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There are several different ways to build the robot, here we have a common example of a mini-sumo. This robot has a pc-board on top that contains most of the electronics. The middle area contains infrared sensors that look for the opponent. A pair of servo motors are used for driving the robot around and the batteries are underneath. Notice the use of a large scoop on the front to push the opponent around with.

In contrast, the maze runner tends to be more complicated, employing more sensors, and in this example, multiple processors to handle not only moving around through the maze, but tracking distance with wheel encoders, and mapping the maze in such a way as to remember where it’s been and choose an optimum return path or finish the route. Again a pair of servos are used for drive motors and an LCD display for operation status or debug information.

You can also make robots from the Lego Mindstorm set. Pictured here is a mini-sumo robot with a light sensor to detect the arena edge. All these robots employ two drive wheels to move the robot around, with a tail wheel, bumper, or front scoop to keep the robot level. This method is the simplest and economical to work with, but you can also use 3 or 4-wheel drive systems.

For the same cost and a simpler mechanical arrangement, you can use an optical sensor that detects the color of the ring surface and signals the controller when the color changes. We can take advantage of the fact that the ring is painted flat black except for a 1” white circle around the parameter. By placing a light source such as an LED and a photo sensor on the bottom of your robot, you can detect that white edge when your light source gets reflected off the white surface and the detector sees the change. For this sensor I would recommend a simple photocell and LED combination. There’s no reason to use a fancy range finding sensor just to see if the floor is black or white. The photocell and LED need to be placed close to the floor and shielded from ambient light sources. You also want to place the sensor at the front of the robot so that the white edge is detected before the robot actually goes over the edge. The easiest way to mount this sensor is behind the front scoop if your robot has one, as shown in this picture.

In addition to the floor sensor you might consider using range sensors to find your opponent. Without some sort of “eyes” for your robot, it must wander around the ring till it happens to run into the opponent. By mounting a pair of range sensors looking forward and angled to the sides, you can tell if the other robot is in front of you or slight off to one side, and adjust your coarse to intercept it.

The drive motors are model airplane servos that have been modified to rotate continuously, instead of 90° side to side. The steps to modify the servo varies slightly between the various brands and from model to model. The appendix has links to some of these servo hacks. You may ask “why bother with servos if we have to hack them up?”. Well these servo are light and compact with excellent torque and they are easy to control with the various controllers previously mentioned. You can purchase them for as little as $10 if you look around. You will need two servos, one for each side of the robot. By turning each servo on and off you can make the robot turn like a tank would. With one servo going forward and the other in reverse, your robot would spin in a tight circle.

The wheel selection is critical for the Mini Sumo since your success is directly related to how much traction you have when pushing the opponent around. The amount of surface area you have in contact with the floor won't automatically give you more traction. In actual application I’ve seen a ½” wheel go head to head with a 1” wheel and both robots were pushing each other with the wheels spinning in place and no forward progress. Another factor in your wheel selection is keeping the overall width under 10cm. A standard servo is about 1.4” tall and when you place 2 of them back to back, you have a width of 2.8”, so your wheels can’t be wider then ½”. You could use a low profile servo which measures just over 1”, which would allow you to use a ¾” wheel. The down side of using this servo is it’s price, $27 compared to $12 or so for a standard servo. A good source for wheels is model airplane wheels, which you could purchase at a local hobby store. They run from 1” to 6” in diameter, but a Mini Sumo would use something between 2” and 3”, with a thickness of ½” to ¾”. Another type of wheel which is very popular with Mini Sumo is a thin plastic wheel with a rubber o-ring around the perimeter. The advantage of this wheel is being thin allows you to keep your design within that 10cm.

Now that you know what parts to use, you need to put it all together. You could purchase the various parts from different sources and fabricate a frame for the robot from raw materials, or you could by a kit which would typically have all the parts except the controller. Here are several more Mini Sumo’s built by PAReX members.

Let’s talk a little about what we want the Mini Sumo to do. If you have a robot with only a floor sensor, then you need to create a pattern for moving around the arena. If you simply run until you hit the edge of the arena and turn around, then your back and forth pattern doesn’t cover much of the arena.

A better approach would be to turn about 35 degrees or so creating a sort of star pattern that will cover most of the arena surface and give you a better chance of finding your opponent.