Pololu 0J5840, Руководство пользователя

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PWM speed control, showing gradual deceleration.
Speed control is achieved by rapidly
switching the motor between two states in
the table. Suppose we keep PD6 high (at 5 V,
also called a logical “1”) and have PD5
alternate quickly between low (0 V or “0”) and
high. The motor driver will switch between
the “forward” and “brake” states, causing M1
to turn forward at a reduced speed. For
example, if PD6 is high two thirds of the time
(a 67%
duty cycle ), then M1 will turn at
approximately 67% of its full speed. Since
the motor voltage is a series of pulses of
varying width, this method of speed control is
called pulse-width modulation (PWM). An
example series of PWM pulses is shown in the graph at right: as the size of the pulses decreases from
100% duty cycle down to 0%, the motor speed decreases from full speed down to a stop.
In the 3pi, speed control is accomplished using special PWM outputs of the main microcontroller that
are linked to the internal timers Timer0 and Timer2. This means that you can set the PWM duty cycle
of the two motors once, and the hardware will continue to produce the PWM signal, in the background,
without any further attention.
The
set_motors()
function in the Pololu AVR Library (see Section 6 for more information) lets you
set the duty cycle, and it uses 8-bit precision: a value of 255 corresponds to 100% duty cycle. For
example, to get 67% on M1 and 33% on M2, you would call
To get a slowly decreasing PWM sequence like the one shown in the graph, you would need to write
a loop that gradually decreases the motor speed over time.
Turning with a differential drive
The 3pi has an independent motor and wheel on each side, which enables a method of locomotion
called
differential drive. It is also known as a “tank drive” since this is how a tank drives. It is completely
unlike the steering system of automobile, which uses a single drive motor and steerable front wheels.
Turning with a differential drive is accomplished by running the two motors at different speeds. In the
previous
set_motors()
example, the left wheel will spin faster than the right, driving the robot forward
and to the right. The difference in speeds determines how sharp the turn will be, and spinning in place
can be accomplished by running one motor forward and one backward. Spinning is an especially
effective maneuver for a round robot, and you won’t have to worry about parallel parking!
1 set_motors(171,84);
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