Servo - December 2011

sub LegoMotors(A,B,C)
 SerialOut 12,0,0x80,4,0,A,1,0,0,0x20,0,0,0,0
 SerialOut 12,0,0x80,4,1,B,1,0,0,0x20,0,0,0,0
 SerialOut 12,0,0x80,4,2,C,1,0,0,0x20,0,0,0,0
return

FIGURE 9. This RobotBASIC function provides speed and direction control for all three NXT motors.

FIGURE 7. LEGO parts make it easy to use bolts to mount motors to the foam-board base.

can be expensive but worthwhile if you know you want a robot with one of these wheel configurations. This makes having an inexpensive experimental platform even more valuable because it can help you decide if multi-directional wheels are appropriate for your projects.

Nexus has just started looking for distributors here in the US so hopefully their products will be available soon without the high cost of international shipping. If you can’t find a local source for their products, their marketing executive Anny Kong ( Anny@NexusRobot.com) has agreed to provide special pricing for SERVO readers that includes shipping to the US. Three omni wheels are $50; four mecanum wheels are $260; and the robot kit shown in Figure 5 is $1,450.

If you have a LEGO NXT system, you can easily mount Nexus’ omni wheels (or perhaps similar wheels from other sources) to each of three motors. Rather than trying to create a base solely from NXT parts, we chose to use foam board as shown in Figure 6. Foam board can be purchased at nearly any craft store, is easily cut with a razor blade or hobby knife, and can be glued to quickly create relatively strong structures. The NXT motors can be mounted to the foam board as shown in Figure 7. We chose to orient the motors vertically, but foam board is so easy to work with that you can let your imagination run wild. Vertical foam board beams were added next to each motor to provide lateral support for the motors and to act as legs for an upper level. Our finished robot is shown in Figure 8. The second level supports the NXT brick (the computer) and — because it is glued to the vertical foam board supports — it adds significant rigidity to the assembly. Notice that we added an ultrasonic distance measuring sensor and three line sensors to make it easy to experiment with a variety of environmental situations. You could use Mindstorms — LEGO’s graphic oriented language — for your experimentation if you prefer, but we used RobotBASIC (a free language available from www.RobotBASIC.com) to interface with our robot. One of the nice things about the NXT is that communication with the motors and sensors can be handled using direct commands. This allows RobotBASIC programs to control the robot (and read sensor data) without downloading any programs to the robot itself.