Two leading qualities of skeletal muscle that produce good performance in uncertain environments are damage tolerance and the ability to modulate impedance. For this reason, robotics researchers are greatly interested in discovering the key characteristics of muscles that give them these properties and replicating them in actuators for robotic devices. This paper describes a method to harness the redundancy present in muscle-like actuation systems composed of multiple motor units and shows that they have these same two qualities. By carefully choosing which motor units are recruited, the impedance viewed from the environment can be modulated while maintaining the same overall activation level. The degree to which the impedance can be controlled varies with total activation level and actuator length.
Discretizing the actuation effort into multiple parts that work together, inspired by the way muscle fibers work in the human body, produces damage-tolerant behavior. This paper shows that this not only produces reasonably good resolutions without inordinate numbers of units, but gives the control system the ability to set the impedance along with the drive effort to the load.
Design and Realization of the CUFF - Clenching Upper-Limb Force Feedback Wearable Device for Distributed Mechano-Tactile Stimulation of Normal and Tangential Skin Forces
Publication Type
Conference Paper
Year of Publication
2015
Conference Name
IEEE International Conference of Intelligent Robots and Systems - IROS2015
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.
Notes
The authors want to thank Cosimo della Santina, Andrea Di Basco, Riccardo Persichini and Fabio Bonomo for their really valuable support in the development of the hardware prototype. This work is supported in part by the European Research Council under the Advanced Grant SoftHands “A Theory of Soft Synergies for a New Generation of Artificial Hands” (no. ERC-291166), and by the EU FP7 project (no. 601165) “WEARable HAPtics for Humans and Robots (WEARHAP)
Effective and stable execution of a remote task in an uncertain environment requires that the task force and position trajectories of the slave robot be appropriately commanded. To achieve this goal, in teleimpedance control, a reference command which consists of the stiffness and position profiles of the master is computed and realized by the compliant slave robot in real-time. This highlights the need for a suitable and computationally efficient tracking of the human limb stiffness profile in real-time. In this direction, based on the observations in human neuromotor control which give evidence on the predominant use of the arm configuration in directional adjustments of the endpoint stiffness profile, and the role of muscular co-activations which contribute to a coordinated stiffening of the task stiffness in all directions, we propose a novel and computationally efficient model of the arm endpoint stiffness behaviour. With the purpose of real-time tracking of the human arm kinematics, an arm triangle is introduced using three body markers at the shoulder, elbow and wrist joints. In addition, a co-contraction index is defined using muscular activities of a dominant antagonistic muscle pair. Calibration and identification of the model parameters are carried out experimentally, using perturbation-based arm endpoint stiffness measurements in different arm configurations and co-contraction levels of the chosen muscles. Results of this study suggest that the proposed model enables the master to naturally execute a remote task by modulating the direction of the major axes of the endpoint stiffness and its volume using arm configuration and the co-ativation of the involved muscles, respectively.
In this paper, we investigate the role of variable stiffness in the reduction of the energy cost for mechanical systems that perform desired tasks. The objective is to assess the use of Variable Stiffness Actuation (VSA) by determining an optimal stiffness profile and the associated energy cost of performing a desired task. For the analysis we consider mechanical systems of n-Degrees of Freedom (DoF), using VSA. We find an analytical solution that expresses the optimal stiffness profile during the task as a function of joint trajectories. This stiffness profile can be either constant or variable in time, and it minimizes a cost function, when performing a desired task.
We calculate the cost related to the torque of the system and the additional cost of changing or keeping a stiffness actively constant. Additionally, we discuss some cases for which it is worth to change the stiffness during a task and cases for which a constant stiffness may be better solution. Furthermore, from simulations and experiments we show cases in which using a variable stiffness profile allows cost savings w.r.t. constant
stiffness. The use of variable stiffness depends on the task, i.e. on the joint trajectories and their frequency, as well as on the mechanical implementation of the actuator used.
This work was partially supported by the European Research Council under the ERC Advanced Grant no. 291166 SoftHands (A Theory of Soft Synergies for a New Generation of Artificial Hands) and under the grant agreement no. 601165 Wearhap (Wearable Haptics for Humans and Robots), within the FP7/2007-2013 program: Cognitive Systems and Robotics.
This paper presents experimental findings on how humans modulate their muscle activity while grasping objects of varying levels of compliance. We hypothesize that one of the key abilities that allows humans to successfully cope with uncertainties while grasping compliant objects is the ability to modulate muscle activity to control both grasp force and stiffness in a way that is coherent with the task. To that end, subjects were recruited to perform a grasp and lift task with a tripod-grasp device with contact surfaces of variable compliance. Subjects performed the task under four different compliance conditions while surface EMG from the main finger flexor and extensor muscles was recorded along with force and torque data at the contact points. Significant increases in the extensor muscle (the antagonist in the task) and co-contraction levels were found with increasing compliance at the contact points. These results suggest that the motor system may employ a strategy of increasing cocontraction, and thereby stiffness, to counteract the decreased stability in grasping compliant objects. Future experiments will examine the extent to which this phenomenon is also related to specific task features, such as precision versus power grasp and object weight.
Notes
This work is supported in part by the European Research Council under the Advanced Grant SoftHands “A Theory of Soft Synergies for a New Generation of Artificial Hands” no. ERC-291166 and under the EU FP7 project WEARHAP “WEARable HAPtics for Humans and Robots” no. 601165.
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 objects.
