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thermistor: Fallback to using beta if Steinhart-Hart c3 is negative 

If the c3 coefficient is negative it can cause the adc calculations to
fail.  The c3 shouldn't be negative in practice, so fallback to a
simple beta calculation in that case.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
This commit is contained in:
Kevin O'Connor 2018-04-09 12:35:18 -04:00
parent 8c8261ba80
commit 75b89f5261
1 changed files with 11 additions and 4 deletions
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15
klippy/extras/thermistor.py
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@ -3,7 +3,7 @@
# Copyright (C) 2016-2018 Kevin O'Connor <kevin@koconnor.net>
#
# This file may be distributed under the terms of the GNU GPLv3 license.
import math
import math, logging
KELVIN_TO_CELCIUS = -273.15
SAMPLE_TIME = 0.001
@ -13,6 +13,7 @@ REPORT_TIME = 0.300
# Analog voltage to temperature converter for thermistors
class Thermistor:
def __init__(self, config, params):
self.name = config.get_name()
self.pullup = config.getfloat('pullup_resistor', 4700., above=0.)
ppins = config.get_printer().lookup_object('pins')
self.mcu_adc = ppins.setup_pin('adc', config.get('sensor_pin'))
@ -20,7 +21,7 @@ class Thermistor:
self.temperature_callback = None
self.c1 = self.c2 = self.c3 = 0.
if 'beta' in params:
self.calc_coefficients_beta(params)
self.calc_coefficients_beta(params, params['beta'])
else:
self.calc_coefficients(params)
def calc_coefficients(self, params):
@ -40,14 +41,20 @@ class Thermistor:
self.c3 = ((inv_t12 - inv_t13 * ln_r12 / ln_r13)
/ (ln3_r12 - ln3_r13 * ln_r12 / ln_r13))
if self.c3 <= 0.:
beta = ln_r13 / inv_t13
logging.warn("Using thermistor beta %.3f in heater %s",
beta, self.name)
self.calc_coefficients_beta(params, 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 calc_coefficients_beta(self, params):
def calc_coefficients_beta(self, params, beta):
# Calculate equivalent Steinhart-Hart coefficents from beta
inv_t1 = 1. / (params['t1'] - KELVIN_TO_CELCIUS)
ln_r1 = math.log(params['r1'])
self.c3 = 0.
self.c2 = 1. / params['beta']
self.c2 = 1. / beta
self.c1 = inv_t1 - self.c2 * ln_r1
def setup_minmax(self, min_temp, max_temp):
adc_range = [self.calc_adc(min_temp), self.calc_adc(max_temp)]