1 / 56

METHODS OF MEASUREMENT Direct comparison :

METHODS OF MEASUREMENT Direct comparison : Compare unknown quantity (measurand) against a known quantity (standard). Eg: tape measure. Direct comparison not always possible or practical. Eg: measuring sound levels. Indirect comparison (calibrated system):

clare-ware
Download Presentation

METHODS OF MEASUREMENT Direct comparison :

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. METHODS OF MEASUREMENT • Direct comparison: • Compare unknown quantity (measurand) against a known quantity (standard). Eg: tape measure. • Direct comparison not always possible or practical. Eg: measuring sound levels. • Indirect comparison (calibrated system): • Makes use of a sensor or transducing device (transducer) to transform the measurand into an analogous form. • The sensor is connected to a series of instruments which convert the output of the sensor into a useful analogous form which presents the measurand in a useful and practical format.

  2. METHODS OF MEASUREMENT • Processing of the analogous signal is often necessary to extract the desired information from the signal. • For reasons related to signal processing, automatic recording and process control, the majority of measuring systems convert the measurand into an analogous electrical form.

  3. THE GENERALISED MEASURING SYSTEM • Most measuring systems consist of three stages: • Detection-transduction stage (sensor-transducer) • Intermediate stage (signal conditioning) • Terminating stage (readout, display, recorder) Indicator Recorder Conditioned signal (raw) signal measurand Processor Sensor-transducer Signal conditioner Controller Human (eyes, pen, paper)

  4. THE GENERALISED MEASURING SYSTEM • Detection-transduction stage (sensor-transducer) • Function: to detect or sense the measurand without affecting it. • Ideally, must also be insensitive to other variables. Eg: Pressure sensor must be insensitive to acceleration, strain gauge must be insensitive to temperature. • Unwanted sensitivity is a measuring error. • Noise: high frequency (fast) • Drift: low frequency (slow)

  5. THE GENERALISED MEASURING SYSTEM • Signal conditioning stage • Function: to modify (improve) the transduced information for compatibility with the terminating (readout / recording/ processing) stage • These include amplification (most common) , filtering (noise removal), offset adjustment, differentiation, integration, telemetry…. • Sometimes also used to provide electrical power or excitation signal required by sensor.

  6. THE GENERALISED MEASURING SYSTEM • Terminating stage (readout, display, recorder) • Function: to provide information on measurand in a format suitable for the application. • For (immediate) human recognition, the display usually comprises: • A relative displacement indicator. Eg: needle, / pointer, trace in a screen (oscilloscope), stylus on a chart, a level change (thermometer)… • In digital (numeric) form. Eg: counter (odometer), numeric LCD etc.. • For recording or processing purposes, the terminating stage may comprise of magnetic recorders, chart (paper) recorders, digital recorders (PC-based data acquisition systems, digital storage oscilloscopes), process controllers (PLCs, computer-based control systems).

  7. THE GENERALISED MEASURING SYSTEM • Example: Pressure gauge Indicator: Visual scale Signal conditioning: none Sensor-transducer: Piston/cylinder (pressure to force) and spring (force to displacement) measurand: Pressure

  8. THE GENERALISED MEASURING SYSTEM • Example: Rocket velocity Sensor-transducer Recording & display Signal conditioning Accelero-meter Low-pass filter Integrator Amplifier PC Data acquisition Printer

  9. THE GENERALISED MEASURING SYSTEM • Example: Rocket velocity Sensor-transducer Recording & display Signal conditioning Accelero-meter Low-pass filter Integrator Amplifier PC Data acquisition Printer

  10. Dial Gauge

  11. MEASURING SYSTEMS : SELECTED EXAMPLES • Displacement / distance: micrometer Micrometer

  12. MEASURING SYSTEMS : SELECTED EXAMPLES • Displacement / distance: Linear Variable Differential Transformer (LVDT)

  13. MEASURING SYSTEMS : SELECTED EXAMPLES • Displacement / distance: Linear Variable Differential Transformer (LVDT)

  14. MEASURING SYSTEMS : SELECTED EXAMPLES • Displacement / distance: Linear Variable Differential Transformer (LVDT)

  15. MEASURING SYSTEMS : SELECTED EXAMPLES • Displacement / distance: Laser (triangulation)

  16. MEASURING SYSTEMS : SELECTED EXAMPLES • Displacement / distance: Laser (triangulation)

  17. MEASURING SYSTEMS : SELECTED EXAMPLES • Displacement / distance: Time-of-flight distance sensors: • Ultrasonic (Sonar), Radio waves (Radar), Optical (Ladar),

  18. MEASURING SYSTEMS : SELECTED EXAMPLES • Displacement / distance: Time-of-flight distance sensors: • Ultrasonic (Sonar), Radio waves (Radar), Optical (Ladar),

  19. MEASURING SYSTEMS : SELECTED EXAMPLES • Displacement / distance: Time-of-flight distance sensors: • Ultrasonic (Sonar), Radio waves (Radar), Optical (Ladar),

  20. MEASURING SYSTEMS : SELECTED EXAMPLES • Displacement / distance: Time-of-flight distance sensors: • Ultrasonic (Sonar), Radio waves (Radar), Optical (Ladar), • Laser diode is pulsed every microsecond • The reflection is detected by a photo diode Animation courtesy Banner Engineering

  21. MEASURING SYSTEMS : SELECTED EXAMPLES • Strain: (resistance strain gauge) • When the conductor (wire) is strained (extended), its cross-sectional area reduces and the total electrical resistance increases. • The change in resistance is used to measure the strain of the material or component onto which the strain gauge is bonded. Bi-axial gauge Tri-axial gauge (rosette) for Principal strains Bi-axial gauge for normal or shear strains

  22. MEASURING SYSTEMS : SELECTED EXAMPLES • Strain: (resistance strain gauge) R =  L/A or R =  L/CD2  C=1 O: C=/4(1) When the conductor is strained, its geometry will change. Differentiating (1) dR = CD2(L d +  dL) – 2C LD dD / (CD2)2 = [(L d +  dL) - 2 L dD/D] / CD2 (2) Dividing (2) by (1) dR/R = dL/L - 2dD/D + d/ (3) Dividing by dL/L throughout: (dR/R) / (dL/L) = 1 – 2(dD/D)/(dL/L) + (d/)/(dL/L) (4) Since dL/L = a = axial strain, dD/D = L = lateral strain and  = Poisson’s Ratio = (dD/D)/(dL/L), Eqn. (4) can be written to define the Gauge Factor, G: G = (dR/R)/(dL/L) = (dR/R)/ a or a = (dR/R)/G (5)

  23. MEASURING SYSTEMS : SELECTED EXAMPLES Force • Force and torque are often measured by bonding a number of strain gauges on a carefully designed component called a load cell. The load cell is usually manufactured using steel which has very linear (elastic) properties as well as having a high elastic modulus (low deformation under load). • Load cells can be designed to measure a wide variety of forces such as compression, bending, tension, shear and torque. Strain gauge type load cells

  24. MEASURING SYSTEMS : SELECTED EXAMPLES • Strain: (resistance strain gauge) • The Wheatstone bridge

  25. MEASURING SYSTEMS : SELECTED EXAMPLES • Strain gauge type load cells

  26. MEASURING SYSTEMS : SELECTED EXAMPLES • Strain gauge type load cells

  27. MEASURING SYSTEMS : SELECTED EXAMPLES • Strain gauge type load cells

  28. MEASURING SYSTEMS : SELECTED EXAMPLES • Strain gauge type load cells

  29. MEASURING SYSTEMS : SELECTED EXAMPLES • Strain gauge type load cells

  30. MEASURING SYSTEMS : SELECTED EXAMPLES • Strain gauge type load cells

  31. MEASURING SYSTEMS : SELECTED EXAMPLES • Strain gauge type load cells

  32. MEASURING SYSTEMS : SELECTED EXAMPLES • Strain gauge type load cells

  33. MEASURING SYSTEMS : SELECTED EXAMPLES • Strain gauge type load cells

  34. MEASURING SYSTEMS : SELECTED EXAMPLES • Time response

  35. MEASURING SYSTEMS : SELECTED EXAMPLES • Time response

  36. MEASURING SYSTEMS : SELECTED EXAMPLES • Time response

  37. MEASURING SYSTEMS : SELECTED EXAMPLES • Time response

  38. MEASURING SYSTEMS : SELECTED EXAMPLES • Temperature • Thermocouples are based on the principle that when two dissimilar metals are joined a predictable voltage will be generated that relates to the difference in temperature between the measuring junction and the reference junction (connection to the measuring device). • RTDs are wire wound and thin film devices that work on the physical principle of the temperature coefficient of electrical resistance of metals. They are nearly linear over a wide range of temperatures and can be made small enough to have response times of a fraction of a second. They require an electrical current to produce a voltage drop across the sensor that can be then measured by a calibrated read-out device.

  39. MEASURING SYSTEMS : SELECTED EXAMPLES • Velocity: Interferometry (Michelson interferometer)

  40. MEASURING SYSTEMS : SELECTED EXAMPLES • Velocity: Interferometry

  41. MEASURING SYSTEMS : SELECTED EXAMPLES Piezoelectric sensors

  42. MEASURING SYSTEMS : SELECTED EXAMPLES Piezoelectric sensors

  43. MEASURING SYSTEMS : SELECTED EXAMPLES Piezoelectric sensors

  44. MEASURING SYSTEMS : SELECTED EXAMPLES Piezoelectric sensors

  45. MEASURING SYSTEMS : SELECTED EXAMPLES Piezoelectric sensors

  46. MEASURING SYSTEMS : SELECTED EXAMPLES Piezoelectric sensors

  47. MEASURING SYSTEMS : SELECTED EXAMPLES Piezoelectric sensors Accelerometer performance: • Sensitivity (Pc/g or V/g) • Range • Resolution • Transverse sensitivity • Amplitude linearity • Frequency response or frequency range

  48. MEASURING SYSTEMS : SELECTED EXAMPLES Piezoelectric sensors Accelerometer performance:

  49. MEASURING SYSTEMS : SELECTED EXAMPLES Piezoelectric sensors Environmental effects: • Temperature – may affect sensitivity, natural frequency and damping. Effects sometimes characterised by manufacturer. • Humidity – mainly affects high impedance transducers. • Acoustic noise. • Strain sensitivity – may generate spurious signals when the case is strained or distorted (ie. badly mounted)

  50. MEASURING SYSTEMS : SELECTED EXAMPLES Piezoelectric sensors Accelerometer mounting:

More Related