Unstable thermistor readings/values from RAMBo 1.3

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TheIrishScion
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Unstable thermistor readings/values from RAMBo 1.3

Post by TheIrishScion »

Just finished my building my mid-January 2016 Rostock Max V2 (latest version, I believe, with the fancy new PSU and the new arms) and there's _something_ not quite right about my thermistor behavior.

I'm getting jumpy readings from both nozzle and bed thermistors, both when they're up at operating temperature as well as down around room temperature. They both appear to exhibit much the same amount of 'jumpiness' and have consistent _averaged_ temperature readings when they're both at room temperature, but they bounce around independently of each other (one can be reading a degree high, one a degree low at the same time. They have been up to 2 or 3 degrees away from actual temperature as well, though currently for whatever reason they're not behaving like that)

Part of the upshot of this is that I cannot get repeatable calibration runs. Below are 4 full runs on my first attempt.

Code: Select all

Run   Kp   Ki   Kd
1   29.77   7.94   37.13
1   20.48   1.71   61.27
1   29.1    7.76   27.29
2   27.14   2.9    63.53
2   37.37   9.96   35.04
2   31.92   3.55   71.73
3   30.25   8.01   28.58
3   15.93   1.07   59.08
3   25.78   6.87   24.18
4   21.37   1.92   59.51
4   26.94   7.16   25.34
4   14.19   0.81   62.44

The resultant average values were nonsense. Some of the calibration runs even timed out for some reason, and never completed. When I tried to use them, even averaging out all 12 entries from the 4 complete runs above, the nozzle temp swung wildly (+/- 8 degrees indicated, 220-236* with a 228 target)

What appeared to be happening was that the temperature reading was so unstable, the reported temperature effectively jumped above 200, then back below, then back above, then below, then above, etc all in a matter of a second or two, and not in a true response to the heater, thereby greatly confusing the calibration routine which was trying to cross the 200 degree threshold 8 times in a nice controlled fashion. In the end I averaged the numbers from Gene's example screenshot in the instructions and used that, which worked well enough (+/- ~3 degrees).

This jumpy temperature reading is also evident on cooldown where my LED ring spends 10 seconds alternately turning on and off depending on whether the thermistor is reading above 50C at the moment.

Cut to Monday morning, after a night of being turned off, I turn the machine on and both bed and extruder temperatures are dead stable, varying +/- 0.1 degrees at room temperature. 'Whoopee!' sez I, 'it's fixed itself!' (though I was deeply suspicious, and with good reason) and I set about running the calibration procedure again. This time it worked flawlessly, here's two runs worth of data from that;

Code: Select all

Run   Kp   Ki   Kd
1   14.78   0.88   62.2
1   15.59   0.94   64.31
1   15.17   0.92   62.6
2   15.31   0.93   63
2   15.59   0.95   64.1
2   15.59   0.95   64.03

I ran it a third time but the numbers were near as dammit identical so I didn't bother recording them.

I averaged and plugged those numbers in and got printing. Initially it seemed to be flawless, maintaining target temperature to within small fractions of a degree. But after a few minutes of printing it seemed to lose accuracy again and the swings started to creep back in (though again, limited to a couple of degrees this time).

Since I wasn't having serious print quality issues, I kept going until Monday night when I re-ran the calibration routine a couple of times to see how it would behave, and it'd gone back to giving the same nonsense results I was getting initially. The ambient temperature swing has returned as well, though not as pronounced, a little over a degree total variance at room temp, though _occasionally_ 2 degrees. See video;
https://www.youtube.com/watch?v=cTNuTY4c41k

So, here's what I think I know;
  • Very quick back-of-a-fag-packet math (having looked at the simple voltage divider circuit for the thermistors in the RAMBo manual) suggests that the RAMBo board itself should be capable of measuring resistance to within at least 50-100 ohms consistently (not necessarily accurate, but consistent reading to reading) which should I think be roughly .1*C at room temp, but I'm seeing what I think amounts to a 500 ohm+ variance based on the reported room temperature fluctuation. That combined with the brief period where it seemed to work correctly (and calibrate properly) leads me to believe There Is Actually Something Wrong. Can someone verify that I should be seeing more consistency in temperature readings?
  • The fact that both long lead and short lead thermistors are misbehaving in the same way suggests that wiring/shielding/crosstalk in the wiring itself is not the issue. Nor should a simple voltage divider/ADC resistor measurement be susceptible to much crosstalk, especially when you consider that the wiring is running in the same extrusion as the high current wires to the resistor, which in turn are turned off when we're measuring at ambient.
  • It seems like dirty power to the voltage divider driving the thermistors themselves _could_ be causing a problem, though since they're running from VCC, I presume that means they're receiving nice clean regulated power from the on-board low voltage logic PSU. I haven't probed anything to confirm that, and without an o-scope I can't really tell much about power quality anyway.
  • If the PSU is making a _bunch_ of noise, that could be getting coupled into the thermistor lines from the rest of the board, but surely if there was something _that_ noisy going on, other systems would be impacted no? I've been searching and I've found very few reports of this problem, with the notable recent exception of this thread; viewtopic.php?f=37&t=9843&p=85416
  • If the above solution (twisting, ferrite-ing and shielding the snot out of everything you can get your hands on) _is_ the solution, then there's a bigger electrical problem on the board.

I'm likely missing something obvious, but I don't want to go borrow an o-scope and burn a bunch of time trying to diagnose what's going wrong if the answer is obvious to one of you guys. This is not a board, nor a platform I'm familiar with.

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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by Polygonhell »

Frankly IME the autotune in repetier might as well be a random number generator.
I usually run it then fiddle with the values manually to dial them in, once dialed in, I never touch them unless there is an issue.
My hotend holds +/- 0.2 degrees at every temperature between 160 and 260 degrees, but if your within a couple of degrees, I wouldn't worry about it too much.

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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by TheIrishScion »

Polygonhell wrote:Frankly IME the autotune in repetier might as well be a random number generator.
I usually run it then fiddle with the values manually to dial them in, once dialed in, I never touch them unless there is an issue.
My hotend holds +/- 0.2 degrees at every temperature between 160 and 260 degrees, but if your within a couple of degrees, I wouldn't worry about it too much.


Right, printing didn't seem adversely affected, I printed my PEEK fan successfully with an 8 degree swing either side of 128C. However, the fact that sometimes it worked perfectly and sometimes not at all makes me very suspicious.

When you're printer is at idle and room temperature, how much do your displayed temperatures move around?

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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by Polygonhell »

TheIrishScion wrote:
Polygonhell wrote:Frankly IME the autotune in repetier might as well be a random number generator.
I usually run it then fiddle with the values manually to dial them in, once dialed in, I never touch them unless there is an issue.
My hotend holds +/- 0.2 degrees at every temperature between 160 and 260 degrees, but if your within a couple of degrees, I wouldn't worry about it too much.


Right, printing didn't seem adversely affected, I printed my PEEK fan successfully with an 8 degree swing either side of 128C. However, the fact that sometimes it worked perfectly and sometimes not at all makes me very suspicious.

When you're printer is at idle and room temperature, how much do your displayed temperatures move around?


it ticks up and down by 0.1, maybe a bit more, it's pretty consistent.

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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by TheIrishScion »

Polygonhell wrote:
TheIrishScion wrote:
Polygonhell wrote:Frankly IME the autotune in repetier might as well be a random number generator.
I usually run it then fiddle with the values manually to dial them in, once dialed in, I never touch them unless there is an issue.
My hotend holds +/- 0.2 degrees at every temperature between 160 and 260 degrees, but if your within a couple of degrees, I wouldn't worry about it too much.


Right, printing didn't seem adversely affected, I printed my PEEK fan successfully with an 8 degree swing either side of 128C. However, the fact that sometimes it worked perfectly and sometimes not at all makes me very suspicious.

When you're printer is at idle and room temperature, how much do your displayed temperatures move around?


it ticks up and down by 0.1, maybe a bit more, it's pretty consistent.

Right, thanks. There's _definitely_ something wrong with mine then. I'm an order of magnitude more than that.

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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by Captain Starfish »

Given that it's both thermistors and it's at room temperature on an idle machine, it kinda rules out interference from motors, heaters and so on.

I'd be pulling out the power and thermistor connectors and having a really close look at them, particularly how well the ground is connected.

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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by TheIrishScion »

Alright, so how's this for system isolation;

RAMBo sitting naked on the Healing Bench, connected solely to the display, the two thermistors, and some random 2A 12V DC wall wart connected to the B power input (Logic, Heaters and Fans), with the other two power inputs left unconnected. Behavior is interesting. When power cycling it, sometimes it comes up reading accurately. Sometimes it comes up jumping around. I can't detect a pattern. Hitting the Reset button on the board doesn't seem to count as a power cycle from the perspective of flipping between normal and jumpy temperature readings. Only actually cutting and returning power to the board does the trick. I _thought_ I'd experienced it switch between normal and jumpy after a period of being turned on while I was actually calibrating and printing yesterday, but I can't replicate that any more if I did. It seems (for 5-10 minute run-times at least) that whatever state it boots in is where it's set for that power cycle.

I suppose I could try removing the display and monitoring over USB, conceivably it could be something in the display logic upsetting things on the main board? However it's late, I'm tired and I'd quite like some official input from SeeMeCNC on what they'd like me to do next. This _could_ be a software bug (some bit of internal calibration not occurring correctly or within a time window or something) but I'm running the latest 091S build (as instructed in Gene's current Tome of Learning) so whatever software I'm dealing with should be much the same as everybody else with a recent kit.

For what it's worth, when it boots in stable mode, readings are _very_ stable indeed (essentially no movement at all), and it's accurate to within a degree of my Fluke non-contact. Reads about .8C high, for the record. Impressive for a thermistor based setup. :-)

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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by Xenocrates »

It seems a fascinating bug. But SeeMeCNC seems like a less than perfect place to ask about truly strange Rambo behavior. Ultimachine may be able to tell you what's wrong (They design and manufacture the RAMBO), and then once you know the problem, if it's board related you can A) Ask SeemeCNC for replacement/repair, or B) send it directly to Ultimachine for repair (They will charge, but a reasonable fee). Both companies will do right by you, but I feel that for really weird issues with the Rambo, the people with the better knowledge base win as far as initial help.
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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by TheIrishScion »

Agreed, I don't necessarily expect SeeMe to fix the bug (if indeed it is a bug, I should probably delve into the source), but it's a brand new board in a brand new printer so I need to at least make them _aware_ of what's going on, as they have warranty responsibility for board as it currently stands. I don't know how their purchasing relationship with Ultimachine is set up, are they assuming all warranty costs, or are they acting as middle-men for the vendor's own process? I'm assuming that not _all_ that many RAMBos fail in the wild.

If it _is_ a software problem, then getting ahead of that may save them some time and expense as well. If it's just a one-off marginal board, then swapping it out is no big deal.

So, I'd be happy work w/ Ultimachine support if that's the best course, I just don't really have a relationship with them yet and would like an introduction :-)

There's also a component of 'dammit, my shiny new printer is strewn across the bench and not doing any printing!' though I'm fine with that so long as it's to a constructive end.

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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by TheIrishScion »

Right, brief update; SeeMeCNC replaced my RAMBo without any hassle (like the rockstars they are) and you'd think that'd be the end of that right?

Nope. It actually looks now like the one thing I didn't pull off the board during testing may be the culprit; the display/encoder/SD card reader module! When I run the machine headless, temperatures are solid as a rock (via mattercontrol) but when I run it with the display they mostly jump around all over the place.

So I think I'm going to lay hands on a scope and see if I can't see what in the wide world of sports is a goin' on. I'll report back once I know more.

Now, if anyone out there has an old version of the board (white, I imagine, based on the instructions?) lying around that they'd like to throw in a padded baggie and send to me for the sake of comparison, I should be eternally grateful. It's a challenge now, I'm going to get to the bottom of this.

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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by geneb »

I wonder if the leads are picking up noise. If you've got one handy, try looping the ribbon cables through a ferrite core and see if that solves the issue.

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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by TheIrishScion »

I do (a box of 'em), and I will, though not until I've got a scope hooked up. I intend to document the fix with some rigor, if indeed one is available.

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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by Armatus »

I'm having nearly an identical problem with my Rostock, the temperatures are pretty stable when everything is powered down but can fluctuate as much as +/- 8 degrees when running, regardless of how many times I tune it. My SD card reader is also giving me all sorts of problems, I'll be watching this thread closely to see what you come up with.

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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by TheIrishScion »

Armatus wrote:I'm having nearly an identical problem with my Rostock, the temperatures are pretty stable when everything is powered down but can fluctuate as much as +/- 8 degrees when running, regardless of how many times I tune it. My SD card reader is also giving me all sorts of problems, I'll be watching this thread closely to see what you come up with.

Can you post up the output values from a couple of runs of the autotune procedure? I'd like to see if you're seeing equivalent anomalies.

Also, you might consider running the calibration with the display unplugged (unplug the little daughter board with the wires coming from it from the RAMBo, not just the wires from the daughter board, I suggest, that way you don't have to keep track of which way they plug back in :-) )

If it's the same problem, you'll suddenly start getting dramatically more consistent and logical PID values.

Once you've plugged PID values in, you can plug the display back in, and while it won't hold _as_ good a temperature with the display in, it'll be _dramatically_ better than it is with crazy PID values.

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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by Armatus »

I did actually play with PID tuning with the display on and off this weekend, the numbers it generated with the display connected could be as much as +/- 20 for the D value, +/- 5 for P and +/- 2 for I. Once I unplugged it, all the readings were much more consistent, with the biggest range at D being +/- 1.5. I didn't save the exact readings but I can run a few tunes again when I get home from campus this weekend and post them just for comparison. I'm also using a new hexagon hotend rather then the standard one the Rostock ships with, which I normally run between 255-260* to print PETG. With the lcd connected, often it wouldn't finish tuning cycles at all, and I could never get it to tune above 240*, but it tuned great the first time without the LCD connected at 255*.

My temperatures are now much more steady, but they still aren't consistent. If I set my printing temperature to 255*, it normally hovers around 250* +/- 2.5* once the fans are on and it's printing at a good speed (80mm/s). Even if I ramp up my set temperature, it doesn't seem to want to exceed 250* while printing at speed. At slower speeds it's better, but limiting my speed to keep my hotend hot is pretty annoying.

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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by Eaglezsoar »

Armatus wrote:I did actually play with PID tuning with the display on and off this weekend, the numbers it generated with the display connected could be as much as +/- 20 for the D value, +/- 5 for P and +/- 2 for I. Once I unplugged it, all the readings were much more consistent, with the biggest range at D being +/- 1.5. I didn't save the exact readings but I can run a few tunes again when I get home from campus this weekend and post them just for comparison. I'm also using a new hexagon hotend rather then the standard one the Rostock ships with, which I normally run between 255-260* to print PETG. With the lcd connected, often it wouldn't finish tuning cycles at all, and I could never get it to tune above 240*, but it tuned great the first time without the LCD connected at 255*.

My temperatures are now much more steady, but they still aren't consistent. If I set my printing temperature to 255*, it normally hovers around 250* +/- 2.5* once the fans are on and it's printing at a good speed (80mm/s). Even if I ramp up my set temperature, it doesn't seem to want to exceed 250* while printing at speed. At slower speeds it's better, but limiting my speed to keep my hotend hot is pretty annoying.


What is your #define MAXTEMP set to in your configuration.h ?
By default it's set to 240, but with the e3d you can up it to 300 safely.

Also I wrapped my heat block on the hotend with self-fusing silicone tape which insulates it from breezes and air from the layer fan and it did make a difference.
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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by Armatus »

I have my max temp set to 300* and the heater block wrapped in Kapton which did stabilize the readings a bit, but they're still inconsistent. Mind linking to the tape you're using?

I do also have a support ticket open and I've linked them to this thread, they're sending me a new LCD screen and adapter board to test, I'll update this thread when I hear more from them/make any progress on my printer.

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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by Eaglezsoar »

Armatus wrote:I have my max temp set to 300* and the heater block wrapped in Kapton which did stabilize the readings a bit, but they're still inconsistent. Mind linking to the tape you're using?

I do also have a support ticket open and I've linked them to this thread, they're sending me a new LCD screen and adapter board to test, I'll update this thread when I hear more from them/make any progress on my printer.


Please use this link as one source of the silicone tape - http://www.amazon.com/s/ref=nb_sb_noss_ ... icone+tape
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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by TheIrishScion »

Armatus wrote:I do also have a support ticket open and I've linked them to this thread, they're sending me a new LCD screen and adapter board to test, I'll update this thread when I hear more from them/make any progress on my printer.

I'll be very interested to see how the different display works out. I'm slightly suspicious that the new display supplier (the red ones, versus the old white ones) may be a wee bit less consistent. I really do need to try a couple of ferrite cores on mine, see if it makes a difference. I haven't asked for a replacement and I'd prefer to see what's actually going on before I do.

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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by Eric LB »

I am new to 3D printing,have just built a new Rostock max, and am printing layer fan shroud as I write this. I lack enough electronics background to understand the above discussion; but can describe my temp behaviors. I have the red LCD board and had no issues with LCD display or knob directions etc upon firmware upload. I did not do a PID autotune until after installing my PEEK fan/shroud (stock seemecnc hot end, though I have an E3d v6 for future upgrade). At autotune my p values were 17.73, 16.59, 16.19. I values were 1.02, .99, .89. D values were 77.23, 69.68, 73.75. I plugged the averages of these into EEPROM, but wonder why they are soooooo different from the default values in the EEPROM. When my printer is at room temp, the temp readings are usually fairly stable within +/- .1 or .2 degrees. When I heat the hot end the temp overshot the target by 14 degrees without the PEEK fan and before autotune, by 4 degrees with PEEK fan before autotune, by 8 degrees with PEEK fan after autotune. Then the temps cycle, particularly before autotune. Regular cycling between 4 degrees over and 4 degrees under occurred with PEEK fan before autotune. After autotune with PEEK fan and while printing the temps (hotend and bed) are staying within .5 degrees of target @50% of the time, within 1 degree of target 80% of the time, within 2 degrees 99% of the time. But they don't seem to cycle regularly; rather, they spend variable periods of time over and variable periods of time under target. That's the best I can describe the behavior. I can say that when the Rambo cooling fan cycles on, it definitely seems to have a cooling effect on the bed, particularly during the preprint warm up phase.

I was hoping that someone more electronically knowledgeable than I could use these observations and be willing to inform me if my electronics is okay, or what would be recommended. Thanking you in advance for constructive comment/ suggestions.

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Re: Unstable thermistor readings/values from RAMBo 1.3

Post by Xenocrates »

Your electronics themselves are fine. That's alright. What's off is your PID values (The autotune helps but is far from perfect, you'll need to manually tweak, there are lots of guides about), which causes the print head to over/under shoot as it's got off values for predicting the temperature curve.
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