Add tendon actuator force sensor to MJX.

PiperOrigin-RevId: 747344916
Change-Id: Iab513ff953408c002216ffff12e3f8c825cedc10

doc/changelog.rst

@@ -8,6 +8,7 @@ Upcoming version (not yet release)
 MJX
 ^^^
 - Added inverse dynamics.
+- Added tendon actuator force sensor.
 
 Version 3.3.1 (Apr 9, 2025)
 ----------------------------

doc/mjx.rst

@@ -246,7 +246,7 @@ The following features are **fully supported** in MJX:
        ``FRAMEPOS``, ``FRAMEXAXIS``, ``FRAMEYAXIS``, ``FRAMEZAXIS``, ``FRAMEQUAT``, ``SUBTREECOM``, ``CLOCK``,
        ``VELOCIMETER``, ``GYRO``, ``JOINTVEL``, ``TENDONVEL``, ``ACTUATORVEL``, ``BALLANGVEL``, ``FRAMELINVEL``,
        ``FRAMEANGVEL``, ``SUBTREELINVEL``, ``SUBTREEANGMOM``, ``TOUCH``, ``ACCELEROMETER``, ``FORCE``, ``TORQUE``,
-       ``ACTUATORFRC``, ``JOINTACTFRC``, ``FRAMELINACC``, ``FRAMEANGACC``
+       ``ACTUATORFRC``, ``JOINTACTFRC``, ``TENDONACTFRC``, ``FRAMELINACC``, ``FRAMEANGACC``
        (``ACCELEROMETER``, ``FORCE``, ``TORQUE`` not supported with connect or weld equality constraints)
 
 The following features are **in development** and coming soon:

mjx/mujoco/mjx/_src/forward_test.py

@@ -203,15 +203,21 @@ class ActuatorTest(parameterized.TestCase):
     mx = mjx.put_model(m)
     dx = mjx.put_data(m, d)
 
-    dx = dx.replace(ctrl=jp.array([1.0, 1.0, 1.0, -1.0, 1.0, -20.0, 5.0, -5.0]))
+    dx = dx.replace(ctrl=jp.array([1.0, 1.0, 1.0, -4.0, 1.0, -20.0, 5.0, -5.0]))
     dx = mjx.forward(mx, dx)
 
     _assert_eq(
         dx.actuator_force,
-        jp.array([1.0, 1.0, 1.0, -1.0, 1.0, -10.0, 5.0, -5.0]),
+        jp.array([1.0, 1.0, 1.0, -4.0 / 3.0, 1.0 / 3.0, -10.0, 5.0, -5.0]),
         'actuator_force',
     )
 
+    _assert_eq(
+        dx.sensordata,
+        jp.array([3.0, -1.0, -10.0, 0.0]),
+        'sensordata',
+    )
+
 
 if __name__ == '__main__':
   absltest.main()

mjx/mujoco/mjx/_src/sensor.py

@@ -27,6 +27,7 @@ from mujoco.mjx._src.types import DisableBit
 from mujoco.mjx._src.types import Model
 from mujoco.mjx._src.types import ObjType
 from mujoco.mjx._src.types import SensorType
+from mujoco.mjx._src.types import TrnType
 # pylint: enable=g-importing-member
 import numpy as np
 
@@ -552,6 +553,15 @@ def sensor_acc(m: Model, d: Data) -> Data:
       sensor = d.actuator_force[objid]
     elif sensor_type == SensorType.JOINTACTFRC:
       sensor = d.qfrc_actuator[m.jnt_dofadr[objid]]
+    elif sensor_type == SensorType.TENDONACTFRC:
+      force_mask = [
+          (m.actuator_trntype == TrnType.TENDON)
+          & (m.actuator_trnid[:, 0] == tendon_id)
+          for tendon_id in objid
+      ]
+      force_ids = np.concatenate([np.nonzero(mask)[0] for mask in force_mask])
+      force_mat = np.array(force_mask)[:, force_ids]
+      sensor = force_mat @ d.actuator_force[force_ids]
     elif sensor_type in (SensorType.FRAMELINACC, SensorType.FRAMEANGACC):
       objtype = m.sensor_objtype[idx]
 

mjx/mujoco/mjx/_src/types.py

@@ -370,6 +370,7 @@ class SensorType(enum.IntEnum):
     TORQUE: torque
     ACTUATORFRC: scalar actuator force
     JOINTACTFRC: scalar actuator force, measured at the joint
+    TENDONACTFRC: scalar actuator force, measured at the tendon
     FRAMELINACC: 3D linear acceleration
     FRAMEANGACC: 3D angular acceleration
   """
@@ -404,6 +405,7 @@ class SensorType(enum.IntEnum):
   TORQUE = mujoco.mjtSensor.mjSENS_TORQUE
   ACTUATORFRC = mujoco.mjtSensor.mjSENS_ACTUATORFRC
   JOINTACTFRC = mujoco.mjtSensor.mjSENS_JOINTACTFRC
+  TENDONACTFRC = mujoco.mjtSensor.mjSENS_TENDONACTFRC
   FRAMELINACC = mujoco.mjtSensor.mjSENS_FRAMELINACC
   FRAMEANGACC = mujoco.mjtSensor.mjSENS_FRAMEANGACC
 

mjx/mujoco/mjx/test_data/actuator/tendon_force_clamp.xml

@@ -46,4 +46,10 @@
     <motor tendon="fixed1"/>
     <motor tendon="fixed1"/>
   </actuator>
+  <sensor>
+    <tendonactuatorfrc tendon="spatial0"/>
+    <tendonactuatorfrc tendon="spatial1"/>
+    <tendonactuatorfrc tendon="fixed0"/>
+    <tendonactuatorfrc tendon="fixed1"/>
+  </sensor>
 </mujoco>