Canister load cell

A load cell is the actual element that measures force or weight. Standard load cells are metallic bodies that are defined by the force applied to them and deform elastically. The deformation is usually measured by glued-on metal strain gauges and converted into a voltage signal. When the load cell is loaded with a defined force, it emits a normalized signal (mV/V) proportional to the load. 

For small to medium forces, so-called bending or shear bars are used. For larger forces, cylindrical bodies or double shear bars are usually used. 

A weighing module consists of the load cell and a mounting module, also known as a load corner or installation aid. The mounting module has the function of directing the force in the desired direction and optimally into the load cell. For containers, the module has the function of fixing the container on the load cell and preventing it from tipping or shifting. The accuracy of the load cell should not be impaired, or only very slightly. Some weighing modules also have integrated transport, installation, and overload protection. 

Regardless of the accuracy of the load cell, the module is of fundamental importance for the system accuracy of the weighing process. In addition, there are special weighing modules with vibration-damping properties, for example.

To achieve optimum accuracy, all support points should be weighed. Apart from the cost, there is no real limit to the number of load cells. Since the load distribution should be as even as possible, 3 or 4 supports are easier to balance than 8 supports, for example. 

For indoor containers, 3 support points are ideal because, with a symmetrical load and arrangement of the load cells, each support point automatically experiences the same load. For large external containers and silos, a 4-point support is often chosen, as this provides a more stable support situation, e.g., in the event of wind and earthquake loads. When erecting the structure, care must be taken to ensure that the load cells are evenly loaded. 

With analog load cells, due to the parallel connection, care must be taken to ensure that the measuring amplifier can supply the required number of cells.

Guides limit the horizontal movement of a container, but allow vertical movement with as little friction as possible.

The canister weighing module allows horizontal displacement of approx. +/- 10 mm in both horizontal axes (floating mounting). In principle, the weighing module can absorb horizontal loads, but in practice, on-site guides are often required to absorb the sometimes considerable horizontal loads (e.g., caused by wind or in the event of an earthquake). 

A lift-off protection device is used on silos and containers when, for example, wind loads or earthquake loads must be taken into account and there is a risk that the silo or container could tip over in certain situations. In the case of wind, for example, the wind exposure area is large in relation to the total weight of the silo, especially when the fill level is low, which is why the tendency to tip over is greatest in these cases. In practice, lift-off protection is often implemented by means of threaded rods that absorb lifting forces and thus prevent the silo from tipping over. Lift-off protection is often not necessary for indoor containers and silos.

The canister weighing module is designed to absorb lifting forces. However, we recommend that a lifting safety device be provided on site. In practice, lifting safety devices are often implemented using threaded rods. 

It is the responsibility of a qualified structural engineer to check the individual fastening of each silo, taking into account the environmental conditions (earthquakes, wind speed, etc.). 

Lateral forces such as wind loads are largely balanced out by the parallel connection of the load cells. When one side of the silo is relieved by the wind, the load on the opposite side increases. The sum of the forces therefore remains the same. Wind loads are only noticeable in strong gusts when the averaging (damping) of the measuring amplifier is greatly reduced. 

All "modern" measuring amplifiers have a so-called "sense" input. Only four wires are required to determine the weight with standard load cells. Two wires are provided for the power supply (5–15 V) and two wires for the measurement signal. For distances greater than 10 m, we recommend 2 additional wires for the so-called "sense" lines. The measuring amplifier "measures" how much voltage is lost along the line via the sense line and compensates for the voltage loss mathematically. With a sufficient cable cross-section and separate laying of the shielded connection cable, distances of up to 200 m are possible. For longer distances, we recommend using an amplifier on site at the silo or a bus connection via the intelligent terminal box. 

There are also other options for transmitting the weight signal over longer distances. These include Profinet, Ethernet interfaces, radio, etc.