Pololu 3pi Robot - Manual (Page 14)

The 30:1 gearmotor used on the
3pi.
Motor operation: current and speed vs. torque.
The free-running speed of a small DC motor is usually many thousands of
rotations per minute (rpm), much higher than the speed we want the wheels of
a robot to turn. A gearbox is a system of gears that converts the high-speed,
low-torque output of the motor into a lower-speed, higher-torque output that
is a much better suited for driving a robot. The gear ratio used on the 3pi is
30:1, which means that for every 30 turns of the motor shaft, the output shaft
turns once. This reduces the speed by a factor of 30, and (ideally) increases
the torque by a factor of 30. The resulting parameters of the 3pi motors are
summarized in this table:
Gear ratio: 30:1
Free-running speed: 700 rpm
Free-running current: 60 mA
Stall torque: 6 oz·in
Stall current: 540 mA
The two wheels of the 3pi each have a radius of 0.67 in, which means that the maximum force it can produce with
two motors when driving forward is 2×6/0.67 = 18 oz. The 3pi weighs about 7 oz with batteries, so the motors are
strong enough to lift the 3pi up a vertical slope or accelerate it at 2 g (twice the acceleration of gravity). The actual
performance is limited by the friction of the tires: on a steep enough slope, the wheels will slip before they stall – in
practice, this happens when the slope is around 30-40°.
Driving a motor with speed and direction control
One nice thing about a DC motor is that you can change the direction of rotation by switching the polarity of the
applied voltage. If you have a loose battery and motor, you can see this for yourself by making connections one way
and then turning the battery around to make the motor spin in reverse. Of course, you don’t want take the batteries out
of your 3pi and reverse them every time it needs to back up – instead, a special arrangement of four switches, called
an H-bridge, allows the motor to spin either backwards or forwards. Here is a diagram that shows how the H-bridge
works:
Pololu 3pi Robot User's Guide © 2001–2014 Pololu Corporation
5. How Your 3pi Works Page 14 of 63