The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Month: May 2013

Thermocouple Ensemble Comparison
Although I don’t have a good way to put thermocouples in a known temperature environment (ie, yes, I can freeze and boil water, but I doubt the trustworthiness of any measurements made therein), I can compare the results from several different thermocouples held at the same (unknown) temperature: if they all agree to within a reasonable tolerance, I assume that they’re all reporting the correct temperature. Even better, if some of the temperatures come from different thermocouple amplifiers, then the electronics cross-check each other.

With that in mind, I attached a 4 Ω 25 W aluminum-body power resistor to the back of the same isothermal block I built for the Thing-O-Matic extruder tests, atop a dab of heatsink compound for good measure:

Isothermal block with 25 W power resistor

Then I tucked four sensors into the drilled holes:

Sensors in heated aluminum block

Clockwise from the front left corner:
- T1: Fluke meter, eBay TC (black sheath)
- T2: Fluke meter, Fluke TC (brown sheath)
- TM5: TC4+Arduino, eBay 100 kΩ thermistor (invisibly fine bare wire leads)
- TC1: TC4+Arduino, eBay TC (black sheath)
The TC4 Shield handles both thermocouple and thermistor sensors, so I added a thermistor from my collection just to see how it worked. The datasheet gives these parameters:
- 100 kΩ resistance at 25 °C
- beta = 3950 (from 25 °C to 50 °C)
- beta = 3990 (from 25 °C to 85 °C)
Unfortunately, there’s no way to include the eBay thermocouple epoxied to the nozzle in this test, but it’s from the same lot as the two in this test, so I assume it’ll produce the same results. The consistency in earlier tests suggests they’re all really Type K thermocouples and produce the same results as the Fluke thermocouple and meter that I assume produces accurate readings.

The closed-cell foam insulating the block from the vice jaws seemed like a good idea at the time.

I connected the resistor to the bench power supply, channeled the true spirit of DIY 3D printing (ie, ignored the power derating curves), and fired it up:

Multiple Sensor Calibration – vs time

The successive steps correspond to power levels of 0 W (the ambient temperature), 1 W, 2 W, 3W, 4W, and 6 W. The last point established that the foam melts at slightly over 100 °C, whereupon the test terminated. Eyeballometrically, the time constant of the resistor + block is on the order of 20 minutes, so these few points represent a rather tedious Basement Laboratory session.

Plotting the temperatures against the reading for T2, the Fluke thermocouple connected to the Fluke meter, and thinning the lines makes the results a bit more obvious:

Multiple Sensor Calibration – vs T2

The three thermocouples and Fluke meter / TC4 Shield pretty much overlay each other, with the thermistor reporting a somewhat higher temperature. Given that TM5 is an eBay thermistor, I’ll let you judge whether the beta value I got from its listing matches the beta of the actual thermistor.

In any event, I’d say the thermocouples report a temperature within at most a degree or two of the actual temperature. Plus, I didn’t get a steam burn in the process…
2013-05-17
Arduino Leonardo + TC4 Thermocouple Shield + TC4 Server

Because I want to measure several fairly high temperatures in and around the extruder with the LinuxCNC controller for the M2, I need a multi-channel thermocouple input. LinuxCNC’s hal_input interface module exposes the values produced by USB HID peripherals as HAL pins, which seems to be a nice way to add devices. A bit of searching revealed the TC4 Arduino Shield four-channel thermocouple + four-channel thermistor + assorted I/O board produced by the homeroasters.org folks as part of their DIY coffee roast controller.

The gotcha: ordinary Arduino boards cannot (without extraordinary effort) become USB HID peripherals, as their USB interface works only as a serial data device. The solution: the new-ish Arduino Leonardo has an on-chip USB interface that can act as a USB HID peripheral as well as the usual serial device.

Dan Newman, of Sailfish firmware fame, conjured up an a Arduino program (which, IMO, is far more complex than a mere sketch) that provides both a human-readable terminal interface and a LinuxCNC HAL-compatible USB HID interface using the Leonardo’s USB capabilities; the TC4Server project repository is on my GitHub account to keep it close to all the other stuff that should appear over the course of this project. His firmware builds on the libraries for the homeroaster’s tc4-shield Google Code project, but is intended for use with LinuxCNC, rather than as part of an Arduino controller.

It’s worth noting that the Leonardo has a mere 32 kB of program storage, so the extensive help documentation built into the program helped prevent feature creep.

Although I’m not yet using LinuxCNC with the M2, I can use TC4-Server’s serial terminal interface to read four channels of thermocouple data to help figure out what’s going on with the M2’s extruder thermistor. The TC4 shield has screw terminals for the thermocouples, but I also added a Proto ScrewShield board for thermistor resistors and easier connections:

TC4 on ProtoScrewShield on Leonardo

The TC4 Shield PCB layout assumes it’s being used with the original Arduino series of boards that bring the I²C (aka I2C, IIC, etc) SDA and SCL signals out on the A4 and A5 analog input pins, respectively. The newer Leonardo board brings SDA and SCL out in a separate header, so you must hotwire them across the board. The green and blue wires (stripped from a ribbon cable) accomplish that purpose: they’re plugged into the new Leonardo header through bent male header pins and clamped into the ScrewShield terminals. This assumes the Leonardo’s A4 and A5 pins remain as inputs, which is true for Dan’s firmware. If you actually need those pins for analog inputs, then you must remove the header pins that interconnect the boards and hotwire directly to the TC4 headers.

The TC4 shield includes an on-board temperature sensor that serves as the thermocouple cold-junction compensation reference. In my simple tests, the board has about 1 °C of self-heating, so I also use it to report the ambient temperature.

With all that in hand, I connected the thermocouple epoxied to the M2’s nozzle to the TC4 and re-ran the previous test with the modified thermistor table:

Rescaled extruder – thermocouple in TC4 board

The TC4 shield produces the same result as the Fluke meter with the same thermocouple, so we know those results will be consistent.

The modified thermistor table produces results that overlay thermocouple data. The questions come down to the accuracy of the thermocouple and whether bending the thermistor table actually represents an inherent property of the thermistor or just compensates for another problem.

Part of my motivation for using thermocouples, rather than thermistors, is that thermocouples avoid the whole dance of matching a given thermistor with a set of properties. Given the uncertain provenance of most thermistors, I have no reason to believe any of them match their alleged datasheets…

2013-05-16
Monthly Image: Turkey Vulture Visitation

We often see Turkey Vultures circling high overhead in thermals rising from, in these parts, sun-heated asphalt parking lots and roads, always on the alert for roadkill. A trio paused for a rest in the trees out front and I managed to get one mediocre portrait against an overcast sky:

Turkey Vulture in tree

They’re staggeringly ugly up close and awkward on the ground, but graceful in their natural element…

2013-05-15

Makergear M2: Thermistor Tables

The Marlin firmware used by the M2 has these thermistor selections in Configuration.h:

//// Temperature sensor settings:
// -2 is thermocouple with MAX6675 (only for sensor 0)
// -1 is thermocouple with AD595
// 0 is not used
// 1 is 100k thermistor
// 2 is 200k thermistor
// 3 is mendel-parts thermistor
// 4 is 10k thermistor !! do not use it for a hotend. It gives bad resolution at high temp. !!
// 5 is ParCan supplied 104GT-2 100K
// 6 is EPCOS 100k
// 7 is 100k Honeywell thermistor 135-104LAG-J01

#define TEMP_SENSOR_0 1
#define TEMP_SENSOR_1 1
#define TEMP_SENSOR_2 0
#define TEMP_SENSOR_BED 1

The first table in thermistortables.h looks like this:

#define OVERSAMPLENR 16

#if (THERMISTORHEATER_0 == 1) || (THERMISTORHEATER_1 == 1)  || (THERMISTORHEATER_2 == 1) || (THERMISTORBED == 1) //100k bed thermistor

const short temptable_1[][2] PROGMEM = {
{       23*OVERSAMPLENR ,       300     },
{       25*OVERSAMPLENR ,       295     },
{       27*OVERSAMPLENR ,       290     },
{       28*OVERSAMPLENR ,       285     },
{       31*OVERSAMPLENR ,       280     },
{       33*OVERSAMPLENR ,       275     },
{       35*OVERSAMPLENR ,       270     },
{       38*OVERSAMPLENR ,       265     },
{       41*OVERSAMPLENR ,       260     },
{       44*OVERSAMPLENR ,       255     },
{       48*OVERSAMPLENR ,       250     },
{       52*OVERSAMPLENR ,       245     },
{       56*OVERSAMPLENR ,       240     },
{       61*OVERSAMPLENR ,       235     },
{       66*OVERSAMPLENR ,       230     },
{       71*OVERSAMPLENR ,       225     },
{       78*OVERSAMPLENR ,       220     },
{       84*OVERSAMPLENR ,       215     },
{       92*OVERSAMPLENR ,       210     },
{       100*OVERSAMPLENR        ,       205     },
{       109*OVERSAMPLENR        ,       200     },
{       120*OVERSAMPLENR        ,       195     },
{       131*OVERSAMPLENR        ,       190     },
{       143*OVERSAMPLENR        ,       185     },
{       156*OVERSAMPLENR        ,       180     },
{       171*OVERSAMPLENR        ,       175     },
{       187*OVERSAMPLENR        ,       170     },
{       205*OVERSAMPLENR        ,       165     },
{       224*OVERSAMPLENR        ,       160     },
{       245*OVERSAMPLENR        ,       155     },
{       268*OVERSAMPLENR        ,       150     },
{       293*OVERSAMPLENR        ,       145     },
{       320*OVERSAMPLENR        ,       140     },
{       348*OVERSAMPLENR        ,       135     },
{       379*OVERSAMPLENR        ,       130     },
{       411*OVERSAMPLENR        ,       125     },
{       445*OVERSAMPLENR        ,       120     },
{       480*OVERSAMPLENR        ,       115     },
{       516*OVERSAMPLENR        ,       110     },
{       553*OVERSAMPLENR        ,       105     },
{       591*OVERSAMPLENR        ,       100     },
{       628*OVERSAMPLENR        ,       95      },
{       665*OVERSAMPLENR        ,       90      },
{       702*OVERSAMPLENR        ,       85      },
{       737*OVERSAMPLENR        ,       80      },
{       770*OVERSAMPLENR        ,       75      },
{       801*OVERSAMPLENR        ,       70      },
{       830*OVERSAMPLENR        ,       65      },
{       857*OVERSAMPLENR        ,       60      },
{       881*OVERSAMPLENR        ,       55      },
{       903*OVERSAMPLENR        ,       50      },
{       922*OVERSAMPLENR        ,       45      },
{       939*OVERSAMPLENR        ,       40      },
{       954*OVERSAMPLENR        ,       35      },
{       966*OVERSAMPLENR        ,       30      },
{       977*OVERSAMPLENR        ,       25      },
{       985*OVERSAMPLENR        ,       20      },
{       993*OVERSAMPLENR        ,       15      },
{       999*OVERSAMPLENR        ,       10      },
{       1004*OVERSAMPLENR       ,       5       },
{       1008*OVERSAMPLENR       ,       0       } //safety
};
#endif

The OVERSAMPLENR constant determines the number of successive ADC samples added together into a single value, which is then used to search the table for the corresponding entry. The table entries are pairs of:
{nominal ADC value * number of samples, temperature in C}
which means that if we know the temperature, we can work backwards to find the ADC value and then compute the actual thermistor resistance.

However, before doing that, I created a modified version of the thermistor table that simply scales the temperatures down by 0.878:

#if (THERMISTORHEATER_0 == 8) || (THERMISTORHEATER_1 == 8) || (THERMISTORHEATER_2 == 8) || (THERMISTORBED == 8) // M2 thermistors on RAMBO
const short temptable_8[][2] PROGMEM = {
	{23*OVERSAMPLENR, 263.51},
	{25*OVERSAMPLENR, 259.12},
	{27*OVERSAMPLENR, 254.73},
	{28*OVERSAMPLENR, 250.34},
	{31*OVERSAMPLENR, 245.94},
	{33*OVERSAMPLENR, 241.55},
	{35*OVERSAMPLENR, 237.16},
	{38*OVERSAMPLENR, 232.77},
	{41*OVERSAMPLENR, 228.38},
	{44*OVERSAMPLENR, 223.98},
	{48*OVERSAMPLENR, 219.59},
	{52*OVERSAMPLENR, 215.2},
	{56*OVERSAMPLENR, 210.81},
	{61*OVERSAMPLENR, 206.42},
	{66*OVERSAMPLENR, 202.03},
	{71*OVERSAMPLENR, 197.63},
	{78*OVERSAMPLENR, 193.24},
	{84*OVERSAMPLENR, 188.85},
	{92*OVERSAMPLENR, 184.46},
	{100*OVERSAMPLENR, 180.07},
	{109*OVERSAMPLENR, 175.67},
	{120*OVERSAMPLENR, 171.28},
	{131*OVERSAMPLENR, 166.89},
	{143*OVERSAMPLENR, 162.5},
	{156*OVERSAMPLENR, 158.11},
	{171*OVERSAMPLENR, 153.71},
	{187*OVERSAMPLENR, 149.32},
	{205*OVERSAMPLENR, 144.93},
	{224*OVERSAMPLENR, 140.54},
	{245*OVERSAMPLENR, 136.15},
	{268*OVERSAMPLENR, 131.76},
	{293*OVERSAMPLENR, 127.36},
	{320*OVERSAMPLENR, 122.97},
	{348*OVERSAMPLENR, 118.58},
	{379*OVERSAMPLENR, 114.19},
	{411*OVERSAMPLENR, 109.8},
	{445*OVERSAMPLENR, 105.4},
	{480*OVERSAMPLENR, 101.01},
	{516*OVERSAMPLENR, 96.62},
	{553*OVERSAMPLENR, 92.23},
	{591*OVERSAMPLENR, 87.84},
	{628*OVERSAMPLENR, 83.45},
	{665*OVERSAMPLENR, 79.05},
	{702*OVERSAMPLENR, 74.66},
	{737*OVERSAMPLENR, 70.27},
	{770*OVERSAMPLENR, 65.88},
	{801*OVERSAMPLENR, 61.49},
	{830*OVERSAMPLENR, 57.09},
	{857*OVERSAMPLENR, 52.7},
	{881*OVERSAMPLENR, 48.31},
	{903*OVERSAMPLENR, 43.92},
	{922*OVERSAMPLENR, 39.53},
	{939*OVERSAMPLENR, 35.13},
	{954*OVERSAMPLENR, 30.74},
	{966*OVERSAMPLENR, 26.35},
	{977*OVERSAMPLENR, 21.96},
	{985*OVERSAMPLENR, 17.57},
	{993*OVERSAMPLENR, 13.18},
	{999*OVERSAMPLENR, 8.78},
	{1004*OVERSAMPLENR, 4.39},
	{1008*OVERSAMPLENR, 0}
};
#endif

I inserted that table, changed the thermistor selection, reloaded the firmware, and ran the same test as before, which produced this result:

The stock thermistor and the thermocouple now report essentially the same values, which is entirely due to the new table. The two additional lines come from two more thermocouples taped to the nozzle and dangling downward toward the platform:

M2 - Hot end with additional thermocouples — M2 – Hot end with additional thermocouples

Given that I simply taped those thermistors in place, they don’t contact the nozzle nearly as well as the epoxied sensors. The fact that one reads a bit higher and the other much lower could be explained by handwaving, but one possibility is that the various thermocouples don’t quite agree with each other.

Time for some calibration along those lines, methinks…

2013-05-14

Makergear M2: Thermistor vs. Thermocouple
With the stock thermistor and my added thermocouple epoxied to the M2’s nozzle, I stepped the temperature upward, let it settle, and recorded the temperature from the Pronterface status display and my Fluke thermocouple meter:

First Heat – M2 thermistor – Fluke with thermocouple

Because the firmware servos the temperature through the stock thermistor, that line is dead straight at the exact setpoint values: the reference never disagrees with itself. The thermocouple, however, reads low by about 12%: according to it, the nozzle runs much cooler than the thermistor value.

Huh?

Several explanations come to mind:
- The firmware is using a lookup table that doesn’t match the thermistor
- The Fluke thermocouple meter reports the wrong value
- The thermocouple junction is defective
- Despite the epoxy blob, the two sensors aren’t at the same temperature
- Something else is kaflooie
This obviously calls for more data…
2013-05-13
Zinc-air Cell Corrosion

For reasons that, alas, have little to do with normal age-related hearing degeneration, I’ve been wearing Etymōtic (that should be a long o symbol) MP-915 High-definition Electronic Earplugs (aka, Martian Ear Beetles) when I need a 6 dB boost for normal conversations. The key advantage: a price 10 dB under full-throttle hearing aids.

Anyhow, each one runs for about two weeks on a 312 zinc-air primary cell, so I picked up a batch from the usual eBay vendor. These were short-dated, which let me figure out how long after the rated shelf life they’d be good for, so I know what’s the largest batch I should buy.

One cell arrived with its air vent seal dislodged:

Corroded zinc-air cell

The cell in the middle is used, with several scratches from the contact point inside the earplug. The cell on the right has a good seal.

Assuming a good seal, the cells seem to work about as well as the long-dated fresh cells included with the earplugs.

Although I found several datasheets (Duracell Energizer Rayovac), there seems to be no way to relate the actual cell you purchase to any particular datasheet; the part numbers do not correspond to anything on the package and the nomenclature varies wildly both between manufacturers and within product lines.

2013-05-12
Bio-Geography Quiz

Found this deep inside a college fieldhouse not too long ago:

North Pole Penguin

Given the slight emphasis drawn into North, the author may be poking fun at the Track & Field team, but …

2013-05-11

Month: May 2013

Thermocouple Ensemble Comparison

Arduino Leonardo + TC4 Thermocouple Shield + TC4 Server

Monthly Image: Turkey Vulture Visitation

Makergear M2: Thermistor Tables

Makergear M2: Thermistor vs. Thermocouple

Zinc-air Cell Corrosion

Bio-Geography Quiz