Re: [CAD_CAM_EDM_DRO] Re: Calculating stepper power

Marcus and Eva wrote:

>Hi All:
> I have an application for a stepper that needs to be able to lift and
>lower a load repeatedly at frequent intervals for long durations using a
>ballscrew.
>Here's the stats:
>Weight to lift...100 lb max
>Pitch of screw...0.200"
>Speed of lift or lower... 20" per min max
>Belt reduction...1:2.
>Typical motion stroke... 0.020"
>Typical duration of action ...8 hours
>
>My question is: how do I set up the calculation to determine the size of
>stepper I need assuming negligible friction in the guide rails (linear ways)
>and screw (ballscrew) and allowing a decent margin of safety.
>The application demands good response time and I don't want to let the
>system get hot, so I'm thinking to overbuild rather than go too skimpy.
>
>

First, you need to figure out the acceleration required. Will this be run
by a motion control program, or a 555 timer or somthing? A motion control
program that can ramp up the step rate at the beginning and down at the
end of the move will perform much better than something that just suddenly
turns on at a constant step rate.

If you assume the screw is really just a drum with a string wrapped
around it,
with a circumference equal to .2", you'll have the same torque as the screw.
Since you spec a 2:1 belt reduction, you can assume the torque needed by
the motor is half that at the screw. That diameter would be .064", for
a radius
of .032" and the steady state torque would be 100 Lbs * .032" = 3.2 Lb-In,
or 51.2 Oz-In. With the reduction, 25.1 Oz-In would be needed. Assuming no
great acceleration is required, and you won't be starting abruptly at
constant
velocity, something around 250 Oz-In should do fine.

The stroke is .020" ? That's only 1/10th of a turn of the ballscrew!
1/5th of
a turn of the motor = 40 full steps. Hmmm, not a lot of time for
acceleration-
constant vel-deceleration ramps. Worst case is 20 steps accelerating,
20 steps
decel. So, you have to go from zero speed to 20 IPM in 20 steps, or from
zero to 667 steps/second in 3 milliseconds. I'm not sure this is a good
application for a stepper motor! I think maybe you should start looking at
servo systems.

Jon