Load Cell HX711

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George_B
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Load Cell HX711

Postby George_B » Fri Aug 19, 2016 10:35 pm

Hello all,

I would like to read the analogue to digital converter in the link below.


https://cdn.sparkfun.com/datasheets/Sen ... nglish.pdf


Would it be possible to know how to get the data from HX711 chip?

Is there any FC6 example ?


Best Regards
George

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Re: Load Cell HX711

Postby petesmart » Sun Aug 21, 2016 11:58 am

Hi George,

Attached is a macro is created in fc6, it will also work in fc7. It reads 24 bits of data from the hx711 chip....the project I used this for was a 4 load cell scale connected in a Wheatstone bridge config. Import the macro into your project and call it as a routine.

Note - given the sensitivity of the hx711 you will need to average the output .

Let me know how you go

Best

Pete
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HX711.FCM
Hx711 macro
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Re: Load Cell HX711

Postby George_B » Sun Aug 21, 2016 3:26 pm

Hi Pete! Thanks for your reply and your time!

I tried to make up a program including your macro. Unfortunately i ended up with no good results although i wont be disappointed at all.

In order to avoid a lot of texting here, i will upload the FC6 program with this post so you all can have a look and/or improve the file.

I will also include the wiring i used between load cell and HX711 module .

The weight value in LCD is not stable and seems to fluctuate up and down a lot.


Thanks again!
George

http://www.ebay.co.uk/itm/291644547509? ... EBIDX%3AIT

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HX711 Weight ADC Module.fcfx
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Re: Load Cell HX711

Postby petesmart » Mon Aug 22, 2016 11:54 am

Hi George,

The fluctuation is pretty normal to a degree - hence my reason to suggest an averaging methodology.....Also check your PSU - do you have filter caps close to the HX711 Chip/board?... this can be a source of noise...

The other thing you can do is reduce the gain and scale up... this may also assist.

When I get moment I will have a look at your project

best

Pete
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Re: Load Cell HX711

Postby petesmart » Mon Aug 22, 2016 12:05 pm

ok... had a look at your project....

when I created the macro, I left all of the config options disabled :oops: - you will need to enable the icons in the red circle and then it should work...the data sheet will give you a good insight as to the config options vs function

let me know how you go

best

Pete
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enable these icons
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Re: Load Cell HX711

Postby George_B » Tue Aug 23, 2016 9:18 pm

Hi Pete,

I tried your suggestion but again the results were not very good.

I get at least a number on the LCD and i assume that the Load Cell may be a crappy one...therefore i will look for a better load cell to make my test.

Is there any load cell which operates well and been tested with this module ?


Thank you
George

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Re: Load Cell HX711

Postby petesmart » Wed Aug 24, 2016 11:04 am

Hi George,

I used 4 load cells from a bathroom scale I purchased for $25...very cheap and cheerful

4 cells wire in a Wheatstone bridge config is good for 150kg with a resolution of 1 gram (ish)

Can you describe what not very good means?..do you know the scaling factor?

Best

Pete
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Re: Load Cell HX711

Postby chipfryer27 » Thu Nov 17, 2016 9:00 pm

Hi Pete / George

First off, thanks for posts on this subject. I have found them to be very helpful.

I recently obtained one of the HX711 modules from eBay so armed with your posts and a lot of gung-ho I set about experimenting.

I see that the modules come in a variety of flavours, mine is the XFW-HX711, a green board with headers for the four Load cell connections, nothing for the screen. I also have quite a few load cells (each is its own wheatstone bridge) rated 5Kg and 20Kg. Each of these cells come with a calibration certificate stating that their accuracy is class C3.

Ages ago, using these and an Instrumentation Amp, I could get quite accurate and repeatable readings just by feeding the output of the amp into an ADC port on the uC directly. I could differentiate between the addition and removal of a "few tens of grams" at a time, and readings were quite stable in that each "read cycle" returned the same values more or less (I would like to stress that absolute accuracy was not a consideration in this application, just a means to know when certain criteria had been met, and I did have to average out readings per cycle) so I was keen to see what this module would offer.

Hooking up the HX711 to an EB-006 (PIC18F4520) via a plug-in expander on Port C and an EB083 connected for its LCD, running Pete's code with mods made to connect over Port C gave immediate results. Scaling and calibration variables in the macro I didn't care about for testing, so they were set to 1. Loop delay was set at 10ms as per Georges posted Flowchart and I varied the average, counting 5, 10, 20, 100 and 200 times.

In each instance the results returned after averaging, had a variation of approx +/- 7% between readings. By that I mean I took 20, separate "averaged" readings, and each reading varied by around 7% from each other. Not good. Repeating with other load cells gave similar results and clearly something was amiss and I initially thought the module to be at fault. Perhaps this is what George saw too?

Reading the datasheet (which is always a last resort <s>) suggested that I might be accessing too quickly, therefore I increased the loop delay from 10ms to 400ms which gave an immediate improvement. Averaging 5 times gave a variation between readings of approx +/- 0.6%, averaging 10 times gave a variation of +/- 0.3% whilst averaging 100 times returned a variation of +/- 0.12%, although admittedly it does take "ages" to update :)

As this module is far cheaper than the Instrumentation Amplifier I used, and is capable of repeatable accuracy (for my needs anyway) I think it will be included in my armoury.. :)

I hope this may help.
Regards
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