A Long-Legged, Direct-Drive Monopedal Robot Achieves Exceptional Jump Height

Gihyeok Na; Justin K. Yim

cs.ROSep 24, 2026

A Long-Legged, Direct-Drive Monopedal Robot Achieves Exceptional Jump Height

Authors: Gihyeok Na, Justin K. Yim

Organizations: Department of Mechanical Science and Engineering, University of Illinois Urbana-Champaign, IL 61801 USA

Abstract

We demonstrate a jumping robot that reaches high (7.6 m) and fast (190 ms stance time) jumps from a single long leg driven by a direct-drive transmission, without elastic energy storage. At 281 g, it achieves the highest jump yet reported for an electrically actuated, spring-free system. The leg uses a new fabric-wrap transmission that provides a variable mechanical advantage, keeping a small electric motor near its peak power output through the stroke while bracing the long, lightweight leg against buckling. A balancing module at the top of the leg uses small propellers to control the leg's orientation on the ground and in the air, where the long leg provides a large moment arm for the control torques. The robot is validated outdoors with vertical jumps, attitude control on the ground and in flight, and tilted jumps.

Figures & tables

Fig. 1: Overlay of a single vertical jump, composited from frames $166.7\text{\,}\mathrm{ms}$ apart, with a person of height $1.83\text{\,}\mathrm{m}$ for scale.

Fig. 2: Overview of the robot and its three subsystems: the body, the balancing module, and the leg. (a) The full robot, with a leg stroke of $1867\text{\,}\mathrm{mm}$ and a body height of $83\text{\,}\mathrm{mm}$ . (b) The balancing module, mounted at the top of the leg. (c) The body, containing the main actuator.

Fig. 3: Actuation strategies, each drawn as a rack and pinion. The colored part is the actuated element, the rack is the leg, and the red arrow marks the actuator output. (a) Series-elastic (SE) actuation: elastic element in series. (b) Parallel-elastic (PE) actuation: elastic element in parallel, with a latch. (c) Direct-drive (DD) actuation: rigid coupling, no elastic element.

Type	Reference	Mass	Leg stroke	Jump height	VJA
Type	Reference	( $\text{\,}\mathrm{kg}$ )	( $\text{\,}\mathrm{cm}$ )	( $\text{\,}\mathrm{m}$ )	( $\text{\,}\mathrm{m}\text{\,}{\mathrm{s}}^{-1}$ )
DD	This work	0.28	187	7.6	5.30
	Wagner and Yim [ 8 ]	1.25	20	0.59	1.19
	Suzuki et al. [ 28 ]	10.3	80	1.6	2.00
	Kau et al. [ 23 ]	4.8	16	1.14	2.23
	Park et al. [ 25 , 26 , 27 ]	33	80	0.5	1.11

TABLE I: Comparison of Electrically Actuated Jumping Robots

Subsystem	Component	Mass ( $\text{\,}\mathrm{g}$ )
Body	Motor	53
	Battery	88
	Chassis, electronics, wiring	51
	Subtotal	192
Balancing module	Motors ( $4\times$ )	12
	Batteries ( $2\times$ )	14

TABLE II: System Mass Breakdown

Fig. 4: Schematic of the fabric-wrap transmission principle. (a) The fabric spirals around the leg at a spiral wrap angle

\phi

, converting motor torque

\tau

into tension

T

and the vertical ground reaction force (GRF) that drives the motion

x

along the leg. (b) At the start of stance, the fabric wraps at radius

r_{0}

. (c) As the leg extends, the radius grows to

r(x)

, decreasing the mechanical advantage (MA). (d) A rack and pinion [ 8 ] , by contrast, keeps

r

fixed, causing a constant mechanical advantage.

Fig. 5: Attitude-control model.

L

is the length from foot to the balancing module,

F

is the net thrust at the module, and

\psi

is the lean from vertical. (a) Contact phase: the system is an inverted pendulum about the foot, with the body center of mass at

x_{0}

and a tripod contacting the ground for support. (b) Aerial phase: the system rotates about its center of mass at

x_{\text{COM}}

, with the body at

x

Fig. 6: Top-down layout of the vertical jump test setup, showing the placements of the two GoPro cameras, the meter stick length references, and the robot.

Fig. 7: Body displacement during a single vertical jump, reconstructed by stereo triangulation.

\Delta Z

is the vertical component;

\Delta X

and

\Delta Y

are horizontal. The body reached a peak height of

7.6\text{\,}\mathrm{m}

Fig. 8: Main actuator telemetry during the jump: winding current, motor shaft velocity, bus voltage, and mechanical power. After an initial ramp, the power holds near its peak for the rest of the stroke.

Fig. 9: On-ground attitude control. (a) Overlay of the robot setting a pitch lean angle. (b) Commanded (dashed) and measured (solid) roll and pitch, with the thruster duty cycle below.

Fig. 10: Roll and pitch telemetry of the balancing module during a vertical jump (top); the gray region marks the post-liftoff phase. The controller switches from ground to aerial gains after liftoff. The bottom plot shows the thruster duty cycle.

Fig. 11: Tilted jump. (a) Overlay of the lean and jump, with frames $166.7\text{\,}\mathrm{ms}$ apart after liftoff; the reference stick on the ground is $1\text{\,}\mathrm{m}$ long. (b) Snapshot sequence of the stance and liftoff phase at $20.8\text{\,}\mathrm{ms}$ intervals.

Fig. 12: Balancing-module telemetry (top): solid lines are measured values, dashed lines are setpoints. The robot balances, tilts, and then jumps. The bottom plot shows the body speed following the jump command issued at

0\text{\,}\mathrm{s}

; the robot lifts off at approximately

190\text{\,}\mathrm{ms}

Legged Robotics

Legged robotics studies how robots with legs move, perceive their surroundings, and perform useful tasks across varied terrain.

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Mar 6, 2026J/K move · Enter open · S save

Underactuated multimodal jumping robot for extraterrestrial exploration

Neil R. Wagner, Justin K. Yim

Department of Mechanical Science and Engineering, University of Illinois Urbana-Champaign

Robot Locomotion Legged Robotics Space Robotics Robotic Control

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We present a rolling and jumping underactuated monopedal robot designed to explore multimodal locomotion on low-gravity bodies. It uses only two reaction wheels to control its spatial orientation with two controllers: a balancing controller which can aim the robot's jump direction on the ground, and an aerial reorientation controller which can aim the robot's leg for landing after flight. We demonstrate rolling, targeted jumping and landing, and self-righting using only three actuators total, keeping system size to 0.33m and 1.25kg. Simple switching between locomotion modes enables the system to deal with differing landscapes and environmental conditions.

Abstract

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