Add test checking if link forces are applied correctly

tests/test_simulations.py

@@ -0,0 +1,92 @@
+import jax.numpy as jnp
+import pytest
+
+import jaxsim.api as js
+import jaxsim.integrators
+import jaxsim.rbda
+from jaxsim import VelRepr
+
+
+def test_box_with_external_forces(
+    jaxsim_model_box: js.model.JaxSimModel,
+    velocity_representation: VelRepr,
+):
+    """
+    This test simulates a box falling due to gravity.
+    We apply to its CoM a 6D force that balances exactly the gravitational force.
+    The box should not fall.
+    """
+
+    model = jaxsim_model_box
+
+    # Build the data of the model.
+    data0 = js.data.JaxSimModelData.build(
+        model=model,
+        base_position=jnp.array([0.0, 0.0, 0.5]),
+        velocity_representation=velocity_representation,
+    )
+
+    # Compute the force due to gravity at the CoM.
+    mg = data0.standard_gravity() * js.model.total_mass(model=model)
+    G_f = jnp.array([0.0, 0.0, mg, 0, 0, 0])
+
+    # Compute the position of the CoM expressed in the coordinates of the link frame L.
+    L_p_CoM = js.link.com_position(
+        model=model, data=data0, link_index=0, in_link_frame=True
+    )
+
+    # Compute the transform of 6D forces from the CoM to the link frame.
+    L_H_G = jaxsim.math.Transform.from_quaternion_and_translation(translation=L_p_CoM)
+    G_Xv_L = jaxsim.math.Adjoint.from_transform(transform=L_H_G, inverse=True)
+    L_Xf_G = G_Xv_L.T
+    L_f = L_Xf_G @ G_f
+
+    # 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.
+    with references.switch_velocity_representation(VelRepr.Body):
+        references = references.apply_link_forces(
+            forces=jnp.atleast_2d(L_f),
+            link_names=model.link_names()[0:1],
+            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 = 0.5
+    dt = 0.001
+    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 didn't move.
+    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())