In this contribution the problem of vibration control is studied on the basis of a fundamental oscillatory system consisting of a mass spring system and an additional mass. The proposed control strategy couples the orbits of the two masses such that both masses stop, while simultaneously stabilizing the second mass to a desired equilibrium. Using a coordinate and input transformation, the control strategy is directly transferred to an n-link manipulator mounted on a base with linear translational stiffness. Using semidefinite Lyapunov functions and a conditional stability argument, it is shown that the proposed control strategy damps out base vibrations, while additionally achieving a desired configuration in the task-space. Finally, the proposed method is compared to a state-of-the-art approach using numerical simulations.

This paper tackles a friction compensation problem without using a friction model. The unique feature of the proposed friction observer is that the nominal motor-side signal is fed back into the controller instead of the measured signal. By doing so, asymptotic stability and passivity of the controller are maintained. Another advantage of the proposed observer is that it provides a clear understanding for the stiction compensation which is hard to be captured in model-free approaches. This allows to design observers that do not overcompensate for the stiction. The proposed scheme is validated through simulations and experiments.

Kinematic redundancy is a characteristic and beneficial property in collaborative robots nowadays as it enhances the flexibility and dexterity of the system. While the robot is manipulating an object, it is often necessary to kinematically reconfigure the robot, for example, when it obstructs the human. For this, internal or so-called null space motions can be carried out which do not affect the main task. In general, it is desirable that the human coworker can anticipate how the robot will move at any time. However, for null space motions this is typically not the case as they are non-intuitive and not suitable for interaction. In this work, we develop intuitive null space interaction behaviors for redundant manipulators, where the human can easily guide the robot. We want to provide users with a tool, that is straightforward to implement and solves real-world problems effectively. Two practical applications for an eight- and ten-DOF robot demonstrate the performance of the proposed method.

Ensuring safety is a primary goal in physical human-robot interaction. In various collision experiments it was found that the robot's effective mass, velocity, and geometry are the key parameters which influence the human injury severity during an impact. Recently, a velocity controller was proposed that limits the robot speed to a biomechanically safe value, taking into account the mass and the curvature in the direction of movement for a given point of interest. The mass and the geometry depend on the mechanical design, however, the effective mass also depends on the robot configuration. In this paper, we exploit the redundant degree(s) of freedom of a joint torque controlled seven- and eight-DOF robot to minimize the effective mass without affecting the desired Cartesian end-effector trajectory and with the goal to improve the performance of the safe velocity controller at the same time. Given recent results in robotics injury analysis, we analyze when such a redundancy resolution scheme actually improves safety. For the considered robots, we find reflected mass extrema that can be obtained by null space motions, and propose a real-time, torque-based redundancy resolution scheme, which is finally verified in experiments.