DC gearmotor + encoder vs stepper (for extruder)

Has anyone tried using a torquey DC gearmotor with encoder feedback for the extruder rather than a stepper?

The biggest advantage I see is that the gearmotor can be 50%+ lighter than a stepper, thus allowing for faster print speeds.

Also, in the case of a bowden extruder, the encoder could be placed on some idler wheel at the print head itself to compensate for the hysteresis.

Thoughts?

Yes reprap and makerbot started with DC gearmotors. The problem is the motor spins very fast so it cannot reverse instantaneously like a stepper spinning slowly can. The small motors also wear out and quickly and also generate a lot of RF noise which has to be suppressed.

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[www.hydraraptor.blogspot.com]

Even a small motor? i.e., [www.pololu.com]

I can of course suppress the RF noise with some caps. Just thinking that it would make the extruder MUCH lighter (and probably smaller as well), giving a mix of benefits between a wades and a bowden extruder.

Yes even a small motor spinning at say 3000 RPM cannot reverse in the time it takes a stepper to do one step. When we extrude the motor is moving relatively slowly but at the start and end of a line we fast forward / reverse about 10 times faster, so you actually need quite a powerful motor and a wide speed range.

Build quality is much better since the reprap world moved to stepper driven extruders.

Here is how I did it: [hydraraptor.blogspot.com]

And here is how I suppressed the RF, it takes more than just caps: [hydraraptor.blogspot.com].

Here is a lightweight stepper design [hydraraptor.blogspot.com]. It could extrude well enough, but the fast forward and rewind were a little slow making the ends of lines a bit blobby. I might revisit it sometime.

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[www.hydraraptor.blogspot.com]

Thanks a lot, nophead! Your work is great!

What do you think about bowden extruders? I see the clear advantage of making the moving head smaller/lighter/faster, but keep hearing about problems with hysteresis. I was thinking having an idler wheel pressed on the moving filament right before entering the extruder nozzle, with an encoder mounted there. Then we can use a PID loop to compensate for the hysteresis. What are your thoughts?

The hysteresis is pretty non-random.

I don't think you would need an encoder, just a proper mathematical model of the flow rate, with parameters tuned experimentally.

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www.Fablicator.com

I don't have any experience with Bowden extruders. I prefer quality over speed so they have never appealed to me.

Yes I am sure an encoder could be made to work but you always have a dead band that you have to jump past, so you need a very fast drive to mitigate it. PID loops are great in the hands of experts, but as they need tuning they don't work well when rolled out to a community of reprappers with wildly different machines. As it is many people struggle to get decent quality prints when all you have to do is use the correct temperature, layer height and flow rate. Just think what it would be like if users had to tune a PID loop to get good quality.

I think the way Ultimaker get round it is to not try to stop the filament flow, they just move very fast between extrusion so it doesn't leave strings.

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[www.hydraraptor.blogspot.com]

I do not want to sacrifice quality, either. I do, however, want to try to push the envelope of speed. Especially with larger prints.

I agree that PID tuning can be a mess when everyone has different configurations. However, if we can normalize the PID inputs by scaling factors such as extruder gear sizes, feed rates, etc. then maybe default PID gains could work for most people.

That is a good point about ultimaker, though. I suppose with fast enough movements, you can mitigate the need for extruder reversal by simply "jerking" the hot end away from the print when you need to move to another part of the print.

asifjahmed Wrote:
-------------------------------------------------------
> Has anyone tried using a torquey DC gearmotor with
> encoder feedback for the extruder rather than a
> stepper?
>
> The biggest advantage I see is that the gearmotor
> can be 50%+ lighter than a stepper, thus allowing
> for faster print speeds.
>
> Also, in the case of a bowden extruder, the
> encoder could be placed on some idler wheel at the
> print head itself to compensate for the
> hysteresis.
>
> Thoughts?

Hi,

the topic is old, however, IÄm also currently investigating bowden extruder setups. For me, the goal is neither increasing speed, nor decreasing print quality - my goal is to have more room/much smaller print heads, as Im targeting multi heads.
The hysteresis seems to be the hardest problem for this type of extruder, hence, an encoder at the head would be a pretty nice idea. However, instead of coupling it to a DC motor, I would just use a stepper and some electronic... Does anybody try that or has made progress in that directions?

Cheers