inexpensive tension sensor (load cell)?

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Parallax charging too much is nothing new. I believe Digikey sells them, or something very similar, for $7 or $8.
Chris S.

order
or
100
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.
http://dkc3.digikey.com/PDF/T033/1154.pdf

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Si Ballenger wrote:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
shb*NO*SPAM*@comporium.net (Si Ballenger) writes:

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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
(Si Ballenger) writes:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.