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

Joe Zeglinski


I was thinking of scenarios where you could put the OTA into an unintended
collision, "while" in the process of backing up.

Lets say the scope had no DEC motion, and the scope was approaching the
pier, at tracking rate. If you followed you original thinking, and reversed
both motors, then the OTA might have been fine, except that it is now
mispositioned during a reverse, where it might now hit the OTA, from a
different direction. Now the OTA does strike the pier, because of the DEC
adjustment. Or, if it did hit, during this second unnecessary swing, the
circuit now reverses the reversal.

Of course, I may have misinterpreted your original intention, of collision
avoidance during tracking, ONLY, and inaccurately extended the discussion to
dual motor reversal. At the very least, even quickly stopping all motion, with
the pier switch approach, will be a large success. Besides, if the OTA were to
hit the pier during slew, I suspect the momentum would be too great to avoid a
minor dent, with the circuit. I'm not sure how quickly it could be emergency
stopped, during a fast slew.

The really bad news is that a pier based pneumatic switch doesn't cover
all possibilities. When I took my new AP900 for it's first ride in the rec
room, I let carefully slewed the scope to see how far it would go before it
hit RA limits. Well, since it was also tracking when I approached the far RA
extent, the OTA got a scratch when it touched the AP's Pier Adapter azimuth
adjust knob - no matter how carefully meticulous I was trying to be. In this
case, the OTA was nowhere near the tripod (pier), but it still would have
gotten a dent, rather than just a minor scratch, from the "pier adapter", on
top of the tripod. If the tracking mount strikes the mount parts itself - gear
box motor housing, or base fork parts, the protection we have been discussing
will fail - not even a loosely engaged clutch will prevent some damage from

... Then again ... wouldn't it be nice ...

It would be nice to have a software application where you could "train it"
to identify the free space of OTA operation - train it the way they train a
robotic arm to work on an assembly line. I believe the trainer swings the arm
around (manually) in all "task" directions to map out the free zone. An
example would be the paint sprayer robot on an automotive assembly line - a
human programmer "shows" the robot where to spray, by literally taking it by
the hand during it's training session. Then a collision avoidance profile, or
map, is vectorized and extrapolated from the manually tested positions. In
operation, after human training, as other applications command the robot to
slew to a target position, the servo driver software constantly checks that
the encoders are showing the robot is still within the "safe envelope", mapped
for the job. This is a 3-D profile, just as would be required for the
telescope mount. In this case, the ASCOM driver used by the application (The
Sky, etc.), would make sure that the OTA is within it's trained 3D envelope.
This would be done for each OTA used on the mount, with perhaps a "safe band"
added, to allow your repositioning of the OTA to balance accessories at some
later date, after training.

The good news is that something basically similar already done in 2D - the
CP3 program allows you to define a "horizon limit" below which the scope will
not GOTO. Now, second stage, create a piece of software to gather AZ-ALT
coordinates as you slew the scope (during daylight) up and down over tree
tops, buildings, and other obstructions on your 360 degree horizon, and
"vectorize that path". This CP3 extension profile is still 2D, but at least it
enhances the DEC positioning, to avoid "the trees", instead of just "the
straight treeline".

Finally, to extend this to 3D (pier/mount avoidance), you would swing the
OTA vertically (+/- 90 degrees), in Dec, at some increment of Hour Angle,
indicating to the training program, how far the OTA will be allowed to slew,
at that specific HA step position. You now have a series of safe limit
coordinates (two Elevation limits at each HA), which can be joined into a 3D
safe envelope, for future use. This envelope not only defines the 2D curve of
your visible horizon, it also defines the third level - of safe limits of OTA
positioning. If you travel to a different site, the horizon curve limiting
position can be reset to the old "treeline" method, as is done now in CP3, but
the third level - collision avoidance curve will still be correct, as long as
you are polar aligned - the same as when it was trained in daylight back home.
Otherwise, if you go to a somewhat different site, at a significantly
different latitude, the third level curve could be rotated by the delta in
geographic latitude, and would still be sufficient to prevent the OTA from
striking the mount or tripod.

Just a thought ...

Good luck with the basic system.


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