Interesting reading - Presure in advance
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@fma said in Interesting reading - Presure in advance:
It is also nice to be able to print fast when prototyping, making several iterations to tune dimensions. I go up to 120-150mm/s, with 1 perimeter, 10% infill, so small parts take less that 15 minutes to print.
As you, I don't print with nobody at home!
Yes I do that a lot too. That's why I print on removable glass and why I have 3 sheets of it - so that I can slide one out when a print finishes, slide a new one in and be printing again, in less than 2 minutes.
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The irony here is that I checked my direct drive delta kit's printer settings and found my accel and "jerk" are also 1000mm/s^2 and 20mm/s:
M201 X1000 Y1000 Z1000 E8000 ; /sec, not /min
M566 X1200 Y1200 Z1200 E1600 ; /min, not /secI never realized that they were this high, but I never had problems so never tinkered with them. On the other hand, I've never even tried a print speed of 300mm/s. (I've done 120mm/s successfully, but prefer to stay around 80mm/s max print.)
So, I owe you a "thank you" for making me look at my configuration and realizing something I hadn't paid attention to.
(I wouldn't even dream of running my duet converted FFCP at these speeds. For that printer, I start to see quality losses at 50mm/sec.)
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@garyd9 Spooky how you happen to have the same values.
Actually, in the grand scheme of things, 1,000 mm/sec^2 is nothing much. Acceleration due to gravity is 9.81metres/sec^2 so we are are only talking 1/10th G.
In a previous life I did a lot of stuff with internal combustion engines. You probably know that a typical road car engine can do 6,000 rpm plus. For the sake of keeping the maths simple, the stroke might be 100 mm, and every piston has to start at the top, travel 100mm, reverse direction and travel back up to the top. So have you considered that at 6,000 rpm, it does that 100 times per second. Or 5ms for a complete up-down-up cycle or 2.5ms to go from rest, travel 100mm and slow down to a stop, before reversing direction and repeating. And that's nothing special either. F1 engines are now limited to (I think) 12,000 rpm but 18,000 rpm has been rumoured to be true in the days of 1.5 litre turbo engines. That's 2 to 3 times faster still. So in the world I used to inhabit, 1,000 mm/sec^2 is bu**er all.
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@deckingman ahh, but how much acceleration does your printer actually see during a corner jerk? Hmm
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@rcarlyle said in Interesting reading - Presure in advance:
@deckingman ahh, but how much acceleration does your printer actually see during a corner jerk? Hmm
Nobody likes a smart are*e
All jesting apart, I have mulled this over from time to time. It won't be infinitely high acceleration - that's impossible. All else being equal, I guess it depends on what "gives" (and something must). When a ball is struck by a foot or a club, the ball itself deforms at the moment of impact, then it accelerates until the force is no longer applied.
As an aside but interesting none the less, is that my (now deceased) father-in-law used to make arrows for traditional long bows. He was somewhat of an expert in his field and wrote a book on it. What is interesting is that, when an arrow is "loosed", it bends as it accelerates forwards. The optimum "bend rate" for accurate shooting is such that the arrow does 1 1/2 deflections before it leaves the bow. I won't go into the details but it's called "the archers paradox". The stiffness of the arrow is called the "spine" and a matched set of arrows will all not only have the same weight, thickness, taper and length, but also have the same "spine", and that "spine" will be matched to the draw weight of the bow. Surprising how much science there is in a simple thing like an arrow.....
So maybe the optimum "jerk" (I hate that term) for a 3D printer is one that matches the printer equivalent of an arrows "spine".
Another thing I wondered about (and you will know the answer to this better than I) is does motor rotor inertia play a part? I'm just thinking along the lines that the motors (on a CoreXY) will be spinning in one direction, then when there is a corner jerk, they will have to reverse direction, which can't happen instantly because of the inertia of the rotor. My gut feel is that the rotor inertia would be insignificant in comparison to the inertia of all the other moving mass but do you know differently?
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@deckingman said in Interesting reading - Presure in advance:
My gut feel is that the rotor inertia would be insignificant in comparison to the inertia of all the other moving mass but do you know differently?
If I remember correctly, the resulting torque is moment of inertia (kg·m²) x angular acceleration (rad.s⁻²).
Moment of inertia of the rotor is: mass (kg) x radius² (m²), where radius is taken at center of gravity of the section of the half rotor (R/2 for a plain cylinder).
Correct me if I'm wrong...
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http://www.softschools.com/formulas/physics/torque_formula/59
A quick check shows that the torque due to moment of inertia is 3 or 4 orders of magnitude lower...
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@fma Yes (and no). I wasn't thinking about torque, just inertia. i.e. when changing motor direction, does the inertia of the rotor play any significant part in how fast that change of direction can take place? Given that the motor is coupled to the gantry I suspect not, as the inertia of the gantry will be significantly higher.
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Mmm, for me, what prevents direction change is the needed torque due to inertia... same as linear accelerations, where you have a resulting force, which in turn gives a torque...
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If the drivetrain is designed properly, the load angular inertia seen at the motor (“reflected inertia”) should be between 1x and perhaps 10x the rotor inertia. If you’re outside that range, your motor is either too big or too small. (The rule of thumb for servos is 1-5x but they have closed-loop feedback stability issues to worry about which open-loop steppers do not.) * You get maximum possible low-speed acceleration capacity when the reflected inertia equals the rotor inertia. * If the rotor inertia is greater, it’s putting too much energy into accelerating its own rotor and not enough into the load. If the reflected inertia is greater, a different gearbox / belt / screw ratio would allow the motor to accelerate the load faster.
In practice, we don’t like to add gearboxes due to parts count and backlash, so the reflected inertia will be a lot larger than the rotor inertia. If it’s more than about 10x larger and you don’t want to change the drivetrain ratios, your motor is probably undersized, but that’s just a loose guideline.
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Thanks for the explanation. So, in Ian's config, the load inertia is (if I do the maths correctly):
m x r² = 4 x 0.01² = 4.10⁻⁴ kg.m² (where r is the radius of the pulley)
I previously estimated the rotor inertia to 4,5.10⁻⁵ kg.m². So we are about 1:10 ratio, which is good?
