Load cells are measuring devices that monitor and gauge forces of compression, tension and shear. They are a type of transducer that converts sensed mechanical force into electrical signals for measurement used in mechanical testing, ongoing system monitoring and as components in devices such as industrial scales.
Of the three major categories by which load cells measure force, compression and tension are the most commonly employed. Sometimes a load cell will measure an object through both of these applications, rather then one or the other.
Both compression and tension loadcells often use strain gauges; strain gauges are very small devices that measure the strain of an object by converting internal deformation into electrical signals, precisely measuring weight, force or strain. Force gauges use strain gauges in push-pull testing and flow measurement. While most load cells measure and test with strain gages, some use piezoelectric sensors, which utilize piezoelectric crystals to measure weight, strain, movement and vibrations.
Pressure sensors and force sensors are transducers very similar to load cells that measure pressure, applied force and strain in gas pressure, altitude and liquid pressure. These sensors are often piezoelectric sensors. Many of these sensors, although quite small, are built to support or to hold as much as several tons; miniature load cells are built to provide precise measurements for much smaller applications. Equipment such as force transducers, torque sensors and load sensors all sustain strain gauges or load cells that measure and convert energy. Digital load cell technology is the most popular way to access the information gained through the sensors.
Various types of load cells, pressure sensors and gauges are used in manufacturing, processing and testing industries. Pressure sensors and load cells are used in food processing industries to precisely measure ingredients and to properly distribute the products during packaging. In industrial warehouse environments, where pallets of inventory are shuffled around, load cells are often used to determine the precise weight of loaded pallets, which is crucial for the filling and accepting of orders.
Other load cell applications include the testing of bridge building materials such as beams for tension strength, as well as in railcar weighing and truck scales. Load cells are essential components in many calibration systems, as well as for fatigue testing in research and development laboratories. With reading accuracies within 0.25%, load cells, sensors and gauges provide accurate mass, weight and pressure measurement of very small loads to loads of several thousand tons.
After load cells transduce mechanical stress into electrical energy, the information that loadcells monitor is then signaled to a recorder or other computerized data collection system. Analog or digital load cell technology is used for the recording and transferring of information. Digital load cells have become more popular than analog load cells in recent years because they work faster, have a higher accuracy rate and better resolution.
When load cells are used to measure any variance in certain ongoing systems, the load cells can sound an alarm or shut down the system itself until the discrepancy is corrected. Load cells can vary greatly in size and shape depending on the industrial arena they will be utilized in. The two basic components of a load cell are the sensing element and circuit. The sensing element is most often a strain gauge, which is comprised of coil. However, it can also be a piezoelectric sensor that utilizes crystals. The circuit is the connection of these gauges or sensors throughout the load cell.
Load cell outputs include analog voltage, analog current, analog frequency, switch or alarm, serial and parallel. The most basic designs consist of four gauges, which make up the measuring circuit. More complex and detailed cells can have up to thirty gauges as part of the measuring circuit. The arrangement of gauges is usually done according to the Wheatstone bridge equation, which was developed in 1833 by Samuel Hunter Christie. It was not until ten years later though that the equations’ namesake improved upon it and made it popular.
The more gauges inside the load cell, the more sensitive the cell is in recording and monitoring variance in measurement. In calculating capacity of a load cell, factors that must be considered are: the maximum force value, the dynamics of the system (i.e. frequency response), the effect that placing the transducer in the force path will have and the maximum extraneous loads that the load cell will handle.
When mounting load cells, such factors must be considered: whether the load cell be in the primary load path or whether it will see the forces indirectly; whether there are any physical constraints that should be met for size and mounting; what level of accuracy is required, and what environmental elements the load cell will be subjected to that may cause special problems. These complexities are necessary to have the correct measuring force load cell in place, to ensure the safety and productivity of the industries employing them.