Development of a Novel Impedance-Controlled Quasi-Direct-Drive Robotic Hand

TL;DR

This work tackles robust, dexterous manipulation in unstructured environments by presenting a quasi-direct-drive robotic hand with variable impedance control in both joint and Cartesian spaces. The design integrates backdrivable differential gear trains with belt reductions to deliver adequate torque via direct motor current sensing, controlled through Field-Oriented Control (FOC). By using impedance control and minimal sensing (current-based torque regulation), the hand achieves force-closure and form-closure grasps, in-hand manipulation, and safe environmental interactions demonstrated through tasks such as smack-and-snatch, object rotation/translation, and coin pickup, all on a low-cost, largely 3D-printed platform. The combination of a differential drive with quasi-direct-drive actuators preserves the benefits of direct-drive actuation while enhancing torque capabilities, enabling high-bandwidth, compliant manipulation suitable for industry, humanoid robotics, and prosthetic applications. Future work targets mechanical downsizing, integrated, compact electronics, thermal management, an extra finger, and mounting the hand to a standard robotic arm for broader task evaluation.

Abstract

Most robotic hands and grippers rely on actuators with large gearboxes and force sensors for controlling gripping force. However, this might not be ideal for tasks that require the robot to interact with an unstructured and unknown environment. In this paper, we introduce a novel quasi-direct-drive two-fingered robotic hand with variable impedance control in the joint space and Cartesian space. The hand has a total of four degrees of freedom, backdrivable differential gear trains, and four brushless direct current (BLDC) motors. Motor torque is controlled through Field-Oriented Control (FOC) with current sensing. Variable impedance control enables the robotic hand to execute dexterous manipulation tasks safely during environment-robot and human-robot interactions. The quasi-direct-drive actuators eliminate the need for complex tactile/force sensors or precise motion planning when handling environmental contact. A majority-3D-printed assembly makes this a low-cost research platform built with affordable, readily available off-the-shelf components. Experimental validation demonstrates the robotic hand's capability for stable force-closure and form-closure grasps in the presence of disturbances, reliable in-hand manipulation, and safe dynamic manipulations despite contact with the environment.

Figures (15)

• Figure 1: The impedance-controlled quasi-direct-drive robotic hand holding a ball.
• Figure 2: Classification of electric drive trains, (a) Series elastic actuators, (b) Variable stiffness actuator, (c) Geared motors with force/torque sensors, (d) Proprioceptive actuators (direct-drive and quasi-direct-drive).
• Figure 3: Model of the designed 2-DOF finger with a differential drive gear train, (a) 3D printed model (see Fig. \ref{['fig:HandDrawings']} for more details), (b) Kinematic model (including 2 BLDC motors).
• Figure 4: (a) Block diagram of the Cartesian-space impedance controller implementation for each finger, (b) Expanded detailed view of FOC-based torque controller.
• Figure 5: Quasi-direct-drive hand design drawings, (a) Hand CAD assembly, (b) CAD rendering of the hand mounted to the Franka Emika Panda robot arm franka2, (c) Labeled section view of the finger.