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

There is no 'spring' in a stepper motor. With a stepper motor, with a
microsteppering drive as you command it to move from microstep to microstep
the stepper motor will not go 'exactly' to where you tell it; it will be
within a few microsteps. With a servo motor you have much the same problem,
it will never be 'exactly' where you tell it to go but it is close (within
the following error). A servo motor actually never stops moving either it
oscillates to maintain its position.

'Getting closer to the target' is a function of the drive reduction, the
deg/step of the stepper (or for a servo the CPR of the encoder combined with
the motors tendency to cog).

Neither a servo nor stepper motor is inherently better or more accurate.
Both have advantages and disadvantages and both will work equally well when
used properly.

Jeff Birt
Soigeneirs.com

-----Original Message-----
From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Roland Jollivet
Sent: Friday, February 25, 2011 12:49 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] How do you compare steppers to servos

Well, a stepper system is open loop, and there is 'spring' in stepper motor.
With a micro-step drive, giving say 1600 steps per rev (200x8), you could
turn the rotor slightly, and no-one will be the wiser. A servo system
however, will hold position far closer to target position.

If the rest of the system is sloppy, then it's a moot point.

Regards
Roland

On 25 February 2011 20:13, Jeffrey T. Birt <birt_j@...> wrote:

>
>
> 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
> >
>
> [Non-text portions of this message have been removed]
>
> [Non-text portions of this message have been removed]
>
>
>


[Non-text portions of this message have been removed]



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