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Re: Park position .......... G11 + Gemini GoTo in


Sep 18, 2005

 


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

#27168 Sep 18, 2005

Hi,



Has anybody else made the experience that when the mount is parked the

motors heat up ?



DEC axis Horizontal and Telescopes also horizontal.



What could be the cause for it ?



best regards Rainer



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

#27169 Sep 18, 2005

Rainer



I've had my Dec motor warm up when I haven't moved the Dec axis for about an

hour. I thought it may be the Gemini trying to hold the motor in a position

it doesn't like be - but I was shot down my Rene, apparently the DC servos

don't have 'cogging' like the steppers. (thinks... But if that is true why

do Celestron make a big deal out of their 'lo-cog' servos in the CGE

mounts?)



Nobody came up with any explanation other than your balance was out (which

it wasn't). I've only noticed it a couple of times, but each time a 'tap'

on one the Dec buttons to move the axis slightly caused the motor to cool

off again. So if the balance was out, why would it cool when you change the

position a couple of arc mins?



Mark



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

#27170 Sep 18, 2005

Mark,

I've seen the same thing with the dec motor. I suspect that after

the last move, the dec motor didn't quite get positioned correctly -

maybe off by a "tick" or 2. So, Gemini still applies some small

current to the motor to get it where it needs to be, but not enough

to actually move it. By bumping a dec button, you move it slightly,

and this time it DOES come to rest in the correct position, so

Gemini doesn't apply any more current.



Now, the problem with my theory is that Rene' says that the Gemini

display won't say "Finished" until both servos are in the correct

position and all current has turned off. If this is in fact the

case, then my theory doesn't work.



Perhaps next time my dec motor gets hot after not having been moved

for a while, I'll try and apply a voltmeter to the motor leads and

see if there's any voltage across them.



Dave S.



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

mark.crossley@d...> wrote: > Rainer

>

> I've had my Dec motor warm up when I haven't moved the Dec axis

for about an > hour. I thought it may be the Gemini trying to hold the motor in

a position > it doesn't like be - but I was shot down my Rene, apparently the

DC servos > don't have 'cogging' like the steppers. (thinks... But if that is

true why > do Celestron make a big deal out of their 'lo-cog' servos in the

CGE > mounts?)

>

> Nobody came up with any explanation other than your balance was

out (which > it wasn't). I've only noticed it a couple of times, but each time

a 'tap' > on one the Dec buttons to move the axis slightly caused the motor

to cool > off again. So if the balance was out, why would it cool when you

change the > position a couple of arc mins?

>

> Mark



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

#27173 Sep 18, 2005

Ranier, I forgot to turn off my mount one night after parking it. The next

morning I came out to dry out and found RA had moved about 2hrs in the

5-6hrs since I closed the obs. Although I've forgotten to turn off the

Gemini a couple more times it hasn't moved like it did that time.

Curious...joe :)





"May You Go Among The Imperishable Stars"



Joe Mize: jmize@...

StarFields Observatory www.cav-sfo.com

Chiefland, FL 29:24'33.4"N 82:51'37.7"W

Moon Phase: tycho.usno.navy.mil/cgi-bin/phase.gif ----- Original Message -----

From: "Rainer" rsbfoto@...>

To: Losmandy_users@yahoogroups.com>

Sent: Sunday, September 18, 2005 1:10 PM

Subject: [Losmandy_users] Park position .......... G11 + Gemini GoTo in





> Hi,

>

> Has anybody else made the experience that when the mount is parked the

> motors heat up ?

>

> DEC axis Horizontal and Telescopes also horizontal.

>

> What could be the cause for it ?

>

> best regards Rainer

>

>

>

>

>

>

> Yahoo! Groups Links

>

>

>

>

>

>

>



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

#27178 Sep 20, 2005

Dave



I think the problem with that theory is that the servo loop should keep

applying more voltage until the motor does move - or if it still doesn't move

you end up with a 'stalled' message.



My theories are...



1. That the servo does reach the desired position, but then overshoots very

slightly (even just by one 'tick') moves back and overshoots again, so it end

up oscillating between two very close positions.



or



2. The servo reaches the desired position, but it naturally 'notches' to a

position slightly off. The servo loop then has to apply a small constant

voltage across the servo to hold it in position.



I think the only way to resolve this problem would be tap into the lines to

the servo and see what is happening. As this condition happens so rarely for

me I don't think I'm going to persue it.



Mark



Quoting Losmandy_users@yahoogroups.com:

> Date: Sun, 18 Sep 2005 21:00:33 -0000

> From: "Dave Sandage" daves@...>

> Subject: Re: Park position .......... G11 + Gemini GoTo in

>

> Mark,

> I've seen the same thing with the dec motor. I suspect that after

> the last move, the dec motor didn't quite get positioned correctly -

> maybe off by a "tick" or 2. So, Gemini still applies some small

> current to the motor to get it where it needs to be, but not enough

> to actually move it. By bumping a dec button, you move it slightly,

> and this time it DOES come to rest in the correct position, so

> Gemini doesn't apply any more current.

>

> Now, the problem with my theory is that Rene' says that the Gemini

> display won't say "Finished" until both servos are in the correct

> position and all current has turned off. If this is in fact the

> case, then my theory doesn't work.

>

> Perhaps next time my dec motor gets hot after not having been moved

> for a while, I'll try and apply a voltmeter to the motor leads and

> see if there's any voltage across them.

>

> Dave S.







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

#27187 Sep 21, 2005

Rainer

>

> I've had my Dec motor warm up when I haven't moved the Dec axis for about

> an

> hour. I thought it may be the Gemini trying to hold the motor in a

> position

> it doesn't like be - but I was shot down my Rene, apparently the DC

> servos

> don't have 'cogging' like the steppers. (thinks... But if that is true

> why

> do Celestron make a big deal out of their 'lo-cog' servos in the CGE

> mounts?)

The key here is cogging in the movements themselves. A DC motor, tends to

apply varying amounts of thrust, as it moved, according to the position of

the coils and commutator. 'Lo-cog' motors, have more commutator segments,

and have them 'angled', rather than parallel to the drive shaft, so that

several coils are activated at once, by varying amounts. This gives

smoother thrust, and avoids vibration in the drives.

Whether a servo 'cogs', is dependant on the gearing between the DC motor,

and the sensor used, and to some extent on the elements used in the control

algorithm. If (for instance), the optical detector has more poles, that the

motor to which it is attached, and the algorithm (assuming it is based on

a'PID' system), has a strong 'P' or 'I' term, then a DC servo can cog.

However provided there is a good number of physical motor poles, to each

detectable output position of the quadrature encoder, cogging won't occur.

Lack of cogging is not therefore 'inherent' in it being a servo, but is

down to the design.

> Nobody came up with any explanation other than your balance was out

> (which

> it wasn't). I've only noticed it a couple of times, but each time a

> 'tap'

> on one the Dec buttons to move the axis slightly caused the motor to cool

> off again. So if the balance was out, why would it cool when you change

> the

> position a couple of arc mins?

>

> Mark

It does actually sound as if the 'I' term may be slightly too low in the

servo algorithm, relative to the 'P' term, and what is happening is that

the motor wants to move a tiny amount, but not enough power is being

applied to actually generate the movement, given the friction/loads

involved. Normally the 'I' term will build with time if this happens,

resulting in the power being increased till the required movement occurs.

However if the 'I' term is small, this won't happen, and motor heating will

result. It suggests that your gearing may be a little stiff to start in

this particular axis, or as if the algorithm assumes a slightly 'freer'

movement.



Best Wishes



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