Creamy
Posts: 1
Joined: Mon May 21, 2018 2:53 pm

Building a Wheatstone bridge

Mon May 21, 2018 2:59 pm

Good day everyone,

I am very, very new to using RPs and I need some guidance on where to start with this. I am currently trying to measure strain from crack growth on some plastics in our lab with strain gauges and I have been told Raspberry Pi could be useful, or even an Arduino. I am not sure what would be required to attempt this, so please, any help is appreciated! Thank you.

drgeoff
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Joined: Wed Jan 25, 2012 6:39 pm

Re: Building a Wheatstone bridge

Mon May 21, 2018 4:53 pm

Why don't you ask whoever told you that a RPi or Arduino would be useful?

What usefulness are you desirous to have?

A Wheatstone bridge is analogue. RPis have no analogue inputs.

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bensimmo
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Location: East Yorkshire

Re: Building a Wheatstone bridge

Mon May 21, 2018 7:22 pm

Something like this. (Good old google and terms ' strain gauge Raspberry Pi ' )
https://electronics.stackexchange.com/q ... d-accuracy?

Everything will depend on your strain gauges.
Which will then need balancing (w.r.t. your title)
And then amplifying.
And then converting to digital.
And then the Pi will get some data


But have a read here
viewtopic.php?t=173854

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Z80 Refugee
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Re: Building a Wheatstone bridge

Mon May 21, 2018 9:43 pm

bensimmo wrote:
Mon May 21, 2018 7:22 pm
But have a read here
viewtopic.php?t=173854
The concluding post in that thread looks rather like hope over practicality, flying in the face of advice further up. Just throwing any old op amp at a strain bridge won't cut the mustard.
Military and Automotive Electronics Design Engineer (retired)

For the best service: make your thread title properly descriptive, and put all relevant details in the first post (including links - don't make us search)!

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Ferdinand
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Location: Leiderdorp, NL

Re: Building a Wheatstone bridge

Tue May 22, 2018 7:38 am

Z80 Refugee,
The concluding post in that thread looks rather like hope over practicality, flying in the face of advice further up. Just throwing any old op amp at a strain bridge won't cut the mustard.
Explain your post!

How would you measure such small voltages?
Success with your project!
Ferdinand

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Z80 Refugee
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Re: Building a Wheatstone bridge

Tue May 22, 2018 8:10 am

The advice (as per the posts in the referenced thread) is to use an instrumentation amplifier.

The concluding post sounds like the OP Intended to experiment with an op amp rather than an instrumentation amp. An ordinary op amp has too great an offset and drift for this kind of application - you could trim it to requirements, but the trim would not remain constant over a long period of time and require frequent calibration. At the very least it needs a zero drift amp, but an instrumentation amp also buffers the inputs so that there is no load on the sensor outputs (which the gain control feedback around unbuffered op amp circuits imposes).

Far easier to buy in an interface designed for the purpose, and as some kind of A-D conversion is required for input to the RPi, a ready made strain gauge digital interface seems like the best idea (unless all you really want to do is have a play for minimum cost with no expectation of long-term reliability).
Military and Automotive Electronics Design Engineer (retired)

For the best service: make your thread title properly descriptive, and put all relevant details in the first post (including links - don't make us search)!

Phrantic
Posts: 2
Joined: Tue May 22, 2018 12:20 pm

Re: Building a Wheatstone bridge

Tue May 22, 2018 2:20 pm

Sounds like a 'student project'. :)

May I recommend Horrowitz & Hill. The Art of Electronics. I once owned what became a well thumbed copy of the first edition which I kindly but foolishly gave to someone else. The second edition included lots of extra stuff about microprocessors and switch mode power supplies but retained much of the nice analogue proper electronics. I do not know whether the following link is 'hooky' but it has been up for some time.

https://pearl-hifi.com/06_Lit_Archive/1 ... ronics.pdf

You will have to poke about a bit but you will find a wealth of information dealing with what you are trying to achieve. Chapter 7), Chapter 15)... Also have a look for Dual Slope Integration A/D conversion.

As others have mentioned you are going to need an instrumentation amplifier, the three op-amp one, to interface the strain gauge bridge to your A/D converter, dual slope integration... You can build one of those including auto-zero with three/four op-amps, a comparator and a CD4066.

http://www.ti.com/product/CD4066B

Look at the legs on that beauty.

You drive the CD4066 with the I/O pins of your Pi whilst watching the output of the comparator. I'll try to remember what one of those looks like. Should be good for at least 14 bits assuming the Pi and your programming is up to it... :)

Don't forget to read the stuff about operational amplifiers, comparators and voltage references and you will have to bone up on what strain gauges are and what they actually measure. You will also need a soldering iron and some veroboard. Do not expect to poke bits of wire in an s-dec.

Nice project. Lots to learn. However...
I am currently trying to measure strain from crack growth on some plastics in our lab with strain gauges
I might be inclined to question the proposed methodology as applied to that particular application. You might translate that as I do not understand but if it is just a 'student project' perhaps the person who has set it does not really care about the application beyond the possibility that it sounds like something that might make sense.

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Ferdinand
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Re: Building a Wheatstone bridge

Thu May 24, 2018 3:19 pm

Hi Creamy,

Perhaps you can use this HX711 board, see discussion about "rpi3 and SparkFun Load Cell Amplifier - HX711".

https://www.raspberrypi.org/forums/vie ... 3#p950998
Success with your project!
Ferdinand

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