RoboRay hand: A highly backdrivable robotic hand with sensorless contact force measurements

This paper presents a 14-DOF robotic hand including 5 fingers and a wrist. The hand has a new tendon-driven mechanism which minimizes frictional loss and maximizes efficiency and backdrivability. In order to accomplish high efficiency and backdrivability as well as human-like payload and dexterity in a compact size, two novel mechanical concepts are proposed. Firstly, the actuators are placed according to the functions of fingers - high power grasping and precise manipulation - instead of positioning at each joint. For the high power grasping, 7 high payload motors are positioned in the forearm, and 5 small size motors are positioned in the palm for the precise manipulation. Secondly, a new tension decoupling mechanism is proposed to the 2-DOF wrist joint, which delivers wire motions of the forearm motors to the fingers without frictional loss or coupling with wrist motion. A total weight including the forearm is 1.59kg which is similar to human. The fingertip force is 15N which is sufficient for most of household work. High backdrivability enables the contact force sensing by measuring a motor current without additional sensors. A detectable minimum contact force was 0.735N. In order to enhance the contact force sensing capability, a friction compensation algorithm was applied, which resulted to the minimum contact force as 0.196N. Theoretical and experimental analyses are also performed.