Fundamentals of Measurement: Transducers and Instrumentation, Lecture notes of Electronics

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Transducers and Instrumentation

Lecture 1

Measurement

• Measurements provide us with a means of describing various phenomena in quantitative terms.
• "whatever exists, exists in some amount".
• The determination of the amount is measurement all about.

Fundamental Measuring Process

Measurement System

• Measurement systems are used for many detailed purposes in a wide variety of application areas.
• The term “measurement system” is meant to include all components in a chain of hardware and software that leads from the measured variable to processed data.

Reference Value

• Exemplar Method
  • The method agreed on by experts as being sufficiently accurate for the purposes to which the data will ultimately be put.
• True value is then referred to as Reference Value when measured by an exemplar method

Nature of Measurement Process

• Measurement process, with assumed identical conditions, repeated over and over, will not produce the same output(s) always. - Never possible to ensure identical conditions for each trial
• Random Sequence-in state of statistical control
• For future use, numerical estimates of error associated with collected data set can be calculated

Bias and Uncertainty in Measurement

Performance Characteristics

• Selection of most suitable instrument
  • (proposed measurement)
• Design of Instrument
  • (For specific measuring tasks)
• Performance Criteria
  • Quantitative bases for comparing one instruments with possible alternatives

Static Characteristics

• Accuracy
  • closeness of the instrument output to the true value of the measured quantity
  • 1 g with an error of 10
    • 2 g
  • inherent limitations of the instrument
  • shortcomings in the measurement process
  • systematic or cumulative errors

Static Characteristics

• Precision
  • ability of the instrument to reproduce a certain set of readings within a given accuracy
  • Repeatability of same measured quantity, made by the same observer, using the same instrument, under the same conditions.
  • Random or accidental errors
  • Determine extent of random errors using Statistical analysis

Static Characteristics

• Resolution
  • smallest increment in the measured value detected with certainty by the instrument.
  • Degree of fineness with which a measurement
• Threshold
  • minimum value of input below which no output can be detected.
  • friction between moving parts
  • looseness in joints (more correctly termed as backlash),
  • inertia of the moving parts
  • length of the scale,
  • spacing of graduations,
  • size of the pointer, etc.

Static Characteristics

• Static Sensitivity (Gain/Scale factor)
  • determined from the results of static calibration
  • sensitivity is represented by the slope of the input-output curve
  • sensitivity of a typical linear spring 400N/mm
  • Deflection factor

Drift

• It is defined as the variation of output for a given input caused due to change in the sensitivity of the instrument due to certain interfering inputs like temperature changes, component instabilities, etc.

Linearity

• Desirable Feature
  • Linear indicating scale
• Departure from linearity
• Independent of input
• Proportional to input