Re: age old question

I have a question about belt reduction for servos...Assuming the
servo has enough torque in each example below. What setup will have
the least positional error. I am looking at this from the prospect
of using cheap DC motors as servos. Or asked in another way, what
setup with these cheap motors will give better positional accuracy
with the same control...

Assume we want 1000 rpm on ballscrew.

1) 3000 rpm servo 100 oz-in continuous, with 3-1 reduction...

2) 2000 rpm servo 150 oz-in continuous, with 2-1 reduction...

And finally, were will encoders give us the best control on these
setups, on the motor, or on the ballscrew...


My personal thoughts are the 3-1 reduction with the encoder on the
ballscrew will give better positional control...

With the encoder on the ballscrew the control will see a higher line
rate per rev of the motor.

With a 3-1 reduction the motor itself can have a larger positional
error (1/3 more) than the 2-1 reduction.


Wally


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis"
<mariss92705@y...> wrote:

>
> I really hate it when only torque is mentioned for a motor.:-)
>
> What gets things done is power, as in Watts. Power is torque times
> RPM. Specifically, Watts = RPM times in-oz / 1351.
>
> One horse-power is 746 Watts. One HP is the ability to lift (or
> shove) 550 lbs to the tune of 720 IPM.
>
> Servos are not step motors. Step motors are high torque, low RPM
> motors. Servos are the opposite, high RPM, low torque motors.
>
> They need reduction gearing to match a load. The servo must be
> turning 80% of no-load speed when your mechanism is moving at its

max

> design speed.
>
> Small NEMA-23 servos can provide more power than a NEMA-42 motor;
> 250W versus 150W. The thing is the small NEMA-23 servo develops

that

> power at 4,800 RPM (80% of a 6,000 RPM no-load).
>
> You want your big machine to motor along at 180 IPM on a 5 TPI
> leadscrew; that is 900 RPM on the screw. The reduction must be
> 4,800/900 or about 5.33:1.
>
> The NEMA-23 servo motor will have about 70 in-oz of continuous

rated

> torque. The screw will see 5.33 times that or a respectable 370 in-

oz

> and will produce 730 lbs of "push".
>
> Being a servo, it will provide 5 times as much torque and "push"

for

> short periods of time (<1 sec), 350 in-oz and 3,660 lbs

respectively.

>
> Reduction gearing is of paramount importance with servos. They are
> not steppers, don't treat them as if they were. You do, you will

burn

> down your motor.
>
> Mariss