In manipulation tasks interaction forces are often decomposed to be able to control robustness-reflective and accelerating forces separately. While this decomposition is typically performed for the synthesis of interaction forces to be applied for example in the context of robotic grasping, less attention has been paid to the analysis of measured, human interaction forces. Here, we introduce a physically-motivated bounding constraint, based on the law of energy conservation, and present a new decomposition approach for interaction force analysis with rigid objects. The decomposition extends the intuitive solution known in literature for the two finger grasp by maximizing robustness-reflective forces while respecting the bounding constraint. Advantages of our approach are illustrated in numerical examples and experiments and by comparing it to existing decomposition approaches. In contrast to existing approaches, our new approach is not limited in the number of interaction points and incorporates only individual interaction forces which are physically plausible.

Imitation learning, also known as Programming by Demonstration, allows a non-expert user to teach complex skills to a robot. While so far researchers focused on abstracting kinematic relations, only little attention has been paid to force information. In this work we study imitation learning of human grasping skills from motion and force data. For this purpose a teleoperation system is realized that allows a human to control a simulated robotic hand and to grasp objects in a virtual environment. Haptic rendering algorithms are implemented to calculate interaction forces that occur when touching the virtual object. While learning of fingertip interaction forces is shown to result in physical inconsistency compared to the demonstrations, we show that learning of internal tensions leads to stable reproductions of the demonstrated grasping skill. Obtained results further indicate an enlarged generalisation capability of grasping skills learnt on the basis of motion and force data compared to grasping skills that encode kinematic relations only.