Stalled on the Ramp: Questions About a Robot's Motor

Robotics Team Notes: Why Does Courier Stall?

1The robotics team at Pinecrest Middle School built Courier, a small robot that carries trays of art supplies from the supply closet to classrooms. Courier works well on flat hallways, but when it carries a full tray up the ramp to the art room, its wheels slow down and sometimes stop turning. Team captain Ximena Thibodeaux and builder Rashid Vandermeer wanted to find out how to make the wheels push harder.

2Each of Courier's wheels is turned by a small electric motor. Inside the motor, a coil of copper wire sits between magnets. When an electric current flows through the coil, the magnets and the coil push on each other, and the push turns the motor's shaft. The motors in Courier are sealed, so the team's coach, Mr. Olumide Haugen, helped them build a larger test motor that they could take apart. Its coil sits between two stacks of disc magnets, and the gap between the magnets and the coil can be changed. A battery pack with a control dial sets the current, and a meter shows the current in amperes (A).

3To measure the push, the pair fixed a wheel to the test motor, turned on the current and held the wheel still with a force gauge pressed against its rim. The gauge showed how hard the wheel pushed on it, in newtons. They ran three tests, changing one thing at a time, and repeated each setup three times. The tables show the averages. In Test 1 they changed the number of magnets in each stack. In Test 2 they changed the current. In Test 3 they changed the gap between the magnets and the coil.

4Rashid noticed two more things. When he swapped the two battery wires, so the current flowed through the coil the other way, the wheel pushed on the gauge just as hard but in the opposite direction. And after five minutes at 1.0 A, the coil felt warm.

5Courier needs a push of at least 1.8 N at each wheel to climb the ramp with a full tray. Its motors now work like the test motor with two magnets in each stack, a current of 0.5 A and a gap of 2 mm. The gap cannot be made smaller than 2 mm, or the coil would rub on the magnets.