On Tue, 13 Nov 2012 13:28:04 -0600, 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

The thing in action, with the right link:
https://docs.google.com/open?id=0B5l...0MxMEM3b1l2c0k

--
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

On 11/13/2012 12:50 PM, Tim Wescott wrote:
On Tue, 13 Nov 2012 13:28:04 -0600, 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

The thing in action, with the right link:
https://docs.google.com/open?id=0B5l...0MxMEM3b1l2c0k

Is there a possibility of mounting a second pot on the other side of the
circuit board so as to act as a second regular bearing and a thrust
bearing. Any "Z" motion of the shaft would be restrained by the dummy pot.

Paul