About acceleration in general, be it steppers or servos. Most people accelerate way too slow. In most cases it shouldn't take longer than 1/4 second to go from zero to full speed. Here's the math:
Let's say you are using a NEMA-23 stepper capable of 50W output (easy). Let's also assume you are shoving around a frictionless 100 lb mass and you want it reach 120 IPM. How long should it take?
Let's also assume a linear rate of acceleration. This is safe because most CNC programs have only this option. It's not by any means optimum, but what it lacks in complexity it makes up for in simple mathematical analysis.
We solve this problem backwards. What force is applied to a 100 lb mass at 120 IPM when 50W is available?
What we have to work with is this identity: Watts = (Lbs * IPM) / 531
Rearrainged: Lbs = (531 * Watts) / IPM = (531 * 50) / 120 = 221 Lbs.
That is a lot of force on a 100Lb load! What does that mean? 100 Lbs of force on a 100 Lb mass will accelerate it at 1G. 221 Lbs of force will accelerate a 100 Lb mass at 2.21 G. That's really accelerating.
What is 1G of acceleration? It's 32 feet/sec^2. Or, it's about 23,040 IPM per second squared.
A lot of people get hung up on the "/second" or "per sec^2" stuff. I know I did until I finally understood math and physics years after I was out of school. Like all really important stuff, it's actually easy.
You light the fuse on a rocket that will accelerate someting at 1G. You look at it 1 second later. It will be moving at 21.8 MPH. Ten seconds later, 281 MPH, 100 seconds later, 2,182 MPH and so on.
Anyway, you are not going to thousands of miles per hour, you are going to 120 IPM (sigh). To make up for it though, your acceleration is 2.21 G. So how long will it take already!!?
Simple. 1G = 32-ft/sec^2 = 23,040 IPM/sec^2. 2.21G = 50,918 IPM/sec^2.
You are going to only 120 IPM so, it will take 120 / 50,918 or 0.0023567 seconds to get to 120 IPM.
That is only 2/1000th of a second! Your CNC software almost certainly can't accelerate that fast in it's program. But that's what a puny 50W NEMA-23 motor working into a 100 Lb can do.
A lot of practical things can tarnish this theoretical value, inertial load (mostly in the motor and direct-coupled load). Even 100 times (practical), it means you should be able to accelerate to full speed in 200mS (100 times 2mS).
1/5 of second to 120 IPM. Only 1% of an inertial-less theoretical system capability.
Mariss  |