Arduino + strain gauge

[netrocos]

I have these 2 strain gauges as you can see in the image (code 632-180) and I want to connect it to the Arduino so I can read the deformation.

They should be in Half Bridge Wheatstone.

I read that I would have to connect it to an operational amplifier so i can be able to read the data, so i have already an INA122P from TI.

With the data sheet i managed to draw this schematic:



Is it correct?

I connect 2 or 3 wires to the strain gage? In the schematic there are 2, but all strain gages i saw they had 3 wires coming of...

This is how they are now:



Sorry if i can't explain better, im a starter and im not a native speaker. Thanks in advance

 

[esp1]

hi,
Your circuit looks basically OK, the calculation of 200uA and 100mV are way out.

With a 5V bridge supply you will have a DC level of 5V/2 at the IA [OPA] inputs.

The passive resistor half of the bridge will require careful/fine adjustment if you are to get 'close' to 0V out for no strain.
Also look at the uses of the Vref pin #5 of the IA.
E

 

[netrocos]

Hi again guys,

I changed the configuration of the second strain gage, so my sketch is this:

This is what i did here:


The problem is that i now read the tension value in the arduino and the value is always 0, no matter how hard i press it.

Is something missing?

Thanks in advance

 

[esp1]

hi,
Have you checked that as one SG is in tension the other is in compression.?
Also check that the output, in say compression drives the OPA output in a positive voltage direction.

As you are using a single voltage supply it will not be possible to drive the OPA output below 0V.
Typically with 0mV input the OPA output will be at approx +100mV or so.

E

EDIT:
Looking at your latest diagram, what is the resistance value of the strain gauges.?
Also I see no provision for nulling out any bridge zero imbalance.?
If the 120R's are not precisely matched you may have a bridge signal which is trying to drive the OPA below 0V.

 

[netrocos]

Hi again, guys.

I have this configuration in the breadboard:



The instrumentation amplifier is the ina122p and not that one. That 3 wires are the 2 strain gages.

The arduino is running this program:


I read 0.36 or 0.4 (tension) and it doesn't change as i apply load.

If i use the multimeter i read little changes when applied directly to the strain gage.

Can someone help me?

Thanks

 

[KlausST]

Hi,

That 3 wires are the 2 strain gages.

Am I missing something? You always need four wires for a strain gauge. 2 for supply and 2 for signals.

Klaus

 

[BradtheRad]

When you think about it, the wheatstone bridge is a difference amplifier.

And the op amp is a difference amplifier.

This common op amp arrangement fulfils the concept of a wheatstone bridge.