Chapter6

We learned how to assemble the car and create python scripts to move the car. In this chapter, let's make an interactive program to control the car through the key board.

Instructions

We will use the keyboard and let the user to input different keys to control the car movement. The following function prints the instructions.

def printscreen():
        # Print the motor speed just for interest
        os.system('clear')
        print("w/s: forward/backward")
        print("a/d: left/right")
        print("q: stops the motors")
        print("x: exit")

    # Infinite loop
    # The loop will not end until the user presses the
    # exit key 'x' or the program crashes...

Interactive Loop

We use an infinite loop to control the car movement. At each iteration, function getch() is called to get a keyboard input from the user. The user can input single character from 'w'/'s'/'a'/'d'/'q' to control the car movements or press 'x' to exit the program. Once any key is pressed, python program can find the corresponding car movement function and drive the car.

def getch():
        fd = sys.stdin.fileno()
        old_settings = termios.tcgetattr(fd)
        try:
                tty.setraw(sys.stdin.fileno())
                ch = sys.stdin.read(1)
        finally:
                termios.tcsetattr(fd, termios.TCSADRAIN, old_settings)
        return ch

def test():
      printscreen()
      while True:
    # Keyboard character retrieval method. This method will save
    # the pressed key into the variable char
    char = getch()

    # The car will drive forward when the "w" key is pressed
    if(char == "w"):
        forward()
        printscreen()

        # The car will reverse when the "s" key is pressed
    if(char == "s"):
        backward()
        printscreen()

    # Stop the motors
    if(char == "q"):
        stop()
        printscreen()

        # The "d" key will toggle the steering right
    if(char == "d"):        
        right()
        printscreen()

        # The "a" key will toggle the steering left
    if(char == "a"):
        left()
        printscreen()

    if(char == "q"):
        stop()
        printscreen()

    # The "x" key will break the loop and exit the program
    if(char == "x"):
        break

        # The keyboard character variable char has to be set blank. We need
    # to set it blank to save the next key pressed by the user
    char = ""

lesson4.py is complete python program for controlling the car movement through user interaction.

#lesson4.py
import RPi.GPIO as GPIO
import sys, tty, termios, os
import time

#Raspberry GPIO PIN for car control
#use GPIO 2, 3 for left motors direction control , 4 for power control
LEFT_FORWARD = 2
LEFT_BACKWARD= 3
LEFT_PWM = 4

#use GPIO 27, 17 for right motors direction control , 18 for power control
RIGHT_FORWARD = 27
RIGHT_BACKWARD= 17
RIGHT_PWM = 18

#left motor speed initial value
speedleft = 0 

#right  motor speed initial value
speedright = 0 

#power for motor ranges from 0 to 100, 0 is stop, 100 is maximum
PWM_MAX = 100


def forward():
        GPIO.output(LEFT_BACKWARD, GPIO.LOW)
        GPIO.output(LEFT_FORWARD,  GPIO.HIGH)
        leftmotorpwm.ChangeDutyCycle(100)

        GPIO.output(RIGHT_BACKWARD, GPIO.LOW)
        GPIO.output(RIGHT_FORWARD,  GPIO.HIGH)
        rightmotorpwm.ChangeDutyCycle(100)

def backward():
        GPIO.output(LEFT_BACKWARD, GPIO.HIGH)
        GPIO.output(LEFT_FORWARD,  GPIO.LOW)
        leftmotorpwm.ChangeDutyCycle(100)

        GPIO.output(RIGHT_BACKWARD, GPIO.HIGH)
        GPIO.output(RIGHT_FORWARD,  GPIO.LOW)
        rightmotorpwm.ChangeDutyCycle(100)

def left():
        GPIO.output(LEFT_BACKWARD, GPIO.HIGH)
        GPIO.output(LEFT_FORWARD,  GPIO.LOW)
        leftmotorpwm.ChangeDutyCycle(100)

        GPIO.output(RIGHT_BACKWARD, GPIO.LOW)
        GPIO.output(RIGHT_FORWARD,  GPIO.HIGH)
        rightmotorpwm.ChangeDutyCycle(100)

def right():
        GPIO.output(LEFT_BACKWARD, GPIO.LOW)
        GPIO.output(LEFT_FORWARD,  GPIO.HIGH)
        leftmotorpwm.ChangeDutyCycle(100)

        GPIO.output(RIGHT_BACKWARD, GPIO.HIGH)
        GPIO.output(RIGHT_FORWARD,  GPIO.LOW)
        rightmotorpwm.ChangeDutyCycle(100)

def stop():
        GPIO.output(LEFT_BACKWARD, GPIO.LOW)
        GPIO.output(LEFT_FORWARD,  GPIO.LOW)
        GPIO.output(RIGHT_BACKWARD, GPIO.LOW)
        GPIO.output(RIGHT_FORWARD,  GPIO.LOW)
        leftmotorpwm.ChangeDutyCycle(0)
        rightmotorpwm.ChangeDutyCycle(0)

#initialization
GPIO.setmode(GPIO.BCM)
GPIO.setup(LEFT_FORWARD, GPIO.OUT)
GPIO.setup(LEFT_BACKWARD, GPIO.OUT)
GPIO.setup(LEFT_PWM, GPIO.OUT)
GPIO.setup(RIGHT_FORWARD, GPIO.OUT)
GPIO.setup(RIGHT_BACKWARD, GPIO.OUT)
GPIO.setup(RIGHT_PWM, GPIO.OUT)
leftmotorpwm = GPIO.PWM(LEFT_PWM,100)
rightmotorpwm = GPIO.PWM(RIGHT_PWM,100)
leftmotorpwm.start(0)
leftmotorpwm.ChangeDutyCycle(0)
rightmotorpwm.start(0)
rightmotorpwm.ChangeDutyCycle(0)


#my test starts here
def printscreen():
        # Print the motor speed just for interest
        os.system('clear')
        print("w/s: forward/backward")
        print("a/d: left/right")
        print("q: stops the motors")
        print("x: exit")

    # Infinite loop
    # The loop will not end until the user presses the
    # exit key 'x' or the program crashes...

    #class method can determine which key has been pressed
    # by the user on the keyboard.
def getch():
        fd = sys.stdin.fileno()
        old_settings = termios.tcgetattr(fd)
        try:
                tty.setraw(sys.stdin.fileno())
                ch = sys.stdin.read(1)
        finally:
                termios.tcsetattr(fd, termios.TCSADRAIN, old_settings)
        return ch

def test():
      printscreen()
      while True:
    # Keyboard character retrieval method. This method will save
    # the pressed key into the variable char
    char = getch()

    # The car will drive forward when the "w" key is pressed
    if(char == "w"):
        forward()
        printscreen()

        # The car will reverse when the "s" key is pressed
    if(char == "s"):
        backward()
        printscreen()

    # Stop the motors
    if(char == "q"):
        stop()
        printscreen()

        # The "d" key will toggle the steering right
    if(char == "d"):        
        right()
        printscreen()

        # The "a" key will toggle the steering left
    if(char == "a"):
        left()
        printscreen()

    if(char == "q"):
        stop()
        printscreen()

    # The "x" key will break the loop and exit the program
    if(char == "x"):
        break

        # The keyboard character variable char has to be set blank. We need
    # to set it blank to save the next key pressed by the user
    char = ""

printscreen()
test()
stop()
# End
Status