3D printer thermo controller for extruder

ManAtWork · 2016-02-11 12:16

Hi,

I'm looking for a simple way to control the temperature of an MK8 extruder. It has an 100k NTC thermistor and a heating element for 24V. It should be no big deal to measure the temperature with an RC decay circuit and switch the heating resistor with a MOSFET and a bit of PID software running in a propeller, or even a simple 2-point on/off controller. Given enough time I could develop one myself. But I'm sure somebody has already done this before. Any hints or pointers welcome.

idbruce · 2016-02-11 21:14

ManAtWork

This is not for an RC decay circuit, but you may change your mind.

Here is some C snippets, if you want the full files, just let me know.

// The number of value pairs in the table.
#define TABLE_SIZE 53
//
// This is a table of ADC readings and corresponding temperature values for both the
// extruder and heated build platform.  In determining the thermistor resistance values,
// data was extracted from the EPCOS B57560G1104F000 datasheet, because that is the
// thermistor being utilized for both the extruder and heated build platform.
//
// During the calculations of this table, a resistor having a nominal value of 4.7K,
// with a 10% tolerance was utilized, however the calculations were performed and based
// upon the actual resistance, which was 4,602 Ohms.
LOOKUPTEMP lookup_temp[TABLE_SIZE] = {
	{.sample = 1591, .temp = 20},
	{.sample = 1577, .temp = 25},
	{.sample = 1560, .temp = 30},
	{.sample = 1540, .temp = 35},
	{.sample = 1517, .temp = 40},
	{.sample = 1490, .temp = 45},
	{.sample = 1459, .temp = 50},
	{.sample = 1424, .temp = 55},
	{.sample = 1386, .temp = 60},
	{.sample = 1343, .temp = 65},
	{.sample = 1296, .temp = 70},
	{.sample = 1245, .temp = 75},
	{.sample = 1191, .temp = 80},
	{.sample = 1135, .temp = 85},
	{.sample = 1076, .temp = 90},
	{.sample = 1016, .temp = 95},
	{.sample = 956, .temp = 100},
	{.sample = 895, .temp = 105},
	{.sample = 835, .temp = 110},
	{.sample = 776, .temp = 115},
	{.sample = 719, .temp = 120},
	{.sample = 665, .temp = 125},
	{.sample = 612, .temp = 130},
	{.sample = 563, .temp = 135},
	{.sample = 517, .temp = 140},
	{.sample = 474, .temp = 145},
	{.sample = 434, .temp = 150},
	{.sample = 397, .temp = 155},
	{.sample = 362, .temp = 160},
	{.sample = 331, .temp = 165},
	{.sample = 302, .temp = 170},
	{.sample = 276, .temp = 175},
	{.sample = 252, .temp = 180},
	{.sample = 231, .temp = 185},
	{.sample = 211, .temp = 190},
	{.sample = 193, .temp = 195},
	{.sample = 177, .temp = 200},
	{.sample = 162, .temp = 205},
	{.sample = 149, .temp = 210},
	{.sample = 136, .temp = 215},
	{.sample = 125, .temp = 220},
	{.sample = 115, .temp = 225},
	{.sample = 106, .temp = 230},
	{.sample = 98, .temp = 235},
	{.sample = 90, .temp = 240},
	{.sample = 83, .temp = 245},
	{.sample = 77, .temp = 250},
	{.sample = 71, .temp = 255},
	{.sample = 66, .temp = 260},
	{.sample = 61, .temp = 265},
	{.sample = 57, .temp = 270},
	{.sample = 53, .temp = 275},
	{.sample = 49, .temp = 280},
};

double extruder_celsius;
double bed_celsius;

/**************************************************************************/
/*!
    @brief  Given the appropriate channel, update the proper external
    temperature variables.
*/
/**************************************************************************/
void UpdateTemp(uint8_t channel)
{
     double samples[NUMSAMPLES];
	uint8_t i;
	double average; 

	// take N samples in a row, with a slight delay
	for (i = 0; i < NUMSAMPLES; i++)
	{
          // read channel the specified channel
		samples[i] = readADC_SingleEnded(channel);
          pause(10);
	}

	// average all the samples out
	average = 0;

	for(i = 0; i < NUMSAMPLES; i++)
	{
		average += samples[i];
	}

	average /= NUMSAMPLES;   

	if(channel == 0)
	{
     	extruder_celsius = lookupTemp(lookup_temp, average);
	}
	if(channel == 1)
	{
     	bed_celsius = lookupTemp(lookup_temp, average);
	}
}

/**************************************************************************/
/*!
    @brief  Given an ADC sample average, lookup the appropriate temperature.
*/
/**************************************************************************/
double lookupTemp(const LOOKUPTEMP *table, double sample)
{
	int index;  

	index = 0;

	// find the two points in the table to use
	while(index < TABLE_SIZE && sample < table[index].sample)
     {
		index++;
	}

	// make sure the point isn't past the end of the table
	if( index == TABLE_SIZE)
	{
		return table[index - 1].temp;
	}

	// make sure the point isn't before the beginning of the table 
	if(index == 0)
	{
		return table[index].temp;
	}

	return table[index].temp;
}

ManAtWork · 2016-03-08 08:04

Hello Bruce,

thanks for the code but I finally wrote my own because I'm not using C, I use a different sensor and I have no real ADC on my board. Because of the exponential R vs. temperature curve it's still quite accurate with only the RC decay circuit. I use a PD controller because a simple on/off or P controller results in enormous overshot due to the delay in the thermal path. PD gives a much better response and PID is not necessary becausse nominal/actual deviation is less than 1°C.

{{ ExtruderHeater.spin temperature controller for 3D-printer extruder
uses an RC decay timer circuit (single slope EDC) and a 100k NTC resistor for
temperature measurement

33nF capacitor to ground
NTC + 100R to VDD

}}
CON
  _clkmode = xtal1 + pll16x 'Standard clock mode * crystal frequency = 80 MHz
  _xinfreq = 5_000_000
  clkfrq   = 80_000_000
  baudRate = 115_000

  pinRC  = 0    ' PA0 I/O RC decay circuit
  pinOut = 1    ' PA1 Out Smart FET for heating

  modeA = %01100<<26 + pinRC ' counter mode: NEG detect
  t_1ms = clkfrq / 1000
  t_10ms = clkfrq / 100

  Tnom = 190    ' nominal temperature in °C
  kp   =   5    ' proportional gain in %/°C
  kd   =  20    ' differential gain in %/(°C/s)
  
VAR
  long  buffer[10]
  long  t   ' temperature in 0.1°C steps
  long  dt  ' temperature rise/fall in 0.1°C/s
  long  bi  ' buffer index
  long  pn  ' nominal power in %
  long  pa  ' actual power
   
OBJ
  'com: "FullDuplexSerial"
  
PUB main | r, lt, c
  'com.Start(31, 30, 0, baudRate)
  DIRA[pinOut]:= 1
  c:= CNT
  repeat
    r:= MeasureR
    t:= RtoTemp (r)
    lt:= buffer[bi]
    buffer[bi++]:= t
    bi//= 10
    dt:= t - lt
    pn:= ((Tnom*10 - t) * kp - dt * kd) / 10
    pn#>= 0
    pn<#= 100
    pa+= pn
    if pa => 100
      pa-= 100
      OUTA[pinOut]:= 1
    else
      OUTA[pinOut]:= 0
    {com.str (String ("r="))
    com.dec (r)
    com.str (String ("  t="))
    com.dec (t/10)
    com.tx (",")
    com.dec (t//10)
    com.str (String ("  d="))
    com.dec (dt*kd)
    com.tx (13)
    }
    waitcnt (c+= clkfrq / 10)
     
PRI MeasureR : R
' measure NTC resistor value
  DIRA[pinRC]:= 1
  FRQA:= 1
  PHSA:= 0
  waitcnt (t_1ms + CNT)
  CTRA:= modeA
  DIRA[pinRC]:= 0
  waitcnt (t_10ms + CNT) 
  CTRA:= 0
  R:= (PHSA-563) * 100 / 142 ' gain and offset compensation
   
PRI RtoTemp (R): Temp | i, l, h
' calculate temperature from resistance value
' unit is 0.1°C
  h:= tableR[0]
  if R > h
    return -999 ' underflow
  repeat i from 1 to 42
    l:= tableR[i] 
    if R > l
      return i*50 - (R-l)*50 / (h-l)
    h:= l 
  return +999 ' overflow
   
DAT
tableR  long  325600, 253400, 198700, 157000, 124900, 100000, 80590, 65350, 53300, 43720 ' 0..45°C
        long  36060, 29890, 24900, 20840, 17350, 14810, 12560, 10700, 9154, 7860 ' 50..95°C
        long  6773, 5858, 5083, 4426, 3866, 3387, 2977, 2624, 2319, 2055, 1826   ' 100..150°C
        long  1623, 1442, 1281, 1138, 1011, 899, 799, 710, 630, 560, 498, 442    ' 155..210°C

The R vs. temp table is just a rough guess. I extrapolated it from a datasheet of a different 10k NTC I had. If somebody has the exact values please let me know.

3D printer thermo controller for extruder

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