building a digital optical protractor

Yes, that would be the way to do it how ever, you need to get a high resolution type for that and maybe shieve it up to increase the ratio mechanically.. to keep the slack out of it, use a mini steel pulley with tension spring in there. this will give you a positive response. Or, you can use a direct contact rubber grip roller, large one on the arm side and small one on the encoder shaft..

There are super-high-resolution rotary optical encoders, numbers like

Why do you need this resolution?

John

Greetings Rick, You can do this mechanically but the system will need to be very rigid and virtually lash free. Any slop will need to be less than the resolution and accuracy desired. You will also need to account for hysteresis in the moving parts. Assuming you can do this then listed below are some links that may help.

Hmm, the job calls for you to measure rotational movement with an accuracy of one ten thousandth of an inch over a radius of six inches. This poses mechanical challenges as much as electronic, seems to me...

I used one of those electronic protractors for carpenters the other day. The protractor has a resolution (ostensibly, accuracy) of a tenth of a degree.

These protractors cost on the order of a hundred dollars which reflects economy of scale in mass production, and the amortization of initial development costs over many thousands of units manufactured. Wonder how many kilodollars went into engineering it? You want to build something with an accuracy a couple of orders of magnitude greater. I say if you can lick this thing on your own with nothing much more than some advice you solicited over the internet, you will have my profound admiration.

This arm is a spindle that holds a gem stone against an abrasive disk. The facets of the gemstone are cut this way. I was wrong on the .001 and .01 would be fine. I have studied the encoders. My hangup is I don't understand what to do with the output or pulse count from the encoder. thanks

Thanks for the response. I mis stated my accuracy need. It only needs to be .01 not .001.

Greetings Rick, Did you see my post about the devices from US Digital? They sell the counter you need. ERS

Okay, if I couldn't afford to make a mistake and had to grind a something valuable like a gemstone with great accuracy I wouldn't try to do it all in one throw by measuring the position of the spindle. I'd grind some material off the stone, measure the stone, then grind some again, in several steps. Grind a few stones, you will adjust pressure and time empirically. Then after you've got it down, you can shave your stones within a few thousandths in only a couple passes if you're lucky. Hmm, "Shave your stones," that don't sound right...

Now that I've had some time to think about it, I'd advise against using a feedback loop of any kind in an attempt to get the accuracy you need. Instead, just use a mechanical stop on the action. The screw from a micrometer will make an adjustable set-stop with micrometer accuracy. Using controlled force from some pneumatic cylinder or the like to advance it, the stone holder keeps moving until a lever or boss on it comes in contact with the micrometer set-stop, and ceasing travel. Maybe you can work out the design. If your budget allows, perhaps a mechanical engineer. Or hire a very talented mechanic. If your app requires automation, use a stepper motor to turn the screw.