RobotChallenge in Vienna
Me and Vilse from CRF spent about three weeks before RobotChallenge building this robot (the robot is called Sleipner and is an autonomous 3kg sumo robot). As we had other thing going on, such as our usual studies at Chalmers, this was done quite in a rush. The first time we tested the robot autonomously was a few hours before the first matches.
Issues with the design
The competition didn’t go as well as planned. The biggest problems were that the magnets were too far away from the ground and that the blade was really weak. The robots with the better blades always picked up Sleipner in front collisions. Some of them had blades made from really sharp knives. The blade on Sleipner was a 1mm thick sheet of spring steel. Recently I made a few attempts at fixing a few of those issues.
The improvements
Firstly, I milled a new base plate for Sleipner. As the wheels are adjustable in height this allowed the magnets to be placed less than 1mm from the ground. Also, they could be distributed in a way to get a better centre of gravity (or, centre of magnet force in this case). The benefits of this action can clearly be seen in the video below.
The next major improvement was the new blade. I bought a couple of knives for a thickness planer made from HS-steel. This was not tested against another robot, but it is most likely a major improvement.
Watch the video for more details
The next 3kg sumo robot
This robot really has enough power with the BLDC motors, but it would be nice to have motors with lower RPM to avoid the gear boxes and gain 400 (the weight limit is 3kg) grams for more magnets. Brushless outrunner motors are perfect RPM-wise, but they have a major drawback: they are always sensorless. This means that they have to be run in open loop at low RPM and thus have very low torque before they speed up. This is no problem for air planes, but for sumo robots that change direction all the time it is completely useless.
I bought a couple of these outrunner motors and managed to mount hall-effect sensors in them. This works really well and gives the motors great torque at the start. In addition, they are also more powerful than other motors in similar size. The ones I modified are 192 grams heavy and are rated 1000 watts each. Also, the KV (RPM per volt) is perfect to run the motor with a small gear against the gears currently mounted on the wheels, which saves 400 grams in total for the gearboxes on the robot. I will write a bit more about this motor modification in another post soon.