Interesting thought about encoders..

[JamesF]

Death rays?

Lasers had crossed my mind, so you may not be too far from the truth

James

[NickK]

Made a minor miscalculation in the order of magnitude 0.13 mm radius with a 650nm - so this technique may be too precise. Nothing like doing some skunkworks..

So first .. test out the laser tape measure providing a class 2 <1mW 620-650nm. Cost £0 - already had one.

So it's definitely a coherent light source

Next Monster Moshi optic fiber light from Argos.. cost £5.99 - enough fiber that does enough for now and it's proved that the mechanism could work!

[NickK]

So for the proof of concept I've located a perfect starting point for the axis - dead 3.5" HDs of which I have a couple. The platters on the HD are about 9.5cm diameter and spin with a pretty good accuracy.

[NickK]

Mwahahaha

So just checking if the Titan will pick up a 650nm beam through two defraction points.. that's a yes.

I've made a HD test right that I'm about to fire up with the titan (the ms lifecam compressed the images thus destroying any interference patterns).

[NickK]

Only one minor problem with this laser measure.. it eats AAA batteries! 

[NickK]

As I spent my entire weekend doing the bathroom I have had some timeoff (for good behaviour!) so I thought I'd get on with this

So I've knocked up a "test" of two pieces of fiber stuck together then stuck to the desk in such a fashion that one end is fixed with the laser measure and the other I can move (the titan is also stuck to the desk - masking tape!). The two pieces of fiber (side by side with about 4mm between them) then point at the titan some 40 cm or so away.

So here is the result - firstly with a piece of card (ok.. my passport as it was to hand) over one output fiber:

The next is with the card removed:

So the CCD is picking up a fringe pattern - but I was expecting this to be circular. Altering the fibers to sit side by side in a different plane results in a change in the direction. There's also a pattern behind that's more in tune with a 2D interference pattern - my guess here is this is the cover glass reflection...

Hmm something to think about.. onwards!

[NickK]

Had a bit of a blinding clear bit of vision.. think this may be worth patenting..

[OzDave]

Patents are only of use to make lawyers richer. They do nothing to protect an individual with an invention. If you hold a patent, nothing stops large companies from copying your invention, and when they do you need a war chest of money to defend yourself.

I'd spend the money for the patent on a fantastic piece of astronomy kit. Actually you could probably get several pretty fantastic bits of kit.

Sent from my iPhone using Tapatalk

[NickK]

Upon investigating the idea further, it has some promise but not for the accuracy required here.. I'm continuing with the previous idea!

[orly_andico]

I started a very long thread on this over at CN.

Started with a Baumer analog sin/cos encoder. That didn't work too well due to mechanical misalignments.

Moved to a dual read head Renishaw set up using the magnetic RLS read heads. This worked, the dual read heads cancel out mechanical misalignments. But the resolution is low and the SDE very high. I got it to do +- 1.6" with a 910mm circumference wheel.

Source code and the schematics are on the thread.

[orly_andico]

Seems the URL didn't take.

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Board/lxd55/Number/5756299/page/1/view/collapsed/sb/5/o/all/fpart/16

[NickK]

Hi - I looked at the mechanical mechanism with an encoder but the difficulty is you need the thing accurately scribed.

Using a self reference works out far better and modelled over consecutive uses will learn the variations (i.e. period error from the non-perfect alignment of the components. If they're constant then it's possible to use that as a form of encoding..

Good to see there's a few people looking at it