I would think that an air pressure switch would be greatly effected by temperature and barometric pressure.
You could get a accelerometer/gyroscope/magnetometer break-out module from Sparkfun and always know where your scope is pointing and when combined with an Arduino (or a better microcontroller), be able to stop the mount when it moved into any of your defined danger zones. Not to mention getting a way to verify your scope's (low precision) pointing remotely with data directly from the module.
Waikoloa, HI 96738
From: ap-gto@... [mailto:ap-gto@...]
Sent: Monday, September 01, 2014 10:13 AM
Subject: Re: [ap-gto] AP1200 "weird" parking behaviour
For some time now, I have been considering an idea like your head & counterweight interaction, below, to protect (not my head), but the telescope from a pier collision.
It should be possible to purchase – perhaps from a science shop like Edmund Scientific, etc – an “air pressure control switch” based sheet, like those floor mats that some stores use to start up a kiosk PC when a customer steps onto it. The idea is to wrap the sheet fairly loosely around the pier and mount – as best you can, and tape the end seam forming a cylindrical vertical collar – or even two of these, one for the pier and another smaller section around the mount, if shape is a problem. . If the mount runs the scope or camera anywhere into that contact zone, (or even into iself, as mine ran the scope into the AZ adjust knobs), the air pressure difference in its bladder activates a KILL switch to remove power from the mount, perhaps along with a flashing warning light.
Or, with a bit of Ray’s programming, the contact closure could also be tied to a spare unused signal pin on the CP3 DB-9 port, which the AP Driver could read and emergency stop the mount, or at the user’s option, PARK it.
I think such an air pressure switch might provide a potentially “fail safe operation” of any mount, and might make for a good AP accessory product. It is water and weather proof, shouldn’t freeze in the worst winter, and can be tucked around any size mount, or pier, to handle any length OTA & camera, if made in various lengths. It could be based on an air bladder, or two conductive sheets (like the “old technology”, touch sensitive PC screens), which act as a switch when they are brought into contact to the scope, anywhere on its surface. A 3/4 inch thick foam switch-pad, might also provide some paint & dent protection, as the scope slows down into the collision.
The only difficulty might be wrapping it under a rats nest of cables, some people have. But probably worth the effort in tucking it under the mess..
Might be worth buying one of those “kiosk mats” for testing.
I wrap several layers of thick foam around my counterweights and countwerweight shaft to prevent accidentally hitting my head in the darkness.