Abstract
To physically interact with a rich variety of environments and to match situation-dependent requirements, humans adapt both the force and stiffness of their limbs. Reflecting this behavior in prostheses may promote a more natural and intuitive control and, consequently, improve prostheses acceptance in everyday life. This pilot study proposes a method to control a prosthetic robot hand and its impedance, and explores the utility of variable stiffness when performing activities of daily living and physical social interactions. The proposed method is capable of a simultaneous and proportional decoding of position and stiffness intentions from two surface electro-myographic sensors placed over a pair of antagonistic muscles. The feasibility of our approach is validated and compared to existing control modalities in a preliminary study involving one prosthesis user. The algorithm is implemented in a soft under-actuated prosthetic hand (SoftHand Pro). Then, we evaluate the usability of the proposed approach while executing a variety of tasks. Among these tasks, the user interacts with other 12 able-bodied subjects, whose experiences were also assessed. Several statistically significant aspects from the System Usability Scale indicate user's preference of variable stiffness control over low or high constant stiffness due to its reactivity and adaptability. Feedback reported by able-bodied subjects reveal a general tendency to favor soft interaction, i.e., low stiffness, which is perceived more human-like and comfortable. These combined results suggest the use of variable stiffness as a viable compromise between firm control and safe interaction which is worth investigating further.
Highlights
An upper limb amputation leaves a person with limited ability to perform work and daily living activities, and hinders social interaction and the perception of self-image (Atkins et al, 1996)
The bottom panel for each condition reports a reconstruction of the torque (τ ) that the motor exerts on the hand along its synergistic direction of motion, which is directly proportional to the force applied to the object
In the case of secondary subjects, the opinion between control modalities was dependent, while for the System Usability scale (SUS) test was considered dependent, even if all answers came from the same prosthesis user, as we found differences at the SUS data distribution over time
Summary
An upper limb amputation leaves a person with limited ability to perform work and daily living activities, and hinders social interaction and the perception of self-image (Atkins et al, 1996). The quality and safety of Human-Robot interaction, are aspects that cannot be underestimated in prosthetics, especially in upper limb, due to the inherently interactive nature of Exploring Stiffness Modulation in Prosthetic Hand the (artificial) hand. In the context of robotic manipulation, impedance control was introduced about 35 years ago by Hogan (1985). Today Social Human-Robot Interaction is an emerging field of investigation. It gave rise to substantial research, including studies on grip force control of robotic hands interacting with humans (Garate et al, 2018; Vigni et al, 2019). Feldman (1986) introduced the idea that considering a single muscle, setting the threshold value of the tonic stretch reflex (λ) leads to a dependence of active muscle force on muscle length (an invariant characteristic function, IC). Either passively or actively varying λ, which is linked-to the stiffness, and changes with antagonistic coactivation, the original EP shifts to a new location that may involve a change in length, force, or both
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