# Influence of the Approach Direction on the Repeatability of an Industrial Robot
## 3 Description of the Measured Robot  
The ABB IRB1200 5/0.9 industrial robot was chosen for the experiments. It is a compact angular six-axis robot with IP40 protection, which is designed for work in clean indoor spaces, for applications such as packaging, handling, assembly, etc. According to the manufacturer, the repeatability of the manipulator is 0.025 mm and the robot can be set in any orientation at any angle. Table 1 contains the basic technical parameters of the robot. The forward reach of the robot is 901 mm with a load capacity of 5 kg (with the center of gravity at a maximum distance of 100 mm from the flange). The experiments are performed at the workstation with a pair of the ABB IRB1200 robots--the robots are mounted on a pedestal so that the bases of the robots are not in a horizontal position but are inclined by an angle of 40\({}^{\circ}\) in the \(y\)-axis of the coordinate system of each of the robots. This is not a standard mounting position of the robot, in this case, the first axis of the robot is loaded by torque even if it maintains a position (any other than 0\({}^{\circ}\)) or rotates at a constant speed. This fact directly affects the accuracy and repeatability of the robot. The robot was warmed-up before every measurement and kept at a constant temperature as much as possible. This is a standard routine for every precise robot programming task. Before the robot targets are precisely calibrated, the robot is kept in the movement for some time, to warm up its structure in a way it will be warmed during its production  
For measurement, the robot is equipped with payloads of various parameters (Figure 1, Table 2), which simulates real operating conditions, where robots rarely carry minimal mass. There is a black-and-white speckle pattern for subsequent verification of repeatability by the DIC method glued on the front surface of the payload. \begin{table}
\begin{tabular}{c c} \hline \hline
**Position Repeatability** & **0.025 mm** \\ \hline Max. TCP speed & 8.9 m/s \\ Max. TCP acceleration & 36 m/s\({}^{2}\) \\ Acceleration time—1 m & 0.06 s \\ Robot weight & 54 kg \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Basic parameters of IRB1200 5/0.9 [1]. The flowchart in Figure 4 describes the experiment process. The ABB IRB 1200 robot equipped with a cylindrical payload whose TCP lies at the intersection of the front plane of the cylinder and its axis of symmetry cycles continuously to the same point, but from different directions (from different starting points). The movement definition is entered parametrically into the control application (written in C #), which runs on a computer and collects data from the robot. The parameters of the robot during the measurement are contained in Table 3.  
Figure 4: Experiment control system flowchart. Figure 3: The stabilization process of the robot effector, recorded after a stop at the target point. All starting positions within one cycle are located at the same distance from the measured target--the default positions form a spherical surface created according to the specified diameter and density using the principle of spherical Fibonacci lattice [34]. The coordinates (x, y, z) are updated by the control application and correspond to points on the spherical surface with the defined diameter. The orientation at the default positions is not changed. Due to the orientation of the manipulator, only the front hemisphere is considered. The spherical Fibonacci lattice is used as the simplest solution for the even distribution of the defined number of points on a spherical surface. Points are distributed evenly regardless of the diameter. By the even distribution, it is meant that the distance between every two adjacent points is the same. The precise description for every single default position is not necessary, the result should be the suitability of arrival from all the directions to the destination target, shown by a gradient between multiple points.