XPod Bridge - Strain gauge

How to set up XPOD conditioners for strain gages

Hard Connection

Let us see how to proceed a half bridge connection.

Use a male SUB D 9 pins connector and a "bullet proof" cable like Ethernet cable to link the strain gages to the connector.

Then weld the connector pins according to the pin-out diagram below:

Set up NVGate

Sensitivity

In quarter bridge configuration, the gauge is usually glued toward the beam direction to measure the bending for example. In this case the right formula is

 

Where

-       S = Em/V is the sensitivity (*V / µdef)

-        e , 2, 3 et 4 are the resistivity of the gage or generated by the hardware depending on the type of the

bridge.Here, 2, 3 and 4 are XPOD internal resistors

-       V, the excitation voltage (in NVGate, the settings is  "Expender module/Bridge voltage")

-        ΔEm, the strain measurement voltage

-         r, equal to R1/R4=R2/R3 often equal to 1.

-        Sg is the gage factor (often equal to 2 x 1E-06 volts per micro-def

-	n, the number of gages (For full bridge, it’s 4. For ¼ bridge it’s 1.)

You can notice that the sensitivity is not linear with the stress  = L/L. For low strain, the formula is equivalent to:

For a half-bridge configuration, it is possible to glue 2 gauges toward the beam direction as shown in the figure below:

In this case, the sensitivity is linear and is equal to:

So the fact that 2 gages are used instead of only one gets the bridge sensitivity twice better.

NVGate setup

It is advised to first add a "transducer" in NVGate database in order to register the bridge configuration.

Browse the Home menu  Transducer tab  Database

Add a new transducer which is in fact a bridge configuration.

In the Conditioner area, fill the Gauge bridge type, the gauge resistance, the excitation voltage and so on.

The sensitivity to be filled is the bridge’s one (in red below). So, it is necessary to compute this sensitivity according to the above formula.

Measurement

The XPOD contains 8 inputs. In case of XPOD connection, the 8 BNC inputs are disabled for using the XPOD inputs.

These XPOD inputs can be connected to different plug-ins like FFT or Time domain analyzer for example.

Then the bridge has to be balanced through the Bridge Auto-zero feature (this option has to be setup to ON in the Input settings).

Press on « Bridge Auto-zero » from the "measurement" tab to start the bridge balancing. This step may last about 30 seconds.

Now, the measurement is ready to start.