On Mon, 12 Nov 2012 17:12:28 -0800, anorton wrote:
"Tim Wescott" wrote in message
...
On Mon, 12 Nov 2012 16:03:47 -0800, mike 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.
Expecting that pot to survive shipment with a big lever attached is
optimistic.
Switch to a real pot that mounts by the shaft bearing, not the element
contacts and a setscrew to remove the lever for shipment.
Or you could leave it the way it is and demonstrate that the
electrical solution is worthless if the mechanical implementation
fails. That's probably a more valuable lesson.
That pot is less than $2.00, and very low friction.
Your "real" pots are big, clunky, expensive, high friction, and require
brackets which must be purchased which further ads to the cost.
So finding a way to isolate that pot from damage while still having it
work would be a big plus.
Perhaps the other useful lesson is that it is best to nail down and
publish all the desired specs before having a design review or calling
the consultants. I am still not clear about what sort of weight and
torque is on that small pot. Just how little friction do you need? How
cheap does the solution have to be?
I should have posted a 3/4 view of the assembly; much would have been
clear.
The shaft is supported by bearings in the framework, and the pot has some
lateral play. So in theory, the pot sees no lateral force at all. Where
I fell down on this is that when the pot sees no axial force it has too
much play in rotation; taking up that play means that the pot, rather
than the bearings, is locating the shaft.
Your idea of using a flex coupling has merit. I'm not sure if I can do
it cheaply, but I'll think on it.
--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com