Re: [CAD_CAM_EDM_DRO] Re: age old question

cnc_4_me <cnc4me@...> wrote:


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

/// Mounting the encoder on the motor shaft is the same as a fine feed adjustment of any measuring instrument. 3:1 ratio reduces any motor positioning error by a factor of three due to the increased reslolution. Ex: 3000 counts on the motor vs. 1000 counts mounted on the screw. Which would be more accurate radially? Timing belts are a very accurate means of transmission in terms of positional accuracy. They dont stretch or shift on the pulleys. I zero out a handwheel on an axis exactly on the hashmark then MDI 2" and the zero is lined up exactly with the hashmark again. Unfortunately, no Bridgeport is able to take advantage of all this precision under any cutter loading. Very close but not that precise. Even the supersonic machining centers only claim .0005" repeatability. My B'port is repeating within .0001" at the screw. They wont perform it at the cutter though. Lock the table down with a handcrank on the ballscrew then rotate it back and forth. It feels springy, thats deflection
in the yoke and endplates and torsional twisting of the screw itself. The same reason they deflect during machining and hold a thou or two at best. Any better than that involves grinding. I'm in the process of designing a set of servo mounts for Bridgeports. I'm going to retain the factory steel endplates in lieu of the common one-piece cast aluminum housing. Aluminum deflects under load much more than steel. Ron///


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis"
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

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