The documentation of the Yocto-MaxiBridge and of the Yocto-Bridge recommend that you use wires as short as possible to connect the load cells to the interface module. But in some applications, for practical reasons, you simply can't use short wires. You can then use a six-wire cabling to connect the load cells and thus compensate the error introduced by the long wires. Let's see how...
As a reminder, the measure of a load cells consists in producing an excitation voltage at the terminals of the cell and to measure the resulting potential difference between the two measuring wires, of which the difference depends on the mechanical stress to which the cell is subjected. This difference is directly proportional to the measured load and to the excitation voltage.
The measured voltage is proportional to the load and to the excitation voltage
The interest of this system is that the load can be directly computed by the ratio between the small potential difference and the excitation voltage. This is called a ratiometric measure and it so happens that it's the most fundamental and thus the most accurate manner to perform a digital voltage measure. It has the advantage to provide a constant result even in case of small variations in the excitation voltage, as it is only the ration between the two voltages that we measure.
So, where does the problem with long wires come from? It comes from the voltage drop in the wires which bring the excitation voltage to the terminals of the load cells. Indeed, from the moment where the resistance of the wires is not insignificant with regards to the resistance of the cell, we note that the measured signal is no more fully proportional to the excitation voltage:
The wire resistance can distort the 4-wire measure
The solution to this issue is to compute the ratio using the effective excitation voltage available at the load cell, instead of the voltage produced at the origin. But to do so, we need an extra pair of measuring wires:
Measure of the excitation voltage with a 3rd pair of wires
So we need to have this additional pair of measuring wires, both on the load cell and on the measuring module.
With regards to the load cell, high-end cells are often directly equipped with six wires. If you have a cell with four wires only, you can simply duplicate each excitation wire with another wire soldered at the origin: the wire with the excitation current is no different from the wire measuring the excitation, it goes to the same place. The key is to not use the same wire to bring the excitation current and to measure the resulting excitation voltage at the cell.
Converting a 4-wire cell into a 6-wire cell
For the connection on the Yoctopuce module, there is no other choice but to use an input dedicated to the measure of the excitation voltage. We have therefore included in the configuration of the Yocto-MaxiBridge an option enabling you to transform the fourth input of the module into a measure of the excitation voltage.
Enabling the measure with 6 wires
You can then connect your 6-wire cell on a Yocto-MaxiBridge as shown below:
Connecting a 6-wire cell
Note that in the diagram, we on purpose used the symbol denoting the use of twisted pairs. Indeed, if the third pair prevents the issue with voltage drop, it doesn't prevent the problem of long wires which have a tendency to collect all kinds of electromagnetic emissions which add noise to your measure. We therefore recommend to use a shielded cable made of twisted pairs, making sure to couple the correct wires in each pair. A rather simple way to reach this goal consists in using a good shielded RJ45 network cable and to use the RJ45-Adapter to connect it to the cell. And don't forget that for the shield to be truly efficient, we recommend to ground it with the fixation screws of the RJ45-Adapter on both sides!