Pololu 0J5840, Руководство пользователя

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The 30:1 gearmotor used on the
3pi.
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 to 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 © 2001–2019 Pololu Corporation
5. How Your 3pi Works Page 17 of 85