Metrology and Gauging Dynamic Characteristics.ppt

Metrology and Gauging
Instructed by: Dr. Sajid Zaidi
PhD in Advanced Mechanics, UTC, France
MS in Advanced Mechanics, UTC, France
B.Sc. in Mechanical Engineering, UET, Lahore
B.TECH Mechanical Technology
IQRA COLLEGE OF TECHNOLOGY (ICT)
INTERNATIONAL ISLAMIC UNIVERSITY, ISLAMABAD

Definition of Metrology
 Metrology
◦ Comes from Greek “metron” (measure) and –logy.
◦ It refers to science of measurement, including all theoretical and
practical aspects of measurement.
 Measurement
◦ It is the procedure of comparing an unknown quantity to a known
standard by means of consistent system of units.
◦ Provides a numerical value of quantity within certain limits of
accuracy and precision.
 Inspection
◦ It is the procedure in which part characteristics (e.g. dimension) are
examined whether they conform to design specification.
◦ Many inspection procedures rely on measurement techniques while
others use gauging methods (simply determine whether the part passes
or fails inspection).
Metrology
and
Gauging
Introduction

Definition of Quality
 Quality
◦ It refers to a product’s fitness for use.
◦ It is the totality of features that bear on a product’s ability to satisfy
a given need.
 Need of Inspection
◦ To ensure that part and components are confirmed to required
standards.
◦ To meet the need of Interchangeability of parts.
◦ To maintain good customer relationship by ensuring that No faulty
product reaches the customer.
◦ The result of inspection are forwarded to the manufacturing
department, thus helps in improving the quality.
◦ It helps to purchase good quality raw material, tool and equipment.
◦ It led to development of precision measuring instruments.
Metrology
and
Gauging
Introduction

Measure of Quality
Metrology
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Gauging
Introduction

Measure of Quality – What to Inspect
Metrology
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Gauging
Introduction

Measure of Quality – When to Inspect
Metrology
and
Gauging
Introduction

Measure of Quality – How to Inspect
Metrology
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Gauging
Introduction

Characteristics of Measurement Systems
 The performance characteristics of a measuring system
may be broadly divided into two groups, namely ‘static’
and ‘dynamic’ characteristics.
 Static characteristics are the values given when steady-
state conditions occur, i.e. the values given when the
system has settled down after having received some
input. Static characteristics also refer to the measurement
of quantities which are constant or vary slowly with
time. For example, the resistance of a resistor etc.
 Dynamic characteristics refer to the behavior between
the time that the input value changes and the time that
the value given by the system settles down to the steady
state value.
Metrology
and
Gauging
Introduction

Static Characteristics
Accuracy
 Accuracy determines the closeness of an instrument
reading to the true value of the physical variable.
 Accuracy is often expressed as a percentage of the full
range output or full-scale deflection (f.s.d).
 For example, a system might have an accuracy of ±1%
of f.s.d. If the full-scale deflection is 10 A then the
accuracy is ±0.1 A.
 This means that when the instrument is reading 1.0 A,
the possible error is 10% of this value.
 The accuracy is a summation of all the possible errors
that are likely to occur.
Metrology
and
Gauging
Introduction

Precision
 The term precision is used to describe the degree of
freedom of a measurement system from random errors.
 Precision is related to the repeatability of the instrument
reading.
 Thus, a highly precise measuring instrument will give
only a small spread of readings if repeated readings are
taken of the same quantity.
 A low precision system will give a large spread of
readings.
 For example, consider the following two sets of readings
obtained for repeated measurements of the same quantity
by two different instruments:
Metrology
and
Gauging
Introduction

Precision
◦ 20.1 mm, 20.2 mm, 20.1 mm, 20.0 mm, 20.1 mm, 20.1 mm
◦ 19.9 mm, 20.3 mm, 20.0 mm, 20.5 mm, 20.2 mm, 19.8 mm
 The results of the measurement give values scattered about some
value.
 The first set of results shows a smaller spread of readings than the
second and indicates a higher degree of precision for the
instrument used for the first set.
 Precision is often, though incorrectly, confused with accuracy.
 High precision does not imply anything about measurement
accuracy. A high precision instrument may have a low accuracy.
 Low accuracy measurements from a high precision instrument are
normally caused by a bias in the measurements, which is
removable by recalibration.
Metrology
and
Gauging
Introduction

12
PrecisionVs Accuracy
Metrology
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Gauging
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13
Measuring
a
fixed
target
position
from
a
satellite
PrecisionVs Accuracy
Satellite
Apollo 13:
Low
precision
low accuracy
Satellite PolyU
18:
High precision
low accuracy
Satellite CYT 118:
High precision,
High accuracy
Metrology
and
Gauging
Introduction

Repeatability and Reproducibility
 Repeatability describes the closeness of output readings when
the same input is applied repetitively over a short period of
time, with the same measurement conditions, same
instrument and observer, same location and same conditions
of use maintained throughout.
 Reproducibility describes the closeness of output readings
for the same input when there are changes in the method of
measurement, observer, measuring instrument, location,
conditions of use and time of measurement.
 Both terms thus describe the spread of output readings for the
same input. This spread is referred to as repeatability if the
measurement conditions are constant and as reproducibility if
the measurement conditions vary.
Metrology
and
Gauging
Introduction

Range
 The range or span of an instrument defines the minimum
and maximum values of a quantity that the instrument is
designed to measure. For example, a thermometer might
be quoted as having a range of -200 to +800°C.
Threshold
 If the instrument input is very
gradually increased from zero there
will be a minimum value required
to give a detectable output change.
This minimum value defines the
threshold of the instrument.
input
Output
Threshold
Metrology
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Gauging
Introduction

Linearity
 It is normally desirable that the output reading of an
instrument is linearly proportional to the quantity being
measured.
 The X’s marked on the Figure show a plot of the typical
output readings of an instrument when a sequence of
input quantities are applied to it.
 Normal procedure is to draw a good fit straight line
through the X’s, as shown in Figure.
 The non-linearity is then defined as the maximum
deviation of any of the output readings marked X from
this straight line. It is usually expressed as a percentage
of full-scale reading.
Metrology
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Gauging
Introduction

Linearity
Metrology
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Gauging
Introduction

Sensitivity
 The sensitivity indicates how much the output of an
instrument system changes when the quantity being
measured changes by a given amount, i.e. the ratio of
ouput to input.
 For example, a thermocouple might have a sensitivity of
20 μV/ºC and so give an output of 20 μV for each 1ºC
change in temperature.
 If we take a series of readings of the output of an
instrument for a number of different inputs and plot a
graph of output against input, the sensitivity is the slope
of the graph.
Metrology
and
Gauging
Introduction

19
Load Cell
Force, F
Output, Vo
Output, Vo (V)
Input, Fi (kN)
Slope = Output / Input
Sensitivity
By plotting a graph and finding the best
straight line, you can obtain the slope m
from the expression y = mx + c
Metrology
and
Gauging
Introduction

Error
 The term error is used for the difference between the
result of the measurement and the true value of the
quantity being measured, i.e.
error = measured value - true value
 Thus if the measured value is 10.1 when the true value is
10.0, the error is +0.10
 If the measured value is 9.9 when the true value is 10.0,
the error is -0.10
 Errors can arise in a number of ways and the following
describes some of the errors that are encountered in
specifications of instrumentation systems.
Metrology
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Gauging
Introduction

Hysteresis Error
 The term hysteresis error is used for the difference in
outputs given from the same value of quantity being
measured according to whether that value has been
reached by a continuously increasing change or a
continuously decreasing change.
 A different value may be obtained
from a thermometer used to measure
the same temperature of a liquid if it
is reached by the liquid warming up
to the measured temperature or it is
reached by the liquid cooling down
to the measured temperature.
Metrology
and
Gauging
Introduction

Non-linearity Error
 The term non-linearity error is used for the error that
occurs as a result of assuming a linear relationship
between the input and output over the working range,
i.e. a graph of output plotted against input is assumed to
give a straight line.
 Linearity error is usually expressed as a percentage error
of full range or full scale output.
Metrology
and
Gauging
Introduction

Insertion Error
 When a cold thermometer is put in to a hot liquid to
measure its temperature, the presence of the cold
thermometer in the hot liquid changes the temperature of
the liquid. The liquid cools and so the thermometer ends
up measuring a lower temperature than that which
existed before the thermometer was introduced.
 This effect is called loading and the consequence is the
insertion error.
 Loading is a problem that is often encountered when
measurements are being made. For example, when an
ammeter is inserted into a circuit to make a measurement
of the circuit current, it changes the resistance of the
circuit and so changes the current being measured.
Metrology
and
Gauging
Introduction

 Application
◦ A commercial pressure measurement system is quoted
in the manufacturer's specification as having:
 Range 0 to 10 kPa
 Supply Voltage ±15 V dc
 Linearity error ±0.5% FS
 Hysteresis error ±0.15% FS
 Sensitivity 5 V dc for full range
 Thermal sensitivity ±0.02%/ºC
 Thermal zero drift 0.02%/ºC FS
 Temperature range 0 to 50ºC
Metrology
and
Gauging
Introduction

Dynamic Characteristics
 The dynamic characteristics refer to the behavior
between the time that the input value changes and the
time that the value given by the system settles down to
the steady state value.
 For example, the figure
shows how the reading of
an ammeter might change
when the current is
switched on. The meter
pointer oscillates before
settling down to give the
steady-state reading.
Metrology
and
Gauging
Introduction

Dynamic Characteristics
Response Time
 This is the time which elapses after the input to a system
is abruptly increased from zero to a constant value up to
the point at which the system gives an output
corresponding to some specified percentage, e.g. 95%,
of the value of the input.
Rise Time
 This is the time taken for the output to rise to some
specified percentage of the steady-state output. Often the
rise time refers to the time taken for the output to rise
from 10% of the steady-state value to 90 or 95% of the
steady-state value.
Metrology
and
Gauging
Introduction

Metrology and Gauging Dynamic Characteristics.ppt

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