thermistor_func.cpp

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/**
 * @author Matthew Kennedy, (c) 2019
 */
 
#include "pch.h"
 
#include "thermistor_func.h"
 
#include <cmath>
 
SensorResult ThermistorFunc::convert(float ohms) const {
    // This resistance should have already been validated - only
    // thing we can check is that it's non-negative
    if (ohms <= 0) {
        return UnexpectedCode::Low;
    }
 
    float lnR = logf(ohms);
 
    float lnR3 = lnR * lnR * lnR;
 
    float recip = m_a + m_b * lnR + m_c * lnR3;
 
    float kelvin = 1 / recip;
 
    float celsius = convertKelvinToCelcius(kelvin);
 
    // bounds check result - please don't try to run rusEfi when colder than -50C
    // high end limit is required as this could be an oil temp sensor on an
    // air cooled engine
    if (celsius < -50) {
        return UnexpectedCode::Low;
    }
 
    if (celsius > 250) {
        return UnexpectedCode::High;
    }
 
    return celsius;
}
 
void ThermistorFunc::configure(thermistor_conf_s &cfg) {
    // https://en.wikipedia.org/wiki/Steinhart%E2%80%93Hart_equation
    float l1 = logf(cfg.resistance_1);
    float l2 = logf(cfg.resistance_2);
    float l3 = logf(cfg.resistance_3);
 
    float y1 = 1 / convertCelsiusToKelvin(cfg.tempC_1);
    float y2 = 1 / convertCelsiusToKelvin(cfg.tempC_2);
    float y3 = 1 / convertCelsiusToKelvin(cfg.tempC_3);
 
    float u2 = (y2 - y1) / (l2 - l1);
    float u3 = (y3 - y1) / (l3 - l1);
 
    m_c = ((u3 - u2) / (l3 - l2)) / (l1 + l2 + l3);
    m_b = u2 - m_c * (l1 * l1 + l1 * l2 + l2 * l2);
    m_a = y1 - (m_b + l1 * l1 * m_c) * l1;
 
    float resistance10percent = cfg.resistance_1 + 0.1 * (cfg.resistance_2 - cfg.resistance_1);
    float tempAt10percentPoint = convert(resistance10percent).Value;
 
    if (tempAt10percentPoint < cfg.tempC_1) {
#if EFI_UNIT_TEST
  throw std::logic_error("Bad thermistor configuration at the left");
#endif
      criticalError("Thermistor configuration has failed 10% test");
    }
 
    float resistance90percent = cfg.resistance_2 + 0.9 * (cfg.resistance_3 - cfg.resistance_2);
    float tempAt90percentPoint = convert(resistance90percent).Value;
    if (tempAt90percentPoint > cfg.tempC_3) {
#if EFI_UNIT_TEST
  throw std::logic_error("Bad thermistor configuration at the right");
#endif
      criticalError("Thermistor configuration has failed 90% test");
    }
}