Position-force combination control with passive flexibility for versatile in-hand manipulation based on posture interpolation

In-hand manipulation is often needed to accomplish a practical task after grasping an object. In-hand manipulation of variously sized and shaped objects in multi-fingered hands without dropping the object is challenging. In this paper we suggest a combined strategy of force control and passive adaptation through soft fingertips with simple interpolation control to achieve in-hand manipulation between various postures and with various objects. While passive compliance can be achieved in numerous ways, this paper uses soft skin, as it does not require complex mechanisms and was easy to integrate in the robot hand (Allegro hand). Softness has proven to significantly ease object grasping, and the current paper shows the importance of softness also for in-hand manipulation. In particular, the simple interpolation strategy between various postures is successful when combined with soft fingertips, with or without force control, but fails with hard fingertips. Objects of varying size, shape and hardness were reliably manipulated. While the soft fingertips enabled good results in our experiments, a sufficiently precise definition of the postures and object size was required. When combining the interpolation control with a force control strategy, bigger errors in defining the posture and object size are possible, without deforming or dropping the object, and the resultant force is lower. As a result, we achieved robust in-hand manipulation between various postures and with objects of different size, shape and hardness.