Add comments for Aloha (#417)

Co-authored-by: Simon Alibert <75076266+aliberts@users.noreply.github.com>

lerobot/common/robot_devices/robots/manipulator.py

@@ -496,6 +496,17 @@ class ManipulatorRobot:
             # Set a velocity limit of 131 as advised by Trossen Robotics
             self.follower_arms[name].write("Velocity_Limit", 131)
 
+            # Use 'extended position mode' for all motors except gripper, because in joint mode the servos can't
+            # rotate more than 360 degrees (from 0 to 4095) And some mistake can happen while assembling the arm,
+            # you could end up with a servo with a position 0 or 4095 at a crucial point See [
+            # https://emanual.robotis.com/docs/en/dxl/x/x_series/#operating-mode11]
+            all_motors_except_gripper = [
+                name for name in self.follower_arms[name].motor_names if name != "gripper"
+            ]
+            if len(all_motors_except_gripper) > 0:
+                # 4 corresponds to Extended Position on Aloha motors
+                self.follower_arms[name].write("Operating_Mode", 4, all_motors_except_gripper)
+
             # Use 'position control current based' for follower gripper to be limited by the limit of the current.
             # It can grasp an object without forcing too much even tho,
             # it's goal position is a complete grasp (both gripper fingers are ordered to join and reach a touch).

lerobot/common/robot_devices/utils.py

@@ -1,13 +1,19 @@
+import platform
 import time
 
 
 def busy_wait(seconds):
-    # Significantly more accurate than `time.sleep`, and mandatory for our use case,
-    # but it consumes CPU cycles.
-    # TODO(rcadene): find an alternative: from python 11, time.sleep is precise
-    end_time = time.perf_counter() + seconds
-    while time.perf_counter() < end_time:
-        pass
+    if platform.system() == "Darwin":
+        # On Mac, `time.sleep` is not accurate and we need to use this while loop trick,
+        # but it consumes CPU cycles.
+        # TODO(rcadene): find an alternative: from python 11, time.sleep is precise
+        end_time = time.perf_counter() + seconds
+        while time.perf_counter() < end_time:
+            pass
+    else:
+        # On Linux time.sleep is accurate
+        if seconds > 0:
+            time.sleep(seconds)
 
 
 class RobotDeviceNotConnectedError(Exception):