On Tue, 13 Nov 2012 15:02:26 -0800, rangerssuck wrote:

On Tuesday, November 13, 2012 4:47:14 PM UTC-5, Tim Wescott wrote:
On Tue, 13 Nov 2012 13:14:02 -0800, rangerssuck wrote:



On Tuesday, November 13, 2012 2:28:05 PM UTC-5, Tim Wescott wrote:

OK, here it is again with more pictures and explanations. Sorry for



starting a new thread, but it seemed reasonable given that I did
such a



poor job of getting the design problem stated in the last one.







I'm building a gizmo, which, for lack of a better name I'm calling a

"fan



trainer". It consists of an arm about 30" long with a small DC
motor

and



propeller on one end, and a pivot and counterweight on the other.
The



thing is pivoted on a frame which has a controller circuit board.
The



controller monitors the motor current and the arm position, and
drives



the motor voltage.







This isn't an executive toy that you sit back and watch move (that

would



get old very fast). Rather, it is a platform to provide a series of



exercises for a student to tune the control system, and to do it in
a

way



that you can see and feel it working. I'd use it in conjunction
with



seminars, and perhaps make it part of a "seminar at home" package.







So my larger problem is to manufacture this in small quantities (20
to



100 a year), and sell them at a reasonable price without losing my
ass.



"Not losing my ass" translates to a total bill-of-materials cost of
$50



or so, and if I could get it way lower then I'd just sell it for
less

and



increase my potential market.







My immediate problem is that the potentiometer that senses the arm



position is getting punched off of the board in shipping. The



potentiometer has a hole, through which you pass a 4mm shaft with a

flat



milled into it. If you mill the flat so that you can easily pass
the



shaft through the pot then the control is not smooth -- the arm
hunts



across the rotational slop caused by the shaft rotating within the
pot.



If you shim the flat for a snug fit (very very light press fit?)
then



when you chuck the thing into a box and fly with it in checked
baggage,



the pot gets punched off of the board, apparently by getting hit
from



behind.







The shaft on which the arm pivots is restrained in the housing by a



couple of model airplane wheel collars. On the trip out, both of
these



collars, and the arm, loosened on the shaft, and the pot was punched



out. On the return trip I loosened the arm and tied it to the back

plate



of the frame -- the wheel collars were fine then, but the pot was
still



punched out.







I really like that pot: it costs less than $2.00 for onsies (less,



obviously, in higher quantities), it has undetectably small
friction,

it



isn't noisy, and because it's board mounted it saves me from needing
a



bunch of brackets which would just drive up my BOM cost. So any



alternative that involves not using the pot has to compete with that



price, and being practical to do in small quantities in an
environment



where labor is not free.







So I'm thinking at this point that perhaps I just need to be happy
with



what I have, and to warn people not to drop the thing off of a table
or



to ship it without disassembling it first. But it would be nice if

there



was a way to make it more robust (by isolating the pot from the
shaft



somehow).







The best suggestion I got from the other thread, assuming that I can
do



it cheaply, is to put a slit in the end of the shaft, so I basically

have



a D-shaped shaft with some spring. I would like that idea a lot if
I



knew what it would cost to have a batch of 20 shafts made with the

slit,



vs. without, and if that cost wasn't much greater than just making
the



shafts.







The second-best suggestion is to use a flat coupling. This would

require



(essentially) two shafts and the coupling, which is clearly going to



drive up the BOM cost, but I'm still toying with how to make it
cheap.







Several people suggested reinforcing the mounting of the pot to the



board: it probably doesn't show in the pictures, but it is clear
from

the



construction of the thing that this would just result in _part_ of
the



now-destroyed pot remaining on the board.







Here's a general arrangement shot:



https://docs.google.com/open?id=0B5l...0MxMEM3b1l2c0k







And the thing in action:



https://docs.google.com/file/d/0B5lS...0MxMEM3b1l2c0k







And, finally, a close-up of the potentiometer in question:



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







--



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



I think you ought to focus on *why* the wheel collars loosened. I am

assuming that had they stayed secure on the shaft that the pot would

have survived. If that is a correct assumption, how about a dimple in

the shaft for the setscrew, or perhaps some loctite?



This is a pretty neat training gizmo for control loops. Sort of like
the

ball balancer, but better.



I considered doing a ball balancer, but I figured this would cost less.



For that matter, I really wanted to do an inverted pendulum -- but I

couldn't see how to do the mechanism cheaply.



The wheel collars stayed tight on the flight home, yet the pot was
still

punched off the board. Not by as far -- but all leads broken is still

all leads broken. I suspect that any sort of collar that rides close
to

the board to limit travel in that direction will either rub or have too

much play to be safe -- but I could be wrong.



At this point what I see is a choice between some rotational slop
between

shaft and pot (which messes up the educational value of the thing), the

current setup (which leaves it fragile, but possibly manageably so),
some

sort of a flex coupling or spring (assuming I can figure out how to do

that well and cheaply), or some sort of a spring-loaded means of
holding

the shaft-pot joint to be snug in rotation, but still low friction to

axial motion.



--

Tim Wescott

Control system and signal processing consulting

www.wescottdesign.com

How about mounting the pot semi-ridgedly on kinked or s-bent pieces of
bus wire? Pretty much free (except for 30 seconds or so of extra
assembly) and easily repairable. If the pot really has very low
rotational friction, the wires shouldn't bed in normal usage, but will
give enough to prevent damage. You could even try some springy piano
wire.

Hmm. Good thought, but it'd take a lot more than 30 seconds. It's a
surface-mount part, so mounting it as designed is very quick while
mounting it just about any other way isn't so.

--
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