Arrangement and Method for the Model-Based Calibration of a Robot in a Working Space

Abstract

An arrangement for the model-based calibration of a mechanism in a workspace with calibration objects that are either directed laser radiation patterns together with an associated laser radiation-pattern generator or radiation-pattern position sensors. Functional operation groups made up of at least one laser radiation pattern and at least one position sensor interact in such a way when a radiation pattern impinges on the sensor that measured sensor position information values are passed along to computing devices that determine the parameters of a mathematical mechanism model with the aid of these measured values. In the process, at least two different functional operation groups are used to calibrate the mechanism, and at least two calibration objects from different functional operation groups are rigidly connected to one another.

Claims

1. A device for model-based calibration of a mechanism, said device comprised of functional operation groups that each include a set of calibration objects made up of one or more lasers, each emitting an individual laser beam or a bundle of individual laser beams, one or more laser beams as projected light patterns and one or more sensors, each with a two dimensional light sensitive surface, wherein, when one of the one or more laser beams strikes one of the one or more sensors, said one of the one or more sensors transmits measured values with position information of one or more laser light images formed by the projected light patterns on the two-dimensional light-sensitive surface to computing devices that determine parameters of a mathematical mechanism model with the aid of said measured values, wherein at least two different functional operation groups are used to calibrate the mechanism, and at least two calibration objects from different functional operation groups are rigidly connected to one another.

2. The device according to claim 1, wherein two or more calibration objects from different functional operation groups are rigidly connected via a carrier unit.

3. The device according to claim 1, wherein at least two calibration objects are rigidly connected and fastened to a carrier unit in a predetermined spacing range or a predetermined orientation range relative to one another, wherein the range limits are determined by the manner in which the specific arrangement is realized and, when a robot is used with the device, by the type of robot, by the size of the robot, by the specific task that the robot is supposed to carry out, by the size of a workspace section in which high precision is required and by a user-specific weighting of position and orientation errors.

4. The device according to claim 1, wherein a subset of the calibration objects is made up of stationary calibration objects and all of the calibration objects that are stationary are mounted on a single carrier unit.

5. The device according to claim 1, wherein a subset of the calibration objects is made up of stationary calibration objects and calibration measurement values of at most two sensors among the sensors that are stationary are recorded and transmitted to the computing unit.

6. The device according to claim 1, wherein a subset of the lasers is made up of rigidly connected lasers and two or more laser beams from lasers that are rigidly connected are arranged in a set, wherein at least one said laser beam is nearly parallel with at least one other said laser beam.

7. The device according to claim 1, wherein the two dimensional light sensitive surface receives two dimensional light patterns.

8. The device according to claim 7, wherein the two dimensional light pattern received by the two dimensional surface is made up of two or more points in a two dimensional pattern.

9. The device according to claim 7, wherein the two dimensional light pattern received by the two dimensional surface is made up of lines in a two dimensional pattern.

10. The device according to claim 7, wherein the two dimensional light pattern received by the two dimensional surface is made up of crosses in a two dimensional pattern.

11. A method for the model-based calibration of a robot in a workspace with several calibration objects and computing devices in accordance with claim 1, comprised of the steps: rigidly connecting at least two calibration objects from different functional operation groups, precisely identifying and recording information on the relative poses of the rigidly connected calibration objects with respect to one another, storing the information on the relative poses of the rigidly connected calibration objects with respect to one another, and obtaining more accurate subsequent mechanism calibrations with this information.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Various embodiments of this invention will be described in more detail below with the aid of the drawings. The following are shown in the figures:

[0063] FIG. 1 Arrangement with maximum information per measurement,

[0064] FIG. 2 Standard calibration system for limited requirements with three rigidly combined sensors on a single carrier unit,

[0065] FIG. 3 Identification of the deviation from linearity in the case of linear joints,

[0066] FIG. 4 Calibration variant with a stationary laser with splitting optics, and

[0067] FIG. 5 Initial estimation of heterogeneous calibration object combinations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0068] FIG. 1 shows an implementation in accordance with the invention with a carrier unit 5 on the effector 6, to which four simple lasers 3 are attached in a rigid pose relative to one another, and a reference object that is comprised of a carrier unit 5 with two rigidly connected sensors 4. There are a total of 4*2=8 different functional operation groups. Four laser-light points are obtained on the light-sensitive surface 7 of the sensor 4 in suitable (calibration) measurement poses of the effector. The amount of effector poses in which all four beams hit a sensor is limited. The prerequisite for successful mechanism calibration, however, is a wide range of the most diverse measurement poses. To combine the requirements for a maximum amount of information per measurement and for a wide range of calibration measurement poses in an optimal fashion, the measurement series are designed in such a way that the sensors are hit by as many laser beams or radiation patters as possible in a few measurement poses, and are hit by fewer beams or by only one laser beam in the most extreme case in other additional measurement poses that result from an optimization of the measurement series. Two laser beams in FIG. 1 are arranged as a rigidly connected pair of parallel beams in one embodiment.

[0069] The example in FIG. 2 shows an effector laser with a stationary carrier unit 5 with three sensors 4 and cross optics that project a cross-shaped radiation pattern 9 onto sensors. The single carrier unit 5 in the example can be easily transported and quickly installed. If the relative poses of the sensors are precisely measured vis-a-vis one another in advance, the carrier unit is suitable for being a length standard with high error attenuation because of the large spacing between the sensors. Only one sensor is irradiated in each case in all of the measurement poses of the mechanism. The calibration method proposed here and the method in [P1] as well as laser-sensor systems in general do not require the unambiguous reconstruction of the respective pose of the effector or of the effector objects from the measured values that are obtained in one measurement pose. Partial information with regard to the respective effector pose from a single measurement is sufficient for a perfect parameter identification which is obtained from the mathematical evaluation of the totality of all measurements.

[0070] FIG. 3 shows a linear or translational joint 10 that stands in the place of more complex mechanisms with several linear joints, e.g. gantry robots or machine tools. Linear joints have slight deviations from linearity in practice that have to be identified and compensated for. Both effector objects and reference objects in FIG. 3 are rigid combinations of one laser 3 and one sensor 4 each in FIG. 3. For the purpose of more efficient calibration, the lasers as per the figure are aligned in a nearly parallel fashion with the joint axis and the sensors are positioned in such a way that both of them can be hit by the respective laser during the entire joint movement. The information yield is twice as high as in the technology according to [P1]. The maximum information of six equations per measurement can be obtained with a third calibration object pair that is likewise aligned in parallel with the joint.

[0071] In FIG. 4, a laser with splitting optics 8 that emits several beams 2 at different angles is mounted in a stationary fashion at the edge of the workspace and a carrier unit 5 with two rigidly connected sensors 4 is mounted on the effector 6. An exchange of the effector object and the reference object in this example results in a different calibration variant than the preceding examples with other advantageous characteristics. The two sensors can be simultaneously hit by different beams of the laser in some of the calibration measurement poses.

[0072] In FIG. 5, a laser 3 is rigidly connected to a sensor 4 in each case, both on the effector 6 and in a stationary fashion in the workspace. Both calibration measurements of the type in FIG. 1 and those of the type in FIG. 4 are possible in this example. Although the measurements are simultaneously taken at the sensors in FIG. 3, that is not the primary goal for the robot with rotary joints in FIG. 5. In this case, the rigid connection above all supports the initial estimation or the initial identification of the pose of the calibration objects, as follows. Let us assume, for instance, that the user puts the reference object 3, 4, 5 in FIG. 5 into the workspace with a position from the laser to the sensor that is precisely measured in advance. As soon as the position of the sensor is determined in the robot coordinate system, the position of the laser that is rigidly connected with it can be calculated immediately afterwards. The poses of the reference objects relative to the robot base, and of the effector objects relative to the effector, have to be determined in an approximate fashion in laser-sensor systems before practical calibration measurement series can be calculated in which the laser really hits the sensor.

BACKGROUND LITERATURE

[0073] [Dynalog] see: www.dynalog.com [0074] [Gatla] C. S. Gatla, R. Lumia, J. Wood, G. Starr, An Automated Method to Calibrate Industrial Robots Using a Virtual Closed Kinematic Chain, IEEE TRANSACTIONS ON ROBOTICS, Vol. 23, No. 6 (2007). [0075] [Höerbach] J. M. Höllerbach, “The Calibration Index and Taxonomy for Robot Kinematic Calibration Methods,” Int. J. Robot. Res., Vol. 15, No. 12, pp. 573-591 (1996). [0076] [P1] U.S. Pat. No. 6,529,852 B2, Knoll et al., Method and Device for the Improvement of the Pose Accuracy of Effectors on Mechanisms and for the Measurement of Objects in a Workspace, 2001 [0077] [P2] Patent FR 2729236 A1, Thomson Broadband Systems, Robot Positioning in Three-Dimensional Space by Active Lighting, 1995 [0078] [P3] Patent Application WO 2010094949 A1, Demopoulos, Measurement of Positional Information for a Robot Arm, 2010 [0079] [P4] Patent DE 202005010299 U1, Beyer, Measurement Device for Use with Industrial Robots has Two Cameras Fixed in Given Angular Relationship and Focused on Reference Object, and has Universal Adapter Plate, 2005 [0080] [P5] Patent Publication U.S. Ser. No. 02/011,0280472 A1, Liu Lifeng et al., System and Method for Robust Calibration between a Machine Vision System and a Robot, 2010 [0081] [Schröer] K. Schröber, Identifikation von Kalibrationsparametern Kinematischer Ketten [Identification of Calibration Parameters of Kinematic Chains]. Hanser Verlag, 1993

LIST OF REFERENCE NUMERALS

[0082] 1. Robot [0083] 2. Radiation pattern (point image) [0084] 3. Laser (radiation-pattern generator) [0085] 4. Sensor (radiation-pattern position sensor) [0086] 5. Carrier unit [0087] 6. Effector [0088] 7. Light-sensitive sensor surface [0089] 8. Laser with splitting optics [0090] 9. Radiation pattern (cross-shaped image) [0091] 10. Linear joint