inexpensive tension sensor (load cell)?

[Kyler Laird]

I need to sense the change in tension on a couple of bungee
cords. This seems basic - just put a load cell between each
cord and my device (a wheelchair).

I've never used a load cell though, and the ones I'm finding
are expensive ($300+) and require a separate controller. I
see consumer devices that are almost exactly what I need
http://www.nationalfishingsupply.com/ber50lbdigsc.html
for one tenth the price, so I suspect there's something out
there that's more suitable for me.

Anyone know of a small (<50 pounds of force) tension sensor
with serial (RS-232/USB) output that doesn't cost a lot? I
do need on the order of 10 samples per second, but I'm just
looking at changes (not absolute accuracy) and precision of
under half a pound is probably fine.

Thank you.

--kyler

 

[Jose Cuervo]

Kyler,
Have you considered using a spring and a potentiometer? With a little
ingenuity you can use a spring that stretches under tension and a
potentiometer that "measures" the stretch. The spring can actually be a
portion of the bungee cord.


"Kyler Laird" <Kyler@news.Lairds.org> wrote in message
news:hn84b1-arg.ln1@jowls.lairds.org...

I need to sense the change in tension on a couple of bungee
cords. This seems basic - just put a load cell between each
cord and my device (a wheelchair).

I've never used a load cell though, and the ones I'm finding
are expensive ($300+) and require a separate controller. I
see consumer devices that are almost exactly what I need
http://www.nationalfishingsupply.com/ber50lbdigsc.html
for one tenth the price, so I suspect there's something out
there that's more suitable for me.

Anyone know of a small (<50 pounds of force) tension sensor
with serial (RS-232/USB) output that doesn't cost a lot? I
do need on the order of 10 samples per second, but I'm just
looking at changes (not absolute accuracy) and precision of
under half a pound is probably fine.

Thank you.

--kyler

 

[Kyler Laird]

"Jose Cuervo" <zorro310@comcast.net> writes:

Have you considered using a spring and a potentiometer?

Yes. I've even considered using a couple of them (tension and
direction) and just using a single cable.

With a little
ingenuity you can use a spring that stretches under tension and a
potentiometer that "measures" the stretch. The spring can actually be a
portion of the bungee cord.

I can see it. It doesn't seem as elegant though. I've been
around way too many potentiometers with noisy outputs, so I
was hoping to come up with something cleaner.

I'm most interested in position, not tension, so I could even
use a spool of cable (Flexi-lead) with a rotary encoder.

Thank you for the ideas.

--kyler

 

[Si Ballenger]

http://www.parallax.com/detail.asp?product_id=30056


On Wed, 17 Dec 2003 23:11:50 GMT, Kyler Laird
<Kyler@news.Lairds.org> wrote:

"Jose Cuervo" <zorro310@comcast.net> writes:

Have you considered using a spring and a potentiometer?

Yes. I've even considered using a couple of them (tension and
direction) and just using a single cable.

With a little
ingenuity you can use a spring that stretches under tension and a
potentiometer that "measures" the stretch. The spring can actually be a
portion of the bungee cord.

I can see it. It doesn't seem as elegant though. I've been
around way too many potentiometers with noisy outputs, so I
was hoping to come up with something cleaner.

I'm most interested in position, not tension, so I could even
use a spool of cable (Flexi-lead) with a rotary encoder.

Thank you for the ideas.

--kyler

 

[Garrett Mace]

"Si Ballenger" <shb*NO*SPAM*@comporium.net> wrote in message
news:3fe110aa.9801073@news.comporium.net...

http://www.parallax.com/detail.asp?product_id=30056

Wow, thanks, that's pretty neat! I was in a discussion with a bunch of hobby
CNC'ers a month or two ago, and we were looking into force sensing to do
things like detect tool crash, compensate for irregular materials, etc. The
load cells we could find were all too expensive or couldn't handle much
force.

Parallax is charging a bit extra at $25.00 each, if all you want is the
sensor. This( http://www.tekscan.com/pdfs/FlexiOrderForm.pdf ) is the order
form to get the sensors directly from the manufacturer, a 4-pack for $55 or
an 8-pack for $99. The sensors are calibrated for 1 pound, 25 pounds, or 100
pounds and there is no price difference.

 

[Kyler Laird]

shb*NO*SPAM*@comporium.net (Si Ballenger) writes:

http://www.parallax.com/detail.asp?product_id=30056

Ah! A good place to start. I can imagine putting one of these on
the inside of the robot's frame and connecting each cable to an arm
that would press against the sensor.

Interesting...

Thank you.

--kyler

 

[Chris S]

Parallax charging too much is nothing new. I believe Digikey sells them, or
something very similar, for $7 or $8.

Chris S.

Parallax is charging a bit extra at $25.00 each, if all you want is the
sensor. This( http://www.tekscan.com/pdfs/FlexiOrderForm.pdf ) is the
order
form to get the sensors directly from the manufacturer, a 4-pack for $55
or
an 8-pack for $99. The sensors are calibrated for 1 pound, 25 pounds, or
100
pounds and there is no price difference.

 

[Si Ballenger]

On Thu, 18 Dec 2003 11:42:05 -0500, "Chris S" <chrisks@udel.edu>
wrote:

Parallax charging too much is nothing new. I believe Digikey sells them, or
something very similar, for $7 or $8.

Chris S.

Check the bottom of the below pdf page. The pressure ranges could
probably be changed by using some type of linkage gizmo to
increase or decrease the force. A digikey search for force sensor
brings up a number of items.

http://dkc3.digikey.com/PDF/T033/1154.pdf

Parallax is charging a bit extra at $25.00 each, if all you want is the
sensor. This( http://www.tekscan.com/pdfs/FlexiOrderForm.pdf ) is the
order
form to get the sensors directly from the manufacturer, a 4-pack for $55
or
an 8-pack for $99. The sensors are calibrated for 1 pound, 25 pounds, or
100
pounds and there is no price difference.

 

[Gordon McComb]

Si Ballenger wrote:

Check the bottom of the below pdf page. The pressure ranges could
probably be changed by using some type of linkage gizmo to
increase or decrease the force. A digikey search for force sensor
brings up a number of items.

These aren't the same as the Flexiforce sensors, but they're also quite
handy. CUI manufacturers "point force" sensors that in some instances
are a better pick than the "band" style.

For the OP and any followups: For instantaneous sensing (no DC) there's
piezo cable. It's available from the manufacturer, with a $100 minimum
order, but a couple outfits on the Web resell it. A simple comparator,
resistor, zener can be used for the input interface. Because piezo
sensors only register change of force, the cable cannot detect
continuous pressure. But it would readily detect a tool hitting its
stops. In fact, that could be readily done with a 50 cent piezo disc.
The harder it hits, the stronger the voltage spike.

-- Gordon
Author: Constructing Robot Bases,
Robot Builder's Sourcebook, Robot Builder's Bonanza

 

[Peter]

Maybe this site is more help:

http://www.imagesco.com/catalog/sensors/pressure.html

I think its the same stuff (and they have more) for half the price of parallax.

Peter

Kyler Laird <Kyler@news.Lairds.org> wrote in message news:<53t8b1-ra3.ln1@jowls.lairds.org>...

shb*NO*SPAM*@comporium.net (Si Ballenger) writes:

http://www.parallax.com/detail.asp?product_id=30056

Ah! A good place to start. I can imagine putting one of these on
the inside of the robot's frame and connecting each cable to an arm
that would press against the sensor.

Interesting...

Thank you.

--kyler