fix(robots): restore joint clipping and wrist_yaw fallback in ReBot B601 send_action

- Add configurable arm control mode (mit default, pos_vel fallback) with tunable mit_kp / mit_kd
- Add optional gripper control mode (force_pos default, mit optional) with gripper_mit_kp / gripper_mit_kd
- Update tests for MIT arm routing, gripper mode routing, and revised joint limits

src/lerobot/datasets/sampler.py

@@ -30,6 +30,7 @@ class EpisodeAwareSampler:
         drop_n_first_frames: int = 0,
         drop_n_last_frames: int = 0,
         shuffle: bool = False,
+        generator: torch.Generator | None = None,
     ):
         """Sampler that optionally incorporates episode boundary information.
 
@@ -41,6 +42,10 @@ class EpisodeAwareSampler:
             drop_n_first_frames: Number of frames to drop from the start of each episode.
             drop_n_last_frames: Number of frames to drop from the end of each episode.
             shuffle: Whether to shuffle the indices.
+            generator: Generator used for shuffling. Exposing this attribute (even when None) lets
+                       `accelerate` register it as the synchronized RNG in distributed training, so
+                       every rank draws the same permutation and batch shards stay disjoint. When
+                       None, shuffling falls back to the global torch RNG.
         """
         if drop_n_first_frames < 0:
             raise ValueError(f"drop_n_first_frames must be >= 0, got {drop_n_first_frames}")
@@ -73,10 +78,11 @@ class EpisodeAwareSampler:
 
         self.indices = indices
         self.shuffle = shuffle
+        self.generator = generator
 
     def __iter__(self) -> Iterator[int]:
         if self.shuffle:
-            for i in torch.randperm(len(self.indices)):
+            for i in torch.randperm(len(self.indices), generator=self.generator):
                 yield self.indices[i]
         else:
             for i in self.indices:

src/lerobot/robots/bi_rebot_b601_follower/bi_rebot_b601_follower.py

@@ -52,7 +52,13 @@ class BiRebotB601Follower(Robot):
             cameras=config.left_arm_config.cameras,
             motor_can_ids=config.left_arm_config.motor_can_ids,
             pos_vel_velocity=config.left_arm_config.pos_vel_velocity,
+            control_mode=config.left_arm_config.control_mode,
+            mit_kp=config.left_arm_config.mit_kp,
+            mit_kd=config.left_arm_config.mit_kd,
+            gripper_control_mode=config.left_arm_config.gripper_control_mode,
             gripper_torque_ratio=config.left_arm_config.gripper_torque_ratio,
+            gripper_mit_kp=config.left_arm_config.gripper_mit_kp,
+            gripper_mit_kd=config.left_arm_config.gripper_mit_kd,
             joint_limits=config.left_arm_config.joint_limits,
         )
 
@@ -67,7 +73,13 @@ class BiRebotB601Follower(Robot):
             cameras=config.right_arm_config.cameras,
             motor_can_ids=config.right_arm_config.motor_can_ids,
             pos_vel_velocity=config.right_arm_config.pos_vel_velocity,
+            control_mode=config.right_arm_config.control_mode,
+            mit_kp=config.right_arm_config.mit_kp,
+            mit_kd=config.right_arm_config.mit_kd,
+            gripper_control_mode=config.right_arm_config.gripper_control_mode,
             gripper_torque_ratio=config.right_arm_config.gripper_torque_ratio,
+            gripper_mit_kp=config.right_arm_config.gripper_mit_kp,
+            gripper_mit_kd=config.right_arm_config.gripper_mit_kd,
             joint_limits=config.right_arm_config.joint_limits,
         )
 

src/lerobot/robots/rebot_b601_follower/config_rebot_b601_follower.py

@@ -65,18 +65,33 @@ class RebotB601FollowerConfig:
         }
     )
 
-    # Target velocity for joints running in POS_VEL mode, in degrees/s. A scalar is
-    # applied to every joint; a list provides one value per joint (in motor order).
-    pos_vel_velocity: float | list[float] = field(default_factory=lambda: [150.0] * 7)
+    # Max speed (deg/s) per joint for POS_VEL arms and FORCE_POS gripper (motor order).
+    pos_vel_velocity: float | list[float] = field(
+        default_factory=lambda: [150.0, 150.0, 150.0, 150.0, 150.0, 150.0, 500.0]
+    )
 
-    # Torque/current ratio for the gripper's FORCE_POS mode, in range [0, 1].
-    gripper_torque_ratio: float = 0.1
+    # Arm control: "mit" or "pos_vel".
+    control_mode: str = "mit"
+
+    # MIT kp/kd per arm joint (motor order). Unused when control_mode="pos_vel".
+    mit_kp: float | list[float] = field(default_factory=lambda: [45.0, 45.0, 45.0, 8.0, 9.0, 8.0, 8.0])
+    mit_kd: float | list[float] = field(default_factory=lambda: [12.0, 12.0, 12.0, 1.0, 1.0, 1.0, 1.0])
+
+    # Gripper control: "force_pos" or "mit".
+    gripper_control_mode: str = "force_pos"
+
+    # FORCE_POS only: max grip force, in [0, 1].
+    gripper_torque_ratio: float = 0.05
+
+    # MIT only.
+    gripper_mit_kp: float = 8.0
+    gripper_mit_kd: float = 0.3
 
     # Soft joint limits (degrees). These are clipped against on every action.
     joint_limits: dict[str, tuple[float, float]] = field(
         default_factory=lambda: {
-            "shoulder_pan": (-145.0, 145.0),
-            "shoulder_lift": (-170.0, 1.0),
+            "shoulder_pan": (-150.0, 150.0),
+            "shoulder_lift": (-200.0, 1.0),
             "elbow_flex": (-200.0, 1.0),
             "wrist_flex": (-80.0, 90.0),
             "wrist_yaw": (-90.0, 90.0),

src/lerobot/robots/rebot_b601_follower/rebot_b601_follower.py

@@ -169,11 +169,25 @@ class RebotB601Follower(Robot):
         print(f"Calibration saved to {self.calibration_fpath}")
 
     def configure(self) -> None:
+        if self.config.control_mode not in ("pos_vel", "mit"):
+            raise ValueError(
+                f"Unsupported control_mode '{self.config.control_mode}'. Use 'pos_vel' or 'mit'."
+            )
+        if self.config.gripper_control_mode not in ("force_pos", "mit"):
+            raise ValueError(
+                f"Unsupported gripper_control_mode '{self.config.gripper_control_mode}'. "
+                "Use 'force_pos' or 'mit'."
+            )
+        use_mit = self.config.control_mode == "mit"
+        gripper_use_mit = self.config.gripper_control_mode == "mit"
         self.bus.enable_all()
         for motor_name, motor in self.motors.items():
-            target_mode = (
-                MotorBridgeMode.FORCE_POS if motor_name == GRIPPER_MOTOR else MotorBridgeMode.POS_VEL
-            )
+            if motor_name == GRIPPER_MOTOR:
+                target_mode = MotorBridgeMode.MIT if gripper_use_mit else MotorBridgeMode.FORCE_POS
+            elif use_mit:
+                target_mode = MotorBridgeMode.MIT
+            else:
+                target_mode = MotorBridgeMode.POS_VEL
             for attempt in range(_ENSURE_MODE_RETRIES + 1):
                 try:
                     motor.ensure_mode(target_mode)
@@ -252,22 +266,34 @@ class RebotB601Follower(Robot):
             goal_present_pos = {key: (g, present_pos.get(key, g)) for key, g in goal_pos.items()}
             goal_pos = ensure_safe_goal_position(goal_present_pos, self.config.max_relative_target)
 
+        use_mit = self.config.control_mode == "mit"
         for motor_name, position_deg in goal_pos.items():
             motor = self.motors.get(motor_name)
             if motor is None:
                 continue
             idx = self.motor_names.index(motor_name)
-            vel_deg_s = (
-                self.config.pos_vel_velocity[idx]
-                if isinstance(self.config.pos_vel_velocity, list)
-                else self.config.pos_vel_velocity
-            )
             pos_rad = math.radians(position_deg)
-            vel_rad = math.radians(vel_deg_s)
             if motor_name == GRIPPER_MOTOR:
-                motor.send_force_pos(pos_rad, vel_rad, self.config.gripper_torque_ratio)
+                if self.config.gripper_control_mode == "mit":
+                    motor.send_mit(pos_rad, 0.0, self.config.gripper_mit_kp, self.config.gripper_mit_kd, 0.0)
+                else:
+                    vel_deg_s = (
+                        self.config.pos_vel_velocity[idx]
+                        if isinstance(self.config.pos_vel_velocity, list)
+                        else self.config.pos_vel_velocity
+                    )
+                    motor.send_force_pos(pos_rad, math.radians(vel_deg_s), self.config.gripper_torque_ratio)
+            elif use_mit:
+                kp = self.config.mit_kp[idx] if isinstance(self.config.mit_kp, list) else self.config.mit_kp
+                kd = self.config.mit_kd[idx] if isinstance(self.config.mit_kd, list) else self.config.mit_kd
+                motor.send_mit(pos_rad, 0.0, kp, kd, 0.0)
             else:
-                motor.send_pos_vel(pos_rad, vel_rad)
+                vel_deg_s = (
+                    self.config.pos_vel_velocity[idx]
+                    if isinstance(self.config.pos_vel_velocity, list)
+                    else self.config.pos_vel_velocity
+                )
+                motor.send_pos_vel(pos_rad, math.radians(vel_deg_s))
 
         return {f"{motor}.pos": val for motor, val in goal_pos.items()}
 

src/lerobot/scripts/lerobot_train.py

@@ -232,15 +232,18 @@ def train(cfg: TrainPipelineConfig, accelerator: "Accelerator | None" = None):
         torch.backends.cudnn.benchmark = True
     torch.backends.cuda.matmul.allow_tf32 = True
 
-    # Dataset loading synchronization: main process downloads first to avoid race conditions
-    if is_main_process:
-        logging.info("Creating dataset")
+    # Dataset loading synchronization: each node's local main process downloads first to avoid
+    # race conditions (the global main process only exists on node 0, so gating on it would let
+    # all ranks of the other nodes download and build the Arrow cache concurrently).
+    if accelerator.is_local_main_process:
+        if is_main_process:
+            logging.info("Creating dataset")
         dataset = make_dataset(cfg)
 
     accelerator.wait_for_everyone()
 
-    # Now all other processes can safely load the dataset
-    if not is_main_process:
+    # Now all other processes can safely load the dataset from the local cache
+    if not accelerator.is_local_main_process:
         dataset = make_dataset(cfg)
 
     # Create environment used for evaluating checkpoints during training on simulation data.
@@ -386,12 +389,19 @@ def train(cfg: TrainPipelineConfig, accelerator: "Accelerator | None" = None):
     # create dataloader for offline training
     if hasattr(active_cfg, "drop_n_last_frames"):
         shuffle = False
+        # A dedicated generator (rather than the global torch RNG) lets accelerator.prepare
+        # synchronize the shuffle permutation across ranks, keeping batch shards disjoint even
+        # when ranks consume the global RNG asymmetrically (e.g. eval on the main process only).
+        sampler_generator = torch.Generator()
+        if cfg.seed is not None:
+            sampler_generator.manual_seed(cfg.seed)
         sampler = EpisodeAwareSampler(
             dataset.meta.episodes["dataset_from_index"],
             dataset.meta.episodes["dataset_to_index"],
             episode_indices_to_use=dataset.episodes,
             drop_n_last_frames=active_cfg.drop_n_last_frames,
             shuffle=True,
+            generator=sampler_generator,
         )
     else:
         shuffle = True

src/lerobot/teleoperators/rebot_102_leader/config_rebot_102_leader.py

@@ -65,7 +65,7 @@ class RebotArm102LeaderConfig:
     joint_ranges: dict[str, list[int]] = field(
         default_factory=lambda: {
             "shoulder_pan": [-150, 150],
-            "shoulder_lift": [-170, 1],
+            "shoulder_lift": [-200, 1],
             "elbow_flex": [-200, 1],
             "wrist_flex": [-80, 90],
             "wrist_yaw": [-90, 90],

tests/datasets/test_sampler.py

@@ -114,6 +114,30 @@ def test_shuffle():
     assert set(sampler) == {0, 1, 2, 3, 4, 5}
 
 
+def test_shuffle_with_generator_is_deterministic():
+    # Two samplers shuffling with same-seed generators must yield identical permutations.
+    # This is what keeps batch shards disjoint across ranks in distributed training, where
+    # accelerate synchronizes the sampler's generator state instead of the global torch RNG.
+    sampler_a = EpisodeAwareSampler([0], [6], shuffle=True, generator=torch.Generator().manual_seed(42))
+    sampler_b = EpisodeAwareSampler([0], [6], shuffle=True, generator=torch.Generator().manual_seed(42))
+    assert list(sampler_a) == list(sampler_b)
+
+    # Desyncing the global RNG must not affect the permutation.
+    sampler_c = EpisodeAwareSampler([0], [6], shuffle=True, generator=torch.Generator().manual_seed(42))
+    order_before = list(sampler_c)
+    sampler_c.generator.manual_seed(42)
+    torch.randperm(1000)  # consume global RNG, as rank-asymmetric code (e.g. eval) would
+    assert list(sampler_c) == order_before
+
+
+def test_generator_attribute_defaults_to_none():
+    # accelerate detects synchronizable samplers via `hasattr(sampler, "generator")`,
+    # so the attribute must exist even when no generator is passed.
+    sampler = EpisodeAwareSampler([0], [6], shuffle=True)
+    assert sampler.generator is None
+    assert set(sampler) == {0, 1, 2, 3, 4, 5}
+
+
 def test_negative_drop_first_frames_raises():
     with pytest.raises(ValueError, match="drop_n_first_frames must be >= 0"):
         EpisodeAwareSampler([0], [10], drop_n_first_frames=-1)

tests/robots/test_rebot_b601_follower.py

@@ -91,10 +91,11 @@ def test_get_observation_converts_to_degrees(follower):
 
 
 def test_send_action_clips_to_joint_limits(follower):
-    # shoulder_pan limit is (-145, 145); request beyond the upper bound.
+    # shoulder_pan limit is (-150, 150); request beyond the upper bound.
     returned = follower.send_action({"shoulder_pan.pos": 999.0})
-    assert returned["shoulder_pan.pos"] == 145.0
-    follower.motors["shoulder_pan"].send_pos_vel.assert_called_once()
+    assert returned["shoulder_pan.pos"] == 150.0
+    # Default control_mode is "mit", so arm joints are driven via send_mit.
+    follower.motors["shoulder_pan"].send_mit.assert_called_once()
 
 
 def test_send_action_routes_gripper_to_force_pos(follower):
@@ -103,6 +104,22 @@ def test_send_action_routes_gripper_to_force_pos(follower):
     follower.motors["gripper"].send_pos_vel.assert_not_called()
 
 
+def test_gripper_mit_mode_routes_to_send_mit():
+    bus_mock = _make_bus_mock()
+    with (
+        patch(f"{_MODULE}.require_package", lambda *a, **kw: None),
+        patch(f"{_MODULE}.MotorBridgeController") as controller_cls,
+        patch(f"{_MODULE}.MotorBridgeMode", MagicMock()),
+    ):
+        controller_cls.from_dm_serial.return_value = bus_mock
+        cfg = RebotB601FollowerRobotConfig(port="/dev/null", gripper_control_mode="mit")
+        robot = RebotB601Follower(cfg)
+        robot.connect(calibrate=False)
+        robot.send_action({"gripper.pos": -10.0})
+        robot.motors["gripper"].send_mit.assert_called_once()
+        robot.motors["gripper"].send_force_pos.assert_not_called()
+
+
 def test_bimanual_prefixes_features():
     with patch(f"{_MODULE}.require_package", lambda *a, **kw: None):
         cfg = BiRebotB601FollowerConfig(