Strain Gauge Amplifying problems

Thread Starter

Motaz Ayyad

Joined Mar 10, 2014
31
no i haven't
here is the circuit that i build , J1 is the strain gauge
i have 100 and 500 ohm pot resistor
where i should put it ??
 

Attachments

t06afre

Joined May 11, 2009
5,934
What you have in your setup is an unbalanced Wheatstone bridge https://www.google.com/search?q=wheatstone+bridge

To balance the bridge you can make a setup by following the principle in the picture. Do not pay attention to values. Let us say the left side of the bridge is the side with gauge. Then use the setup on the right side to tune your bridge so it shows a s close to zero with no load
 

ericgibbs

Joined Jan 29, 2010
18,766
no i haven't
here is the circuit that i build , J1 is the strain gauge
i have 100 and 500 ohm pot resistor
where i should put it ??
This is image is the circuit I posted in post #13, R1 and R2 represent POT.

I assume that your strain gauge element is 270R.?
That ALL the fixed the resistors, in the bridge and the LM358 OPA's, are at least 0.1% tolerance.??

Which type of IA or OPA's are you now using.?

I have combined the two 10R's and the 270R, in the bridge arms, into a single value of 290R shown in your original circuit, for simulation ONLY
 

Attachments

ericgibbs

Joined Jan 29, 2010
18,766
Your circuit is an improvement over the previous version.

Using a 100R pot for precise bridge balancing will be difficult.
Is it a single or multi-turn potentiometer.?
 

Thread Starter

Motaz Ayyad

Joined Mar 10, 2014
31
it is 100 ohm single turn , i haven't seen a muti-turn pot.
i tried the circuit and i reach 0 or 0.1mv at some point
but i found out some problems
1- the output voltage either .62V constant(with no change) or higher(with change in loads) while i expected it to be zero or around that
2- huge instability 20 to 50mv change

any ideas about solutions?

and thanks for all your help
 

ericgibbs

Joined Jan 29, 2010
18,766
hi MA,
With a single voltage supply to the IA you will not get a zero voltage output.
What you can do is to use the Vref pin on the IA, connect the pin to a +1V low impedance source, that will set the IA output to +1V, make the +1V source adjustable.

The +1V output will have to be a 'virtual zero' level, if you are using an ADC and MCU, subtract this fixed +1V from the ADC readings.

If you must have a true 0V output from the IA, you need a dual supply.
Which type of IA are you using.??

I would add some filtering to both of the IA input pins, say a 4k7 in series from the bridge outputs, +sig and -sig and across the IA input pins a 10n or 47nF capacitor.
 

Thread Starter

Motaz Ayyad

Joined Mar 10, 2014
31
Hi
i am still using the 3 O/P(TL072 or LM358) circuit not an IA
i was looking for an dual voltage rail and i found out that the computer power supply has +12 0 -12 , so can i use it ??

and about the filters ,47K and 10nf as LPF right?
 

t06afre

Joined May 11, 2009
5,934
Regarding dual supply power supplies. They are very easy to create out of two single supply powers. Just look at the picture. I have used two 9 volt batteries. But the concept will apply to most types of power supplies.
 

ericgibbs

Joined Jan 29, 2010
18,766
also i have seen a circuit that split the single voltage rail to dual
from 12 0 to +6 0 -6 using transistors
This transistor circuit splitter will mean that you have Virtual ground for 0V, which I would not recommend for this application.

You say you have a 'true' +/-12V supply with a Common/0V, I would recommend that you use that.
 
Last edited:

Thread Starter

Motaz Ayyad

Joined Mar 10, 2014
31
thank you
i tried to use 2 9V battery as in your pic and it works , i can now reach range of 50 to 100mv instead of 640mv constant

but you told me that zeroing the output using the pot in the Wheatstone bridge is difficult , so what is the second solution??

also i found out new problem which is when i setting the output near zero it keep incrementing without any thing effecting it
 

Attachments

ericgibbs

Joined Jan 29, 2010
18,766
thank you
i tried to use 2 9V battery as in your pic and it works , i can now reach range of 50 to 100mv instead of 640mv constant

but you told me that zeroing the output using the pot in the Wheatstone bridge is difficult , so what is the second solution??
You could have 100R pot as a Coarse adjustment and say a 10R multiturn as a fine adjustment

also i found out new problem which is when i setting the output near zero it keep incrementing without any thing effecting it
What type of fixed resistors do you have in the other arms of the bridge.?
and their tolerance.?
It could be a temperature problem.
Are you using +/-battery to excite the bridge.?

Would you please post a full circuit showing the bridge and the IA circuit.?
 

Thread Starter

Motaz Ayyad

Joined Mar 10, 2014
31
umm there is no 10R muti-turn in the electronic stores here

i am using 5.1K silver (+- 10% i think) and 10K Gold (+- 5%)
the bridge 320 ohm silver too

the bridge is using +5V and 0

the circuit attached in the previous post and here is it again
 

Attachments

ericgibbs

Joined Jan 29, 2010
18,766
The bridge and LM358 resistors should be 0.1% or at least 1% tolerance to ensure temperature stability and good common mode rejection within the IA.

For the 10R, you could use a 100R pot and connect a 10R in parallel with it.
It will not be linear but it should enable a finer adjustment.

If you have an ADC after the IA, use the same 5V for ADC Vref as you have for the bridge.
 

Thread Starter

Motaz Ayyad

Joined Mar 10, 2014
31
it's more clear now
i was able to find some type of resistors called cermet and as being said they have small tolerance
i have 1K, 5K and 10K cermet resistors

about the bridge if i higher up the resistors to kilo ohm for example (three 1K 5% and (290R of strain gauge) + 1K Cermet resistor

i am not using ADC yet, after completing the circuit maybe i will use LabView with DAQ
 

ericgibbs

Joined Jan 29, 2010
18,766
If you have 1K in three of the bridge arms and 290Rsg +1K in the fourth arm, the bridge will be unbalanced.

What type of resistors are the 5% .??
 

ericgibbs

Joined Jan 29, 2010
18,766
OK, I read your post wrong, I thought that you meant that the 1K is series with 290R SG was a fixed 1K.!

the 5% are 4 color normal resistors
Are they Carbon or Metal film resistors.?

EDIT:
Do you realise by making the bridge resistors 1K and adding a 710R [ 1K adjusted] in series with the 290R SG, will decrease the bridge sensitivity to ~30% of its present value..??

Example: Original bridge gave ~3mV for 1R change in the 290R SG, the 1K bridge will give ~1mV for a 1R change in SG.!!
 
Last edited:
Top