Bed heating, multiple sensors

paralepsis

Hi, I'm got a railcore with a Keenovo heater on the bottom of a thick aluminum plate. There's a thermistor placed on top of the bed that gives me a pretty good "actual bed" temperature, and the Keenovo has a built-in sensor that gives me what is going on at the bottom surface.

Clearly my target is related to the top surface, so that is the sensor that I have associated with the heater. However, with this configuration I routinely see the bottom sensor read as high as 150C.

The adhesive on the bottom surface is rated to stand a significant amount of time at 150C, but I'm trying to find a good/simple way to heat the bed to a target top temperature (which typically is 70-80C for me; PETG), while not exceeding some upper bound (e.g., 120C) for the bottom surface.

Any thoughts on doing this without shenanigans like redefining the heater each print, etc.? Thanks!

dc42

@paralepsis check the M143 command, in particular the A2 parameter. https://docs.duet3d.com/en/User_manual/Reference/Gcodes#m143-maximum-heater-temperature

paralepsis

@dc42 Thanks, I'll give that a shot.

paralepsis

@paralepsis I could use some guidance. My setup:

; Thermistors
M308 S0 P"temp0" Y"thermistor" A"bed_heat" T100000 B4240 H0 L0 		;Bed thermistor - connected to temp0
M308 S1 P"temp1" Y"thermistor" A"e0_heat" T100000 B4725 C7.06e-8 H0 L0	;duet3 e3d thermistor - connected to e0_heat
M308 S3 P"temp3" Y"thermistor" A"bed_heat2" T100000 B3950 H0 L0      ; Bed thermistor - Keenevo builtin


; Heaters
M950 H0 C"out0" T0				;Bed heater is on out0
M950 H1 C"out1" T1				;Hotend heater is on out1

; Bed
M140 H0

Reading the documentation on M143, I think what I want to be able to do is use M143 H0 A2 C0 S125 <something> to temporarily switch off the heating of the bed when exceeding 125C.

I'm confused about the <something>. The documentation indicates that there can be multiple "monitors" for a heater, but I cannot find in the documentation any mention of how to define an additional monitor? Is it as simple as multiple T parameters to the M950 definition of the heater?

Thanks!

OwenD

@paralepsis said in Bed heating, multiple sensors:

The documentation indicates that there can be multiple "monitors" for a heater, but I cannot find in the documentation any mention of how to define an additional monitor? Is it as simple as multiple T parameters to the M950 definition of the heater?

The monitor is defined by the P parameter of M143
You've already defined two thermistors.
So I believe you need to do two different sets of M143 commands
eg

M143 H0 P0 S90 A2 T0; switch off heater 0 temporarily if it exceeds 90C on themistor 0 
M143 H0 P1 S100 A0 T0; raise a heater fault if it exceeds 100C
M143 H0 P2 S125 A2 T3; switch off heater 0 temporarily if it exceeds 125C on themistor 3 
M143 H0 P3 S140 A0 T3; raise a heater fault if it exceeds 140C

paralepsis

@OwenD Thanks -- clearly I was making this more complicated than it is :).

paralepsis

@paralepsis works great!