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    Air pump too fast at minimum pwm

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    • Nxt-1undefined
      Nxt-1 @alankilian
      last edited by

      @alankilian You are right, the low side switching has gotten me several times over the years. Below is a trace of the motor running at 1% pwm with the scope gnd connected to actual gnd and the signal probe to the negative terminal of the motor.

      For those who requested it, here a link to the pump I got.

      a4c6288b-2c42-4899-aa1c-96b301071cec-image.png

      Duet3D and delta printer enthousiast. Buildlog
      Looking for Duet3D configuration support, check out Nxt-3D

      fcwiltundefined 1 Reply Last reply Reply Quote 0
      • fcwiltundefined
        fcwilt @Nxt-1
        last edited by

        @Nxt-1

        Hi,

        How are you setting the 1%?

        The scope image simple doesn't look like 1% to me - unless I'm completely looking at it wrong.

        Thanks.

        Frederick

        Printers: a E3D MS/TC setup and a RatRig Hybrid. Using Duet 3 hardware running 3.4.6

        Nxt-1undefined 1 Reply Last reply Reply Quote 0
        • Nxt-1undefined
          Nxt-1 @fcwilt
          last edited by

          @fcwilt I am setting the 1% in the DWC machine control dashboard tab

          Duet3D and delta printer enthousiast. Buildlog
          Looking for Duet3D configuration support, check out Nxt-3D

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          • fcwiltundefined
            fcwilt
            last edited by fcwilt

            I wonder if this is part of the problem.

            This is a screen shot of the DWC Object Model showing the fan data for a Duet 3 Mini 5 I am testing running firmware 3.2.

            Notice the min and max values.

            IF they are being enforced this fan cannot be set to 1%.

            That might explain why the scope image doesn't look like 1% to me.

            I need to do some testing.

            Duet v3 Fan Data.png

            Frederick

            Printers: a E3D MS/TC setup and a RatRig Hybrid. Using Duet 3 hardware running 3.4.6

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            • fcwiltundefined
              fcwilt
              last edited by

              OK I issued M106 P0 S0.01

              This is what the object model shows now.

              Does this object model reflect what the fan output is actually doing?

              I don't know.

              But if the object model DOES correctly indicate what the fan output is doing it would explain why 1% is as not slow as you expected.

              Duet v3 Fan Data 2.png

              Frederick

              Printers: a E3D MS/TC setup and a RatRig Hybrid. Using Duet 3 hardware running 3.4.6

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              • fcwiltundefined
                fcwilt
                last edited by

                To the M106 command in my config.g file for this fan I added this parameter and value L0.

                Then I issued the M106 P0 S0.01.

                This is the object model now.

                At this point it might help if @dc42 would jump in and clarify default fan settings like min speed when no M106 L parameter is present.

                In the meantime add L0 to the appropriate M106 in your config.g file and see it it makes a difference in the behavior of the pump.

                Duet v3 Fan Data 3.png

                Printers: a E3D MS/TC setup and a RatRig Hybrid. Using Duet 3 hardware running 3.4.6

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                • fcwiltundefined
                  fcwilt
                  last edited by

                  Hi,

                  OK I took the time to configure out1 on the Duet 3 Mini as a fan.

                  M950 F0 C"out1" Q500
                  M106 P0 S0 H-1 C"F0" 
                  

                  Notice there is no L parameter.

                  I connected a 1K resistor across the outputs and connected my battery powered scope leads across the resistor.

                  The scope has the ability to measure the duty cycle of a waveform and I enabled that feature.

                  I re-booted the board and issued a M106 P0 S0.01 command.

                  The object model reported that the actual value was 0.1.

                  The scope reported the duty cycle to be 10%.


                  I changed the fan setup to this:

                  M950 F0 C"out1" Q500
                  M106 P0 S0 H-1 C"F0" L0
                  

                  I re-booted the board and issued a M106 P0 S0.01 command.

                  The object model reported that the actual value was 0.01.

                  The scope reported the duty cycle to be 1%.


                  So at least with firmware 3.2 on a Duet 3 Mini 5, the default L value for a fan is 0.1.

                  And that explains why the pump would not go as slow as you wanted.

                  Add the L parameter using whatever min value is suitable and you should be good to go.


                  That was fun.

                  Frederick

                  Printers: a E3D MS/TC setup and a RatRig Hybrid. Using Duet 3 hardware running 3.4.6

                  Nxt-1undefined 1 Reply Last reply Reply Quote 0
                  • dc42undefined
                    dc42 administrators
                    last edited by

                    Is scope ground connected to Duet ground, i.e. VIN- or the GND pin of any of the I/O connectors? I have a feeling that it isn't, which is why the trace is still odd.

                    Duet WiFi hardware designer and firmware engineer
                    Please do not ask me for Duet support via PM or email, use the forum
                    http://www.escher3d.com, https://miscsolutions.wordpress.com

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                    • Nxt-1undefined
                      Nxt-1 @fcwilt
                      last edited by Nxt-1

                      @dc42 In the last trace I posted the scope ground was connected with a wire straight to the psu negative terminal.

                      @fcwilt I never knew that there was a default min value, thanks for pointing that out. I have put it at 0 now and that definitely helped. The trace looked quite similar though, ie. as saw tooth.

                      At this point I was ready to call it a day and slap a power resistor in series (which I tried and worked decently well) to drop the voltage. Out of curiosity and recalling that dc pointed out that the 22.5kHz switching frequency was very high, I tested again at 1kHz. (this was at like 4% or something similar). The trace speaks for itself I guess.

                      I draw two conclusions from this. 1) The previous trace was not a measurement error as the setup did not change, only the frequency of operation. 2) The E1 mosfet on the duet cannot switch fast enough, there are unwanted capacitances or something in that area. At least the conclusion is something does not like higher switching speeds.

                      EDIT: just to avoid confusion, all the traces I post here are scope gnd to actual gnd and scope signal to the negative terminal of the pump.
                      4782e202-3e35-4daf-8549-e5bc586f5e76-image.png

                      Duet3D and delta printer enthousiast. Buildlog
                      Looking for Duet3D configuration support, check out Nxt-3D

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                      • Nxt-1undefined
                        Nxt-1
                        last edited by

                        Here are two more traces:

                        5% @2kHz
                        94f3497c-1710-460c-b521-03a9475bdb3f-image.png

                        5% @5kHz
                        983bdf0a-c3e8-47b6-b516-c7f939fe9a67-image.png

                        Duet3D and delta printer enthousiast. Buildlog
                        Looking for Duet3D configuration support, check out Nxt-3D

                        fcwiltundefined 1 Reply Last reply Reply Quote 0
                        • alankilianundefined
                          alankilian
                          last edited by

                          @alankilian said in Air pump too fast at minimum pwm:

                          I think things will look better if you connect the scope ground to ground and scope-signal to E1-.
                          You'll see a zero-volt signal when the pump is "on" and a woozy-doozy signal probably getting up to 24 Volts when the pump is "off".

                          PERFECT!

                          I think you've demonstrated that everything is working exactly as it's supposed to even at 22 KHz.

                          That minimum of 10% was certainly buried in the details. Thanks @fcwilt for finding that.

                          Now, how do we get your darn motor to slow down more?

                          Or, can you just make a "leak" in the output hose to let off 50% of the air and then you'll be able to control it enough?

                          Or can you restrict the output through an orifice to limit the air?

                          We'll get you there somehow.

                          Great investigative work.

                          SeemeCNC Rostock Max V3 converted to V3.2 with a Duet2 Ethernet Firmware 3.2 and SE300

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                          • fcwiltundefined
                            fcwilt @Nxt-1
                            last edited by

                            @Nxt-1

                            Hi,

                            Are you sure the L0 parameter took hold?

                            The image labeled 5% at 2kHz looks like10% - which is the default minimum - what you get with no L parameter.

                            Frederick

                            Printers: a E3D MS/TC setup and a RatRig Hybrid. Using Duet 3 hardware running 3.4.6

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                            • Nxt-1undefined
                              Nxt-1 @fcwilt
                              last edited by

                              @fcwilt said in Air pump too fast at minimum pwm:

                              @Nxt-1

                              Hi,

                              Are you sure the L0 parameter took hold?

                              The image labeled 5% at 2kHz looks like10% - which is the default minimum - what you get with no L parameter.

                              Frederick

                              You are right in between setting the different frequencies the L parameter got overridden. If I set it again and look at the trace it has a half as long off period than before, as it should be. The rise time the same or similar.

                              Duet3D and delta printer enthousiast. Buildlog
                              Looking for Duet3D configuration support, check out Nxt-3D

                              fcwiltundefined 1 Reply Last reply Reply Quote 1
                              • fcwiltundefined
                                fcwilt @Nxt-1
                                last edited by

                                @Nxt-1

                                So can you get the pump running slow enough now?

                                Frederick

                                Printers: a E3D MS/TC setup and a RatRig Hybrid. Using Duet 3 hardware running 3.4.6

                                Nxt-1undefined 1 Reply Last reply Reply Quote 0
                                • Nxt-1undefined
                                  Nxt-1 @fcwilt
                                  last edited by

                                  @fcwilt said in Air pump too fast at minimum pwm:

                                  @Nxt-1

                                  So can you get the pump running slow enough now?

                                  Frederick

                                  Yes, no, maybe? I think it's slow enough now, it does make some progressively unhappy noises as low percentages as I drop the frequency. For now, I have some things I really need printed so I am going to roll with it @2kHz I think 🙂

                                  For sure many thanks for those who contributed.

                                  Duet3D and delta printer enthousiast. Buildlog
                                  Looking for Duet3D configuration support, check out Nxt-3D

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                                  • fcwiltundefined
                                    fcwilt @Nxt-1
                                    last edited by

                                    @Nxt-1 said in Air pump too fast at minimum pwm:

                                    Yes, no, maybe? I think it's slow enough now, it does make some progressively unhappy noises as low percentages as I drop the frequency.

                                    If a motor is designed to run on pure DC then PWM DC can make it sound unwell.

                                    There are filter circuits you can add between the board connections and the motor that can smooth out the pulses giving something more like DC.

                                    It might make a suitable improvement.

                                    Frederick

                                    Printers: a E3D MS/TC setup and a RatRig Hybrid. Using Duet 3 hardware running 3.4.6

                                    alankilianundefined 1 Reply Last reply Reply Quote 1
                                    • alankilianundefined
                                      alankilian @fcwilt
                                      last edited by

                                      @fcwilt said in Air pump too fast at minimum pwm:

                                      PWM DC can make it sound unwell.

                                      Also, 2,000 Hz is a pretty annoying frequency.

                                      SeemeCNC Rostock Max V3 converted to V3.2 with a Duet2 Ethernet Firmware 3.2 and SE300

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                                      • dc42undefined
                                        dc42 administrators
                                        last edited by

                                        Those traces look much better than the previous ones. Can you confirm that the motor was connected when you took those traces? If so, then I suspect that the slow rise time is caused by an interference suppression capacitor connected across the motor terminals, possibly inside the motor itself.

                                        Duet WiFi hardware designer and firmware engineer
                                        Please do not ask me for Duet support via PM or email, use the forum
                                        http://www.escher3d.com, https://miscsolutions.wordpress.com

                                        Nxt-1undefined 1 Reply Last reply Reply Quote 0
                                        • Nxt-1undefined
                                          Nxt-1 @dc42
                                          last edited by

                                          @dc42 said in Air pump too fast at minimum pwm:

                                          Those traces look much better than the previous ones. Can you confirm that the motor was connected when you took those traces? If so, then I suspect that the slow rise time is caused by an interference suppression capacitor connected across the motor terminals, possibly inside the motor itself.

                                          Yes, motor and diode were connected for those traces.

                                          Duet3D and delta printer enthousiast. Buildlog
                                          Looking for Duet3D configuration support, check out Nxt-3D

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                                          • dc42undefined
                                            dc42 administrators
                                            last edited by dc42

                                            In that case, I suspect an interference suppression capacitor is causing the slow rise times, or possibly just the capacitance of the motor windings. When the mosfet turns off, the motor inductance, resistance and capacitance from a LCR resonant circuit that start to ring; but the diode clamps the oscillation it when it reaches 24V.

                                            The basic issue you are having is that lightly-loaded DC motors respond more to the peak voltage they are supplied with than to the average voltage.

                                            If you never want to drive the pump as maximum RPM then the simplest fix may be a power resistor in series with the pump; or you could power the pump positive terminal from 12V from a buck regulator instead of 24V. If you do need the full range then I suggest an LC filter between the Duet and the pump, with the flyback diode connected to the input (Duet end) of the filter. However, you will need to use quite a large inductor. Even at 65kHz PWM frequency you will need at least 1mH with a suitably-high rated current.

                                            Duet WiFi hardware designer and firmware engineer
                                            Please do not ask me for Duet support via PM or email, use the forum
                                            http://www.escher3d.com, https://miscsolutions.wordpress.com

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