Digit Force Control for Dexterous Manipulation: Effects of Contact Surface Stiffness and Object’s Center of Mass

Abstract

Dexterous manipulation requires well-controlled digit forces. This study aimed to investigate the strategy of digit force control with different contact surface stiffness and different center of mass (COM) during object manipulation. Twenty healthy subjects participated in the experiment. They were instructed to reach, grasp and lift a custom-made grip device with the thumb and index finger. The kinematics and kinetic parameters such as grasp force, load force, fingertip center of pressure (COP), complementary torque were compared in all conditions during grasping. Meanwhile, the coordination angle (CA) and projection angle (PA) were also calculated to evaluate the adaptation of force vectors. Results showed that the force signals were significantly higher when the COM in the left direction. The load force and COP had a significant increase when grasping a soft contact surface. The complementary torque and CA were associated with the stiffness of contact surface; and the PA was principally modulated by object’s COM. These results suggested that the digit force control for dexterous manipulation could be affected by object’s COM and the stiffness of contact surface. A reinforced control mechanism would be engaged when grasping an object with unbalanced load and softer contact surface. This study provides a novel approach to quantify the hand-object interaction during dexterous manipulation.