hphp/util/pidcontroller.h

   1 /*
   2    +----------------------------------------------------------------------+
   3    | HipHop for PHP                                                       |
   4    +----------------------------------------------------------------------+
   5    | Copyright (c) 2010-present Facebook, Inc. (http://www.facebook.com)  |
   6    +----------------------------------------------------------------------+
   7    | This source file is subject to version 3.01 of the PHP license,      |
   8    | that is bundled with this package in the file LICENSE, and is        |
   9    | available through the world-wide-web at the following url:           |
  10    | http://www.php.net/license/3_01.txt                                  |
  11    | If you did not receive a copy of the PHP license and are unable to   |
  12    | obtain it through the world-wide-web, please send a note to          |
  13    | license@php.net so we can mail you a copy immediately.               |
  14    +----------------------------------------------------------------------+
  15 */
  16
  17 #ifndef incl_HPHP_PIDCONTOLLER_H_
  18 #define incl_HPHP_PIDCONTOLLER_H_
  19
  20 #include <cmath>
  21 #include <limits>
  22
  23 namespace HPHP {
  24 /**
  25  * Like anything useful, we benefit from using a pid controller in certain
  26  * applications.  This is the 10 millionth implementation of a naive one.
  27  */
  28
  29 struct PIDController {
  30   PIDController() = default;
  31   PIDController(double dt, double max, double min,
  32                 double kP, double kI, double kD)
  33       : dt(dt)
  34       , max(max)
  35       , min(min)
  36       , kP(kP)
  37       , kI(kI)
  38       , kD(kD) {}
  39
  40   // This returns the new value for the controlled variable, and takes
  41   // `curValue`, the process variable, and the `setpoint`.
  42   double determineOutput(double setpoint, double curValue) {
  43     double error = setpoint - curValue;
  44
  45     double proportionalTerm = kP * error;
  46
  47     double integralDelta = error * dt;
  48     m_integral += integralDelta;
  49
  50     double integralTerm = kI * m_integral;
  51     // This stops integral windup issues in cases the controlled variable is
  52     // temporarily not influenced by the contoller output.
  53     //
  54     // You may need to widen the max an min values slightly to have proper pid
  55     // behavior near those areas.
  56     if (integralTerm > max) {
  57       m_integral = max / kI;
  58       integralTerm = max;
  59     }
  60     if (integralTerm < min) {
  61       m_integral = min / kI;
  62       integralTerm = min;
  63     }
  64
  65     // The derivative is based on values rather than error to not overrespond
  66     // during setpoint changes.
  67     double derivativeTerm = 0;
  68     if (m_initialized) {
  69       derivativeTerm = (m_lastValue - curValue) / dt * kD;
  70     } else {
  71       m_initialized = true;
  72     }
  73
  74     m_lastValue = curValue;
  75
  76     auto const output = proportionalTerm + integralTerm + derivativeTerm;
  77     if (output > max) return max;
  78     if (output < min) return min;
  79     return output;
  80   }
  81
  82   double dt;
  83   double max;
  84   double min;
  85   double kP;
  86   double kI;
  87   double kD;
  88
  89 private:
  90   double m_integral{0};
  91   double m_lastValue{std::numeric_limits<double>::max()};
  92   bool m_initialized{false};
  93 };
  94
  95
  96 }
  97
  98 #endif // incl_HPHP_PIDCONTOLLER_H_