RE: [CAD_CAM_EDM_DRO] How do you compare steppers to servos

How are servos 'more accurate'? It makes no difference if you have 1500 CPR
encoders with a high theoretical resolution as the actual 'accuracy' is a
function of the entire system (screws, frame, etc.)



Jeff Birt



From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of CNC 6-axis Designs
Sent: Friday, February 25, 2011 12:04 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] How do you compare steppers to servos





Same-same, but different, as they say.

If you need a high dynamic range, you need servos.
If you need lots of torque at speed, you need servos.
If you need about 100W or more of power, you need servos.

If you need either speed, or high resolution, you can do it with steppers.

For example, I am now making a very large industrial router, with
steppers. Money is not an issue.
Steppers are better in this app due to
-safety
-reliability
-simplicity
BUT, I only need high torque (very high max top speed) and low
resolution (woodworking).

260 oz-in steppers are usually pretty good Nema 23 ones, today.
So you can get a fairly large usable speed range out of them. Upto
1000-2000 rpm. depending (SS, 70 V DC, geckos).

Also, you can figure about 2/3 or maybe 200 oz-in of torque as needed
for the servos, as the stepper system will have needed some headroom.
Now, depends again very much on what you want to do.
If the system needs the continuous torque all the time (grinder, sander)
or sometimes (point-point machine, could be pick n place etc), the servo
requirements could be very different, for maybe 250-300 oz-in peak
torque or continuous torque.
The difference in servo size and cost is huge between 300 oz-in peak and
continuous.

A lathe would need little continuous (power) push, for example, as would
a plasma (peak loads, acceleration).
A router might need lots. 100 kg for me.
A mill might be 50 kg (Bp M-head, or 11 mm end mill ) to 200 kg (20 mm
end mill, Bridgeport Series 2).
My mill runs very well on nema 23 steppers. I have servos, unused, but
wont benefit from using them in any way.
This is because the mill is limited by end mill size (12.7 mm), milling
accuracy (0.01 mm) to physical lmits which I already have achieved
(more/less).
Using servos would not make better pieces, but could accelerate as much
faster as I wanted to pay for.
However, maximum smart rpm is at around 1000 rpm for the ballscrews
which means it would be an almost complete waste of money.
Even though the mill is big, servos would not help as I do not get lots
of back-n-forth rapids, and the time taken is spent machining.

Servos are inherently, usually, more accurate, faster, better
acceleration, much better top end torque.
They are more complex, much more dangerous, more sensitive.

So the correct answer is, it depends on how much torque does you system
require, at what speeds and what resolution.
The best solutions are balanced systems.
Making them is easy, but needs taking all 3 factors into account.

> Suppose a system is advertised using nema23 steppers of some size, say
> 260 oz-in.
>
> You want to build the same system but instead use servos.
>
> How do you compare the compare power of the the steppers to the servos
>
> thanks
> Glenn
>

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