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building a digital optical protractor

i am needing to measure the angle of an arm. I would like the readout
to be very accurate to .001 degrees. Has this been done. I thought
of measuring the optical pulses through a digital encoder like used in
a digital mouse. thanks in advance for the help.

Rick Wells
 
J

Jamie

Jan 1, 1970
0
i am needing to measure the angle of an arm. I would like the readout
to be very accurate to .001 degrees. Has this been done. I thought
of measuring the optical pulses through a digital encoder like used in
a digital mouse. thanks in advance for the help.

Rick Wells
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..
 
J

John Larkin

Jan 1, 1970
0
i am needing to measure the angle of an arm. I would like the readout
to be very accurate to .001 degrees. Has this been done. I thought
of measuring the optical pulses through a digital encoder like used in
a digital mouse. thanks in advance for the help.

Rick Wells

There are super-high-resolution rotary optical encoders, numbers like
5 million counts per revolution. But they are mechanically precise and
definitely not cheap. Google "high resolution encoder."

Why do you need this resolution?

John
 
E

Eric R Snow

Jan 1, 1970
0
i am needing to measure the angle of an arm. I would like the readout
to be very accurate to .001 degrees. Has this been done. I thought
of measuring the optical pulses through a digital encoder like used in
a digital mouse. thanks in advance for the help.

Rick Wells
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.
https://sdp-si.com/eStore/Direct.asp?GroupID=205
Anti-backlash gears from SDP/SI, about $100.00 US for 72 tooth gear
that might work.
https://sdp-si.com/eStore/Direct.asp?GroupID=590
Metal spur gears from SDP/SI, about $12.00 US for a 20 tooth gear that
might work with the above gear.
http://www.usdigital.com/products/ed3/
A digital readout from US Digital that will read rotary encoders in
quadrature. About $104.00 US.
http://www.usdigital.com/products/s6/
A 2500 count digital encoder from US Digital that will return 10,000
pulses when read in quadrature with the above readout.

360/10,000=.036 degrees. With the above gears and a 10 tooth gear you
can reduce further the system to attain .001 degree resolution. It's
up to you now to build something accurate enough. Maybe your best bet
is to instead figure out what the range is going to be, how many
degrees total you will be moving, and use a laser micrometer to
measure the position of the end of the arm. A 1 inch long arm rotated
1 degree will raise one end about .017". This means that you can use
the arm length as a reducer, similar to a gear reduction.
ERS
 
K

kell

Jan 1, 1970
0
i am needing to measure the angle of an arm. I would like the readout
to be very accurate to .001 degrees. Has this been done. I thought
of measuring the optical pulses through a digital encoder like used in
a digital mouse. thanks in advance for the help.

Rick Wells

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.
 
There are super-high-resolution rotary optical encoders, numbers like
5 million counts per revolution. But they are mechanically precise and
definitely not cheap. Google "high resolution encoder."

Why do you need this resolution?

John

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
 
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.

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

Eric R Snow

Jan 1, 1970
0
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
Greetings Rick,
Did you see my post about the devices from US Digital? They sell the
counter you need.
ERS
 
K

kell

Jan 1, 1970
0
Thanks for the response. I mis stated my accuracy need. It only
needs to be .01 not .001.- Hide quoted text -

- Show quoted text -

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...
 
K

kell

Jan 1, 1970
0
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.


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.
 
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