Scaling when using constant current excitation with strain gage

Hello DEWESoft !

I am currently trying to find out what is the formula used to scale a strain gage measurements when providing the gage factor (k) in DEWESoft. in CONSTANT CURRENT excitation of the bridge.

I know you use this formula : Strain (µdef) = (4000 / k) * Measured Sensitivity (in mV/mA). But can you develop or point me to a page / article where the calculation is done ?

I have to use this method when I am only used to constant voltage method but can't find the same result. I have to inform my customer about this coefficient and develop all the calculations.

Thanks a lot !

Regards,

Max

Hello,

When configuring a strain measurement (in µdef, microstrain) in quarter bridge mode with current excitation (not voltage excitation) we have to set the gage factor « k » in DEWESoft, thus DEWESoft computes by itself the scaling coefficient with this formula :

But when using current excitation mode it should be (see attached pdf) :

With the nominal resistance of the bridge (350 or 120 Ohms).

Do you confirm ? Did I do something wrong in my calculations ? I tested it with a strain gage simulator and it seems right.

Hi,

Thank you for your questions. I did tests to evaluate your concerns.

Start with normal setup for bridge FULL/350Ohm/10mA Excitation/Scale=1

​

​Now switch to Strain:

Leave all settings as is, means gage factor(k) = 2.

​

And the scale shows now what it should, based on the wanted formula from the beginning.

Scale = (4 * 10^6) / (k * BridgeRes) = (4000000/(2*350)) = 5714,286

The calculations for other modes are:

• For FullBridge it is: ( 4 * 10^6 ) / ( k * Rbridge)
• For HalfBridge it is: ( 2* 10^6 ) / ( k * Rbridge)

And QuarterBridge is basically a HalfBridge with an internal fixed resistor, in this case, so same formula.

Hi Matic,

Thanks for your answer, but I still do not understand why you are dividing by 2 the scaling coefficient in quarter bridge (or half bridge) mode. Maybe there are some things I don't know about what is happening inside the hardware.

Sm is the theoretical sensitivity measured by DEWESoft.

So :

So when using a SIRIUS STG with a quarter bridge configuration (with 3 completion resistors inside the hardware) or a full bridge configuration (with 3 completion resistors or 3 non active gages) the scaling shouldn’t change and the measurement too.

Is there something I am missing or don't know about the hardware in current excitation mode ? (see attached PDF).

Thanks a lot !

Regards,

Hi,

I got an explanation and confirmation that bridge measurement with current excitation works correctly. Here is the explanation:

Formulas are different from Full against to Half/Quarter, but in fact this is related to hardware.

In FullBridge the exc current goes parallel on 2 sides of the resistors, so left 2 resistors and right 2 resistors.

In HalfBridge the exc current controller controls only for the connected HalfBridge so it is only 1 side (2 resistors) where the current goes through.

And QuarterBridge can be seen as HalfBridge, where the current goes through the internal completion resistor and the external quarter resistor.

The point is: the scale looks different but the measurement result is correct due to how the current goes through resistors.

BR

Hi Matic !

Thanks for your answer ! Now it is clear to me since there is an explanation.

Can you provide a bit of schematic and / or calculations that lead to the formula ?

I need to explain how calculations are made in order for our customer to compute their scalings according to the different configuration of strain gages they have.

When I simulate a single source for 2 strain gages I don't get the division by 2.

Thanks !

Hi,

In the previous reply, I mentioned that current goes only through the connected bridge, e.g. the resistors that are on the right side (Sensor side), if you refer to the schematic picture from the Ch. setup. The voltage across this part of the bridge is doubled when the right side is a quarter or a half bridge. There is no current through the internal completion resistors (amplifier side). This is the reason that formulas are different. I think that the below schematics explain this design very well.

Do not hesitate if you need further help.

Hi Matic,

Thanks again for your answer. I guess this is the last time I annoy you.

I think that I got what was wrong with how I calculated the scaling, it is all about the reference (IN+ / IN- / GND) do you confirm the method and calculations below (or attached pdf)

Thanks;

Maxime