A Quasi-Direct-Drive Underactuated Asymmetric Hand for Dexterous and Efficient Grasping and Manipulation
Authors: Benjamin Davis, Chase Kidder, Hannah S. Stuart
Organizations: Embodied Dexterity Group, Dept. of Mechanical Engineering, University of California Berkeley, Berkeley, CA, USA · General Motors, Warren, MI, USA
In this paper, we present the Berkeley QUAD (Quasi-direct-drive, Underactuated, Asymmetric Design) Hand, a four-finger anthropomorphic robotic hand with 11 degrees of freedom and 8 degrees of actuation. The design utilizes QDD actuation at the base of each finger, enabling high force transparency for dexterous, adaptive performance. However, the low torque density of these actuators traditionally presents major issues with size, weight, and thermal limits. We overcome this by applying bio-inspired asymmetry, delegating dexterity to the radial fingers through individual QDD actuation, and strength to the ulnar finger through an underactuated, compliantly coupled transmission driven by a larger QDD motor. A novel preloaded, linkage-based transmission permits this ulnar coupling in a way that preserves human-like workspace reachability. Under light loads, the ulnar motor drives the third (middle) finger directly for dexterity while the fourth (ring) finger mirrors its motion. However, under larger loads, the middle finger complies while the motor drives the ring finger further downwards and inwards toward the center of the grasp to apply better closure forces. Hardware evaluations validate this architecture, demonstrating that the hand achieves 29 out of 33 Feix taxonomy grasps and exhibits backdrive forces as low as 50 g for delicate interactions. Additionally, the underactuated fourth finger improves grasp closure and provides the spatial efficiency necessary for larger actuation, yielding up to a 96-fold reduction in heat generation during sustained loading. Webpage: https://benudavis.github.io/berkeley-quadhand/
Figures & tables
Fig. 1: We introduce the Berkeley QUAD Hand, an anthropomorphic hand design that utilizes mechanically intelligent joint coupling with backdrivable QDD actuation for passive grasp adaptability, dexterity, and strength.
Fig. 2: The Berkeley QUAD Hand ( A ) has four fingers, 11 DoF, and 8 DoA, with approximately human dimensions. ( B ) Joints are actuated asymmetrically, with the effective transmission of the middle and ring MCP joints depending on the ulnar coupling mechanism ( C ). This mechanism enables an inward-sweeping trajectory of the ring finger once the middle finger becomes obstructed.
Actuator
SteadyWin GIM3505-08 [ 8 , 9 ]
CubeMars GL40 [ 6 ]
CubeMars AKE60-8
Diameter (mm)
43
46.5
69
Thickness (mm)
30
∼ 40
35
Weight (g)
97
∼ 160
260
Gear Ratio
8:1
5.23:1
8:1
Rphase ( Ω )
5.06
2.25
0.289
Rated Speed (rpm)
225
82.2
180
TABLE I: Comparison of QDD actuators.
Fig. 3: Kinematic chains for the coupled middle and ring fingers are shown together on the left. The ulnar transmission is formed by points A - H ; the middle MCP linkage chain is OHIJK ; the ring base spatial linkage involves points L,M,N,P ; and the underactuated ring finger linkage chain is NQRS . The right depicts independent flexion ( O′ABC ) and abduction ( DEFG ) linkages for the index finger.
Fig. 4: Reachable workspace of each finger ( A, B ), and the passive workspace of the ring finger (shown with points) for three fixed middle MCP joint angles (shown with solid lines) ( C, D ).
Fig. 5: A shows the QUAD hand achieving 29 (out of 33) grasps from the Feix taxonomy [ 17 ] . In line with [ 17 ] , grasps are provided with original numberings and sorted into power, intermediate, and precision categories. B shows the hand performing the Kapandji test [ 30 ] , missing only one pose (#5) due to the lack of a fifth finger.
Fig. 6: A shows an example test setup used to perform the minimum backdrive force and sustained load evaluations for each finger. The average ( ± SD) minimum backdrive force per finger is shown in B , while the average current and heat energy dissipated per finger during sustained load are plotted in C and D , respectively.