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# Temperature measurements with thermistors
#
# Copyright (C) 2016-2019 Kevin O'Connor <kevin@koconnor.net>
#
# This file may be distributed under the terms of the GNU GPLv3 license.
import math, logging
from . import adc_temperature
KELVIN_TO_CELSIUS = -273.15
# Analog voltage to temperature converter for thermistors
class Thermistor:
def __init__(self, pullup, inline_resistor):
self.pullup = pullup
self.inline_resistor = inline_resistor
self.c1 = self.c2 = self.c3 = 0.0
def setup_coefficients(self, t1, r1, t2, r2, t3, r3, name=""):
# Calculate Steinhart-Hart coefficients from temp measurements.
# Arrange samples as 3 linear equations and solve for c1, c2, and c3.
inv_t1 = 1.0 / (t1 - KELVIN_TO_CELSIUS)
inv_t2 = 1.0 / (t2 - KELVIN_TO_CELSIUS)
inv_t3 = 1.0 / (t3 - KELVIN_TO_CELSIUS)
ln_r1 = math.log(r1)
ln_r2 = math.log(r2)
ln_r3 = math.log(r3)
ln3_r1, ln3_r2, ln3_r3 = ln_r1**3, ln_r2**3, ln_r3**3
inv_t12, inv_t13 = inv_t1 - inv_t2, inv_t1 - inv_t3
ln_r12, ln_r13 = ln_r1 - ln_r2, ln_r1 - ln_r3
ln3_r12, ln3_r13 = ln3_r1 - ln3_r2, ln3_r1 - ln3_r3
self.c3 = (inv_t12 - inv_t13 * ln_r12 / ln_r13) / (
ln3_r12 - ln3_r13 * ln_r12 / ln_r13
)
if self.c3 <= 0.0:
beta = ln_r13 / inv_t13
logging.warning("Using thermistor beta %.3f in heater %s", beta, name)
self.setup_coefficients_beta(t1, r1, beta)
return
self.c2 = (inv_t12 - self.c3 * ln3_r12) / ln_r12
self.c1 = inv_t1 - self.c2 * ln_r1 - self.c3 * ln3_r1
def setup_coefficients_beta(self, t1, r1, beta):
# Calculate equivalent Steinhart-Hart coefficients from beta
inv_t1 = 1.0 / (t1 - KELVIN_TO_CELSIUS)
ln_r1 = math.log(r1)
self.c3 = 0.0
self.c2 = 1.0 / beta
self.c1 = inv_t1 - self.c2 * ln_r1
def calc_temp(self, adc):
# Calculate temperature from adc
adc = max(0.00001, min(0.99999, adc))
r = self.pullup * adc / (1.0 - adc)
ln_r = math.log(r - self.inline_resistor)
inv_t = self.c1 + self.c2 * ln_r + self.c3 * ln_r**3
return 1.0 / inv_t + KELVIN_TO_CELSIUS
def calc_adc(self, temp):
# Calculate adc reading from a temperature
if temp <= KELVIN_TO_CELSIUS:
return 1.0
inv_t = 1.0 / (temp - KELVIN_TO_CELSIUS)
if self.c3:
# Solve for ln_r using Cardano's formula
y = (self.c1 - inv_t) / (2.0 * self.c3)
x = math.sqrt((self.c2 / (3.0 * self.c3)) ** 3 + y**2)
ln_r = math.pow(x - y, 1.0 / 3.0) - math.pow(x + y, 1.0 / 3.0)
else:
ln_r = (inv_t - self.c1) / self.c2
r = math.exp(ln_r) + self.inline_resistor
return r / (self.pullup + r)
# Create an ADC converter with a thermistor
def PrinterThermistor(config, params):
pullup = config.getfloat("pullup_resistor", 4700.0, above=0.0)
inline_resistor = config.getfloat("inline_resistor", 0.0, minval=0.0)
thermistor = Thermistor(pullup, inline_resistor)
if "beta" in params:
thermistor.setup_coefficients_beta(params["t1"], params["r1"], params["beta"])
else:
thermistor.setup_coefficients(
params["t1"],
params["r1"],
params["t2"],
params["r2"],
params["t3"],
params["r3"],
name=config.get_name(),
)
return adc_temperature.PrinterADCtoTemperature(config, thermistor)
# Custom defined thermistors from the config file
class CustomThermistor:
def __init__(self, config):
self.name = " ".join(config.get_name().split()[1:])
t1 = config.getfloat("temperature1", minval=KELVIN_TO_CELSIUS)
r1 = config.getfloat("resistance1", minval=0.0)
beta = config.getfloat("beta", None, above=0.0)
if beta is not None:
self.params = {"t1": t1, "r1": r1, "beta": beta}
return
t2 = config.getfloat("temperature2", minval=KELVIN_TO_CELSIUS)
r2 = config.getfloat("resistance2", minval=0.0)
t3 = config.getfloat("temperature3", minval=KELVIN_TO_CELSIUS)
r3 = config.getfloat("resistance3", minval=0.0)
(t1, r1), (t2, r2), (t3, r3) = sorted([(t1, r1), (t2, r2), (t3, r3)])
self.params = {"t1": t1, "r1": r1, "t2": t2, "r2": r2, "t3": t3, "r3": r3}
def create(self, config):
return PrinterThermistor(config, self.params)
def load_config_prefix(config):
thermistor = CustomThermistor(config)
pheaters = config.get_printer().load_object(config, "heaters")
pheaters.add_sensor_factory(thermistor.name, thermistor.create)
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