ME 311 Mechanical Measurements Instrumentation I 4/29/03

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ME 311Mechanical MeasurementsInstrumentation
I4/29/03
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Announcements

• The attendance sheet is being circulated. Please
  sign it before you leave!
• We are not meeting for lab tomorrow. Time to
  finish up testing and prepare for oral
  presentation.
• How are your projects coming along? How many
  groups have finished their testing? Any general
  question?
• If you have specific question, dont hesitate to
  contact your professors! We are here to help you.

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Road Map of Lecture 7

• Overview of a measurement system
• sensor/transducer
• signal conditioning (will be covered in greater
  details in ME 421)
• data acquisition (will be covered in greater
  details in ME 421)
• Focus on the sensor/transducer system in this
  course
• How does it work? (operating principle)
• Issues on range, resolution, sensitivity
• System characteristics
• Need for calibration
• Terminology with system characteristics
• Homework assignment (as a group)

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Overview of a Measurement System
e.g. pressure, temperature, light intensity
Measurand
e.g. pressure gauge, thermocouple, photo-cell
e.g. amplification, band-pass filtering
e.g. display, record, storage
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Basic Terminology

• The measurand is a physical quantity, property,
  or condition that is being measured.
• A sensor is the portion of a measurement system
  that responds directly to the measurand.
• In the general sense, a transducer is a device
  which transforms one physical variable into
  another.
• In most cases, the physical variable is
  transformed into an electrical signal.

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Advantages of Electrical Transducers

• Amplification or attenuation can be obtained
  easily
• Inertial effects are minimized
• Frictional effects are minimized (no moving part)
• Remote indication of recording is feasible
• Commonly susceptible to miniaturization
• Majority of output/recording devices require
  electrical input

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The Sensor/Transducer System

• The fundamental function of a sensor/transducer
  system is to sense the desired input, excluding
  all others, and provide analogous output.
  (Example microphone for sound detection)
• In reality, the sensor/transducer system always
  measures both the quantity of interest and some
  noise.
• Can you name a few examples of transducer which
  you worked with in the first four labs and your
  project? What is the variable transformation in
  each case?
• thermocouple (from temperature difference to
  potential difference)
• thermistor (from temperature difference to
  resistance difference)
• piezoelectric transducer (from stress/deformation
  to potential difference)
• strain gauge (from strain/deformation to change
  in resistance)
• photo-cell (from light intensity to change in
  resistance)
• manometer (from pressure difference to height
  difference between fluid columns)

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Issues to Consider

• Knowledge of the measurand
• static or dynamic (mean versus fluctuations)
• time scale
• amplitude scale
• frequencies present (How large is the range?)
• magnitudes
• Knowledge of the sensor
• operating principle (Does it imply any limitation
  on what can be measured?)
• resolution (How does the resolution compare with
  the magnitude of the measurand?)
• sensitivity
• settling time
• bandwidth (Is it large enough to capture the
  variation in the measurand?)

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Characteristics of a Sensor/Transducer System

• Viewing from a black-box perspective, every
  instrumentation is a dynamical system which has
  its own frequency response to the change in the
  measurand (amplitude and phase characteristics)
• always has a lag time, no perfect (instant,
  exact) data transmission
• recall the plotting device in Professor Olsons
  lab (Does anyone recall the highest frequency
  which can be tracked?)
• the measurement system may be of first or higher
  order ranging from simple to complex response

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More Vocabulary on System Characteristics

• Time scales
• time constant (for 1st order system)
• rise time
• settling time
• Frequency response
• Transient response
• Bandwidth
• Accuracy, range
• Overshoot

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Need for Calibration

• One of the most overlooked procedures in
  conducting an experiment. (Did you do it for
  your project?)
• Rather than just accepting the reading of an
  instrument, calibration firmly establishes the
  accuracy of the instruments.
• Sometimes the manufacturers specifications may
  not be taken at face value.
• It involves a comparison of the instrument with
  either
• a primary standard (may be difficult to obtain),
• a secondary standard with a higher and known
  accuracy than the instrument to be calibrated,
• a known input source.

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Many Types of Sensor/Transducer System

• Acceleration
• piezoelectric accelerometer
• strain gage accelerometer
• Force Torque
• shaft torque sensor
• dynamometer
• Pressure transducer
• Temperature
• thermocouple
• resistance temperature detector (RTD)
• Flow rate
• rotameter
• turbine flowmeter

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Homework Assignment

• Each group chooses one of the sensor/transducer
  systems to research on.
• Address the following key questions
• What is the operating principle?
• Range, sensitivity, resolution (based on the
  chosen model)
• Any required accessories (signal conditioning)?
• This homework is an attempt to get you involved
  in our second lecture on Instrumentation. It
  will be due at the beginning of the next lecture.

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Suggested References

• T.G. Beckwith, R.D. Marangoni, and J.H. Lienhard
  V, Mechanical Measurements, Addison-Wesley.
• R.S Figliola D.E. Beasley, Theory and Design
  for Mechanical Measurements, Wiley.
• J.P. Holman, Experimental Methods for Engineers,
  McGraw Hill.
• A.J. Wheeler A.R. Ganji, Introduction to
  Engineering Experimentation, Prentice Hall.
• SENSOTEC www.sensotec.com
• OMEGA www.omega.com
• DYNOmite www.land-and-sea.com/dyno.htm