In the recent years, a clear trend towards simplification emerged in the development of robotic hands. The use of soft robotic approaches has been a useful tool in this prospective, enabling complexity reduction by embodying part of grasping intelligence in the hand mechanical structure. Several hand prototypes designed according to such principles have accomplished good results in terms of grasping simplicity, robustness, and reliability. Among them, the Pisa/IIT SoftHand demonstrated the feasibility of a large variety of grasping tasks, by means of only one actuator and an opportunely designed tendon driven differential mechanism. However, the use of a single degree of actuation prevents the execution of more complex tasks, like fine pre-shaping of fingers and in-hand manipulation. While possible in theory, simply doubling the Pisa/IIT SoftHand actuation system has several disadvantages, e.g. in terms of space and mechanical complexity. To overcome these limitations we propose a novel design framework for tendon driven mechanisms, where the main idea is to turn transmission friction from a disturbance into a design tool. In this way the degrees of actuation can be doubled with little additional complexity.
By leveraging on this idea we design a novel robotic hand, the Pisa/IIT SoftHand 2. We present here its design, modeling, control, and experimental validation. The hand demonstrates that by opportunely combining only two degrees of actuation with hand softness, a large variety of grasping and manipulation tasks can be performed only relying on the intelligence embodied in the mechanism. Examples include rotating objects with different shapes, opening a jar, pouring coffee from a glass.