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Joseph Gwinn

In article ,
(dan) wrote:

What's that Lassie? You say that Joseph Gwinn fell down the old
rec.crafts.metalworking mine and will die if we don't mount a rescue
by Sun, 07 Dec 2008 19:22:55 -0500:

Or was the head stationary? With a stationary head, what would
determine witch direction to go on the line?

The head would be stationary, and all adjustment would be electronic.

Let's first deal with one axis at a time.

There are two rectangular cells side by side. If the line to be
followed is parallel to the boundary between cells, then the
differential amplifier output will be signed and proportional to the
offset of the line from the border. If the line is instead
perpendicular to the border, the output of both cells will be reduced,
but their difference will remain zero.

Now, add a second pair of cells, so we have four cells total laid next
to one another like tiles in a square pattern. Let us number the tiles
by row and column:

S11 S12

S21 S22

We have two differential amplifiers. The X amplifier has plus inputs
from S12 and S22, and minus inputs from S11 and S21. The Y amplifier
has plus inputs from S11 and S12, and minus inputs from S21 and S22.

In math:

X output = +(S12 + S22) -(S11 + S21)

Y output = +(S11 + S12) -(S21 + S22)

It's clear what will happen if the follow line is vertical or
horizontal, so let's consider the case of a diagonal line crossing the
centers of S21 and S12. If perfectly centered, X and Y outputs are both
zero. If the line drifts towards S22, what happens? S12 and S21 will
remain about the same, while S22 will grow and S11 will shrink. This
will cause X output to become more positive, while the Y output becomes
more negative.

One can go through this exercise for any line orientation, and get the
same answer, so the X and Y outputs provide a sufficient steering signal.

Joe Gwinn

Joe,... you're my hero. I get it.

I didn't think I would be able to understand a stationary head
tracker. Especially from one posting alone.

Just one thing. What makes the axes move at all?
Once centered on the line, why does it follow the line. And witch
way does it go? On a vertical line would it go up or down?

It's propelled at a constant speed as set by the operator, and the servo
only follows the line.

And do you know anything about the rotating head type.
I get how the tracker head sees the line and turns to follow it, but
how is the rotation of the tracker head turned into X and Y outputs?
Is it like the stationary head sensors, but fed by something attached
to the rotating head?

A rotating head can use one or more eccentric sensors to do the work of
all four sensors in my example (or nine sensors in the larger array for
edge following), so long as the sensor head rotates fast enough that the
curves to be followed are sampled very densely.

Joe Gwinn