pid_autotuner.cpp

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#include "pid_autotuner.h"
#include "esphome/core/log.h"
#include <cinttypes>
#include <numbers>
 
namespace esphome::pid {
 
static const char *const TAG = "pid.autotune";
 
/*
 * # PID Autotuner
 *
 * Autotuning of PID parameters is a very interesting topic. There has been
 * a lot of research over the years to create algorithms that can efficiently determine
 * suitable starting PID parameters.
 *
 * The most basic approach is the Ziegler-Nichols method, which can determine good PID parameters
 * in a manual process:
 *  - Set ki, kd to zero.
 *  - Increase kp until the output oscillates *around* the setpoint. This value kp is called the
 *    "ultimate gain" K_u.
 *  - Additionally, record the period of the observed oscillation as P_u (also called T_u).
 *  - suitable PID parameters are then: kp=0.6*K_u, ki=1.2*K_u/P_u, kd=0.075*K_u*P_u (additional variants of
 *    these "magic" factors exist as well [2]).
 *
 * Now we'd like to automate that process to get K_u and P_u without the user. So we'd like to somehow
 * make the observed variable oscillate. One observation is that in many applications of PID controllers
 * the observed variable has some amount of "delay" to the output value (think heating an object, it will
 * take a few seconds before the sensor can sense the change of temperature) [3].
 *
 * It turns out one way to induce such an oscillation is by using a really dumb heating controller:
 * When the observed value is below the setpoint, heat at 100%. If it's below, cool at 100% (or disable heating).
 * We call this the "RelayFunction" - the class is responsible for making the observed value oscillate around the
 * setpoint. We actually use a hysteresis filter (like the bang bang controller) to make the process immune to
 * noise in the input data, but the math is the same [1].
 *
 * Next, now that we have induced an oscillation, we want to measure the frequency (or period) of oscillation.
 * This is what "OscillationFrequencyDetector" is for: it records zerocrossing events (when the observed value
 * crosses the setpoint). From that data, we can determine the average oscillating period. This is the P_u of the
 * ZN-method.
 *
 * Finally, we need to determine K_u, the ultimate gain. It turns out we can calculate this based on the amplitude of
 * oscillation ("induced amplitude `a`) as described in [1]:
 *   K_u = (4d) / (πa)
 * where d is the magnitude of the relay function (in range -d to +d).
 * To measure `a`, we look at the current phase the relay function is in - if it's in the "heating" phase, then we
 * expect the lowest temperature (=highest error) to be found in the phase because the peak will always happen slightly
 * after the relay function has changed state (assuming a delay-dominated process).
 *
 * Finally, we use some heuristics to determine if the data we've received so far is good:
 *  - First, of course we must have enough data to calculate the values.
 *  - The ZC events need to happen at a relatively periodic rate. If the heating/cooling speeds are very different,
 *    I've observed the ZN parameters are not very useful.
 *  - The induced amplitude should not deviate too much. If the amplitudes deviate too much this means there has
 *    been some outside influence (or noise) on the system, and the measured amplitude values are not reliable.
 *
 * There are many ways this method can be improved, but on my simulation data the current method already produces very
 * good results. Some ideas for future improvements:
 *  - Relay Function improvements:
 *    - Integrator, Preload, Saturation Relay ([1])
 *  - Use phase of measured signal relative to relay function.
 *  - Apply PID parameters from ZN, but continuously tweak them in a second step.
 *
 * [1]: https://warwick.ac.uk/fac/cross_fac/iatl/reinvention/archive/volume5issue2/hornsey/
 * [2]: http://www.mstarlabs.com/control/znrule.html
 * [3]: https://www.academia.edu/38620114/SEBORG_3rd_Edition_Process_Dynamics_and_Control
 */
 
PIDAutotuner::PIDAutotuneResult PIDAutotuner::update(float setpoint, float process_variable) {
  PIDAutotuner::PIDAutotuneResult res;
  if (this->state_ == AUTOTUNE_SUCCEEDED) {
    res.result_params = this->get_ziegler_nichols_pid_();
    return res;
  }
 
  if (!std::isnan(this->setpoint_) && this->setpoint_ != setpoint) {
    ESP_LOGW(TAG, "%s: Setpoint changed during autotune! The result will not be accurate!", this->id_.c_str());
  }
  this->setpoint_ = setpoint;
 
  float error = setpoint - process_variable;
  const uint32_t now = millis();
 
  float output = this->relay_function_.update(error);
  this->frequency_detector_.update(now, error);
  this->amplitude_detector_.update(error, this->relay_function_.state);
  res.output = output;
 
  if (!this->frequency_detector_.has_enough_data() || !this->amplitude_detector_.has_enough_data()) {
    // not enough data for calculation yet
    ESP_LOGV(TAG, "%s:   Not enough data yet for autotuner", this->id_.c_str());
    return res;
  }
 
  bool zc_symmetrical = this->frequency_detector_.is_increase_decrease_symmetrical();
  bool amplitude_convergent = this->amplitude_detector_.is_amplitude_convergent();
  if (!zc_symmetrical || !amplitude_convergent) {
    // The frequency/amplitude is not fully accurate yet, try to wait
    // until the fault clears, or terminate after a while anyway
    if (!zc_symmetrical) {
      ESP_LOGVV(TAG, "%s:   ZC is not symmetrical", this->id_.c_str());
    }
    if (!amplitude_convergent) {
      ESP_LOGVV(TAG, "%s:   Amplitude is not convergent", this->id_.c_str());
    }
    uint32_t phase = this->relay_function_.phase_count;
    ESP_LOGVV(TAG, "%s: >", this->id_.c_str());
    ESP_LOGVV(TAG, "  Phase %" PRIu32 ", enough=%" PRIu32, phase, enough_data_phase_);
 
    if (this->enough_data_phase_ == 0) {
      this->enough_data_phase_ = phase;
    } else if (phase - this->enough_data_phase_ <= 6) {
      // keep trying for at least 6 more phases
      return res;
    } else {
      // proceed to calculating PID parameters
      // warning will be shown in "Checks" section
    }
  }
 
  ESP_LOGI(TAG, "%s: PID Autotune finished!", this->id_.c_str());
 
  float osc_ampl = this->amplitude_detector_.get_mean_oscillation_amplitude();
  float d = (this->relay_function_.output_positive - this->relay_function_.output_negative) / 2.0f;
  ESP_LOGVV(TAG, "  Relay magnitude: %f", d);
  this->ku_ = 4.0f * d / (std::numbers::pi_v<float> * osc_ampl);
  this->pu_ = this->frequency_detector_.get_mean_oscillation_period();
 
  this->state_ = AUTOTUNE_SUCCEEDED;
  res.result_params = this->get_ziegler_nichols_pid_();
  this->dump_config();
 
  return res;
}
void PIDAutotuner::dump_config() {
  if (this->state_ == AUTOTUNE_SUCCEEDED) {
    ESP_LOGI(TAG,
             "%s: PID Autotune:\n"
             "  State: Succeeded!",
             this->id_.c_str());
    bool has_issue = false;
    if (!this->amplitude_detector_.is_amplitude_convergent()) {
      ESP_LOGW(TAG, "  Could not reliably determine oscillation amplitude, PID parameters may be inaccurate!\n"
                    "    Please make sure you eliminate all outside influences on the measured temperature.");
      has_issue = true;
    }
    if (!this->frequency_detector_.is_increase_decrease_symmetrical()) {
      ESP_LOGW(TAG,
               "  Oscillation Frequency is not symmetrical. PID parameters may be inaccurate!\n"
               "    This is usually because the heat and cool processes do not change the temperature at the same "
               "rate.\n"
               "    Please try reducing the positive_output value (or increase negative_output in case of a cooler)");
      has_issue = true;
    }
    if (!has_issue) {
      ESP_LOGI(TAG, "  All checks passed!");
    }
 
    auto fac = get_ziegler_nichols_pid_();
    ESP_LOGI(TAG,
             "  Calculated PID parameters (\"Ziegler-Nichols PID\" rule):\n"
             "\n"
             "  control_parameters:\n"
             "    kp: %.5f\n"
             "    ki: %.5f\n"
             "    kd: %.5f\n"
             "\n"
             "  Please copy these values into your YAML configuration! They will reset on the next reboot.",
             fac.kp, fac.ki, fac.kd);
 
    ESP_LOGV(TAG,
             "  Oscillation Period: %f\n"
             "  Oscillation Amplitude: %f\n"
             "  Ku: %f, Pu: %f",
             this->frequency_detector_.get_mean_oscillation_period(),
             this->amplitude_detector_.get_mean_oscillation_amplitude(), this->ku_, this->pu_);
 
    ESP_LOGD(TAG, "  Alternative Rules:");
    // http://www.mstarlabs.com/control/znrule.html
    print_rule_("Ziegler-Nichols PI", 0.45f, 0.54f, 0.0f);
    print_rule_("Pessen Integral PID", 0.7f, 1.75f, 0.105f);
    print_rule_("Some Overshoot PID", 0.333f, 0.667f, 0.111f);
    print_rule_("No Overshoot PID", 0.2f, 0.4f, 0.0625f);
    ESP_LOGI(TAG, "%s: Autotune completed", this->id_.c_str());
  }
 
  if (this->state_ == AUTOTUNE_RUNNING) {
    ESP_LOGD(TAG,
             "%s: PID Autotune:\n"
             "  Autotune is still running!\n"
             "  Status: Trying to reach %.2f °C\n"
             "  Stats so far:\n"
             "    Phases: %" PRIu32 "\n"
             "    Detected %zu zero-crossings\n"
             "    Current Phase Min: %.2f, Max: %.2f",
             this->id_.c_str(), setpoint_ - relay_function_.current_target_error(), relay_function_.phase_count,
             frequency_detector_.zerocrossing_intervals.size(), amplitude_detector_.phase_min,
             amplitude_detector_.phase_max);
  }
}
PIDAutotuner::PIDResult PIDAutotuner::calculate_pid_(float kp_factor, float ki_factor, float kd_factor) {
  float kp = kp_factor * ku_;
  float ki = ki_factor * ku_ / pu_;
  float kd = kd_factor * ku_ * pu_;
  return {
      .kp = kp,
      .ki = ki,
      .kd = kd,
  };
}
void PIDAutotuner::print_rule_(const char *name, float kp_factor, float ki_factor, float kd_factor) {
  auto fac = calculate_pid_(kp_factor, ki_factor, kd_factor);
  ESP_LOGD(TAG,
           "    Rule '%s':\n"
           "      kp: %.5f, ki: %.5f, kd: %.5f",
           name, fac.kp, fac.ki, fac.kd);
}
 
// ================== RelayFunction ==================
float PIDAutotuner::RelayFunction::update(float error) {
  if (this->state == RELAY_FUNCTION_INIT) {
    bool pos = error > this->noiseband;
    state = pos ? RELAY_FUNCTION_POSITIVE : RELAY_FUNCTION_NEGATIVE;
  }
  bool change = false;
  if (this->state == RELAY_FUNCTION_POSITIVE && error < -this->noiseband) {
    // Positive hysteresis reached, change direction
    this->state = RELAY_FUNCTION_NEGATIVE;
    change = true;
  } else if (this->state == RELAY_FUNCTION_NEGATIVE && error > this->noiseband) {
    // Negative hysteresis reached, change direction
    this->state = RELAY_FUNCTION_POSITIVE;
    change = true;
  }
 
  float output = state == RELAY_FUNCTION_POSITIVE ? output_positive : output_negative;
  if (change) {
    this->phase_count++;
  }
 
  return output;
}
 
// ================== OscillationFrequencyDetector ==================
void PIDAutotuner::OscillationFrequencyDetector::update(uint32_t now, float error) {
  if (this->state == FREQUENCY_DETECTOR_INIT) {
    bool pos = error > this->noiseband;
    state = pos ? FREQUENCY_DETECTOR_POSITIVE : FREQUENCY_DETECTOR_NEGATIVE;
  }
 
  bool had_crossing = false;
  if (this->state == FREQUENCY_DETECTOR_POSITIVE && error < -this->noiseband) {
    this->state = FREQUENCY_DETECTOR_NEGATIVE;
    had_crossing = true;
  } else if (this->state == FREQUENCY_DETECTOR_NEGATIVE && error > this->noiseband) {
    this->state = FREQUENCY_DETECTOR_POSITIVE;
    had_crossing = true;
  }
 
  if (had_crossing) {
    // Had crossing above hysteresis threshold, record
    if (this->last_zerocross != 0) {
      uint32_t dt = now - this->last_zerocross;
      this->zerocrossing_intervals.push_back(dt);
    }
    this->last_zerocross = now;
  }
}
bool PIDAutotuner::OscillationFrequencyDetector::has_enough_data() const {
  // Do we have enough data in this detector to generate PID values?
  return this->zerocrossing_intervals.size() >= 2;
}
float PIDAutotuner::OscillationFrequencyDetector::get_mean_oscillation_period() const {
  // Get the mean oscillation period in seconds
  // Only call if has_enough_data() has returned true.
  float sum = 0.0f;
  for (uint32_t v : this->zerocrossing_intervals)
    sum += v;
  // zerocrossings are each half-period, multiply by 2
  float mean_value = sum / this->zerocrossing_intervals.size();
  // divide by 1000 to get seconds, multiply by two because zc happens two times per period
  float mean_period = mean_value / 1000 * 2;
  return mean_period;
}
bool PIDAutotuner::OscillationFrequencyDetector::is_increase_decrease_symmetrical() const {
  // Check if increase/decrease of process value was symmetrical
  // If the process value increases much faster than it decreases, the generated PID values will
  // not be very good and the function output values need to be adjusted
  // Happens for example with a well-insulated heating element.
  // We calculate this based on the zerocrossing interval.
  if (zerocrossing_intervals.empty())
    return false;
  uint32_t max_interval = zerocrossing_intervals[0];
  uint32_t min_interval = zerocrossing_intervals[0];
  for (uint32_t interval : zerocrossing_intervals) {
    max_interval = std::max(max_interval, interval);
    min_interval = std::min(min_interval, interval);
  }
  float ratio = min_interval / float(max_interval);
  return ratio >= 0.66f;
}
 
// ================== OscillationAmplitudeDetector ==================
void PIDAutotuner::OscillationAmplitudeDetector::update(float error,
                                                        PIDAutotuner::RelayFunction::RelayFunctionState relay_state) {
  if (relay_state != last_relay_state) {
    if (last_relay_state == RelayFunction::RELAY_FUNCTION_POSITIVE) {
      // Transitioned from positive error to negative error.
      // The positive error peak must have been in previous segment (180° shifted)
      // record phase_max
      this->phase_maxs.push_back(phase_max);
    } else if (last_relay_state == RelayFunction::RELAY_FUNCTION_NEGATIVE) {
      // Transitioned from negative error to positive error.
      // The negative error peak must have been in previous segment (180° shifted)
      // record phase_min
      this->phase_mins.push_back(phase_min);
    }
    // reset phase values for next phase
    this->phase_min = error;
    this->phase_max = error;
  }
  this->last_relay_state = relay_state;
 
  this->phase_min = std::min(this->phase_min, error);
  this->phase_max = std::max(this->phase_max, error);
 
  // Check arrays sizes, we keep at most 7 items (6 datapoints is enough, and data at beginning might not
  // have been stabilized)
  if (this->phase_maxs.size() > 7)
    this->phase_maxs.erase(this->phase_maxs.begin());
  if (this->phase_mins.size() > 7)
    this->phase_mins.erase(this->phase_mins.begin());
}
bool PIDAutotuner::OscillationAmplitudeDetector::has_enough_data() const {
  // Return if we have enough data to generate PID parameters
  // The first phase is not very useful if the setpoint is not set to the starting process value
  // So discard first phase. Otherwise we need at least two phases.
  return std::min(phase_mins.size(), phase_maxs.size()) >= 3;
}
float PIDAutotuner::OscillationAmplitudeDetector::get_mean_oscillation_amplitude() const {
  float total_amplitudes = 0;
  size_t total_amplitudes_n = 0;
  for (size_t i = 1; i < std::min(phase_mins.size(), phase_maxs.size()) - 1; i++) {
    total_amplitudes += std::abs(phase_maxs[i] - phase_mins[i + 1]);
    total_amplitudes_n++;
  }
  float mean_amplitude = total_amplitudes / total_amplitudes_n;
  // Amplitude is measured from center, divide by 2
  return mean_amplitude / 2.0f;
}
bool PIDAutotuner::OscillationAmplitudeDetector::is_amplitude_convergent() const {
  // Check if oscillation amplitude is convergent
  // We implement this by checking global extrema against average amplitude
  if (this->phase_mins.empty() || this->phase_maxs.empty())
    return false;
 
  float global_max = phase_maxs[0], global_min = phase_mins[0];
  for (auto v : this->phase_mins)
    global_min = std::min(global_min, v);
  for (auto v : this->phase_maxs)
    global_max = std::max(global_max, v);
  float global_amplitude = (global_max - global_min) / 2.0f;
  float mean_amplitude = this->get_mean_oscillation_amplitude();
  return (mean_amplitude - global_amplitude) / (global_amplitude) < 0.05f;
}
 
}  // namespace esphome::pid