⚛️ Physics Simulation¶

OrcaGym's physics simulation is based on the MuJoCo engine, providing high-precision rigid body dynamics and constraint solving.

See OrcaPlayground examples/euler/ for complete runnable code.

🔧 MuJoCo Backend — Model loading, solver configuration, stepping control
📐 State Management — qpos/qvel/qacc, state setting and reading
💥 Contacts and Forces — Contact detection, force querying, external force injection
🔄 Force Application and IK — External force application, state writing, Jacobians, inverse kinematics
🔗 Equality Constraints — WELD/CONNECT constraints, mocap anchoring, grasp operations
🧤 Soft Bodies and Flexibles — MuJoCo Flex system (experimental)

Quick Reference¶

Operation	API	Description
Step + sync (recommended)	`env.do_simulation(ctrl, n)`	Atomic operation, auto-syncs data
Advance n physics steps	`env.mj_step(n)`	Execute n physics steps
Forward update	`env.mj_forward()`	Refresh derived quantities (body poses, sensors)
Sync view	`env._sync_view()`	Internal method; `do_simulation()` already auto-syncs, so public users typically do not need to call this manually
Jacobian matrix	`env.mj_jacBody(jacp, jacr, name)`	Position/rotation Jacobian
Site Jacobian	`env.mj_jacSite(jacp, jacr, name)`	Site point Jacobian
Apply external force	`env.apply_body_force(name, f, τ)`	Apply force/torque to a body
Clear body force	`env.clear_body_force(name)`	Clear external force on a specific body
Clear all forces	`env.clear_all_forces()`	Clear all external forces
Set friction	`env.set_geom_friction({name: arr})`	Set geom friction coefficients
Set mocap	`env.set_mocap_pos_and_quat({name: {...}})`	Set mocap body pose
Set solver	`env.sim_config.timestep = 0.002`	Timestep configuration

Configure via env.sim_config:

Parameter	Default	Description
`timestep`	0.001 (G1) / 0.002	Physics simulation step size (seconds)
`iterations`	100	Solver iteration count
`integrator`	0 (Euler)	Integrator type (0=Euler, 1=RK4)
`gravity`	`[0, 0, -9.81]`	Gravity vector
`tolerance`	1e-8	Solver tolerance

The G1 Euler examples (Lessons 4–9) use the following standard parameters:

Parameter	Value	Description
`time_step`	0.001s	Physics step 1ms (MuJoCo 1000Hz)
`frame_skip`	20	20 physics steps per control cycle
`dt`	0.02s	Control frequency 50Hz
`integrator`	0 (Euler)	Semi-implicit Euler integration