When Dahua Instrument Factory of Shanghai uses multiple sensors in the electronic weighing system, the commonly used combination methods based on the principle of weighing sensors include three methods: serial operation, parallel operation, and series-parallel operation.
The series work method is that each sensor uses separate power supply for the bridge and the output is connected in series. Set their bridge arm resistances as R1 and R2 respectively, and the sensitivities are S1 and S2 respectively. The bridge voltages are U1 and U2 respectively, and the full scale is F. In order to ensure a normal series working state, S1U1 = S2U2 needs to be satisfied. Similarly, it can be proved that when n sensors work in series, S1U1=S2U2=...=SnUn needs to be satisfied to ensure normal operation. This is the basic condition for tandem work. From this formula, it can be seen that for sensors working in series, regardless of the parameters of each sensor, theoretically, the normal operating state can be established by adjusting the supply voltage. Of course, the discreteness of parameters in practical use cannot be too large.
The disadvantage is that after the series connection, in the case of DC power supply, each sensor needs to be independent of the bridge voltage power supply, otherwise it will destroy the original relationship of the bridge circuit, increase the complexity of the equipment and increase the cost. In the case of AC power supply, the secondary winding of the power transformer needs to be exactly the same for an electronic scale with a high requirement for weighing accuracy, which is difficult in actual production. In addition, the output impedance of the sensor increases in series, which easily causes interference. At present, most of the domestic electronic scales still use this type of connection, but they are rarely used abroad and have almost been eliminated.
All-parallel working mode This mode is parallel input of each sensor, using a common power supply for the bridge, the output also works in parallel. Let their sensitivity be S1, S2 respectively, the arm resistance is R1, R2 separately, the bridge voltage is U1, U2 respectively, the full scale is all F. The conditions for the parallel operation of these two sensors are S1R1=S2R2. Obviously, the parameter requirements of the sensors in parallel are relatively high. Similarly, when n sensors work in parallel, S1/R1=S2/R2=...=Sn/Rn.
In the condition that the weighing display has high sensitivity or high resolution, the parallel connection method is preferable because it requires only one bridge power supply and the system is simple and economical. However, it requires that the average deviation of the output impedance of each sensor is small, and the tolerance of the sensor coefficient cannot be too large. Otherwise, when the stress of several sensors is not uniform, the average value of the output voltage will produce an error. When working in parallel in foreign countries, most of the two isolation resistors are connected between the two outputs of each interpreter and the attached weight indicator. Since the internal resistance of the sensor is a function of the output signal, the series resistance can reduce the effect of the resistance change on the output. The total resistance of the two isolated resistors of each sensor must be equal, the two resistors must be equal to each other, and the tolerance must be small. This can reduce the influence on the total output of the sensor due to the unequal output impedance of the sensor or inconsistent load cell parameters.
Tests have shown that when the sensors working in parallel are loaded unevenly, the average error of the sensor is less than 0.05%.
Series and parallel mixed working method This is the synthesis of the above two working methods, both series and parallel. In general, each sensor uses a separate bridge power supply, but the outputs are connected in parallel. This type of operation is also called series-parallel connection.
It can be seen that the series-parallel working mode has both the characteristics of parallel working modes and similar places in series work mode. Regardless of the sensor parameters used, it can establish the normal working state by adjusting the bridge voltage supply. Of course, the discreteness of the sensor parameters cannot be too large. It can also be proved that the output signal Un=Ui when n sensors work in parallel, and the total output impedance is Rn=R/n. As for the four sensors of an electronic scale, two or two grounds are connected in parallel or two or two grounds are connected in parallel, and although they are also possible, they are not used in practical applications.
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