Assynchronous AC motor as Axis Drive in CNC

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Thread Starter

Bhagwan Swaroop

Dear friends,
I have come across a typical application where, I need to move a big mass, the load inertia is big. The load or Column of the machine is 25
tons in weight. A lot of friction in machine slides. I need to control the axis through the CNC. The Synchronous AC servo motor is costly
and I don't need very dynamic response, the moment of inertia of motor is another problem. I am thinking of using Assynchronous AC in
duction motor in Close loop using Linear scales as feed back device. but my worry is the position accuracy. I need high position accuracy.
Any suggesions ?? Ask for more details kindly mail me at : [email protected]
 
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Many people confuse accuracy with repeatability. Accuracy is like always hitting the bull's-eye (under many varying conditions) and repeatability
is like always hitting near the bull's-eye (under limited varying conditions). Most apps only need repeatability. A repeatable system can be [re]scaled accordingly. First, you need to select the resolution of the linear scale according to the desired accuracy/repeatability. Most all modern closed loop drives and controls can be tuned to keep steady state pos error within 1-2 encoder counts. One key issue is the repeatability / accuracy and stability of your rotary to linear mechanics. The mechanics has to be suitable for what your trying to do. One reason why more and more OEMs are going to Linear Motor Servo Systems is to make major performance improvements by totally eliminating the rotary / linear mechanics that has a lot of accuracy/performance drawbacks. High force linear motors are on the market and well proven in CNC for many years now. However, a machine must be designed from the ground up to properly apply LM technology.
 
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Michael Batchelor

> Many people confuse accuracy with repeatability. Accuracy is like always
> hitting the bull's-eye (under many varying conditions) and repeatability
> is like always hitting near the bull's-eye (under limited varying
> conditions). Most apps only need repeatability. A repeatable system can

Not quite. Accuracy is being right. Repeatability is being consistent.

Consider a system which hits within a 10cm circle of the center of the bull's-eye in a grouping randomly dispersed verses another system which hits 2m to the left of the bull's-eye in a grouping within 2 microns.

The first system is more "accurate" than the second, while the second is more "repeatable" than the first. If my control system is trying to insert something into a 1mm hole, the "accurate"
system hasn't got a chance, while with the "repeatable" I merely need to index the target to the right by 2m.

For a more real world example of how repeatability is the main focus take a look at the number of old paper mills around the world that are run by keeping the processes "at the little pencil mark" to make good product. The numbers on the indicators in some of those plants could be removed and no one would notice.

MB
 
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Mark Blunier

> Many people confuse accuracy with repeatability. Accuracy is=20
> like always
> hitting the bull's-eye (under many varying conditions) and=20
> repeatability
> is like always hitting near the bull's-eye (under limited varying
> conditions).

No, that's accuracy again. Repeatability is how close to hitting the same spot, not how close to hitting the target spot. If you aim at the bull's-eye and always hit 3 inches high of the bull's-eye, that's high repeatability, but poor accuracy.

> Most apps only need repeatability.

No, most apps need accuracy.

> A repeatable system can be [re]scaled accordingly.

Which gives you the accuracy you want. If you have poor repeatability, you also have poor accuracy. If you have good accuracy, you will
have good repeatability. You can have good repeatability with poor accuracy, but you can't have good accuracy with poor repeatability.

Mark Blunier
Any opinions expressed in this message are not necessarily those of the company.
 
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Well, I'm not sure I agree your analogy is better than mine. You are comparing a 10cm bull's-eye target to a repeatable grouping within 2 microns. Apples to oranges comparison regarding real world motion concepts. In my analogy, it is logical to assume the accuracy bull's-eye is on same scale or even smaller scale as the repeatability scale - real world. OK from a dictionary definition but we are discussing motion concepts here.
 
I normally don't mix in when it comes to politics, religion, and sex, but the current difference of opinion reminds me of the story of 3
hermits living together in the same cave:

One day a horse passes the cave. A year later one of the hermits slowly says, "did... yuh... see... the... cow?" Another year passes by and the 2nd hermit says, "T'weren't... a... cow... t'were... a... horse!" Still another year goes by and the 3rd hermit says, "If... you... two... don't... stop... this... consarn... bickerin... I'm... gonna... leave!"

I suggest as a arbitrator, the IEEE Std 100-1992 "The New IEEE Standard Dictionary of Electrical and Electronics Terms" for the definition of Accuracy and of Repeatability! BTW, if you don't have access to the std, let me know, and I'll send you the definitions!

Regards,
Phil Corso, PE
(Boca Raton, FL)
 
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You should be able to do this application with a 10KW or less motor if properly sized, geared, etc. In similar applications, our experience with the synchronous AC motor has been excellent.
You can certainly use an AC induction motor with a vector controlled drive as long as you are willing to accept less dynamic performance (bandwidth). I don't really think you'll save a great deal in this size, though.

Tom Bullock

Thomas B. Bullock, President
Bull's Eye Marketing Incorporated
Industrial Controls Consulting Division
104 S. Main Street, Suite 320
Fond du Lac, WI 54935
PH: 920: 929-6544
FX: 920: 929-9344
E-mail: [email protected]
 
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Dobrowolski, Jacek

Hi,
Go to site: "www.sew.de/englisch":http://www.sew.de/englisch and look for tutorial: 'Drive Engineering - Practical Implementation Volume 7: Servo-Drives' on the Documentation page. It contains short comparison of different types of servo-drives. There's also presented way to calculate the positioning accuracy depending on the drive type, selected gearbox, etc.

Regards,

Jacek Dobrowolski, M. Sc. E. Eng.
Software Eng.
 
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