In article ,
says...
On Mon, 3 Oct 2005 11:30:22 -0400, Ned Simmons
wrote:

In article ,
says...
This may seem off topic but it's not really. Measuring is important to
machining. And I need to make a measurement without putting pressure
on the item being measured. Now, for the purists, I know that light
will put a little pressure on what I'm measuring, but this will have
way less effect than any kind thing I can measure. I have an encoder
that outputs 10,000 pulses per revolution. With a shaft measuring
.19515" (radius) each pulse equals .0001 on the circumference. So, if
the radius keeps multiplying by 10 then the amount traveled by the
circumference will increase by 10. Here's the plan:
Put a mirror on the shaft.
Shine a laser at the mirror.
Adjust the mirror until the laser is visible on a target 1591.5"
(132.625 feet) away.
This will increase the amount traveled to .1" for every pulse.
So, measuring the distance the spot travels will show how many pulses
should be generated.
The reason for this measurement is to rule out backlash in the
encoder. It appears that there is a 9 pulse error. In other words, if
the encoder shaft is turned one way and the count is noted, and then
turned the other way until the count changes, it appears that the
shaft turns an amount equal to 9 pulses. I need to rule this out
because this is the error I'm getting is a mechanical system and it
appears that all the mechanical lash has been reduced to less than
.0001". The last thing seems to be the encoder itself.
Thank You,
Eric R Snow,
E T Precision Machine


It sounds like it would work, but aren't you off by a factor of 10 on
the distance to the target? It looks to me like 1/10000 of a rev at the
shaft would move the spot 1" at 1591".

Do you have access to a microscope you could fixture to look at a paper
disk with a reference mark attached to the encoder shaft? Or if the guts
of the encoder are accessible, you could look directly at the encoder
disc. You don't need much magnification to resolve .001".

All that said, unless something is very wrong with the encoder, I doubt
you'll find anything other than a very small amount of hysteresis in the
detector.

I apologize for not responding further to your earlier requests, but
really didn't have much to add other than to stress the need to pay
attention to everything that could possibly go wrong when working to
tenths, which I know you already know. Which method of driving the
encoder did you settle on?

Ned Simmons


Boy, I need to proofread my posts. There were a lot of errors in the
above. Especially the times ten answer. You are right, 132 feet would
show 1 inch. Which is what I want. The driving method so far is a
belt. I can detect no slipping at all. But I keep getting this .0009
error. I don't know if this is a display error because it's reading
the encoder disc in quadrature. I thought about looking at the disc
itself. It's 2.000 inches in diameter with 2500 marks on it. which
makes the spacing about .0025". This is readable with a magnifier. But
I thought that amplifying the error would be easier because I can see
the movement at the same time as the display changes. I can very
easily see 9 inches movement at 132 feet. And it turns out I have a
target just about that far away. A nice clean cement wall. Thanks for
reading and responding.
Eric


If you've got an o'scope you can look directly at the
encoder channels to eliminate any possibility that it's the
readout, if that's what you mean by display error. Less
convenient, but workable, would be a voltmeter or two. Or
even an LED on each channel, as long as you're careful to
limit the current based on the encoder's drive capability.

Ned Simmons