A load cell is a transducer which converts force into a measurable electrical output. Although there are many varieties of force sensors, strain gauge load cells are the most common load cell – and, except for certain laboratory applications where precision mechanical balances are still used, strain gauge load cells dominate the weighing industry. Pneumatic load cells are sometimes used where intrinsic safety and hygiene are desired, and hydraulic load cells are considered in remote locations, as they do not require a power supply. Strain gauge load cells offer accuracies from within 0.03% to 0.25% full scale and are suitable for almost all industrial applications.
How Do Load Cells Work?A load cell works by converting mechanical force into digital values that the user can read and record. The inner working of a load cell differs based on the load cell that you choose – there are, for example, hydraulic load cells, pneumatic load cells, and strain gauge load cells. With their wide range of features and capabilities, strain gauge load sensors are the most commonly used among the three types, specifically for weighing applications – including industrial scales, medical scales, and even retail scales. Strain gauge load cells contain strain gauges within them that send up voltage irregularities when under load. The degree of voltage change is covered to digital reading as weight.
When to Use a Load CellA load cell measures mechanical force, mainly the weight of objects. Today, almost all electronic weighing scales use load cells because of the accuracy with which they can measure weight. Load cells find their application in a variety of fields that demand accuracy and precision. There are different classes to load cells (Class A, Class B, Class C, and Class D), and with each class, there is a change in both accuracy and capacity.
Load Cell TypesLoad cells represented the first major design change in weighing technology. In today’s processing plants, electronic force sensors are preferred in most applications. Types of load cells can be distinguished according to the type of output signal generated (pneumatic, hydraulic, electric) or according to the way they detect weight (bending, shear, compression, tension, etc.).
Hydraulic Load CellsMiniature load cell Hydraulic load cell sensors are force-balance devices, measuring weight as a change in pressure of the internal filling fluid. In rolling diaphragm type hydraulic force sensors, a load or force acting on a loading head is transferred to a piston that in turn compresses a filling fluid confined within an elastomeric diaphragm chamber.
As force increases, the pressure of the hydraulic fluid rises. This pressure can be locally indicated or transmitted for remote indication or control. Output is linear and relatively unaffected by the amount of the filling fluid or by its temperature.
If the load cells have been properly installed and calibrated, accuracy can be within 0.25% full scale or better, acceptable for most process weighing applications. Because this sensor has no electric components, it is ideal for use in hazardous areas.
Typical hydraulic load call applications include tank, bin, and hopper weighing. For maximum accuracy, the weight of the tank should be obtained by locating one force sensor at each point of support and summing their outputs.
Pneumatic Load CellsPneumatic load cells also operate on the force-balancing principle. These devices use multiple dampener chambers to provide higher accuracy than hydraulic load cells. In some designs, the first dampener chamber is used as a tare weight chamber.
Pneumatic load cells are often used to measure relatively small weights in industries where cleanliness and safety are of prime concern. Miniature load cell
The advantages of this type of load cell include their being inherently explosion proof and insensitive to temperature variations. Additionally, they contain no fluids that might contaminate the process if the diaphragm ruptures. Disadvantages include relatively slow speed of response and the need for clean, dry, regulated air or nitrogen.
Strain Gauge Load CellsStrain gauge load cells are a type of load cell where a strain gauge assembly is positioned inside the load cell housing to convert the load acting on them into electrical signals. In order to measure strain, these types of load cells must be connected to an electric circuit that is capable of measuring the minute changes in resistance corresponding to strain. Strain gauge load cells usually employ four strain gauge elements electrically connected to form a Wheatstone bridge circuit (Figure 1).
The weight on the load cell is measured by the voltage fluctuation caused in the strain gauge when it undergoes deformation.
The gauges themselves are bonded onto a beam or structural member that deforms when weight is applied. Modern load cells have 4 strain gauges installed within them to increase the measurement accuracy. Two of the gauges are usually in tension and two in compression – and they are wired with compensation adjustments.
When there is no load on the load cell, the resistance of each strain gauge will be the same. However, when under load, the resistance of the strain gauge varies, causing a change in output voltage. The change in output voltage is measured and converted into readable values using a digital meter.
Piezoresistive Load CellsMiniature load cell Similar in operation to strain gauges, piezoresistive force sensors generate a high level output signal, making them ideal for simple weighing systems because they can be connected directly to a readout meter. The availability of low-cost linear amplifiers has diminished this advantage, however. An added drawback of piezoresistive devices is their non-linear output.
Inductive and Reluctance Load CellsBoth of these devices respond to the weight-proportional displacement of a ferromagnetic core. One changes the inductance of a solenoid coil due to the movement of its iron core; the other changes the reluctance of a very small air gap.
Magnetostrictive Load CellsThe operation of this force sensor is based on the change in permeability of ferromagnetic materials under applied stress. It is built from a stack of laminations forming a load-bearing column around a set of primary and secondary transformer windings. When a force is applied, the stresses cause distortions in the flux pattern, generating an output signal proportional to the applied load.
This is a rugged sensor and continues to be used for force and weight measurement in rolling mills and strip mills.
Weighing load cell applicationsLoad cells represented the first major design change in weighing technology. In today's processing plants, electronic force sensors are preferred in most applications, although mechanical lever scales are still used if the operation is manual and the operating and maintenance personnel prefer their simplicity.
In this page you find a weighing system design with load cells.