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Posted by Frnak McKenney on August 18, 2009, 9:47 am
Did anyone else notice John Iovine's article "How to Make
Bi-Directional Flex Sensors" in the June issue of Nuts & Volts
magazine?
While resistive flex sensors have been available for several years,
they haven't been cheap, and the shape options have been a bit
limited. Mr. Iovine's article suggests that these could be
home-built, shaped-to-order, and inexpensive enough that that many
of these could be added to a robot without blowing a hobbyist's
budget.
The sensor design seemed simple enough: a sandwich made up of a
strip of thin and flexible plastic (stiffener), two strips of thin
copper surrounding a resistive layer of some sort, and shrink-wrap
tubing to keep the ham and cheese... er, "conductive layers" from
squishing out. The resistive layer might be tricky (_cloth_??), but
I was fairly sure I had some black conductive bags around here
"somewhere" (translation: only a day or two of digging <grin!>).
And it looks like Mr. Iovine's approach would make it possible to
design (say) a flex sensor that could report, with reasonable
accuracy, human finger flexing by using a "club sandwich"
flex/copper/resistance design:
flex: ==============================
Cu1: ----------
Res1: oooooooooo
Cu2: --------------------
Res2: oooooooooo
Cu3: --------------------
Res3: oooooooooo
Cu4: ----------
Layer Cu3 might look like this from above (not to scale) if you
wanted all terminals to be at one end of the sensor:
-----------------------------------+
----------------+R2R2R2R2 |
|R2R2R2R2 |
|R2R2R2R2 |
|R2R2R2R2 |
|R2R2R2R2 |
|R2R2R2R2 |
+------------------+
So I went exploring Saturday afternoon. It looks like I need a new
set of places to look for oddments: the two big local art/craft
stores (Micheals, Ben Franklin) could provide sheet copper for
"embossing", but two thicknesses of their 36ga stuff seemed a bit
stiff for small robot sensors. No luck on flexible plastic.
Home Depot had some rolls of copper sheet ("roof flashing") that
seemed a bit thinner, but I didn't need 12"x20' of it, not at
$33/roll. No plastic there, either. Fortunately we have one "real"
hardware store here in Richmond -- Pleasants -- but I foolishly
didn't start with them, and now I'm temporarily sidetracked.
The article mentions that parts and pre-built sensors can be ordered
from the Images Scientific Instruments 'web site:
Images Scientific Instruments
http://www.imagesco.com/sensors/flex-sensor.html
(Based on the prices quoted there, it looks like $12 would purchase
enough material for 12 0.3"x5" sensor strips.)
Has anyone else tried making one of these? If so, what did you use,
and what were your results?
Frank McKenney
--
The vice of the modern notion of mental progress is that it is
always concerned with the breaking of bonds, the effacing of
boundaries, the casting away of dogmas. But if there is such a
thing as mental growth, it must mean the growth into more and
more definite dogmas. The human brain is a machine for coming to
conclusions; if it cannot come to conclusions it is rusty.
-- G.K. Chesterton: Concluding Remarks on the
Importance of Orthodoxy (1905)
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)
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Posted by Rich Webb on August 18, 2009, 10:13 am
On Tue, 18 Aug 2009 08:47:23 -0500, Frnak McKenney
[snippety snip]
>Has anyone else tried making one of these? If so, what did you use,
>and what were your results?
It's on the "to do" list (somewhere)
Small Parts http://www.smallparts.com/ has copper sheet but I don't see
any that has acetate backing but that doesn't seem to be a necessary
characteristic.
And, of course, McMaster-Carr (which seems to have pretty much
everything) has copper foil in various form-factors.
http://www.mcmaster.com/#copper-foil/8py95
(BTW - stick a space character after your double-dash for the "standard"
usenet sig delimiter. I.e., "-- " vice "--")
--
Rich Webb Norfolk, VA
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Posted by Frnak McKenney on August 19, 2009, 8:32 am
Hi, Rich. Thanks for the response.
> On Tue, 18 Aug 2009 08:47:23 -0500, Frnak McKenney
>
> [snippety snip]
Thank you. <grin!>
>>Has anyone else tried making one of these? If so, what did you use,
>>and what were your results?
>
> It's on the "to do" list (somewhere)
Hm. Wonder if that's the same "somewhere" where I stored my black
electrostatic bags, the "safe place where I can find it when I need
it"?
> Small Parts http://www.smallparts.com/ has copper sheet but I don't see
> any that has acetate backing but that doesn't seem to be a necessary
> characteristic.
No. I'm not exactly sure why the author specified "laminated"
copper, since the laminate's insulating properties aren't used, and
the separate strip of acetate (or whatever) provides the needed
stiffness/springiness. Also not sure what happens to laminated
copper when you solder to it: does it separate?
You did prompt me to dig up the relationship between PC board copper
in "ounces" and sheet metal thickness: 1oz ~= .0012in
> And, of course, McMaster-Carr (which seems to have pretty much
> everything) has copper foil in various form-factors.
> http://www.mcmaster.com/#copper-foil/8py95
Thanks.
The copper tape listed in McMaster-Carr is .0035", which would make
it roughly equivalent to "3oz copper" on a printed circuit board.
With conductive adhesive, it's only $13.03 for 1/2" wide, $8 for
1/4" wide; wonder how it would change the sensor's characteristics
if you put the adhesive side toward the resistive material?
Presumably that wouold mean better electrical contact, but in the
original (glueless) design the copper and plastic can slide a bit
against one another; I wonder what happens when the three layers are
"glued" together and it _can't_ slide?
Copper mesh (screen) would have a different "contact pattern" than
foil/sheet: it would press against the resistive material only at
the acreen's "peaks". Fewer points of contact, but more pressure at
those points... hard to guess how that would affect the sensor.
Oh, and then there's copper 101, copper 110, and beryllium copper. I
seem to recall beryllium-copper being used in springs; wonder if
using that would let us omit the acetate strip?
> (BTW - stick a space character after your double-dash for the "standard"
> usenet sig delimiter. I.e., "-- " vice "--")
Thanks for reminding me. It's not easy to see the extra space
sometimes... most times... okay, ever. <grin!>
Frank
--
...[S]tigmas are the corollaries of values. If work, indpendence,
responsibility, respectability are valued, then their converse must
be devalued, seen as disreputable.
-- Gertrude Himmelfarb/The De-Moralization of Society
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)
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Posted by Gordon McComb on August 18, 2009, 1:24 pm
Frnak McKenney wrote:
>
> So I went exploring Saturday afternoon. It looks like I need a new
> set of places to look for oddments: the two big local art/craft
> stores (Micheals, Ben Franklin) could provide sheet copper for
> "embossing", but two thicknesses of their 36ga stuff seemed a bit
> stiff for small robot sensors. No luck on flexible plastic.
Haven't seen John's article but I'd opt for foil tape, a fairly common
find at craft stores (but maybe not Michael's). Widths are from 1/4" to
maybe 1", with thicknesses from 1 to 2 mil or so.
Masterfoil is a popular brand. It's used most in stained glass work, but
it has many uses. I've made *capacitative* sensors out of these kinds of
tapes, but never resistive. Might be an interesting experiment.
-- Gordon
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Posted by Frnak McKenney on August 19, 2009, 8:41 am
Hi, Gordon. Thanks for joining in.
On Tue, 18 Aug 2009 10:24:47 -0700, Gordon McComb
> Frnak McKenney wrote:
>>
>> So I went exploring Saturday afternoon. It looks like I need a new
>> set of places to look for oddments: the two big local art/craft
>> stores (Micheals, Ben Franklin) could provide sheet copper for
>> "embossing", but two thicknesses of their 36ga stuff seemed a bit
>> stiff for small robot sensors. No luck on flexible plastic.
>
> Haven't seen John's article but I'd opt for foil tape, a fairly common
> find at craft stores (but maybe not Michael's). Widths are from 1/4" to
> maybe 1", with thicknesses from 1 to 2 mil or so.
>
> Masterfoil is a popular brand. It's used most in stained glass work, but
> it has many uses. I've made *capacitative* sensors out of these kinds of
> tapes, but never resistive. Might be an interesting experiment.
Hm. Stained glass... that sparks a memory. I may have a roll of
that around here... um, "somewhere". Perhaps if I look for the
electrostatic bags first...
Frank
P.S. Thank you (and Rich) for not pointing out that I misspelled
the magazine name. It should have been "June 2009 SERVO" rather
than "June 2009 Nuts & Volts". Sorry about that.
--
It is the business of education to implant insight and respon-
sibility. It must turn irresponsible opinion into responsible
judgement and lead from chance and arbitrariness to the rational
lucidity of an intellectual order. -- Mies Van der Rohe
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)
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