This paper presents a study of haptic perception of virtual stiffness and the influence of visual feedback in virtual reality. In this study, a novel and lightweight haptic glove (ExoTen-Glove) based on Twisted String Actuation (TSA) system is presented and evaluated. This system has two independent TSA modules with integrated force sensors and small-size DC motors. ExoTen-Glove provides force feedback to the users during the execution of grasping virtual objects. The overall design, the controller and the preliminary experimental evaluation of the ExoTen-Glove have been shown in this paper. Different experiments have been performed in virtual reality environment using HTC VIVE headset with 2 degrees of freedom grasping tasks, squeezing a pair of virtual springs with the participant's thumb and index fingers. The aim of this study is to illustrate the benefit of using ExoTen-Glove to distinguish stiffness of a pair of virtual springs and the role of the visual feedback. The results show that the users use not only haptic cues but also use visual cues in detecting spring stiffness difference.

This paper presents a novel lightweight and simple TSA-based (twisted string actuation) wearable haptic glove (ExoTen-Glove). This system is using two independent twisted string actuators with integrated force sensors and small-size DC motors. The proposed system can provide users force feedback during the execution of grasping virtual objects. The design of the TSA-based ExoTen-Glove, description of the TSA system, the controller and the preliminary experimental evaluation of the proposed system has been presented in this paper. This device has been evaluated by an experiment in virtual reality environment using HTC VIVE headset with 2 degrees of freedom grasping tasks, where the participants were squeezing a real spring with their thumb and index finger and compare it with a virtual spring stiffness. The results prove the applicability of the ExoTen-Glove for rehabilitation and haptic purposes.

In this paper, an ongoing work for verifying the behavior of a twisted string actuator in contact with a sliding surface or guided through a sheath is presented. The twisted string actuation system is particularly suitable for very compact and light-weight robotic devices, like artificial limbs and exoskeletons, since it allows the implementation of powerful tendon-based driving systems, based on small-size DC motors characterized by high speed, low torque and very limited inertia. One of the major limitations of this actuation system is by now related to the fact that the string should not be in contact with any obstacle, because this contact will alter the twisting angle propagation along the string and, eventually, completely stop the string twisting. This design constraint imposes a straight path between the motor and the linear load attached to the other string end. After the presentation of the basic properties of the twisted string actuation system, the model of the twisted string in contact with a sliding surface is discussed. The behavior of the system has been then experimentally verified and discussed. A preliminary evaluation of control strategies for compensating the side effects generated by the contact of the twisted string with the sliding surface is also presented.

In this paper, an ongoing work for the implementation of a variable stiffness joint actuated by a couple of twisted string actuators in antagonistic configuration is reported. The twisted string actuation system is particularly suitable for very compact and light-weight robotic devices, like artificial limbs, exoskeletons and robotic hands, since it renders a very low apparent inertia at the load side, allowing the implementation of powerful tendon-based driving systems, using as actuators small-size DC motors characterized by high speed, low torque and very limited inertia. The basic properties of the twisted string actuation system are firstly presented, and the way how they are exploited for the implementation of a variable stiffness joint is discussed. A simple control algorithm for controlling the joint stiffness and position simultaneously is discussed, and a the feedback linearization of the device is taken into account and validated in simulation.

In this paper, the implementation of a variable stiffness joint actuated by a couple of twisted string actuators in antagonistic configuration is presented. The twisted string actuation system is particularly suitable for very compact and light-weight robotic devices, like artificial limbs and exoskeletons, since it renders a very low apparent inertia at the load side, allowing the implementation of powerful tendon-based driving systems, using small-size DC motors characterized by high speed, low torque and very limited inertia. After the presentation of the basic properties of the twisted string actuation system, the way how they are used for the implementation of a variable stiffness joint is discussed. A simple PID-based motor-side algorithm for controlling simultaneously both the joint stiffness and position is discussed, then the identification of the system parameters is performed on an experimental setup for verifying the proposed model and control approach.