Servo - May 2009

Institute of Industrial Technology bears little resemblance to the Hubo bipedal humanoid developed by KAIST. SeRoPi uses a far more stable wheeled mobile base than Hubo and Asimo’s bipedal structure. SeRoPi’s wheels allow it to move about at a fairly fast 4. 5 mph — a fast walk for humans.

What distinguishes SeRoPi from the others is its dexterous arms and ‘hands’ that allow it to reach down to the floor to pick up objects. Using dual stereoscopic camera eyes to recognize objects, SeRoPi can use both hands to manipulate these objects using computer vision.

The 132 pound, 50 inch tall robot maneuvers itself around on two differentially-driven wheels with a passive castor wheel for balance.

FIGURE 8.

FIGURE 9.

The Robotis

Dynamixel Actuators

Robotis Bioloid Dynamixel servos are different from all the other model-aircraft types of servos because of their onboard intelligence. In fact, they’re referred to as actuators instead of servos.

The basic AX- 12 Dynamixel actuators (see Figure 10) have the usual motor and associated geartrain that we are all familiar with, but the similarity ends there. Each AX- 12

FIGURE 10.

actuator has its own serial ID over a ‘Robotis TTL multi-drop digital packet communications network’ and because of this, a single set of three wires — data, 9.6V VDC, and ground — runs to all actuators in series. Or they can be daisy chained together as shown in Figures 11A and 11B (courtesy of CrustCrawler).

There are two identical Molex type connectors on the sides to allow each successive actuator to be connected with a jumper cable. Typical PWM model aircraft servos usually require three leads to each individual servo, or at least an individual 1.0-2.0 ms PWM signal lead to each, and shared power and ground leads.

Digital packet communication is possible with all Dynamixel actuators to control direction, speed, and torque with actual feedback of these parameters and — depending on the model — outputs of internal temperature, voltage, light, and sound detection. The individual user manuals are very well written. The higher-end models — DX-117, EX-106, RX- 10, RX- 28, and RX- 64 — use RS-485 network communication and have metal gears.

The Dynamixel AX-S1 smart sensor module integrates a three-directional infrared distance sensor, an infrared remote control receiver, a sound sensor, a light sensor, and a

buzzer, as well as the network interface to communicate with the actuators and controller. It can also detect small internal temperature changes, the system’s voltage, and convey all readings into a range of 1,024 increments at up to 1 Mbps. It utilizes the same daisy chain interconnections as the series of actuators.

As with the AX- 12, the recommended operating voltage for the AX-S1 is 9. 6 volts; not the 4. 8 to 6 volts with model aircraft servos. The sensor module shown in Figure 12 looks a lot like the AX- 12 and is mounted in a similar manner, but the round flange is just a mounting area, not a rotating disc.

The Robotis CM- 5 main controller is based on the popular Atmel ATMega 128 (128K of Flash memory) running at 17 MHz. Also configured for 9. 6 VDC, it can communicate with the AX series of Dynamixels, their two-axis gyro module, the AX-S1, the Zigbee ZIG-100 standard, RS-232 or RS-485 serial, and by six command buttons on the panel. The controller also has an LCD display for data display and control. There is a stand-alone CM-2+ evaluation board with the same Atmel controller and similar functions that is fully compatible with the Bioloid components for dedicated control systems.