🐛 Fix MAX31865 PT1000 normalization (#24407)

Co-authored-by: Scott Lahteine <thinkyhead@users.noreply.github.com>
This commit is contained in:
John Lagonikas 2022-06-29 01:53:05 +03:00 committed by Scott Lahteine
parent 57c137a60f
commit c076094fa9
2 changed files with 45 additions and 33 deletions

View File

@ -133,13 +133,13 @@ SPISettings MAX31865::spiConfig = SPISettings(
/** /**
* Initialize the SPI interface and set the number of RTD wires used * Initialize the SPI interface and set the number of RTD wires used
* *
* @param wires The number of wires in enum format. Can be MAX31865_2WIRE, MAX31865_3WIRE, or MAX31865_4WIRE. * @param wires The number of wires as an enum: MAX31865_2WIRE, MAX31865_3WIRE, or MAX31865_4WIRE.
* @param zero The resistance of the RTD at 0 degC, in ohms. * @param zero_res The resistance of the RTD at 0°C, in ohms.
* @param ref The resistance of the reference resistor, in ohms. * @param ref_res The resistance of the reference resistor, in ohms.
* @param wire The resistance of the wire connecting the sensor to the RTD, in ohms. * @param wire_res The resistance of the wire connecting the sensor to the RTD, in ohms.
*/ */
void MAX31865::begin(max31865_numwires_t wires, float zero_res, float ref_res, float wire_res) { void MAX31865::begin(max31865_numwires_t wires, const_float_t zero_res, const_float_t ref_res, const_float_t wire_res) {
zeroRes = zero_res; resNormalizer = 100.0f / zero_res; // reciprocal of resistance, scaled by 100
refRes = ref_res; refRes = ref_res;
wireRes = wire_res; wireRes = wire_res;
@ -437,42 +437,61 @@ float MAX31865::temperature() {
* *
* @return Temperature in C * @return Temperature in C
*/ */
float MAX31865::temperature(uint16_t adc_val) { float MAX31865::temperature(const uint16_t adc_val) {
return temperature(((adc_val) * RECIPROCAL(32768.0f)) * refRes - wireRes); return temperature(((adc_val) * RECIPROCAL(32768.0f)) * refRes - wireRes);
} }
/** /**
* Calculate the temperature in C from the RTD resistance. * Calculate the temperature in C from the RTD resistance.
* Uses the technique outlined in this PDF:
* http://www.analog.com/media/en/technical-documentation/application-notes/AN709_0.pdf
* *
* @param rtd_res the resistance value in ohms * @param rtd_res the resistance value in ohms
* @return the temperature in degC * @return the temperature in °C
*/ */
float MAX31865::temperature(float rtd_res) { float MAX31865::temperature(float rtd_res) {
rtd_res *= resNormalizer; // normalize to 100 ohm
// Constants for calculating temperature from the measured RTD resistance.
// http://www.analog.com/media/en/technical-documentation/application-notes/AN709_0.pdf
constexpr float RTD_Z1 = -0.0039083,
RTD_Z2 = +1.758480889e-5,
RTD_Z3 = -2.31e-8,
RTD_Z4 = -1.155e-6;
// Callender-Van Dusen equation
float temp = (RTD_Z1 + sqrt(RTD_Z2 + (RTD_Z3 * rtd_res))) * RECIPROCAL(RTD_Z4); float temp = (RTD_Z1 + sqrt(RTD_Z2 + (RTD_Z3 * rtd_res))) * RECIPROCAL(RTD_Z4);
// From the PDF...
// //
// The previous equation is valid only for temperatures of 0°C and above. // The previous equation is valid only for temperatures of 0°C and above.
// The equation for RRTD(t) that defines negative temperature behavior is a // The equation for RRTD(t) that defines negative temperature behavior is a
// fourth-order polynomial (after expanding the third term) and is quite // fourth-order polynomial (after expanding the third term) and is quite
// impractical to solve for a single expression of temperature as a function // impractical to solve for a single expression of temperature as a function
// of resistance. // of resistance. So here we use a Linear Approximation instead.
// //
if (temp < 0) { if (temp < 0) {
rtd_res = (rtd_res / zeroRes) * 100; // normalize to 100 ohm #ifndef MAX31865_APPROX
float rpoly = rtd_res; #define MAX31865_APPROX 5
#endif
temp = -242.02 + (2.2228 * rpoly); constexpr float RTD_C[] = {
rpoly *= rtd_res; // square #if MAX31865_APPROX == 5
temp += 2.5859e-3 * rpoly; -242.02, +2.2228, +2.5859e-3, -4.8260e-6, -2.8183e-8, +1.5243e-10
rpoly *= rtd_res; // ^3 #elif MAX31865_APPROX == 4
temp -= 4.8260e-6 * rpoly; -241.96, +2.2163, +2.8541e-3, -9.9121e-6, -1.7152e-8
rpoly *= rtd_res; // ^4 #elif MAX31865_APPROX == 3
temp -= 2.8183e-8 * rpoly; -242.09, +2.2276, +2.5178e-3, -5.8620e-6
rpoly *= rtd_res; // ^5 #else
temp += 1.5243e-10 * rpoly; -242.97, +2.2838, +1.4727e-3
#endif
};
float rpoly = rtd_res;
temp = RTD_C[0];
temp += rpoly * RTD_C[1];
rpoly *= rtd_res; temp += rpoly * RTD_C[2];
if (MAX31865_APPROX >= 3) rpoly *= rtd_res; temp += rpoly * RTD_C[3];
if (MAX31865_APPROX >= 4) rpoly *= rtd_res; temp += rpoly * RTD_C[4];
if (MAX31865_APPROX >= 5) rpoly *= rtd_res; temp += rpoly * RTD_C[5];
} }
return temp; return temp;

View File

@ -73,13 +73,6 @@
#define MAX31865_FAULT_RTDINLOW 0x08 // D3 #define MAX31865_FAULT_RTDINLOW 0x08 // D3
#define MAX31865_FAULT_OVUV 0x04 // D2 #define MAX31865_FAULT_OVUV 0x04 // D2
// http://www.analog.com/media/en/technical-documentation/application-notes/AN709_0.pdf
// constants for calculating temperature from the measured RTD resistance.
#define RTD_Z1 -0.0039083
#define RTD_Z2 0.00001758480889
#define RTD_Z3 -0.0000000231
#define RTD_Z4 -0.000001155
typedef enum max31865_numwires { typedef enum max31865_numwires {
MAX31865_2WIRE = 0, MAX31865_2WIRE = 0,
MAX31865_3WIRE = 1, MAX31865_3WIRE = 1,
@ -103,7 +96,7 @@ private:
uint16_t spiDelay; uint16_t spiDelay;
float zeroRes, refRes, wireRes; float resNormalizer, refRes, wireRes;
#if ENABLED(MAX31865_USE_READ_ERROR_DETECTION) #if ENABLED(MAX31865_USE_READ_ERROR_DETECTION)
millis_t lastReadStamp = 0; millis_t lastReadStamp = 0;
@ -160,7 +153,7 @@ public:
int8_t spi_clk); int8_t spi_clk);
#endif #endif
void begin(max31865_numwires_t wires, float zero_res, float ref_res, float wire_res); void begin(max31865_numwires_t wires, const_float_t zero_res, const_float_t ref_res, const_float_t wire_res);
uint8_t readFault(); uint8_t readFault();
void clearFault(); void clearFault();
@ -168,6 +161,6 @@ public:
uint16_t readRaw(); uint16_t readRaw();
float readResistance(); float readResistance();
float temperature(); float temperature();
float temperature(uint16_t adc_val); float temperature(const uint16_t adc_val);
float temperature(float rtd_res); float temperature(float rtd_res);
}; };