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- """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, division
- from math import *
- import sys, getopt
- "Constants"
- ZERO = 273.15
- VADC = 5
- VCC = 5
- ARES = pow(2,10)
- VSTEP = VADC / ARES
- TMIN = 0
- TMAX = 350
- 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)
- 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
- self.c2 = b
- self.c3 = c
- self.rp = rp
- 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
- def resist(self, adc):
- "Convert ADC reading into a resistance in Ohms"
- r = self.rp * self.voltage(adc) / (VCC - self.voltage(adc))
- return r
- def temp(self, adc):
- "Convert ADC reading into a temperature in Celsius"
- l = log(self.resist(adc))
- Tinv = self.c1 + self.c2*l + self.c3* l**3
- return (1/Tinv) - ZERO
- 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
- r1 = 100000
- t2 = 150
- r2 = 1641.9
- t3 = 250
- r3 = 226.15
- rp = 4700
- num_temps = 36
- 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 = int((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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