112 lines
3.1 KiB
C
112 lines
3.1 KiB
C
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/*
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* @Date: 2025-07-04 13:05:24
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* @Author: mypx
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* @LastEditors: mypx mypx_coder@163.com
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* @LastEditTime: 2025-07-10 13:28:55
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* @FilePath: pt100x.c
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* @Description:
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* Copyright (c) 2025 by mypx, All Rights Reserved.
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*/
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#include "pt100x.h"
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#include <math.h>
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/*
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* Low temperature region (T < 0°C):
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* R(T) = R0(1 + A*T + B*T^2 + C*(T - R0)T^3)
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* High temperature region (T >= 0°C):
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* R(T) = R0(1 + A*T + B*T^2)
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*/
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#define A 3.9083e-3
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#define B -5.775e-7
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#define C -4.183e-12
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#define PT100_R0 100
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#define PT1000_R0 1000
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/**
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* @brief Calculate the temperature based on the resistance value of a PT100
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* platinum resistor (precise calculation).
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* @param resistance Measured resistance value (Ω).
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* @param ptType Type of the PT sensor (PT100 or PT1000).
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* @return Temperature value (°C), returns NaN if the resistance value is invalid.
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*/
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double pt_precise_temperature(double resistance, PtType ptType)
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{
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double R0 = (ptType == PT100) ? PT100_R0 : PT1000_R0;
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if (resistance < R0)
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{
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// Build the cubic equation: a*T³ + b*T² + c*T + d = 0
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double a = C * R0;
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double b = B - 100 * C;
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double c = A;
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double d = 1 - resistance / R0;
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// Use Newton's iteration method to solve the equation
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double T = 0.0; // Initial guess value
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int max_iterations = 100;
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double tolerance = 1e-6;
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for (int i = 0; i < max_iterations; i++)
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{
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// Calculate the polynomial value and its derivative
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double f = a * T * T * T + b * T * T + c * T + d;
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double df = 3 * a * T * T + 2 * b * T + c;
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// Check if the derivative is close to zero
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if (fabs(df) < tolerance) break;
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// Iterative update
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double delta = f / df;
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T -= delta;
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// Check convergence
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if (fabs(delta) < tolerance) break;
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}
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return T;
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}
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else
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{
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// High temperature region (T ≥ 0°C): Solve the quadratic equation
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// Quadratic equation: B*T² + A*T + (1 - R/R0) = 0
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double discriminant = A * A - 4 * B * (1 - resistance / R0);
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if (discriminant < 0)
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{
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return NAN;
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}
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// Take the positive root
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return (-A + sqrt(discriminant)) / (2 * B);
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}
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}
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/**
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* @brief Calculate the temperature based on the resistance value of a PT100 platinum resistor
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* (engineering approximation).
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* @param resistance Measured resistance value (Ω).
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* @param type Type of the PT sensor (PT100 or PT1000).
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* @return Temperature value (°C).
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*/
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double pt_approximate_temperature(double resistance, PtType type)
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{
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double R0 = (type == PT100) ? PT100_R0 : PT1000_R0;
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return (resistance / R0 - 1) / 0.00385;
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}
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float pt_temperature_to_resistance(double t, PtType type)
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{
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float resistance;
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float r0 = (type == PT100) ? PT100_R0 : PT1000_R0;
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if (t >= 0.0f)
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{
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resistance = r0 * (1 + A * t + B * t * t);
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}
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else
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{
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resistance = r0 * (1 + A * t + B * t * t + C * (t - r0) * t * t * t);
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}
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return resistance;
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}
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