Re: Servo motor selection

Mariss,

Thanks again for your great 'top 10' post. Really awesome stuff. I
have it printed out and sitting right next to me actually...

I've gone through your calculations and they make perfect sense.
Except for one thing, I'm not sure where you got 6000 RPM, but
according to the Camtronics site, these motors are 900 RPM. Of
course, this is significantly less, and things don't look good from
there.

Did I miss something? I do see that they say 50 oz-in per amp...
but they don't mention how many amps the motors are rated for as far
as I can find. Looking at the power supply, they show 25amps. Is
it a good assumption to think each motor is about 5 amps? That
would mean 250 oz-in, which would mean about 155 watts.

Also, the machine I'm getting is 10tpi. If I use 2:1 reduction
instead of 4:1, that would pretty much make the equations come out
the same. These motors can come with a 500 CPR encoder so I think
I'm good there for .0001 accuracy.

I'm probably off in left field somewhere, so please set me straight
if I've managed to confuse the situation again... I'll get it
sooner or later.

Thanks again.

Jake

--- In CAD_CAM_EDM_DRO@y..., "mariss92705" <mariss92705@y...> wrote:
> Jake,
>
> Those are good motors. Observe the continuous torque rating of 50
in-
> oz. Think in terms of power, as in Watts. 50 in-oz at 6,000 RPM is
> 222W (torque times RPM divided by 1351 equals Watts). That also
> equals 3/10 of a HP (222 / 746 = .297HP). 1HP also equals 550 ft-
lbs
> per second or 396,000 in-lbs per minute. You have 118,800 in-
lbs/min
> at your disposal. This means if you want 300 IPM, you have 396 lbs
> of "push" available (118,800 / 300). That takes care of the power
> part.
>
> Now for the reduction ratio. From the previous example, you want
300
> IPM. Assuming you have a 5 turns per inch (5 TPI) screw, it would
> have to turn at 1,500 RPM (IPM times TPI). The motor turns at
6,000
> RPM so the reduction ratio is 4:1 (6,000 / 1,500).
>
> Now a reality check. A 4:1 reduction on your 50 in-oz motor
applies
> 200 in-oz on the leadscrew. Force (lbs) = in-oz times 6.28 times
TPI
> divided by 16. For you that is 200 times 6.28 times 5 divided by
16
> equals 392.5 lbs; close enough to 396. Use non-backlash reduction
> (toothed belt).
>
> Finally, the encoder. Say you want 0.0001" resolution (10,000
counts
> per inch). You have a 4:1 reduction and a 5 TPI screw, so the
motor
> has to turn 20 revs to move 1". The encoder has to have 500 counts
> per rev (10,000 / 20). Since there are 4 counts per encoder line,
use
> a 125 line encoder (500 / 4).
>
> The motor has a 600 in-oz stall torque. This is 12 times the
> continuous torque. That means for brief periods of time (1/2 sec)
you
> would have 4750 lbs of "push" available (12 times 396).
>
> Mariss
>
>
>
>
> --- In CAD_CAM_EDM_DRO@y..., "jbordens" <jake@a...> wrote:
> > First, an introduction...
> >
> > I'm Jake, and I'm new to the group. I'm currently waiting for
my
> new
> > Grizzly G3012 mill to arrive. Its backordered, so I've got a
few
> > weeks till it arrives.
> >
> > I'd like to do a motion control setup for it. I'm pretty handy
> with
> > electronics, and I understand most of how this works. However,
as
> > for chosing motors I'm at a loss.
> >
> > I'd like to thank Mariss for the wonderful top 10 list. Its
helped
> > me greatly. To quote:
> >
> > "7. Bewildering choice of motors, encoders, servodrives"
> >
> > I'm offcially bewildered. I'm deathly afraid of taking all this
> time
> > (and money) and having underpowered motors.
> >
> > The camtronics people sell peak-600oz/in motors 50oz/in per amp
for
> a
> > price I can afford. My question is: is this a good way to go?
> Will
> > these be strong enough to handle a mill this size? Should I go
> with
> > a direct drive, or timing belts? Is there a good conversion
factor
> > from stepper to servo oz-in ratings?
> >
> > Any help is appreciated, and I do appologize for the post. I
> haven't
> > seen a "guide to chosing motors", other than a really confusing
bit
> > about coiled ropes and such that I found in this forum. Maybe
I'll
> > put something together with all my notes once I've gained some
> > experience.
> >
> > Thanks for your time.