Ensure consistent link forces and fix static joint friction

src/jaxsim/api/common.py

@@ -87,7 +87,8 @@ def inertial_to_other_representation(
         array: jtp.Array,
         other_representation: VelRepr,
         transform: jtp.Matrix,
-        is_force: bool = False,
+        *,
+        is_force: bool,
     ) -> jtp.Array:
         r"""
         Convert a 6D quantity from inertial-fixed to another representation.
@@ -153,7 +154,8 @@ def other_representation_to_inertial(
         array: jtp.Array,
         other_representation: VelRepr,
         transform: jtp.Matrix,
-        is_force: bool = False,
+        *,
+        is_force: bool,
     ) -> jtp.Array:
         r"""
         Convert a 6D quantity from another representation to inertial-fixed.

src/jaxsim/api/ode.py

@@ -113,7 +113,7 @@ def system_velocity_dynamics(
     ).astype(float)
 
     # Build link forces if not provided
-    W_f_L = (
+    O_f_L = (
         jnp.atleast_2d(link_forces.squeeze())
         if link_forces is not None
         else jnp.zeros((model.number_of_links(), 6))
@@ -125,7 +125,7 @@ def system_velocity_dynamics(
 
     # Initialize the 6D forces W_f ∈ ℝ^{n_L × 6} applied to links due to contact
     # with the terrain.
-    W_f_Li_terrain = jnp.zeros_like(W_f_L).astype(float)
+    W_f_Li_terrain = jnp.zeros_like(O_f_L).astype(float)
 
     # Initialize the 6D contact forces W_f ∈ ℝ^{n_c × 6} applied to collidable points,
     # expressed in the world frame.
@@ -183,7 +183,7 @@ def system_velocity_dynamics(
 
         # Compute the joint friction torque
         τ_friction = -(
-            jnp.diag(kc) @ jnp.sign(data.state.physics_model.joint_positions)
+            jnp.diag(kc) @ jnp.sign(data.state.physics_model.joint_velocities)
             + jnp.diag(kv) @ data.state.physics_model.joint_velocities
         )
 
@@ -194,6 +194,17 @@ def system_velocity_dynamics(
     # Compute the total joint forces
     τ_total = τ + τ_friction + τ_position_limit
 
+    references = js.references.JaxSimModelReferences.build(
+        model=model,
+        joint_force_references=τ_total,
+        link_forces=O_f_L,
+        data=data,
+        velocity_representation=data.velocity_representation,
+    )
+
+    with references.switch_velocity_representation(VelRepr.Inertial):
+        W_f_L = references.link_forces(model=model, data=data)
+
     # Compute the total external 6D forces applied to the links
     W_f_L_total = W_f_L + W_f_Li_terrain
 

tests/test_simulations.py

@@ -1,3 +1,4 @@
+import jax
 import jax.numpy as jnp
 import pytest
 
@@ -90,3 +91,83 @@ def test_box_with_external_forces(
     assert data.time() == t_ns / 1e9 + dt
     assert data.base_position() == pytest.approx(data0.base_position())
     assert data.base_orientation() == pytest.approx(data0.base_orientation())
+
+
+def test_box_with_zero_gravity(
+    jaxsim_model_box: js.model.JaxSimModel,
+    velocity_representation: VelRepr,
+    prng_key: jnp.ndarray,
+):
+
+    model = jaxsim_model_box
+
+    # Split the PRNG key.
+    key, subkey, subkey2 = jax.random.split(prng_key, num=3)
+
+    # Build the data of the model.
+    data0 = js.data.JaxSimModelData.build(
+        model=model,
+        base_position=jax.random.uniform(subkey2, shape=(3,)),
+        velocity_representation=velocity_representation,
+        standard_gravity=0.0,
+        soft_contacts_params=jaxsim.rbda.SoftContactsParams.build(K=0.0, D=0.0, mu=0.0),
+    )
+
+    # Generate a random linear force.
+    L_f = (
+        jax.random.uniform(subkey, shape=(model.number_of_links(), 6))
+        .at[:, 3:]
+        .set(jnp.zeros(3))
+    )
+
+    # Initialize a references object that simplifies handling external forces.
+    references = js.references.JaxSimModelReferences.build(
+        model=model,
+        data=data0,
+        velocity_representation=velocity_representation,
+    )
+
+    # Apply a link forces to the base link.
+    references = references.apply_link_forces(
+        forces=jnp.atleast_2d(L_f),
+        link_names=model.link_names(),
+        model=model,
+        data=data0,
+        additive=False,
+    )
+
+    # Create the integrator.
+    integrator = jaxsim.integrators.fixed_step.RungeKutta4SO3.build(
+        dynamics=js.ode.wrap_system_dynamics_for_integration(
+            model=model, data=data0, system_dynamics=js.ode.system_dynamics
+        )
+    )
+
+    # Initialize the integrator.
+    tf = 1.0
+    dt = 0.010
+    T = jnp.arange(start=0, stop=tf * 1e9, step=dt * 1e9, dtype=int)
+    integrator_state = integrator.init(x0=data0.state, t0=0.0, dt=dt)
+
+    # Copy the initial data...
+    data = data0.copy()
+
+    # ... and step the simulation.
+    for t_ns in T:
+
+        data, integrator_state = js.model.step(
+            model=model,
+            data=data,
+            dt=dt,
+            integrator=integrator,
+            integrator_state=integrator_state,
+            link_forces=references.link_forces(model=model, data=data),
+        )
+
+    # Check that the box moved as expected.
+    assert data.time() == t_ns / 1e9 + dt
+    assert data.base_position() == pytest.approx(
+        data0.base_position()
+        + 0.5 * L_f[:, :3].squeeze() / js.model.total_mass(model=model) * tf**2,
+        rel=1e-4,
+    )