LEGO® EV3 Python (Pybricks) Cheat sheet

Drive the Robot (Control two motors at same time)

• Move then stop (drive straight or spin turn)
  • settings(straight_speed, straight_acceleration, turn_rate, turn_acceleration) (modifies next straight or turn command)
  • straight(distance), then stop
  • turn(angle) in place, then stop
• Move forever (straight or turn until a stop() or another drive() command, or until program ends)
  • drive(drive_speed, turn_rate)
  • How to call drive:

    • drive() then wait(milliseconds), then stop()

      robot.drive(150, 0) # drive at 150mm/sec, in straight line (because turn_rate=0)
      wait(1000) # wait one second
      robot.stop() # not required if program ends after wait command 
      

      —>try it out (copy code and paste it under Python tab)

      • Note: at faster speeds, the robot will not accurately drive in a straight line - you will need to use a sensor to fix this (e.g. gyro)

    • call drive() many times inside while loop using distance() or angle():

      robot.reset()
      # keep driving until either robot has travelled 100cm or turned 120degrees
      while robot.distance() < 1000 and robot.angle() < 120:
          robot.drive(150,30) # drive for 150mm/s, while turning left at 30deg/sec 
          wait(10) # wait for a short time or do something else
      robot.stop()
      

      —>try it out (copy code and paste it under Python tab)

  • reset() - distance is calculated from start of program, so reset() it before using distance in a while loop

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Single motor control (for Attachments)

• Action
  • run(speed)

    • Runs the motor at a constant speed, no stopping. If it is the only command, make sure you use a wait command, otherwise the program will end before run command can execute:

      motorC.run(200)
      wait(500) # run the lift motor command for half a second
      motorC.stop()
      

      —>try it out (use Tow Truck robot)

  • run_angle(speed, rotation_angle, then=Stop.HOLD, wait=True)
    • rotates motor X degrees relative to its current angle.
  • run_target(speed, target_angle, then=Stop.HOLD, wait=True)
    • rotates motor X degrees until they reach the absolute angle you specify.
• Stopping
  • stop() - glide to a stop
  • brake() - motor friction stop
  • hold() - stop motor rotation and actively hold

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Sensors

• classGyroSensor(port, positive_direction=Direction.CLOCKWISE)

  • Drive Straight Using Gyro - Forward: (Proportional algorithm)

    gyro_sensor = GyroSensor(Port.S3) # Assumes gyro is connected to port 3
    
    distance = 1000 # millimetres
    robotSpeed = 150 # mm/sec
        
    robot.reset() 
    gyro_sensor.reset_angle(0)
        
    PROPORTIONAL_GAIN = 1.1    
    while robot.distance() < distance:
      angle_correction = -1 * PROPORTIONAL_GAIN * gyro_sensor.angle()
      robot.drive(robotSpeed, angle_correction) 
      wait(10)
    robot.stop()
    

    —>try it out (use Single Sensor Line Robot and select Gyro Challenges from Worlds under the Simulator tab)

    • Notes:
      • Remember to call robot.reset() before using robot.distance() as a test in a loop
      • If your gyro is attached backwards on your robot, use Direction.COUNTERCLOCKWISE when setting up GyroSensor
      • Distance must be far enough so that Gyro code can make correction (e.g. 20mm distance is too short)

  • Drive Straight Using Gyro - Forward or Backwards: (Proportional algorithm)

    gyro_sensor = GyroSensor(Port.S3) # Assumes gyro is connected to port 3    
    
    distance = -1000 # millimetres
    robotSpeed = 150 # mm/sec    
    
    robot.reset() 
    gyro_sensor.reset_angle(0)
    
    PROPORTIONAL_GAIN = 1.1
    if distance < 0: # move backwards
        while robot.distance() > distance:
            reverseSpeed = -1 * robotSpeed        
            angle_correction = -1 * PROPORTIONAL_GAIN * gyro_sensor.angle()
            robot.drive(reverseSpeed, angle_correction) 
            wait(10)
    elif distance > 0: # move forwards             
        while robot.distance() < distance:
            angle_correction = -1 * PROPORTIONAL_GAIN * gyro_sensor.angle()
            robot.drive(robotSpeed, angle_correction) 
            wait(10)            
    robot.stop()
    

    —>try it out (use Single Sensor Line Robot and select Gyro Challenges from Worlds under the Simulator tab)

  • See also: Driving straight using a gyro and PID algorithm

  • Spin Turn Using Gyro - Right Turn Only

    gyro_sensor = GyroSensor(Port.S3)    
    
    angle = 90 # degrees
    speed = 150 # mm/s
    
    gyro_sensor.reset_angle(0)
    while gyro_sensor.angle() < angle:
        motorA.run(speed=speed)
        motorB.run(speed=(-1 * speed))
        wait(10)  
    
    motorA.brake()
    motorB.brake()    
    

    —>try it out (copy code and paste in under Python tab)

  • Spin Turn Using Gyro - Either Direction

    gyro_sensor = GyroSensor(Port.S3)    
        
    angle = -90 # degrees
    speed = 150 # mm/s
    
    gyro_sensor.reset_angle(0)
    if angle < 0:
        while gyro_sensor.angle() > angle:
            motorA.run(speed=(-1 * speed))
            motorB.run(speed=speed)
            wait(10)
    elif angle > 0:  
        while gyro_sensor.angle() < angle:
            motorA.run(speed=speed)
            motorB.run(speed=(-1 * speed))
            wait(10)  
    else:
        print("Error: no angle chosen")
    
    motorA.brake()
    motorB.brake()    
    

    —>try it out (copy code and paste in under Python tab)

  • Notes:

    • reset_angle(0) - remember to reset gyro angle before using it as a test in a loop
    • Gyro must be plugged to EV3 before power up. Make sure brick does not move on power up. Never plug in gyro to an already powered up EV3 brick - it messes up the calibration.
• classColorSensor(port)
  • 3-level line follower using color sensor:
    • (not very accurate)

    color_sensor_in1 = ColorSensor(Port.S1)    
    
    while True:
        if color_sensor_in1.reflection()  > 95: # white
            robot.drive(drive_speed=75, turn_rate=-60)
        else: 
            if color_sensor_in1.reflection()  < 10: # black
                robot.drive(drive_speed=75, turn_rate=60)
            else: #straight
                robot.drive(drive_speed=75, turn_rate=0)
    

    —>try it out (use Single Sensor Line Robot and select Line Following Challenges from Worlds, under the Simulator tab)

  • Proportional line follower

    color_sensor_in1 = ColorSensor(Port.S1)
    
    BLACK = 10
    WHITE = 95
    threshold = (BLACK + WHITE) / 2
    PROPORTIONAL_GAIN = 1.5
    while True:
        deviation = color_sensor_in1.reflection() - threshold
        angle_correction = PROPORTIONAL_GAIN * deviation
        robot.drive(drive_speed=100, turn_rate=angle_correction)
        wait(10)     
    

    —>try it out (use Single Sensor Line Robot and select Line Following Challenges from Worlds, under the Simulator tab)

Sample programs

• ROBOT EDUCATOR PROGRAMS
  • Basic Movement
  • Obstacle Avoidance
  • Line Following

Documentation

• LEGO® MINDSTORMS Education EV3 MicroPython
• see also: pybricks cheatsheet By FLL TechTacos