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#!/usr/bin/python
"""Thermistor Value Lookup Table Generator
Generates lookup to temperature values for use in a microcontroller in C format based on:
http://en.wikipedia.org/wiki/Steinhart-Hart_equation
The main use is for Arduino programs that read data from the circuit board described here:
http://make.rrrf.org/ts-1.0
Usage: python createTemperatureLookup.py [options]
Options:
-h, --help show this help
--rp=... pull-up resistor
--t1=ttt:rrr low temperature temperature:resistance point (around 25C)
--t2=ttt:rrr middle temperature temperature:resistance point (around 150C)
--t3=ttt:rrr high temperature temperature:resistance point (around 250C)
--num-temps=... the number of temperature points to calculate (default: 36)
"""
from math import *
import sys
import getopt
class Thermistor:
"Class to do the thermistor maths"
def __init__(self, rp, t1, r1, t2, r2, t3, r3):
t1 = t1 + 273.15 # low temperature (25C)
r1 = r1 # resistance at low temperature
t2 = t2 + 273.15 # middle temperature (150C)
r2 = r2 # resistance at middle temperature
t3 = t3 + 273.15 # high temperature (250C)
r3 = r3 # resistance at high temperature
self.rp = rp # pull-up resistance
self.vadc = 5.0 # ADC reference
self.vcc = 5.0 # supply voltage to potential divider
a1 = log(r1)
a2 = log(r2)
a3 = log(r3)
z = a1 - a2
y = a1 - a3
x = 1/t1 - 1/t2
w = 1/t1 - 1/t3
v = pow(a1,3) - pow(a2,3)
u = pow(a1,3) - pow(a3,3)
c3 = (x-z*w/y)/(v-z*u/y)
c2 = (x-c3*v)/z
c1 = 1/t1-c3*pow(a1,3)-c2*a1
self.c1 = c1
self.c2 = c2
self.c3 = c3
def res(self,adc):
"Convert ADC reading into a resolution"
res = self.temp(adc)-self.temp(adc+1)
return res
def v(self,adc):
"Convert ADC reading into a Voltage"
v = adc * self.vadc / (1024 ) # convert the 10 bit ADC value to a voltage
return v
def r(self,adc):
"Convert ADC reading into a resistance in Ohms"
v = adc * self.vadc / (1024 ) # convert the 10 bit ADC value to a voltage
r = self.rp * v / (self.vcc - v) # resistance of thermistor
return r
def temp(self,adc):
"Convert ADC reading into a temperature in Celcius"
v = adc * self.vadc / (1024 ) # convert the 10 bit ADC value to a voltage
r = self.rp * v / (self.vcc - v) # resistance of thermistor
lnr = log(r)
Tinv = self.c1 + (self.c2*lnr) + (self.c3*pow(lnr,3))
return (1/Tinv) - 273.15 # temperature
def adc(self,temp):
"Convert temperature into a ADC reading"
y = (self.c1 - (1/(temp+273.15))) / (2*self.c3)
x = sqrt(pow(self.c2 / (3*self.c3),3) + pow(y,2))
r = exp(pow(x-y,1.0/3) - pow(x+y,1.0/3)) # resistance of thermistor
return (r / (self.rp + r)) * (1024)
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 = int(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)
max_adc = (1024 ) - 1
min_temp = 0
max_temp = 350
increment = int(max_adc/(num_temps-1));
t = Thermistor(rp, t1, r1, t2, r2, t3, r3)
tmp = (min_temp - max_temp) / (num_temps-1)
print tmp
temps = range(max_temp, min_temp + tmp, tmp);
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
print "#define NUMTEMPS %s" % (len(temps))
print "const short temptable[NUMTEMPS][2] PROGMEM = {"
for temp in temps:
print " { (short) (%7.2f * OVERSAMPLENR ), %s\t}%s // v=%.3f\tr=%.3f\tres=%.3f degC/count" % ( t.adc(temp), temp, \
',' if temp != temps[-1] else ' ', \
t.v( t.adc(temp)), \
t.r( t.adc(temp)), \
t.res(t.adc(temp)) \
)
print "};"
def usage():
print __doc__
if __name__ == "__main__":
main(sys.argv[1:])