Re: Servo sizing, determining required acceleration
Originally Posted by
double0jimb0
@joeaverage Great info on inertia ratio, there is almost zero discussion of this on the online/hobbiest forums, and I think a lot of disappointment after builds don't meet expectations. I don't know the math, maybe you can add to this:
A servo engineer explained to me that the "control authority" (my words, not his) of the servo goes up at something like a x^3 rate with increased gear ratio. So a screw that is driven by a 1:1 coupler with have 8x less control authority than a screw with 2:1 coupler. This comes down to the increased available torque and the increased # of resolution steps for the servo do its algorithmic magic. This translates hugely into stiffness/accel/cut quality/MRR/etc.
Also, what max rapids can you achieve with the 3205 ground screw you have? What length of screw? And fixed-simple/floating bearing config? I'm trying to spec a ball screw for 1650mm of travel, 40kg moving mass, .25g accel min (.5g would be great, rest of machine weighs 800kg, bolted to ground, boxed steel construction). Did you find the online calculators to be accurate with respect to critical speed? (no one lists what mounting accuracy requirements are needed to achieve these numbers, so I have no idea how to add a fudge/safety factor).
Thanks!
Reflected inertia changes by the square of the gear ratio.
Look up "reflected inertia"
This affects the inertia that the servo motor "sees" / the inertia ratio
Inertia ratio is important for control stability.
Too high a ratio and the servo will not perform as well / longer settling times / resonance.
But gearboxes / belt reductions often have downsides. Backlash, flex etc.
(I am just a hobbyist, not an engineer, so my understanding is limited)
7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)