Multidigit force control during unconstrained grasping in response to object perturbations

Abstract

Because of the complex anatomy of the human hand, in the absence of external constraints, a large number of postures and force combinations can be used to attain a stable grasp. Motor synergies provide a viable strategy to solve this problem of motor redundancy. In this study, we exploited the technical advantages of an innovative sensorized object to study unconstrained hand grasping within the theoretical framework of motor synergies. Participants were required to grasp, lift, and hold the sensorized object. During the holding phase, we repetitively applied external disturbance forces and torques and recorded the spatiotemporal distribution of grip forces produced by each digit. We found that the time to reach the maximum grip force during each perturbation was roughly equal across fingers, consistent with a synchronous, synergistic stiffening across digits. We further evaluated this hypothesis by comparing the force distribution of human grasping vs. robotic grasping, where the control strategy was set by the experimenter. We controlled the global hand stiffness of the robotic hand and found that this control algorithm produced a force pattern qualitatively similar to human grasping performance. Our results suggest that the nervous system uses a default whole hand synergistic control to maintain a stable grasp regardless of the number of digits involved in the task, their position on the objects, and the type and frequency of external perturbations. We studied hand grasping using a sensorized object allowing unconstrained finger placement. During object perturbation, the time to reach the peak force was roughly equal across fingers, consistently with a synergistic stiffening across fingers. Force distribution of a robotic grasping hand, where the control algorithm is based on global hand stiffness, was qualitatively similar to human grasping. This suggests that the central nervous system uses a default whole hand synergistic control to maintain a stable grasp.

Role of Tactile Noise in the Control of Digit Normal Force

“…With the TACtile-sensorized Object (TACO), we are able to record the position and the normal force exerted by each finger on its sensorized surface, while it allows participants to freely choose the position for digit placement on the object, thus enabling unconstrained grasping (Schürmann et al, 2011). For details on using this device for gasping studies, see also Naceri et al (2017). For this study, an additional force/torque sensor was integrated in the center of the TACO to additionally record the net torque and force vectors (see below).…”

“…It is possible to grasp the same object in an infinite number of ways, due to the multiple combinations of digit placement on the object and the level of force exerted by each digit. Grasping and manipulating an object requires that the central nervous system masters this redundancy in degrees of freedom (Bernshtein, 1967;Naceri et al, 2014Naceri et al, , 2017. Sensory feedback from proprioception, touch, and other modalities such as vision plays a key role in the control of grasping.…”

“…Additionally, we evaluated whether the thimble modulates the contact force locally, i.e., the force exerted by the covered finger, or globally, when grasping with three and four digits. We exploited the technical advantages of an innovative sensorized object to study the above question during unconstrained hand grasping (Naceri et al, 2017). In each trial, participants were required to grasp, lift, transport, and replace the sensorized object.…”

The interaction between motion and texture in the sense of touch

“…Another pivotal role of the sense of touch is the delivery of direct information about surface friction, compliance, and microslip, which are important for dexterous manipulation of objects and grasp control (102,103). Indeed, the tactile channel plays an important role in the control of digit force (104,105) and in the perception, execution and planning of fine hand movements (3). Several studies demonstrated the crucial role of cutaneous information in other motor tasks, such as in the control of grasp and finger posture (106), and guiding hand reaching (5).…”

Evaluation of Pressure Capacitive Sensors for Application in Grasping and Manipulation Analysis

“…The most significant of them are load cells [ 16 ], strain gauges [ 7 ] piezoresistive sensors, such as Force Sensing Resistor (FSR) sensors [ 13 , 14 ]. Also ad hoc developed sensor modules based on resistive measurements have been used, where the resistance of a conductive elastometer is changed by the applied normal force [ 15 , 17 ]. However, it is shown that the capacitive sensing principle has better performance than other technologies in terms of simple structure, stability and temperature independence [ 18 , 19 ].…”