Charging Contacts

The robots have two skis soldered to the bottom of the PCB and two charging prongs on the top. As well as supporting the front of the robot, the skis are ground terminals for charging. They make contact with a foil pad in the charging bay. The angled charging prongs are designed to wedge under the aluminium rail in the bay, which is powered.

These exposed contacts pose little electrical risk. The charging contact is isolated from the battery, so there is no exposed voltage on the prong. No current flows when any combination of prongs and skis meet in a robot-to-robot collision. Shorting one robot's skis to its own prongs is also safe. 

We used 1.2mm copper wire for the skis and prongs. This was a poor choice. The copper is so malleable that the skis bend when a robot crashes into a wall at full speed - a regular occurence which the rest of the robot is amply robust to withstand. This sometimes makes the robot drive in circles until the bend is corrected. If you build some Formica robots; use steel wire instead. The same diameter of steel wire would be far more rigid. However, bare steel will not take solder (definitely not PbSn, but I'm not so sure about lead free solders). Also, it will rust. Try nickel- or tin-plated steel wire, or perhaps stainless steel with some kind of magic solder.

To improve the reliability of the connection between robot and charger, some software tricks are used. If the connection is broken while charging, as reported by the status signals from the charge controller, the robot drives forwards until it is reestablished. Sometimes the robots bounce in the charger and start to oscillate. The situation would probably be better if there were some "give" in the contact. A flexible metal comb or brush in place of the aluminium rail in the charger might improve matters.

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