Underactuated tendon-driven fingers are a simple, yet effective solution, for realizing robotic grippers and hands. The lack of controllable degrees of actuation and precise sensing is compensated by the deformable structure of the finger, which is able to adapt to the objects to be grasped and manipulated, and also to implement grasping strategies based on environmental constraint exploitation. One of the main drawbacks of these robotic fingers is that, due to the limited number of actuators, they can only realize a limited number of movements. Finger closure motion realized by activating the tendon depends on finger mechanical properties, and in particular on elastic joint stiffness. In this paper, we introduce a passive elastic joint to be implemented in monolithic fingers in which the stiffness can be actively regulated by applying a pre-compression to the structure, controlled by a twisted-string actuator (TSA). The paper describes the working principle of the joint, investigates the relationship between pre-compression and flexural stiffness, and finally shows its application to a robotic finger composed of three phalanges.

Nowadays, especially with the Covid-19 pandemic, researchers are focusing their attention on the remote delivery of devices designed for rehabilitation purposes, allowing people to recover without the physical presence of a doctor. Manual therapy is a physical treatment that is used by therapists for the treatment of musculoskeletal pain and/or disabilities. The aim of this work is to present HAPP, a new haptic portable device, designed to help patients suffering of different patholohgies, as for instance the Complex Regional Pain Syndrome type-I disease, and more in general to investigate the effects of manual therapy for diseases of the carpus and metacarpus, by mimicking traditional mechanical and rhythmic stimuli characteristics of manual treatments. Its structure consists of a plate oriented by revolute-prismatic-spherical joints, with a rack-pinion mechanism that actuates the end-effector, stimulating the user’s hand palm. We provide details about the device, such as the mechanical design, the mathematical model and a graphical user interface. Preliminary studies in order to evaluate the device force exerted at the user’s palm were carried out.