Strain Gauge Amplification Circuit

[v1nd]

So I'm working with some 350? strain gauge's, and I'm looking to buy some industrial, pre-made amplification circuits. I have a couple of one's I designed for 120? and I understand that it's really quite easy to make the switch, but I'd rather try to see if I can get an industrial one (that I can trust more than my own) for less than $200-$300 before I risk making my own. I don't have any indication of where the gauge was bought or anything, just measured the ends with an ohmmeter, so there's not really anymore information to be had. Any suggestions?

Thanks,

[Simon]

what is a strain guage and why should you need an amplifier ?

[naxxfish]

I wouldn't have thought it'd be too much effort to re-match the impedances on your own design - I am a bit rusty with  opamp design, but I remember making a strain gauge amplifier for a project a while ago. 

I guess the main thing to consider would be the balance between the advantages you get from buying OTS amplifiers or building your own.  I guess it also depends on the application - if you were using it to measure static strain (e.g. as a scale) then I wouldn't have thought you'd get much advantage buying one.  If it's something more complicated you might find you can get a better response from a industrial amplifier. 

[rf-loop]

If want cheap and also some DIY working but still good guality there are lot of instrumentational amplifiers.

These are extremely easy to use "building blocks":

Here is one and good example: http://focus.ti.com/lit/ds/symlink/ina128.pdf  just as simple as 1-2-3-go

Also INA101 (high accuracy) http://focus.ti.com/lit/ds/symlink/ina101.pdf

Also there are better models for single supply and extremely low supply voltages if need.

[v1nd]

A strain gauge just measures the strain on any member you wish to measure strain through; this is done through change in resistance, the change is generally in the milliohm's so an amplifier is required, typically one that allows you to control the gain to match the forces you put through the member.

I use a combination of AD623, TLC272, and TS921 op-amps with a 10x pot to allow for gain control; I understand the current design would be quite easy to change...I'd still rather get an industrial, pre-made amplifier circuit (specific to 350? strain gauge's) that's ready to use. If not, I'll change the current design, just thought it couldn't hurt to ask.

Thanks,

[Simon]

by member do you mean parts of the human body as in arms legs... err... I'll stop there 

[v1nd]

I just say member to be general, in essence you could put it anywhere, in my case it's on a steel tube so I can measure the forces through it.

[Simon]

so it measures how much the member stretches ?

[v1nd]

In some cases, it can represent just that; if a load is applied coaxially (parallel to the member, ie. along the member's length) you end up gaining length along the axis and loosing girth. In practice, the magnitude of the strain is very small, hence the amplifier.

In my case the strain measures the tensive and compressive forces through the member. So when you apply a load at the end of a tube, there will be bending proportional to the force at the middle. Using a gauge at the top, you measure tension in the tube; at the bottom, you'd measure compression.

In specific, strain is the fractional change in length of the member. After knowing the strain (along with other information relating to the member), we can work backwards to calculate the forces along a member. In this way, we're able to - in my case - optimize the type of material as well as the thickness of tubing we use at certain areas so that they correspond with the forces, allowing for savings in weight and cost.

In the end, it's always about saving money. 

[Simon]

After knowing the strain (along with other information relating to the member), we can work backwards to calculate the forces along a member. In this way, we're able to - in my case - optimize the type of material as well as the thickness of tubing we use at certain areas so that they correspond with the forces, allowing for savings in weight and cost.

In the end, it's always about saving money. 

ah now it makes sense, my next question / observation was going to be how do you quantify that change unless you know the properties of the material without "cutting the member and putting the gauge in it  "

[djsb]

http://en.wikipedia.org/wiki/Young%27s_modulus

and mechanical properties in

http://en.wikipedia.org/wiki/List_of_materials_properties

David.

[TechGuy]

So I'm working with some 350? strain gauge's, and I'm looking to buy some industrial, pre-made amplification circuits. I have a couple of one's I designed for 120? and I understand that it's really quite easy to make the switch, but I'd rather try to see if I can get an industrial one (that I can trust more than my own) for less than $200-$300 before I risk making my own. I don't have any indication of where the gauge was bought or anything, just measured the ends with an ohmmeter, so there's not really anymore information to be had. Any suggestions?

Thanks,

Try Omega Engineering http://www.omega.com/
McMaster might also sell them too http://www.mcmaster.com

[qno]

I have just been working with one of these. They are OK.

Look here:

http://www.magtrol.com/datasheets/lmu209.pdf