Load cell
Load cells are a special form of force transducers (force sensors) for the construction of weighing devices , i. H. 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.
construction
Load cells contain as load cells usually have a spring body , d. H. a suitably shaped piece of metal, the geometry of which changes slightly under the influence of weight. This elastic deformation is recorded by strain gauges for weights ranging from a few grams to several thousand tons and converted into an electrical signal.
Typical spring body shapes for load cells are
- Double bending beam for small loads
- Shear bar for larger loads
- columnar spring body (compression rod or hollow cylinder)
- Ring torsion spring body for high loads
- Multi-beam spring body for very high-quality scales
- Diaphragm spring body: small size with high stiffness
Mechanical properties
Depending on the spring body, load cells have different dynamic properties. A high level of dynamics is 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 stiff load cells and electronically matched filters in the downstream measuring device. In the case of static applications, it usually only depends on the size and the type of load application (assembly), e.g. B. whether the load cells should be integrated harmoniously or whether the construction height of a container should not change etc.
Metrological properties
This includes the nominal load up to which the load cell (scale) is to be operated, the limit load at which permanent destruction of the transducer occurs (deformation of the spring body) and the characteristic value, which is the output signal at nominal load (usually 2 mV / V) . When specifying the measurement errors , reference is usually made directly to the installation in a scale and the number of parts (steps) that a scale could achieve with this load cell: 1000, 3000, 4000, 6000 or 10000 parts according to OIML (Organization Internationale de Métrologie Légale). The second important value for the balance designer is the minimum division value v min of the load cell, from which the smallest possible increment follows via v min x (number of load cells) 1/2 . For a load cell with v min = 10000, a nominal load of 10 t and a number of parts of 3000 z. B. a scale with a resolution of 1 kg and a load of 3 t can be built. Since the load cell itself can be loaded up to 10 t, a tare load (preload, e.g. container weight) of up to 7 t can be accepted.
The corner load dependency is also important for platform scales. This means how large a surface mounted on the load cell can be without placing a weight in one of the corners leading to an (impermissible) deviation.
Particularly for the non-verifiable area, information about the non-linearity of the characteristic curve with increasing and decreasing load as well as the hysteresis of the zero point is made or combined and given as a cumulative error (composite error). Deviations caused by temperature fluctuations are specified separately.
Other important parameters are the load creep (change in the output signal over 30 minutes at nominal load) and the output resistance or the ratio of output resistance and electrical characteristic (signal at nominal load). This information is important if several load cells are to be connected in parallel, e.g. B. with a platform scale or container weighing, in which several load cells are positioned at the corners.
Areas of application
When it comes to weighing devices, a distinction is made between the areas subject to verification (see verification office ) and areas not subject to verification. The area subject to verification includes B. all scales in the trade (meat, fruit and vegetables etc.). Here, the law on measurement and calibration (see calibration ) of the EU regulates the requirements for scales and, accordingly, for the load cells and the downstream electronics. Load cells that are intended for this application must have certain properties with regard to measurement errors and repeatability.
Apples, chicken drumsticks or other foods are sorted by mass in order to put together containers with a certain total mass. The faster and more precisely the weighing and categorization can take place here, the less “surplus” is filled into the container. Here, even saving a small amount of only 1% of the total content can lead to major savings.
In addition to these dynamic areas of application, there are also many static applications in which the weight of a container is constantly weighed in order to record the inflow and outflow of the material it contains. Applications are storage tanks with the ingredients in the chemical industry or in food production. In the food industry, load cells have to be washable and in the chemical industry some of them have to be explosion-proof .