# Influence of the Approach Direction on the Repeatability of an Industrial Robot
## 7 Conclusions  
The hypothesis that the repeatability of the robot at the target depends, among other things, on the direction of approach to it, has been proven.  
The graphs (Figures 7 and 11) show that the error does not occur randomly (the deviation of the approach to the target was the same regardless of the order in which the starting points were evaluated). By recalculating the results of the measured values of the joint variables to the positioning deviation of the tool center point, the repeatability was determined (inaccuracies that belong to the individual directions of approach at the target).  
Many influences affect the repeatability value. Especially the thermal expansion will always affect any high-precision device. For this reason, the robot was warmed-up before every measurement and kept at a constant temperature as much as possible.  
The data given by the resolvers cannot be used to determine the most accurate direction for an approach to a target. It is caused by the fact, that in these data, the arm and shaft deflection is not contained. Resolver data have been used only to prove the hypothesis. The real accurate data could be given only by the external measurement device, DIC in this case.  
To verify the ascertainment and especially to obtain real and accurate values of robot repeatability, all measurements were replicated with the use of the 3D DIC methodology. The results of these measurements are displayed not only in the form of box-plot graphs but also in 3D maps, that better illustrate the achieved results.  
The dependence of the direction of approach on the repeatability, achievable at the target, was verified at several points. In Measurements 19-24, the target point was shifted by 150 mm in both directions of all the world coordinate system axes. The hypothesis was, therefore, verified not only under different parameters of the robot's movement but also at seven different targets.  
The degree of this dependence varies with respect to other parameters, such as the weight carried or the approach speed. For example, in Measurement 20 (Figures 24 and 25), the difference from the (zero, exact) value is +0.019 (max) and +0.106 (min). If, for example, depending on the nature of the process and the workplace, it is possible to approach from the most advantageous direction, there may be a refinement compared to the least advantageous direction, by 0.087 mm in the order of the total deviation of repeatable accuracy.  
This research presents a new opportunity to increase the repeatability of any robotic arm. According to the knowledge of the surrounding space and its influence on the tool center point deviation, the trajectory could be slightly changed or improved so that the repeatability will be increased. Authors in this research used the DIC cameras to measure the real state, but any external device, which is accurate enough would be used.  
The provided measurement is not permanent, regular calibration will be required as in any other accurate device.  
In a practical application, a high precision assembly task, for example, the procedure would be as follows:  
1. Prepare the robot and external technologies to an operational state;
2. Set DIC cameras, define a control application inputs (orientation of measured target, number of points on sphere, number of repetitions from every single initial point, etc.);
3. Run the robot's movement to the measured target, measure the reached position by DIC;
4. Evaluate the sphere, create the \"3D map\", place it into a figure of workstation, to get an orientation of sphere according to the surrounding installation;
5. With the information of the most and least suitable approaches of direction, re-program the robot motion to get the better repeatability.