Collaborative robotics: analysis of influence of the tool and the characteristics of the task on the upper limbs joint angles and task precision

Abstract

Industrial challenges described in the Industry 4.0 projects are focused on the importance of the human in a collaboration with the system and particularly with a robot. In this case, the collaborative robotics situation is analysed. By understanding the constraints and the capabilities of workers, the robot-human collaboration can be designed in accordance with the companies need. Moreover, collaborative robotics, and in particular restrained free physical assistance robot (Cobot), is presented as a possible solution in order to reduce work related musculoskeletal disorders. Scientific literature on this field is very limited and in field solutions have not yet been deployed on a large scale. In this context, the aim of this contribution was to analyse joint angles of the upper limbs and the precision of the task with regard with the tool (traditional grinding tool or cobot) and different characteristics of the task (required force level and movement direction) during industrial grinding. Five professional grinders were asked to perform grinding tasks, on the horizontal plane and two levels of force (F1 = 35N, F2 = 70N). The height of the fixing device of the workpiece was adjustable (to take into account the anthropometry of the subject). The task was exerted at a given speed (10 mm/s). A force plate was used to collect the forces applied by the participants during the experiment and a real time visual feedback (led light) was given to the participant to inform him about the exerted force level. This experiment was carried out under two conditions: manually (traditional grinding using a manual grinding wheel) and assisted by a Cobot. The inter-segmental angles were estimated using a magneto-inertial sensors system. To identify links between the mean of each joint angle of the upper arm (8 degrees of freedom: flexion/extension, abduction/adduction and rotation of the shoulder, flexion/extension and rotation of the elbow, flexion/extension, abduction/adduction and rotation of the hand) and the level of the exerted force and the used tool (traditional grinding tool and cobot), a linear mixed model was performed. The results showed that the tool has a significant influence on the flexion/extension and the rotation of the left shoulder, the rotation of the left and right wrist, the flexion of the left wrist and the rotation of the left elbow. No significant influence of the exerted force was identified. Moreover, there was no interaction between the used tool and the exerted force. These results show that the analysis of the task is of greater importance in the design the tool, for instance the cobot. In conclusion, it is necessary to consider the role of humans as earlier as possible in the design of a tool in order to create the best functional devices.