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refactored termistor table calculation to be in line with wikipedia's article about Steinhart-Hart coefficients

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This commit is contained in:
Steffen Vogel 2015-01-06 08:24:59 +01:00
parent 21205cc3d8
commit 95f4a55820
1 changed files with 32 additions and 40 deletions
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72
Marlin/scripts/createTemperatureLookupMarlin.py
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@ -23,68 +23,60 @@ 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):
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
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 * (pow(l1,2) + pow(l2,2) + l1*l2)
a = y1 - (b + pow(l1,2)*c)*l1
self.c1 = a # Steinhart-Hart coefficients
self.c2 = b
self.c3 = c
self.rp = rp # pull-up resistance
def res(self,adc):
def res(self, adc):
"Convert ADC reading into a resolution"
res = self.temp(adc)-self.temp(adc+1)
return res
def v(self,adc):
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
return adc * VSTEP # convert the 10 bit ADC value to a voltage
def r(self,adc):
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
r = self.rp * self.v(adc) / (VCC - self.v(adc)) # resistance of thermistor
return r
def temp(self,adc):
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
r = self.rp * self.v(adc) / (VCC - self.v(adc)) # resistance of thermistor
lnr = log(r)
Tinv = self.c1 + (self.c2*lnr) + (self.c3*pow(lnr,3))
return (1/Tinv) - 273.15 # temperature
return (1/Tinv) - ZERO # temperature
def adc(self,temp):
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)
x = (self.c1 - (1.0 / (temp+ZERO))) / (2*self.c3)
y = sqrt(pow(self.c2 / (3*self.c3),3) + pow(x,2))
r = exp(pow(y-x,1.0/3) - pow(y+x,1.0/3)) # resistance of thermistor
return (r / (self.rp + r)) * ARES
def main(argv):
"Default values"