56 research outputs found
Modeling and Control of a novel Variable Stiffness three DoF Wrist
This paper presents a novel design for a Variable Stiffness 3 DoF actuated
wrist to improve task adaptability and safety during interactions with people
and objects. The proposed design employs a hybrid serial-parallel configuration
to achieve a 3 DoF wrist joint which can actively and continuously vary its
overall stiffness thanks to the redundant elastic actuation system, using only
four motors. Its stiffness control principle is similar to human muscular
impedance regulation, with the shape of the stiffness ellipsoid mostly
depending on posture, while the elastic cocontraction modulates its overall
size. The employed mechanical configuration achieves a compact and lightweight
device that, thanks to its anthropomorphous characteristics, could be suitable
for prostheses and humanoid robots.
After introducing the design concept of the device, this work provides
methods to estimate the posture of the wrist by using joint angle measurements
and to modulate its stiffness. Thereafter, this paper describes the first
physical implementation of the presented design, detailing the mechanical
prototype and electronic hardware, the control architecture, and the associated
firmware. The reported experimental results show the potential of the proposed
device while highlighting some limitations. To conclude, we show the motion and
stiffness behavior of the device with some qualitative experiments.Comment: 13 pages + appendix (2 pages), 19 figures, submitted to IJR
A Novel Skin-Stretch Haptic Device for Intuitive Control of Robotic Prostheses and Avatars
Without proprioception, i.e., the intrinsic capability of a body to perceive its own limb position, completing daily life activities would require constant visual attention and it would be challenging or even impossible. This situation is similar to the one experienced after limb amputation and in robotic tele-operation, where the natural sensory-motor loop is broken. While some promising solutions based on skin stretch sensory substitution have been proposed to restore tactile properties in these conditions, there is still room for enhancing the intuitiveness of stimulus delivery and integration of haptic feedback devices within user's body. To contribute to this goal, here, we propose a wearable device based on skin stretch stimulation, the Stretch-Pro, which can provide proprioceptive information on artificial hand aperture. This system can be suitably integrated in a prosthetic socket or can be easily worn by a user controlling remote robots. The system can imitate the stretching of the skin that would naturally occur on the intact limb, when it is used to accomplish motor tasks. Two versions of the system are presented, with one and two actuators, respectively, which deliver the stretch stimulus in different ways. Experiments with able-bodied participants and a preliminary test with one prosthesis user are reported. Results suggest that Stretch-Pro could be a viable solution to convey proprioceptive cues to upper limb prosthesis users, opening promising perspectives for tele-robotics applications
Design and realization of the CUFF - clenching upper-limb force feedback wearable device for distributed mechano-tactile stimulation of normal and tangential skin forces
Rendering forces to the user is one of the main goals of haptic technology. While most force-feedback interfaces are robotic manipulators, attached to a fixed frame and designed to exert forces on the users while being moved, more recent haptic research introduced two novel important ideas. On one side, cutaneous stimulation aims at rendering haptic stimuli at the level of the skin, with a distributed, rather than, concentrated approach. On the other side, wearable haptics focuses on highly portable and mobile devices, which can be carried and worn by the user as the haptic equivalent of an mp3 player. This paper presents a light and simple wearable device (CUFF) for the distributed mechano-tactile stimulation of the user's arm skin with pressure and stretch cues, related to normal and tangential forces, respectively. The working principle and the mechanical and control implementation of the CUFF device are presented. Then, after a basic functional validation, a first application of the device is shown, where it is used to render the grasping force of a robotic hand (the Pisa/IIT SoftHand). Preliminary results show that the device is capable to deliver in a reliable manner grasping force information, thus eliciting a good softness discrimination in users and enhancing the overall grasping experience
Dexterity augmentation on a synergistic hand: the Pisa/IIT SoftHand+
Soft robotics and under-actuation were recently demonstrated as good approaches for the implementation of humanoid robotic hands. Nevertheless, it is often difficult to increase the number of degrees of actuation of heavily under-actuated hands without compromising their intrinsic simplicity. In this paper we analyze the Pisa/IIT SoftHand and its underlying logic of adaptive synergies, and propose a method to double its number of degree of actuation, with a very reduced impact on its mechanical complexity. This new design paradigm is based on constructive exploitation of friction phenomena. Based on this method, a novel prototype of under-actuated robot hand with two degrees of actuation is proposed, named Pisa/IIT SoftHand+. A preliminary validation of the prototype
follows, based on grasping and manipulation examples of some object
A synergy-driven approach to a myoelectric hand
In this paper, we present the Pisa/IIT SoftHand
with myoelectric control as a synergy-driven approach for
a prosthetic hand. Commercially available myoelectric hands
are more expensive, heavier, and less robust than their bodypowered
counterparts; however, they can offer greater freedom
of motion and a more aesthetically pleasing appearance. The
Pisa/IIT SoftHand is built on the motor control principle of
synergies through which the immense complexity of the hand
is simplified into distinct motor patterns. As the SoftHand
grasps, it follows a synergistic path with built-in flexibility to
allow grasping of a wide variety of objects with a single motor.
Here we test, as a proof-of-concept, 4 myoelectric controllers:
a standard controller in which the EMG signal is used only as
a position reference, an impedance controller that determines
both position and stiffness references from the EMG input, a
standard controller with vibrotactile force feedback, and finally
a combined vibrotactile-impedance (VI) controller. Four healthy
subjects tested the control algorithms by grasping various
objects. All controllers were sufficient for basic grasping,
however the impedance and vibrotactile controllers reduced
the physical and cognitive load on the user, while the combined
VI mode was the easiest to use of the four. While these results
need to be validated with amputees, they suggest a low-cost,
robust hand employing hardware-based synergies is a viable
alternative to traditional myoelectric prostheses
Teleimpedance Control of a Synergy-Driven Anthropomorphic Hand
In this paper, a novel synergy driven teleimpedance
controller for the Pisa–IIT SoftHand is presented. Towards
the development of an efficient, robust, and low-cost hand
prothesis, the Pisa–IIT SoftHand is built on the motor control
principle of synergies, through which the immense complexity
of the hand is simplified into distinct motor patterns. As the
SoftHand grasps, it follows a synergistic path with built-in
flexibility to allow grasping of objects of various shapes using
only a single motor. In this work, the hand grasping motion
is regulated with an impedance controller which incorporates
the user’s postural and stiffness synergy profiles in realtime.
In addition, a disturbance observer is realized which estimates
the grasping contact force. The estimated force is then fedback
to the user via a vibration motor. Grasp robustness and
transparency improvements were evaluated on two healthy
subjects while grasping different objects. Implementation of
the proposed teleimpedance controller led to the execution of
stable grasps by controlling the grasping forces, via modulation
of hand compliance. In addition, utilization of the vibrotactile
feedback resulted in reduced physical load on the user. While
these results need to be validated with amputees, they provide
evidence that a low-cost, robust hand employing hardwarebased
synergies is a viable alternative to traditional myoelectric
prostheses
Preliminary results toward a naturally controlled multi-synergistic prosthetic hand
Robotic hands embedding human motor control
principles in their mechanical design are getting increasing
interest thanks to their simplicity and robustness, combined
with good performance. Another key aspect of these hands
is that humans can use them very effectively thanks to the
similarity of their behavior with real hands. Nevertheless,
controlling more than one degree of actuation remains a
challenging task.
In this paper, we take advantage of these characteristics in
a multi-synergistic prosthesis. We propose an integrated setup
composed of Pisa/IIT SoftHand 2 and a control strategy which
simultaneously and proportionally maps the human hand movements
to the robotic hand. The control technique is based on
a combination of non-negative matrix factorization and linear
regression algorithms. It also features a real-time continuous
posture compensation of the electromyographic signals based
on an IMU. The algorithm is tested on five healthy subjects
through an experiment in a virtual environment. In a separate
experiment, the efficacy of the posture compensation strategy is
evaluated on five healthy subjects and, finally, the whole setup
is successfully tested in performing realistic daily life activities
Relaying the High-Frequency Contents of Tactile Feedback to Robotic Prosthesis Users: Design, Filtering, Implementation, and Validation
It is known that high-frequency tactile information conveys useful cues to discriminate important contact properties for manipulation, such as first contact and roughness. Despite this, no practical system, implementing a modality matching paradigm, has been developed so far to convey this information to users of upper-limb prostheses. The main obstacle to this implementation is the presence of unwanted vibrations generated by the artificial limb mechanics, which are not related to any haptic exploration task. In this letter, we describe the design of a digital system that can record accelerations from the fingers of an artificial hand and reproduce them on the user's skin through voice-coil actuators. Particular attention has been devoted to the design of the filter, needed to cancel all those vibrations measured by the sensors that do not convey information on meaningful contact events. The performance of the newly designed filter is also compared with the state of the art. Exploratory experiments with prosthesis users have identified some applications where this kind of feedback could lead to sensory-motor performance enhancement. Results show that the proposed system improves the perception of object-salient features such as first-contact events, roughness, and shape
Implementation and Control of the Velvet Fingers: a Dexterous Gripper with Active Surfaces
Since the introduction of the first prototypes of
robotic end-effectors showing manipulation capabilities, much
research focused on the design and control of robot hand and
grippers. While many studies focus on enhancing the sensing
capabilities and motion agility, a less explored topic is the
engineering of the surfaces that enable the hand to contact
the object.
In this paper we present the prototype of the Velvet Fingers
smart gripper, a novel concept of end-effector combining
the simple mechanics and control of under-actuated devices
together with high manipulation possibilities, usually offered
only by dexterous robotic hands. This enhancement is obtained
thanks to active surfaces, i.e. engineered contact surfaces able
to emulate different levels of friction and to apply tangential
thrusts to the contacted object. Through the paper particular
attention is dedicated to the mechanical implementation, sense
drive and control electronics of the device; some analysis on
the control algorithms are reported. Finally, the capabilities
of the prototype are showed through preliminary grasps and
manipulation experiment
Variable stiffness control for oscillation damping
In this paper a model-free approach for damping control of Variable Stiffness Actuators is proposed. The idea is to take advantage of the possibility to change the stiffness of the actuators in controlling the damping. The problem of minimizing the terminal energy for a one degree of freedom spring-mass model with controlled stiffness is first considered. The optimal bang-bang control law uses a maximum stiffness when the link gets away from the desired position, i.e. the link velocity is decreasing, and a minimum one when the link is going towards it, i.e. the link velocity is increasing. Based on Lyapunov stability theorems the obtained law has been proved to be stable for a multi-DoF system. Finally, the proposed control law has been tested and validated through experimental tests
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