extruder steps per mm

I'm using a direct drive extruder with a beefy high-torque stepper. However, missed extruder steps have been an issue for me with this design -- not sever, but definitely an issue, especially as the feedrate goes up.

I had been driving the stepper in 1/16 microstepping mode. Microstepping trades off torque for resolution, so I decided to try extruding in full and half stepping modes, just to see the impact on performance.

The motor seems to be doing better in half stepping mode, however, something now seems to be wrong with the feedrate. When I call for 5mm of filament to be extruded via PronterFace, the motor speed is *way* too fast: I've set it down to 1mm/m, and it still extruders 5mm in about 5 seconds. I have updated the steps_per_mm array in my firmware (an old version of marlin). Is there another variable somewhere that I've forgotten about?

Going from 1/16 to 1/2 step, you should be able to use your old value divided by 8. Cant think of anything else.

Quote

Microstepping trades off torque for resolution

Not really. 16 microsteps at 1/16 result in the exact same motor currents as a full step. To avoid step losses, watch out for mechanical fricition and make sure your firmware works properly. Acceleration is often set too high by default.

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Traumflug Wrote:
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> Microstepping trades off torque for resolution
>
> Not really. 16 microsteps at 1/16 result in the
> exact same motor currents as a full step. To avoid
> step losses, watch out for mechanical fricition
> and make sure your firmware works properly.
> Acceleration is often set too high by default.


+1 Micro stepping does not decrease your torque per full motor step. If it doesn't have the torque at 1/16 then there is no reason to do with micro stepping which would help it at 1/2.

The 1/16 step itself moves with less torque, but if you held your motor shaft still and kept stepping for eight 1/16th steps, the torque trying to turn it would be the same as a half step, and if you did 16 steps the torque would now be the same as a full step.

My reading suggests that it's actually a fairly complex issue: [www.micromo.com] :

"...if the load torque plus the motor’s friction and detent torque is greater than the incremental torque of a microstep successive microsteps will have to be realized until the accumulated torque exceeds the load torque plus the motor’s friction and detent torque.

Simply stated, taking a microstep does not mean the motor will actually move! And if reversing direction is desired a whopping number of microsteps may be needed before movement occurs. That’s because the motor shaft torque must be decremented from whatever positive value it has to a negative value that will have sufficient torque to cause motion in the negative direction."

There's also this article, which goes to the trouble of normalizing the input current into the motor under different stepping modes, in order to analyize impact on torque: [www.designnews.com] (not something that the pololu drivers can do, AFAIK, but interesting.)

I'm definitely not an expert here, and may well not fully understand the issue.

What I do know is:
- my extruder motor loses steps,
- the thumb screw which holds the delrin bearing against the filament has a tendency to loosen. I've recently put semi-permanent thread-locker on it, but it still seems to be a problem on long prints.
- I suspect that the vibration from missed extruder steps is one of the major elements that works to loosen the thumbscrew.
- In half stepping mode, a manual extrusion command doesn't seem to work correctly via pronterface. Printing a gcode file, though, does.

I'm in the process of getting parts together to switch to a different extruder (maybe Greg's Wade's) but I'd like to keep the machine up and running till then.

The first article isn't relevant because with only 1/16 microstepping the incremental torque is always enough to overcome the friction in the motor and causes it to move.

The second article doesn't make any sense. It isn't the average currents that need to be the same, for the same torque, it is the vector sum.

With pololu drivers the micro step setting does not affect the torque because the vector sum of the currents is always the same in any position.

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