Nevio Luigi Tagliamonte

A Novel Compact Torsional Spring for Series Elastic Actuators for Assistive Wearable Robots

The introduction of intrinsic compliance in the actuation system of assistive robots improves safety and dynamical adaptability. Furthermore, in the case of wearable robots for gait assistance, the exploitation of conservative compliant elements as energy buffers can mimic the intrinsic dynamical properties of legs during locomotion. However, commercially available compliant components do not generally allow to meet the desired requirements in terms of admissible peak load, as typically required by gait assistance, while guaranteeing low stiffness and a compact and lightweight design. This paper presents a novel compact monolithic torsional spring to be used as the basic component of a modular compliant system for series elastic actuators. The spring, whose design was refined through an iterative FEA-based optimization process, has an external diameter of 85 mm, a thickness of 3 mm and a weight of 61.5 g. The spring, characterized using a custom dynamometric test bed, shows a linear torque versus angle characteristic. The compliant element has a stiffness of 98 N·m/rad and it is capable of withstanding a maximum torque of 7.68 N·m. A good agreement between simulated and experimental data were observed, with a maximum resultant error of 6%. By arranging a number of identical springs in series or in parallel, it is possible to render different torque versus angle characteristics, in order to match the specific applications requirements.

Design and Characterization of a Novel High-Power Series Elastic Actuator for a Lower Limb Robotic Orthosis

A safe interaction is crucial in wearable robotics in general, while in assistive and rehabilitation applications, robots may also be required to minimally perturb physiological movements, ideally acting as perfectly transparent machines. The actuation system plays a central role because the expected performance, in terms of torque, speed and control bandwidth, must not be achieved at the expense of lightness and compactness. Actuators embedding compliant elements, such as series elastic actuators, can be designed to meet the above-mentioned requirements in terms of high energy storing capacity and stability of torque control. A number of series elastic actuators have been proposed over the past 20 years in order to accommodate the needs arising from specific applications. This paper presents a novel series elastic actuator intended for the actuation system of a lower limb wearable robot, recently developed in our lab. The actuator is able to deliver 300 W and has a novel architecture making its centre of mass not co-located with its axis of rotation, for an easier integration into the robotic structure. A custom-made torsion spring with a stiffness of 272.25 N·m·rad–1 is directly connected to the load. The delivered torque is calculated from the measurement of the spring deflection, through two absolute encoders. Testing on torque measurement accuracy and torque/stiffness control are reported.

Passivity constraints for the impedance control of series elastic actuators

Robots operating in close contact with humans require actuators capable of accurately and safely modulating the delivered torque. To this aim, rotary series elastic actuators are largely adopted. Torque control is often implemented using a cascade control scheme involving proportional–integral regulators (velocity controller nested in a torque controller) for its simplicity and its potential of ensuring coupled stability. A high-level impedance control loop is also commonly added to regulate the interaction with the external agents. In the present work, conservative passivity conditions are derived when a cascade-controlled series elastic actuator is used to haptically display different models of virtual impedance. In particular, the case of a null impedance, of a pure spring and of series and parallel spring–damper systems (corresponding to standard linear viscoelastic bodies) are analyzed in order to derive design guidelines useful for the selection of the control gains as well as for determining the ranges of renderable virtual impedance.

Design of a variable impedance differential actuator for wearable robotics applications

In the design of wearable robots, the possibility of dynamically regulating the mechanical output impedance is crucial to achieve an efficient and safe human-robot interaction and to produce useful emergent dynamical behaviors.

Human-robot interaction tests on a novel robot for gait assistance

This paper presents tests on a treadmill-based non-anthropomorphic wearable robot assisting hip and knee flexion/extension movements using compliant actuation. Validation experiments were performed on the actuators and on the robot, with specific focus on the evaluation of intrinsic backdrivability and of assistance capability. Tests on a young healthy subject were conducted. In the case of robot completely unpowered, maximum backdriving torques were found to be in the order of 10 Nm due to the robot design features (reduced swinging masses; low intrinsic mechanical impedance and high-efficiency reduction gears for the actuators). Assistance tests demonstrated that the robot can deliver torques attracting the subject towards a predicted kinematic status.

Rendering viscoelasticity with Series Elastic Actuators using cascade control

Rotary Series Elastic Actuators (SEAs) are largely adopted to safely and accurately modulate interaction torques when robots operate in close contact with humans. Torque control is often based on a cascade scheme including PI regulators for a velocity controller nested in a torque controller. This solution is simple, robust and can potentially guarantee coupled stability. A high-level impedance control loop is also commonly added to regulate the behavior of the interaction port as a desired virtual viscoelastic body. In the present work, passivity is analyzed when a cascade controlled SEA is employed to display a virtual parallel spring-damper system. The case of a null desired impedance and of a pure spring are also tackled. The range of renderable mechanical impedance and guidelines for the selection of the control gains are derived.

Robomorphism: A Nonanthropomorphic Wearable Robot

This article describes a novel wearable robot (WR) intended to assist hip and knee flexion/ extension through series elastic actuators (SEAs). A nonanthropomorphic (NA) design was pursued to improve ergonomics while optimizing dynamic properties through a smart distribution of swinging masses. Once the anthropomorphism constraint is relaxed, the number of possible architectures becomes very high, and a methodology must be defined to point out the best options. To this purpose, a design methodology, which includes a novel approach to kinematic synthesis, topology selection, and morphological optimization, is also presented. The advantages offered by the novel architecture are demonstrated both theoretically and experimentally. In particular, the results show a low reflected inertia on the users body, a high backdrivability, and an intrinsic tolerance to misalignments. Such advantages make the proposed robot a promising platform for the development of assistive and rehabilitation systems.

pVEJ: A modular passive viscoelastic joint for assistive wearable robots

In complex dynamical tasks human motor control notably exploits the possibility of regulating joints mechanical impedance, both for stability and for energetic optimization purposes. These biomechanical findings should translate in design requirements for wearable robotics joints, which are required to produce adaptable intrinsic viscoelastic behaviors. This paper describes the design of a purely mechanical, rotary, passive ViscoElastic Joint (pVEJ), functionally equivalent to a torsional spring connected in parallel to a rotary viscous damper. The device has a modular design, which allows to modify the stiffness characteristics by replacing cam profiles. Damping coefficient can be also regulated off-line, manually acting on a valve. Prototype performances are characterized using a custom-developed dynamometric test-bed. Results demonstrate the capability of the system to render both the desired stiffness and damping values, in a range of impedance and peak torque compatible to that of wearable robotics for gait assistance.

Design of a rotary passive viscoelastic joint for wearable robots

In the design of wearable robots that strictly interact with the human body and, in general, in any robotics application that involves the human component, the possibility of having modular joints able to produce a viscoelastic behaviour is very useful to achieve an efficient and safe human-robot interaction and to give rise to emergent dynamical behaviors. In this paper we propose the design of a compact, passive, rotary viscoelastic joint for assistive wearable robotics applications. The system integrates two functionally distinct sub-modules: one to render a desired torsional stiffness profile and the other to provide a desired torsional damping. Concepts and design choices regarding the overall architecture and the single components are presented and discussed. A viscoelastic model of the system has been developed and the design of the joint is presented.

Kinematic synthesis, optimization and analysis of a non-anthropomorphic 2-DOFs wearable orthosis for gait assistance

This paper describes the optimization of a planar wearable active orthosis for hip and knee assistance during overground walking. A non-anthropomorphic design is pursued in order to improve ergonomics and to reduce torque requirements. Based on a previously-developed systematic search algorithm of the admissible generalized solutions for the selected problem, a solution is selected and optimized by means of genetic algorithms and constrained non-linear optimization. The optimized design allows to conveniently redistribute mechanical power through different actuators, i.e. peak torque and velocity requirements for each actuator can be modulated, thus promoting a lighter design. A detailed analysis of the resulting mechanism workspace is carried out, including the evaluation of kinematic singularities, in order to verify the adequateness of the design in real-world scenarios. The developed model and optimization results are validated through a numerical analysis and experiments in a mock-up system.