Hi, I'm planning to make a small set of self designed swarm bots.
In terms of a mechanical platform, what's my best bet. My criteria are cheapness and easiness. Ideally, it should cost < $25 per platform, and be easy to make.
So far, I'm thinking of pairing two solarbotics GM9 gearmotors and wheels with a thin wood base, and a third caster-esque wheel.
Is this the best way to go?
Sensors first
I discovered that any budget robot should focus on the sensor(s) first. They are harder to find and more expensive than motors. And your demands may be more specific too.
In a pinch, you could always pirate a toy for motors. Sensors are very rare in cheap toys.
nice 
25$ is very low budget… But still possible.
I agree on the sensors. I think the cheapest way to get sensors is to make them yourself(bump, ir).
On the motors I recommend 9g servos from ebay. Ebay is the first and only shop for you when using so low budget and 9g servos are the cheapest servos on eBay. You will have to hack them but its still worth the trouble and money because they have gearbox and motor controller built in.
I plan to use an IR
I plan to use an IR transceiver on each of the robot’s four sides. Id
The ebay 9g servos are amazing! That might be the way to go.
dagu
dagu has a cheap robot platform, for 15 usd ( i think), https://www.robotshop.com/letsmakerobots/node/9419
