Re: [CAD_CAM_EDM_DRO] Re: Motor vs. Servo

All very good questions.

All the moving components- the motor, the screw, the table
it drives- have inertia of course.

The rotating part inertias are measured by
moment of inertia as I described before. The linear parts
such as the table and work can just be measured by mass.

It is usual to convert all these to equivalent moment of inertia
when selecting motors. That is... a heavy table driven by
a screw is the same as a larger screw with no table.It adds
flywheel effect. Imagine the coasting load backdriving the screw. (although
it doesn't have to backdrive to make this so)So all the inertial loads are
added to make total flywheel
effect at the point where you would connect the motor.

Ok, now lets imagine an expensive servo that has a rotor
moment of inertia a quarter of the equivalent moment of the shaft it is to
be hooked to. As you imply, this would be a waste of money. If you spent
even more for a motor with
half the inertia there would be virtually no improvement
in the acceleration of the system because the load would
dominate.

Now lets imagine that the servo moment of inertia is EQUAL
to the load moment. This is often considered optimum for
perforfance. The electrical losses in the servo are minimum at this point.
Doing this is called INERTIA MATCHING. It is often NOT the most economical
however. It is not required either.

In the third case imagine a servo that has twice the moment of inertia of
the load. In this case most of the torque is needed
just to wind up the servo rotor. The load is relatively small
and is just hanging on for the ride. The servo dominates. This is also a
waste of money because the machine probably is
not being used anywhere near its capability.

You might think that a heavy mill table would have a much
higher equivalent moment of inertia than any reasonably sized
servo armature but this is often not the case! Just to put some rough
numbers on it a typical 3/4 hp 1750 rpm dc brush motor has enough POWER and
TORQUE to drive a Bridgeport type leadscrew/table directly but its rotor
moment is about the equivalent of a several THOUSAND pound load. So this is
a
case 3 type of thing. The motor just can't accelerate its
own rotor fast enough (even unhooked from everything)
to move the table in a manner that will allow reasonable
feeds and speeds in cnc milling.

Will it work? Yes probably. But feed and spindle speeds
will have to be much much slower than a typical machine.

So then what motor/servo specs are right for say a milling machine x-axis?
Well as I said that has to be calculated from
an analysis of the friction, speed, inertia, gear ratios, leadscrew pitch
and critical speed.

I can say that for a bridgeport longitudinal table travel the
D.C. brush motor (for direct drive) will roughly be as follows:

Rotor moment of inertia: .05 - .2 in-oz/sec^2
Continuous(stall) torque: 300 in-oz +
Max rpm at rated voltage: 1500-2000 rpm
voltage and torque constant to match driver

This is just for an example- don't use it to select a motor.
It might be different depending on how tight the gibs are,
whether it has ballscrews, and what type of work is to be done. Again it is
safest to measure the parameters and
calculate. Given the information, I and others on the list
can do that and even have or can find the right motor.
I have to charge a little bit for this or I will starve.

Uh oh... UPS just drove up with my SERVO TO GO card!!!
Yikes that is a light small package for $900 worth of stuff.

Bye!

Les



Leslie Watts
L M Watts Furniture
Tiger, Georgia USA
http://www.rabun.net/~leswatts/wattsfurniturewp.html

----- Original Message -----
From: "jtfrimenko" <jtf@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Friday, December 28, 2001 10:37 AM
Subject: [CAD_CAM_EDM_DRO] Re: Motor vs. Servo


> Since the motor is connected ridgedly, through the lead screw, to the
> work, how does the inertia of the work affect motor inertia? A
> bridgeport table must weight several hundred pounds and is sliding on
> gibs. Driving a high inertial load with a low inertia motor seems to
> be inefficient. If we were doing pick-and-place with a mill table,
> then a fast motor would be necessary.
>
> The next question, given a typical feed rate and screw pitch, what is
> the minimum motor characteristics required. High speed slew is not
> necessary for a home machine.
>
> john
>
> --- In CAD_CAM_EDM_DRO@y..., "Les Watts" <leswatts@r...> wrote:
> > To me the biggest distinction between a dc brush motor
> > being called a servo or just a motor is its armature
> > MOMENT OF INERTIA.
> >
>
>
>
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