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How to measure distances ?

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almtree

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Hi,

I have a focuser that has a 2 inch travel between each end. I need to measure the focus position with the best accuracy possible, what kind of sensors are there to make this type of measurement?


Thanks

P.A.
 

ee_freak

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I think that the more accurate method would be laser of some sort of optical device but I 't know almost nothing about that.

The practical method might be a decoder. I had seen some miniature machine tool use a radial decoder to read the position of the dial.Its accuracy is about 0.001" +-0.0002" or something like that. What kind of focuser you're using? A rack type or Crayford style? BTW I guess that you're talking about telescope focuser :wink:.

good luck,
ee_freak
 

almtree

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On 2002-04-01 23:48, ee_freak wrote:
I think that the more accurate method would be laser of some sort of optical device but I 't know almost nothing about that.

The practical method might be a decoder. I had seen some miniature machine tool use a radial decoder to read the position of the dial.Its accuracy is about 0.001" +-0.0002" or something like that. What kind of focuser you're using? A rack type or Crayford style? BTW I guess that you're talking about telescope focuser :wink:.

good luck,
ee_freak

Hi,

Laser was my first thought, but since it's a so small distance that it would by overkill and expensive.

Yap, it's a JMI NGF focuser (crayford style) with motor that a use with my LX200 telescope.

Thanks for the help :smile:

Regards

P.A.
 

Cathay

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Xi|inx has a contest on C00lrunner CPLD
application design for PALM last year.
One of the finalist is using Ultrasound
for distance finding. The whole design
was post on their website and you can search
for it. Hope it helps!
 

IBall

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You might get good results using LVDT, as in this appnote.


Uploaded file: AN1182.pdf
 

jjljajp

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What kind of source is that? What is the NA? There are many methods available depends on the Wave Length used
 

harri_el_sucio

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sorry for language

In the magazine CIRCUIT CELLAR exist circuits
using ultrasound for distance finding and
with optical device . It is Free

http://www.circuitcellar.com

Visit Articles on line
 

jetmarc

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I have a focuser that has a 2 inch travel between each end. I need to measure the focus position with the best accuracy possible, what kind of sensors are there to make this type of measurement?
Have you thought of using a permanent magnet
on one end and a hall sensor on the other
end?
 

glasstire

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Laser interferometer would be overkill but extremely precise. Ultrasonic, Hall effect, TDR techniques probably too crude. LVDT sensors good. Also try Mitutoyo Digimatic dial indicators (www.mitutoyo.com). Model 543-462B has 50mm range, 0.0001mm resolution, but expensive.
 

Rainbow Wizard

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The easiest way to solve this problem is to use a simple interferometer made with a LED and appropriare collimator. Then use a photodiode coupled to an up/down counter to count the interference fringes as the focuser moves.
 

Rainbow Wizard

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Interferometer for guaging distance

The easiest way to solve this problem is to use a simple interferometer made with a LED and appropriare collimator. Then use a photodiode coupled to an up/down counter to count the interference fringes as the focuser moves.
 

Robby

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Hello,

You need two parts: one laser diode and one direction sense photo diode.

The technique is called -> triangulation.

regards

Robby

By the way, it's also possible with a normal led, but less accuracy.
 

snowman53

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twinsen

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what about a rotary encoder?

I once made a DRO for a milling machine by using a shaft encoder (US Digital)
You can use a cable running over a pully on the encoder or drive the encoder directly on the focuser with a rubber wheel with a bit of tension (A VCR pinch roller works nicely)
I guess it all comes down to the resolution you want...
 

snowman53

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Motor w/ Position Feedback

If you decide to do a mechanical feedback system instead of non-contact measurement, I might suggest looking at either the Maxon line of brushless motors. And/or the Smartmotor line of brushless motors & controllers.

We have used Maxon motors to provide an optical positioning stage with sub micron repeatability and positional accuracy. In truth our customer could not measure any error using a $50K interferometer.

www.maxonmotorusa.com/products/

They are small, can be ordered with a 1024 count encoder and up to a 6400:1 planetary gear reduction. That would give you 64,000 counts per rev at the output shaft. You would need to provide a linear gear & optical guides.

You can use the Maxon controller, but we like the Smartmotor controller for its ease of programming. Just send it an RS232 rate or position command & it takes care of the rest.

www.animatics.com/

Smartmotor also offers an integrated motor, encoder and controller package. But the package is larger and if I re-call you can only get ~2000 counts per rev. Adequate for many applications.

Good Luck
 

glasstire

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I think LED triangulation has bad resolution over a *2 inch* range with standard PSD. I think standard PSD spatial linearity not that good, maybe 1% FS.

Rainbow Wizard, I never see interferometer with LED source. Any refs, it looks interesting.

Motor/leadscrew with encoder feedback maybe ideal. But how to get absolute position ref once power is removed? Could use hall effect switch? Below URL describes positional hall switch with .002 inch abs. accuracy.

http://content.honeywell.com/sensing/prodinfo/solidstate/technical/chapter7.pdf
 

Pim

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If being able to refine the mechanical precision of the parts, why not use a stepper motor and a lead-screw? You will need something to set a reference point somewhere (optical, magnetical, mechanical contact). Then only apply a measured number of pulses to the stepper motor and you can determine where you want to go. Berger&Lahr makes 5-phase stepper motors that are able to turn fractions of a degree very well controlled by any microcontroller (and driver) circuitry.

/Pim
 

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