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 Cell
A 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 Types
Load 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 Cells
Miniature 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 Cells
Pneumatic 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 Cells
Strain 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 Cells
Miniature 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 Cells
Both 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 Cells
The 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.
Choose the right load cell for your application
Compression Load Cells
A compression load cell will often have an integral button design. They are ideal for mounting where space is restricted. They offer excellent long term stability.
Compression/Tension Load Cells
Compression/Tension load cells can be used for applications where the load may go from tension to compression and vice versa. They are ideal for space restricted environments. Threaded ends facilitate easy installation.
S-Beam Load Cells
S-Beam load cells get their name from their ‘S’ shape. S-Beam force sensors can provide an output if under tension or compression. Applications include tank level, hoppers, and truck scales. They provide superior side load rejection.
Bending Beam Load Cells
Bending Beam load cells can be used in multiple force applications, tank weighing, and industrial process control. It features low profile construction for integration into restricted areas.
Platform and Single Point force sensors
Platform and Single Point force sensors are used in commercial and industrial weighing systems. They provide accurate readings regardless of the position of the load on the platform.
Canister Load Cells
Canister load cells are used for single and multi-weighing applications. Many feature an all stainless steel design and are hermetically sealed for washdown and wet areas.
Weighing load cell applications
Load 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.
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