Definition
Load Cell is a specific type of transducer or sensor capable of transforming force (load) into a measurable electrical output. Load cells measure tension, compression, or shear. A typical load cell device comprises of four strain gauges in a Wheatstone bridge configuration.
Working
Force conversion into an electrical signal by a load cell is usually carried out indirectly and in two stages. The first stage conversion is done through a mechanical arrangement in which the force being sensed deforms the strain gauge. This strain gauge then converts the deformation (strain) to electrical signals. One can also get load cells with one or two strain gauges. The electrical signal generated by strain gauge is in the order of a few millivolts which needs further amplification by an instrumentation amplifier. The output of the transducer is fed to an algorithm calculator to calculate the force applied to the transducer.
Classification of Load Cells
Load cell designs are classified on various bases. They are classified according to the
Type of output signal generated
- Pneumatic
- Hydraulic
- Electric
Way they detect weight
- Compression: Compression load cells measure a pushing together force along a single axis
- Tension: Tension load cells measure a pulling apart force along a single axis
- Shear: Shear load cells measure the displacement of a structural element to find out force. Shear cell types for load sensors include shear beam, bending beam, or single point bending beam.
Hydraulic Load Cells
Hydraulic load cells are also referred to as hydro-static load cells. These are mainly used in industrial applications. These are force -balance devices, measuring weight as a change in pressure of the internal filling fluid. They give linear output reasonably unaffected by the amount of the filling fluid or by its temperature. Accuracy can be achieved within 0.25% full scale or better, provided the load cells have been correctly installed and calibrated. This range of accuracy is suitable for most process weighing applications. Since this sensor has no electric components, it is perfect for use in hazardous areas. Typical hydraulic load cell applications include tank, bin, and hopper weighing. These load cells can eliminate troubles encountered with strain gauge load cells. Hydraulic load cell is probably the best load cell to use in an outdoor environment because it is immune to transient voltage (lighting).
Pneumatic Load Cells
Pneumatic load cells also work on the force-balance operating principle. These devices make use of multiple dampener chambers to offer higher accuracy as compared to a hydraulic device. In several designs, the first dampener chamber is utilized as a tare weight chamber. Pneumatic load cells are frequently employed to measure moderately small weights in industries where cleanliness and safety are principal concerns.
Advantages
- Inherently explosion proof
- Insensitive to temperature variations
- They contain no fluids hence there is no chance of contamination of the process if the diaphragm ruptures.
Disadvantages
- Comparatively slow speed of response
- Need for clean, dry, regulated air or nitrogen
Strain-gauge Load Cells
Strain-gauge load cells are bonded onto a beam or structural member which gets deformed when weight is applied. In majority of the cases, four strain gauges are employed to achieve maximum sensitivity and temperature compensation. Two of the gauges remain generally in tension, and two in compression, and are wired with compensation adjustments. When weight is applied, the strain changes the electrical resistance of the gauges in proportion to the load.
Advantages: As compared to other load cells strain gauge load cells offers more accuracy and lower unit costs. Although there are several varieties of load cells available, strain gauge based load cells are the most widely used type.
Piezo-electric Load Cells
In these types of load cells a piezoelectric material is compressed which then generates a charge that is conditioned by a charge amplifier. It is valuable for dynamically measuring force. Geo mechanical applications entail vibrating wire load cells owing to their low amounts of drift. All load cell rings when they are subjected to an abrupt load change and this is caused by the spring like behavior of the load cell.
Selection of a Load Cell
Following points must be kept in mind while selecting a proper load cell:
- It is crucial to decide whether one wants a tension or a compression load cell. A tension load cell measures the amount of weight pulling it, whereas a compression load cell measures the weight by pushing against it. A compression load cell is generally placed beneath the object that needs to be weighed. Keeping all this in mind, one can easily choose an appropriate load cell.
- The number of cells should be decided based upon the number of supports used. In ideal situations, each support should have a load cell and this is more essential if the weight is not equally distributed between the supports.
Sources
interweigh