Subversion Repositories MK-Marlin

#!/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://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 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 = 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:])