Acceleration as a function of speed
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Yes, thanks and i will add it to the wish list. maybe Christmas is coming sooner or later.
In between, i can process gcode to do a proof (or fail) of concept. How many paths segments (or lines) can the Duet handle per second ?I still think it is not clear how this should help. dc42 you are still thinking and comparing "state of the art".
I repeated already several times, that most benefit comes when traveling not when printing - on this point we agree, but if "acceleration as a function of speed" is there because of travel moves, it would be stupid not to use it when printing.
I also think that because of missing stiffness, only some users will benefit.
The reason why i use 24 V, 2 steppers per axis and 1 one driver per stepper (it would be easy to connect 2 steppers in series to one driver at 24 V) is that i see no need at all to bottleneck my possibilities. I don´t build this printer to print at 50 mm/s, I bought such a printer long ago.The only way not to loose torque at 12 V is to waste resources by reducing the current to e.g. 50 % of design - like in your example. In this case, i could be proud that the torque doesn´t drop until maybe 5-10 rps, but only because i lost already 50 % of the initial torque. My target is not to keep the torque as much as possible constant by reducing it artificially, i am hunting for e.g. super fast travel moves.
I used the same calculation as you to choose my steppers, but we should never forget the difference between theory and real life.
check the torque curves -> http://www.electrocraft.com/products/stepper/TPP17/
TPP1729-A20 TPE17-45A15 TPE17-45A20
5 rps 18 Ncm 21 Ncm 26 Ncm
10 rps 15 Ncm 9 Ncm 16 Ncm
20 rps 7 Ncm 0 Ncm 5 Ncm
L mH 1,0 3,4 1,9
R Ohm 1,2 2,0 1,1
L/R 0,8 1,7 1,7
current 1,0 A 1,5 A 2,0 A1. It is wrong that torque doesn´t drop significantly with speed, just look above. Running at 80 % current doesn´t change these curves a lot. The faster you run in average the more the current is limiting itself, so you could increase the current to maybe 100 % because in average you don´t reach it anyhow - but the 100% torque at low speed would be there.
2. It is wrong to choose the smallest "possible stepper" IF somebody wants performance in terms of speed. The smallest one above TPP1729-A20 with 1 A has the highest torque at 20 rps - but i am not interested in the highest high-speed torque, i am interested in high torque from beginning to end, most moves are"slow" and therefore the biggest (TPE17-45A20 2 A) one is the winner in this example. The mass of inertia is missing here, so the winner of the netto high speed torque is still the smallest one, but it doesn´t change anything that until very high speed, the smallest (as well as the mid sized) is the "loser". In terms of price is the smallest not more or less expensive than the biggest (maybe 1-2 EUR…).
With a 16 T GT2 pulley at 1.8° 5 rps are 160 mm/s, 20 640 mm/s. With 0.9° 5 rps is about in the printing range.
In case of the TPE17-45A20 the torque drops from 26 to 5 Ncm from 5 to 20 rps (640 mm/s) - these values are not unreachable, in my case 1000 mm/s and above are usable. In other words, when i would hunt for 1000 mm/s, i would also have to reduce the low speed acceleration to about 1/10 of what would be possible..... this is just not necessary.
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Can you post a video showing that you can actually print reasonable quality parts at 1,000mm/sec using 5g acceleration please? Up until now, I thought that everything you were saying was based on some sort of theoretical scenario and conjecture. I didn't realise that you have actually done tests printing at 1,000mm/sec using 5g acceleration. So if you can actually prove that it works under real life conditions then that adds a whole new dimension to this discussion. I think all the makers of hot ends and extruders would love to see how their items can be used to print at far higher speeds than anyone else has been able to achieve. In fact, this would completely revolutionise the entire world of 3D printing and you will be hailed a hero. All you have to do is prove it in a rational scientific way, rather than quote theoretical. possibilities.
As mentioned earlier above, my complete z-axis "power train" is missing, i am waiting for parts, but in between, i am doing test.
So until now i didn´t print a single piece and i don´t think i will use 5g for printing because the deflections are too big (at least in theory, but jerk should also not work as good as it does in theory).Right now > 10g at up to 500 mm/s are no problem (no missing steps). I am now on/going to vacation and afterwards the postman should ring the bell.
In principle, if you use the full available torque when traveling and you don´t have z-deflections (that means you don´t hit the print, bad luck for deltas…), in my case 20 g could be possible in the low speed region.5g when printing could be maybe interesting to get rid of jerk, that means instead of bumping it, "gentle" accelerating it, for the 1st 0.0... mms.
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Ah, sorry. I thought when you said you were testing at 1,000mm /sec you were actually testing under real printing conditions.
By the way, you still don't understand how instantaneous speed change (what has become known as Jerk) works in 3d printers but hey, it's easy enough to set it to zero to test your theory that you can do away with it. Looking forward to seeing a video of your 1,000mm/sec 5g (or as you say in your case 20g) accelerations.
