Mini Sumobots Assembly and Construction Tips
The Parallax Sumobot kits are great, and the instructions clear. I am very happy with the quality of the materials used, as so far all the kits have withstood the ravages of multiple twelve year olds and countless combats. The following are things we discovered as we went along during the assembly and modification process.
One of the most important things I learned is that the batteries should not be issued until the robot has been inspected for correct wiring. It is possible to destroy the line detection sensors if the leads are reversed. Unfortunately, we learned this one the hard way. As soon as I heard a students say "What is that smell?", I knew I was in trouble. I also made a rule that before wires are disconnected for any reason, the batteries are removed and given to me. Before reissuing the batteries, I check the wiring carefully.
A minor detail is the mounting of the battery holder. If mounted as far forward as possible, the robot will tip over more easily. On the other hand, it will allow the robot to right itself much more easily if given a chance. If the battery holder is mounted as far back as it can go, the robot is less likely to tip over but less likely to right itself. So far, opinion over which is better is divided and much time, testing and discussion has been given to the subject. When installing the battery holder, make sure you have already firmly bent the leads in the correct direction leading to the connection points. The leads need to be carefully bent to make sure that they do not contact and rub the wheel during operation.
It is wise to round up a few spare machine screws, nuts and washers to have on hand. It is easy for one to get dropped and lost, and they can come off unnoticed during combat. We actually wound up finding all of them in the end, but I certainly don't expect to always be that lucky!
The only detail of the kit that I found needs to be modified right out of the box is the mounting of the opponent detection infrared L.E.D.. The way it is mounted causes it to be knocked off frequently during matches, blinding the combatant. I solved this issue by mounting the L.E.D.'s under the main controller board using 1/4 inch wire holders available at any hardware store. A close up of this arrangement can be seen below.
Note that one should place washers on the mounting standoffs on the opposite side to compensate for the added height of the wire holders. I cut the leads of the L.E.D.s short and then soldered wires about 5 cm long to the leads of the L.E.D.s and insulated with shrink tubing. You can see in the picture that I cut off the top mounting part of the wire holders. The L.E.D. and holder will not fit without this modification. Testing showed no discernible difference in opponent detection range or sensitivity using this configuration, and the eyes can can no longer be dislodged during combat. However, it is important to secure the wires well close to the body of the 'bot, or they can be snagged during combat.
Alignment of the scoop is an important aspect to successful competition. If the scoop has a gap between the scoop and the surface of the ring the 'bot will tend to get tipped over by their opponent. The scoop alignment can be occasionally knocked out of whack during matches or while working on the robot. Students quickly learn that they had better check the scoop before and after matches.
One of the most common modifications that students want to make is to the scoop. Scoops with a curve designed to allow the opponent to ride up and tip over, scoops designed to get under an opponents scoop, scoops with white paper to confuse the opponents line detectors, you name it. We used copper to make custom scoops, because it is soft and easy to work with. One issue with all of the scoop designs is that the mounting bolts tend to get caught by the opponent's scoop. This presents a leverage point to tip the other robot over and prevented the curved scoop idea from operating as intended. Next session I plan on trying scoops with the mounting bolts soldered to the back.
One issue with scoop modification does arise if students modify the scoop in such a way as to cause the angle of the robot to change. As the kit comes with the stock scoop, the Sumobot's eyes are perpendicular to the surface of the ring. Scoop modifications may change this angle, and if the Sumobot winds up slanted downwards it may cause the robot to see the playing surface as an an opponent. If the Sumobot is slanted upwards, it may miss detecting opponents. This problem stumped us for a little while before we figured it out. All of a sudden, 'bots that had been working well were having trouble locking in on their opponent. Within fairly wide parameters this can be compensated for by bending the leads of the opponent detection receivers to make them perpendicular to the playing surface.
Another popular modification is to put some type of support on the back of the robot to help prevent tip over. These came in a number of different forms. One good place we found to attach such hardware is the rear battery holder screw. These devices can be pretty effective, but it is important to make sure they are not scratching the surface of the playing area. I have students use files to round the corners and smooth the edges, then put tape on to decrease scratching.
The kits weigh in well under 500 grams, the maximum legal weight. This is done intentionally to allow modification and additions to the robot to stay within the weight limit. As this is a beginners Sumobot class, we don't add a lot of weight with modifications. Since one does seem to want the 'bots at the legal maximum, we used sheet lead to add mass. The weights are generally attached using two sided tape and/or masking tape.
An important issue we ran into using lead weights was that if the weights become dislodged during combat they can short out the leads on the bottom of the controller board. We ruined a battery pack this way, with the short causing the holder to melt, and I am very grateful that is all that got ruined. To prevent this problem, all weights are wrapped in masking tape before being attached, and then are inspected by me to make sure they are secure. Any modifications or additions to the area in-between the top of the servos and the bottom of the controller board should watch carefully for this issue, as it could trash the controller.
Unfortunately, I am limited as to how extensively students can be allowed to modify the robots because they must be disassembled and used again for the next class. Glue or any other permanent type of thing can't be allowed. This is a pity, as students have many good ideas to modify and build up their robots that we can't do. To this end, I am working on developing an advanced Sumobot class in which the students keep their robot. This will probably be in the form of a summer class or camp. Upon discussing this idea with parents, one thing that became clear is that if the parents are to make such an investment, there needs to be follow up classes during the school year. This will allow students to continue using their robot and make the investment worthwhile.
I made a ring out of a sheet of Marlite, available at most hardware stores. Marlite is covered with some kind of white coating, so all I needed to do was paint the ring. The ring I made isn't a legal mini sumobot ring. A legal ring is supposed to use a certain type of linoleum for the playing surface, but I have not as yet been able to locate any. Instead of the linoleum, I painted a black circle on the Marlite. I used Krylon's new plastic paint for the job, and it worked pretty well. It does get scratched during combat, but so far an occasional repainting and touch ups with Sharpie markers has kept it functional. The problem is that a relatively small scratch can spoof sumobots into thinking they have encountered the edge.
To make a neat circle, I started by using a large compass to mark a 77 cm circle on the board. Then I covered the line with many 20 cm or so pieces of blue masking tape, making sure to put about half of each strip of tape over the line I had drawn. I drilled a hole for a small bolt in the center of the ring. I then took a yardstick, fortunately marked in cm's on one side, and drilled a hole in one end and attached it to the bolt in the center of the ring. At the correct distance to make a 77 cm ring, I carefully worked an Exacto blade through the yardstick until it stuck through about 5 mm. By rotating the yardstick a nice circle was cut out of the tape. Peel the inside tape off and complete masking the outside, and you are ready to paint. I wish I had taken some pictures of this process, as it is much easier than it sounds. Allow the paint to dry for a couple of days before using the ring.
Questions? Comments? Contact me and let me know!