Application example: load cell

The following example shows the connection of a full bridge with external 10V supply and the calculation of the weight value in the control.

Connection of the load cell

The EL9510 power supply terminal with 10 V is used to supply the 350 Ω full bridge. The full bridge is therefore supplied with U_V = 10 V. The current flow causes a voltage drop on the supply lines (red, dark blue) to the measuring bridge, so that U_Ref < U_V returns to the EL3351 on the sense lines (orange, light blue). Since U_Ref and U_Diff are the "true" voltages acting at the measuring bridge, these must be measured and used for the calculated evaluation.

Connection of a load cell with 10 V supply via the EL9510

Parallel connection of load cells If several load cells are connected in parallel to the EL3351 and the internal 5V supply is used, the total current consumption must be observed. It must not exceed the specified maximum output current. Only load cells that are appropriately calibrated and approved for this operation may be used.

Further calculation in the controller (TwinCAT)

Calibration of the system Before the measured value calculation, a calibration of the entire system is required due to the tolerances of the load cells/force transducers. This calibration must be carried out with a reference weight!

U_Diff and U_Ref are transported to the controller as INT values via the EtherCAT process data of the EL3351. The voltage values are to be calculated there according to

U_Diff [mV] = PDO "Ch. 1 value_dec" x (20 mV / 2147483647_dec)

U_Ref [V] = PDO "Ch. 2 value_dec" x (12 V / 2147483647_dec)

The gross weight calculation (incl. scale tare weight) is then done according to

Y_R = (U_Diff / U_Ref) x (E_max / C_n)

with

Any necessary corrective interventions (tare, adjustment, zero) must then be made on the control side on Y_R.

Concrete example in numbers:

Data of the sensor

• Characteristic value C_n = 2 mV / V
• Nominal load E_max = 50 kg
• Supply voltage U_V = 10 V

Weight calculation

At full load (load cell is loaded with 50 kg) U_{Diff_max} is calculated to:

U_{Diff_max} =  U_{Ref_max}  x  C_n = 10 V x 2 (mV / V)  = 20 mV

Assumption: With G = 7 kg load
U_Diff = U_{Diff_max} / E_max x G = 20 mV / 50 kg x 7 kg = 2,8 mV

Fictitious measured values 

• Bridge: „value1“ (Inputs bridge) = 0x11EB7026 = 300.642.342_dec
• Supply: „value2“ (Inputs supply) = 0x6AA9D8F2 = 1.789.516.018_dec

Calculation of the voltages with the measured values

U_Diff = 300642342_dec x (20 mV / 2147483647) = 2,79995 mV

U_Ref = 1789516018_dec x (12 V / 2147483647) = 9,9997 V

G = U_Diff / U_Ref x E_max / C_n = 2,79995 mV / 9,9997 V x 50 kg / 2(mV / V)) = 7,00009 kg