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correcting bed temp offset due to thermistor placement

Posted by shadowphile 
correcting bed temp offset due to thermistor placement
October 27, 2018 04:18PM
The math is in my head but no place to execute.
My bed surface and the thermistor reading are about 5 deg different after I allow them some time. This is due to the thermistor not being directly on the surface.
1) i would prefer to command the actual desired surface temp than have to manually (eyes roll) add 5 degrees to my commanded temp.
2) Its not actually 5 degrees except at one specific temperature. The hotter the heater, the greater the difference (ie they are the same when the heater is off). So I actually need a table (or use a fixed percentage) to know how much compensation to add.
3) Until they are stable the difference is also not predictable.
4) If the temp is changed further in the gcode then the initial compensation wont be right anymore.
4)This is all predictable and therefore compensatable but I cant see any way to implement the math (which is simple btw). PID alone is not enough. The math requires not only the current sensor temp but also its history, therefore the the calculation has to be ongoing and start from a known state (most likely with no heat when the sensor and the bed surface have settled to the same temp)
(other factors with impact include the ambient air temp, critically so for a heated chamber, and the presence of the print object itself as an insulator, which has a lesser impact, especially on the first layer when it wont change much so can be ignored for now).

Anybody see a way to compensate in this way? Would be a state of the art feature in the duet firmware. Just like PID, the parameters for each print setup would need to be characterized one time.

Edited 1 time(s). Last edit at 10/27/2018 04:27PM by shadowphile.
Re: correcting bed temp offset due to thermistor placement
October 27, 2018 08:51PM
The thermistor samples the temperature at a single point on the underside of the bed. You print on the top side, on glass or whatever surface is there for the plastic to stick to. The temperature of the bed is different everywhere that the thermistor is not sampling.

Here's a thermal image of my printer's bed, an 8mm thick piece of cast aluminum covered with 0.7 mm of PEI, with temperature set to 100C.



If your printer's bed is not a cast aluminum plate and/or has glass on top of it, you can expect the temperature variation to be much greater.

Which point has the correct bed temperature?

Print temperature settings marked on filament spools are rough guidelines. You have to run a few experiments to see what settings in your printer get the print layers to stick to each other and get the print to stick to the bed. Don't worry about the numerical value. Set the temperature(s) to whatever works.

Edited 1 time(s). Last edit at 10/27/2018 08:58PM by the_digital_dentist.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: correcting bed temp offset due to thermistor placement
October 28, 2018 12:23AM
Boy, which *I* had a FLIR!
I'm aware of all this. I have a 6mm MIC-6 plate myself. I would prefer the numbers be accurate, then play with *those* rather than just trying to see what works. Why have two uncertainties when one of them could be removed? In an ideal future everything is calibrated and we won't have to dial in every freakin parameter.
Re: correcting bed temp offset due to thermistor placement
October 28, 2018 03:12AM
Most users just get to know the indicated temperatures that they need to get good prints with a particular filament, rather than trying to get then firmware to correct the reading. But if you do want to try to correct the reading, you could change the thermistor B value that you use in the M305 P0 command in config.g.

Edited 1 time(s). Last edit at 10/28/2018 03:12AM by dc42.



Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: correcting bed temp offset due to thermistor placement
October 29, 2018 12:45PM
thanks, I hadnt thought of redefining the thermistor.

btw, anybody here that uses a heavy tooling plate: how do u deal with the extremely slow cooling rate?
Re: correcting bed temp offset due to thermistor placement
October 29, 2018 03:00PM
I deal with it by letting it cool off at its own rate. My printer is enclosed and heated, and like the bed, the enclosure takes a while to cool off, too. You'll find that if you leave the bottom of the bed uninsulated it will cool a bit faster than if you insulate it. People insulate the bottom mainly for two reasons: to protect whatever is under the bed from radiated heat -the need for which depends on the printer's construction, and to get the bed to heat up faster to print temperature, especially if it has a low or under powered heater. You can get a pretty good estimate of the time to heat or power required at this site: [jscalc.io] If you don't like the numbers, either insulate the underside of the bed or get a higher powered heater.

My 300x300x8mm bed has a 750W heater and gets to 100C in 4.5 minutes without any added insulation on the underside of the plate. Adding insulation to the underside of the heater reduces heat-up time and increases cool-down time. My 420 liter printer's 500W enclosure heater gets the temperature up to 50C in about 15-20 minutes, depending on ambient temperature. Heat "lost" from the underside of the bed in my printer helps heat up the enclosure.

I print a lot of ABS and letting the print cool off slowly helps prevent the warping and delamination problems that many people experience. I often start prints at night before I go to bed and when I get up in the morning, they're usually done and the printer is cool. The machine is capable of fast printing, but I usually keep the speed down to 60-100 mm/sec to maximize print quality. If I'm asleep while the print is running, what difference does it make if it finishes in 6 hours or only 3?

Speed is over rated. If you frequently find the cool-off time to be a problem, you might be better served by running two printers, especially if you're doing commercial work. If your mass production work doesn't generate enough revenue to allow you to get/build a second printer, you aren't charging enough for your highly skilled services and the wear and tear on your printer. That's not a sustainable business model.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: correcting bed temp offset due to thermistor placement
October 30, 2018 03:23AM
Quote
shadowphile
btw, anybody here that uses a heavy tooling plate: how do u deal with the extremely slow cooling rate?

Here are two solutions:

1. Print on removable glass plates that you clamp on to the bed plate. You can put a PEI, PrintBite or other surface on top of the glass plate. When the print is finished, remove the plate+print, fit another one, then you are ready to print again. This works well for small and medium size printers. You can also put the bed+print in the freezer if necessary to help the print release from the bed.

2. Use a couple of blower fans to blow air over the bed after the print has finished.



Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: correcting bed temp offset due to thermistor placement
October 30, 2018 11:57AM
thanks for the inputs. Cool down speed isnt a big concern for me either and I dont do ABS (yet) but Im used to a 500w silicone pad directly laminated to a glass plate so temps were dead on and cooled almost immediately. I love printing on glass but I broke two and decided a heavy tooling plate was the way to go.
Re: correcting bed temp offset due to thermistor placement
October 31, 2018 07:24AM
Mounting a heater directly on glass is a bad idea for a print bed. Glass is a thermal insulator so you get wildly varying temperature across the surface of the glass. Here's a thermal image of a Taz printer that came from the factory with a heater glued to a glass bed. There's a 30C+ variation in temperature across its surface. We never could get prints to stick to it reliably. Fortunately, someone broke the glass and I replaced it with a piece of 1/4" tooling plate. No more problems.

Before (heater on glass):



After (same heater glued to tooling plate):




Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: correcting bed temp offset due to thermistor placement
November 01, 2018 03:29PM
well I cant argue with those pics! one of the problems I had with the metal plate is it gets much hotter in the middle than the setting before the heat evens out but I had the sensor at the edge cause I thought the thick plate would conduct well enough. Wrong!
So recently slotted a thin plate from the edge to the middle to lay down the sensor in the middle, sandwiched between the heater and main plate. More thermal mass and extra resistance between the plates but at least I dont have dangerous overshoots. I tried to get the sensor from directly touching the heat pad and I filled the cavity with thermal grease so that leaves me with about 5 deg loss on the surface with room temp ambience.
Re: correcting bed temp offset due to thermistor placement
November 01, 2018 10:24PM
Slicers tend to arrange prints at or around the center of the bed. That would make the center the most sensible place for the temperature sensor and is probably why manufacturers of bed heaters with integrated sensors tend to put them there.


Son of MegaMax 3D printer: [www.instructables.com]
Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
Re: correcting bed temp offset due to thermistor placement
November 02, 2018 03:24AM
E3D makes PCB bed heaters with greater heating power at the edges, to counter the increased heat loss around the edge and achieve a more uniform temperature. There are companies in China who will make a silicone heater to your own size, voltage and power specifications. Maybe they would also do increased heating power at the edges if asked.



Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: correcting bed temp offset due to thermistor placement
November 03, 2018 04:37PM
I actually thought about bolting power resistors to the underside of the bed in an uneven distribution for that very reason but that comes with its own issues. (coupling to the plate is not as good; hotspots; resistors are thick and need extra insulation, for starters)
Re: correcting bed temp offset due to thermistor placement
February 06, 2021 02:39PM
Yep, good discussion. I got a infared temperature thermometer. Or what ever you call the product. My Tevo Tornado 2018 Bed I found out is 10 or 20 degrees from what I set the temperature in Simplify3D. I see that the outer edges are around 5-7 degrees cooler than the center. I also found that if I let the bed stableize for a few minutes the bed temp rases to about 10 degrees off. So far it seems consistent. Maybe that is the reason smallish prints print perfectly and bigger prints like 6" and larger keep failing. My question is...
I keep having a slight layer shift on even small 3" parts. As I said smaller but taller parts print perfectly. I have changed just about everything else that could result in the Layer shift issue. Also, One big circular print 288mm. Kept coming up unsticking on one of 2 places on the print. I just built a wood platform with adjustable feet to make sure my desk was dead on perfectly level.

Any thoughts:
Re: correcting bed temp offset due to thermistor placement
February 07, 2021 02:38AM
@Johnmg, if these layer shifts are happening using a Duet controller, then please ask for help at [forum.duet3d.com].



Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
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