Servo - December 2013

outside the robot with LiPo chargers and use them wisely. Store them outside your robot when not in use, as they can ignite when being charged or when the robot is in a dormant state.

Model helicopters, quadcopters, and robots with LiPo batteries have set many car’s backseats on fire. That’s the price you pay for otherwise great batteries.

Ready-built Robot Bases

Parallax built a large 18" diameter robot base for their Eddie robot, and also for their MadeUSA (see Figure 15). This particular photo shows an added Eddie controller board seen on the bottom platform. It was a bit pricey for most builders as it had expensive-to-make machined wheels and mounts made in the US-based Parallax facility, and 10 Ping))) and IR sensors on the periphery. I've consulted with Ken Gracey and his engineers on the design of a low cost yet high quality large base and they have come up with a $336 base called the Arlo Robot. (They are hoping to drop the price even lower.)

The new base uses the same gearmotors and 18" platforms, but uses molded plastic mounts and wheels as shown in Figure 16.

Adding their HB- 25 fan-cooled motor controllers, a second platform, IR and ultrasonic sensors, quadrature encoders, and even using machined aluminum wheels and mounts are available as upgrades or options.

The Wild Thumper robot base shown in Figure 17 is an ideal mid-size base at 16. 5” long by 11. 5” wide at the wheels, and 5” high. It weighs six pounds and has a 5 kg/11 pound payload capacity. It features a 2. 5” ground clearance when lightly loaded.

Several robot suppliers carry this base and some add their own drivers. The $350 Parallax version includes two of the Parallax HB- 25 fan-cooled motor controllers and also a Propeller project board. The six wheels are independently spring-loaded to traverse rough terrain but do have Ackermann steering.

To drive the robot, the three right motors and the three left motors are driven differentially by the two motor controllers. It is definitely skid steering. Drawing 6. 6 amps each at stall, that’s 19. 6 amps for each side.

Internally, there is room for the two controllers in one compartment and a LiPo or NiMH battery in the other compartment. You can drive the wheels at 350 RPM for a 34:1 ratio gearmotor — that is a fairly high speed for a robot. It is one great base for the price.