Hello Folks,

I am reading in this forum for quite a while using it for my projects and always found the answers I needed. But for my current project I need some additional help.

The project:

I try to do some DAQ from a strain gauge. Therfore my first step was to test a basic application with a breadboard and an Arduino.

I have follwoing problem:
The values I read with the Arduino fluctuate with time which seems to be due to temperature. According to my calcs, the resistors should't be influenced that much by temp.

Further, as soon as I knock on the table the whole arrangement is atttached to, the values show me a peak and fluctuate for quite a while before they begin to stabalize to the "usual" slope.

Attached you can find a few figures and the circuit. Maybe anybody can give me some hints where this fluctuations come form an how to get rid of them.

THX
Dan

 

What does the circuit do when you actually apply strain? Are these variations minute compared to the strain that you are trying to measure?

I'm not surprised that something happens when you bump the table apart from anything else those breadboard circuits are not that reliable.

Resistors will have some amount of temperature dependence you should check the spec of the ones you are using and see if the output variation is just due to the expected resistor change due to the temperature shift. Don't forget a hair dryer will actually be pretty hot if you focus it on the resistors, perhaps you need to measure the temperature change too.

 

The variations with time are minute compared to the actual increase / decrease when strain is applied.
Howerver, the variations make it impossible to set a "Zero" point.

The variations which occur when knocking on the table are in the same range as if strain is applied and it seems they are caused by some contact problems in the breadboard. (How could that be identified?)

If I touch the Resistors, the values also "jump" as if the R value would suddenly change.

Further I am not shure whether it is a good idea to use a variable resistor to set calibrate the bridge to "zero" since it gives me additional Temp- dependence. It also seems to be a major cause of the "fluctuations". Any idea how to calibrate the bridge differently?

THX, Dan

 

I think most of the instability you report are due to how you have set up the mechanical arrangement of the strain gauge. The knife with the handle will act as an amplifier of any vibration occuring on the table. You have to short the blade and remove the handle and re-check stability with the new strain gauge condition.

Cheers

Alberto

 

Thx for your comment Alberto!

You are right, the handle is amplifying all kinds of vibrations. But this occurs with the "natural frequency" which can be seen quite good when exciting the knife at the end.

The frequncies observed when knocking on the table seem to be completely random which makes me think it might be due to some defects or wrong setup of the electronics. The resisotors and the variable resistor are very suspicious to me but I don't know any alternative.

Also the ADC conversion might influence the setup. I might give an Optocoupler a try...

THX, Dan