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Transducer

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Instruments and Process Control -Transducer

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Transducer

  1. 1. Process Control and Instrumentation Dr. Debasis Sarkar Department of Chemical Engineering Indian Institute of Technology Kharagpur
  2. 2. Transducers
  3. 3. Transducers • Transducers are devices that transform signals in one form to a more convenient form • Not just conversion of energy! Diaphragm produces displacement on application of pressure. Note that displacement and pressure are both manifestation of energy – but displacement is more convenient from the measurement point of view • Transducers can be of various types: Mechanical, Electrical, Optical, Acoustic, etc. • Electrical transducers are always preferred:  signal can be conditioned easily (modified/amplified/modulated etc.)  easy remote operation
  4. 4. Transducers • Here we are concerned with Electrical Transducers that produces an electrical output due to an input of mechanical displacement or strain • Mechanical strain or displacement may be produced by a primary sensor due to various input physical variables such as temperature, pressure, flow etc. Primary Sensor Electromechanical Transducer Mechanical displacement/strain Electrical outputInput Temp, Pr, etc Force Displacement Pressure Diaphragm
  5. 5. Transducers We will briefly discuss:  Pneumatic Transducer: – Flapper/Nozzle system  Electromechanical Transducers: – Linear Variable Differential Transducer (LVDT) • Inductance type transducer: magnetic characteristics of an electrical circuit changes due to motion of an object – Resistance Strain Gauge • If a conductor is stretched/strained, its resistance will change – Capacitive Type Transducer • There is a change in capacitance between two plates due to motion – Piezo-electric Transducer • An electrical charge is produced when a crystalline material (quartz/barium titanate) is distorted
  6. 6. Flapper/Nozzle System • Flapper/Nozzle system is a pneumatic transducer • Pneumatic control system operates with air. The signal is transmitted in the form of variable air pressure in the range of 3 – 15 psi. • Early days, we had only pneumatic control systems • With advent of modern electronics, many pneumatic control systems have now been replace by electronic control systems • However, even these days many industrial actuators are still pneumatic in nature • Advantage: Safe/Low cost/can generate more torque to its own weight compared to electrical actuators • Disadvantage: Slow response
  7. 7. Flapper/Nozzle System Due to the presence of flapper, there will be a back pressure that will alter the output pressure or signal pressure (P0) Altering the gap between nozzle and flapper (x) alters the resistance to airflow and hence the output pressure Increase in x will lower the resistance and fall in output pressure (P0) Po can be calibrated in terms of gap (x) , that is, (displacement) Ps P0, T0 Flapper/Nozzle system is the basis of all pneumatic transmitters Consists of a fixed flow restriction (orifice) and a variable restrictor (nozzle and flapper) Air at a fixed pressure (Ps) flows through a nozzle past a restriction in the tube
  8. 8. Flapper/Nozzle System
  9. 9. Flapper/Nozzle System Approximate static sensitivity calculation: • Assume flow through the restrictions incompressible • Let, orifice diameter: do, nozzle diameter: dn • Fluid density: ρ; assume equal discharge coefficient (Cd) 4 1 2 ( ) 4 o d s o d q C P P π ρ   −    2 0( ) 2 ( )d n i ambq C d x P Pπ ρ= − 1 2 0 2 2 4 1 16 1 n is o q q P d xP d = ⇒ = + The sensitivity dP0/dxi thus varies with xi. It has maximum at: 2 0.14 o i n d x d = When xi is sufficiently large, P0/Ps becomes almost constant. P0/Ps is linear between 0.15 and 0.75. For Ps = 20 psi, this corresponds to 3-15 psi and this is the limits of industrial control pressure. Flow through orifice: Flow through nozzle: Assuming flow continuity and Pamb = 0 gage:
  10. 10. Flapper/Nozzle System Flapper/Nozzle system Plot of Signal pressure Vs gap Ps P0 P0, T0 Approx linear range in 3 – 15 psi Gap, x Outputpressure
  11. 11. Flapper/Nozzle System
  12. 12. Flapper/Nozzle System: Electro- Pneumatic Signal Converter
  13. 13. Flapper/Nozzle System: Electro- Pneumatic Signal Converter
  14. 14. Flapper/Nozzle System: Electro- Pneumatic Signal Converter
  15. 15.  Electromechanical device that produces electrical output proportional to displacement of a movable core: Displacement Transducer  Most commonly used variable inductance transducer in industry A soft iron core provides magnetic coupling between a single primary coil (A) and two identical secondary coils (B), connected in series opposition When core slides through transformer, a certain portion of the coils are affected. This induces a unique voltage Linear Variable Differential Transformer (LVDT) Primary coil Second ary coils Ferromagne tic core Ferromagnetic core is physically connected to the object whose displacement is to be measured
  16. 16. Linear Variable Differential Transformer (LVDT)
  17. 17. Transducers: LVDT Core: Nickel Iron Alloy, Ferrite Primary coil is excited by a sinusoidal voltage of amplitude 1 V to 15 V and frequency 50 Hz to 20 kHz The sensitivity of typical LVDTs is in the rage of 1 to 5 v/v/cm Displacement: ±0.002 cm to several cm The whole sensor is enclosed and shielded so that no field extends outside it and hence cannot be influenced by outside fields LVDT Circuit diagram Core centrally located
  18. 18. Transducers: LVDT Secondary coils are NOT connected in series opposition:
  19. 19. Transducers: LVDT When core is central, the induced voltage in the secondary coils are equal in magnitude. But the output voltage is zero as they are connected in series opposition. As the core moves up/down, the induced voltage of one secondary coil increases, while that of other decreases. The output voltage is proportional to the displacement of the core Output voltage on either side of null position is 180° out of phase
  20. 20. Rotary Motion Type LVDT S1 P S2
  21. 21. Rotary Motion Type LVDT
  22. 22. Inductive Pressure Sensor: Use of LVDT
  23. 23. Advantages of LVDT • LVDT is a very sensitive transducer • Over a range of motion, the output is linear • Essentially frictionless measurement • Very long mechanical life • Very high resolution • Null repeatability
  24. 24. Strain Gauge • When we apply force to a solid at rest, it will be mechanically deformed to a certain extent. If the force is tensile, the length of the solid will increase. If the force is compressive, the length of the solid will decrease. • The longitudinal or axial strain is defined as: ε = ΔL/L • Longitudinal stress: σ = F/A (force F applied on area A) • Stress-strain relationship within elastic limit: Hooke’s Law: E = σ/ε E = Young’s modulus [if σ is in kg/m2, so will be E] • When a body of length is elongated, its transverse (perpendicular) dimension will contract. Lateral strain: εt = ΔD/D • Poisson’s ratio: ν = Lateral strain/Longitudinal strain = εt/εa Poisson’s ratio lie between 0 and 0.5. And mostly, it is 0.3
  25. 25. Strain Gauge • Strain measurement is essentially measurement of very small, about 1 micrometer, displacement • Methods: – Mechanical: Use levers and gears to measure ΔL after magnification [early days: extensometer uses many levers to magnify strain so that it becomes readable] – Electrical: Change in resistance or inductance or capacitance – Optical: Use interference, diffraction, and scattering of light waves • Most commonly used method: Electrical: change in resistance: Resistive Strain Gauges
  26. 26. Strain Gauge Theory For a wire of cross-sectional area A, resistivity ρ, and length L the resistance is given by: L R A ρ = fractional changein resistance fractional changein length R R L L ∆ = ∆ = / / / a R R R R G L L ε ∆ ∆ = = ∆ To provide a means of comparing performance of various gauges, the gauge factor, or strain sensitivity, of a gauge is defined as: Higher gauge factors are generally more desirable -- the higher the gauge factor the higher the resolution of the strain gauge This equation holds good for many common metals and nonmetals at room temperature when subjected to direct or low frequency current When the wire is stretched, the cross-sectional area A is reduced, which causes the total wire resistance to increase. In addition, since the lattice structure is altered by the strain, the resistivity of the material may also change, and this, in general, causes the resistance to increase further.
  27. 27. Strain Gauge Theory ( , , ) L R R R L A A R R R R L A L A ρ ρ ρ ρ = ⇒ =  ∂ ∂ ∂    ∆ = ∆ + ∆ + ∆      ∂ ∂ ∂      2 L L R L A A A A ρ ρ ρ       ⇒ ∆= ∆ − ∆ + ∆            Dividing throughout by R R L A R L A ρ ρ ∆ ∆ ∆ ∆ = − + 1st term: Length change 2nd term: Area change 3rd term: Resistivity change 2 2 If , then 2 2 2 2 t A CD A CD D A CD D D A CD D ε = ∆= ∆ ∆ ∆ ∆ = = = 2a t R R ρ ε ε ρ ∆ ∆ = − +
  28. 28. Strain Gauge Theory 2a t R R ρ ε ε ρ ∆ ∆ = − + / Therefore,Gauge Factor, / 2 / 1 2 1 2 a t a a R R F L L E ρ ε ε ρ ε ρ ρ ν ε ν ψ ∆ = ∆ ∆ − + = ∆ =+ + =+ + Poisson's ratiot a ε ν ε =− = / / /a E L L ρ ρ ρ ρ ψ ε ∆ ∆ = = ∆ Constant for a material, directly proportional to Modulus of Elasticity, E. Ψ = Bridgeman coefficient Material Composition Gauge Factor Advance Cu 55%, Ni 45% 2 – 2.2 Nichrome Ni 80%, Cr 20% 2.2 – 2.5 Pure Platinum Pt 100 ~4.8 Semiconductor type 100 - 200
  29. 29. Strain Gauge A strain gauge is a passive type transducer whose electrical resistance changes when it is stretched or compressed The wire filament is attached to a structure under strain and the resistance in the strained wire is measured by Wheatstone Bridge principle  Un-bonded Type  Bonded Type  Semiconductor Type
  30. 30. Strain Gauge Operation Un-bonded Strain Gauge: Movable base Fixed base Wire: 25 mm length, 25 micrometer diameter Electrically insulated pins Stretched un-bonded wire Connec ted to object (Input motion or force)  A set of preloaded resistance wire is stretched between two frames: one movable and the other fixed  A small motion of the movable base increases tension in two wires while decreasing it in two others.  Change in resistance cause Wheatstone bridge unbalance  The output voltage is proportional to input displacementA very small motion (say 50 µm) and very small forces can be measured
  31. 31. Strain Gauge Operation Bonded Strain Gauge: Wire-type Foil type Bonded strain gauges are directly bonded to the surface of the specimen being tested, with a thin layer of adhesive cement. They use paper or bakelite as baking material. Useful for measurement of strain, force, torque, pressure, vibrations, etc. They are very sensitive and can measure strains as low as 10-7. Bonded strain gauges are also made of semiconductor material. Usually, silicon doped with boron (p-type) or silicon doped with arsenic (n-type) are used. High gauge factor, small gauge length are advantages. High temperature sensitivity and nonlinearity are disadvantages.
  32. 32. Strain Gauge Operation Bonded Strain Gauge:
  33. 33. Strain Gauge Strain gauge circuit with temperature compensations Temperature compensation: Strain gauge circuit without temperature compensations
  34. 34. Application of Strain gauge • Strain gages are used to measure displacement, force, load, pressure, torque or weight • Strain gages may be bonded to cantilever springs to measure the force of bending • Strain-gage elements also are used in the design of pressure transmitters using a bellows type or diaphragm type pressure sensor • Semiconductor type strain gauge – high gauge factor
  35. 35. Capacitive Transducers d Area=A 0 A C d ε ε= C: capacitance, pF ε0: dielectric constant (relative permittivity) of free space (vacuum) = 8.85 pF/m ε: dielectric constant of insulating material A: area of plates, m2 d: distance between plates, m Two parallel metal plates separated by a dielectric or insulating material: d A A Plate displacedd changes Dielectric material moves (a) (b) (c) There are 3 ways to change the capacity: (1) variation of distance between the plates (d) [Fig. a] (2) variation of the shared area of the plates (A ) [Fig. b] (3) variation of dielectric constant (ε) [Fig. c]
  36. 36. – Keep one plate is fixed while the other is physically attached to the moving object and thus moves with the moving object – The position of the moving object causes a change in the distance between the plates (d) and hence changes the capacitance, C – Capacitance is inversely proportional to the motion Capacitive Transducers Moving object Fixed plate 0 A C d ε ε= We can use pressure to vary the distance between two plates and measure the change in capacitance by a suitable electric bridge circuit. The capacitance is proportional to the pressure. (Capacitive Pressure Transducer)
  37. 37. Capacitive Transducers
  38. 38. Capacitive Transducers
  39. 39. • One plate is attached to the moving object and the other is kept stationary • Capacitance is: • sensitivity is • This relationship is nonlinear - but can be made linear by using an op-amp circuit 2 C K S d d ∂ = = − ∂ Capacitive Transducers 0 A K C d d ε ε = = Moving Plate Position d vo Fixed Plate Capacitance Bridge
  40. 40. Displacement measurement by changing dielectric: • Displacement can be measured by attaching the moving object to a solid dielectric element placed in between the plates Liquid Level Measurement: • Liquid level as shown below can be measured as the dielectric medium between the plates changes with the liquid level vo Fixed Plates Liquid Level h Liquid Capacitance Bridge Tank Capacitive Transducers
  41. 41. DC Output vo Capacitance Bridge Rotating Plate A Fixed Plate Rotation θ • One plate rotates and the other is stationary • Common area is proportional to the angle, θ 0 A C K d ε ε θ= = • The relationship is linear and K is the sensor constant • Sensitivity is K C S = ∂ ∂ = θ Capacitive Transducers Rotational Sensor
  42. 42. + + + + + + + + + - - - - - - - - - Force/pressure Accumulation of charge at the surface If the dimensions of some polarized crystalline materials are changed as a result of mechanical force (longitudinal/transverse/shear), electric charges proportional to the imposed force are accumulated on the surface upon which the force is imposed. This property can be exploited to measure many physical variables such as force, pressure, strain, torque, acceleration, sound, vibration, etc. The materials characterizing this property are known as piezoelectric materials. Piezoelectric materials deform when a voltage is applied Piezoelectric Sensors Longitudinal effect Transverse effect Shear effect
  43. 43. Piezoelectric Transducers • Materials – Natural occuring highly polar crystal • Quartz, Rochelle salt, ammonium dihydrogen phosphate – Synthesized • Barium titanate, Ceramic • Lead zirconate titanate • When a crystalline material like quartz is distorted an electric charge is produced • Application of a force P causes deformation xi producing a charge Q, where Q = Kxi where K = charge sensitivity constant • Crystal behaves like a capacitor, carrying a charge across it. Voltage across crystal E0 is: 0 ( / )i i KxQ E kx k K C C C = = = = Force xi E0t
  44. 44. Piezoelectric Transducers
  45. 45. Piezoelectric Sensors Advantages:  Low cost, small size  High sensitivity and High mechanical stiffness  Broad frequency range  Good linearity and repeatability  High linearity, negligible hysteresis Disadvantages:  High Impedance  Low Power  Drift with temperature and pressure
  46. 46. Differential Pressure Transmitter • A DP cell is a differential pressure cell. It is used to measure the differential pressure between two input points. It consists of a sensor, a transducer and a transmitter combined in a single device.
  47. 47. Differential Pressure Transmitter • A DP cell is a differential pressure cell. It is used to measure the differential pressure between two input points. It consists of a sensor, a transducer and a transmitter combined in a single device.
  48. 48. Working Principle of a D/P Cell In DP cell a diaphragm is present which remains in normal condition when the forces on both sides of diaphragm are equal. The unequal forces (pressure difference) create deformation in the diaphragm. By the extent of deformation, the differential pressure is calculated. There are two main types of DP Cells:  Pneumatic DP Cell  Electrical/Electronic DP Cell
  49. 49. Working Principle of D/P Cell
  50. 50. Pneumatic Transmitter: Basic Idea Only a limited volume of air can pass through the restriction, thus we need a way to boost the volume in order to drive a signal any distance.
  51. 51. Pneumatic Transmitter: Relay Volume booster relay
  52. 52. Pneumatic D/P Cell • The diaphragm capsule is held between two flanged castings which form chambers on either side. • These are designated as the high and low pressure sides of the DP cell. • Air is supplied to keep the force bar in horizontal position and in this way differential pressure is calculated.
  53. 53. Pneumatic D/P Cell • The diaphragm capsule is held between two flanged castings which form chambers on either side. • These are designated as the high and low pressure sides of the DP cell. • Air is supplied to keep the force bar in horizontal position and in this way differential pressure is calculated.
  54. 54. Electrical D/P Cell
  55. 55. Electrical D/P Cell
  56. 56. Use of a D/P Cell Transmitter The differential pressure cell is one of the most common methods of measuring level. Open Tank Measurement • Low side of the d/P cell is left open to atmosphere. • High side measures the hydrostatic head pressure which is proportional to the height of the liquid and its density. Low side open to atmosphere 24 VDC mA 4 – 20 mA To PLC or Controller
  57. 57. Use of a D/P Cell Transmitter The differential pressure cell is one of the most common methods of measuring level. Open Tank Measurement • Low side of the d/P cell is left open to atmosphere. • High side measures the hydrostatic head pressure which is proportional to the height of the liquid and its density.
  58. 58. Use of a D/P Cell Transmitter In a closed tank, the Low side of the d/P cell is connected to the top of the tank and will cancel the effects of the vapour pressure above the surface. Closed Tank Measurement • Low side of the d/P cell measures the vapour pressure above the surface. • High side measures the hydrostatic head pressure which is proportional to the height of the liquid and its density + vapour pressure 24 VDC mA 4 – 20 mA To PLC or Controller H L
  59. 59. Use of a D/P Cell Transmitter Flow-rate measurement Interface measurement
  60. 60. Electrical Pressure Transducer

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