Wax That Melts on Purpose: Cooling a Delivery Robot

A Robot That Overheats

1The robotics team at Maple Grove Middle School built a wheeled robot that carries mail and packages between the school's front office and the classroom wings. On long runs, its motor controller, a small circuit board that sends power to the motors, gets hot. The board sits on a metal heat sink. If the heat sink goes above 70 °C, the controller shuts itself off to avoid damage, and the robot stops in the hallway. The team noticed that this happened most often on afternoon runs, after the robot had already made several trips.

2Team captain Sofia Lindqvist read that some electronics are protected with a wax that melts at a useful temperature. With help from their coach, Mr. Bashir Rahimi, the team bought 20 g of a wax blend that melts near 54 °C. Sofia sealed it in a small aluminum can, filling the can only about 85 percent full, and the team clamped the can against the heat sink. Aluminum carries thermal energy well, so the wax inside stays close to the temperature of the heat sink.

3To test the idea, the team ran the robot on the same 28-minute route twice in a 22 °C hallway: once without the wax can and once with it. Both runs used the same 2 kg box, the same speed and a fully charged battery. A temperature probe on the heat sink took a reading every 4 minutes. During the second run, Darius Mensah peeked through a small clear window in the can's lid to see the wax, and he wrote down what it looked like each time the probe took a reading. By the end of that run, the liquid wax had risen almost to the lid.

4After the second run, the team parked the robot with its power off and kept recording the heat-sink temperature as it sat in the hallway. Those readings are in Table 2.