Bed Auto Tuning

  • I have been trying to tune the heater of my Print bed. The bed is 300mm X 600mm 12mm thick aluminum. I have a 110v silicone heater for the Bed. It is controlled by a SSR
    From what I understand the PWM for the bed heater is running at about 10HZ which should work on and SSR
    When I ran the auto tune I get the error that auto tune has been cancelled because temperature is not falling.
    I know it takes some time for the bed to start to reduce in temperature due to its mass but is there some way to overcome this error
    I have adjusted the initial settings M307 H0 B0 D400
    Should it be something different

  • Never seen that. It sounds as if you are trying to tune the bed heater when it's already hot. Try staring with the bed at ambient.

    I'm a bit confused by your statement that you adjusted the initial settings but then you list M307 which is where you would normally put the results of the auto tuning. For auto tuning, you need to be looking at M303 - instructions are here

    It'll work fine with an SSR and mains powered bed. Mine is fairly similar to yours - 400mm x 400mm x10mm thick with 240V silicone heater and it works fine.


  • administrators

    I presume you are using firmware 1.18beta because earlier versions don't produce that message. When the temperature reaches your target temperature, the heater is turned off. The firmware then expects the temperature to go on rising for a while, but to start falling again no later than 60 seconds after the heater was turned off. I can increase that timeout if necessary. Please can you do some tests to establish how long after turning the heater off it takes to reach peak temperature and start cooling.

  • That'll be why I haven't seen the message then. I'm not in a position to be able to test this myself but I'd guess that I'd have the same problem.

  • Update. Just did a quick test (I'm restricted to using Pronteface at the moment). Heated the bed to 60 then turned the heater off and recorded the temperature ever 15 secs.
    Time 0, Temp=60.0
    15 - 60.1
    30 - 60.1
    45 - 60.1
    60 - 60.1
    75 - 59.9
    90 - 59.6
    105 - 59.4
    120 - 59.3

    That's with a 10mm thick bed but also with 10mm of insulation under.

  • administrators

    Thanks Ian, looks like I need to increase that timeout. I hope percar will run the same test.

  • Start Temp 19.4C
    M303 H0 S75

    Start Time 6:41:11
    Heater Shut Off 6:48:32 (75C)
    Max Temp. 6:49:52 (79.4C)

    The error message came up at 6:49:32

    So if what you are saying that it is expecting the temperature to fall within 60 seconds then I can see why it fails
    Perhaps 120 seconds would be nice. I had to increase the dead time to 120 seconds because it also takes a while for the bed heat to start to rise as well.

  • administrators

    I've put a firmware build for the Duet WiFi with the timeout increased to 120 seconds at

  • Thanks David

    That worked

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