We use shunt calibration for two purposes. To check if the measurement chain is working and to cancel out the effect of lead wire compensation
Bridge balancing is the function of the bridge amplifiers to eliminate the bridge sensor offset. Mathematically it means to simply remove the initial offset of the sensor on the side of the amplifier. For the demo, we connect a quarter bridge strain sensor.
In the scope screen above the unscaled value currently read (left side) is 0,9 mV/V. That is because the strain gauge does not have exactly 350 ohms, which is normal, due to tolerances.
Click on "Balance sensor" to zero the bridge.
The unbalance will be measured and shown. Amplifier will choose the right setting to achieve full-scale range.
The input signal is now 0 mV/V
We normally do the balancing just before we perform the measurement. If we want to do the balancing on number of different channels, we can do that by using the GROUP OPERATIONS (described at the end of the topic).
When sensor balance is used, we can always check what is the value of the unbalance by using the short function. When using the SHORT ON operation, the pins 2 and 7 (the input pins of the amplifier) are internally shorted.
We simply click "Short on".
The resulting value will show the sensor unbalance.
Disable the short again by clicking "Short off"
Please note that we can remove the sensor balance by clicking the Reset button. After that, the sensor offset will be removed and the amplifier will be set into the initial state.
On the screen, we can see the unbalance of the sensor again, which is 0,9 mV/V.
"Balance" and "Reset" are, therefore, opposite operations.
There is also a function called "Zero" which is similar to "Balance sensor". Let's look at the difference bellow.
Imagine, we have a force transducer with a strain full bridge output. It will measure the weight in our experiment. In the first picture, we measure the unbalance of the bridge sensor, e.g. 35 N. Let's do a "Balance sensor". The output is now 0 N. A vehicle is put on the test bed. We measure its weight, which is 12 000 N. For our measurement only the changing of the weight is of interest, so we cancel out the fix offset with the Zero function.
Click the "Zero" button in Channel Setup (can be reset by right mouse-click). The output is now zero again. Note that this is a pure software subtraction. If the range was set to "Automatic", the range is automatically adapted to -52000...+28000.
The range can be set to "Automatic" in the channel setup window of the appropriate channel (right mouse-click).
Now all offsets are canceled and we start the measurement. This function can also be accessed in the Measure mode (but NOT while storing!).
With shunt calibration we can:
- check if the amplifier is working properly (excitation and value readout);
- check if strain gauge is connected and working properly;
- we can compensate the length of the lead wires.
The SHUNT ON operation is meant for checking if the connected strain gauge is OK. From the wiring schematic in Dewesoft X you see that the amplifier already comes with the integrated shunt resistor.
The idea behind is to "shunt"/connect a resistor of known value parallel to one resistor of the bridge to achieve a known, calculable unbalance.
With the "Shunt calibration," we can automatically check the measured value against a predefined one (from sensor database or TEDS). For the measurement, this internal shunt resistor is disconnected again of course.
Let's check out the formulas:
R3 and R4 are part of the bridge completion, mounted internally in the amplifier. The R1 is the connected strain gauge. The R2 is also mounted internally in the amplifier together with the shunt Rs. With a 350 ohms quarter bridge and an internal shunt resistor of 175kohm (SIRIUS STG module) the
expected unbalance should be:
The resistor value in one leg, when shunt is connected, is calculated from the equation of parallel resistors:
From the equation of the bridge we get the bridge unbalance:
If the bridge factor is 2, the resulting strain will be:
If the bridge is out of balance, we need to click first on "Balance Sensor", because the formula is only valid on a balanced bridge (both Rb = 350 ohms).
Then we click "Shunt on".
The output value comes very close to the expected value (0,495 mV/V).
But how big is the error exactly?
We can define the size of this error by making shunt cal check. Before we do that, we need to make needed SHUNT CAL PREPARATION.
Shunt cal check is done on scaled values, therefore, we take a look on the strain scaling. The max input from the graph above is 2 mV/V; the max scaled output signal is 4000 um/m. So, the scaling factor is 2000.
Our target value of 0,495 mV/V would equal (x 2000) = 1000 um/m.
After all the preparations are done, we can make SHUNT CAL CHECK.
The result of our shunt cal check looks very promising in this case (-0,2 %). That means that the strain gauge is OK.