Here is some Data:
The load cells has a total resistance of 1965 and 1966 Ohms
I'm exciting the cells with 5V
With 150Ohm resistors on Rf and Rg the baseline Voltage on Vout = 870mV
R1 and R2 are both 1KOhm
Applying a load does not change the Value in either case.
What am I doing wrong is the Resistor values completely off or what?
After some meditating yesterday I thought I had figured it out,
If Rf=Rg and R1=R2
Vout=Rf/R1 x (V1-V2)
So I changed the resistor values,
Vout = 5V
V1-V2=0.100V (This i wasn't actually sure about but i figured that it'd be something like that)
Which meant that:
5=Rf/R1 x 0.100
5/0.100=Rf/R1
50=Rf/R1
Based on that I chose the new resistor values Rf=4.7K R1=100
4700/100=47 (which is close to 50 so I thought it'd be enough)
Sadly this still isn't enough gain to get any reading on the voltage change, which I find strange but I'm going to try to double it and see if that works.
Using a better multimeter I get a 3mv change with 11,5Kg keep in mind that I'm still using the LM358 to subtract the values only with zero gain i.e no resistors,
I might try using 3 OPamps in a Instrumental amplifier setup and see where that gets me.
I think you need WAYmore gain and a proper instrumentation amp (very very good CMRR). The change in resistance of a strain-gauge is measured in tiny fractions of a percent, so gains of 1000 or more aren't unreasonable.
This might be useful: http://cerulean.dk/words/?page_id=42 there a gain-resistor of 10 ohms is used, meaning a gain of 6000. Sounds more likely than 50!
MarkT:
I think you need WAYmore gain and a proper instrumentation amp (very very good CMRR). The change in resistance of a strain-gauge is measured in tiny fractions of a percent, so gains of 1000 or more aren't unreasonable.
This might be useful: http://cerulean.dk/words/?page_id=42 there a gain-resistor of 10 ohms is used, meaning a gain of 6000. Sounds more likely than 50!
I agree, still I was hoping to see some small change at least with a gain of 470, but it might be the amp as well, I'm thinking of trying one of the INA amps preferably with an easy gain adjust
With a very small differential signal like this you cannot compromise on CMRR (common-mode-rejection-ratio), PSRR (power supply rejection ratio) and Voffs (input offset voltage). This is exactly the situation a precision instrumentation amp like the INA125 is designed for, most opamps will have way too much input offset and offset variability to have a hope here - you'll just end up amplifying the input offset voltage to saturation or have a highly temperature-sensitive output value.
YES! I got it to work. Using two LM358Ps set up as a three Op Amp Instrumentation Amplifier. The problem I had with that circuit before must have been the low resistor values I used, This time having done the math, and using high resistor values (100K, 10K, 150K, 6.7K) I got it to work really nicely. Next thing to do is to try the two Op Amp Instrumentation Amplifier, Why you ask? Well because then i only need to use one LM358.
I'm very happy right now. And thanks to all of you guys for helping me out.
I noticed that the equation: Vo=(sig+ - sig-)(1 + R1/R2 + 2R2/Rg) doesn't make sense since they state that R1=R2 and R3=R4
If R1=R2 then Vo=(sig+ - sig-)(1 + 2R2/Rg) would be true, but then there wouldn't be anything that defined the relation between (R1, R3) & (R2, R4)
After further experimentation I get less and less certain that I managed to read a weight change, Seems more like I've built a great EMI antenna and amplifier, Putting a weight on the sensor does change the value, but it seems I can accomplish the same change by resting my finger on the sensor.
