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david.f42
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Quieting the R3 blower design

Jul 21, 2018 8:24 pm

I'm needing to run the part cooling blower at around 25% (maybe even less) to get a good print now. As a consequence, I guess beause of the PWM approach to fan speed (?), the blower motor makes a ton more noise than previously when it was running closer to 100% speed.

Is there a good way to muffle this new undesirable sound? Would maybe using a suitable size capacitor across the motor leads solve the issue? Or is there a better way?

BEST ANSWER by david.f42 » Aug 11, 2018 5:48 pm
Version 2 hardware completed:
MK3_version2.jpg
This added the OLED display and a red reset button.

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david.f42
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Re: Quieting the R3 blower design

Jul 22, 2018 10:50 pm

Here's an o-scope shot of the voltages being applied to the blower fan when the fan speed is set to 25% by the g-code:
NewFile1.png
NewFile1.png (35.35 KiB) Viewed 1205 times
So, pretty much classic PWM, with a max of 5v and a min of 2v and a duty cycle of 25%.

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david.f42
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Re: Quieting the R3 blower design

Jul 22, 2018 11:37 pm

Here it is with the blower attached (and making a lot of noise):
NewFile2.png
NewFile2.png (39.72 KiB) Viewed 1201 times
and here it is with a 2200uF capacitor across the blower's power leads (resulting in ~95% less audible noise):
NewFile3.png
NewFile3.png (33.45 KiB) Viewed 1201 times
That last 5% of audible noise is still somewhat annoying, though I suppose I could live with it. It is certainly much better now than before the mod.
Last edited by david.f42 on Jul 23, 2018 1:37 am, edited 1 time in total.

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david.f42
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Re: Quieting the R3 blower design

Jul 23, 2018 1:34 am

Final thoughts: The PWM is operating at 500Hz. It is audible because human hearing is in the 20hz to 20Khz frequency range. So, if (?) the firmware can support a higher frequency PWM--something higher than the human hearing range--then that would solve the issue at the source.

flaviu.i
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Re: Quieting the R3 blower design

Jul 23, 2018 7:21 am

Thanks for the analysis. I will also try a 2200uF capacitor to reduce the noise.

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Knickohr
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Re: Quieting the R3 blower design

Jul 23, 2018 8:15 am

Be carefull with adding such a big Elko ! Now your fan is running with more then 25%.

Thomas

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david.f42
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Re: Quieting the R3 blower design

Jul 26, 2018 2:27 pm

I did some experiments and found that if I drive the PWM at frequencies greater than 7kHz, then the blower is completely quiet. The default for the Prusa Einsy is just 490Hz. Moreover, at 7kHz PWM I can still drive the blower over its full range of ~200RPM to ~3000RPM.

Therefore, I'll be doing exactly that, using an Arduino Pro Mini to measure the duty cycle output by the Einsy board and then converting that at an equivalent duty cycle at 7kHz frequency to the blower motor. This will work standalone without needing to change the Einsy board or the Einsy firmware. As you can see in the schematic below, the material cost is quite low. By changing the prescaler on the Pro Mini, a 7kHz frequency PWM is created.
7Khz_3.png
7Khz_3.png (23.53 KiB) Viewed 899 times

Meanwhile, entirely separate from the above, the blower motor's tachometer output is truly strange:
NewFile5.png
NewFile5.png (48.59 KiB) Viewed 899 times
I can't make much sense out of it. Can anyone reading this interpret it?

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ejberg
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Re: Quieting the R3 blower design

Jul 27, 2018 7:54 am

david.f42 wrote:
Jul 26, 2018 2:27 pm
Meanwhile, entirely separate from the above, the blower motor's tachometer output is truly strange:
Indeed.
The problem is that you're PWM regulating on the GND connection to the fan. The tacho signal is referenced to GND, so you are seeing a mix of the tacho and your PWM duty cycle. You should PWM regulate the 5V supply to the fan instead.

EDIT:
Maybe I am wrong about the above. Reading the Einsy schematic, it seems to be PWM regulating on fan GND too.
But then I don't get how you can read the duty cycle output from the Einsy by taking the positive fan output and connect it to a port on your microprocessor (and connecting the PWM regulated GND to your common GND).

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ejberg
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Re: Quieting the R3 blower design

Jul 27, 2018 9:36 am

david.f42 wrote:
Jul 23, 2018 1:34 am
Final thoughts: The PWM is operating at 500Hz. It is audible because human hearing is in the 20hz to 20Khz frequency range. So, if (?) the firmware can support a higher frequency PWM--something higher than the human hearing range--then that would solve the issue at the source.
It does look like there is support for a higher frequency. In Configuration.h there is a define "FAST_PWM_FAN" that isn't enabled. It looks like enabling it will increase the frequency to 7.8 kHz, but also increase the temperature of the fan driving FET (Q2).

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david.f42
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Re: Quieting the R3 blower design

Jul 27, 2018 10:58 am

ejberg wrote:
Jul 27, 2018 9:36 am
david.f42 wrote:
Jul 23, 2018 1:34 am
Final thoughts: The PWM is operating at 500Hz. It is audible because human hearing is in the 20hz to 20Khz frequency range. So, if (?) the firmware can support a higher frequency PWM--something higher than the human hearing range--then that would solve the issue at the source.
It does look like there is support for a higher frequency. In Configuration.h there is a define "FAST_PWM_FAN" that isn't enabled. It looks like enabling it will increase the frequency to 7.8 kHz, but also increase the temperature of the fan driving FET (Q2).
It works! This is great. I had thought about changing the prescaler in firmware, but I was concerned about unintended consequences. This does it though, so I guess it must be OK.

The main consequence seems to be that of setting fan speeds. For instance, "M106 S100" now produces only 480RPM on the blower, and "M106 S120" produces maximum RPM. "M106 S101" produces 800RPM. So, as you can infer, the specifying range is both different and a lot narrower.

However, it is quiet! That's the important thing. It's a relatively small sacrifice to change my cooling settings in the slicer to get that benefit. :D :D :D

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