Pages: [1] 2   Go Down
Author Topic: Arduino-Load-cell interface?  (Read 9577 times)
0 Members and 1 Guest are viewing this topic.
0
Offline Offline
Newbie
*
Karma: 0
Posts: 9
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Does anyone have advice on interfacing a loadcell with Arduino? If so what loadcell was used and where can it be obtained?
Mactitan.
Logged

Bristol, UK
Offline Offline
Edison Member
*
Karma: 1
Posts: 1197
Exhibitor at UK Maker Faire
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

I'm working on a weighing device, which will work in a similar way to a load cell.  Try searching the forums for "strain gauge".  I got the strain gauge bridge out of a set of inexpensive kitchen scales (Tesco's, £10).  Some photos of the disassembly are here:

http://www.flickr.com/photos/anachrocomputer/sets/72157612291470816/
Logged

0
Offline Offline
Newbie
*
Karma: 0
Posts: 9
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Thank you. I wonder if a cheap electronic bathroom scale will have the same loadcell or strain guage?
Logged

Bristol, UK
Offline Offline
Edison Member
*
Karma: 1
Posts: 1197
Exhibitor at UK Maker Faire
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

I opened up my bathroom scales, and they did have a similar beam with bridge-connected strain gauges.  But they also had a complicated mechanical arrangement to take the load from the user any convey it to the beam.  Plus, there was a sort of "mechanical advantage" thing going on, using classical principles of leverage to reduce the force on the sensor beam.
Logged

0
Offline Offline
Newbie
*
Karma: 0
Posts: 9
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Thank you for the reply, Anachrocomputer.
Looks like I will have to look for a kitchen scale with a LC. We don't have Tesco in Canada, however I am sure Wal-Mart will have something.
Logged

0
Offline Offline
Newbie
*
Karma: 0
Posts: 24
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Hello,

For amplifier i use this:
http://home.people.net.au/~aircommand/day67.htm#circuitdiagram

I have heard that these guys are good, i got mine from
ebay.


http://www.chinaeleccomp.com/

//Lnxrkthkr

« Last Edit: February 03, 2009, 05:49:37 pm by lnxrkthkr » Logged

Left Coast, CA (USA)
Offline Offline
Brattain Member
*****
Karma: 361
Posts: 17261
Measurement changes behavior
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

I have a Omerga engineering load cell (0-20lbs). It has a 4 wire bridge output. You wire a positive voltage and ground (5-10vdc) to two of the wires and the other two wires generate a DC millivolt output voltage (2mv/v) meaning if using a 5vdc voltage source the bridge output range would be 0-10 millivolts. This would have to be amplified by an op amp to be useful to the Arduino A/D inputs.

Lefty

Logged

0
Offline Offline
Newbie
*
Karma: 0
Posts: 9
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Thank you for the super info. I will post my progress once I get the hardware setup.
Regards
mactitan
Logged

0
Offline Offline
Newbie
*
Karma: 0
Posts: 19
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Load Cell and Arduino connected using the ina125p ic.

I have just made a small project using this ic to interface to the analoginput of the Arduino board.
The ina125p is very very easy to work with, and can be used in many different configuration.
Just add one component: A resistor to set gain.

I have tryed to make a small page about the project:
http://cerulean.dk/words/?page_id=42

Good luck with yours. :-)
Logged

0
Offline Offline
Jr. Member
**
Karma: 0
Posts: 94
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Do load cells suffer from any form of drift? If I was to put a 20Kg weight onto a bathroom scales and come back in a year (assuming infinite battery life) would the scales still be reading 20Kg? I have an application where the weigh would be altering slowly and have long periods of no weight change at all. I don't need fantastic accuracy but would need no drift or very little so that in the periods of inactivity the reading won't change much between viewings.
 I'm thinking there wouldn't be significant drift as these things are pretty linear and repeatability is high but most bathroom and kitchen scales require you to zero them before each use and whilst that maybe due to you moving it around I'm wondering if there is anything else going on.
Logged

0
Offline Offline
Tesla Member
***
Karma: 141
Posts: 9470
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

If you use elastic propertys of materials to support the load and allow measurable movement, then there will probably be some non recoverable deformation over time when left under load. You might want to look at setups that don't depend on material deformation to generate a measurable change.
Logged

Consider the daffodil. And while you're doing that, I'll be over here, looking through your stuff.   smiley-cool

Left Coast, CA (USA)
Offline Offline
Brattain Member
*****
Karma: 361
Posts: 17261
Measurement changes behavior
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Kind of depends on the quality of the specific load cell, the better ones have pretty good repeatablity and one reason they can be so costly. However a good design would utilize a software 'tare weight' function that would 'zero' out any calibration drift and is also useful for factoring out a container's weight when you just want to know the weight of the material you place or pour into the container. Tare compensation is easy to do in software.

Lefty
Logged

0
Offline Offline
Tesla Member
***
Karma: 141
Posts: 9470
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
Tare compensation is easy to do in software.
That's correct, but the difficult part in this instance is to determine what would be the appropriate compensation to apply in this DIY setup. If commercial load cells are used, then they may have performance data sheets available for calibration based on time/load variables without "re-zeroing" (doubtful).
Logged

Consider the daffodil. And while you're doing that, I'll be over here, looking through your stuff.   smiley-cool

Left Coast, CA (USA)
Offline Offline
Brattain Member
*****
Karma: 361
Posts: 17261
Measurement changes behavior
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Quote
If commercial load cells are used, then they may have performance data sheets available for calibration based on time/load variables without "re-zeroing" (doubtful).

When I was working in a oil refinery we had many expensive load cell applications installations, from several high end suppliers. We always had to calibrate (both zero and gain) periodically using traceable calibration weights (certified weights in grams up to 50 pound weights). This commerical equipment always came with internal calibration adjustments and procedures. For internal use we would calibrate yearly, but for custody transfer use (meaning real money changes hand  smiley-wink we would do it more often. Analog instrumentation electronics should always have a calibration requirement that needs to be evaluated for every specific application. But then again how many of us check/verify the calibration of our personal test equipment, DMM, signal generators, scopes, etc.  smiley-wink

PS: I wonder if drug dealers calibrate their scales?  smiley-wink

PS2: Damn, 3700 posts, time sure flies in the Arduino world.
Lefty
« Last Edit: July 23, 2010, 01:37:45 pm by retrolefty » Logged

0
Offline Offline
Jr. Member
**
Karma: 0
Posts: 94
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Thanks for your responses. I can see that if you use springs for example to stabilise a platform that is then connected to a load cell the change in the springs over time would alter the reading. How likely is this if the platform is directly resting on the load cell i.e. no mechanism to transmit load or stabilise just a load cell? I also appreciate that even with an expensive load cell you are still deforming something to be able to measure it. Using a tare function is what the normal kitchen or bathroom scales use but I'm not sure how I could do this without removing the weight (which isn't an option for me). I could incorporate some sort of mechanism that retracted the load cell every now and then to reset the zero but this would of course increase the complexity of the project considerably. Any ideas on what sort of magnitude I'd be looking at with drift? I'd be measuring say 50 Kg on a load cell out of a bathroom scales, what drift would I expect over 6 months? I'm not really too concerned about anything up to about 500 grams but a kilo would be a problem. Thanks again fellas.
Logged

Pages: [1] 2   Go Up
Jump to: