Hi RCarlyle,
thanks for your comments, as with all longer threads it gets difficult to catch up.
ad 1) I have described the problem, as short as possible writing above:
The steppers can only provide a certain power, which in the end decreases with speed. Because of power = force * velocity = mass * acceleration * velocity --> if the power is constant, the max possible acceleration has to drop with 1/v. This gets more and more important when moving at higher speeds (e.g. > 150 mm/s), because there the stepper power falls off significantly - but we would need more.
This is relevant for quick printers when printing and especially for travel moves. The problem as with nearby all new improvements is, that people have problems to stop thinking the old way. We should not care how things are done now, if we can do it better in future.
You claim:
More practically, if you actually run the numbers on nominal acceleration force demands versus motor torque capacity, most 3D printers have 10-50x torque safety factors
What is your base/reference torque ? The given numbers are just not correct, for a somehow efficient printer, they are far too high and i assume you just judging by "how much torque is needed in theory to accelerate the inertias compared to not losing a 1/16th micro step". If one compares the needed torque at 1/16 micro stepping to accelerate with 0.1g at 100 % stepper current (nobody uses), it might look that way - but why should we bottleneck us artificially ? For sure, one can buy a Ferrari and limit the 500 kW engine to 50 kW - but why should we ? And would we buy a DUET ? For sure you could build a printer only using 1% of its possible torque - but we should make it as good and not as bad as possible.
Yet, the only way to avoid ringing with "old school jerk" (without using low jerk values) is adding damping by friction. I use e.g. pre tensioned igus tribo tapes and if you measure the force needed to slide them and not losing 1/16th micro step, you will notice that there is no 10-50x factor at all. In the end increasing damping (and jerking the old way) gives more or less "junction deviation" results in terms of print quality.
Also, we don´t talk only about fdm, tell a CNC guy that he has 10-50x more torque as he needs at higher speeds....
There are not only printing moves. 1/16th micro doesn´t matter at all when traveling. Deltas may need to z-hop, but especially xy/z printers can utilize their full potential. The only way i know to avoid stringing with TPU/TPE etc. are damn fast travel moves. If you accelerate with 10g up to 400-500 mm/s, there is not much stringing left. But it would still decrease if i could use 20g and 1000 mm/s. Also, decreasing travel time can dramatically reduce printing time - depending on the model. These values are for my printer, but you can adjust them to any other printer.
You talk about safety factors - i want to increase them when it is needed (at higher speeds) and not to choose them completely inefficient and still not having enough safety when we would need it.
Don´t forget the new move plan. When the negative side of jerk is reduced, people will utilize this potential and use "the new jerk tuning parameter (junction deviation)" to increase speeds and accelerations. Not to forget, that the new move plan - if it should not slow down printing speed - needs to use higher acceleration values. By that everybody will dig dipper into the area where torque drops with speed although we would need more torque.
People want G2/G3 - although as shown above - G2/G3 influences a very small percentage of the total printed filament length. If G2/G3 is included but not acceleration = f(speed), than this is not efficient at all.
I did design my printer from the very beginning to be able to print at max. possible acceleration to get constant extrusion. My fine printing acceleration is 1g and for draft prints i use up to 5g. Travel moves are done with 10-20g. Because of travel moves i have to do gcode processing and split large moves into more segments with different allowed max speeds, or i have to limit the max. speed to avoid coming into the "torque drop zone".
The problem nowadays is not to build a stiff printer, the problem is the hotend/extrusion part. Why should we artificially bottleneck us ?
ad 2) Ringing only occurs when braking (at the end of a move) and not when the speed increases!
I have written above:
I assume, that as long as we don´t notice ringing, we don´t care about it.
With the old move plan when increasing speed at the start of the ramp, when the acceleration is increased stepwise, this will generate a bump and oscillations - but these oscillations are in the "line of the path" (at least for cartesian printers). So all they do is to vary the amount of extrusion per length, which is compared to the overall extrusion error nothing.
ad 3) With junction deviation coming there is no need to talk about that. But i disagree, in short terms, if what you say would be correct, all adaptive car suspension systems would not work;) In general the cheaper ones of these systems work only by adjusting the damping factor and not the spring stiffness, but it doesn´t matter how you reduce/increase the force at the right point in time as long as you adjust it properly.
The point is to start with a lower spring stiffness (beginning jerk) and end with a higher spring stiffness (when oscillation would start) compared to normal. If you think in a step response way, the low stiffness at the beginning would lead to a later and slower crossing of set point and actual value. After crossing the set point value the overshoot would start and than (or just before) the spring stiffness is increased. This would lead to a bigger "junction deviation". Without reducing the stiffness at the beginning, the speed - when the overshoot occurs - would we much higher and by that the kinetic energy the system has to absorb also --> more ringing.
Low stiffness at start (--> low speed when overshooting) and high stiffness at the end (nail it down) would definitely reduce the negative side of jerk. As you and i have written, the torque stiffness (stepper) is also only one part of the game. But the stiffer the printer, the more important it gets.
The potential of the above is small and it is obsolete with the new move plan including junction deviation.
