Re: [CAD_CAM_EDM_DRO] Re: Rotary to linear translation - an interesting idea.

Sorry David, but I do not think that is correct.

Think about what happens with an infinitely thin tape. There would be no error in the decoder pickup wheel since the average
contact distance is the same as the wheel radius. That would be true even if wrapped around a full turn. The tape is still
moving around a radius in that case so the "speed up" should be there if your explanation was true.

I really do not believe it will speed up since if one part of the tape speeds up but not the other part, it really must be
stretching and I think we all agree that the tape is not changing its average length.

The tape is stretching on the outside and compressing on the inside. We are measuring on the compressed shorter inside and
therefor will read a shorter than true distance.

Your explanation implies that the tape moves faster and the wheel will read too high while I think it looks like a larger radius
wheel and therefore will rotate slower.

As usual, one must watch those lawyers......
Bertho Boman
===================

"David A. Forsyth" wrote:

> ........
> because the tape is attached to the table and must move at the same
> speed, and when you deflect the tape with the wheel it now
> travels around a radius, and that circumferential distance is
> larger than the straight line distance, the tape must travel around
> the radius faster in order to maintain it's length since the tape is
> not stretchy (to that degree anyway).
>
> now, if you increase the deflection, more tape is around the wheel =
> greater circumferential distance, ergo it must go past faster,
> turning the wheel faster = increasing the counts per linear unit
> moved by the table/tape assembly.
>
> that's why you make the encoder wheel slightly smaller than you
> think hence giving you too few counts per linear unit, and adjust for
> correct count by pressing it further into the tape.
>
> hope that clarifies?
>
> PS: I say 'unit' above because some work in inches and some in thou
> and some in mm and if I use one it becomes too specific and I have to
> give actual values that are true but I'm trying to convey the concept
> and theory, not actual values....
>
> BTW I spoke this problem and my explaination of it to my wife last
> night and she understood it and agreed with me on the principal.
> she's into dogs and horses and so I must think that either
> a) I'm correct
> b) I should have been a lawyer cos my argument was convincing even if
> incorrect (-:
>
> IMHO it's not about how the tape stretches on the inside/outside etc,
> but just about the circumferential distance travelled as compared to
> the straight line distance the tape would have traversed if the wheel
> wasn't there.
>
> also note the whole system depends on the spacing of the support
> rollers. if one moves, you'll have to recalibrate....
>
> --
> Living in South Africa
> Flying power Kites
> Chasing 3'6" gauge steam trains
> http://www.ru.ac.za/departments/iwr/staff/daf/