Servo - April 2012

FIGURE 3. An early photo of Maxwell, a human-scale mobile manipulator using Dynamixel and ROS.

FIGURE 4. The TurtleBot arm.

42 SERVO 04.2012

4,096 positions over 360 degrees of motion, or a resolution of 0.087 degrees. This greatly improves our ability to place the robot gripper exactly where we want it.

The new MX-servos also have a 32-bit microcontroller in them, unlike the older eight-bit controller found in previous series. This allows a maximum bus rate of up to 3 MBps which could be useful if you want to very finely control your robot arm. The improved controller also offers real PID control and access to the individual P, I, and D term gains which opens up a number of cool possibilities for tweaking the servo performance. In all cases, the MX-series servo has mechanical dimensions compatible with its equally-sized RX predecessor. The MX- 28 servo is available now; MX- 64 and MX-106 servos should be on shelves shortly.

Dynamixel Controllers

Since the Dynamixel servos have a serial bus, you can actually connect them directly to a serial port or to a USB port using a USB-to-serial adapter such as the Robotis USB2Dynamixel. However, we can get even better performance by placing a stand-alone controller in between the USB and Dynamixel buses.

Several years ago (when I helped start the Mech Warfare competition at RoboGames), I found myself wanting an easy-to-use microcontroller board with integrated XBEE wireless radio and the ability to control Dynamixel servos. Out of this need grew the ArbotiX RoboController. After Mech Warfare that year, I found that a number of other people had a similar need. The ArbotiX RoboController is available from Trossen Robotics, and all of the software is released under an open source license.

Why do I bring this up? Because over the past year and a half, I have been using the ArbotiX to connect my robot hardware to a laptop running ROS. I have stopped using the XBEE radio connection (as was used for remote control in Mech Warfare) and started using a direct hard-link wire (a USB-to-serial cable from FTDI) to achieve a more robust connection, and I have added larger motor drivers to support a larger drivetrain for my 25 pound, 5 ft tall mobile manipulator, Maxwell (Figure 3). Maxwell is currently undergoing a series of upgrades after which he will have a new arm consisting of mostly MX- 64 servos, instead of the current mix of both AX and RX servos which has caused a nightmare of wiring issues. I'll have more details about Maxwell in next month's SERVO Magazine.

I built the ArbotiX-ROS drivers from the ground up with the intention that users could quickly configure a robot using only a few lines of a configuration file without writing much (if any) code. The drivers are documented at www.ros.org/wiki/arbotix and are released under an open source license. The controllers offer code that allows Dynamixel-based robots to easily leverage the navigation and arm navigation capabilities of ROS.