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