Electric transducer
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Transcript of Electric transducer
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Electric transducer(Load cell)
Prepared by: Uday A. Korat (B.E. 3rd year(EC),LDCE)
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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.
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Types of Electric transducer
• Sound -> Electrical (microphones)
• Force -> Electrical (Load Cells)
• Kinetic -> Electrical (piezoelectric, generators)
• Light -> Electrical (solar panels) • Thermal -> Electrical (thermocouple)
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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
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• 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".
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Strain gauge
• When external forces are applied to a stationary object, stress and strain are the result.
• Stress is defined as
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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
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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.
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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.
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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 wire's 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)
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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.
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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.
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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.
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Strain gauge
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� 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.
Physical Principle
� Second term: change in resistivity due to the change in the crystal latticeof the material under strain (piezoresistive effect). Dominant insemiconductors (but expensive).
� Foils/filaments inside the strain gauge are ~1/1000th inchdiameter, made up of basic metal conductors.
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Strain gauge
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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 outputvoltage 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).
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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.
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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.
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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.
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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.