This paper presents two methods for the computation of the null space velocity command in redundant robots. Both these methods resort to the solution of a constrained optimization problem. The first one is a formalization of the traditional Gradient Projection Method (GPM) which guarantees the respect of the joint bounds and a gradual activation/deactivation of the null space command. The second one, called Null Space Basis Optimal Linear Combination Method (NSBM), finds the optimal coefficients of a basis of the null space of the Jacobian, ensuring in turn that the joint bounds are respected and that the null space is activated and deactivated gradually. The two methods are applied to the case study of a welding application in which the null space command must avoid the collision between the robot and an obstacle. The comparison of the results of the case study shows that NSBM performs better than GPM. The proposed algorithms are also tested on a real robotic platform to demonstrate that their computational time is compatible with the real-time requirements of the robot.

This paper presents a system designed to induce, in healthy subjects, artificial hand-tremor that is observed in persons affected by neurological impairments. The objective is to allow a healthy user to feel in first person the effect of the impairment while performing common manipulative tasks in order for her/him to understand and gain empathy with the impaired person. The developed tool is based on a wrist-attached desktop haptic interface with a workspace that is comparable to that of the arm of the user. Such device is able to exert controlled forces on the user's wrist and induce hand-tremor whose frequency and amplitude is correlated with different pathologies. The control of the device is based on the recording and playback of tremor signals acquired by a motion tracker. In this paper, we present the system with its dynamic characteristics and three different types of controller are experimentally tested and compared.

This paper presents the development of an interaction-torque control architecture for a rehabilitation exoskeleton (RehabExos) with flexible joints equipped with internal torque sensors. The architecture consists of an outer control loop, which is based on the kineto-statics of the RehabExos, and on four identical-independent inner full-state joint-torque controllers (IJTFC). The considered IJTFC demonstrate good stability, responsiveness and accuracy in tracking the desired torques generated by the outer controller. Comparison with other control strategies available from the literature is shown via analytical and experimental results, which highlight the improved performances of the proposed IJTFC method in controlling the RehabExos robot.

A linear-quadratic-Gaussian regulator is proposed for the torque control of flexible robotic joints with built-in torque sensor. The regulator requires the joint-torque sensor information only and features: (1) a Kalman filter that, beside reducing the noise and evaluating the derivative of the torque sensor measure, is able to estimate all the external and internal torques acting on the joint; 2) a controller that optimizes system stability, responsiveness, accuracy and effort. The regulator is implemented on a flexible joint of a rehabilitation exoskeleton. Simulation and experimental results are provided which demonstrate regulator performance and efficacy.

A conically shaped Dielectric Elastomer (DE) linear actuator is presented which is obtained by coupling a DE film with a compliant mechanism. The compliant mechanism is designed, by means of a pseudo-rigid-body model, to suitably modify the force generated by the elastomer film. The resulting actuator provides a nearly constant force along the entire actuator stroke when the DE film is activated and returns to an initial rest position when the DE film is deactivated. Experimental activity fully validates the proposed concept. Possible applications of this kind of actuator are Braille cells, light weight robots and haptic devices.

This paper presents an optimization procedure conceived to design parallel mechanisms (PMs) with legs of constant and/or variable length connected, at their endpoints, to a fixed base and a movable platform through universal and spherical joints, respectively. In the proposed procedure, the first natural frequency of the mechanism is the objective function to be maximized. The optimization problem is formulated by using dimensionless variables in order to identify the optimal geometry independently of mechanism size, platform density, and leg cross-sectional area and material. As a case study, the procedure is employed to find the optimal geometry of a 2-DOF spherical PM to be used as an orienting device in future space missions.

This paper deals with a class of over-constrained parallel manipulators that can be obtained from architecturally singular parallel mechanisms with legs of type US (U and S stand for universal and spherical joints respectively) by the addition of legs of type UPS (P stands for actuated prismatic pair). These manipulators can be internally preloaded in order to tune the system stiffness, to diminish system backlash and to control the state of tension of their structural elements. The paper, in particular, addresses the static and stiffness analyses of this class of manipulators. A symmetric stiffness matrix defined as the Hessian of the elastic potential energy of the system is used, as an alternative to the Cartesian stiffness matrix used in this field, to describe the stiffness characteristic of these manipulators. An over-constrained 2-dof parallel spherical wrist is analyzed as a case study.