For this year's Fire Fighting robot I decided to improve on the theme from Gamadome, the 'bot I made with Fred Martin for the 2002 contest. But instead of Ban Roll-Ons for casters, this year I'm going to use omni-wheels. A little more professional looking. I also want to use the new motor controllers I designed that will give me PID control over the motors. Some other aspects of the design:
Here you can see the very first sketches of the mechanical design concepts. It's a simple disc, with two drive wheels and two omni-wheel casters. The diameter is limited by the length of the motors, and the ground clearance requires at least 2.5" wheels - I'm using 3" diameter wheels. This sketch is a side view, and a little confusing until you see the next sketch, which is the top view.
Here's the top view:
For all the other parts, they will stack on similar disks, mounted on top of the base disk, using plastic standoffs. This is sometimes called a "wedding cake" design.
At the time of these sketches, I had the following a pretty good idea of which parts I would use, based on previous experience and the discovery of a couple of cool new finds. Here's the preliminary bill of materials:
Bill of Materials
Motors: Surplus Globe motors, 1.5" diameter, with 500CPR encoders. They are 24V motors, so if 12V isn't speedy enough, I can add another 12V battery. There is no gearhead, and I'm worried about the torque. They are only rated at about 3-5 oz-in torque without a gearhead. And I'm only going to run them at 250 rpm max, and the top speed of these puppies is around 5000 rpm. Sounds like a challenge, but the built-in encoders are just too good to pass up. Last time I had a lot of trouble tweaking the home-made encoders.
Wheels: I'm using 3" pulleys from smallparts.com. They have a 1/4" bore, and my motor shafts are closer to 1/8" diameter. Another challenge. For the "tires" I used common buna (black rubber) O-rings from the hardware store. The O-rings are 2.5" diameter, and stretched nicely to fit the 3" wheels. I was lucky that if the wheels were any larger, I would have had to special order the O-rings. I would like O-rings that were a little thicker, because they're just barely thick enough to extend beyond the wheel rims.
Casters: Omni-wheels from Acroname. 1.5" diameter, one in front, one in back. Need to devise a way of mounting them to allow free rotation, and maybe a little bit of spring, to keep contact with the ground. The challenges are adding up!
Power Supply: Nicad 10-pack (12V) from Digikey. This worked well in the past. Last time I used the 1000mAH flavor, this time I'm trying the 1500mAH version, for a little more money, but hopefully it will last longer. I'm using the same charge circuit - just a 5W 47-ohm resistor in series with the battery, with an LED across the resistor, and coaxial wall-wart connector to plug it into the wall. When the LED goes out, it's basically charged.
Motor Controller: I know this great company, Gamatronix, that sells this great controller that I designed, the Gamoto. I think I'll use those ;). One for each motor. This will give me precise PID control over position, speed, and acceleration, along with accurate tracking, so I can do dead-reckoning.
Brain: HandyCricket, designed by my friend Fred Martin. Logo is just too fun and easy to program to use anything else.
Wall Sensors: The now-standard Sharp GP2D12 IR ranger, analog version. These will plug nicely into the spare analog input ports of the Gamoto. There are four ports on each Gamoto, but I will only use two IR sensors: one straight ahead, one to the right.
Flame sensor: OK, I'm finally breaking down and buying the Hamamatsu/UVtron flame detector package from Acroname. There is no more sensitive sensor for detecting flames. The simple photocells I used last time were just too finicky.
Fire-putter-outer: This year I'm sticking with the tried and true motor with a fan, but I will be adding a Darlington transistor (TIP104) between the Cricket's motor output and the motor, allowing me to use a much higher voltage (12V), while still having some speed control from the Cricket. I bought this motor and fan kit from SuperDroidRobots.com.