# Industrial Robot Positioning Performance Measured on Inclined and Parallel Planes by Double Ballbar
## 1 Introduction  
The constant increase in automation in industrial areas is also related to the increasing share of industrial robotics applications. According to a press release issued by IFR (International Federation of Robotics), almost two million new robots are expected to be installed in manufacturing plants worldwide between 2020 and 2021 [1]. One of the first steps in creating a robotic application is a selection of a suitable robot according to certain parameters [2]. Key features include workspace size, load capacity, and also the number of controlled axes. Depending on the specific task that will be automated by the industrial robot, the so-called performance criteria are also important. Performance criteria allow a more detailed look at the robot features. One of the best-known performance criteria of industrial robots is the pose repeatability, which expresses the ability of the robot TCP (Tool Center Point) to repeatedly return to a defined position in the same direction [3, 4]. Pose repeatability is also the performance criterion most frequently mentioned in technical data sheets by industrial robot manufacturers [5]. In addition to the performance criterion above, there are many others, all of which are defined in the ISO 9283 standard. This standard defines the individual performance criteria, the method of their calculation, together with the recommended conditions for their measurement [6]. ISO 9283 is further elaborated on in the respective technical report ISO/TR 13309, which aims to provide an overview of metrological methods and measuring equipment for measuring performance criteria [7]. Numerous scientific papers also deal with the issues related to measurement of the performance criteria of industrial robots. In most cases, these measurements are performed using indicators [8, 9, 10, 11], laser trackers [12, 13, 14], vision systems [15, 16, 17], or laser interferometers [18, 19, 20] primarily intended for measuringthe properties of CNC machine tools. These measuring devices satisfy the classifications and methods described in the above technical report to varying degrees. Furthermore, in professional publications, a double ballbar type of a device is experimentally used to measure performance parameters of industrial robots [19; 21; 22; 23], which, same as the laser interferometers, are primarily intended for evaluating the condition of CNC machine tools [24]. At the Department of Automation and Production Systems, several papers have also been published related to the possibilities of using the given measuring equipment in connection with industrial robots [25; 26; 27; 28]. The principle of measurement with the Renishaw Ballbar is based on movement along a circle or an arc with a fixed point of rotation, while a real distance of moving part from the center of the rotation is compared to the ideal one and then the deviations are recorded. Actual measurement utilizes a precise linear encoder [29]. The use of this device in connection with industrial robots is not specified in the standard (ISO 9283) nor in the technical report (ISO/TS 13309) nor in the standard that describes intended usage of such device (ISO 230-4). The necessity for knowledge of real values of performance criteria is important not only in the creation of a new robotic cell, in connection with the selection of a suitable industrial robot, but also in robotic applications where the industrial robot has been performing a given task already for some time. These are mainly applications that are more demanding on performance criteria, their stability, or predictability of their development thru time. Such tasks are, for example, precision assembly, a dimensional inspection that implements 3D scanners or measuring probes, etc. [25; 30]. In such cases, the measurement of performance criteria is necessary to monitor the robot's technical condition or to troubleshoot existing problems.