Re: Anyone designed a limit switch or similar to prvent pier col...

Joe Zeglinski

Hi Rick,

I've had a chance to sleep on the idea and I think I have a practical and
do-able solution (relying on my past 40 years of computer systems and hardware

Think about how "Touch Sensitive Screens" work on PC monitors at kiosks.
These are really just "continuous surface microswitches".

There is a high tech optical based design - your finger interrupts either
ultrasonic surface waves or infra red beams, as you point onto the screen
surface. X/Y encoders on the bottom and side edges, behind the bezel,
calculate your finger pointing cursor position. This is one solution.

The low tech, mechanical, solution, is a "two layer Mylar sheet" covering
the monitor screen, with resistance coated inner surfaces facing each other,
and held apart by very sparse clear plastic grid, so they don't collapse and
short out. When you "depress" the front Mylar surface, your finger shorts the
two facing, inner surfaces at that point, and similar X/Y encoders to the
optical design, calculate finger position by the decrease in the X/Y
resistance, like a centre tapped potentiometer.

Your idea of using "microswitches" to detect collision of the OTA with the
pier, can be done in a similar way, to the previous mechanical approach.

First, you purchase (or even make) a long, rectangular sheet of the Touch
Sensitive Display screen material. The length is somewhat longer than the
bottom half of your OTA . Effectively, the T.S.D. sheet only needs to be as
tall as the extent where the end of the scope will strike the pier - doesn't
need to be as long as the pier is tall - certainly not all the way to the
ground (unless you have a long Newtonian).

Next, fasten (glue), a Velcro strip along the vertical edges, to act as a
zipper. The width of the T.S.D. material should be about an inch or so wider
than the circumference of the pier, it will wrap around. This is to allow some
foam inner backing.

Finally, glue a half inch, or so, foam backing sheet to the inner side of
this T.S.D. "pier wrapper" - to act as a cushion for the OTA, as it
approaches the pier (somewhere along the pier's upper length), and presses
softly into the foam backed Touch Sensitive Display sheet. The thickness and
stiffness of the backing foam determines how much time you will allow the OTA
to be hitting the pier - tracking motion can be handled with thin foam, but if
you are slewing, extra thickness gives more TSD wrapper crush time warning.
The stiffness has to be such that it will push the TSD back against the OTA
edge, but soft enough not to damage the mount motor gears.

There you have it - a scope version of a finger pointing to a kiosk
monitor screen. Same principle, using existing, easily obtained material.
Plus, the scope will be detected ANYWHERE it hits the pier, closing the
contacts at that point, with this one TSD "switch".

This "pier cushion/wrapper" can handle a small range of pier diameters,
since the Velcro zipper can be tighten up, to fit. The wrapper can also be
removed, rolled up, or transferred to another pier. The TSD material cushion
will be Velcro zippered facing North, where the OTA will never hit the stripe
that does not have TSD protection (just the Velcro strip itself), so you can
economise by using a narrower sheet of TSD, and allow a wider width of Velcro
zipper. The wider Velcro gives you variable overlap, and a wider range of pier
diameters, that it can be used on.

The length doesn't have to be exact, since you can slide the cushion into
place up or down, perhaps to the mid height of the pier - since the OTA isn't
likely to hit the pier between there and the base of the mount (that is an
unused length of pier space, so why pay for it).

You can also make small diameter versions, to apply as "TSD material Tube
Socks" to the three legs of your field tripod, and join the three collision
detector cables together. When the OTA hits any one of the legs, the common
signal gives you a warning, or if wired in series with the mount, kills (or
reverses) the power to the drive motors.

I think this "Touch Sensitive Pier Wrapper" will work - it can be wired
into a logic circuit for complex software reporting and warning, which will
take automatic collision reversal actions, or it can simply be a non powered,
conductive switch surface, wired in series with your mount's drive power
source, or hand controller buttons.

As I alluded to earlier, rather than paying a few hundred dollars for the
specific computer monitor TSD material, you can probably use your ingenuity to
fabricate your own. All you need is two Mylar conductive sheets, and separate
them with badminton netting (or other smaller hole spacing thin netting). Glue
the edges, add two wires to complete the circuit - you have a TSD sheet for
your pier wrapper.

You can make your own "conductive Mylar sheets" by spraying the inner
facing surfaces of the two sheets with "conductive paint" sold as EMI
shielding conductive paint, in a spray can (for a few dollars). Basically,
it's either silver or copper particles in solution. It's a very inexpensive
way of making your own TSD continuous surface microswitch sheeting. The entire
TSD pier cushion might cost you under $30, as a guess.

Just a thought ... Comments?


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