Pendulum

A single-link pendulum hanging from a fixed pivot.

PendulumSwingup

Property	Value
Canonical ID	mjx/pendulum_swingup-v0
Action space	Box(-1.0, 1.0, (1,), float32)
Observation space	Box(-inf, inf, (3,), float32)
Episode length	1000
Config	{"ctrl_dt": 0.02, "sim_dt": 0.01, "naconmax": 0, "njmax": 0}

Description

The pendulum starts hanging straight down. The agent applies torque at the pivot to swing it up to and balance it at the upright position. Available torque is small enough that the pendulum can't be lifted in a single push — energy has to build up through several swings before it crosses the top, which is what makes the task interesting despite the simple body.

Rewards

Uses a dense reward with a tolerance indicator over the pole's vertical alignment:

reward = tolerance(pole_vertical, (COSINE_BOUND, 1.0))

pole_vertical is the z-component of the pole's frame z-axis (i.e. cos(angle_from_upright)), and COSINE_BOUND marks the lower edge of the upright tolerance band. tolerance is DM Control's smooth indicator. It returns:

• 1.0 while pole_vertical sits inside (COSINE_BOUND, 1.0) (pole near upright).
• A smooth decay below COSINE_BOUND as the pole tilts away from vertical.
• 0.0 when the pole points straight down.

Starting state

obs = [0.2815 0.9596 0.    ]

(cos(angle), sin(angle), angular velocity — pole near hanging-down with a small randomisation.)

Termination

Episode ends when step >= max_steps (default 1000). No early termination.

Usage

import envrax
env = envrax.make("mjx/pendulum_swingup-v0")

Reference

Upstream: mujoco_playground/_src/dm_control_suite/pendulum.py.