Load Cells - A History

What is a Load Cell?

A Load Cell is a particular kind of transducer or sensor which converts force into measurable electrical output. Your typical load cell device consists of four strain gauges in a Wheatstone bridge configuration. On an industrial scale, this conversion consists of a load being transformed into an analog electrical signal. While there are many different types of load cells, the strain gauge load cell is the classical design. In this type of load cell, the conversion is accomplished by the physical deformation of strain gauges which are bonded into the load cell beam and wired into a Wheatstone bridge configuration.

The History of Load Cells

As a part of the measuring instrument family, the weighing scale is basically a lever. Its classic form is the equal arm balance which consists of a bar with two pans hanging from each end and a fulcrum at the centre of the bar upon which the bar can balance. Used by the ancient Egyptians, these balances were used to measure everything from people to gold. Another classic form is the steelyard invented by the Greeks (Archimedes) and Romans. Unlike the equal arm balance, its fulcrum was placed just off-center. An object of unknown weight was hung from the shorter arm, and a known weight, called the poise or counterpoise, was attached to the longer arm in such a way that it could slide back and forth. When the load was in place, the poise's position was adjusted until it balanced the load attached to the shorter arm. The weight of the unknown object was determined by comparing the position of the poise to the markings on the longer arm.

Advancements in scale technology took a huge step forward when Leonardo Da Vinci used positions of calibrated counterweights on a mechanical lever to balance and determine unknown weights. Variations of his designs used multiple levers, each of a different length and balanced with a single standard weight. Before hydraulic and electronic strain gauge load cells replaced mechanical levers for industrial weighing applications, these mechanical lever scales were widely used. They were used to weigh everything from pills to railroad cars and did so accurately and reliably, provided they were properly calibrated and maintained. They involved the use of a weight balancing mechanism or the detection of the force developed by mechanical levers. The earliest pre-strain gauge force sensors included hydraulic and pneumatic designs.

In 1843, the British physicist Charles Wheatstone devised a bridge circuit that could measure electrical resistances. The Wheatstone bridge circuit is ideal for measuring the resistance changes that occur in strain gages. Although the first bonded resistance wire strain gauge was developed in the 1940s, it was not until modern electronics caught up that the new technology became technically and economically feasible. Since that time, however, strain gages have proliferated both as mechanical scale components and in stand-alone load cells. Today, except for specific laboratories where precision mechanical balances are still used, strain gauge load cells dominate the weighing industry. Pneumatic load cells are sometimes used where intrinsic safety and hygiene are desired, and hydraulic load cells are considered in remote locations as they do not require a power supply. Strain gauge load cells offer accuracies from within 0.03% to 0.25% full scale and are suitable for almost all industrial applications.

How Load Cells Work

Load cell designs are classified according to the type of output signal generated (pneumatic, hydraulic, electric) or according to the way they detect weight \(compression, tension, or shear). Hydraulic load cells are force-balance devices measuring weight as a change in pressure of the internal filling fluid. Pneumatic load cells also operate on the force-balance principle. These devices use multiple dampener chambers to provide higher accuracy than can a hydraulic device. Strain-gage load cells convert the load acting on them into electrical signals. The gauges themselves are bonded onto a beam or structural member that deforms when weight is applied.

Kinds of Load Cells

Load cells measure tension, compression, or shear. Compression load cells measure a pushing together force along a single axis. Tension load cells measure a pulling apart force along a single axis. Shear load cells measure the displacement of a structural element to determine force.? Shear cell types for load sensors can be a shear beam, bending beam, or single point bending beam. The most common sensor technologies are piezoelectric and strain gauges.

1. Piezoelectric: a piezoelectric material is compressed and generates a charge that is conditioned by a charge amplifier. 2. Strain Gauge: strain-sensitive variable resistors are bonded to parts of the structure that deform when making the measurement. Are typically used as elements in a Wheatstone bridge circuit, which is used to make the measurement.