Jan 092014

I’ve always been interested in competitive robotics. Two (or more) robots duking it out on the field of honor. Trying to solve a problem as fast or efficiently as possible, all the while trying to avoid, out score, or destroy the competitions robot.

Competitive robotics can generally be divided into to groups, direct combative and competitive problem solving. Both are a lot of fun, although combative can get pretty ugly, and expensive, in a hurry.

Competitive robotics puts a lot of pressure on developers and implementers to find optimal solutions in unstructured environments where other participants are actively trying to out score, block, or destroy your robot. You often need to come up with ad-hoc solutions in the middle of competitions to account for variations in the environment and to counter competitors robots.

Things we’ve had to overcome include changes in playing field surface, broken parts, changes in starting conditions, changes in competition rules that you don’t find out about until you get to the competition, and as always, differences in competitors robots that require you to adapt and modify your strategies on the fly.

In competitive robotics you push, pull, climb, and race to get the task at hand done before your competitor does. Changes in traction on different fields requires adapting your motion platform’s drive system. You can change your wheels, from slicks to diggers, or the other way round. Sometimes simply cleaning the tires, washing the dust and dirt off, improves traction enough to make a difference. Sometimes you need more. We’ve created studded tires by lacing plastic wire ties around the wheel and tire and cutting off the tails to leave the knobs pointing out. You do what you have to do to keep your robot moving and stop the competitors robot from moving.

Broken parts are always a bane. Hopefully you’ve built your robot to be maintainable. Motors die, drivers burn out, axles twist and break, screws fall out and pieces fall off. Broken parts require more an act of Nascar like pit crew skills than design changes. How fast can you swap out or adapt parts, under pressure in the middle of a competition, while maintaining a competitive robot. Sometimes you just work around the broken part, changing code to not use the broken functionality or adapting other sensors or actuators to try to work around the broken parts.

The four robot platforms I’m actively using are the

IFI Vex Robotics line,

Pololu’s Zumo Mini Sumo bot,

Pololu’s 3Pi,

and Sparkfun’s Redbot.

All of these platforms are extensible in various ways. Each of them have varying degrees of physical reconfigurability, with the VEX robot being the most physically reconfigurable, the Redbot next, then a toss up between the 3Pi and the Zumo (at the level of not much). All four platforms support adding sensors and of course full reprogrammability. The 3Pi, Zumo, and Redbot benefit from having the same microcontroller, the Atmel Atmega328p, and the same programming environment, Arduino or avr-gcc. The VEX controller is based on an ARM Cortex controller and uses the quite capable (but not free) RobotC development environment.

The VEX ARM processor arguably provides a much more powerful computing platform but is a more closed environment. The Redbot was designed to be modified with multiple sensor and actuator mount points and a fully open design. The 3Pi and Zumo have an open software development environment but suffer from a more closed hardware platform. Both the Zumo and 3Pi robots are able to be extended physically but not as simply as the Redbot or VEX robots.

The VEX robot, while powerful and benefiting from a reasonable line of accessories, is pretty much closed. The manufacturer, IFI, provides scant documentation on the VEX controller internals.

Each platform has different strengths and weaknesses.

The VEX line is expensive and relatively closed. The platform is physically very reconfigurable and being targeted at middle and high school students is pretty robust. The IFI provided sensors are mechanically robust and easy to interface. The downside of being targeted at students is the platform is not easily extended. IFI / RobotC provide few opportunities to step outside the bounds of the RobotC development environment. The upside is, it is harder for a student to get too far afield. The downside is, it is harder for a student to bring new innovations to the platform.

The Redbot line is fully open and extensible. The schematics, board layouts, and platform components are open and documented on Sparkfun’s website. The Redbot is probably the least robust physically. The Redbot is a great development platform but with an eye towards high extensibility and low cost it is probably at the highest risk of taking a catastrophic hit in a competition. That said the Redbot might be the best price to performance platform of the four. The Redbot’s size makes it too big to qualify for mini sumo competitions.

The 3Pi platform is hands down the best choice for line following competitions. It is pretty much designed to be fast and excel at line following. The 3Pi is low to the ground, is probably the fastest by far of the four robots mentioned here, and has a built in integrated line sensing array. The 3Pi platform provides for some extensibility. The 3Pi can be readily extended with sensors and other electronics but the motion platform is pretty fixed. Pololu sells a prototyping shield for adding sensors and other electronics. They also sell replacement motors with different gear ratios to achieve different speed to torque characteristics, but that is about it. Two wheel pivot steering with a caster is pretty much all you get. The 3Pi also requires a pretty clean and unobstructed field. The wheels are on the smaller side and not made for climbing or rough surfaces. If you are doing line following, maze solving, swarm bots, stuff that relies mostly on interesting software solutions and limited environmental interactions the 3Pi is the way to go.

The final robot I actively use is the Pololu Zumo. This bot is specifically a mini sumo pusher bot. Like the 3Pi, the Zumo is optimized for mini sumo competitions. Like the 3Pi the physical platform is not readily extended. The mini sumo rules limit the weight and footprint of the bot, so large physical changes to the bot are probably not in the cards anyhow. The Zumo allows for some sensor extension but the base platform with the sensor array consumes most of the available IO of the 328p microcontroller. Like the 3Pi the motors can be changed to achieve different gear ratios. Also similar to the 3Pi the Zumo relies on two motor slip skid steering. The Zumo is a tracked platform and has a pretty good grip but again like the 3Pi it requires a relatively smooth obstruction free playing field.

Each of these robots solves a different problem in a different way. For line following competitions and newer roboticists the 3Pi is the way to go. For mini sumo the Zumo is great platform. For the home hobbyist the Redbot is probably the best answer. For a more physically extensible and more powerful (and significantly more costly) platform the VEX line is the way to go.

In the end all of the robots are great solutions each with it’s individual strengths and weaknesses. Pick the one that best fits your style or target competition and go for it!

Here’s hoping to see your robot on the competition field.