Controlled bed heating



  • Bit of a strange request (and could probably be easily done in gcode or a macro)

    But my bed is around 400x400 and 8mm thick the AC heat pad is about 350x350.

    The issue I'm having is when heating the bed gets full power and reaches the required temperature within a minute or so. But the outside edges are still cool.

    This is causing the bed to wrap really bad.

    I've replaced the bed and it was straight. Heated it and again it wrapped.

    It would be nice to have a controlled heating curve.

    Something like if bellow a set temperature heat at at lower pwm untill a set temperature is reached and so on

    Craig



  • @craigb244 A few random thoughts. Funnily enough, my bed is almost exactly the same size 400 x 400 x 10 with a heater that is 360 x360 but I have no problem with the bed warping. Is that what you mean by wrapped? Is the bed over constrained against thermal expansion?

    Reaching temperature in a minute sounds like you have the thermistor at the junction between the bed heater and the aluminium plate (I'm assuming that it is aluminium). If so, then I'll wager that the top surface of the plate isn't anything like as hot after only one minute - not just the edges. What I did was drill a hole in the edge of the plate, as deep as possible and as close to the top surface as possible and fitted the temperature sensor in there.

    If the temperature sensor is really sensing the top surface temperature (or close to it) then to get that amount of thermal mass up to temperature within a minute would indicate that you have a seriously over powered heater.



  • Thanks Ian.

    I'm only heating to 50 at the moment,

    The thermistor is built into the silicone heat pad so I do know the reading is a LOT faster than what the actual bed is at.

    And I also agree I do think the pad is a bit over powered. It can easily get the bed over 100c without any effort.

    I might have to do some more testing before changing the plate again.

    Maybe setting a fixed maximum pwm will solve the problem, just worried that it might cause stability problems at higher temperatures.



  • @craigb244 How is the bed mounted? As @deckingman mentioned, it might just be overconstrained, so when the bed expands in all directions, it's forced into warping (no matter how quickly or slowly it's heated up).
    Maybe post a picture of your setup.



  • @craigb244 A few things. With PID control, if the actual temperature is a long way away from the set point, then initially heating will start at 100% PWM and only reduce when the temperature approaches the set point. So in a way, your assumption is correct and you could possibly use a macro to reduce the maximum PWM value. One way would be like this (note that I haven't tried it so it might not work):

    M307 H0 S0.5 ; sets the maxi PWM for the bed heater to 50%
    M190 S30 ; Heat bed to 30 and wait for temperature to be reached.
    M307 H0 S0.75; sets heater PWM to a maximum of 75%
    M190 S40; Heat be to 40 and wait for temperature to be reached
    M307 H0 S1.0; set heater to use up to 100% PWM
    M190 S50; complete heating to 50.

    But I don't think this will help your warping problem. The reason I say that is with the temperature sensor in the middle of the silicone heater, when you start to heat the bed, the temperature of the silicone heater rises rapidly to the set point but there hasn't been enough time for that heat to conduct through the rest of the plate. But the sensor and control system "don't care" so the heater gets turned off (or PWM kicks in at a very low value). PWM only increases once the temperature at the silicone heater drops below the set point. So in a way, having the sensor where it is in the centre of the heater, effectively reduces the PWM value for the aluminium plate as whole. By switching off early, it is already doing what you want to do by other means.

    So I don't think any of that will help with your warping issue. i.e. - I don't think the warping is due to the rate of change of temperature and is simply a function of increased temperature.

    As a side note, what size (wattage) is your heater? For info, mine is 800 Watt and that's a bit on the high side. I also have 12mm of semi rigid insulation underneath the heat pad. With my sensor in the edge of the bed as described, it take about 5 1/2 minutes to go from from ambient of around 22 to 50 degC. PWM actually cuts in at around 49 degrees or so. Before I moved the sensor, it would take more than twice that long for the top surface to reach that temperature due to the heater switching off prematurely.


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