I did choose WANTAI 42BYGHW811, in my setup, they should be able to achieve nearly 1g close to about 1500mm/s, without losing 1/16 micro step with a light direct driven extruder/hot end. I don´t have a "problem" at 160mm/s, this is just an example to show that if the torque drop starts with 12 V at "0" rps, this problem is existing in the normal operating range of 3D printer, just check the curves.
1. In 3D printers we normally run the motors well below their maximum current, because at maximum current the motors get very hot. At reduced current, the back emf due to inductance is lower, so the (reduced) torque is maintained to higher speeds. For example, if you halve the current, then the torque at low speeds will be half what is shown on the torque/speed curves and will be almost flat until it intercepts the torque/speed curve shown for the same motor and same driver voltage at full current.
This is not special for 3d printing, it is a good rule of thumb for every stepper application to use at max about 80 % of the specified amps. The steppers itself are normally rated to > 100°C surface temperature, I operate my steppers at about 80 % of the rated current and they get at max about 50 °C (also NEMA32s), which is at least for me no problem. Because P = IIR it is for sure sensitive to the chosen current - but it is also no trick to cool them and in the long term i will use mine at maybe > 100 % current.
Who operates his steppers at half of the design current? This 50 % comparison is completely misleading and even at 50 % and 12 V there is also before 5rps a torque drop. If somebody operates at 50 % i would suggest him to go for different (smaller and faster) steppers. But do these people buy a premium product like the Duet Wifi ?
2. Because we want precise motion during printing, we use 1/16 or greater microstepping, and to get position accuracy this means that the motors need to have far more holding torque than is needed to produce the acceleration demanded, so that we can maintain a position accuracy of around one 1/16 microstep. But high-speed travel moves don't need this accuracy during the high speed part of the motion, it's only the end of the move that needs to be accurate. So quite a large loss of torque is acceptable during the middle part of a travel move. If you know the mass of the print head or bed or whatever the motor is moving, you can work out the torque needed to produce the desired acceleration, and you just need to make sure the motor can provide this torque with a sufficient safety margin e.g. 2x.
Yes, that is one reason for the firmware change request, now we get back on track.
But i would say we don´t use 16 microsteps for finer resolution (with a 16 T GT2 pulley 16th means in theory 0.01 mm), because this fine steps never happen with this accuracy (e.g. stepper nonlinearities and all other mechanical errors of a 3D printer are much bigger) we just do it because it runs smoother and less noisy - and because it doesn´t cost anything. Maybe it can reduce some fancy patterns on deltas, but there only relative accuracy is needed. On xyz printers 16th stepping doesn´t improve accuracy at all.
I have to show more clearly what this request is for because it still seems to be not clear:
1. Many speed as well quality problems are extrusion related. Often people claim their E3D v6 cannot go above e.g. 50 mm/s because then under extrusion and so on is happening. They say the extrusion capacity is not enough - which is wrong. The problem is, that the hot end print speed varies and by that the extrusion rate. Unfortunately, molten plastics have a viscoelastic material law which brings the time factor in play. So any change in extrusion rate/speed is going to make problems - but this is not the root cause. The root cause is the varying printing speed. The root cause for the varying printing speed is a lack of useable acceleration which is limited by the mass/stiffness of the system and the stepper torque which is a function of speed .
Especially at lower layer heights and small nozzles or with e.g. a volcano, the extrusion capacity is enough for speeds far above 100 mm/s. But as soon as the speed drops the extrusion problems will occur. On youtube are enough videos showing damn fast prints - with a miserable print quality. I want fast prints with good quality - this is not possible without the maximum acceleration.
2. Many problems are retraction related. Why do we need retraction at all ? Because too much plastic would ooze out. The smartest way I know to overcome this problem is to give the oozing process no time. Half the time and you half the amount which could ooze out. If you print flexible material that is the only way to go. If you don´t need to retract (or less), problems of 1. are also reduced. Solve the problem by just increasing the speed of travel moves.
3. Think the other way round. With a Duet Wifi capable of acceleration as a function of speed, one could use smaller (faster) and cheaper steppers. This could mean one uses NEMA14 for normal printers or NEMA 17 instead of NEMA23 for big size printers.
4. printing time
Many prints need excessive amounts of travel moves. Normally most travel moves are short distance moves. Using a histogram to show the distribution of travel move length, it would be very heavy on the left side (much more short than long moves). What does short distance mean ? With 2g it takes 25 mm to reach 1000 mm/s, with 5g 10 mm. The problem is, that long travel moves need more time and increasing max travel speed would reduce the time significant. To increase max travel speed, i would have to reduce the max torque - also in the low speed region were most of the moves happen. Increasing max travel speed ends up to give no benefit, if the max torque on the low side has to be decreased.
A higher acceleration in the low-speed region (printing) means a higher average printing speed and again a reduction of printing time.
5. Jerk. Jerk was introduces to get away from blobs on every corner (bleeding edges) and to decrease printing time. Unfortunately jerk is the most brutal way and is mostly based on backlash and low stiffness. In the old days both was existing at much higher levels than nowadays. Using too much jerk shows ringing/resonance artifacts in the print (as well as too high accelerations). Using 2 g it takes 0.0xyz mm to get to e.g. 20 mm/s. But it is much more gentle and gives by that less artifacts - or at same amount of artifacts much more printing speed. Jerk could be reduced to nearby 0 and substituted by "soft" acceleration. Doing so would be like a poor mans s-curve acceleration in the very low speed region.
6. Future developments like s-curve acceleration/moves. Although some people thinks it is too CPU intensive, for me it is clear that in future we will use something like s-curve like moves. This gives much more possible printing speed at same artifact level. But using s-curve like moves reduces at the same max acceleration the average acceleration - so in total the benefit is much smaller as it cold be. To overcome this problem, acceleration has to be really maximized - according speed.
Finally:
I don´t care at all, what people are doing now, i question for what is possible, i want to push the limits - and premium products like the Duet are bought because the "normal" stuff is not as good as it could be. It is not true that printing speed is limited by extrusion rate to e.g. 100 mm/s. It is limited by too much varying printing speed, caused by too low acceleration settings.
Why should we bottleneck our printers? Why should we waste resources?
I don´t care if I am able to use 5g in the low-speed region (<= 1000 mm/s) already now if I can get 8g…. or travel with 2000 mm/s instead of 1000 mm/s.
BTW: On travel moves, the deflection of the printer parts doesn´t matter (nearby). When printing they become very significant at higher accelerations (e.g. > 1g), and more stiffness (a stiffer system means more mass) doesn´t help more. So finally the firmware will also have to address this problem because there is a mechanical material limit where engineering cannot increase the stiffness vs weight ratio anymore (see 6.).
[Edit: i added 5 and 6]
