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Using Arduino => Project Guidance => Topic started by: mz243100 on Jan 13, 2018, 01:55 pm

Title: calibrating a straight-bar load cell
Post by: mz243100 on Jan 13, 2018, 01:55 pm
I am currently utilizing a straight bar load cell with the hx711 amplifier for a project of mine. I was wondering if anyone knew the easiest way to calibrate a straight bar load cell? My load cell has a max load rating of 20kg. I am using it for grip strength measurements and have fashioned a grip. I can easily remove the grip of the straight bar if need be in order to calibrate. My current problem is I am really not sure what would work to calibrate the load cell. With a ring type load cell it is way easier as there is a single load point. But it is slightly different with the straight bar. Any help will be much appreciated.
Title: Re: calibrating a straight-bar load cell
Post by: jremington on Jan 13, 2018, 05:27 pm
Please post a picture of your cell, and explain how you intend to measure weights or forces with it.
Title: Re: calibrating a straight-bar load cell
Post by: mz243100 on Jan 13, 2018, 11:32 pm
I have attached an image of my cell, it is a straight bar type load cell. The max load is 20kg. Essentially, I am combining the load cell with the myoware muscle sensor. I wanted to investigate correlation between force applied and muscle potential. I am focused specifically on the muscles that control grip so I am essentially using the load cell to measure applied grip force. If need be, I can easily remove the grip if that is required in order to calibrate. Note: please ignore the numbers on the image of the load cell
Really appreciate the help
Title: Re: calibrating a straight-bar load cell
Post by: jremington on Jan 14, 2018, 01:17 am
I wonder if calibration will be meaningful or useful. What did you have in mind?

I guess from the picture that the red bars are meant to be gripped by someone's hand, in which case different ways of gripping the setup might lead to large variations in the observed strain, even if the effort is the same. Have you already investigated this, and if so, what was the outcome?

If the above test leads to repeatable measurements, you can do comparisons without having to calibrate (which seems problematic in any case).
Title: Re: calibrating a straight-bar load cell
Post by: DangerToMyself on Jan 14, 2018, 03:17 am
20kg max? (44lbs)

Don't let anyone who works with their hands try it. I can squeeze more than 5 times that (both hands combined).

Just putting that out there in case you're designing something to gather real world data for some sort of research. 20kg is no where near large enough, if you are.
Title: Re: calibrating a straight-bar load cell
Post by: mz243100 on Jan 16, 2018, 08:26 pm
Hey guys really sorry for the late reply. Have been a bit swamped with some other work. Yes, I definitely agree that 20kg is way too low, unfortunately,  messed up and ordered the wrong straight bar, I asked for a 50-100kg one.  I am more interested in looking at the correlation between stronger applied grip force and muscle potential, so it does not necessarily need to be the max grip force but just progressively stronger grip force between each level(if that makes sense). In regards to jremington's reply, the test is leading to repeatable measurements. To be honest I am not very interested in calibrating, but it is an assessed piece of work where they ask on how to calibrate it, it seems that calibrating it will get me marks. The criteria says that if the device is not calibrated, mention how you would calibrate it. If anyone has any idea on how I would hypothetically do that, I would appreciate it. Thanks alot for the help guys
Title: Re: calibrating a straight-bar load cell
Post by: TonyWilk on Jan 16, 2018, 08:56 pm
If you mounted the sensor on a bar (of wood or metal) and arranged it like this:

Diagram:
(https://forum.arduino.cc/index.php?action=dlattach;topic=522406.0;attach=241470)

Then you would be able to measure more than the 20Kg of just the sensor alone
and you could simply sit it on the edge of a bench and hang weights where the "GRIP" goes to do your calibration.


Yours,
  TonyWilk
Title: Re: calibrating a straight-bar load cell
Post by: jremington on Jan 16, 2018, 09:04 pm
You could calibrate the grip for a particular well defined configuration, like applying a known force at two points in the center of each hand grip bar. Consider drilling holes at those positions, insert a screw eye or bolt to attach a luggage scale, or apply weights.

If you have access to some professional device for measuring grip force, calibrate your device empirically by asking various people to grip both, applying the "same" force to both. With lots of trials and averaging, you can come up with a reasonable approximation to a scale factor.

Adafruit has a good tutorial on sensor calibration.
Title: Re: calibrating a straight-bar load cell
Post by: mz243100 on Jan 16, 2018, 09:32 pm
Hey guys thanks for the quick replies. Jremington, thanks, I will take a look, my main problem is the deadline is coming up soon so am not sure If I will have time to acquire a professional device to measure grip force and calibrate using that method. I will check adafruit's website, appreciate the help. Tonywilk, thanks for the diagram,  I was wondering what do those black lines between both the load cell and the wooden/metal bar represent. Does that mean I should attach them both to each other, or should I screw the load cell directly into the metal/wooden bar in this situation, and the black lines represent screws?
Title: Re: calibrating a straight-bar load cell
Post by: TonyWilk on Jan 16, 2018, 10:17 pm
Tonywilk, thanks for the diagram,  I was wondering what do those black lines between both the load cell and the wooden/metal bar represent. Does that mean I should attach them both to each other, or should I screw the load cell directly into the metal/wooden bar in this situation, and the black lines represent screws?
Sorry, I meant the load cell should be bolted at both ends to the metal/wooden bar - so that it bends with the bar. The two bars are bolted to some block at the left hand end.

Yours,
  TonyWilk
Title: Re: calibrating a straight-bar load cell
Post by: mz243100 on Jan 16, 2018, 10:24 pm
Thanks again Tonywilk, one more question, so does that mean that the straight bar is bolted to both bars?

Title: Re: calibrating a straight-bar load cell
Post by: TonyWilk on Jan 17, 2018, 01:20 am
Thanks again Tonywilk, one more question, so does that mean that the straight bar is bolted to both bars?
Is this any better....
Crappy scribbled Diagram:
(https://forum.arduino.cc/index.php?action=dlattach;topic=522406.0;attach=241479)


Yours,
 TonyWilk
Title: Re: calibrating a straight-bar load cell
Post by: TomGeorge on Jan 17, 2018, 09:09 am
Thanks again Tonywilk, one more question, so does that mean that the straight bar is bolted to both bars?


Hi,
A load-cell doesn't actually measure force, it measures the change in dimension of the cell, using resistive sensors.
So deflection of the loadcell can be calibrated to read force.

By fixing another bar alongside it, you are now measuring the deflection of load-cell and bar, this will need a greater force to deflect the load-cell compared to it when it is on its own.

When calibrating, I'm mot sure how accurate you want to be,  1 litre of water == 1kg.

Tom... :)
Title: Re: calibrating a straight-bar load cell
Post by: TonyWilk on Jan 17, 2018, 01:27 pm
By fixing another bar alongside it, you are now measuring the deflection of load-cell and bar, this will need a greater force to deflect the load-cell compared to it when it is on its own.
Yeh, did that on purpose;  earlier the OP said (re: having a 20kg load cell):
Quote
Yes, I definitely agree that 20kg is way too low, unfortunately,  messed up and ordered the wrong straight bar, I asked for a 50-100kg one.
Yours,
 TonyWilk

Title: Re: calibrating a straight-bar load cell
Post by: TomGeorge on Jan 17, 2018, 01:35 pm
Yeh, did that on purpose;  earlier the OP said (re: having a 20kg load cell):
Yours,
 TonyWilk
Yup, just agreeing and explaining the method in the madness... :) :)


Tom... :)
Title: Re: calibrating a straight-bar load cell
Post by: mz243100 on Jan 20, 2018, 05:38 pm
Thanks alot for the help guys, appreciate all the help
Mazin