Fixed heating fault detection for heaters with slow cooling rates

3 files changed, 8 insertions, 9 deletions

diff --git a/src/Configuration.h b/src/Configuration.h
index 89765445..bf047bf5 100644
--- a/src/Configuration.h
+++ b/src/Configuration.h
@@ -124,9 +124,10 @@ constexpr float DefaultThermistorC = 7.06e-8;
 
 // Parameters used to detect heating errors
 constexpr float DefaultMaxHeatingFaultTime = 5.0;		// How many seconds we allow a heating fault to persist
-constexpr float AllowedTemperatureDerivativeNoise = 0.25;	// How much fluctuation in the averaged temperature derivative we allow
+constexpr float AllowedTemperatureDerivativeNoise = 0.12;	// How much fluctuation in the averaged temperature derivative we allow
 constexpr float MaxAmbientTemperature = 45.0;			// We expect heaters to cool to this temperature or lower when switched off
 constexpr float NormalAmbientTemperature = 25.0;		// The ambient temperature we assume - allow for the printer heating its surroundings a little
+constexpr float LowAmbientTemperature = 15.0;			// A lower ambient temperature that we assume when checking heater performance
 constexpr float DefaultMaxTempExcursion = 15.0;			// How much error we tolerate when maintaining temperature before deciding that a heater fault has occurred
 constexpr float MinimumConnectedTemperature = -5.0;		// Temperatures below this we treat as a disconnected thermistor
 
diff --git a/src/Heating/LocalHeater.cpp b/src/Heating/LocalHeater.cpp
index 2803d49b..d2b6bde3 100644
--- a/src/Heating/LocalHeater.cpp
+++ b/src/Heating/LocalHeater.cpp
@@ -260,7 +260,7 @@ void LocalHeater::Spin() noexcept
 					else if (gotDerivative)
 					{
 						const float expectedRate = GetExpectedHeatingRate();
-						if (derivative + AllowedTemperatureDerivativeNoise < expectedRate
+						if (derivative + AllowedTemperatureDerivativeNoise < expectedRate * 0.75
 							&& (float)(millis() - timeSetHeating) > GetModel().GetDeadTime() * SecondsToMillis * 2)
 						{
 							++heatingFaultCount;
@@ -463,12 +463,10 @@ float LocalHeater::GetAveragePWM() const noexcept
 // Get a conservative estimate of the expected heating rate at the current temperature and average PWM. The result may be negative.
 float LocalHeater::GetExpectedHeatingRate() const noexcept
 {
-	// In the following we allow for the gain being only 60% of what we think it should be, to avoid false alarms
-	const float maxTemperatureRise = 0.60 * GetModel().GetGainFanOff() * GetAveragePWM();	// this is the highest temperature above ambient we expect the heater can reach at this PWM
-	const float initialHeatingRate = maxTemperatureRise/GetModel().GetTimeConstantFanOn();	// this is the expected heating rate at ambient temperature
-	return (maxTemperatureRise >= 20.0)
-			? (maxTemperatureRise + NormalAmbientTemperature - temperature) * initialHeatingRate/maxTemperatureRise
-			: 0.0;
+	const float initialHeatingRate = GetModel().GetHeatingRate() * GetAveragePWM();
+	return (temperature > LowAmbientTemperature)
+				? initialHeatingRate - (temperature - LowAmbientTemperature) * GetModel().GetCoolingRateFanOn()
+					: initialHeatingRate;
 }
 
 // Auto tune this heater. The caller has already checked that no other heater is being tuned and has set up tuningTargetTemp, tuningPwm, tuningFans, tuningHysteresis and tuningFanPwm.
diff --git a/src/Heating/LocalHeater.h b/src/Heating/LocalHeater.h
index c107c2d3..f9988ff4 100644
--- a/src/Heating/LocalHeater.h
+++ b/src/Heating/LocalHeater.h
@@ -21,7 +21,7 @@ class HeaterMonitor;
 
 class LocalHeater : public Heater
 {
-	static const size_t NumPreviousTemperatures = 4;		// How many samples we average the temperature derivative over
+	static const size_t NumPreviousTemperatures = 8;		// How many samples we average the temperature derivative over
 
 public:
 	LocalHeater(unsigned int heaterNum) noexcept;