Abstract
The concept of postural synergies of the human hand has been shown to potentially reduce complexity in the neuromuscular control of grasping. By merging this concept with soft robotics approaches, a multi degrees of freedom soft-synergy prosthetic hand [SoftHand-Pro (SHP)] was created. The mechanical innovation of the SHP enables adaptive and robust functional grasps with simple and intuitive myoelectric control from only two surface electromyogram (sEMG) channels. However, the current myoelectric controller has very limited capability for fine control of grasp forces. We addressed this challenge by designing a hybrid-gain myoelectric controller that switches control gains based on the sensorimotor state of the SHP. This controller was tested against a conventional single-gain (SG) controller, as well as against native hand in able-bodied subjects. We used the following tasks to evaluate the performance of grasp force control: (1) pick and place objects with different size, weight, and fragility levels using power or precision grasp and (2) squeezing objects with different stiffness. Sensory feedback of the grasp forces was provided to the user through a non-invasive, mechanotactile haptic feedback device mounted on the upper arm. We demonstrated that the novel hybrid controller enabled superior task completion speed and fine force control over SG controller in object pick-and-place tasks. We also found that the performance of the hybrid controller qualitatively agrees with the performance of native human hands.
Highlights
Restoring hand function through prostheses in individuals with upper limb loss is critically important to help them regain independence and improve quality of life
Force Control of Prosthetic Hand how the human neuromuscular system solves the sensorimotor complexity of controlling hand movements during grasping tasks, it was found that hand postures used to grasp a large set of common objects can be approximated by a few finger joint coordination patterns, i.e., synergies (Santello et al, 1998, 2002)
The SHP demonstrated human-like motion during reach-to-grasp (Fani et al, 2016), its capability to interact with objects with human-like force remains to be systematically validated. Such human-like force control is important in activities of daily living (ADL), which includes but are not limited to moving delicate objects, modulating grasp force to object weight, and manipulate compliant objects
Summary
Restoring hand function through prostheses in individuals with upper limb loss is critically important to help them regain independence and improve quality of life. By combining the concept of synergy with soft robotics technologies, a prosthetic hand, the SoftHand-Pro (SHP), was developed to simultaneously maximize simplicity and functionality (Godfrey et al, 2013) This hand employs an under-actuated design where the number of synergies, and the number of actuators, was reduced to one, i.e., the first principal component observed in human grasping data that accounts for more than 50% of the variance in grasp posture data (Santello et al, 1998). Such human-like force control is important in activities of daily living (ADL), which includes but are not limited to moving delicate objects, modulating grasp force to object weight, and manipulate compliant objects
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