## 4 Discussion  
The reported experiments show that the developed haptic tele-operation system allows for improving the tele-operation experience of the user. The processing of the different point clouds acquired using 3D LIDARs manages to correctly separate the rocks from the grizzly. The haptic controller and control interface manage to provide enough force feedback to the operator to get their attention, so that they can perceive elements close to the end-effector, thus increasing the perception of the environment in the operating area. In addition, the user interface improves the visual perception of the elements, improving the control over the robot, even if the haptic feedback part is not used. The results depicted in Table 2 allow for comparing the different configurations, and in this way the different features that the proposed haptic tele-operation system includes. When comparing configurations, A and B, a significant improvement of the performance of the task is observed when the end-effector controller is used (configuration B). The main reason for this, is that controlling an arm by actuating directly the joints is a difficult task that requires much preparation and practice. This way, controlling the end-effector makes the operation more affordable for new users. When comparing configurations B and C, it can be observed a slight improvement in the case of using a 3D model of the environment (configuration C). The reason for this is that in case B, the operator cannot change the perspective, and therefore, in some cases it is not possible to have a proper visualization to avoid colliding. When the operator uses configuration C, they have the possibility of changing the viewpoint and in this way avoid collisions. Finally, when comparing configurations C and D, a more notorious performance improvement is detected in the case of using haptic feedback. The reason for that is that the operator does not have to worry about colliding with the environment or rocks. This produces that the operator is able to increase the speed, and can move the arm without worrying about collisions. By comparing the different configurations using the hammer tip trajectories (see Figure 19), it is possible to visualize the effect of each configuration on the end effector path. In this aspect, configuration A and B generate a larger deviation when approaching the target, due to the poor perspective that make it more comfortable to first raise the end-effector and then approaching the target from a higher position in order to get a better perspective. In contrast, configurations C and D generate a path that approach the end-effector directly to the target, as they are able to manage the perspective, but then finish with different maneuvers of positioning. In particular, configuration D presents a slight rise over the rock when approaching as a result of haptic feedback that allow the operator to perform fine maneuvers more easily, unlike configuration C which tends to perform wider maneuvers at the final stage of the positioning process. Similar to [9; 10; 11], the results show that using a haptic system improves the performance and safety of performing a certain task using a robot manipulator. In addition to analyzing the number of collisions, our study evaluates the level of precision of the task being executed, and this type of consideration is not studied in other works and it is relevant for the context of this type of task in particular, which is positioning the chisel for breaking rock. In addition, our system uses the virtual environment to generate multi-perceptive views to aid in performing a task. This feature is not analyzed in the other studies describing haptic tele-operation based on point-clouds [9; 10; 11]. It should also be noted that although the development is intended for industrial environments with a real impact hammer, a small-scale robotic arm was used for the laboratory tests, which implies different behaviors due to the different construction of these elements (electric vs. hydraulic).