dynamixel ax-18a servos not responding reliably [closed]

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Hi all,

we are trying to control 8 Dynamixel AX-18A servo motors using the USB2Dynamixel dongle, using the Ubuntu 10.10 packages for ROS Electric. We created a launch file according to the dynamixel_controllers tutorial.

When we start it, we get output like

[INFO] [WallTime: 1321639123.057186] Pinging motor IDs 1 through 8...
[INFO] [WallTime: 1321639123.094411] Found motors with IDs: [1, 2, 3, 4, 5, 6, 8].
[WARN] [WallTime: 1321639123.118003] Checksum of 230 does not match the checksum from servo of 246
Exception in thread Thread-5:
Traceback (most recent call last):
  File "/usr/lib/python2.6/threading.py", line 532, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.6/threading.py", line 484, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/opt/ros/electric/stacks/dynamixel_motor/dynamixel_driver/src/dynamixel_driver/dynamixel_serial_proxy.py", line 277, in __publish_diagnostic_information
    status.values.append(KeyValue('Model Name', str(self.motor_static_info[mid] ['model'])))
KeyError: 1

Every time we start the launch file, one or more random motor ids are not found. In the example above, it is motor number 7 that is missing. The process is still running after giving the above output. Also, the checksum error is a little different each time.

The launch file:

<launch>
    <node name="dynamixel_manager" pkg="dynamixel_controllers" type="controller_manager.py" required="true" output="screen">
        <rosparam>
            namespace: dxl_manager
            serial_ports:
                pan_tilt_port:
                    port_name: "/dev/ttyUSB1"
                    baud_rate: 1000000
                    min_motor_id: 1
                    max_motor_id: 8
                    update_rate: 10
        </rosparam>
    </node>
</launch>

Output of "rosparam get /dynamixel" (in this run, servo 6 is missing):

pan_tilt_port:
  '1': {degrees_per_encoder_tick: 0.29296875, encoder_resolution: 1024, encoder_ticks_per_degree: 3.4133333333333336,
    encoder_ticks_per_radian: 195.56959407132098, max_angle: 1023, max_torque: 1.3974476250000001,
    max_velocity: 8.0416045193750012, min_angle: 0, model_name: AX-18F, model_number: 18,
    radians_per_encoder_tick: 0.0051132692929521375, radians_second_per_encoder_tick: 0.0078608059817937455,
    range_degrees: 300.0, range_radians: 5.2359877559829888, torque_per_volt: 0.14709975,
    velocity_per_volt: 0.84648468625000006}
  '2': {degrees_per_encoder_tick: 0.29296875, encoder_resolution: 1024, encoder_ticks_per_degree: 3.4133333333333336,
    encoder_ticks_per_radian: 195.56959407132098, max_angle: 1023, max_torque: 1.3827376500000002,
    max_velocity: 7.9569560507500006, min_angle: 0, model_name: AX-18F, model_number: 18,
    radians_per_encoder_tick: 0.0051132692929521375, radians_second_per_encoder_tick: 0.0077780606556696,
    range_degrees: 300.0, range_radians: 5.2359877559829888, torque_per_volt: 0.14709975,
    velocity_per_volt: 0.84648468625000006}
  connected_ids: [1, 2, 3, 4, 5, 7, 8]

We double checked the baud rate settings in both the launch file and the motor settings using the Dynamixel Commander tool for Windows. Does anybody know what could be wrong here?

Update: When we use the latest revision of dynamixel_motors stack (rev 1555), all the motors are found. There are still checksum errors, though:

[INFO] [WallTime: 1321969002.397966] pan_tilt_port: Pinging motor IDs 1 through 8...
[ERROR] [WallTime: 1321969002.718987] Exception thrown while getting attributes for motor 4 - Invalid response received from motor 4. Wrong packet prefix ['\xff', '\xef']
[ERROR] [WallTime: 1321969002.738579] Exception thrown while getting attributes for motor 4 - Checksum received from motor 4 does not match the expected one (43 != 47)
[ERROR] [WallTime: 1321969002.750398] Exception thrown while getting attributes for motor 4 - Invalid response received from motor 4. Wrong packet prefix ['\x7f', '\xff']
[ERROR] [WallTime: 1321969002.802157] Exception thrown while getting attributes for motor 4 - Invalid response received from motor 4. Wrong packet prefix ...