Update MJX inverse dynamics for tendon armature.

PiperOrigin-RevId: 775658246
Change-Id: Ie6a71b971d4b165e0235689b7b4ee3246486e10d

mjx/mujoco/mjx/_src/inverse.py

@@ -93,11 +93,15 @@ def inverse(m: Model, d: Data) -> Data:
     d = discrete_acc(m, d)
 
   d = inv_constraint(m, d)
-  d = smooth.rne(m, d, flg_acc=True)
+  d = smooth.rne(m, d)
+  d = smooth.tendon_bias(m, d)
   d = sensor.sensor_acc(m, d)
 
   qfrc_inverse = (
-      d.qfrc_bias + m.dof_armature * d.qacc - d.qfrc_passive - d.qfrc_constraint
+      d.qfrc_bias
+      + support.mul_m(m, d, d.qacc)
+      - d.qfrc_passive
+      - d.qfrc_constraint
   )
 
   if m.opt.enableflags & EnableBit.INVDISCRETE:

mjx/mujoco/mjx/_src/inverse_test.py

@@ -19,6 +19,7 @@ from jax import numpy as jp
 import mujoco
 from mujoco import mjx
 from mujoco.mjx._src import support
+from mujoco.mjx._src import test_util
 import numpy as np
 
 # tolerance for difference between MuJoCo and MJX calculations - mostly
@@ -109,6 +110,41 @@ class InverseTest(parameterized.TestCase):
     self.assertLess(fwdinv1, 1.0e-3)
     _assert_eq(dxinv.qacc, dx.qacc, 'qacc')
 
+  def test_inverse_tendon_armature(self):
+    m = test_util.load_test_file('tendon/armature.xml')
+
+    d = mujoco.MjData(m)
+    d.qvel = np.random.uniform(low=-0.01, high=0.01, size=d.qvel.shape)
+    d.ctrl = np.random.uniform(low=-0.01, high=0.01, size=d.ctrl.shape)
+    d.qfrc_applied = np.random.uniform(
+        low=-0.01, high=0.01, size=d.qfrc_applied.shape
+    )
+    d.xfrc_applied = np.random.uniform(
+        low=-0.01, high=0.01, size=d.xfrc_applied.shape
+    )
+    mujoco.mj_step(m, d, 10)
+
+    mx = mjx.put_model(m)
+    dx = mjx.put_data(m, d)
+
+    dx = mjx.forward(mx, dx)
+    dxinv = mjx.inverse(mx, dx)
+
+    fwdinv0 = jp.linalg.norm(
+        dxinv.qfrc_constraint - dx.qfrc_constraint, ord=np.inf
+    )
+    fwdinv1 = jp.linalg.norm(
+        dxinv.qfrc_inverse
+        - (
+            dx.qfrc_applied + dx.qfrc_actuator + support.xfrc_accumulate(mx, dx)
+        ),
+        ord=np.inf,
+    )
+
+    self.assertLess(fwdinv0, 1.0e-3)
+    self.assertLess(fwdinv1, 1.0e-3)
+    _assert_eq(dxinv.qacc, dx.qacc, 'qacc')
+
 
 if __name__ == '__main__':
   absltest.main()

mjx/mujoco/mjx/_src/smooth_test.py

@@ -321,32 +321,7 @@ class TendonTest(parameterized.TestCase):
   @parameterized.parameters(JacobianType.DENSE, JacobianType.SPARSE)
   def test_tendon_armature(self, jacobian):
     """Tests MJX tendon armature matches MuJoCo."""
-    m = mujoco.MjModel.from_xml_string("""
+    m = test_util.load_test_file('tendon/armature.xml')
-        <mujoco>
-          <worldbody>
-            <site name="site0" pos="1 0 1"/>
-            <body>
-              <joint type="slide" axis="0 0 1"/>
-              <joint type="hinge" axis="0 1 0"/>
-              <geom type="box" size="0.1 0.1 0.1" mass="1" pos="1 0 0"/>
-              <site name="site1"/>
-            </body>
-          </worldbody>
-          <tendon>
-            <spatial armature="123">
-              <site site="site0"/>
-              <site site="site1"/>
-            </spatial>
-            <spatial armature="456">
-              <site site="site0"/>
-              <site site="site1"/>
-            </spatial>
-          </tendon>
-          <keyframe>
-            <key qpos="1.2345 1.2345" qvel="1.2345 1.2345"/>
-          </keyframe>
-        </mujoco>
-        """)
     m.opt.jacobian = jacobian
     d = mujoco.MjData(m)
     mujoco.mj_resetDataKeyframe(m, d, 0)

mjx/mujoco/mjx/test_data/tendon/armature.xml

@@ -0,0 +1,24 @@
+<mujoco>
+  <worldbody>
+    <site name="site0" pos="1 0 1"/>
+    <body>
+      <joint type="slide" axis="0 0 1"/>
+      <joint type="hinge" axis="0 1 0"/>
+      <geom type="box" size="0.1 0.1 0.1" mass="1" pos="1 0 0"/>
+      <site name="site1"/>
+    </body>
+  </worldbody>
+  <tendon>
+    <spatial armature="123">
+      <site site="site0"/>
+      <site site="site1"/>
+    </spatial>
+    <spatial armature="456">
+      <site site="site0"/>
+      <site site="site1"/>
+    </spatial>
+  </tendon>
+  <keyframe>
+    <key qpos="1.2345 1.2345" qvel="1.2345 1.2345"/>
+  </keyframe>
+</mujoco>