Part cooling fan speed variation - is this normal?  

RE: Part cooling fan speed variation - is this normal?

S100 is setting the fan to 2/5th roughly of its top speed (0-255).  Id say its the pwn isnt quite perfect at that speed so theres a bit of waver in the output.  

RE: Part cooling fan speed variation - is this normal?

@neophyl

But the fact that it is so consistent and the repetetive pattern seems odd to me.  It does the same at higher speeds too (like 50% and 75%).  

RE: Part cooling fan speed variation - is this normal?

As I mentioned I’d make a guess that the firmwares pwm is not perfect, maybe there’s some form of interrupt that the software is servicing or its busy doing other housekeeping tasks and that causes jitter on the output. Pwm is basically turning the output pin on and off really really fast (otherwise you can get audible switching), while it’s doing that it has to monitor the inputs, drive the lcd, drive any other outputs and loads of other things even when it’s not printing. Oh and service the serial connection you have open when you connect with pronterface. Basically even when it’s not printing the embedded chip is still really busy.   
these sorts of processors split the available time between all the tasks and often there’s different priority tasks too. If the fan pwm is lower down the pecking order things like that can easily happen. 
If your printer is printing fine then do t worry about it. If it bothers you a lot then open an issue over on the Prusa Github and maybe one of the devs can fix it. Some how I doubt it will be a high priority with them given all the other stuff they need to do. 

RE: Part cooling fan speed variation - is this normal?

@neophyl

Yeah - makes sense.  I was mainly curious to see if others saw the same thing.  I just reimaged my octoprint server to get octopi up to date (it refused to update plugins anymore), and restored a backup of the server i made with the internal backup utility.  Not sure if this had anything to do with it, so just floated this out there to see if others saw the same behavior.  I am not super concerned about it - just curious...

RE: Part cooling fan speed variation - is this normal?

Hello,

these speed changes are used to measure the fan speed. During the PWM, a reliable speed measurement is not possible, so the fan is operated for a short time with 100% PWM.

This picture makes it a bit clearer:
Channel 1 (yellow): PWM signal to drive the fan.
Channel 2 (blue): Speed signal from the fan. Almost identical to the PWM signal! The speed signal is almost lost.

RE: Part cooling fan speed variation - is this normal?

@holger-s3

Ah - very interesting - that would explain the regularity of the blips in speed.  Thanks for the info!

RE: Part cooling fan speed variation - is this normal?

The solution would be a 4-pole PWM fan whose 4th line provides a constant supply voltage for the speed measurement.