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Lec 07(sensors 2)

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Mechatronics 2 course Ain Shams University

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Lec 07(sensors 2)

  1. 1. Faculty of Engineering Ain Shams University Mohammed Ibrahim 3/31/2017 1 MDP: Mechatronics (2) Lecture 07: Sensors II
  2. 2. Potentiometers
  3. 3. Potentiometers •Construction Electromechanical device containing – Conductive film (resistive element) – Wiper – Slider (a) Potentiometer (b) Schematic diagram of the potentiometer
  4. 4. •Construction Potentiometers
  5. 5. Potentiometers
  6. 6. Potentiometers Vo Vs V=0 to VexRp Rx xmax x Vo X Ideal Actual According to the slider position [against a fixed resistive element] the resistive element is “divided” at the point of wiper contact. •Can be Linear or Rotary potentiometer
  7. 7. Linear Potentiometer • Terminal A connected to the supply voltage • Terminal B connected to the Ground • X=L*(Vout / VSupply) X: Travelled Distance L: Total Length
  8. 8. Angular Potentiometer Single turn Multi-Turns • The Same Like Linear Potentiometer • But Vout is a Function of the Angular Position • ⊖= Ф *(Vout / VSupply) ⊖: Rotated angle, Ф: Total angle
  9. 9. Differential Transformers
  10. 10. Linear Variable Differential Transformer (LVDT)  LDVT is a robust and precise device which produce a voltage output proportional to the displacement of a ferrous armature for measurement of robot joints or end-effectors. It is much expensive but outperforms the potentiometer transducer. Linear Variable Differential Transformer (LVDT)
  11. 11. LVDT An inductor is basically a coil of wire over a “core” (usually ferrous) It responds to electric or magnetic fields A transformer is made of at least two coils wound over the core: one is primary and another is secondary Primary Secondary Inductors and tranformers work only for ac signals A B A B BAout VVV 
  12. 12. LVDT
  13. 13. Vo=V1-V2 Vi V1 V2 V1 > V2 Vo Vi How does a LVDT work? Core moves toward Sec1 Sec1 Voltage goes up In-Phase with Input voltage
  14. 14. Vi Vo Vo=V1-V2 Vi V1 V2 V1 = V2 Core in central position No difference output voltage How does a LVDT work?
  15. 15. Vo=V1-V2 Vi V1 V2 Vi Vo V2 > V1 How does a LVDT work? Core moves toward Sec 2 Sec 2 Voltage goes up out-Phase with Input voltage
  16. 16. Signal conditioning Scheme A signal conditioning circuit that removes these difficulties is shown in Figure , where the absolute values of the two output voltages are subtracted. Using this technique, both positive and negative variations about the center position can be measured.
  17. 17. LVDT for Force Measurement  Force transducers are often based on displacement principles. There various type force and torque transducer available commercially A force-measuring device based on a compression spring and LDVT.
  18. 18. Rotary Variable Differential Transformer (RVDT)
  19. 19. Strain Gauge
  20. 20. Common usages of strain gauge • Used standalone for  testing  diagnostic  monitoring But the most common usage is as primary transducer in the creation of another transducer Elastic structure Strain Gauges Force Pressure Displacement Acceleration Strain
  21. 21. Strain gauges: resistive principle FF Sensitive element Assumptions: • Strain gauge perfectly glued to the measured surface • Strain gauge electrically insulated • Planar deformation state A L R   • R  resistance of the sensor [] •   resistivity of the material [m] • L  conducer length [m] • A  conducer area [m2]
  22. 22. Common values: • Nominal resistance: R  120 , 350  nominal resistance production tolerance: ± 1% • Base length: 0,6-200 mm • Materials: Constantan (Cu-Ni alloy), Karma, Ni-Cr alloy, semi-condutors... base Strain gauges: resistive principle
  23. 23. Measuringbase longitudinal axis transversal axis Pigtail connections terminals support grid reference markings Strain gauges: resistive principle Measuring direction
  24. 24. A L R   The gauge resistance varies due to two different effects: • Dimentional alteration (L, A) due to strain; • Resistivity variation () due to volume alteration (piezoresistive effect). 2 A LdA A Ld A dL dR   A dAd L dL R dR    Strain gauges: resistive principle ELASTIC RANGE
  25. 25. ELONGATION PIEZORESISTIVITY )/( )/( 21 )/( )/( LdLLdL RdR GF     Common value: k=2 (for metallic alloys). GAGE FACTOR Strain gauges: resistive principle 2 22 2 V dVL V dL V LdL dR   L dL R dR 2 00  ddV V L2   A L R   2 )/( )/(  LdL RdR GF PLASTIC RANGE
  26. 26. Strain gauges: resistive principle  Calibration information  Manufacturer provides “gage factor” or GF  2.x typical dL/L dR/R m=GF
  27. 27. Painting layers removal Strain gauges application:
  28. 28. Application spot cleansing Strain gauges application:
  29. 29. Strain gauge positioning Strain gauges application:
  30. 30. Adhesive glue application Strain gauges application:
  31. 31. Strain gauge application: BE CAREFUL! AVOID BENDING! Strain gauges application:
  32. 32. Pressure application (the thinnest glue layer possible) Strain gauges application:
  33. 33. Terminal welding Strain gauges application:
  34. 34. Cables strain release fixing Strain gauges application:
  35. 35. Protective layers application Strain gauges application:
  36. 36.  fast acting glues: (short duration measurement application) • cyanacrilate: • short time polymerization • ambient temperature  slow acting glues: (long duration measurement application) • epossidic glue: • a catalyst is needed • high temperature accelerates polymerization • fenolic glue: • high temperature • high pressure Adhesive used:
  37. 37. Strain-gage temperature compensation
  38. 38. measure 1 2 3 4 5 I5 dummy E Temperature effect: DUMMY GAGE
  39. 39. Wheatstone Bridge                     43 4 21 2 21 43 4 2 21 2 1 RR R e RR R e VVe RR R eV RR R eV ii o ii ei
  40. 40. Steel beam with E  210000 Mpa Stress applied a=100 Mpa uniaxial R=120  Gage factor (GF): k=2 Resistance variation: R=0.114  MEASURING R/R requires a workaround a a   E m m4 762. /x10m / m = 476 -4 R R GF  9 5. x10 -4 1 Measuring resistance variation
  41. 41. VOLTAGEREADING 11 22 33 44 II55 EE AA BB CC DD E RR R VAB 21 1   E RR R VAD 43 3   Wheatstone bridge: principle    E RRRR RRRR V 4321 3241    Introducing resistance variations and assuming small variation form the same nominal resistance we have: 0 4321 4R RRRR E V   ii RRR  0 ii RR 
  42. 42.  Opposing branches signals add themselves up R1+R1 R4+R4 V R2 R3 E 21 Wheatstone bridge: principle 0 41 4R RR E V   04 2 R R E V   If the signal is the same we have:
  43. 43. R1+R1 R4 V R2 R3+R3 E 22  Adjacent branches signals are subtracted Wheatstone bridge: principle 0 31 4R RR E V   0 E V If the signal is the same we have:
  44. 44. R1, R2, R3, R4 having the same nominal resistance As a first step a balancing resistance is introduced, whose resistance can be altered until the reading of the UNSTRAINED configuration is null THIS allows for offset compensation and makes the actual brigde closer to satisfy the assumptions made in the model Rbal I5 1 2 3 4 E Wheatstone bridge: principle
  45. 45. QUARTER BRIDGE 1 2 3 4 E V Wheatstone bridge: configuration
  46. 46. HALF BRIDGE 1 2 3 4 E V Wheatstone bridge: configuration
  47. 47. HALF BRIDGE Wheatstone bridge: configuration
  48. 48. FULL BRIDGE 1 2 3 4 E V Wheatstone bridge: configuration
  49. 49. Connection cables resistance is not compensated by the dummy (RL) 1 2 3 4 dummy RL RL E Wheatstone bridge: 2 wire connection
  50. 50. 1 2 3 4 3 wires connection and shielding dummy E Wheatstone bridge: 3 wires connection
  51. 51. 1 2 3 4 V+ V- S+ S- To be used with a short connection cable Wheatstone bridge: 4 wires connection
  52. 52. 1 2 3 4 V+ V- S+ S- SENS+ SENS- V I  0 I  0 Wheatstone bridge: 6 wires connection Suitable for long connection cables AKA: REMOTE SENSING
  53. 53. Bridge calibration: offset nulling 1 2 3 4 E V Rbalance As a first step any discrepancy between the actual resistance and the nominal one is balanced introducing a variable resistance between two adjacent elements and reading the output. The resistance is changed until a null reading of V is reached. OFFSET NULLING AND GAIN CALIBRATION CAN BE PERFORMED ONLY WHEN ALL ELEMENTS ARE UNLOADED
  54. 54. Bridge calibration: gain calibration 1 2 3 4 E V Rcalibration As a final step a calibrating resistance is introduced in parallel with one of the elements, in order to create a known resistance variation. The reading of V as a result of the calibration resistance introduction is used to compute a gain compensation for the measuring circuitry reading V and nominal resistance uncertainty. 1/R1=1/R0+1/Rcal V*=GV
  55. 55. Strain Gauges connection: bridge 1 2 3 4 E V Some likely assumption to make calculation easier: - ΔR/R is small - All gage factors are equal - All nominal resistances are equal -an equivalent strain is computed by dividing the ratiometric output by the common gage factor V/E=(ΔR1+ΔR4-ΔR2-ΔR3)/R0 V/E=GF(ε1+ε4-ε2-ε3) εT=(V/E)/GF=ε1+ε4-ε2-ε3 Δrj/R0=GFεj 1 2 3 4 V
  56. 56. Bridge configuration: traction and bending As cantilever beam is subject to traction N, bending B, and to temperature variation A. Effects of torque are usually negligible Strain along the principal axis on the upper side of the beam is therefore: εI=εN+εB+εA While on the transversal axis strain on the upper side will be given by: εII=-νεN-νεB+εA On the lower sides the strains become respectively: εIII=εN-εB+εA εT=(V/E)/GF=ε1+ε4-ε2-ε3 N B 1 2 3 4 εI εII εIII εIV εIεII I εII εIV
  57. 57. Strain Gauge configuration: traction Single strain gauge Suitable only if: •bending moment is negligible •temperature is constant or autocompensated ε1=εI=εN+εB+εA ε2=0 ε3=0 ε4=0 εT=εN+εB+εA N B 1 2 3 4 QUARTER BRIDGE 1
  58. 58. Strain Gauge configuration: traction Orthogonal strain gauges Compensates: •Temperature Is affected by: •Bending moment and amplifies sensitivity ε1=εN+εB+εA ε2=-νεN-νεB+εA ε3=0 ε4=0 εT=(1+ν)εN+(1+ν)εB N B 1 2 3 4 HALF BRIDGE 1 2
  59. 59. Strain Gauge configuration: traction Temperature dummy gauge Compensates: •Temperature Is affected by: •bending moment ε1=εI=εN+εB+εA ε2=εA ε3=0 ε4=0 εT=εN+εB N B 1 2 3 4 HALF BRIDGE 1 2
  60. 60. Strain Gauge configuration: bending Opposed faces Compensates: •Temperature •Normal traction and amplifies sensitivity ε1=εN+εB+εA ε2=εN-εB+εA ε3=0 ε4=0 εT=2εB N B 1 2 3 4 HALF BRIDGE 1 2
  61. 61. Strain Gauge configuration: bending Orthogonal strain gauges Compensates: •Temperature •Normal traction and amplifies sensitivity N B 1 2 3 4 FULL BRIDGE 1 4 2 3
  62. 62. Questions Questions 3/31/2017 62

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