Principle of operation of load cells

The conversion is achieved by the physical deformation of strain gauges, which are combined to the load cell structure. The strain gauge is connected to a bridge circuit with four strain gauges (full bridge), two gauges (half bridge) or one gauge (quarter bridge). With half and quarter bridges, the bridge is completed using a precision monitor.

Additional bondable resistors are configured in the bridge circuit to compensate for the effect of temperature on the zero or no-load signal as well as sensitivity (due to the output caused by the applied load, plus another resistor that renders the bridge ineffective at no load) .

The completed Wheatstone bridge requires a stable DC power supply to excite the circuit. This is typically 5 Vdc or 10 Vdc, but can be any value from 1 Vdc to 18 Vdc.

When stress is applied to the bonded strain gauge, a change in resistance occurs that unbalances the bridge. This provides an output signal that is linearly proportional to the stress value. The value of this signal is also proportional to the excitation voltage, typically a few millivolts.

Load Cell Output Options

These low-level millivolt signals are compatible with a variety of custom strain gauge instruments. These instruments include digital displays, analog and digital amplifiers. A typical analog amplifier will produce higher level voltages (0-5Vdc, 0-10Vdc) or currents (0-20mA, 4-20mA) for subsequent processing.

Digital amplifiers typically provide RS232, RS422 or RS485 outputs using either a common ASCII protocol or a range of more specialized industry-specific protocols that have been developed (e.g. Modbus). Both analog and digital instruments have been reduced in size (miniaturized) enough to allow items to be mounted within the body of a load cell or force transducer.

Load cell designs are distinguished either by the type of output signal they produce (pneumatic, hydraulic or electrical) or by the way they detect loads or weights (bending, shear, compression, tension, etc.).

Load cells can take many forms in order to meet a wide variety of uses in R&D and industrial applications. However, most designs today use strain gauges as sensing elements, where foils or semiconductors are used.

Foil gauges offer the widest selection of load cell designs and therefore tend to be the most commonly used. Strain gauge patterns provide measurements of tensile, compressive and shear forces.

Semiconductor strain gauges have a smaller pattern range, but offer the advantage of being smaller and provide a large gauge factor, resulting in a larger output for the same given stress. Because of these characteristics, they tend to be used in miniature load cell designs. Proof rings are used for load measurement, using calibrated metal rings whose motion is measured with precision displacement transducers.

Load cell operating principle

Strain gauge load cells convert the load acting on them into an electrical signal. The gauges themselves are bonded to a beam or structural member that deforms when a force is applied. In most cases, four strain gauges are used to obtain maximum sensitivity and temperature compensation. Two gauges are usually in tension and two in compression and are wired by compensation adjustment. When a load is applied, the strain causes the resistance of the load to change in proportion to the load. As strain gauge versions continue to improve their accuracy while reducing unit cost, other load cells are becoming less popular.

Early load cell designs simply used strain gauges to measure the direct stress that is introduced into the metal element when subjected to tensile or compressive forces. Bending beam type designs used strain gauges to monitor stresses in the sensing element when subjected to bending forces. More recently, shear stress measurements have been adopted as a more efficient method of load determination because the method is less dependent on the direction of the force applied to the load cell.

Other types of load cells include hydraulic (or hydrostatic), pneumatic, piezoelectric, and vibroelectric load cells.

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