追小球的小车之pid

 pid.py >>https://github.com/wagnerc4/flight_controller/blob/master/pid.py

from pyb import millis
from math import pi, isnan

class PID:
  _kp = _ki = _kd = _integrator = _imax = 0 
  _last_error = _last_derivative = _last_t = 0
  _RC = 1/(2 * pi * 20)
  def __init__(self, p=0, i=0, d=0, imax=0):
    self._kp = float(p)  #比例
    self._ki = float(i)   #积分
    self._kd = float(d)   #微分
    self._imax = abs(imax)  
    self._last_derivative = float('nan')
  def get_pid(self, error, scaler):
    tnow = millis()
    dt = tnow - self._last_t
    output = 0
    if self._last_t == 0 or dt > 1000:
      dt = 0
      self.reset_I()
    self._last_t = tnow
    delta_time = float(dt) / float(1000)
    output += error * self._kp
    if abs(self._kd) > 0 and dt > 0:
      if isnan(self._last_derivative):
        derivative = 0
        self._last_derivative = 0
      else:
        derivative = (error - self._last_error) / delta_time
      derivative = self._last_derivative + \
                   ((delta_time / (self._RC + delta_time)) * \
                    (derivative - self._last_derivative))
      self._last_error = error
      self._last_derivative = derivative
      output += self._kd * derivative
    output *= scaler
    if abs(self._ki) > 0 and dt > 0:
      self._integrator += (error * self._ki) * scaler * delta_time
      if self._integrator < -self._imax: self._integrator = -self._imax
      elif self._integrator > self._imax: self._integrator = self._imax
      output += self._integrator
    return output
  def reset_I(self):
    self._integrator = 0
    self._last_derivative = float('nan')

pid是什么？