Patent Application: US-201514625643-A

Abstract:
the invention relates to 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 .

Description:
the object of this invention is to therefore provide a further development of an arrangement and a method of the type described at the outset that eliminates the above - mentioned drawbacks . the problem is solved as per the invention by having at least two calibration objects taken from different functional operation groups rigidly connected to one another . the crucial advantage of this rigid connection is a maximum increase in the information or efficiency per measurement as follows : the impact point of a laser beam on a sensor of a laser - sensor system provides , in accordance with [ p1 ], two equations for the parameter identification : one each for the x and y coordinates of the impact point in the sensor coordinate system . in contrast , the four rigidly connected beams of the example in fig1 , for instance , provide 4 * 2 = 8 equations per measurement . two of them are dependent upon the remaining six and provide important redundant information . six independent equations are the maximum amount of information available per measurement , because six coordinates unambiguously determine an effector pose . the well - known elementary geometric relationships will not be explained in more detail here . in the case of the example in fig1 , 4 * 4 laser parameters and 2 * 6 sensor parameters — and thus an additional 28 parameters in total — had to be identified up to now during each robot calibration in addition to the robot parameters . if , in contrast , the rigid , relative pose of the calibration objects vis - a - vis one another on their carrier units is precisely determined with highly accurate devices , for instance by the manufacturer of the calibration system before the carrier unit is delivered , only the pose of the two carrier units has to be determined for subsequent robot calibrations , requiring 6 + 6 = 12 parameters . the following important features of the invention result from that : the exact knowledge of the relative poses of the rigidly connected calibration objects provides the following advantages : the efficiency of the calibration process is increased or the process is accelerated because fewer calibration measurements are required : instead of identifying the poses of a number of independent calibration objects relative to one another and to the robot , only the pose of a carrier object ( with known calibration objects ) has to be identified . the period of time for recalibration after a crash , after wear and tear , after a replacement of the robot or its components is shortened because of that , for instance . the resulting mechanism precision increases because fewer parameters have to be identified . the smaller the number of parameters , the less complicated the parameter identification , which requires extremely complex calculations . in particular , all of the mathematical algorithms have a tendency to “ smear ” unavoidable residual errors over a number of parameters . this effect is dampened by the instant invention . practical advantages also arise during the initial installation of the calibration objects and the initial estimation in that objects that are rigidly connected to a carrier unit are easier to handle than several unconnected objects . in particular , the ( re ) installation of the calibration system on the robot in the workcell of the user is substantially curtailed because only a few carrier objects have to be transported , set up in a useful way and subjected to initial estimation . the initial estimation is one of the most time - consuming and difficult operations in the ( re ) calibration . more information can be obtained per measurement . that increases the efficiency of the calibration process and the resulting mechanism precision as follows : several measurements can be recorded in each measurement pose simultaneously in case that several different operation pairs provide simultaneous signals . in the case of n different operation pairs as in fig1 , for instance , a single measurement pose provides the n - fold information in contrast to the earlier situation . the same information as before can therefore be obtained with fewer measurement poses . all of the measurement errors in a given pose can be averaged for each measurement pose . the error averaging and the use of redundancy as in fig1 , for instance , make a reduction of the measurement errors possible and this consequently leads to more consistent , more accurate measurement data , which then leads to an increase in the resulting mechanism precision . that is crucial for the mathematical parameter identification . the initial , one - time , exact determination of the relative poses of the rigidly connected calibration objects increases the efficiency of subsequent ( re ) calibrations : the calibration process becomes more efficient because measurement efforts are saved with every subsequent robot calibration or recalibration of the robot . that is especially crucial for temperature compensation , where in - process calibration has to be performed on a continual basis . the resulting mechanism precision increases because increased precision can be obtained easily by investing extraordinary effort into the initial , one - time determination of the relative poses of the rigidly connected calibration objects ; e . g . these relative poses may be determined by the calibration system producer with a particularly precise measurement device during their initial , one - time determination . the later mechanism calibrations will provide more precise results because of that . if only a single operation pair is used for the measurements in accordance with the original technology in [ p1 ], the robot has to make several movements and broad movements to obtain the same information as it obtains in the case of an implementation in accordance with the invention , e . g . in accordance with fig1 . because of the large yield of information per measurement , expansive movements of the mechanism can be eliminated without losses to the resulting pose accuracy with a corresponding optimization of the calibration measurement poses . the reduction of the required free space is important , because space is usually limited in robot workcells . 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 : fig2 standard calibration system for limited requirements with three rigidly combined sensors on a single carrier unit , fig3 identification of the deviation from linearity in the case of linear joints , fig4 calibration variant with a stationary laser with splitting optics , and fig1 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 fig1 are arranged as a rigidly connected pair of parallel beams in one embodiment . the example in fig2 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 . fig3 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 fig3 are rigid combinations of one laser 3 and one sensor 4 each in fig3 . 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 . in fig4 , 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 . in fig5 , 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 fig1 and those of the type in fig4 are possible in this example . although the measurements are simultaneously taken at the sensors in fig3 , that is not the primary goal for the robot with rotary joints in fig5 . 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 fig5 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 . 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