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Make disconnected thermistor show temp as absolute zero again

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In a previous commit I changed the temperature calculation to give more
accurate readings at extreme values. This change meant that a
disconnected thermistor no longer shows as absolute zero. Coupled with
the change in thermistor beta value, this meant that a disconnected
extruder thermistor would show as -28.2C, which is above the usual error
threshold of -30C. This commit introduces a special case for a
disconnected thermistor and restores the reading to absolute zero for
that case.
This commit is contained in:
David Crocker 2014-01-15 13:42:59 +00:00
parent 618304c021
commit 82e23d4774
1 changed files with 9 additions and 3 deletions
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12
Platform.cpp
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@ -334,9 +334,15 @@ void Platform::ClassReport(char* className, float &lastTime)
float Platform::GetTemperature(int8_t heater)
{
// If the ADC reading is N then for an ideal ADC, the input voltage is at least N/(ADC_RANGE + 1) and less than (N + 1)/(ADC_RANGE + 1), times the analog reference.
// So we add 0.5 to to the reading to get a better estimate of the input. We don't care whether or not we get exactly zero with the thermistor disconnected.
float r = (float)GetRawTemperature(heater) + 0.5;
// If the ADC reading is N then for an ideal ADC, the input voltage is at least N/(AD_RANGE + 1) and less than (N + 1)/(AD_RANGE + 1), times the analog reference.
// So we add 0.5 to to the reading to get a better estimate of the input. But first, recognise the special case of thermistor disconnected.
int rawTemp = GetRawTemperature(heater);
if (rawTemp == AD_RANGE)
{
// Thermistor is disconnected
return ABS_ZERO;
}
float r = (float)rawTemp + 0.5;
return ABS_ZERO + thermistorBetas[heater]/log( (r*thermistorSeriesRs[heater]/((AD_RANGE + 1) - r))/thermistorInfRs[heater] );
}