Load cell

Load cells are a special form of the force transducer ( force sensors ) for the construction of weighing devices, that is, for weighing with scales. They are in grams (g ) kilograms (kg) or tons (t) calibrated, not in Newtons ( N) as the force transducer. In practice, however, far more load cells are as force transducers in use.

Construction

Load cells such as load cells usually comprise a spring body, i.e., a suitably shaped piece of metal, the geometry of which changes slightly under the action of the weight. This elastic deformation is monitored for weight on some 100 grams to 1000 tons of strain gauge is converted into an electrical signal.

Below about 100 grams can be used other principles, such as the electromagnetic force compensation (see microbalance ).

Typical spring body shapes for load cells are

  • Double bending beam for small loads
  • Shear beam for larger loads
  • Columnar spring body ( push rod or hollow cylinders )
  • Ringtorsionsfederkörper for high loads
  • Multi bending beam spring body for very high quality scales
  • Diaphragm spring body: small size with high stiffness

Mechanical properties

Depending on the spring body have load cells have different dynamic properties. High dynamics is important in many filling and sorting, as for the actual weighing procedure often only fractions of a second are available. Mature load cells working here in the oil fillings as attenuation, in newer systems, one uses a combination of mechanically very stiff load cells and electronically matched filters in the downstream gauge. For static applications, it is usually only on the size and type of load application (assembly ) to, for example, whether the load cells are to be harmoniously integrated or may not change, the height of a container etc.

Metrological properties

These include the rated load, is to be operated up to the load cell (Libra), the limit load at which a permanent destruction of the pickup occurs ( deformation of the spring body ) as well as the characteristic value, which is the output signal at nominal load (usually 2 mV / V). In specifying the measurement errors is usually taken directly to the installation into a scale reference and the number of parts specified ( steps) that could reach a balance with this load cell: 1000, 3000, 4000, 6000 or 10000 Parts International OIML (Organisation Legal Metrology ). The second important value for the scale constructor is the minimum value vmin division of the load cell, from the above vmin x ( number of load cells ) 1/2 the smallest possible increment follows. For a load cell with vmin = 10000, a rated load of 10 t and a part number of 3000 could be for example a balance with a resolution of 1 kg and a load of 3 tons built. Since the load cell may be loaded even up to 10 t, so here is a tare load ( preload, such as container weight) of up to 7 t be accepted.

For the range of platform scales nor the corner load dependence essential, that is, how big a load cell mounted on the surface must be without laying a weight in one of the corners leads to a ( unauthorized ) error.

Especially for non-custody area also details of the non-linearity of the characteristic curve in on ascending and descending burden and the hysteresis of the zero point to be made or combined and reported as the total error (composite error). Deviations, which are caused by temperature fluctuations, however, specified separately.

Other important characteristics are the creep strain ( change in output over 30 minutes at rated load) and the output resistance and the ratio of output resistance and electrical characteristic value (signal at nominal load). This information is important if several load cells to be connected in parallel, eg for a platform scale or hopper weighing, in which several load cells are positioned at the corners.

Areas of application

The weighing devices, a distinction between the legal metrology (see Bureau of Standards ) and the non-approved area. The custody area includes, for example, all the scales on the market ( meat, fruit and vegetables, etc.). Here regulates the law on the measurement and calibration system (see calibration) of the EU requirements for scales and accordingly to the load cell and the downstream electronics. Load cells, which are provided for this application must have certain properties with respect to measurement error and repeatability.

Apples, chicken legs or other foodstuffs are sorted by mass, compiling packages with certain total mass. The faster and more accurate here can be the weighing and categorization, the less " surplus" is filled into the container. This can even lead to big savings effects the saving of a small amount of only 1 % of the total content.

In addition to these dynamic fields of application, there are also many static applications, where the weight of the container is continuously weighed to measure the inflow and outflow of the material contained in it. Applications are storage tanks with the ingredients in the chemical industry or in food production. In the food industry, load cells must be washable and explosion-proof in the chemical industry in part.

  • Scale
  • Sensor
  • Biomechanics
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