First, a force is applied to the mechanical system, which transfers the force to the strain gauge, which is basically an elastic foil connected to a wheatstone bridge configuration. This wheatstone bridge configuration is the electronic device that the strain gauge uses to convert a certain amount of strain into electrical output. Essentially, the mechanical system and strain gauge are used to convert a force into an electrical output.
This electrical output is generally very small, so it is amplified using a form of electric amplification, for which an integrated circuit or transistor may possibly be used. The electrical output is measured and the plugged into a computer algorithm. The algorithm uses the amount of electricity to tell a user how much force was applied to the load cell.
Electric transducer (Load cell) Prepared by: Uday A. Korat (B.E. 3rd year(EC),LDCE)
Electric transducer• A transducer is a device that converts one form of energy to another. Energy types include (but are not limited to) electrical, mechanical, electromagnetic (including light), chemical, acoustic or thermal energy.• Electric transducer: A device which converts any kind of energy into electric signal whether it is analog or digital.
Types of Electric transducer• Sound -> Electrical (microphones)• Force -> Electrical (Load Cells)• Kinetic -> Electrical (piezoelectric, generators)• Light -> Electrical (solar panels)• Thermal -> Electrical (thermocouple)
Load cell• Load cells are integrated sensors that measure weights and output continuous electrical, pneumatic, or hydraulic analog signals.• A load cell is generally comprised of three parts: a mechanical system, a strain gauge, and an electronic amplification device
• The measurement of a force is done by the use of these three parts in the order they are listed. It should be noted that load cells can be configured with multiple "strain gauges".
Strain gauge• When external forces are applied to a stationary object, stress and strain are the result.• Stress is defined as
Strain gauge• Strain is defined as the amount of deformation per unit length of an object when a load is applied. Strain (ε) = ΔL/L• Typical values for strain are less than 0.005 inch/inch and are often expressed in micro-strain units: 1 μstrain = 106 strain
Strain gauge• Strain may be compressive or tensile and is typically measured by strain gages.• It was Lord Kelvin who first reported in 1856 that metallic conductors subjected to mechanical strain exhibit a change in their electrical resistance.• This phenomenon was first put to practical use in the 1930s.
Strain gauge• Fundamentally, all strain gages are designed to convert mechanical motion into an electronic signal.• A change in capacitance, inductance, or resistance is proportional to the strain experienced by the sensor.
Strain gauge• If a wire is held under tension, it gets slightly longer and its cross-sectional area is reduced. This changes its resistance (R) in proportion to the strain sensitivity (S) of the wires resistance. When a strain is introduced, the strain sensitivity, which is also called the gage factor (GF), is given by: GF = (ΔR/R)/(ΔL/L)
Strain gauge• The ideal strain gage would change resistance only due to the deformations of the surface to which the sensor is attached.• However, in real applications, temperature, material properties, the adhesive that bonds the gage to the surface, and the stability of the metal all affect the detected resistance.
Strain gauge• Because most materials do not have the same properties in all directions, a knowledge of the axial strain alone is insufficient for a complete analysis. Poisson, bending, and torsion strains also need to be measured. Each requires a different strain gage arrangement.
Strain gauge• The most widely used characteristic that varies in proportion to strain is electrical resistance. Although capacitance and inductance-based strain gages have been constructed, these devices sensitivity to vibration, their mounting requirements, and circuit complexity have limited their application.
Physical Principle Ohm’s Law R = ρ L/A Combining Ohm’s Law with definition of strain ε: ∆R/R = (1+2v)ε+ ∆ρ/ρ= Gε First term: Under strain, wire changes dimension, and thus the resistance changes. Dominant for metals. Second term: change in resistivity due to the change in the crystal lattice of the material under strain (piezoresistive effect). Dominant in semiconductors (but expensive). Foils/filaments inside the strain gauge are ~1/1000th inch diameter, made up of basic metal conductors.
Load Cell Implementation Change in resistivity under strain is linear when ∆R/R is less than 1% → Small ∆V (mV level) Wheatstone Bridge Circuit is used with a strain gauge as one or more of its resistors: Instrumentation Amplifier Applied force causes small change in resistance in strain gauge → change in output voltage across bridge circuit. Output voltage from bridge circuit is amplified using an instrumentation amplifier, usually to 0-5 or 0-10 V range. Circuitry housed in a mechanical device (the load cell casing itself). Algorithms determine actual force based on output voltage (usually outside load cell).
Application of Strain gauge• Strain gages are used to measure displacement, force, load, pressure, torque or weight. Modern strain-gage transducers usually employ a grid of four strain elements electrically connected to form a Wheatstone bridge measuring circuit.• The strain-gage sensor is one of the most widely used means of load, weight, and force detection.• As the force is applied, the support column experiences elastic deformation and changes the electrical resistance of each strain gage. By the use of a Wheatstone bridge, the value of the load can be measured. Load cells are popular weighing elements for tanks and silos and have proven accurate in many other weighing applications.
Application of Strain gauge• Strain gages may be bonded to cantilever springs to measure the force of bending.• The strain gages mounted on the top of the beam experience tension, while the strain gages on the bottom experience compression. The transducers are wired in a Wheatstone circuit and are used to determine the amount of force applied to the beam.
Application of Strain gauge• Strain-gage elements also are used widely in the design of industrial pressure transmitters. Using a bellows type pressure sensor in which the reference pressure is sealed inside the bellows on the right, while the other bellows is exposed to the process pressure.• When there is a difference between the two pressures, the strain detector elements bonded to the cantilever beam measure the resulting compressive or tensile forces.
Application of Strain gauge• A diaphragm-type pressure transducer is created when four strain gages are attached to a diaphragm.• When the process pressure is applied to the diaphragm, the two central gage elements are subjected to tension, while the two gages at the edges are subjected to compression.• The corresponding changes in resistance are a measure of the process pressure. When all of the strain gages are subjected to the same temperature, such as in this design, errors due to operating temperature variations are reduced.