1,721,148 research outputs found
A Non-invasive Real-Time Method for Measuring Variable Stiffness
Get PDFThe need for adaptability to the environment, energy conservation, and safety during physical interaction with humans of many advanced robotic applications has prompted the development of a number of Variable Stiffness Actuators (VSA). These have been implemented in a variety of ways, using different transduction technologies (electromechanical, pneumatic, hydraulic, but also piezoelectric, active polymeric, etc. ) and arrangements with elastic elements. All designs share a fundamentally unavoidable nonlinear behavior. The control schemes proposed for these actuators typically aim at independently controlling the position (or force) of the link, and its stiffness with respect to external disturbances. Although effective feedback control schemes using position and force sensors are commonplace in robotics, control of stiffness is at present completely open–loop. In practice, instead of measuring stiffness, it is inferred from the mathematical model of the actuator. Being this in most cases only roughly known, model mismatches affect severely stiffness control, undermining its utility. It should be noticed that, while for constant stiffness elements an accurate calibration of the model is possible, the same approach is hardly viable for variable stiffness systems.
We propose a method for estimating stiffness while it is varied, either intentionally or not, hence without knowledge of the command inputs. The method uses instantaneous measurements of force and position at one of the ends of the compliant elements in the system, and derives a measure that asymptotically converges to the current value of stiffness, up to an error which can be bounded by an arbitrarily small value. Simulation and experimental results are provided, which illustrate the performance of the proposed measurement method
A Real-time Parametric Stiffness Observer for VSA devices
No full textWe consider the problem of estimating non-linear time-varying stiffness of a mechanical system based only on force and position measurements. A recent work presented a non-parametric stiffness observer, which converges to within an Uniformly Ultimately Bounded neighborhood of the real stiffness value. The method provides excellent results for applications where the system is persistently excited. In this paper, we provide a parametric identification method that complements the previous solution in that it can provide, after a sufficiently long learning period, a complete model of the nonlinear stiffness, which can be applied henceforth even in the absence of excitation. Convergence conditions for the proposed method are discussed. Simulation and experimental results are provided, illustrating the performance of the proposed algorithm
An Upper-Limb Prosthetic Approach to Reduce Compensatory Motions in Reaching Tasks
No full textIn contemporary studies within the upper-limb prosthetic field, current investigations persistently revolve around the research of innovative control methodologies. These works are driven by the intention to diminish cognitive fatigue experienced by users while simultaneously enhancing the resilience and intuitiveness of control mechanisms. Several investigations have introduced approaches involving the exploitation of corrective movements as indicators of control system error, effectively closing the control loop. In prior studies, a predetermined linkage associates specific compensatory motions with distinct degrees of freedom in prosthetic devices. In contrast to this precedent, our study introduces a methodology that circumvents this intermediary step, enabling a direct mapping between human motions and the number of prosthetic joints. The proposed algorithm has been instantiated and validated using Matlab/Simulink, employing a simulated scenario featuring a trans-humeral prosthetic user
Rendering Softness: Integration of Kinesthetic and Cutaneous Information in a Haptic Device
Get PDFWhile it is known that softness discrimination relies on both kinesthetic and cutaneous information, relatively little work has been done on the realization of haptic devices replicating the two cues in an integrated and effective way. In this paper, we first discuss the ambiguities that arise in unimodal touch, and provide a simple intuitive explanation in terms of basic contact mechanics. With this as a motivation, we discuss the implementation and control of an integrated device, where a conventional kinesthetic haptic display is combined with a cutaneous softness display. We investigate the effectiveness of the integrated display via a number of psychophysical tests and compare the subjective perception of softness with that obtained by direct touch on physical objects. Results show that the subjects interacting with the integrated haptic display are able to discriminate softness better than with either a purely kinesthetic or a purely cutaneous display
A general compensation control method for human–robot integration
No full textThis paper introduces a new generalized control method designed for multi-degrees-of-freedom (DoF) devices to help people with limited motion capabilities in their daily activities. The challenge lies in finding the most adapted strategy for the control interface to effectively map the user’s motions in a low-dimensional space to complex robotic assistive devices, such as prostheses, supernumerary limbs, and even remote robotic avatars. The goal is a system that integrates the human and the robotic parts into a unique system, moving to reach the targets decided by the human while autonomously reducing the user’s effort and discomfort. We present a framework to control general multi-DoFs assistive systems, which translates user-performed compensatory motions into the necessary robot commands for reaching targets while canceling or reducing compensation. The framework extends to prostheses of any number of DoFs up to full robotic avatars, regarded here as a sort of “whole-body prosthesis” of the person who sees the robot as an artificial extension of their own body without a physical link but with a sensory-motor integration. We have validated and applied this control strategy through tests encompassing simulated scenarios and real-world trials involving a virtual twin of the robotic parts (prosthesis and robot) and a physical humanoid avatar
VSA-II: A Novel Prototype of Variable Stiffness Actuator for Safe and Performing Robots Interacting with Humans
No full textThis paper presents design and performance of a novel joint based actuator for a robot run by variable stiffness actuation, meant for systems physically interacting with humans. This new actuator prototype (VSA-II) is developed as an improvement over our previously developed one reported in [9], where an optimal mechanical-control co-design principle established in [7] is followed as well. While the first version was built in a way to demonstrate effectiveness of variable impedance actuation (VIA), it had limitations in torque capacities, life cycle and implementability in a real robot. VSA-II overcomes the problem of implementability with higher capacities and robustness in design for longer life. The paper discusses design and stiffness behaviour of VSA-II in theory and experiments. A comparison of stiffness characteristics between the two actuator is discussed, highlighting the advantages of the new design. A simple, but effective PD scheme is employed to independently control joint-stiffness and joint-position of a 1-link arm. Finally, results from performed impact tests of 1- link arm are reported, showing the effectiveness of stiffness variation in controlling value of a safety metric
An Approach to Exploit Compensatory Motions in Upper-Limb Prostheses Control
No full textAmong the recent investigations in upper-limb prostheses, the research still focuses on exploring new control solutions to reduce the user's mental fatigue and improve the control's robustness and intuitiveness. Some studies present solutions to close the control loop by using compensatory motions as error indexes. A previous relation is established to associate a predefined compensation motion to a given prosthetic degree of freedom. We developed an approach to avoid this step and directly correlate the human motions to as many prosthetic joints as needed. The implemented algorithm is tested in Simulink for a simulated trans-humeral amputee
Planning Robotic Manipulation with Tight Environment Constraints
No full textIn many real-world manipulation problems, the constraints imposed by the environment on an object are tight. In these cases, most state-of-the-art planners struggle to fit satisfactorily in low dimensional sub-manifolds, while still ensuring geometric and force feasibility. On the other hand, humans are at ease with such situations and indeed exploit constraints to manipulate objects proficiently.To face this challenge, we propose to merge state-of-art randomized grasp planning methods with model-based grasp analysis. We use the partial form-closure analysis framework to find the geometrically feasible motions of the object. Then, to ensure that the desired motions are physically realizable by the robot, we resort to an extension of the force-closure analysis framework accounting also for dynamic friction. We use these instruments to construct a random tree in a simplified planning space containing only object-robot configurations that are reachable through effectively applicable contact forces. The algorithm, validated in simulation and in preliminary experiments with a collaborative robot, features the ability to compute solutions for heavily constrained real-world manipulation problems
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