Re: [CAD_CAM_EDM_DRO] Servo Motor Sizing

Greg Maxwell wrote:

>All,
>
>I am looking at the Gecko servo motor drivers. How do I determine
>the right size servo motor per my application? I will be controlling
>the X,Y,Z axis of an older Bridgeport Knee mill.
>
>

You need to figure out the top speed you want to move at, and the
greatest force you will need to apply to the table. For a 1 Hp Bridgeport,
I worked out the worst case linear thrust at about 1000 Lbs, and then worked
from that figure. For a particular motor, your required rapid traverse
speed
and the max RPM of the motor determines the belt reduction ratio to use.
Given that, you then can convert motor peak torque (times belt reduction
ratio) into leadscrew peak torque. You can then convert that to linear
force on the table. Or, working backward from requirements, the
leadscrew torque (in Oz-In) = (8 * Lbs Force) / (Pi * leadscrew TPI).
Motor torque would be leadscrew torque / belt ratio

for 1000 Lbs and a 5 TPI ballscrew, you get 509 Oz-In
With a 2.5:1 belt reduction ratio, you get 509/2.5 = 204 Oz-In
If you want 120 IPM rapid traverse, you need 600 RPM on the leadscrew,
With a 2.5:1 ratio, the motor needs to spin at 1500 RPM. Since the Gecko is
limited to 80 V, the motor must have a Kv of no more than 53 V/1000 RPM
to achieve that speed. (Actually, a Kv of no more than, say, 40 V/1000RPM
would be good to give some margin for losses in the drive and the motor's
IR drop, etc.) The Gecko is good to 20 A, so assuming the motor could
handle
the full 20 A, then a Kt of 204 Oz-In/20 A or 10 Oz-In/Amp would just
work. A Kt greater than this would be desirable, a Kt less than this would
prevent you from developing the desired 1000 Lbs of force.

Jon