Evaluation of Grip Force and Energy Efficiency of the “Federica” Hand

Abstract

The actual grip force provided by a hand prosthesis is an important parameter to evaluate its efficiency. To this end, a split cylindrical handlebar embedding a single-axis load cell was designed, 3D printed and assembled. Various measurements were made to evaluate the performances of the “Federica” hand, a simple low-cost hand prosthesis. The handlebar was placed at different angular positions with respect to the hand palm, and the experimental data were processed to estimate the overall grip force. In addition, piezoresistive force sensors were applied on selected phalanxes of the prosthesis, in order to map the distribution of the grasping forces between them. The electrical current supplied to the single servomotor that actuates all the five fingers, was monitored to estimate the force exerted on the main actuator tendon, while tendon displacement was evaluated by a rotary potentiometer fixed to the servomotor shaft. The force transfer ratio of the whole system was about 12.85 %, and the mean dissipated energy for a complete cycle of closing-opening was 106.80 Nmm, resulting lower than that of many commercial prostheses. The mean grip force of the “Federica” hand was 8.80 N, that is enough to support the user in many actions of daily life, also considering the adaptive wrapping capability of the prosthesis. On average, the middle phalanges exerted the greatest grip force (2.65 N) on the handlebar, while the distal phalanges a force of 1.66 N.

The “Federica” Hand

“…Various tests were carried out to verify the efficacy of the “Federica” hand in performing some actions of daily living (ADL), some of which have been reported in previous studies [ 9 , 10 , 11 , 16 , 24 ]. Healthy volunteers wore the “Federica” hand, holding the rigid metallic handle, suitably mounted on the back of the prosthesis, and some bands were secured to their forearms by the two aluminum bars (see Figure 3 ).…”

“…On the contrary, the “Federica” hand actively performs its hand-opening and closing movements in a closed chain system: the traction of the main tendon from the palm side allows the hand’s closure, while the traction of the main tendon from the back side permits its opening. A powerful grip is made possible by particularly efficient energy transfer from the motor to the prosthetic fingers [ 24 ].…”

“…All the aforementioned [ 19 , 20 , 22 , 23 ] devices made use of underactuated mechanical systems combined with springs and/or elastic elements, which unavoidably absorb a considerable amount of energy supplied by the motors when the hand closes. In general, elastic elements are used for the passive opening of the hand (i.e., fingers extension), while, during the closing of the hand (i.e., fingers flexion), springs have to be loaded, and this requires energy expenditure [ 24 ].…”

“…This study presents the “Federica” hand, an anthropomorphic and adaptive prosthetic device that minimizes the complexity of all components without sacrificing much of the performances. Despite being powered by a single, off-the-shelf servomotor, it has shown high versatility in gripping disparate objects (even deformable ones), very high energy efficiency (for an anthropomorphic prosthesis) [ 24 ] and a speed not matched by any other prosthetic prototypes [ 11 ]. The control system, based on customized FSRs sensors, is innovative, more user-friendly and faster than EMG-based control [ 14 ].…”

Design of Tendon-Actuated Robotic Glove Integrated with Optical Fiber Force Myography Sensor

“…However, as the FSR has a rigid structure, its response may vary with the glove substrate characteristics, as well as with the object shape and texture [31]. An alternative comprises estimating the grip forces from the servo motor current to immediately interrupt the mechanical or FES excitation in response to a torque peak [32]. Ultimately, one may attach a tactile sensor matrix to the glove palm.…”