I am trying to build a thrust stand using a beam and strain gauges. I have attached the four strain gauges (350ohms) and have tested each individual one with the strain indicator box to verify that they work. My issue seems to be combining the four strain gauges into one value. I'm using the DAQ USB6009 to collect the data.
Attached you will find a picture of my schematic of the combinator and additional information regarding the strain gauges. I was getting rather weird values on Labview so I decided to use an oscilloscope to see the output.
The excitation voltage is 10V and I calculated that i should get about 2.41mv/V
When I place the oscilloscope I get values ranging from 2mv to 19mv and the values are bouncing in between. Do I at least have the correct schematic for connecting the strain gauges to the DAQ?
Any help is appreciated. Thank you in advance

 

I don't understand the wiring on the strain gauges. Normally the gauges
are connected in a bridge circuit configuration as below. Use the differential input on the USB6009
SG

 

hi l88,
The long wiring to the strain gauges is not screened, also I do not see any noise filtering on the bridge output connecting to the 6008.
I suspect your bouncing is due to electrical noise pick up, also are the metal plates to which the SG's are fixed connected to Common/0v.??
E

 

I don't understand the wiring on the strain gauges. Normally the gauges
are connected in a bridge circuit configuration as below. Use the differential input on the USB6009
SG

View attachment 172273

I don't have much experience with this. I used three wires because that is what you are supposed to use when using the strain indicator box (P3500), so I assumed it was the same thing.

 

hi,
I would suggest screened cables from the SG's to the 6009.
If the 6009 does not have inbuilt noise filtering, this image shows a typical method of reducing input interference.
E

 

hi l88,
The long wiring to the strain gauges is not screened, also I do not see any noise filtering on the bridge output connecting to the 6008.
I suspect your bouncing is due to electrical noise pick up, also are the metal plates to which the SG's are fixed connected to Common/0v.??
E

View attachment 172284

No, the metal plates to which the strain gauges are applied are not connected to common ground. After ericgibbs mentioned it, now I'm wondering if my three wire setup is even correct and instead should go with two wires. http://www.vishaypg.com/docs/11092/tt612.pdf
I read that three wires are added for bridge balance

 

hi,
Post a circuit schematic showing how the SG's are interconnected , show excitation voltage and signal take off points.
E

 

hi,
Post a circuit schematic showing how the SG's are interconnected , show excitation voltage and signal take off points.
E

Hi Ericgibbs, Thank you for helping me. I apologize for my lack of knowledge with electronics. Attached is the picture of the connection to the DAQ.

 

hi 88,
That wiring looks wrong.
Do you have a datasheet for a strain gauge that you could post.?
E

EDIT:
Note the 'sense' connections on each SG in the bridge.

 

hi 88,
That wiring looks wrong.
Do you have a datasheet for a strain gauge that you could post.?
E

EDIT:
Note the 'sense' connections on each SG in the bridge.

Hi, Ericgibbs,
The datasheet is:
http://ntcomponents.com/Datasheets/Sensors, Transducers/standard-weldable.pdf
I have the 350ohms strain gauges with a GF of 2.115
Thank you

 

hi,
I see that OK.
Post a drawing showing the R1,R2,R3,R4 SG's , how they are connected and which face of the metal sheet they fixed, ie: Compression or Tension.
E

EDIT:
This is the way I see it.
The White wires on each SG are connected to the same SG pad.

 

hi,
I see that OK.
Post a drawing showing the R1,R2,R3,R4 SG's , how they are connected and which face of the metal sheet they fixed, ie: Compression or Tension.
E

Attached is the image showing which ones are in compression and tension. R2 and R4 are in compression, while R1 and R3 are in tension.

 

hi,
Do you agree with my edited version in post #11.?
E

 

hi,
Do you agree with my edited version in post #11.?
E

My apologizes I didn't see the edited version. If I'm reading yours correctly, is this what you mean? It accounts for the three wires
Attached is the image.

 

hi,
There are 3 wires on each SG, but the two White wires are connected to the same point on the SG. ie: linked together.

E

 

hi,
There are 3 wires on each SG, but the two White wires are connected to the same point on the SG. ie: linked together.

E

Oh, I understand what you are saying. I agree with your schematic. I will try this out and see what I get. It looks right. Thank you very much for your help throughout this. Very much appreciated

 

The image posted in #14 is correct. You need to keep the wiring between the gauges as short as possible. For best results connect a 4 conductor shielded cable from the load cell to the 6009. Connect the shield conductor to the metal plate.
SG

 

Hi,

When I tried doing that wheatstone bridge configuration in #14 my output at the S+ and S- using an oscilloscope is 2.55V. Shouldn't my output from the wheatstone be in mV?

Thank you

 

Shouldn't my output from the wheatstone be in mV?

Correct! The voltage should be zero if all the strain gauges have equal resistance and there is no stress.
To test for continuity you should read 2.5 volts from zero volts to the white wires on both sides of the bridge.
SG

 

Correct! The voltage should be zero if all the strain gauges have equal resistance and there is no stress.
To test for continuity you should read 2.5 volts from zero volts to the white wires on both sides of the bridge.
SG

Thank you Sghioto for your quick reply. I uploaded an image of the wheatstone bridge with the formula. My belief was that the output (Vout), or in my case S+ and S-, would be in millivolts. Am I not understanding this correctly?