3. Controlling the Speed of MotorsΒΆ

In the previous project, we can give high and low levels to the motor to make it spin or stop in a constant speed.

But how to set different speed?

About PWM

In this case, you can use Pulse-width Modulation, it allows you to give analogue behaviours to digital devices, such as motors. This means that rather than motor being simply spin or stop, you can control its speed.

Happily, handling Pico PWM in MicroPython is very simple, requiring only three simple commands.

  • Create a pin as a PWM object.

pin = machine.PWM(machine.Pin(17))

  • Tell Raspberry Pi Pico how often to switch the power between ON and OFF.

pin.freq(20000)

  • Tell the pin for how long it should be ON each time. For Raspberry Pi Pico in MicroPython, this can range from 0 to 65535. 655535 would be 100% of the time, a value of around 32727 indicates 50%.

pin.duty_u16(0xFFFF)

Rotating the motor at high speed (motor_3_set_speed.py)

  • Set pinB to high, i.e. PWM value is 0xFFFF (65535).

  • Then the motor will rotate clockwise by setting pinA to any value between 0 and 0xFFFF. In general, the smaller the value, the faster the speed, and the maximum speed is obtained at 0.

import machine

pinA = machine.Pin(17, machine.Pin.OUT)
pinB = machine.Pin(16, machine.Pin.OUT)

pwmA = machine.PWM(pinA) # Create PWM object
pwmB = machine.PWM(pinB) # Create PWM object
pwmA.freq(20000)
pwmB.freq(20000)

# fast
pwmA.duty_u16(0x0000)
pwmB.duty_u16(0xFFFF)

Rotating motor at slow speed

Now turn the left front motor clockwise in the same way, but you will find that it will spin much slower.

import machine

pinA = machine.Pin(13, machine.Pin.OUT)
pinB = machine.Pin(12, machine.Pin.OUT)

pwmA = machine.PWM(pinA)
pwmB = machine.PWM(pinB)
pwmA.freq(20000)
pwmB.freq(20000)

# slow
pwmA.duty_u16(0xFFFF)
pwmB.duty_u16(0xAAAA)

Stop 4 Motors

To stop all motors, write 0xFFFF to all PWM pins.

import machine

for i in range(10,18):
    pin = machine.PWM(machine.Pin(i, machine.Pin.OUT))
    pin.freq(20000)
    pin.duty_u16(0xFFFF)