Robot Docking: Mapping the Fields Around a Charging Dock

Finding the Dock

1The robotics team at Brookhaven Middle School built a small robot that carries parts and tools across its workshop. When its battery runs low, the robot must find its charging dock and line up with two metal contacts. Team captain Ximena Duarte-Holm suggested a magnet: the dock has a strong magnet in its base, and a magnetic sensor inside the robot's plastic body measures the strength of the magnetic field around it in microteslas (µT). Team members Pavel Ruiz and Hanako Abernathy set out to learn how the dock magnet affects things that do not touch it.

2First, Hanako laid the dock magnet on a table and set twelve small compasses in a ring 5 cm from it, and twelve more in a ring 15 cm away. With the magnet removed, the needles in both rings pointed north. With the magnet in place, the needles in the inner ring turned to point along curved paths that ran from one end of the magnet around to the other end. When she slowly spun the magnet, the needles in both rings swung around with it, although the outer needles swung less. Compasses 40 cm away kept pointing north no matter how she turned the magnet.

3Next, Pavel moved the robot straight toward the dock and read the sensor at several distances. He took each reading three times and subtracted the reading he got with the magnet removed. He repeated the 10 cm reading with a sheet of plywood and then with a clear plastic panel standing between the robot and the dock. Both times the reading was 200 µT, the same as with only air between them. The team programmed the robot to switch to its slow docking mode when the reading rises above 100 µT.

4During testing, the team noticed something else. After the robot rolled across the workshop carpet for a few minutes, bits of foam packing jumped up onto its plastic shell before the shell touched them. The longer the robot had been rolling, the farther away the foam bits could be and still jump. After Pavel touched the shell to a metal strip connected to the ground, the foam bits stopped jumping.