Load cells are a special form of force transducers (force sensors) for the construction of weighing devices, i.e. for weighing with scales. They are calibrated in grams (g) kilograms (kg) or tons (t), not in newtons (N) like the force transducers. In practice, however, far more load cells are used as force transducers. Load cells are essential components in various industrial, commercial, and scientific applications, serving as precise transducers for measuring force or weight. Whether it’s monitoring the weight of a vehicle on a bridge, ensuring accurate dosing in pharmaceutical manufacturing, or enabling precise weighing in retail scales, load cells play a crucial role in countless systems.
Principles of Operation
At its core, a load cell is a transducer that converts mechanical force into an electrical signal proportional to the applied force. The fundamental principle behind load cell operation is strain gauge technology. A strain gauge is a device that changes resistance when subjected to mechanical strain or deformation.
Load cells, like force transducers, usually contain a spring body, i.e. a suitably shaped piece of metal whose geometry changes slightly under the influence of weight. This elastic deformation is detected by strain gauges for weights over a few grams up to several 1000 tons and converted into an electrical signal.
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Strain Gauge Arrangement: Load cells typically contain one or more strain gauges arranged in a Wheatstone bridge configuration. This arrangement allows for precise measurement of small changes in resistance resulting from applied force.
Deformation Under Load: When a force is applied to the load cell, it undergoes deformation, causing the strain gauges to stretch or compress. This deformation leads to a change in the electrical resistance of the strain gauges, which is directly proportional to the applied force.
Wheatstone Bridge Circuit: The Wheatstone bridge circuit consists of four resistive elements connected in a diamond-shaped configuration. As the resistance of the strain gauges changes due to applied force, the imbalance in the bridge circuit generates a voltage output proportional to the force.
Output Signal: The output signal from the Wheatstone bridge is typically a low-level voltage or current, which requires amplification and conditioning to be suitable for further processing or display.
Depending on the spring body, load cells have different dynamic properties. High dynamics are important in many filling and sorting systems, as often only fractions of a second are available for the actual weighing process. Older load cells work with oil fillings as damping, while newer systems use a combination of mechanically very rigid load cells and electronically matched filters in the downstream measuring device. In static applications, the only thing that matters is usually the size and the type of load application (assembly), e.g. whether the load cells should be harmoniously integrated or whether the construction height of a container must not change, etc.
Types of Load Cells
Load cells come in various types, each designed to suit specific applications based on factors such as capacity, accuracy, environmental conditions, and mechanical configuration.
Compression Load Cells are designed to measure compressive forces along a single axis. They are commonly used in applications such as tank and hopper weighing, material testing, and hydraulic press monitoring.
Tension Load Cells are used to measure tensile forces or pulling loads. They are often employed in applications such as crane scales, conveyor belt tension monitoring, and cable tension measurement.
Shear Load Cells measure forces applied in shear or parallel to the plane of the load cell. They find applications in tasks such as platform scales, vehicle weighing systems, and aerospace testing.
Bending Beam Load Cells utilize a bending beam or cantilever design to measure bending forces. They are commonly used in bench scales, retail weighing systems, and industrial process control.
Ring and Pancake Load Cells have a compact, low-profile design and are suitable for applications where space is limited. They are often used in robotics, medical devices, and force measurement in confined spaces.
Applications of Load Cells
Load cells form the basis of retail scales used in supermarkets, grocery stores, and specialty shops for weighing produce, bulk goods, and packaged items with accuracy and reliability. The area subject to custody transfer includes, for example, all scales in the trade (meat, fruit and vegetables, etc.). Here, a country’s Act on Metrology and Verification regulates the requirements for scales and, accordingly, for load cells and downstream electronics. Load cells intended for this application must have certain characteristics in terms of measurement deviations and repeatability.
Apples, chicken legs or other foodstuffs are sorted by mass in order to assemble containers with a certain total mass. The faster and more precisely the weighing and categorisation can take place, the less “excess” is filled into the containers. Here, even saving a small amount of just 1% of the total content can lead to big savings.
In addition to these dynamic applications, there are also many static applications in which the weight of a container is constantly weighed to record the inflow and outflow of the material it contains. Applications include storage tanks with the ingredients in the chemical industry or in food production. In the food industry, load cells must be washable and in the chemical industry partially explosion-proof.
Load cells are extensively used in industrial environments for weighing applications such as tank and silo weighing, batch mixing, and inventory management.
They play a critical role in material testing machines, ensuring accurate measurement of tensile, compressive, and flexural properties in materials such as metals, plastics, and composites. Load cells are integral to automotive testing equipment for evaluating vehicle performance, durability, and safety. They are also used in aerospace testing for structural load testing, fatigue analysis, and component validation.
They are employed in medical devices such as infusion pumps, patient scales, and rehabilitation equipment. In pharmaceutical manufacturing, they ensure precise dosing and dispensing of medications and ingredients.
