Mohssen Hosseini

Feedback linearization of variable stiffness joints based on twisted string actuators

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.

Design and implementation of a simple and low-cost optoelectronic force sensor for robotic applications

This paper presents the design and implementation of a force sensor based on optoelectronic components. The sensor is designed for a twisted string actuation module that can be used for a large set of robotic applications where tendon driven mechanisms are required. The proposed sensor measures the force applied by the twisted string actuation system by detecting the deformation of a properly designed compliant structure integrated into the actuation module. The main advantages of the proposed sensor consist in the use of a very compact commercial light fork as a sensing element. This solution is characterized by a very simple assembly procedure and allows a good sensor response in terms of sensitivity, linearity and noise rejection to be achieved using an extremely simple electronics, thereby obtaining in this way a reliable and very cheap sensor that can be easily integrated in actuation modules for robots and can easily adapted to a wide application set. The paper presents the basic working principle and the design of the force sensor. The sensor compliant frame has been verified both by finite element analysis and by experimental measures, and the results have been compared. Moreover, the sensor calibration and the experimental validation have been performed both in static and dynamic conditions.

Modeling and identification of a variable stiffness joint based on twisted string actuators

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.

A Wearable Robotic Device Based on Twisted String Actuation for Rehabilitation and Assistive Applications

The preliminary experimental study toward the implementation of an arm rehabilitation device based on a twisted string actuation module is presented. The actuation module is characterized by an integrated force sensor based on optoelectronic components. The adopted actuation system can be used for a wide set of robotic applications and is particularly suited for very compact, light-weight, and wearable robotic devices, such as wearable rehabilitation systems and exoskeletons. Thorough presentation and description of the proposed actuation module as well as the basic force sensor working principle are illustrated and discussed. A conceptual design of a wearable arm assistive system based on the proposed actuation module is presented. Moreover, the actuation module has been used in a simple assistive application, in which surface-electromyography signals are used to detect muscle activity of the user wearing the system and to regulate the support action provided to the user to reduce his effort, showing in this way the effectiveness of the approach.

Development of sEMG-driven assistive devices based on twisted string actuation

A twisted string actuation module with an integrated force sensor based on optoelectronic components is presented in this paper. This solution is especially suited for very compact, light-weight and wearable robotic devices, such as exoskeletons, but is appropriate for various robotic applications. An in-depth presentation of the proposed actuation module, the description and the basic sensor working principle of the integrated force sensor are portrayed and discussed. Extensive experimental measures have been carried out to verify the actuation module compliant frame design and the related finite element analysis results. Therefore, the proposed actuation module has been used for the implementation of a simple assistive application where surface-electromyographic signals are used to detect the users muscle activity in order to control the support action provided by the actuator, thus reducing his/her effort.

Development of an haptic interface based on twisted string actuators

In this paper, a cable-driven haptic interface able to move in the six-dimensional space, suitable for applications in various robotic scenarios is presented. The device takes advantage of four force-controlled twisted string actuators to generate a linear force along the three Cartesian space dimensions while providing a considerable force-weight ratio and low inertia. The system consists of a frame fixed to the ground, where the twisted string actuation modules are arranged, and by a mechanical interface devoted to the physical connection of the actuators with the forearm of the human operator. This mechanical interface allows to secure the forearm of the user while leaving to her/him the freedom to use the hand to accomplish other tasks, such as teleoperating a robotic gripper. The four twisted string actuators allow to control the three linear DoF of the haptic interface, allowing both Cartesian position and a force regulation. Both the design, the simulation and the preliminary implementation of the haptic interface are presented in this work.

Corrigendum to “A Wearable Robotic Device Based on Twisted String Actuation for Rehabilitation and Assistive Applications”

In the article titled “A Wearable Robotic Device Based on Twisted String Actuation for Rehabilitation and Assistive Applications” [1], in the “4.2. TSA Design Validation” section, the text reading “The maximum deformation with the maximum load of 80N is ΔXmax ≃ 1.08 ⋅ 10 −4 m indicating the compliant frame deformation is within the goal working region Δd < 0.12mm and can be considered linear.” should be corrected as follows. “The maximum deformation with the maximum load of 80N is ΔXmax ≃ 2.34 ⋅ 10 −4 m indicating the compliant frame deformation is within the goal working regionΔd < 0.12mm and can be considered linear.”

Early evaluation of sEMG-driven muscle modelling for rehabilitation and assistive applications based on wearable devices

In this paper, a preliminary identification and evaluation activity about the modeling of the muscle behavior during isometric contraction is reported. The muscle model is based on a second order dynamic system taking as input the surface skin electromyographic signals measured by means of a pair of electrodes placed on the skin of a human subject and providing as output the force generated by the muscle, applied to the limbs through the tendon. Experimental tests have been performed to identify an internal non-linear fitting between the measured sEMG signal and the impulsive contractile element force, in the case of isometric contractions of the biceps brachii. The proposed model is therefore exploited to drive a wearable elbow support device to limit the activity of the users muscle to a maximum desired threshold value. The experimental results show that the proposed model can be effectively used to provide a direct information about the force provided by the muscle and to modulate the support action provided by the wearable device.

Twisted string actuation with sliding surfaces

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.