Servo - December 2013

76 SERVO 12.2013

welding (as is typical for assembling a metal robot) since screws can be removed or placed differently to ‘tweak’ a robot’s chassis — welds cannot.

Aluminum sheet metal — especially 6061-T6 stock — is, in my opinion, the best material for a robot base, though I have also used harder 7075 in applications where little machining is required. Both types are readily available at many locations and are easy to cut with a bandsaw, though I would suggest using a large metal shear at a nearby machine shop or technical college as the edges are so much smoother (especially for thicker stock). Small holes are easily cut into thinner aluminum and steel stock using a chassis punch as shown in Figure 5. Drill a small pilot hole and use a ratchet wrench to tighten the three punch pieces until the hole pops through.

Cutting large holes in any sheet metal is a bit harder. Large bandsaws have a way to cut the blade and insert it in a small pilot hole, and then weld it back together with a welder mounted on the saw. If you don’t have access to one of these, you can use a Sawzall type of reciprocating saw or hand jigsaw fitted with a metal cutting blade to enlarge holes for wheels and such.

It takes a bit of practice to cut a nice hole. You’ll need to do a bit of filing to smooth out the ragged edges. A fly cutter as shown in Figure 6 can cut larger round holes using a drill press.

Make sure your work piece is securely clamped down. Either of these methods work for mild sheet steel, as well. Plywood, HDPE, most plastics, brass stock, and just plain lumber can also be used for robot bases and structure.

Designing Robots for Outdoor Operation

Considering the growing popularity of the RoboMagellan contests around the country, I will start with base designs that are best for these types of autonomous robots that must traverse rough outdoor courses during competitions.

I was just at the SRS’s 2013 Robothon exposition at the Seattle Center (in Seattle) and witnessed off-road configuration RC car chassis like Mark Curry’s robot in the 2012 SRS RoboMagellan competition shown in Figure 7 is typical of most entrants. Large knobby tires coupled with a beefy drive train mounted on a rigid base can easily traverse thick grass and inclines.

A large hobby shop — especially one that deals mostly in off-road RC cars Remember, it is not going to be an RC vehicle anymore; you are going to need special power supply setups for the motor drivers and also for the other electronics. Sensors (cameras and object detection), GPS antenna mounting, shaft encoders, and one or more microcontrollers need unique mounting locations.

If you are mostly interested in RoboMagellan-style robots that generally operate at lower speeds, large shock-absorber mounted wheels are not that important, but that also depends on the area where the competitions are to be held.

At this year’s Robothon, one RoboMagellan contestant’s entry was doing quite nicely until it happened upon a steep 6” edge of grass next to a sidewalk. Instead of smoothly transitioning from the grass to the pavement, the robot dropped at a 45º angle to the edge of the concrete and stalled, unable to move forward or backward.

Look for vehicles that have wheels that protrude past the front and rear portions of the chassis to allow them to grip and climb over obstacles. Remember that larger wheels may have more traction surface, but