cs.ROSep 15, 2026
SaveAthenaZero: A low-inertia, bimanual robot for dynamic manipulation
Organizations: RAI Institute, Cambridge, Massachusetts, 02142, USA
Abstract
AthenaZero is a bimanual manipulator designed to minimize inertia without compromising control authority. By utilizing quasi-direct drive actuation and transmission remotization techniques, the system achieves an effective endpoint mass comparable to that of a human---about an order of magnitude less than conventional robot manipulators. This characteristic, combined with its inherent torque transparency, makes AthenaZero exceptionally well-suited for dynamic manipulation. We describe the methodology} that led to this design and demonstrate the robot's capabilities on three baseball-inspired tasks: throwing, catching, and batting, which showcase complex interactions on human-comparable timescales where milliseconds matter. AthenaZero was capable of throwing at speeds in excess of 30 m/s, with catching and batting at speeds in excess of 14 m/s over a short 7.3 m distance. Batting practice and a game of catch were subsequently performed in robot-to-robot and human-to-robot variations, showcasing the efficacy and adaptability of our system in tasks that require high acceleration.
Figures & tables
Figure 1: AthenaZero throwing a baseball. AthenaZero ’s low-inertia design lends itself to highly dynamic, athletic tasks such as throwing a baseball in excess of 25 m/s.
Figure 2: Motor-level and joint-level architecture of AthenaZero . Nomenclature: Torso (T), Right (R), Left (L), Joint (J), and Motor (M) . Two torso motors (TM1 and TM2) connect directly to the TJ1 torso joint in parallel via a belt. Shoulder motors RM1-RM3 (LM1-LM3) are axially coincident, combining towards RJ1-RJ3 (LJ1-LJ3) and acting as a spherical shoulder joint. Elbow junction actuators RM4-RM5 (LM4-LM5) through a belt transmission are axially perpendicular, mimicking a universal joint RJ4-RJ5 (LJ4-LJ5) at the elbow. Wrist motors RM6-RM7 (LM6-LM7) are differentially coupled, symbolically representing a near-universal joint RJ6-RJ7 (LJ6-LJ7) at the wrist junction. Translational offset between the two axes is 6 mm. A total of 3 motors for each hand are located within the torso, with Bowden transmissions that remote actuation out to three fingers via tendons.
| Group | Joint | Motor Diameter | Gear Ratio | Peak Torque | Peak Velocity | Actuator Inertia* |
|---|---|---|---|---|---|---|
| Torso | TJ1 | 2 x 95 mm | 10.48:1 | 378 Nm | 32 rad/s | 5.49e-2 kg m 2 |
| Shoulder | (R/L)J1/2/3 | 95 mm | 5:1 | 90 Nm | 66 rad/s | 6.25e-3 kg m 2 |
| Elbow | (R/L)J4 | 95 mm | 5:1 | 90 Nm | 66 rad/s | 6.25e-3 kg m 2 |
| (R/L)J5 | 76 mm | 5:1 | 20 Nm | 89 rad/s | 1.28e-3 kg m 2 | |
| Wrist | (R/L)J6 | 38 mm ||38 mm | 10.6:1 | 16.1 Nm | 100 rad/s | 4.09e-4 kg m 2 |
| (R/L)J7 | 38 mm|| 38 mm | 7.5:1 | 11.4 Nm | 142 rad/s | 2.28e-4 kg m 2 |
Table 1: AthenaZero joint specifications. TJ1 is actuated by two 95 mm motors coupled in parallel with a belt reduction. The wrist is a differentially coupled parallel mechanism. Wrist specifications are taken at the neutral position and due to the properties of the linkage, is non-linear. * Actuator Inertia signifies the total reflected rotor inertia of that motor/gearbox combination.
Figure 3: Remoting actuation via transmissions reduces effective mass. Shoulder and Elbow: The majority of the arm’s mass is contained via the combination of motors RM1-RM5 (equivalently LM1-LM5). Shoulder motors are located at the joint, but elbow flexion leverages a belt transmission. Wrist: The wrist is a 2 DoF parallel mechanism which remotes the motors to the elbow junction. Hand: The hand is a 6 DoF, 3-tendon underactuated mechanism that interdigitates. Tactile sensors are placed inside of the fingertips and palm for contact sensing (denoted in blue).
Figure 4: Effective mass analysis of AthenaZero and off-the-shelf manipulators. We compare the effective mass of a human arm, AthenaZero —with a locked torso—and a set of commercially-available manipulators (Barrett WAM, Franka Research 3, UR5e, and KUKA LBR iiwa 14). A. We compute effective mass in a neutral configuration for an operational point at the wrist across a planar sweep of directions. Solid lines report total effective mass (including reflected inertia of the actuators) whereas dashed lines report the effective mass of the links alone. Numerical values for the maximum effective mass across swept directions for each arm are shown. B. To promote whole-body contact, we compare the effective mass along all body structures, computed in the direction normal to the surface at each point. QDD manipulators ( AthenaZero / WAM) elicit a similar effective mass on their upper arms as the high-geared manipulators do at the end-effector. C. Effective mass is configuration dependent. We report the distribution of mean effective mass at the wrist across 2000 random configurations. Box limits indicate the 25th and 75th percentiles, the central line represents the median, and whiskers span the range of non-outlier data, extending to the most extreme values within 1.5 times the interquartile range of the box limits. Points falling beyond the whiskers are plotted individually as outliers.
Figure 5: Pendulum impact test. A. A 1 kg pendulum impact test on AthenaZero results in a relatively large end-effector displacement of 115.3 mm and a positive post-contact pendulum velocity. B. A 1 kg pendulum impact test on on the FR3 results in a much smaller end-effector displacement (13.7 mm) and a negative pendulum velocity (bouncing effect). Error bars indicate 95% confidence intervals calculated from the standard error of the mean for five independent trials per robot.
Figure 6: Robot stiffness evaluation. Resultant positional offset of AthenaZero when commanded to 30 random joint configurations in unloaded and loaded conditions. In the loaded condition, the 1.8 kg mass was affixed to the end effector. Box limits indicate the 25th and 75th percentiles, the central line represents the median, and whiskers span the range of non-outlier data, extending to the most extreme values within 1.5 times the interquartile range of the box limits. Points falling beyond the whiskers are plotted individually as outliers.
| Skill | Maximum Velocity | Accurate Velocity | |
|---|---|---|---|
| Throwing | 30.8 m/s | 21.4 m/s | |
| Maximum Velocity @ 7.3 m | Reaction Time | Extrapolated Velocity @ 18.4 m | |
| Catching | 18.3 m/s | 0.398 s | 46.1 m/s |
| Batting | 13.9 m/s | 0.525 s | 35.0 m/s |
Table 2: Overview of throwing, catching, and batting capabilities. We showcase the maximum velocities of each baseball task, in addition to the equivalent catching and batting velocity according to reaction time if the two robots were placed on a standard pitcher’s mound and in the batter’s box (18.4 m).
Figure 7: Baseball-inspired tasks to showcase dynamic manipulation. A. Throwing a baseball at 13.4 m/s. B. Catching a baseball at 13.4 m/s @ 7.3 m (0.544 s reaction time). C. Batting a tennis ball at 13.4 m/s. @ 7.3 m (0.544 s reaction time).
Figure 8: Cooperative variations of playing a game of catch and participating in batting practice. A. Two robots playing a game of catch with one another. B. A robot and a human playing a game of catch. C. One robot throwing batting practice to another robot. D. A human throwing batting practice to a robot.
| Joint Angle [rad] | AthenaZero | WAM | FR3 | UR5e | iiwa14 |
|---|---|---|---|---|---|
| -0.1396 | 0 | 0 | 0 | 0 | |
| 0.1396 | -1.5708 | -1.5708 | -3.14159 | -1.5708 | |
| 0.5236 | 0 | 0 | 1.5708 | 0 | |
| 1.5708 | 1.5708 | -1.5708 | 3.14159 | -1.5708 | |
| 1.5708 | 0 | 0 | -1.5708 | 0 | |
| 0 | 0 | 3.14159 | 0 | 0 |
Table 3: Neutral robot joint configurations. The specified joint angles place compared robots in similar poses for effective mass analysis.
Figure S1: Layered control architecture of AthenaZero . High-level planners generate motion and control parameters that are evaluated within a deterministic 1 kHz real-time loop to produce joint- or task-space reference signals. These references propagate through the control and runtime layers and are ultimately translated into high-frequency actuator commands over EtherCAT.
Figure S2: Dynamometer characterization of output torque in linear regime. Leveraging a dynamometer, we characterize the torque production capabilities of a QDD actuator. During a locked rotor test, the system swepdf different commanded torques and measures the resultant torque through a load cell. The data are fit via least squares, and a highly linear model representation with an =0.9973 is noted for 2,741 samples.
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