MEASUREMENT AND INSTRUMENTATION BMCC 4743

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MEASUREMENT AND INSTRUMENTATIONBMCC 4743

• LECTURE 2 PERFORMANCE OF STATIC CHARACTERISTICS
  IN MEASUREMENT AND INSTRUMENTATION

Mochamad Safarudin Faculty of Mechanical
Engineering, UTeM 2008
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Contents

• Definition of static
• Requirement of static characteristics
  preservation
• Static calibration
• Passive active sensors

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Definition of Static

• The steady state relationship between input and
  output of an instrument
• Measurement of quantities that are constant or
  vary quite slowly with respect to time.
• It does not involve differential equations.
• All the static performance characteristics are
  obtained by one form or another via a process
  called static calibration.

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Contents

• Definition of static
• Requirement of static characteristics
  preservation
• Static calibration
• Passive active sensors

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Requirement of static characteristics preservation

• Precise Accurate
• Measurements that are close to each other are
  precise
• Measurements that are close to the correct value
  are accurate
• Measurements can be
• Precise but inaccurate
• Neither precise nor accurate
• Precise and accurate

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Examples
Three industrial robots were programmed to place
components at a particular point on a table. The
target point was the center of a circle shown
below. The results are

• Low precision,
• low accuracy

(b) Precise not accurate
(c) Precise and accurate
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Accuracy
Ideal device
Output of device
40
20
Accuracy at of FS
0
20
40
Value of measurand
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Requirement of static characteristics preservation

• Repeatability
• The ability of an instrument to register readings
  accurately repeatedly
• Linearity
• The input and output relationship of a linear
  transducer can be represented by the following
  equation
• where y is the output of transducer, x is the
  input of transducer, m is the slope of curve
  (transfer function), c is the offset.
• Often, the straight line approach is used for
  certain range of operation for a non-linear
  system.

y mx c
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Requirement of static characteristics preservation

• Hysteresis
• Hysteresis results in predictable error. May be
  due to internal friction, freeplay or looseness
  in the mechanism of an instrument.
• Also in electrical phenomena (relation between
  the output voltage and the input field current in
  a d.c. generator) - the effect is due to magnetic
  hysteresis of the iron in the field coils.
• The transfer functions differ with the increase
  and decrease of inputs as shown in the following
  figure.

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Generalized graph of output/input relationship
where hysteresis is present. (From P. H.
Sydenham, Handbook of Measurement Science, Vol.
2, Chichester, U.K., John Wiley Sons, 1983)
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Linearity
It is highly desirable that the measurement
system has a linier relationship between input
and output means that the change in output is
proportional to the change in the value of the
measurand Deviation from true linearity is called
linearity error
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Sensitivity

• Sensitivity is the ratio of change in magnitude
  of the output to the change in magnitude of the
  measurand
• SensitivityD(output)/D(input)

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Sensitivity and Non Linearity
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Contents

• Definition of static
• Requirement of static characteristics
  preservation
• Static calibration
• Passive active sensors

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Static Calibration

• Imagine a situation in which all inputs (desired,
  interfering or modifying) except one are kept at
  some constant values.
• The one input under study is varied over some
  range of constant values which causes the
  output(s) to vary over some range of constant
  values.
• The input/output relationships developed in this
  way comprise a static calibration valid under the
  stated constant conditions of all other inputs.
• The procedure may be repeated for other inputs
  for overall instrument static behaviour
• Ultimate objective is to define measurement
  accuracy

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Contents

• Definition of static
• Requirement of static characteristics
  preservation
• Static calibration
• Passive active sensors

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Passive Active Sensors

• Sensors are often transducer in that they are
  devices that convert input energy of one form
  into output energy of another form.
• Categorisation of sensors depends on how they
  interact with the environment they are measuring
• Passive sensors they do not add energy as part
  of the measurement process but may remove energy
  in their operation. E.g. thermocouple converts a
  physical temperature into a voltage signal
• Active sensors they add energy to the
  measurement environment as part of the
  measurement process. E.g. in a radar or sonar
  system, the distance to some object is measured
  by actively sending out a radio (radar) or
  acoustic (sonar) wave to reflect off some object
  and measure its range from the sensor

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EXERCISE

• What is the meaning of the following words
• Measurand
• Physical quantity
• Data
• Parameter
• Transducer
• Actuator

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ANSWER TO EXERCISE

• Measurand Physical quantity being measured
• Physical quantity Variable such as pressure,
  temperature, mass, length, etc
• Data Information obtained from the
  instrumentation/measurement system as a result of
  the measurements made of the physical quantities
• Parameter Physical quantity within defined
  (numeric) limits.
• Transducer A device that converts one form of
  energy to another
• Actuator Electronic transducer that converts
  electrical energy into mechanical energy

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Next Lecture

• Analysis on Experimental Data
• End of Lecture 2