Servo - January 2007

Select Your Motor

One of the biggest decisions you have to make when building your bipedal robot is what kind of actuators to use. We used Robtis’ Dynamixel servo motors, model DX-117; and will upgrade to the RX- 64 for the next version of DARwIn (Figures 2 and 3). The DX-117 has a maximum torque of 39 kg-cm and the RX- 64

∨ TABLE 2. Table of specifications for Robotis’ Dynamixel DX-117 servo motor.

DYNAMIXEL DX-117

Mass 66 g

Resolution 0.35°

Angle Range 300°

Voltage 12V-16V

Torque 29 kg-cm — 39 kg-cm

SPECS

CPU PC/104 (Planned)

Height 600 mm

Mass 4 kg

Degree of Freedom 21

Power 2 Li-poly @ 7.4V

_SensorsRate gyro, force _sensors

< FIGURE 1/TABLE 1. Here is a photo and statistics table for DARwIn.

has a maximum torque of 64 kg-cm (Table 2). Both have built-in position and speed controllers. All user control is done with RS-485 serial communication.

If you are on a tight budget, regular R/C servo motors are fairly inexpensive, but since you get what you pay for, the performance may be unacceptable. You can also use miniature DC motors, but you need to design your own position feedback controller or purchase one such as the “allmotion” boards, which can be expensive. Different actuators have different torque properties and mass properties; so if keeping your robot lightweight is a must, you may want to put the stronger, heavier motors in the joints that see the greatest load.

Kinematic Design

Once you pick your actuators, you need to decide on your kinematic structure — this is one of the most important things to do in the design process! You can make your own or use ours (Figure 4). Your kinematic model will determine how you design your joints and how you attach the actuators.

Where to Put Stuff

legs. It is possible to house hardware in other cavities in the robot, some such cavities may be in the leg or foot (depending on the design). Though the batteries are in the chest for the current design, next year, we plan on placing the batteries in the feet of the robot. This placement will make room for our new PC/104 board computer, IEEE 1394 PC board camera, rate gyro, DC-DC converter, and protection circuits.

Let’s Be Honest: Materials and Machining

Finally, before you start designing the links and joints, determine what materials you can afford to buy and what machining tools you have access to (milling machines, welding equipment, etc.). This will dictate how ornate (or how simple) your robot’s design can be. If you have access to a four-axis CNC mill and large amounts of bar stock aluminum, you have much more freedom in your design than if you only use sheet aluminum.

For DARwIn, time constraints and availability led us to use sheet aluminum for almost the entire structure. With some experience under our belt, we’re designing and building DARwIn 2.0 in half the time and using a four-axis CNC mill to create most of our parts. Figure 6 shows a CAD drawing of our new hip design. We recommend using aluminum for the structure of the robot because of its price, weight, strength, and the ease of machining it. If you have the funds, using rapid prototyping would be an excellent option.

∨ FIGURE 2. RX- 64 motor.

40 SERVO 01.2007

∨ FIGURE 3. DX-117 motor.

Next, decide on a location for your extra hardware: CPU, batteries, sensors, etc. We decided to house everything except for the force sensors in the chest for simplicity (Figure 5). One issue with placing everything this high is it creates a larger torque on the motors in the

Design

Now that you’ve nailed down the essentials, you’re ready to start designing the links and joints of your robot. You can get as creative as your imagination lets you, but if you need a starting point, here are some tips and examples from DARwIn.