Sensorless collision detection with safe reaction for a robot manipulator.

Abstract: This thesis is about collision detection and reaction for robot manipulators without joint torque sensors. The method for detecting collisions used by [1], [2] is based on an algorithm that estimates the residual torque i.e. the friction- and external torque in each joint by only using position data from encoders and the currents in each joint on the robot. The dynamic models for the robot in the residual torque estimation algorithm are calculated using the Lagrangian formulation. These models consist of different components that influence the joint torques such as inertia, centripetal acceleration, coriolis acceleration and gravity. By modelling the friction torque in the joints, it is possible to compare the modelled friction torque with the estimated residual torque from the algorithm. The difference between these torques results in an estimation of the external torque acting on each joint. Thresholds for the external torques are experimentally determined, when the external torques exceed these, a collision is detected by the robot. After a collision has been detected, the robot needs to react in a safe way to minimize the risk of damaging the robot and its environment. Collision reaction strategies have been proposed by [3], a few of these are implemented and compared between each other for different cases of collision in this thesis. The collision detection and different collision reactions were implemented on a UR5-e robot from Universal robots. The results showed that the simple reaction of braking the robot and then maintaining the position using position control is the quickest way of stopping the motion of the robot after a collision has been detected. Due to the hazard of someone getting stuck between the robot and the environment the former explained strategies were not chosen as an optimal collision strategy, instead a strategy where the robot brakes and then switches to a compliant control mode showed to be a better solution since the stopping time is only slightly longer but the impulse of the collision is the best of all the different strategies that were tested. Where the previous research in the field of sensorless collision detection that has been referenced in this report only has focused on the method of detecting the collision, this thesis continues the research by applying different reaction strategies on a robot without torque sensors and torque control capabilities. By combining the method for collision detection from [1], [2] together with reaction strategies from [3], this thesis has shown how robot manipulators that do not have expensive torque sensors in each joint can be made safer.