Hi All-
First post here. Question for you all, do you think it is possible to increase the sensitivity of these 3 wire load cells you see all over the place by grinding away some of the metal directly below the strain gauge like this:
I will be using (4) of these in the common bridge setup. This will change the resistance under load, but there is no way I can make the cuts accurately so that the resistance stays the same on all four.
Would this imbalance even matter?
Thanks,
Axis
Grinding would be hard to control, with the result that the sensitivities of the individual sensors would all be different. It might be an interesting challenge to calibrate the resulting bridge.
Can you not measure the resistance WHILE you are making the cuts?
Thanks for the idea Paul, that might be a lil tricky to do. The resistance doesn't change until a load is applied which would make it hard to get my dremel where it needs to be. And then the heat from friction during grinding will change the resistance. My application is really suited to this type of load cell except the weight I am working with is less than a tenth the max load.
I admit I haven't completely researched the subject of strain gauges. I assumed when using (4) in a bridge the resistances are additive. So if each load cell had a different linear slope when summed with the other 3 I would have a constant linear function. Like I say I am assuming and it is very easy for me to overlook something.
The output of the bridge is the difference between the voltages output by the left and right voltage dividers (assuming that the bridge amplifier inputs draw no current). In turn those outputs depend on the sensitivities and strain-dependent resistances of the two strain gauges that make up each one.
It would be a good idea to read up on strain gauge bridges, and learn how they work.
You can also adjust the sensitivity by using levers to connect between the cell and the weight.
You would save yourself a lot of grief by buying strain gauges that are appropriate to the task.
I would think it is doable. However if doing it my hand you could get wildly different sensitivities on each. Your electronics would have to have different calculation constants for each sensor.
I know... $500 each is out of my price range. I need 12 of them.
I was looking for an 'easy' way to increase the sensitivity. Each cell is rated at 50kg max. with 4 that is 200kg. These are the mass produced load cells you see in human bath scales.
I need to be around 10kg max for all four. I understand these are not precision instruments by any stretch of the word, but the goods ones really are way out of my price range.
What I have works. The load on the cells is about 4kg. I would like to be accurate to +/- 100 g. Long term. Right now I am achieving +/- 400 g. I have spent countless hours recording measurements and temperatures (the load cells are mounted to a metal frame with thermal grease, also have a temperature sensor mounted to the frame).
I can see a correlation between output and temperature but even that moves around. I can't nail down an accurate temperature compensation equation. There is something else at work here that is beyond my ability to correct with math.
This might be as good as I can do.
Digging a little deeper into the math I still see a linear relationship on the bridge with different sensitivity load cells. I must be missing something important, here is my work:
Why do you think that? What you have done looks superficially OK.
What I'm missing is strain gauges that cost $500 each and why you are looking at them.
You can make your own by gluing dirt-chip strain gauge sensor strips to a bar of metal.
I need low profile gauges, these are about 3/8" thick. All the other pancake or button types I have found are really expensive. The object I am weighing is about 24" in diameter so using four of these types makes sense for balance purposes. To modifying something that is 90% of what I need sounds easier than building something 100% from scratch. I am just here to learn, not to upset anyone.
Seems to be easy enough to give it a shot. The worst that can happen is you scrape one or two sensors.
BTW I calibrate my load sensors by using objects that can be weighed at the local post office, perhaps even the grocery store but my gut feels the post office is more accurate. I would guess and UPS or FedEx office would be OK as well.
You could try hacking this, or something similar:
That is an interesting one. Thanks!
@JohnRob- I am going to give it a try. These load cells are dirt cheap so ruining some is not a big deal. There is probably more than one thing that could cause a failure when I do the hack. I just needed to convince myself that it could work in theory. Appreciate it.
Please post your results. I've been thinking, you could make a "slot" cut or file the whole area where the sensor is bonded. I think the whole area would be your best bet.
Good luck
