fix(calibration): detect inverted gripper drive_mode on SO follower/leader calibration

The SO follower/leader calibration hardcoded drive_mode=0 for all motors.
When the gripper motor is mounted mirrored relative to the other arm's
(raw position increases when closing instead of decreasing), the
0=closed/100=open normalization convention flips: a 'closed' (0%)
command from the leader unnormalizes to the follower's fully-open hard
stop, slamming the gripper open and triggering the Feetech overload
protection (torque off until power cycle).

FeetechMotorsBus already supports per-motor inversion via drive_mode
(apply_drive_mode=True); calibration just never set it. This adds a
step after range recording that asks the user to fully close the
gripper, reads the raw position, and sets drive_mode=1 when the closed
position sits at range_max. No calibration file format change.

Verified against the exact calibration values reported in #3942 using
the real FeetechMotorsBus normalization: with drive_mode=0 a 0% goal
maps to raw 2035 (open, bug reproduced); with drive_mode=1 it maps to
raw 3528 (closed, expected).

Fixes #3942

src/lerobot/robots/so_follower/so_follower.py

@@ -142,11 +142,22 @@ class SOFollower(Robot):
         range_mins[full_turn_motor] = 0
         range_maxes[full_turn_motor] = 4095
 
+        drive_modes = dict.fromkeys(self.bus.motors, 0)
+        input(f"Fully close the gripper of {self} and press ENTER....")
+        gripper_closed_pos = self.bus.read("Present_Position", "gripper", normalize=False)
+        if abs(gripper_closed_pos - range_maxes["gripper"]) < abs(gripper_closed_pos - range_mins["gripper"]):
+            # The closed position is at the top of the recorded range, meaning the raw position increases
+            # when the gripper closes. This is inverted with respect to the expected convention
+            # (0 = closed, 100 = open), which happens when the gripper motor is mounted mirrored.
+            # Invert it in software via drive_mode so both arms share the same convention.
+            logger.info("Gripper motor is inverted, setting drive_mode=1 to compensate.")
+            drive_modes["gripper"] = 1
+
         self.calibration = {}
         for motor, m in self.bus.motors.items():
             self.calibration[motor] = MotorCalibration(
                 id=m.id,
-                drive_mode=0,
+                drive_mode=drive_modes[motor],
                 homing_offset=homing_offsets[motor],
                 range_min=range_mins[motor],
                 range_max=range_maxes[motor],

src/lerobot/teleoperators/so_leader/so_leader.py

@@ -110,11 +110,22 @@ class SOLeader(Teleoperator):
         range_mins[full_turn_motor] = 0
         range_maxes[full_turn_motor] = 4095
 
+        drive_modes = dict.fromkeys(self.bus.motors, 0)
+        input(f"Fully close the gripper of {self} and press ENTER....")
+        gripper_closed_pos = self.bus.read("Present_Position", "gripper", normalize=False)
+        if abs(gripper_closed_pos - range_maxes["gripper"]) < abs(gripper_closed_pos - range_mins["gripper"]):
+            # The closed position is at the top of the recorded range, meaning the raw position increases
+            # when the gripper closes. This is inverted with respect to the expected convention
+            # (0 = closed, 100 = open), which happens when the gripper motor is mounted mirrored.
+            # Invert it in software via drive_mode so both arms share the same convention.
+            logger.info("Gripper motor is inverted, setting drive_mode=1 to compensate.")
+            drive_modes["gripper"] = 1
+
         self.calibration = {}
         for motor, m in self.bus.motors.items():
             self.calibration[motor] = MotorCalibration(
                 id=m.id,
-                drive_mode=0,
+                drive_mode=drive_modes[motor],
                 homing_offset=homing_offsets[motor],
                 range_min=range_mins[motor],
                 range_max=range_maxes[motor],

tests/robots/test_so100_follower.py

@@ -149,3 +149,38 @@ def test_configure_writes_position_pid_coefficients():
     bus_mock.write.assert_any_call("P_Coefficient", "shoulder_pan", 32)
     bus_mock.write.assert_any_call("I_Coefficient", "shoulder_pan", 1)
     bus_mock.write.assert_any_call("D_Coefficient", "shoulder_pan", 16)
+
+
+@pytest.mark.parametrize(
+    "gripper_closed_pos, expected_drive_mode",
+    [
+        (2035, 0),  # closed position at range_min -> raw increases when opening -> not inverted
+        (3528, 1),  # closed position at range_max -> raw increases when closing -> inverted
+    ],
+)
+def test_calibrate_detects_gripper_drive_mode(follower, gripper_closed_pos, expected_drive_mode):
+    """Regression test for #3942: the follower gripper can be mounted mirrored with respect to the
+    leader's, in which case its raw position increases when closing. Calibration must detect this
+    and set drive_mode=1 so that normalized values follow the 0=closed/100=open convention."""
+    follower.connect()
+
+    motors = list(follower.bus.motors)
+    follower.bus.set_half_turn_homings.return_value = dict.fromkeys(motors, 0)
+    follower.bus.record_ranges_of_motion.return_value = (
+        dict.fromkeys(motors, 2035),
+        dict.fromkeys(motors, 3528),
+    )
+    follower.bus.read.return_value = gripper_closed_pos
+
+    with (
+        patch("builtins.input", return_value=""),
+        patch.object(type(follower), "_save_calibration", lambda self: None),
+    ):
+        follower.calibration = {}
+        follower.calibrate()
+
+    follower.bus.read.assert_called_with("Present_Position", "gripper", normalize=False)
+    assert follower.calibration["gripper"].drive_mode == expected_drive_mode
+    for motor in motors:
+        if motor != "gripper":
+            assert follower.calibration[motor].drive_mode == 0