100) according to any one of the preceding claims.
  15. Computer program product comprising program code having computer-executable instructions for performing the method (100) according to any one of claims 1 to 13, particularly when executed on a control unit of the coordinate measuring machine (1) according to claim 14.

Description

The invention pertains to a computer-implemented method of setting control parameters for a coordinate measuring machine (CMM) based on feedback signals from a probe sensor while portions of the CMM are being excited, by oscillating or vibrating the portions at one or more known frequencies. In particular, the method comprises feedback and feedforward control loop parameterization for outer control loops where end-effector sensor data is used as feedback. CMMs are important in various industries, e.g. in production measurement, quality control or reverse engineering. They may be utilized e.g. to determine deviations of the geometry of manufactured products from a design model, in particular to determine whether the deviations are within the manufacturing tolerance. Such measurements are typically carried out automatically or semi-automatically based on a computer generated or operator selected measuring path wherein such measuring path is provided with respect to a design model. Another application of a CMM gaining more prominence is the reverse engineering of an object. In such cases no design model exists, but an operator commands the 3D movement of the probe head by manual steering commands utilizing e.g. a jog box / joystick. Alternatively, the operator may directly steer a handheld sensor. Typically, a CMM comprises a main structure, a probing system, and a data collection and data processingsystem. The main structure usually comprises a set of actuators responsible for positioning the probing system. One widespread example is a three-axis system, as, e.g., disclosed in DE 43 25 347. For instance, the main structure might include a basis with a measuring table and a movable frame. The workpieces might be positioned or mounted on the measuring table. The movable frame is mounted on the basis such that it can be moved along a first axis. The frame comprises an arm mounted such that it can be moved along a second axis perpendicular to the first axis. The probing system comprises the probe head and is mounted on the arm such that it can be moved along a third axis, which is perpendicular to the first and second axes. Such a construction enables the steering of the probe head in all three dimensions allowing to measure the relevant 3D coordinates of an object. Contemporary three-axis systems often further comprise components, e.g. stacked rotary tables, to provide five degrees of freedom (5DoF) regarding the pose of the probe head relative to the workpiece. Another typical embodiment of the CMM is the so-called articulated arm coordinate measuring machine (AACMM). An AACMM comprises a stationary base and an arm comprising multiple arm segments connected by articulations. The articulations provide movability to a movable end of the arm which is opposed to the base and wherein a probe head can be attached. Due to its design principles such a system is less accurate than the above-mentioned 3- or 5-axis system, on the other hand it offers higher flexibility. For instance, EP 2 916 099 B1 discloses such an AACMM instrument. The probing system of the CMM might be based on contact or non-contact techniques. In the first case a mechanical probe, typically realized as a stylus, achieves direct mechanical contact with the workpiece and the probe is guided through a given measuring path, while the endpoint coordinates of the probe are derived from sensor readings regarding the state of the CMM. Non-contact techniques are based on projecting a primary measuring beam on the workpiece and registering a secondary beam emanating from the object surface region. One advantage of non-contact techniques is that damage to the work object is less likely due to the lack of mechanical contact. Furthermore, non-contact methods allow a parallel acquisition of an extended area, unlike to the stylus-based methods where only the coordinates of a single point are registered. To obtain high-quality data the CMM measurement should be performed under steady-state measurement conditions. For contact probes this might be provided e.g. by keeping the contact and/or the friction force between the stylus and the work object within a certain range. Such simple feedback methods are not available for automatically guiding a non-contact probe. This problem is particularly aggravated in the case of a manual measurement of an unknown work object, since in this case it is not even possible to provide a sufficiently good approximation path based on a digital model or previous measurement data. EP 4 386 313 A1 discloses a method for controlling a distance between a non-contact measurement probe head of a CMM and a workpiece during a measurement. EP 3 781 901 B1 discloses dynamically adapting operation of a CMM. For efficient automated control of a CMM, a behaviour of the system should be known, so that the control parameters can be adapted to that behaviour. Known methods for adapting the control loop parameters are mainly based on try and error and so