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#!/usr/bin/env python
"""Thermistor Value Lookup Table Generator
Generates lookup to temperature values for use in a microcontroller in C format based on:
https://en.wikipedia.org/wiki/Steinhart-Hart_equation
The main use is for Arduino programs that read data from the circuit board described here:
https://reprap.org/wiki/Temperature_Sensor_v2.0
Usage: python createTemperatureLookupMarlin.py [options]
Options:
-h, --help show this help
--rp=... pull-up resistor
--t1=ttt:rrr low temperature temperature:resistance point (around 25 degC)
--t2=ttt:rrr middle temperature temperature:resistance point (around 150 degC)
--t3=ttt:rrr high temperature temperature:resistance point (around 250 degC)
--num-temps=... the number of temperature points to calculate (default: 36)
"""
from __future__ import print_function
from __future__ import division
from math import *
import sys
import getopt
"Constants"
ZERO = 273.15 # zero point of Kelvin scale
VADC = 5 # ADC voltage
VCC = 5 # supply voltage
ARES = pow(2,10) # 10 Bit ADC resolution
VSTEP = VADC / ARES # ADC voltage resolution
TMIN = 0 # lowest temperature in table
TMAX = 350 # highest temperature in table
class Thermistor:
"Class to do the thermistor maths"
def __init__(self, rp, t1, r1, t2, r2, t3, r3):
l1 = log(r1)
l2 = log(r2)
l3 = log(r3)
y1 = 1.0 / (t1 + ZERO) # adjust scale
y2 = 1.0 / (t2 + ZERO)
y3 = 1.0 / (t3 + ZERO)
x = (y2 - y1) / (l2 - l1)
y = (y3 - y1) / (l3 - l1)
c = (y - x) / ((l3 - l2) * (l1 + l2 + l3))
b = x - c * (l1**2 + l2**2 + l1*l2)
a = y1 - (b + l1**2 *c)*l1
if c < 0:
print("//////////////////////////////////////////////////////////////////////////////////////")
print("// WARNING: negative coefficient 'c'! Something may be wrong with the measurements! //")
print("//////////////////////////////////////////////////////////////////////////////////////")
c = -c
self.c1 = a # Steinhart-Hart coefficients
self.c2 = b
self.c3 = c
self.rp = rp # pull-up resistance
def resol(self, adc):
"Convert ADC reading into a resolution"
res = self.temp(adc)-self.temp(adc+1)
return res
def voltage(self, adc):
"Convert ADC reading into a Voltage"
return adc * VSTEP # convert the 10 bit ADC value to a voltage
def resist(self, adc):
"Convert ADC reading into a resistance in Ohms"
r = self.rp * self.voltage(adc) / (VCC - self.voltage(adc)) # resistance of thermistor
return r
def temp(self, adc):
"Convert ADC reading into a temperature in Celcius"
l = log(self.resist(adc))
Tinv = self.c1 + self.c2*l + self.c3* l**3 # inverse temperature
return (1/Tinv) - ZERO # temperature
def adc(self, temp):
"Convert temperature into a ADC reading"
x = (self.c1 - (1.0 / (temp+ZERO))) / (2*self.c3)
y = sqrt((self.c2 / (3*self.c3))**3 + x**2)
r = exp((y-x)**(1.0/3) - (y+x)**(1.0/3))
return (r / (self.rp + r)) * ARES
def main(argv):
"Default values"
t1 = 25 # low temperature in Kelvin (25 degC)
r1 = 100000 # resistance at low temperature (10 kOhm)
t2 = 150 # middle temperature in Kelvin (150 degC)
r2 = 1641.9 # resistance at middle temperature (1.6 KOhm)
t3 = 250 # high temperature in Kelvin (250 degC)
r3 = 226.15 # resistance at high temperature (226.15 Ohm)
rp = 4700; # pull-up resistor (4.7 kOhm)
num_temps = 36; # number of entries for look-up table
try:
opts, args = getopt.getopt(argv, "h", ["help", "rp=", "t1=", "t2=", "t3=", "num-temps="])
except getopt.GetoptError as err:
print(str(err))
usage()
sys.exit(2)
for opt, arg in opts:
if opt in ("-h", "--help"):
usage()
sys.exit()
elif opt == "--rp":
rp = int(arg)
elif opt == "--t1":
arg = arg.split(':')
t1 = float(arg[0])
r1 = float(arg[1])
elif opt == "--t2":
arg = arg.split(':')
t2 = float(arg[0])
r2 = float(arg[1])
elif opt == "--t3":
arg = arg.split(':')
t3 = float(arg[0])
r3 = float(arg[1])
elif opt == "--num-temps":
num_temps = int(arg)
t = Thermistor(rp, t1, r1, t2, r2, t3, r3)
increment = int((ARES-1)/(num_temps-1));
step = (TMIN-TMAX) / (num_temps-1)
low_bound = t.temp(ARES-1);
up_bound = t.temp(1);
min_temp = int(TMIN if TMIN > low_bound else low_bound)
max_temp = int(TMAX if TMAX < up_bound else up_bound)
temps = list(range(max_temp, TMIN+step, step));
print("// Thermistor lookup table for Marlin")
print("// ./createTemperatureLookupMarlin.py --rp=%s --t1=%s:%s --t2=%s:%s --t3=%s:%s --num-temps=%s" % (rp, t1, r1, t2, r2, t3, r3, num_temps))
print("// Steinhart-Hart Coefficients: a=%.15g, b=%.15g, c=%.15g " % (t.c1, t.c2, t.c3))
print("// Theoretical limits of thermistor: %.2f to %.2f degC" % (low_bound, up_bound))
print()
print("const short temptable[][2] PROGMEM = {")
for temp in temps:
adc = t.adc(temp)
print(" { OV(%7.2f), %4s }%s // v=%.3f\tr=%.3f\tres=%.3f degC/count" % (adc , temp, \
',' if temp != temps[-1] else ' ', \
t.voltage(adc), \
t.resist( adc), \
t.resol( adc) \
))
print("};")
def usage():
print(__doc__)
if __name__ == "__main__":
main(sys.argv[1:])
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