pid.py

#!/usr/bin/env python3

# *****************************************
# PiFire PID Controller
# *****************************************
#
# Description: This object will be used to calculate PID for maintaining
# temperature in the grill.
#
# This software was developed by GitHub user DBorello as part of his excellent
# PiSmoker project: https://github.com/DBorello/PiSmoker
#
# Adapted for PiFire
#
# PID controller based on proportional band in standard PID form https://en.wikipedia.org/wiki/PID_controller#Ideal_versus_standard_PID_form
#   u = Kp (e(t)+ 1/Ti INT + Td de/dt)
#  PB = Proportional Band
#  Ti = Goal of eliminating in Ti seconds
#  Td = Predicts error value at Td in seconds

# *****************************************

# *****************************************
# Imported Libraries
# *****************************************
import time

# *****************************************
# Class Definition
# *****************************************
class PID:
	def __init__(self,  pb, ti, td, center):
		self._calculate_gains(pb,ti,td)

		self.p = 0.0
		self.i = 0.0
		self.d = 0.0
		self.u = 0

		self.last_update = time.time()
		self.error = 0.0
		self.set_point = 0

		self.center = center

		self.derv = 0.0
		self.inter = 0.0
		self.inter_max = abs(self.center / self.ki)

		self.last = 150

		self.set_target(0.0)

	def _calculate_gains(self, pb, ti, td):
		self.kp = -1 / pb
		self.ki = self.kp / ti
		self.kd = self.kp * td

	def update(self, current):
		# P
		error = current - self.set_point
		self.p = self.kp * error + self.center # p = 1 for pb / 2 under set_point, p = 0 for pb / 2 over set_point

		# I
		dt = time.time() - self.last_update
		# if self.p > 0 and self.p < 1: # Ensure we are in the pb, otherwise do not calculate i to avoid windup
		self.inter += error * dt
		self.inter = max(self.inter, -self.inter_max)
		self.inter = min(self.inter, self.inter_max)

		self.i = self.ki * self.inter

		# D
		self.derv = (current - self.last) / dt
		self.d = self.kd * self.derv

		# PID
		self.u = self.p + self.i + self.d

		# Update for next cycle
		self.error = error
		self.last = current
		self.last_update = time.time()

		return self.u

	def set_target(self, set_point):
		self.set_point = set_point
		self.error = 0.0
		self.inter = 0.0
		self.derv = 0.0
		self.last_update = time.time()

	def set_gains(self, pb, ti, td):
		self._calculate_gains(pb,ti,td)
		self.inter_max = abs(self.center / self.ki)

	def get_k(self):
		return self.kp, self.ki, self.kd