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Merge pull request #126 from ami-iit/add_test_external_force
Add test checking if link forces are applied correctly
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import jax.numpy as jnp | ||
import pytest | ||
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import jaxsim.api as js | ||
import jaxsim.integrators | ||
import jaxsim.rbda | ||
from jaxsim import VelRepr | ||
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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. | ||
""" | ||
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model = jaxsim_model_box | ||
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# 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, | ||
) | ||
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# 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]) | ||
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# 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 | ||
) | ||
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# 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 | ||
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# Initialize a references object that simplifies handling external forces. | ||
references = js.references.JaxSimModelReferences.build( | ||
model=model, | ||
data=data0, | ||
velocity_representation=velocity_representation, | ||
) | ||
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# 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, | ||
) | ||
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# 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 | ||
) | ||
) | ||
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# 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) | ||
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# Copy the initial data... | ||
data = data0.copy() | ||
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# ... and step the simulation. | ||
for t_ns in T: | ||
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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), | ||
) | ||
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# 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()) |