calibration.py

File contents

#!/usr/bin/env python3
import scipy
import numpy as np

__all__ = ["calibrate"]


DEFAULT_PT100_COEFFICIENTS = (
    -264.8895,
    234.1986,
    -2.480645,
    -0.1422663,
)


def polynom(x, coeffs):
    return coeffs[3] * x**3 + coeffs[2] * x**2 + coeffs[1] * x + coeffs[0]


def polynom_with_offset(x, coeffs, offset):
    return polynom(x, coeffs) - offset


def format_command_parameter(value):
    """Forces numbers to fit into 9 characters, so OsTech commands won't get too long."""
    result = ""
    for i in range(9, 0, -1):
        result = "%.*g" % (i, value)
        if len(result) <= 9:
            break
    return result


def get_commands(coeffs, sensor_number=None):
    """Generates OsTech Sensor coefficient commands."""
    sensor_number_string = str(sensor_number) if sensor_number else "[SensorNumber]"
    coeffs_formatted = [format_command_parameter(c) for c in coeffs]
    commands = [
        f"{sensor_number_string}SSC{i}{c}" for i, c in enumerate(coeffs_formatted)
    ]
    return commands


def calibrate(
    temperatures,
    sensor_number=None,
    coeffs=DEFAULT_PT100_COEFFICIENTS,
):
    """Calculate new sensor coefficients from measured temperature pairs and old coefficients.

    :param temperatures: list of temperature pairs [(OsTech device temperature, externally measured temperature)]
    :param sensor_number: number of the temperature sensor (1…7)
    :param coeffs: old sensor coefficients (c0…c3), can be omitted if default coefficients are used
    :return: list of new sensor coefficients (c0…c3), commands to set the new coefficients
    """
    temperatures = np.array(temperatures)
    temp_difference = temperatures[:, 0] - temperatures[:, 1]
    voltages = np.array(
        [
            scipy.optimize.root(
                polynom_with_offset, np.array([1]), args=(coeffs, temp)
            ).x[0]
            for temp, _ in temperatures
        ]
    )

    coeffs_difference = np.array((np.polyfit(voltages, temp_difference, 3))[::-1])
    coeffs_new = [float(c) for c in coeffs - coeffs_difference]

    result = (
        coeffs_new,
        get_commands(coeffs_new, sensor_number),
    )
    return result


# Example
if __name__ == "__main__":
    temperatures = [
        # (temperature measured by OsTech device, externally measured temperature)
        (-5.0, -5.8),
        (35.0, 35.5),
        (75.0, 77.6),
    ]

    coeffs_new, commands = calibrate(temperatures, sensor_number=3)
    coeffs_lines = "\n".join([str(x) for x in coeffs_new])
    print(f"Coefficients:\n{coeffs_lines}")
    command_lines = "\n".join(commands)
    print(f"Commands:\n{command_lines}")