My home-brew focuser.

Here is some info about my DIY Meade LXD55 remote focuser.

I thought others might be interested in how I did it and what mistakes I
made.

http://www.nu-ware.com/Focuser/

Jo

Jo wrote:
Here is some info about my DIY Meade LXD55 remote focuser.

I thought others might be interested in how I did it and what mistakes I
made.

http://www.nu-ware.com/Focuser/

Jo



Neat.

Why don't you close the loop and have proper position feedback ?


Steve

In ,
Steve Taylor typed:

Why don't you close the loop and have proper position feedback ?


I was working with whatever bits and pieces were to hand. Any suggestions on
how to provide a positional feedback loop with typical junkbox items would
be cool. Come to think of it....there is a component in the VCR that *may*
be a linear position sensor..possibly for mechanically detecting how much
tape was remaining on a reel during fast rewind. Not sure of the resolution,
though. Any other ideas?

Jo

"Jo" wrote in message
...
Here is some info about my DIY Meade LXD55 remote focuser.

I thought others might be interested in how I did it and what mistakes I
made.

http://www.nu-ware.com/Focuser/

Jo



That's cool.

Colin.

"Jo" wrote in message
...
In ,
Steve Taylor typed:

Why don't you close the loop and have proper position feedback ?


I was working with whatever bits and pieces were to hand. Any suggestions
on
how to provide a positional feedback loop with typical junkbox items would
be cool. Come to think of it....there is a component in the VCR that *may*
be a linear position sensor..possibly for mechanically detecting how much
tape was remaining on a reel during fast rewind. Not sure of the
resolution,
though. Any other ideas?

Jo




Maybe you could use a potentiometer, that throws over the full range of the
focuser. Then measure the voltage to show the distance.

Probably doesn't resolve enough though.

Col.
www.cjdawson.com

"Jo" wrote:
In ,
Steve Taylor typed:

Why don't you close the loop and have proper position feedback ?

I was working with whatever bits and pieces were to hand. Any suggestions on
how to provide a positional feedback loop with typical junkbox items would
be cool. Come to think of it....there is a component in the VCR that *may*
be a linear position sensor..possibly for mechanically detecting how much
tape was remaining on a reel during fast rewind. Not sure of the resolution,
though. Any other ideas?

An optical mouse can work on a variety of surfaces to a resolution of
a few hundredths of an inch. They work over USB. Which is designed to
share nicely. It's not absolute positioning and I don't know how
accurate they are over an inch or so, but it is junkbox. Well, it is
as soon as you treat yourself to a new mouse


Tim
--
Google is not the only search engine.

Jo wrote:
In ,
Steve Taylor typed:

Why don't you close the loop and have proper position feedback ?



I was working with whatever bits and pieces were to hand. Any suggestions on
how to provide a positional feedback loop with typical junkbox items would
be cool. Come to think of it....there is a component in the VCR that *may*
be a linear position sensor..possibly for mechanically detecting how much
tape was remaining on a reel during fast rewind. Not sure of the resolution,
though. Any other ideas?

How many turns does the final drive from the motor make with the actual
focussing mechanism ? You could just detect position by inference from
the rotation of the drive wheel- use a multi-turn pot ( 6 quid)

Linear plastic pot ?

Shaft encoder from a mouse ? I'm also pretty sure that there are
encoders in VCRs. Have one fixed stop in your focusser. Drive onto the
stop and set your zero reference. That would need get nasty if you stick
with an analogue servo though - you'd need a counter and DAC to get a
voltage to drive the loop....not for the faint hearted.

What kind of positional resolution would you need I wonder ?

Steve

In ,
Steve Taylor typed:

Just to say thank to all who sent suggestions for improvements.

How many turns does the final drive from the motor make with the
actual focussing mechanism ? You could just detect position by
inference from the rotation of the drive wheel- use a multi-turn pot
( 6 quid)


I have a multi-turn pot around somewhere. It is quite big and looks like a
precision device.

Linear plastic pot ?

Shaft encoder from a mouse ? I'm also pretty sure that there are
encoders in VCRs. Have one fixed stop in your focusser.

The nice thing about the current system is that it is an independant module
that I can remove easily for manual focusing and I am reluctant to start
adding limit switches to the Meade focuser. However, there is a way around
this requirement. The controller can simply make some assumptions about the
initial position of the focus tube and I will always do an initial manual
focus before starting. If it is a few mm off, so what?


Drive onto the
stop and set your zero reference. That would need get nasty if you
stick with an analogue servo though - you'd need a counter and DAC to
get a voltage to drive the loop....not for the faint hearted.

You got me thinking, see below...


What kind of positional resolution would you need I wonder ?

Not sure about required resolution at the focuser tube..but I do know that
one turn of the DC motor makes no visible difference to the focus, it takes
quite a few turns before any optical changes occur..and that is after taking
precautions that I'm not looking at backlash in the gear train.. Here is my
plan:

I don't need absolute positional feedback, just relative feedback so I can
tell the remote drive "Move in/out a bit from the current position" Bear in
mind that the focus tube will always be initially set somewhere near optimum
focus before switching the controller on.. The "move" command translates to
"Move in/out xx motor turns" Where xx will always be 1. For this I need to:
1) Count DC motor turns.
2) Stop when the required number of turns has been reached..but not
suddenly.

1) I could use a optical encoder disk/led setup to count DC motor revs.
Alternatively, I may be able to count revs by monitoring the DC feed to the
motor and watching for commutator breaks. In other words, use the motor
itself as a tachometer. There is the complication that I'm currently pulsing
the feed to the motor for speed control but a combination of zener
thresholds and suitable time constants might allow me to see the commutator
open circuits as the motor shaft rotates.

2) Given that I can get a supply of tachometer pulses back from the motor in
step 1) I need to count them. A digital counter is one solution but I have
thought of something simpler. I integrate the pulses and compare the
integrated signal with the output from my multiturn pot that is tied to a
stabilised reference voltage. The comparitor output is then used to
determine the pulse width and polarity of the drive to the motor.

In the above positional resolution will depend on:
a) Motor turns per unit movement of the focuser tube.
b) "Goodness" of the integrator circuit and comparitor.
c) Resolution of the multiturn potentiometer.
d) Stability of the reference voltage.

I need to do a *lot* of prototyping to get the above running...should be
fun. What do you folks think?

Jo

Jo wrote:

The nice thing about the current system is that it is an independant module
that I can remove easily for manual focusing and I am reluctant to start
adding limit switches to the Meade focuser.

Depends how you fit the switches - they can just as easily attach to
your module and touch the focusser tube wherever they can find. Its only
for a reference. A digital position servo would be pretty easy.


Not sure about required resolution at the focuser tube..but I do know that
one turn of the DC motor makes no visible difference to the focus, it takes
quite a few turns before any optical changes occur..and that is after taking
precautions that I'm not looking at backlash in the gear train.. Here is my
plan:

I may be able to count revs by monitoring the DC feed to the
motor and watching for commutator breaks.

If you don't HAVE to use second order effects like that, why bother -
you only need a single flag pulse per rev of the motor and a (ideally) a
direction signal.

OR a rubber wheel on the focusser tube, then a digital pot chip with a
push button up/dn counter....linear output volts with position.


2) Given that I can get a supply of tachometer pulses back from the motor in
step 1) I need to count them. A digital counter is one solution but I have
thought of something simpler. I integrate the pulses and compare the
integrated signal with the output from my multiturn pot that is tied to a
stabilised reference voltage. The comparitor output is then used to
determine the pulse width and polarity of the drive to the motor.

In the above positional resolution will depend on:
a) Motor turns per unit movement of the focuser tube.
b) "Goodness" of the integrator circuit and comparitor.
c) Resolution of the multiturn potentiometer.
d) Stability of the reference voltage.

The "simpler" method is the road to madness - the integrator would droop
too much in practice. The pot won't be a problem, a zener would be
stable enough, but board leakage would knacker the integrator for long
period integration.

If you are an amateur and WANT to mess around, go ahead. If one of my
staff offered this as a method though.....

Steve

"Jo" wrote in message
...
In ,
Steve Taylor typed:

Just to say thank to all who sent suggestions for improvements.

How many turns does the final drive from the motor make with the
actual focussing mechanism ? You could just detect position by
inference from the rotation of the drive wheel- use a multi-turn pot
( 6 quid)


I have a multi-turn pot around somewhere. It is quite big and looks like
a
precision device.

Linear plastic pot ?

Shaft encoder from a mouse ? I'm also pretty sure that there are
encoders in VCRs. Have one fixed stop in your focusser.

The nice thing about the current system is that it is an independant
module
that I can remove easily for manual focusing and I am reluctant to start
adding limit switches to the Meade focuser. However, there is a way
around
this requirement. The controller can simply make some assumptions about
the
initial position of the focus tube and I will always do an initial
manual
focus before starting. If it is a few mm off, so what?


Drive onto the
stop and set your zero reference. That would need get nasty if you
stick with an analogue servo though - you'd need a counter and DAC to
get a voltage to drive the loop....not for the faint hearted.

You got me thinking, see below...


What kind of positional resolution would you need I wonder ?

Not sure about required resolution at the focuser tube..but I do know
that
one turn of the DC motor makes no visible difference to the focus, it
takes
quite a few turns before any optical changes occur..and that is after
taking
precautions that I'm not looking at backlash in the gear train.. Here is
my
plan:

I don't need absolute positional feedback, just relative feedback so I
can
tell the remote drive "Move in/out a bit from the current position" Bear
in
mind that the focus tube will always be initially set somewhere near
optimum
focus before switching the controller on.. The "move" command translates
to
"Move in/out xx motor turns" Where xx will always be 1. For this I need
to:
1) Count DC motor turns.
2) Stop when the required number of turns has been reached..but not
suddenly.

1) I could use a optical encoder disk/led setup to count DC motor revs.
Alternatively, I may be able to count revs by monitoring the DC feed to
the
motor and watching for commutator breaks. In other words, use the motor
itself as a tachometer. There is the complication that I'm currently
pulsing
the feed to the motor for speed control but a combination of zener
thresholds and suitable time constants might allow me to see the
commutator
open circuits as the motor shaft rotates.

2) Given that I can get a supply of tachometer pulses back from the
motor in
step 1) I need to count them. A digital counter is one solution but I
have
thought of something simpler. I integrate the pulses and compare the
integrated signal with the output from my multiturn pot that is tied to
a
stabilised reference voltage. The comparitor output is then used to
determine the pulse width and polarity of the drive to the motor.

In the above positional resolution will depend on:
a) Motor turns per unit movement of the focuser tube.
b) "Goodness" of the integrator circuit and comparitor.
c) Resolution of the multiturn potentiometer.
d) Stability of the reference voltage.

I need to do a *lot* of prototyping to get the above running...should be
fun. What do you folks think?

I think you've lost sight of the objective!

Why do you think you need closed-loop control of this? You haven't
yet said what the arbiter of a 'focused' system is going to be. Why
can't you simply use up and down buttons to drive the system?
After all, it is going to be the camera/MK1 eyeball which decides
if it is in focus, n'est ce pas?


--
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Dorset UK Astro Society's Web pages, Info, Meeting Dates, Sites & Maps
Change 'news' to 'sewn' in my Reply address to avoid my spam filter.