fig1 shows a robot arm 1 comprising seven joints 3 - 9 . the robot arm 1 is connected to a control unit 2 which guides drives ( not shown ) for adjusting the joints 3 - 9 in the robot arm 1 . each drive consists for example of an electric motor which alters the placement of the corresponding joint , wherein a transmission gear is optionally provided . the robot arm 1 also consists of four arm sections 11 , 12 , 13 , 14 and a base part 10 . the base part 10 serves to fasten the robot arm 1 , i . e . the fastening point of the robot arm 1 is situated on the base part 10 . the arm section 14 at the moving end of the robot arm 1 simultaneously forms a holding device 14 for a medical instrument . the spatial position of the holding device 14 can also be referred to as the robot arm position . the arm section 11 can be rotated via the joint 3 in one axis relative to the support part 10 . the arm section 12 can be rotated via the joints 4 , 5 in two axes relative to the arm section 11 . the arm section 13 can be rotated via the joints 6 , 7 about two axes relative to the arm section 12 . the accommodating device 14 can be rotated via the joints 8 , 9 about two axes relative to the arm section 13 . the joints 4 , 5 , 6 , 8 enable a pivoting movement , while the joints 3 , 7 , 9 enable a torsion movement . the holding device 14 serves to accommodate a medical instrument such as for example a biopsy needle or a milling head . the robot arm 1 serves to precisely position and move the medical instrument . due among other things to the material rigidity of the arm sections 11 - 14 and the joints 3 - 9 , a deviation occurs between the robot arm position , and therefore the position of the medical instrument , and the theoretical position which follows from the guidance system for the joints 3 - 9 . this is caused on the one hand by the inherent weight of the robot arm 1 and on the other by external forces acting on the robot arm 1 . the acting forces are ascertained by force sensors ( not shown ) which are for example integrated into the joints 3 - 9 . in order to assess the positional deviations , a rigidity model of the robot arm 1 is stored in the control unit 2 . on the basis of the rigidity model and the acting forces , it is possible to calculate the deviation between the actual robot arm position and the theoretical robot arm position according to the joint placement . the result of this calculation and / or the rigidity alone can be used in various ways . a first option is to predetermine a minimum rigidity and to calculate from this the working space within which the robot arm 1 maintains the required rigidity . when the robot arm 1 is subsequently used , a robot arm position outside of this working space is prevented by the control unit 2 . it is in particular possible to check , before the robot arm 1 is used , whether the working space covered with the desired rigidity contains the space necessary for the application . the desired minimum rigidity is for example input by the operator of the robot arm 1 or automatically chosen on the basis of the application scenario . in accordance with a second option , the anchoring point of the robot arm 1 , i . e . the position of the base part 10 , is calculated from the rigidity and the working space to be traveled , wherein for example different anchoring points are iteratively assumed and the working space covered by the robot arm 1 with the desired rigidity is calculated for each anchoring point . the first anchoring point at which the rigidity is maintained for the whole of a working space to be reached is for example chosen . alternatively , the minimum rigidity for each anchoring point and the working space to be reached is calculated , and the anchoring point with the greatest minimum rigidity is chosen . the minimum rigidity is the rigidity of the robot arm 1 in the position within the working space as a whole for which the rigidity of the arm 1 is at its lowest . in another alternative , the anchoring point with the greatest average rigidity , for which the requirements regarding the minimum rigidity are fulfilled , is chosen . the average rigidity is for example the arithmetical mean of all the rigidities , wherein the rigidities are either weighted equally or are weighted differently for different regions of the working space to be traveled . in accordance with a third option , a warning is output when the robot arm 1 falls below a defined boundary value for the rigidity . if the operator moves the robot arm 1 to a position at which it falls below the boundary value for the rigidity , then a warning tone for example sounds or a warning signal is displayed . in accordance with a fourth option , the joint placement for which defined criteria regarding the rigidity are fulfilled is selected from a number of possible joint placements , in order to reach a robot arm position . this is possible if the number of joints is over - determined , i . e . if a robot arm position can be reached using a number of joint placements . in this case , the optimum joint placement is to be determined . to this end , the number of all possible joint placements is for example determined , and a check is sequentially made for each joint placement as to whether it leads to the desired minimum rigidity . the first joint placement which fulfils the rigidity is then for example selected . alternatively , the rigidity for each possible joint placement is calculated , and the joint placement which achieves the greatest minimum or average rigidity is selected . it is possible within the framework of the present invention to combine individual features of the aforesaid options . it is in particular possible , for example , to respectively calculate the rigidity for different joint placements when determining the anchoring point . computer program elements of the invention may be embodied in hardware and / or software ( including firmware , resident software , micro - code , etc .). the computer program elements of the invention may take the form of a computer program product which may be embodied by a computer - usable or computer - readable storage medium comprising computer - usable or computer - readable program instructions , “ code ” or a “ computer program ” embodied in said medium for use by or in connection with the instruction executing system . within the context of this application , a computer - usable or computer - readable medium may be any medium which can contain , store , communicate , propagate or transport the program for use by or in connection with the instruction executing system , apparatus or device . the computer - usable or computer - readable medium may for example be , but is not limited to , an electronic , magnetic , optical , electromagnetic , infrared or semiconductor system , apparatus , device or medium of propagation , such as for example the internet . the computer - usable or computer - readable medium could even for example be paper or another suitable medium on which the program is printed , since the program could be electronically captured , for example by optically scanning the paper or other suitable medium , and then compiled , interpreted or otherwise processed in a suitable manner . the computer program product and any software and / or hardware described here form the various means for performing the functions of the invention in the example embodiment ( s ). although the invention has been shown and described with respect to one or more particular preferred embodiments , it is clear that equivalent amendments or modifications will occur to the person skilled in the art when reading and interpreting the text and enclosed drawing ( s ) of this specification . in particular with regard to the various functions performed by the elements ( components , assemblies , devices , compositions , etc .) described above , the terms used to describe such elements ( including any reference to a “ means ”) are intended , unless expressly indicated otherwise , to correspond to any element which performs the specified function of the element described , i . e . which is functionally equivalent to it , even if it is not structurally equivalent to the disclosed structure which performs the function in the example embodiment ( s ) illustrated here . moreover , while a particular feature of the invention may have been described above with respect to only one or some of the embodiments illustrated , such a feature may also be combined with one or more other features of the other embodiments , in any way such as may be desirable or advantageous for any given application of the invention .
a method for assessing the positioning accuracy of a medical robot arm comprising at least one joint , wherein the rigidity of the robot arm in a joint placement is calculated and assessed on the basis of a rigidity model .