RE: [CAD_CAM_EDM_DRO] Re: stepper accuracy compared to servos...

This discussion has brought up a number of things that are, in part, true,
but don't bring much insight to the issue. I don't know how to deal with
the misconceptions that are abound in this group, other than to state a few
facts:

Idea: Steppers are worse than steppers because they can lose steps.
False. If a stepper system is faced with a torque requirement greater than
the system can support, it will lose steps. If a servo system is faced with
a torque requirement greater than it can support, it will fault. I think
much of this problem comes from the common idea of running a stepper harder
and until it begins to lose steps, and then backing off a bit. Servo
systems would have a similar bad rap if people ran them so hard that they
faulted on a position error and then backed off a bit. It is simply poor
practice to run a system on the edge of failure, and loss of a single step
is tantamount to failure.

Idea: Steppers will always have lower dynamic response than servo
False. The dynamic response of a system is related to the available torque
and inertia, plus effects of the control law, feedback response, and a wide
variety of factors. Generic responses like "True servo systems will always
have better dynamic responses than any step/dir system" ignore the complex
engineering issues that are involved in such systems.

Idea: Steppers are like a mechanical detent
I would defend the analogy to the following extent: A mechanical detent is
highly under damped, subject to resonance, and is difficult to improve with
feedback or control law improvements. A mechanical detent is an integer
function, much like a step. Any system utilizing a mechanical detent clutch
is best operated at torque levels far below that required to snap over the
detent. A mechanical detent is simple, open loop, dependable, and
inexpensive system, thereby making it both useful and suitable to a wide
variety of applications, as long as the limitations are understood and
properly dealt with.

Idea: Commercial CNC uses servos, so they must be better.
Misleading. In reality, good engineering matches the properties of the
solution to the needs of the problem. The details of engineering control
systems cannot be explained in a few emails. I spend some years getting a
masters degree in such issues and I'm not smart enough to boil it down to a
few sentences. The most expensive CNC systems don't use PM servos, but they
use vector drives on induction motors, yet we do not conclude that we should
put those systems on homebrew CNC? To suggest that servos are better than
steppers is like suggesting that turbines are better than 4 cycle engines.
In fact, they are different. Controversy will exist on applications that
overlap such that either solution might apply, but neither is fundamentally
better than the other.

The real value of steppers:
Cheap and dependable

The real problem with steppers:
1) They only work with relatively small systems. 2) The dynamic response is
difficult or impossible to tune and always under damped. 3) Tuning the
dynamic response will normally require modifications to the mechanical
system. There is really no control law, just the laws of physics. 4)
Position errors beyond a half step will become permanent.

The real value of servos:
1) System dynamics can be controlled through a control law. Feedforward
gains, integral gains, and non-linear gain mapping can allow the control
system to adapt to the mechanical system. This includes close coordination
between the motion profiler and the control law. 2) Position errors can be
sustained over time or space without becoming permanent. 2) Servos can get
big. PM servos can be had to around 30 KW, steppers can barely deliver 0.1
KW. A Bridgeport machine is pretty much the borderline between suitable
applications. Machines larger than a Bridgeport will be very difficult to
do with steppers. Machines smaller may be suitable for steppers.

The real problem with servos:
1) They cost more. 2) They are more complex, more wires, feedback devices,
gain parameters. 3) Servos have higher maintenance issues. Steppers are
similar to induction motors in failure rates. Servos have much higher
failure rates.

Greg

> -----Original Message-----
> From: Paul [mailto:no.spam27@...]
> Sent: Friday, January 30, 2004 12:10 PM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: Re: [CAD_CAM_EDM_DRO] Re: stepper accuracy compared to
> servos...
>
>
>
> At first read, the original post looked like B.S., however, if the servo
> system in question is a G320/340 step/dir type drive, then yes,
> the dynamic
> accuracy is going to be suspect. The one point I would disagree
> with is the
> "you are guaranteed to have a dynamic accuracy of much better
> than one full
> step" statement. Your accuracy is only going to be plus or minus
> one step,
> never "better than". Factor in the angular inaccuracies of a
> typical stepper
> motor, say +/-5% (and even worse when micro stepping) along with dynamic
> errors, and steppers can not compete with TRUE servo systems.
>
> True servo systems, either full analogue or digital SERCOS, will
> always have
> better dynamic responses than any step/dir system. If steppers
> were really
> that good, everyone would be using them.
>
> Regards, Paul.
>
>
>
> On Friday 30 January 2004 5:19 pm, cnczeus wrote:
> > > (If you missed the whole post, it's worth reading IMO...)
> >
> > I believe you refer to the in motion accuracy of steppers to that of
> > servos and I must agree that steppers are better, however once a
> > single step is lost the remainder of the motion is far worse than the
> > worst of the servo drive.
>
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