# AcroMonk: A Minimalist Underactuated Brachiating Robot

arXiv: https://arxiv.org/abs/2305.08373 · HTML: https://mlsanity.com/paper/2305.08373/

## Key facts

- Cited by 4 papers in the map (data through 14 Oct 2026).
- First posted on arXiv 15 May 2023.

## Built on by (most cited first, 4 of 4)

- [End-to-End Reinforcement Learning for Torque Based Variable Height Hopping](https://mlsanity.com/paper/2307.16676/) (2023)
- [Reinforcement Learning for Robust Athletic Intelligence: Lessons from the 2nd 'AI Olympics with RealAIGym' Competition](https://mlsanity.com/paper/2503.15290/) (2025)
- [Bayesian Inverse Physics for Neuro-Symbolic Robot Learning](https://mlsanity.com/paper/2506.08756/) (2025)
- [Robust Brachiation on a Life-Sized Dual-Arm Robot Using Waypoint-Guided Reinforcement Learning](https://mlsanity.com/paper/2608.17320/) (2026)

## Abstract (arXiv)

Brachiation is a dynamic, coordinated swinging maneuver of body and arms used by monkeys and apes to move between branches. As a unique underactuated mode of locomotion, it is interesting to study from a robotics perspective since it can broaden the deployment scenarios for humanoids and animaloids. While several brachiating robots of varying complexity have been proposed in the past, this paper presents the simplest possible prototype of a brachiation robot, using only a single actuator and unactuated grippers. The novel passive gripper design allows it to snap on and release from monkey bars, while guaranteeing well defined start and end poses of the swing. The brachiation behavior is realized in three different ways, using trajectory optimization via direct collocation and stabilization by a model-based time-varying linear quadratic regulator (TVLQR) or model-free proportional derivative (PD) control, as well as by a reinforcement learning (RL) based control policy. The three control schemes are compared in terms of robustness to disturbances, mass uncertainty, and energy consumption. The system design and controllers have been open-sourced. Due to its minimal and open design, the system can serve as a canonical underactuated platform for education and research.