On Tue, 13 Nov 2012 10:22:57 -0800, Paul Drahn wrote:

On 11/12/2012 2:27 PM, Tim Wescott wrote:
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.

I think a little more forensic study is needed. Did the solder break?
Did the traces on the board pull off the board? Remember when we first
learned to solder we were told never to rely on solder to hold anything.

I would suggest you fabricate a tinned clamping piece with a hole for
the shaft and bent over the edges of the pot and then soldered to the
circuit board. Solder the clamp before you solder the legs of the pot so
there is no upward pressure on the clamp.

The solder broke on some pins, and the pins broke elsewhere. But the pot
is clearly not designed to be a bearing, either regular or thrust.
Reinforcing the solder joints enough will just make the housing break.

Isolating the pot from everything except for shaft rotation -- that
should work.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com