A load cell is a transducer that converts force into an electrical signal -- usually measured in mV (millivolts) per V (volts) excitation -- using an instrumentation amplifier. Load cells come in dozens of varieties. The most widely used is the strain gauge (which measures the change in resistance of electrical current, caused by strain).
History and Uses
In 1843, English physicist Sir Charles Wheatstone invented a bridge circuit capable of calculating electrical resistance, which over time developed into the strain gauge load cells used today. In the meantime, hydraulic load cells became useful in outdoor environments susceptible to lightning. Strain gauge load cells dominate today's industrial weighing applications because of their unmatched accuracy. Load cells have many applications, including force measurement, portable weigh scales, platform scales, electronic weighbridge or truck weighing, finding centre of gravity, structural health monitoring, feedback control and impact measurement.
Distinguishing Between Four- and Six-Wire Load Cells
The gauges that aren't stand-alone load cells cannot function without appropriate wiring -- load cell cables -- which are comprised of four or six wires. Both four- and six-wire cells have +/- excitation and +/- signal lines, but six-wire cells have + and - sense lines that must be employed for proper functionality. Unlike four-wire load cells, six-wire cables do not contain a temperature compensating system. Four-wire load cells must perform within a specific temperature range; in six-wire load cells, sense lines are connected to the indicator's sense terminals, which send back the load cell's actual voltage. The indicator then modifies its amplifier or its output voltage to compensate for resistance fluctuations.
Modifying Four- and Six-Wire Load Cell Cables
The benefit of the more direct weighing system found in six-wire load cells is the ability to cut (or extend) such cables to any length, a modification that should be used sparingly. Four-wire load cell cables are calibrated to work under specified conditions and should never be cut.