Observables

Tutorials Contact forces and regularization · Joint reaction forces

Allocate observable arrays once, before capture, and pass them to every step:

from newton.solvers import SolverSteel

solver = SolverSteel(model, enable_sleeping=True)
obs = solver.observables(
    {SolverSteel.ObservableKind.BODY_PARENT_F,
     SolverSteel.ObservableKind.CONTACT_F,
     SolverSteel.ObservableKind.BODY_AWAKE},
    contacts=contacts,
)
solver.step(state_0, state_1, control, contacts, dt, observables=obs)
obs.body_parent_f   # (body_count,) spatial vectors: force [N], torque [N·m], world frame
obs.contact_f       # per rigid contact row: force on shape 0's body [N]
obs.body_awake      # (body_count,) bool

Observable	What it reports	Limits
body_parent_f	The wrench the parent applies to each body through its inbound joint, from the step’s solved motion: inverse dynamics at the post-solve accelerations, less gravity, body_f, and the solved contact, friction, spin and loop-closure impulses of its subtree divided by $h$. Linear force first, torque about the body’s center of mass second.	Built from rigid inertia without armature, like MuJoCo. Loop-closure reactions are left out with pre-elimination. Contact wrenches are summed with atomics, so the last bits can vary run to run. Bodies outside every articulation report zero.
contact_f	Per live rigid contact row, the force on shape 0’s body from the solved normal and friction impulses divided by $h$.	Torque part is zero (torsion is not reported). Dropped rows, rows beyond the contact count and contacts of skipped sleeping islands report zero.
body_awake	True for awake bodies, False for sleeping ones, as of the end of the step that received the container.	Requires enable_sleeping. Bodies welded to the world always read True.
constraint_overflow	Device bool per world plus a global entry: rows, contacts or input lost since the last reset.	Read in a kernel, or call check_constraint_capacity() outside capture.
constraint_row_watermarks()	Per family: retained and requested high-water marks, dropped contact rows, overflow excess, overflowing world-steps; and contact_high_water.	Needs row_watermark=True; host read outside capture.
compliance_contact_count	Compliant contacts prepared in the last step, and compliance_skipped_contact_count for those excluded.	contact_compliance only; host read outside capture.

Steel does not simulate tendons, so it reports no tendon observables.