4. Move the Car

Previously, we have learned how to make a single motor turn at different speeds.

In this project, we will learn how 4 motors work together to make the Pico 4WD car go forward, backward, left and right.

You can push the Pico 4WD car forward to see how the 4 wheels (motors) are turning. You will find that the two wheels (motors) on the left side are turning counterclockwise, while the two wheels (motors) on the right side are turning clockwise.

So you can use a parameter to determine whether the motor is on the left or the right side, and thus set its direction of rotation.

Move Forward (motor_4_forward.py)

• Here, create a function motor_run() to control the movement of the car, and use the parameter position to determine the position of the motors.

• When position > 0, it means the corresponding motor is on the right side, so let the motor turn clockwise.

• When position < 0, it means the corresponding motor is on the left side, so let the motor turn counterclockwise.

import machine
import time

pin = []
motor_pin = [17,16,15,14,13,12,11,10]

for i in range(8):
    pin.append(None)
    pin[i] = machine.PWM(machine.Pin(motor_pin[i],machine.Pin.OUT))
    pin[i].freq(20000)

def motor_run(power,pinA,pinB,position):
    power= int(power/100.0*0xFFFF)
    if position>0:
        # clockwise
        pinA.duty_u16(0xFFFF-power)
        pinB.duty_u16(0xFFFF)
    elif position<0:
        # anticlockwise
        pinA.duty_u16(0xFFFF)
        pinB.duty_u16(0xFFFF-power)

try:

    power = 50

    # forward
    motor_run(power,pin[0],pin[1],-1) #left front
    motor_run(power,pin[2],pin[3],1) #right front
    motor_run(power,pin[4],pin[5],-1) #left rear
    motor_run(power,pin[6],pin[7],1) #right rear
    time.sleep(2)

finally:
        # stop
    power = 0
    motor_run(power,pin[0],pin[1],-1) #left front
    motor_run(power,pin[2],pin[3],1) #right front
    motor_run(power,pin[4],pin[5],-1) #left rear
    motor_run(power,pin[6],pin[7],1) #right rear

• Copy the above code into Thonny or open the motor_4_forward.py under the path of pico_4wd_car-v2.0\examples\learn_modules.

• After running and powering up the Pico 4WD car, you will see the car move forward.

Note

In order to allow the car to move on the ground without the USB cable connected, you need to save this script as main.py to Raspberry Pi Pico, see 5. Run Script Offline(Important) for a tutorial.

Move Backward

• As well as forward, the car also has stop, backward, and steering situations. To reverse the direction of the motor, we would normally write a negative number to power, but the above code would return errors.

• Therefore, the above code requires one more layer of conditional judgment, as shown below.

import machine
import time

pin = []
motor_pin = [17,16,15,14,13,12,11,10]

for i in range(8):
    pin.append(None)
    pin[i] = machine.PWM(machine.Pin(motor_pin[i],machine.Pin.OUT))
    pin[i].freq(20000)

def motor_run(power,pinA,pinB,position):
    value= int(abs(power)/100.0*0xFFFF)
    if position*power>0:
        # clockwise
        pinA.duty_u16(0xFFFF-value)
        pinB.duty_u16(0xFFFF)
    elif position*power<0:
        # anticlockwise
        pinA.duty_u16(0xFFFF)
        pinB.duty_u16(0xFFFF-value)
    elif position*power==0:
        # stop
        pinA.duty_u16(0xFFFF)
        pinB.duty_u16(0xFFFF)


try:

    power = 50

    # backward
    motor_run(-power,pin[0],pin[1],-1) #left front
    motor_run(-power,pin[2],pin[3],1) #right front
    motor_run(-power,pin[4],pin[5],-1) #left rear
    motor_run(-power,pin[6],pin[7],1) #right rear
    time.sleep(2)

finally:
        # stop
    power = 0
    motor_run(power,pin[0],pin[1],-1) #left front
    motor_run(power,pin[2],pin[3],1) #right front
    motor_run(power,pin[4],pin[5],-1) #left rear
    motor_run(power,pin[6],pin[7],1) #right rear

The car can now go backward, turn left or right by changing the positive and negative values of power.

About the Steering

The movement of the Pico 4WD car is controlled by 4 motors. So you have two ways to make it steer.

Take turning right as an example

1. The left motors turn clockwise and the right motors turn counterclockwise.

   power = 50
   
   # turn right
   motor_run(power,pin[0],pin[1],-1) #left front
   motor_run(-power,pin[2],pin[3],1) #right front
   motor_run(power,pin[4],pin[5],-1) #left rear
   motor_run(-power,pin[6],pin[7],1) #right rear
   

2. The speed of the left motors is greater than the speed of the right motors.

   power = 80
   
   # also turn right
   motor_run(power,pin[0],pin[1],-1) #left front
   motor_run(power/2,pin[2],pin[3],1) #right front
   motor_run(power,pin[4],pin[5],-1) #left rear
   motor_run(power/2,pin[6],pin[7],1) #right rear