So, I'm working on a gizmo to go with a series of control systems
training seminars that I'm putting on.

The gizmo is basically a fan on a pivoted stick, with a control system
that works to maintain the angle of the stick relative to its mount at a
commanded value. It does a good job of giving people a visceral
understanding of how a feedback control system works, and I don't think
it's going to cost me much to produce.

But I went and took it on an airline flight for the first time this week,
and it didn't survive well. The position feedback from the pivot is
provided by a nice inexpensive potentiometer with a D-shaped hole, into
which one inserts a shaft of the correct dimensions. Here's a close-up
of the pot mounted on the board, with a shaft (and, if you've sharp eyes,
a little paper shim that keeps things snug).

https://docs.google.com/open?id=0B5l...TBCc3VzWDYxZ0E

On the flight (two flights each way, Portland to Ottawa and back), the
pot broke off the board. It was obviously punched out of the board by
the force of the shaft acting on the back of the pot. Fortunately the
training is for engineers, and it was at a corporate site, so my customer
was able to repair the thing and I was able to use it for demonstrations.

Unfortunately -- even though I thought I had identified the problem and
fixed it -- it broke on the way back, too.

Now, one solution to this may just be that I need to find a different way
of putting the whole thing together so that it's easy to disassemble for
shipping, and then don't ship it assembled.

But I also want to put it out to the group for suggestions: the shaft
needs to be shimmed to a snug fit in the hole of the pot, or the slight
play between pot and shaft messes up the control (the arm will hunt
within the slop of the connection). But shimming things seems to set up
a problem with the shaft transmitting too much force to the pot, and --
ping!!!

This thing has experienced a moderate amount of knocking around in my
shop, and use both on the property and around the local area without
breaking. But as soon as I go and ship the damn thing it breaks. So not
only am I very concerned about shipping, I'm a bit concerned about this
being a point of fragility in an otherwise reasonably stout mechanism.

Comments appreciated.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com