[EN] PID

Resume

The purpose with creating a PID in our project is for the robot to be able to follow a black line to reach an objective. To do that, he will get the color sensor data to give movements orders so that the robot can follow the line.

Multi-nodes communication :

The only goal of PID node is to do the calculations for the trajectory correction, to do that he communicate with multiple nodes as you can se below:

As you can see, the node compute without stopping, but it is the controller who decide if the PID commands are use for movement.

Settings of the PID :

The PID is composed of multiple parameters configured at the creation of the node:

class PID_controller:
    def __init__(self,kp,ki,kd,isat):
        self.kp = kp
        self.ki = ki
        self.kd = kd
        self.isat = isat
        self.old_error = 0
        self.sum_integral = 0

• kp for the proportional component
• ki for the integral component
• kd for the derived component
• isat the maximum value for the error

Node functioning:

Initialisation

def __init__(self):
	super().__init__('turtlebot_pid')
  
	self.publisher_ = self.create_publisher(Twist, 'PID_cmd_vel', 10)
	self.subscription = self.create_subscription(rgbtype,'RGB_Value',self.get_RGBvalue,10)
	self.subscription
  
	timer_period = 0.5  # seconds
	self.timer = self.create_timer(timer_period, self.pid_send)
  
	self.i = 0
	self.valeurgb= rgbtype()#specific topicof the RGB sensor
	self.pidangle=PID_controller(0.9,2.0,0,60)

Permit to subscribe to the sensor topic ('sensor'), and publish movements orders to the controller with the topic ('PID_cmd_vel'). Initialisation of the PID with the given settings.

Compute and send

def get_RGBvalue(self, msgsub):
	self.valeurgb=msgsub

def pid_send(self):
	error=((self.valeurgb.r+self.valeurgb.g+self.valeurgb.b)/(3*2.55))-40
	white=self.valeurgb.c    
	angle=self.pidangle.compute(0.1,error)
     
	msg = Twist() #RGB sensor
	msg.linear.x=2#self.order.straight;
	msg.angular.z=angle
	self.publisher_.publish(msg)
	self.i += 1

Gets the values of the sensor every half seconds with the function get_RGBvalue().

Computes and sends the movements parameters every half seconds with pid_send().