FSR sensors and heated bed

Have anyone been able to use FSR sensors with a heated bed? Is this possible? Or the heat bed puts out too much heat to use both?

We've done the research on this, and the answer is... It depends.

If you use Johann's code, it may be possible... You will need to really insulate the bed well as the readings could exceed what Johann uses to trigger. The problem with more insulation is that the more you place between the FSR and the bed, the less accurate your FSR's are. What fixes one problem, creates another.

If you use a trinket system... you need less insulation as the FSR's can tolerate a decent amount of heat. Is it enough? Will it cause other issues? That part we don't know yet. You will likely have to reset the FSR's once up to temp, or calibrate while cold and I would still expect to use some sort of insulation, which leads back to the insulation inducing slop.

Thank you sheepdog43, you are awesome it seems you always end up answering my questions.
Now that you mention the trinket, I saw it and I was a bit confused since as I understood you connected the FSRs to the RAMPS, could you please tell me what are the differences? Is it worth adding the trinket? For what you posted it seems the trinket compensates for heat.
Once more thank you for all your invaluable help.

You're welcome.

FSR's return a different resistance depending on weight. How Johann's system works is the resistance exceeds a certain number the firmware treats it as being triggered. Heating them changes the number they send, so what will trigger it at room temp is different than what triggers it while hot. Johann's system was never designed for that and cannot compensate. Even changing the room tempo can change their sensitivity.

With a Trinket, you wire the fsr's to the Trinket and the Trinket to the Ramps. When the Trinket boots, it assumes that whatever the FSR is showing for resistance is the baseline and any change triggers it. It then sends a binary signal (on or off) to the Ramps. As fas as the ramps knows, you just have a normal endstop there, no trying to adjust the sensitivity or anything. If you heat the bed after the Trinket has booted, you just reset the Trinket, which once again sets the current position as the baseline. Basically it self adjusts and makes FSR's act like an endstop switch.

If you use Johns FSR board [github.com] you don't have to reset it, as it adjust itself constantly.

Also it has 3 LEDs which helps a lot during setup as you can easily see, which sensor fires and if for example something got under the bed and prevents the trigger on one side or the sensor is not perfectly flat. This makes the FSR usage almost pain free.

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wuz
If you use Johns FSR board [github.com] you don't have to reset it, as it adjust itself constantly.

I think that's just part of his code in general as I think my Trinkets with his code does the same.

Edited 1 time(s). Last edit at 10/17/2014 08:29PM by sheepdog43.

Sheepdog43 thank you one more time you are awesome, I think I be ordering a trinket for my machine
I saw some heat resistant FSR here [www.tekscan.com] there are good up to 200C but at $163 is not cheap.
For what I am understanding is not that the FSR gets damaged is that heat affects the readings of the sensor.
Is there a better FSR to buy, or it does not matter? I am thinking of ordering the board and sensors from here [www.tridprinting.com].

Yes, just the reading changes.

Some standard FSR's can handle PLA temps, but not ABS, most are only rated for 60c. So you will either need to insulate it or buy the more expensive ones. As for quality fo manufacturer, it doesn't matter and there is only a couple manufacturers.

Thank you sheepdog43, so the FSR's can be insulated? Best insulator to use? Thank you once more.

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sheepdog43

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wuz
If you use Johns FSR board [github.com] you don't have to reset it, as it adjust itself constantly.

I think that's just part of his code in general as I think my Trinkets with his code does the same.

Sure, its just code, but then you only have one LED. I had one sensor not firing as a small piece of PLA got stuck next to the sensor, and I would have had a harder time to figure this out, if there is only one LED as the others chime in, at some point.

Also the board comes already programmed, so the install is very very easy.

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wuz
Sure, its just code, but then you only have one LED. I had one sensor not firing as a small piece of PLA got stuck next to the sensor, and I would have had a harder time to figure this out, if there is only one LED as the others chime in, at some point.

Also the board comes already programmed, so the install is very very easy.

You press a sensor light comes on, doesn't matter if there is one light or 3. We've had a few fail and it wasn't hard to diagnose.
An extra light would have made no difference.

At any rate, I meant Trinket in general, John's board is basically a purpose built Trinket. It being pre-flashed is the biggest benefit, Adafruit Trinkets are a pain to do sometimes.

Hey guys any ideas on how to insulate the FSR to use it with ABS? Thank you.

A thought just occured to me. When printing, is there ever an issue with the FSR triggering, thus signalling that the hot-end has hit the bed, even though it might have just pressed down on the part?

As for the heat bed, I was thinking of using a small heat bed with a large sheet of glass. So I would be limited on the size of my ABS prints, but I could still print large PLA prints without a heat bed. Would that solve the heat issue?

Endstops are ignored while printing.

If you plan on only heating one area, it will need to be boro, otherwise you will break the glass.

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sheepdog43
Endstops are ignored while printing.

Depends on ENDSTOPS_ONLY_FOR_HOMING setting in Marlin.
I do not remember about Repetier (despite the fact the printer uses Repetier now).

End stops are ignored while printing. Did not know that, thank-you. Though I will need to double check Repieter as that is what I am using right now.

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sheepdog43
Endstops are ignored while printing.

If you plan on only heating one area, it will need to be boro, otherwise you will break the glass.

Not necessarily. I have a similar setup on one of my core xy machines. I use a 200 x 200 MK2A heat bed pcb but my glass bed is 300 x 300. I have a piece of 1mm aluminium sheet between the pcb and the glass and the glass is held in place with bulldog clips to allow lateral expansion. The glass is normal 4mm window glass and has dozens of hours of print time with no issues.

I run it at 70 for PLA and 100 for ABS. The bed edges I.e not directly over the pcb are cooler than the centre but the aluminium helps even things out a bit. At 70 in the centre the edges are around 60.

Andy

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AndyCart

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sheepdog43
Endstops are ignored while printing.

If you plan on only heating one area, it will need to be boro, otherwise you will break the glass.

Not necessarily. I have a similar setup on one of my core xy machines. I use a 200 x 200 MK2A heat bed pcb but my glass bed is 300 x 300. I have a piece of 1mm aluminium sheet between the pcb and the glass and the glass is held in place with bulldog clips to allow lateral expansion. The glass is normal 4mm window glass and has dozens of hours of print time with no issues.

I run it at 70 for PLA and 100 for ABS. The bed edges I.e not directly over the pcb are cooler than the centre but the aluminium helps even things out a bit. At 70 in the centre the edges are around 60.

Andy

You aren't heating the glass unevenly, or just one area because you are using a heat spreader, some heat difference is acceptable, it's not going to break because of a small difference.

He was wanting to heat only part of the glass and leave the rest cool.


And yes, the endstops do need to be configured that way in some firmware, but not all. In some cases it just may be a matter of it being set by default, others just do. I don't remember even seeing an option for it in Smoothie.

Edited 1 time(s). Last edit at 10/23/2014 06:27PM by sheepdog43.

For those who want to have a heated bed and print with ABS, why not put the FSR's into a hot end setup?

FSR's built into a groovemount designed to be used with an actively cooled hot end, used as an autolevel calibration probe:

I am using a groove mount FSR setup I found on thingiverse: [www.thingiverse.com]. The slighlty modified firmware available here: [github.com]. This setup
uses two FSR's built into the groovemount and the tension can be adjusted with two springs. I am using a fan cooled E3D hot end, so the top of the heatsink should be relatively cool, and
not transfer too much heat into the plastic. This way, the hot end ittself is doing the probing. You can pre-load some of the resistance on the FSR's with adjusting the spring tension i.e. adjust some of
the sensitivity. I have not tested this out yet, so wish me luck......

Picture attached.

I made cups for mounting the FSRs below the heated bed. See here:[forum.seemecnc.com]

Hey rpress that is an interesting design, it looks like the bed is screwed into the caps of the cups and the cups are attached to the printer, so you can just lift the bed up from the cups right? Is the bed with the glass heavy enough to hold everything in place? There is no issues with vibration or the risk of anything moving?

Yes, you could lift up the bed, but now I have double adhesive tape holding it all together. The bed and glass is plenty heavy, even without any double adhesive it doesn't pop up. The tolerance is such that there is not much side movement, I have not seen anything in the print that would indicate the bed is moving.

Suggestions for a good insulator between a heated bed and acrylic? I have a triangular piece of acrylic that sits over my aluminum extrusion frame with the FSR's sandwiched between the two. That part is working perfect with triggering the FSR's, and I have no need/desire to change it. But, I'd like to put a 170mm round heated glass build plate over top, and need to ensure I have sufficient insulation between the two. Suggestions on a hard insulating material that can take the heat from the build plate and not transfer too much to the acrylic?

I use 3mm cork