Temperature drop

My extruder controller is working much better after I cured the motor noise problem. The 10 bit SAR ADC also seems to work better than the 16 bit sigma-delta version did. With the 16 bit one there was a lot of noise on the readings, even when the motor wasn't running. I had to average over many samples to get a consistent reading which delayed the response. With the 10 bit ADC I just read it and compare it with the temperature set point value to decide if the heater should be on or off. That gives a temperature swing of about ± 3°C with the heater going on and off every four or five seconds.

The temperature is calculated from the ADC reading and vice versa by the PC with the following Python class :-

from math import *

class Thermistor:
 "Class to do the thermistor maths"
 def __init__(self, r0, t0, beta, r1, r2):
     self.r0 = r0                        # stated resistance
     self.t0 = t0 + 273.15               # temperature at stated resistance, e.g. 25C
     self.beta = beta                    # stated beta
     self.vref = 1.5 * 1.357 / 1.345     # ADC reference, corrected
     self.vcc = 3.3                      # supply voltage to potential divider
     self.vs = r1 * self.vcc / (r1 + r2) # effective bias voltage
     self.rs = r1 * r2 / (r1 + r2)       # effective bias impedance
     self.k = r0 * exp(-beta / self.t0)  # constant part of calculation

 def temp(self,adc):
     "Convert ADC reading into a temperature in Celsius"
     v = adc * self.vref / 1024          # convert the ADC value to a voltage
     r = self.rs * v / (self.vs - v)     # resistance of thermistor
     return (self.beta / log(r / self.k)) - 273.15        # temperature

 def setting(self, t):
     "Convert a temperature into a ADC value"
     r = self.r0 * exp(self.beta * (1 / (t + 273.15) - 1 / self.t0)) # resistance of the thermistor
     v = self.vs * r / (self.rs + r)       # the voltage at the potential divider
     return round(v / self.vref * 1024)    # the ADC reading

It is instantiated as follows :-

thermistor = Thermistor(10380, 21, 3450, 1790, 2187)

10380 is the resistance of the thermistor measured by my multimeter at a room temperature of 21°C. 3450 is the beta of the thermistor taken from the data sheet. The last two values are the two resistors forming a potential divider with the thermistor wired across the second one, again the values are measured with a multimeter. The fudge factor of 1.357 / 1.345 corrects the MSP430 internal reference voltage so that it agrees with the multimeter.

The result seems to track the temperature measured by a thermocouple to within about 5°C, good enough for me. Just as I had finished checking it, I knocked the thermistor leads with my thermocouple and it fell off. It was stuck to the brass nozzle with JB Weld but I forgot to roughen the surface first. I am now waiting 16 hours for it to set again.

The extruder controller firmware is only about 400 lines of C. As well as temperature control it also controls the DC motor precisely using the shaft encoder and handles the I²C protocol.

I have now completed all the mechanical parts, the electronics and the firmware. I just need to get the RepRap host code to talk to my non standard hardware to complete the machine.