Re: [CAD_CAM_EDM_DRO] cnc,linear encoder design question

See http://www.heidenhain.com/posmeas.html for a good coverage of the topic
of using linear scales for feedback.

IMO linear scales are an excellent way of providing position information as
long as the scale and the reader head have a constant alignment. On a mill
table for example if your table tends to droop when run out to the extremes
of its travel the grating on the scale will shift in relationship to the
grating on the read head as the table tilts. If this occurs you can measure
the error and compensate in your software. Most DRO's have this feature.

You should be able to set the dead band on your servos to avoid oscillation
due to vibration causing the scale readings to fluctuate. Of course the
vibration will still cause anomalies in your part.


At 12:47 AM 3/16/02 +0000, docholliday01201 wrote:

>I have read recently the post regarding the concept of using linear
>encoders for a do-it-yourself CNC conversion. It didn't seem to me
>that I saw clear answer to the questions I have had, myself,
>regarding this concept. Let me state my idea here in detail and
>solicit some specific answers to specific questions.
>
>
>
> My design principle is this: I have a Rong-Fu 35 milling
>machine that I wish to convert to CNC operation. My prior experience
>is with an open loop stepper system on a Sherline CNC machine. What
>I wish to do is to use a PM servo motor which drives through a belt
>reduction system, to turn a zero backlash precision ballscrews to
>move the X, Y,Z axes of the machine. Directly to the table will be
>mounted a linear incremental encoder. my intention is to use a U.S.
>digital system which is composed of a 360 line per inch Mylar encoder
>strip and read head. I am likely to use gecko drivers and as a
>result the encoders will tie directly to the servo drivers of each
>axis.
>
> I heard much talk regarding resolution and accuracy. As I
>see it from an engineering standpoint the accuracy of any given
>system is equal to or less than the most inaccurate part in that
>system. In a system of a servo motor, a belt drive, thrust bearings,
>ballscrew, and ballscrew Mount, the most inaccurate part in this
>system is likely to be the ballscrew. From the specifications I have
>seen ballscrews range in accuracy from approximately 0.004 inches per
>foot, to significantly more accurate. However in the world of do-it-
>yourself CNC the more accurate ballscrews are significantly out of
>the price range of the average user. The ballscrews I had
>entertained using were Thompson units available through Reid tool and
>had a lead screw accuracy of 0.004 inches per foot. That means that
>no matter what I drive the ballscrew with, the accuracy of the system
>can be no greater than the inaccuracy of the screw. Any attempted
>extremely high resolution servo motors and encoders is a waste of
>time, as they will very accurately locate an inaccurate part
>(relatively speaking). For instance in the above example of a lead
>screw accuracy of 0.004 inches per foot the lead screw accuracy is
>approximately 3/10,000 of an inch per inch. A servo motor with a
>rotary encoder of 1024 counts per revolution is capable of resolving
>approximately 2/10,000 of an inch per inch assuming a five turn per
>inch lead screw. If this servo motor where to be running and 5 to 1
>belt reduction the accuracy of the system has not increased by a
>factor of five, but because of the fact that the lead screw is no
>more accurate than 3/10,000 of an inch, then the system regardless of
>any engineering prior to the ballscrew, can never resolve greater
>than this
>
> The highest order of accuracy, it would appear, in the world
>of CNC would be the use of an encoder to directly measuring table
>movement. In a system such as this the inaccuracys downstream from
>the actual table movement are irrelevant, as what is finally measured
>is table movement as opposed to lead screw rotation. As I see it
>only one significant problem exists and that problem is a backlash
>since table movement and the servo motor movement are not
>specifically and directly tied together as they would be with a
>system where a rotary encoder is mounted to the servo motor. then a
>situation of oscillation could potentially exist where a driving
>voltage to a motor would not immediately result in table movement as
>backlash in the screw would need to be absorbed prior to actual
>encoder movement. The logical counter this problem would be to
>design a system with zero backlash therefore any movement of the
>motor would be table movement itself.
>
> The question I have is: in the actual operation on the
>milling machine will the physical shaking on the machine induce
>sufficient jumping of the encoder signal to induce oscillation. This
>question could likely be restated as: is a ballscrew driving a table
>sufficiently rigid so as to allow one near zero actual table movement
>even when forces are applied assuming that the ballscrew is
>completely incapable of rotation. If indeed this system is
>sufficiently rigid then the system will assume that the linear
>encoder being read is indeed the rotary encoder mounted to the back
>of the servo motor. Now the resolution of the system is purely a
>function of only one variable and that is the resolution of the
>encoder. With the encoder I intend on using the resolution is; 360 x
>4 (as this is a quadrature encoder), or approximately 7/10,000 of an
>inch. If my part is one inch long, or one foot long I should be able
>to hold this resolution regardless of length of the part because the
>cumulative error of the ballscrew does not exists and the final
>feedback to the hardware is indeed actual table movement and not
>encoder movement tied to a servo motor. I realize that of course any
>error in the encoder will be replicated in the part, however encoders
>are known to be of extremely high resolution and accuracy.
>
> Any questions, comments, or general gripes with this overall
>design scheme ,I would be interested in your opinions. Especially if
>you give them to me fairly soon as I intend on amassing the parts as
>soon as possible.
>
>
> Bill H
>
>
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