Among the load cells in use for electronic weighing it is certainly a must to name the load cells with electrical strain gauges. In general, an electrical strain gauge is nothing more than a measuring instrument capable of detecting deformations, even minimal, of a material subjected to mechanical or thermal stress. Through the detection of deformations it is possible to obtain the load to which the material is subjected.
The load cells with electrical strain gauges are therefore composed of a metal body on which electrical resistances, the real strain gauges, are glued. When the load is applied, the metal deforms, changing the electrical resistance of the applied strain gauges.
On average, each load cell is equipped with four or eight strain gauges, connected together in a Wheatstone bridge configuration fed at constant voltage on a diagonal. The bridge supplies, on the second diagonal, a signal proportional to both the supply voltage and the resistance variation of the strain gauges. The latter is caused by the applied load, which has deformed the metal.
Therefore, a signal expressed in mV / V will be provided at the output, one milliVolt for each supply Volt.
In order for the strain gauge bridge to be accurate it is necessary to integrate it with the following auxiliary circuits:
1. Bridge zeroing circuit: allows for a zero output when no load is present;
2. Zero drift compensation circuit: it allows to neutralize the variations of the elastic modulus caused by the temperature changes at zero load;
3. Field drift compensation circuit: similar to the one above, this circuit neutralizes the variations of the elastic modulus related to the temperature changes during the load;
4. Output signal calibration circuits: make the signal stable through the use of high stability resistors in series with the power supply voltage;
Finally, it should be noted that the output signal calibration circuits and the field drift compensation circuits must necessarily be divided symmetrically throughout the power supply circuit to avoid the circulation of eddy currents when using systems with multiple cells of load.
Load cells with electrical strain gauges can be of various types. Let's see them.
• Load bearing bending for plates
This type of load cells is suitable for small and medium capacity systems where it is important that the weighing surface remains insensitive to the load application point. Generally made of aluminum, its use in environments with high humidity or in the presence of aggressive chemicals is not recommended due to the impossibility of hermetic sealing due to their particular construction shape.
• Double bending beam
They are cells particularly suitable for weighing at small capacities (2÷300 kg) and offer a good response even in systems with high dynamics. They also have a relative insensitivity to the load application point, which makes them usable also for weighing small platforms and containers with a center of gravity subject to small displacements.
• Shear beam
These cells are used for medium capacity weighing (0.5÷10 t). However, they require solid fixing bases, firmly anchored, in order to withstand the particularly high bending moments generated during the weighing phases. Not suitable for high transverse loads due to their H measurement section.
The membrane load cells are suitable for medium capacity (0.5÷50 t), they are practical to assemble and have a high resistance to transverse loads.
• Universal traction/compression
Used in medium capacity (100 kg÷10 t), these cells, as defined by the name, allow thrust and traction measurements in test equipment on lifting equipment.
• Column compression
The use is usually for medium/large capacity weighing (10÷500 t). They are self-centering and allow the free thermal expansion of the parts.
The pins are used on lifting equipment and operating machines. A great advantage of the pins is the possibility of replacing pins already in place without the need for modifications. Generally, pin load cells are dimensioned ad hoc for each application.
• Other types of cells
Among other types of cells we name the toroidal, double-edged, flange and washer.
Load cell terminology
Let's have a look at the meaning of each word:
• Nominal load: load corresponding to the nominal output
• Initial load: load below which the specification data are no longer valid
• Maximum load: load above which the specification data are no longer valid
• Non-linearity (best line): maximum deviation from the best line of upward measurements
• Hysteresis error: maximum deviation between rising and falling measurements
• Repeatability error: maximum standard deviation out of ten measurements
• Drift in 30 minutes: signal change in 30 min after a rapid load change
• Nominal temperature range: ambient temperature range within which the specification data is valid
• Zero thermal drift: max variation of signal for every 10 ° C variation of ambient temperature with zero load
• Field thermal drift: max variation of signal for every 10 ° C of ambient temperature variation with cell loaded
• Effect of pressure variations: variation of the output signal due to variations in barometric pressure
• Nominal output: output corresponding to the nominal load
• Tolerance on the output at zero: max value that the output can assume with cell without load
• Maximum power supply voltage: maximum cell power supply voltage
• Input resistance: resistance measured between the cell power conductors
• Output resistance: resistance measured between the cell signal conductors
• Insulation resistance at 50 V: resistance measured in d.c. between cell circuit and cash desk
• Breaking load: load beyond which the mechanical destruction of the cell occurs
• Maximum transverse load: load, perpendicular to the measuring axis of the cell, beyond which irreversible mechanical or electrical changes occur
• Nominal deflection: elastic stroke of the load application point from zero to nominal load
• Working temp. range: ambient temperature range within which the cell can work with errors exceeding the specified limits, without irreversible modifications.
• Storage temp. range: ambient temperature range in which the cell, not connected or loaded, can be stored without irreversible modifications
• Protection: type of resistance to dust and water according to EN60529