🐛 Fix MAX31865 PT1000 normalization (#24407)
Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>
This commit is contained in:
John Lagonikas
Scott Lahteine
parent
57c137a60f
commit
c076094fa9
@ -133,13 +133,13 @@ SPISettings MAX31865::spiConfig = SPISettings(
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/**
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* Initialize the SPI interface and set the number of RTD wires used
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*
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* @param wires The number of wires in enum format. Can be MAX31865_2WIRE, MAX31865_3WIRE, or MAX31865_4WIRE.
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* @param zero The resistance of the RTD at 0 degC, in ohms.
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* @param ref The resistance of the reference resistor, in ohms.
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* @param wire The resistance of the wire connecting the sensor to the RTD, in ohms.
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* @param wires The number of wires as an enum: MAX31865_2WIRE, MAX31865_3WIRE, or MAX31865_4WIRE.
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* @param zero_res The resistance of the RTD at 0°C, in ohms.
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* @param ref_res The resistance of the reference resistor, in ohms.
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* @param wire_res The resistance of the wire connecting the sensor to the RTD, in ohms.
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*/
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void MAX31865::begin(max31865_numwires_t wires, float zero_res, float ref_res, float wire_res) {
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zeroRes = zero_res;
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void MAX31865::begin(max31865_numwires_t wires, const_float_t zero_res, const_float_t ref_res, const_float_t wire_res) {
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resNormalizer = 100.0f / zero_res; // reciprocal of resistance, scaled by 100
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refRes = ref_res;
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wireRes = wire_res;
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@ -437,42 +437,61 @@ float MAX31865::temperature() {
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*
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* @return Temperature in C
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*/
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float MAX31865::temperature(uint16_t adc_val) {
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float MAX31865::temperature(const uint16_t adc_val) {
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return temperature(((adc_val) * RECIPROCAL(32768.0f)) * refRes - wireRes);
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}
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/**
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* Calculate the temperature in C from the RTD resistance.
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* Uses the technique outlined in this PDF:
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* http://www.analog.com/media/en/technical-documentation/application-notes/AN709_0.pdf
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*
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* @param rtd_res the resistance value in ohms
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* @return the temperature in degC
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* @return the temperature in °C
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*/
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float MAX31865::temperature(float rtd_res) {
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rtd_res *= resNormalizer; // normalize to 100 ohm
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// Constants for calculating temperature from the measured RTD resistance.
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// http://www.analog.com/media/en/technical-documentation/application-notes/AN709_0.pdf
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constexpr float RTD_Z1 = -0.0039083,
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RTD_Z2 = +1.758480889e-5,
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RTD_Z3 = -2.31e-8,
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RTD_Z4 = -1.155e-6;
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// Callender-Van Dusen equation
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float temp = (RTD_Z1 + sqrt(RTD_Z2 + (RTD_Z3 * rtd_res))) * RECIPROCAL(RTD_Z4);
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// From the PDF...
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//
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// The previous equation is valid only for temperatures of 0°C and above.
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// The equation for RRTD(t) that defines negative temperature behavior is a
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// fourth-order polynomial (after expanding the third term) and is quite
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// impractical to solve for a single expression of temperature as a function
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// of resistance.
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// of resistance. So here we use a Linear Approximation instead.
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//
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if (temp < 0) {
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rtd_res = (rtd_res / zeroRes) * 100; // normalize to 100 ohm
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float rpoly = rtd_res;
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#ifndef MAX31865_APPROX
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#define MAX31865_APPROX 5
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#endif
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temp = -242.02 + (2.2228 * rpoly);
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rpoly *= rtd_res; // square
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temp += 2.5859e-3 * rpoly;
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rpoly *= rtd_res; // ^3
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temp -= 4.8260e-6 * rpoly;
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rpoly *= rtd_res; // ^4
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temp -= 2.8183e-8 * rpoly;
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rpoly *= rtd_res; // ^5
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temp += 1.5243e-10 * rpoly;
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constexpr float RTD_C[] = {
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#if MAX31865_APPROX == 5
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-242.02, +2.2228, +2.5859e-3, -4.8260e-6, -2.8183e-8, +1.5243e-10
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#elif MAX31865_APPROX == 4
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-241.96, +2.2163, +2.8541e-3, -9.9121e-6, -1.7152e-8
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#elif MAX31865_APPROX == 3
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-242.09, +2.2276, +2.5178e-3, -5.8620e-6
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#else
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-242.97, +2.2838, +1.4727e-3
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#endif
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};
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float rpoly = rtd_res;
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temp = RTD_C[0];
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temp += rpoly * RTD_C[1];
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rpoly *= rtd_res; temp += rpoly * RTD_C[2];
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if (MAX31865_APPROX >= 3) rpoly *= rtd_res; temp += rpoly * RTD_C[3];
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if (MAX31865_APPROX >= 4) rpoly *= rtd_res; temp += rpoly * RTD_C[4];
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if (MAX31865_APPROX >= 5) rpoly *= rtd_res; temp += rpoly * RTD_C[5];
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}
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return temp;
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@ -73,13 +73,6 @@
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#define MAX31865_FAULT_RTDINLOW 0x08 // D3
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#define MAX31865_FAULT_OVUV 0x04 // D2
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// http://www.analog.com/media/en/technical-documentation/application-notes/AN709_0.pdf
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// constants for calculating temperature from the measured RTD resistance.
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#define RTD_Z1 -0.0039083
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#define RTD_Z2 0.00001758480889
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#define RTD_Z3 -0.0000000231
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#define RTD_Z4 -0.000001155
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typedef enum max31865_numwires {
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MAX31865_2WIRE = 0,
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MAX31865_3WIRE = 1,
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@ -103,7 +96,7 @@ private:
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uint16_t spiDelay;
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float zeroRes, refRes, wireRes;
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float resNormalizer, refRes, wireRes;
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#if ENABLED(MAX31865_USE_READ_ERROR_DETECTION)
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millis_t lastReadStamp = 0;
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@ -160,7 +153,7 @@ public:
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int8_t spi_clk);
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#endif
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void begin(max31865_numwires_t wires, float zero_res, float ref_res, float wire_res);
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void begin(max31865_numwires_t wires, const_float_t zero_res, const_float_t ref_res, const_float_t wire_res);
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uint8_t readFault();
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void clearFault();
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@ -168,6 +161,6 @@ public:
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uint16_t readRaw();
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float readResistance();
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float temperature();
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float temperature(uint16_t adc_val);
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float temperature(const uint16_t adc_val);
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float temperature(float rtd_res);
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};
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