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.
Highlights
Grip force is an important parameter to evaluate performances and functionalities of both human and prosthetic hands
This study presents the experimental tests carried out on the “Federica” hand, for evaluating its power grip force and energy efficiency
A custom split cylindrical handlebar embedding a single axis load cell was used; it was positioned in various angles during grasp
Summary
Grip force is an important parameter to evaluate performances and functionalities of both human and prosthetic hands. Different grip force measurement systems have been presented and they are principally classified into mechanical, strain gauge and force sensors devices. A study presented a grip-measuring device for neuro-rehabilitation, made of a split cylinder handlebar containing a single axis load cell, with the aim to improve the patient’s ability to modulate the grasp force [1]. Grip performances of the “KIT Prosthetic Hand” were evaluated by using split cylinders with diameters of 31 mm and 49 mm, containing a six degrees of freedom (DOFs) force/torque sensor (Mini 40, ATI Industrial Automation®), that revealed a power grasp of about 24 N [4]. Grasp force tests of the “Soft Hand Prosthesis”, were performed by means of a force/torque sensor (Nano 25, ATI Industrial Automation®) positioned inside a split cylindrical handlebar [5]. Two of the beams are instrumented with strain gauges configured as full Wheatstone bridges, while the third is a static reference beam [6]
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