[SENSOR] Fix actuator torque sensor naming for MuJoCo compatibility

[ShravanTata]

Description

The current logging code uses actuatorfrc_torque_{joint_name} when reading torque sensors from MuJoCo, while the mjcf.py follows the pattern f'actuatorfrc_{actuator.tag}_{actuator.joint}' to set the sensor names. However, this creates a mismatch with MuJoCo's naming conventions as actuator.tag returns motor for torque actuators.

Problem

• Current behavior: Code generates sensor names like actuatorfrc_torque for torque actuators
• Issue: MuJoCo doesn't have "torque" actuators - they are called "motor" actuators
• Working cases: Position and velocity elements work correctly with this naming pattern

Expected Behavior

The sensor naming should align with MuJoCo's actuator terminology:

• Option 1: Torque actuator sensors should be referenced as actuatorfrc_motor instead of actuatorfrc_torque
• Option 2: Torque actuators sensors should be initialized as actuatorfrc_torque instead of actuatorfrc_motor

Impact

This naming mismatch likely causes:

• Failed sensor readings for torque/motor actuators
• Potential runtime errors or silent failures in logging
• Inconsistency with MuJoCo's internal naming conventions

Proposed Solution 1

Update actuator sensor naming in mjcf.py

if actuator.tag == "motor":
    actuator_type = "torque"
else:
   actuator_type = actuator.tag
mjcf_model.sensor.add(
                    'actuatorfrc',
                    # Adapted for muscles
                    name=f'actuatorfrc_{actuator_type}_{actuator.joint}',
                    actuator=actuator_name,
                )

Proposed Solution 2

Update the actuator naming logic to map "torque" to "motor" when generating MuJoCo sensor names, or adjust the naming pattern to match MuJoCo's conventions.

diff@@ -44,7 +44,7 @@ def joints_data(physics, sensor_maps):
                 ['jointvel', 1],
                 ['actuatorfrc_position', 1],
                 ['actuatorfrc_velocity', 1],
-                ['actuatorfrc_torque', 1],
+                ['actuatorfrc_motor', 1],
         ]
     ]
 
@@ -74,7 +74,7 @@ def get_sensor_maps(physics, verbose=True):
         'jointpos', 'jointvel', 'jointlimitfrc',
         'force', 'torque',
         # Joints control
-        'actuatorfrc_position', 'actuatorfrc_velocity', 'actuatorfrc_torque',
+        'actuatorfrc_position', 'actuatorfrc_velocity', 'actuatorfrc_motor',
         # Muscles
         'musclefrc', 'tendonpos', 'tendonvel',
         'musclefiberlen', 'musclefibervel',
@@ -139,7 +139,7 @@ def get_sensor_maps(physics, verbose=True):
                     ['velocities', 'jointvel'],
                     ['torques (position)', 'actuatorfrc_position'],
                     ['torques (velocity)', 'actuatorfrc_velocity'],
-                    ['torques (torque)', 'actuatorfrc_torque'],
+                    ['torques (torque)', 'actuatorfrc_motor'],
                     ['limits', 'jointlimitfrc'],
                     ['forces (total)', 'force'],
                     ['torques (total)', 'torque'],
@@ -250,7 +250,7 @@ def get_physics2data_maps(physics, sensor_data, sensor_maps):
             'jointpos', 'jointvel', 'jointlimitfrc',
             'actuatorfrc_position',
             'actuatorfrc_velocity',
-            'actuatorfrc_torque',
+            'actuatorfrc_motor',
     ]:
         sensor_maps[f'{identifier}2data'] = np.array([
             sensor_maps[identifier]['indices'][
@@ -518,9 +518,9 @@ def physicsactuators2data(physics, iteration, data, sensor_maps, units):
         data.sensors.joints.array[iteration, :, sc.joint_torque] += (
             physics.data.sensordata[sensor_maps['actuatorfrc_velocity2data']]
         )*itorques
-    if len(sensor_maps['actuatorfrc_torque2data']) > 0:
+    if len(sensor_maps['actuatorfrc_motor2data']) > 0:
         data.sensors.joints.array[iteration, :, sc.joint_torque] += (
-            physics.data.sensordata[sensor_maps['actuatorfrc_torque2data']]
+            physics.data.sensordata[sensor_maps['actuatorfrc_motor2data']]
         )*itorques

Notes
Solution 2 is technically a breaking change for any code that explicitly references actuatorfrc_torque, the current implementation is broken because it doesn't actually log the motor torques from MuJoCo. This fix restores the intended functionality. Anyone relying on actuatorfrc_torque will need to update their code to use actuatorfrc_motor instead, but they'll also benefit from having access to the actual motor torque values that were previously missing.

[jarreguit]

This was handled in #6.

[jarreguit]

@ShravanTata, can you please verify if this is the correct issue that was referred?

[ShravanTata]

@jarreguit No, this is a separate issue from that one. Check this commit for the fix: b7cec5e