Strain gage amp circuit and readout

[metiz]

Hi,

I'm working on a project where I have to get a weight readout from a straingage attached to an aluminium box profile. The mechanical part of the assignment is done, but they're realy letting us hanging with the electronics part.

We were handed this circuit scematic (after specifically asking a teacher not involved in the project).


Some mods have been made after advice from other students: Removed the diode, replaced the 390 ohm resistor on the right with a 10K one, replaced the direct-to-ground lead from pin 5 with a 1M ohm resistor. "rekstrookje" means straingage btw (I'm dutch). the straingage is not yet attached, I use a potentiometer.

I'm using an arduino uno with Basic Firmata and Firmata Test to see if I get any signal on ports A0 through A5. Regardless of POT setting, I get a reading between 1000 and 1020. If I hook up to, for example, A3 then it too will show 1000, as well as A4 to A6.

We BARELY have done anything with an arduino, we just sratched the surface of OP AMPS and have basically zero programming experience. The assignment basically says "Figure it out"

Is this circuit usable and if not, how do I change it;
How can I imput the data to an arduino and get an actual weight readout with TARE and ZERO "buttons"? (Since we're expected to "figure it out", I'm all up for a ready-made Github program);
Are there easier ways to do this? an Arduino is not required per-sé

A final thought: If I buy a 2nd hand kitchen scale and hook up our straingage to it, then use an arduino to compensate for whatever deviation in the readout we're getting, would that work?

Thanks,
Joost.

 

[chuckey]

1. With no weight (= no strain) the op amp must be zero'd. That is to say set up to a known bit of its range. As the op amp will not give a true 0 V output something else should be used , say +.5V, by returning the bottom two arms of the bridge to a .5V source, say 90 ohms to +5V and 10 ohms to earth.
2. The change of resistance per unit of strain is very small, .1%, so if a 10mm gauge is stretched by .1mm , the resistance will change by .001 X .01 = very small number, so your opamp must have a very high gain (100,000?) to get a decent change in its output voltage.
Now you can calibrate your micro to produce a useable display = Kgf or tonnes/ins^2 or whatever. The gain of the opamp should be stable , so when you switch it all on again, you just adjust the "zero" control which actually gives the .5V output from the opamp, but 0 units on the display.
Frank

 

[BradtheRad]

If your schematic is a plain op amp, then it is acting at maximum gain, causing it to switch suddenly if it switches at all.

It needs a feedback resistor to the inverting input.

The potentiometer was dialed up and down, to simulate readings from your strain gauge.

Your schematic contains an op amp (top), and also an instrumentation amplifier (bottom left). I have not used the latter.

 

[metiz]

Thanks for the replies guys. Brad, does the 22k resistor not seriously decrease the amplification factor? I have a very sturdy aloy bar to work with, and a tiny 350 ohm straingage. Chuckey mentioned that with a very small stretch of, say .1mm, you'll need a lot of aplification to get a reading. If I put a load of 50 newton on my aloy rod, It'll only give by about 2.7mm at the end. Total elongation is absolutely tiny, in the order of 0.03mm over the total length. If I put a load of anything above about 120 Newtons on the bar, it'll permanently deflect.

How do I read out the the information?

 

[BradtheRad]

Thanks for the replies guys. Brad, does the 22k resistor not seriously decrease the amplification factor? I have a very sturdy aloy bar to work with, and a tiny 350 ohm straingage. Chuckey mentioned that with a very small stretch of, say .1mm, you'll need a lot of aplification to get a reading. If I put a load of 50 newton on my aloy rod, It'll only give by about 2.7mm at the end. Total elongation is absolutely tiny, in the order of 0.03mm over the total length. If I put a load of anything above about 120 Newtons on the bar, it'll permanently deflect.

How do I read out the the information?

Naturally you should not have to exceed device specs.

The feedback resistor can be any value you choose, to give you sufficient amplification. It may end up in the megohms, and the 1k resistor might need to be in the tens of ohms instead, to give you upwards of 100,000 gain (as Chuckey's post #2 suggests).

You may even need to add a second op amp after the first, just to get a usable reading.

Experimenting is an essential part of project development.

My resistor values are all flexible. The main caveat is to avoid letting too much current flow through any component, especially your sensor.

 

[metiz]

I'll give it a try thursday and report back in, thanks!