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Re: Periodic Error Question


Jan 16, 2004

 


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#18733 Jan 16, 2004

I have a question,



How do I measure the periodic error of a mount with a web cam? I

have a general idea, but I would appreciate any advice you have for

simplifying the procedure.



I usually use a mac but I have access to a pc.



Scott

--

Scott Rychnovsky

srychnov@...

ATM Page: tlepage.home.mindspring.com/



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#37023 Apr 4, 2008

Quesytion: I (think I) understand that PE is caused by the worm gear

being out of round, and that PE completes a cycle in one rotation. What

about the gear that drives the worm? Can the worm periodic error itself

have a periodic error caused by the main gear? If so, how does that

factor into PEC?

Bob Hertel



----------------------------

#37025 Apr 4, 2008

Bob,



There's periodic error due to the every gear in the system. The

difference between all of them is the duration of a cycle, or how long

it takes for the error to start repeating.



The large gear rotates once every 24 hours. Any periodic error due to

manufacturing intolerances will only repeat after 24 hours.



The worm rotates once every 4 minutes. For this reason, PEC designers

almost never take into account the large gear: it rotates way to

slowly. Instead, PEC corrects for worm defects only.



Since PEC only corrects for errors that repeat every 4 minutes, it

will not correct for any errors with a different length cycle.



Regards,



-Paul



--- In Losmandy_users@yahoogroups.com, "rjhertel2001"

robert.hertel@...> wrote: >

> Quesytion: I (think I) understand that PE is caused by the worm gear

> being out of round, and that PE completes a cycle in one rotation.

What > about the gear that drives the worm? Can the worm periodic error

itself > have a periodic error caused by the main gear? If so, how does that

> factor into PEC?

> Bob Hertel

>



----------------------------

#37026 Apr 4, 2008

Or more strictly, is not a whole multiple/divisor of worm cycle length.



So the first gear in the gear box has a period of 9.6s which is a

divisor of the worm period (25x), but the second gear has a period of

32s which is not (7.5x).



Mark --- In Losmandy_users@yahoogroups.com, "Paul K" pkane2001@...> wrote:

>...

> Since PEC only corrects for errors that repeat every 4 minutes, it

> will not correct for any errors with a different length cycle.

>...



----------------------------

#37027 Apr 4, 2008

Mark, you're absolutely right. Any periodic error with a cycle that is

an integer multiple of the worm period will also be corrected by PEC.

None of the gears in a G11 have such a cycle.



Regards,



-Paul



--- In Losmandy_users@yahoogroups.com, "Mark Crossley"

mark.crossley@...> wrote: >

> Or more strictly, is not a whole multiple/divisor of worm cycle

length. >

> So the first gear in the gear box has a period of 9.6s which is a

> divisor of the worm period (25x), but the second gear has a period

of > 32s which is not (7.5x).

>

> Mark

> --- In Losmandy_users@yahoogroups.com, "Paul K" pkane2001@> wrote:

> >...

> > Since PEC only corrects for errors that repeat every 4 minutes, it

> > will not correct for any errors with a different length cycle.

> >...

>



----------------------------

#37028 Apr 4, 2008

rjhertel2001 wrote: > Quesytion: I (think I) understand that PE is caused by the worm gear

> being out of round, and that PE completes a cycle in one rotation. What

> about the gear that drives the worm? Can the worm periodic error itself

> have a periodic error caused by the main gear? If so, how does that

> factor into PEC?



If the worm is eccentric you get a variation in speed that cycles once

per revolution of the worm. If the wheel is eccentric you get a

variation in speed that cycles once per revolution of the wheel.

Because the period of revolution of the wheel is approximately 24 hours,

much longer than your PEC can record, eccentricity of the wheel doesn't

contribute to periodic error in a way that PEC can cure. Variations

from tooth to tooth on the wheel produce non-periodic variations in

speed, which you guide out. Variations in the tooth profile on the

wheel and worm produce both periodic and non-periodic errors. The point

of contact between the worm and the wheel has a cyclical movement that

produces a periodic variation in speed. Reduction gears between the

motor and the worm add their own errors. It's a wonder it works as well

as it does.



Bud



----------------------------

#37029 Apr 4, 2008

How about the gear box gears in the motor drive, they also contribute

to the PE, correct? I would think that they might be corrected

somewhat with PEC, but not positive.

Floyd

--- In Losmandy_users@yahoogroups.com, William Hamblen wrhamblen@...>

wrote: >

> rjhertel2001 wrote:

> > Quesytion: I (think I) understand that PE is caused by the worm gear

> > being out of round, and that PE completes a cycle in one rotation.

What > > about the gear that drives the worm? Can the worm periodic error

itself > > have a periodic error caused by the main gear? If so, how does that

> > factor into PEC?

>

> If the worm is eccentric you get a variation in speed that cycles once

> per revolution of the worm. If the wheel is eccentric you get a

> variation in speed that cycles once per revolution of the wheel.

> Because the period of revolution of the wheel is approximately 24

hours, > much longer than your PEC can record, eccentricity of the wheel doesn't

> contribute to periodic error in a way that PEC can cure. Variations

> from tooth to tooth on the wheel produce non-periodic variations in

> speed, which you guide out. Variations in the tooth profile on the

> wheel and worm produce both periodic and non-periodic errors. The

point > of contact between the worm and the wheel has a cyclical movement that

> produces a periodic variation in speed. Reduction gears between the

> motor and the worm add their own errors. It's a wonder it works as

well > as it does.

>

> Bud

>







----------------------------

#37031 Apr 5, 2008

How about the gear box gears in the motor drive, they also contribute

> to the PE, correct? I would think that they might be corrected

> somewhat with PEC, but not positive.

Key thing to remember is that any angular error in earlier gears, is

reduced by the latter gear reductions.

If (for instance), the teeth on the gear driving the worm wheel, had a

1/10th degree error round their cycle (absolutely awful, none would be

this bad...), it'd only give a 1/1800th degree error on the final

position (on a scope with a 180 tooth final drive), and a 1/3600th

degree error (for a scope with a 360 tooth final drive).

Because of the large ratio produced by the worm, earlier errors become

almost insignificant.

However the worm, and final gear themselves, both contribute the same

amount to the final motion accuracy, implying that in terms of precise

pointing, it'd be great to gave a 24 hour PEC. Fortunately however, in

terms of tracking, again because of the way the gearing works, it'd take

a whole cycle of the worm, for an error on a tooth in the main crown

wheel to appear, and another cycle for it to disappear, and because the

worm actually touches several teeth at a time, there is a tendency for

such errors to be averaged out (this is the difference between a true

'worm' gear, driving onto at least a partially hobbed main wheel, and a

simple spiral gear on a crown wheel). So the largest repetitive error in

the final motion, and the one changing fastest, is the error from the

worm wheel. The main wheel introduces just as much error, but at a much

lower rate, while earlier gears introduce higher 'rate' errors, but of

much smaller size. Hence single worm cycle PEC. :-)



Best Wishes

> Floyd

> --- In Losmandy_users@yahoogroups.com, William Hamblen wrhamblen@...>

> wrote:

>> rjhertel2001 wrote:

>>> Quesytion: I (think I) understand that PE is caused by the worm gear

>>> being out of round, and that PE completes a cycle in one rotation.

> What

>>> about the gear that drives the worm? Can the worm periodic error

> itself

>>> have a periodic error caused by the main gear? If so, how does that

>>> factor into PEC?

>> If the worm is eccentric you get a variation in speed that cycles once

>> per revolution of the worm. If the wheel is eccentric you get a

>> variation in speed that cycles once per revolution of the wheel.

>> Because the period of revolution of the wheel is approximately 24

> hours,

>> much longer than your PEC can record, eccentricity of the wheel doesn't

>> contribute to periodic error in a way that PEC can cure. Variations

>> from tooth to tooth on the wheel produce non-periodic variations in

>> speed, which you guide out. Variations in the tooth profile on the

>> wheel and worm produce both periodic and non-periodic errors. The

> point

>> of contact between the worm and the wheel has a cyclical movement that

>> produces a periodic variation in speed. Reduction gears between the

>> motor and the worm add their own errors. It's a wonder it works as

> well

>> as it does.

>>

>> Bud



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