# Industrial Robot Positioning Performance Measured on Inclined and Parallel Planes by Double Ballbar
## 6 Conclusions
### Possible Further Work  
The LR Mate 200 iC robot used had been calibrated several times in the past, with the application of the so-called Zero position master method. The accuracy of this method is based only on the visual adjustment of the individual joints of the robot to the zero position, using reference marks. Therefore, this calibration method is considered inaccurate, that fact is also pointed out in the Fanuc robot's user manual [40]. The manual emphasizes that the method should only be used in exceptional cases. Such a state of affairs was addressed in the past by refining the calibration, using original calibration data. In connection with the identified influence of the motion of the J4 and J5 joints on the final shape of the polar graph and the overall measurement results, it is possible to look for the cause probably in the still persistent inaccurate calibration of the given robot. Based on this, there is room for further work and repetition of the measurements presented, subject to a more detailed specification of the LR Mate 200 iC robot calibration. There is also room for comparison with a robot of a clearly more accurate calibration than that of the robot used in the experiments described in our paper.  
It is possible to use Renishaw Ballbar to evaluate individual properties of industrial robots defined in ISO 9283 standard. The double ballbar measuring technique is based on periodical measuring of deviation of the real radius from programmed one during the movement in a circular path. It is possible to utilize such a measurement technique for the evaluation of so-called path accuracy (AT according to ISO 9283) and path repeatability (RT according to ISO 9283). Path accuracy characterizes the ability of a robot to move its mechanical interface along the command path in the same direction n times [6]. Path repeatability expresses the closeness of the agreement between the attained paths for the same command path repeated n times [6]. Measured values are important for the calculation of both mentioned properties. A good example is a measurement on the inclined plane using NOFLIP configuration, measurement conditions are included in Table 4 and the polar graph is shown in Figure 10a. In this case, the average radius of ten repeated measurements (clockwise) was between 99.594 mm and 100.201 mm. In order to reliably determine the values of RT and AT, it is necessary to analyze measurement records more and to define the correct methodology of evaluation. The measurements collected with Ballbar 20 software can be too distorted for this purpose, as the software subtracts estimated eccentricity represented by the first harmonics of the measured profile processed by fast Fourier transformation. For evaluation and calculation of the performance parameters of the industrial robot, it would be necessary to get values declared by its manufacturer as they usually declare only unidirectional path repeatability (RP according to ISO 9283). Another possible way to verify results would be to compare it to measurement using a laser interferometer or a laser tracker.  
Conceptualization, Methodology: all authors; Writing--original draft: V.T. and I.Z.; Writing--review and editing: M.C., I.K. and M.S. All authors have read and agreed to the published version of the manuscript. This work was supported by the Slovak Research and Development Agency under the contract No. APVV-16-0283.