The center of every scale is the load cell
The quality of the load cells used is crucial for high weighing accuracy and reproducible values. For this reason, the load cells that are part of the weight sensors octogon have an accuracy of at least 0.03, but most of them are certified according to OIML accuracy class C3. This means that ourload cellsthe high demands placed on calibrated scales. Octogon load cells are based on the strain gauge measuring principle with full bridge circuit. The output signal / measuring bridge signal of the load cells is mV / V. All measuring cells are 6-wire technology as standard. Optionally, models with an extended compensated temperature range or ATEX-tested load cells for use in Ex areas can be offered.
Please note: Load cells are just one part of the components required to determine weight. You also need evaluation electronics such as a weighing transmitter or a scale display.
Selecting a Load Cell
Depending on the application, a specific question leads to the right load cell. In crane scales and similar applications, traction sensors. A S-type load cell is used wherever the tensile force is to be determined. For example, shear bars are used when the force is introduced vertically. The nominal load is also crucial when choosing the right load cell. While single-point load cells with nominal loads of up to 2000 kg are mainly used for low loads with a high resolution in the g or mg range, compression load cells offer a weighing range of up to 500t, or more if required.
General functionality of different load cell types
Load cells are required to build weighing devices (scales). The most important component of a load cell is the so-called spring body. This is a metal part that deforms under weight. This deformation is recorded by a strain gauge and converted into an electrical signal.
In the case of the double bending beam, the most commonly used spring element, the acting weight causes the spring element to deform. The multi-bending beam works in a similar way. Here, several bending beams are coupled in such a way that all beams are deformed simultaneously when a load is introduced. The coupling of several bending beams is used in platform scales. Shear beam load cells are used for weighing devices with a higher nominal load. The way they work is similar to the bending beam. The shear beam is suitable for measuring when force is introduced vertically. S-shaped spring elements are always used when the tensile force needs to be determined. The spring element is deformed by pulling. The column-like compression rod or hollow cylinder spring elements are deformed by compression. Membrane spring elements also react to pressure. However, they are smaller and have a higher rigidity.
Mechanical properties
The various load cells have different mechanical properties that are required for a wide range of applications. Load cells with high dynamics are used where many weighing processes have to be carried out in a very short time, such as in automatic sorting systems. In static applications, such as a weighing platform, the dynamics are less important. Here the load cells are selected according to size or mounting direction.
Technical terms and definitions for load cells
Spring body – A spring body is the main component of a load cell. ItIt is a piece of metal that deforms easily under the influence of force or weight.The spring body must be designed in such a way that the deformation process is linear and can be reproduced as often as required. The deformation stretches the surface of the spring body. This stretching is measured by a strain gauge glued to the spring body.
Strain gauges (DMS)– The strain gauge is glued to the spring body. It consists of a measuring grid that is attached to a carrier. When the spring body is deformed by the application of force, the deformation is registered by the measuring grid and converted into an electrical impulse. In order to compensate for measurement errors, four strain gauges are usually connected to form a Wheatstone bridge circuit. The strain gauges are attached to the spring body in such a way that two strain gauges are compressed or stretched at a time.
Electrical impulse– An electrical impulse is created when electrons (negatively charged particles) move towards a positively charged pole. In weighing technology, the electrical impulse is the voltage that is output by the strain gauge when the measuring grid in the strain gauge registers a deformation. This electrical impulse is passed on to a control unit, which calculates a change in weight from this electrical signal.
Control unit– The control unit converts the electrical impulses transmitted by the strain gauges into the actual weight. This means that the control unit calculates a weight from the electrical impulses and then displays this on a display.
Step size– In weighing technology, the step size refers to the accuracy with which weighing can be carried out. The step size of a scale is calculated from the minimum division value of the load cell and the number of load cells used.
Tare weight– The tare weight refers to the packaging weight or the weight of the empty container.
Traction– Tensile force is a force that acts from the attachment point of a body to the point of application of the force, i.e. the force with which a body is pulled. The tensile force is measured in Newtons (N).
Metrological parameters of load cells
In addition to the mechanical properties, the load cells also have specific measurement properties that are relevant for their use. The nominal load is of course very important. This is specified in grams, kilograms or tons. Another value is the limit load at which the spring body is permanently deformed or damaged. The third important value is the characteristic value that describes the output signal at nominal load. The usual value here is 2mV/V. The designer of a scale also needs the minimum division value of the load cell. From these parameters he can then calculate the technical data of the scale, such as the measurement accuracy, the nominal load of the scale or the maximum permissible tare weight.
Other important parameters are the output resistance (especially when several load cells are connected in parallel) or the load creep (fluctuations in the output value under prolonged load).
Since scales are of course often used in the food industry, where there is a legal requirement for scales, the load cells used there must also be able to be calibrated accordingly. The special requirements for a scale or load cell that is subject to legal verification are regulated by law.
Application area of a load cell
Wherever tensile or compressive forces or deformations of solid components are to be measured, load cells in various designs are installed. In addition to the load cells evaluation is also required. In the area evaluation devices for load cells we offer you a comprehensive range of scale electronics and transmitters. We have the right product for every area, regardless of whether you just want to read a value, need an output signal for further processing in a PLC or want to switch relays with potential-free contacts. In particular, individual solutions are possible with our weighing systems. In addition to the existing standardized software packages, adaptations can be made to your existing system.
We would be happy to provide you with our know-how for your planning.
