chapter one.pptx

University of Gondar
Institute of technology
Electrical and Computer engineering
•Mechatronics[IEng4161]
By Getachew D.

1. Introduction to Instrumentation
1.1. Introduction
Measurement is a process of gathering information from a physical world
and comparing this information with agreed standards.
➢ Measurements are carried out by using instruments, which are designed and
manufactured to fulfill specific tasks.
➢ A process is defined as a system which generates information e.g.
 chemical reactor,
 jet fighter,
 submarine
 car and human heart.
➢ Information/measured variables are commonly generated by processes.
 a car generates displacement, velocity and acceleration variables, and
 a chemical reactor generates temperature, pressure and composition
variables.

Then the observer is defined as a person who needs this information from
the process: this could be the car driver, or the plant operator.
➢ The purpose of the measurement system is to link the observer to the
process
➢ The input to the measurement system is the true value of the variable; the
system output is the measured value of the variable.
➢ The measured variables are then expressed in terms of measurement units,
often by using Systems International(SI) units.
➢ These units can be either standard or further set of units derived from
them.

There are four basic elements in a typical measurement system:
a. Sensing element
➢ This is in contact with the process and gives an output which
depends in
some way on the variable to be measured. Examples are:
 Thermocouple where millivolt e.m.f. depends on temperature
 Strain gauge where resistance depends on mechanical strain
 Orifice plate where pressure drop depends on flow rate.

b. Signal conditioning element
➢ This takes the output of the sensing element and converts it into a form
more suitable for further processing, usually a d.c. voltage, d.c. current or
frequency signal.
➢ Examples are:
 Deflection bridge which converts an impedance change into a voltage change
 Amplifier which amplifies millivolts to volts
 Oscillator which converts an impedance change into a variable frequency
voltage

c. Signal processing element
➢ This takes the output of the conditioning element and converts it into a
form more suitable for presentation. Examples are:
Analogue-to-digital converter (ADC) which converts a voltage into a digital
form for input to a computer
Computer which calculates the measured value of the variable from the
incoming digital data.
➢ Typical calculations are:
 Computation of total mass of product gas from flow rate and density data
 Integration of chromatograph peaks to give the composition of a gas stream
 Correction for sensing element non-linearity.

d. Data presentation element
➢ This presents the measured value in a form which can be easily recognized
by the observer. Examples are:
 Simple pointer–scale indicator
 Chart recorder
 Alphanumeric display
 Visual display unit (VDU).

Types of Instruments
1. Active and passive instruments
➢ Passive instrument: output is entirely produced by the quantity being measured.
 Example: pressure-measuring device shown in Figure below. The pressure of the
fluid is translated into a movement of a pointer against a scale.
 The energy expended in moving the pointer is derived entirely from the
change in pressure measured: there are no other energy inputs to the system.
➢ Active instrument: here the quantity being measured simply modulates the
magnitude of some external power source.
 Example: a float-type petrol tank level indicator as sketched below.
 The change in petrol level moves a potentiometer arm, and the output signal
consists of a proportion of the external voltage source.
 The energy in the output signal comes from the external power source:
 the primary transducer float system is merely modulating the value of the
voltage from this external power source.

In active instruments, the external power source is usually in electrical form,but in
some cases, it can be a pneumatic or hydraulic one.
One very important difference between active and passive instruments is the level
of measurement resolution that can be obtained.

For pressure measuring device, it is possible to increase resolution by making
the pointer longer, such that the pointer tip moves through a longer arc.
➢ But the scope for such improvement is clearly restricted by the practical
limit of how long the pointer can conveniently be.
➢ In an active instrument, however, adjustment of the magnitude of the external
energy input allows much greater control over measurement resolution.
➢ In terms of cost, passive instruments are normally of a more simple construction
than active ones and are therefore cheaper to manufacture.
➢ Therefore, choice between active and passive instruments for a particular
application involves tradeoff b/n the resolution against cost.

2. Null-type and deflection-type instruments
➢ In a deflection type of instrument, the value of the quantity being measured
is displayed in terms of the amount of movement of a pointer.
➢ An example for this type of instrument is the pressure gauge discussed before.
➢ An alternative type of pressure gauge is the deadweight gauge shown in
Figure below, which is a null-type instrument.
➢ Here, weights are put on top of the piston until the downward force balances
the fluid pressure.
➢ Weights are added until the piston reaches a datum level, known as the null point.
➢ Pressure measurement is made in terms of the value of the weights needed to reach
this null position.

The accuracy of these two instruments depends on different things.
➢ For the first one it depends on the linearity and calibration of the spring, whilst for
the second it relies on the calibration of the weights.

Calibration of weights is much easier than careful choice and calibration of
a linear-characteristic spring.
➢ This means that the second type of instrument will normally be the more
accurate.
➢ This is in accordance with the general rule that null-type instruments are
more accurate than deflection types.
➢ In terms of usage, the deflection type instrument is clearly more convenient.

3. Analogue and digital instruments
➢ An analogue instrument gives an output that varies continuously as the
quantity being measured changes.
➢ The output can have an infinite number of values within the range that the
instrument is designed to measure.
➢ The deflection-type of pressure gauge described earlier is a good example
of an analogue instrument.
➢ A digital instrument has an output that varies in discrete steps and so can
only have a finite number of values.
➢ The rev counter sketched in Figure below is an example of a digital instrument

A cam is attached to the revolving body whose motion is being measured, and on
each revolution the cam opens and closes a switch.
➢ The switching operations are counted by an electronic counter.
➢ This system can only count whole revolutions and cannot discriminate any
motion that is less than a full revolution.

4. Indicating instruments and instruments with a signal output
➢ Some instruments give an audio or visual indication of the magnitude of
the physical quantity measured.
➢ Others give an output in the form of a measurement signal whose magnitude is
proportional to the measured quantity.
➢ The class of indicating instruments normally includes all null-type instruments
and most passive ones.
➢ A common analogue indicator is the liquid-in-glass thermometer.
➢ One major drawback with indicating devices is that human intervention is
required to read and record a measurement.
➢ Instruments that have a signal-type output are commonly used as part of
automatic control systems.

1.3. Static performance characteristics of instruments
➢ In this lecture, the characteristics that typical elements may possess and
their effect on the overall performance of the system will be discussed.
➢ Knowledge of these characteristics is essential to select the most suitable
instrument for a specific task.
➢ The broad classification of the characteristics can be static( steady-state) or dynamic
characteristics.
➢ Static characteristics establish relationships b/n the output O and input I
of an element when I is either at a constant value or changing slowly.
➢ These characteristics can be systematic characteristics, that can be exactly
quantified by mathematical or graphical means.
➢ On the other hand, statistical characteristics are those which cannot be exactly
quantified.

The systematic characteristics are: range, span, sensitivity, resolution, hysteresis,
and dead space.
➢ Where as the statistical characteristics include repeatability, tolerance
➢ The ways in which an element responds to sudden input changes are termed
its dynamic characteristics.

Range
➢ The input range of an element is specified by the minimum and maximum values
of I , i.e. Imin to Imax
➢ The output range is specified by the minimum and maximum values of O,
i.e. i.e. Omin to Omax
 Thus a pressure transducer may have an input range of 0 to 104 Pa and an output
range of 4 to 20 mA;
 a thermocouple may have an input range of 100 to 250C and an output range of
4 to 10 mV .
Span
➢ Span is the maximum variation in input or output, i.e. input span is Imax −Imin,
and output span is Omax − Omin
 Thus in the above examples the pressure transducer has an input span of 104 Pa
and an output span of 16 mA;
 the thermocouple has an input span of 150◦C and an output span of 6mV

Linearity
➢ An element is said to be linear if corresponding values of I and O lie on a straight
line.
➢ It is normally desirable that the output reading of an instrument is linearly
proportional to the quantity being measured.
➢ The output equation is given by:

Non-linearity
➢ The non-linearity is defined as the maximum deviation of any of the output
readings from the straight line.
➢ The difference between actual and
ideal straight-line behaviour is:
N (I ) = O(I ) − (KI + α)
= ⇒ O(I ) = KI + α + N (I )
➢ Non-linearity is often quantified in
terms of the maximum non-linearity;
➢ It is expressed as a percentage of full-scale deflection (f.s.d.), i.e. as a
percentage of span. Thus

Threshold
➢ Threshold is the minimum level of detectable input that cause change in
the output readings, if the input is gradually increased from zero.
➢ For example, consider a typical car speedometer that has a threshold of
about 15 km/h.
➢ This means that, if the vehicle starts from rest and accelerates, no output
reading is observed on the speedometer until the speed reaches 15 km/h.
Resolution
➢ The resolution is the smallest change in the input value that will produce
an observable change in the output.
➢ One of the major factors influencing the resolution of an instrument is how
finely its output scale is divided into subdivisions

Sensitivity to disturbance
➢ All calibrations and specifications of an instrument are only valid under
controlled conditions of temperature, pressure etc.
➢ As these condition vary, certain characteristics change and the sensitivity
to disturbance is a measure of the magnitude of this change.
➢ Such environmental changes affect instruments in two main ways, known
as zero drift (bias) and sensitivity drift.
➢ Zero drift or bias describes the effect where the zero reading of an instrument is
modified by a change in ambient conditions.
➢ This causes a constant error that exists over the full range of measurement of the
instrument.
➢ Sensitivity drift (scale factor drift) defines the amount by which an instrument’s
sensitivity of measurement varies as ambient conditions chang

Dead space
➢ Dead space is defined as the range of different input values over which there is
no change in output value.
➢ Any instrument that exhibits hysteresis also displays dead space, as marked on
Figure above.
➢ Some instruments that do not suffer from any significant hysteresis can still
exhibit a dead space in their output characteristics, however.
➢ Backlash in gears is a typical cause of dead space, and results in the sort of
instrument output characteristic shown in Figure below.

Accuracy
➢ The accuracy of a measurement of a variable is the closeness of the measurement
to the true value of the variable.
➢ It is quantified in terms of measurement error, i.e. the difference between
the measured value and the true value.
➢ Inaccuracy is the extent to which a reading might be wrong, and is often
quoted as a percentage of the full-scale (f.s.) reading of an instrument.
➢ Assume a pressure gauge of range 0–10 bar has a quoted inaccuracy of
±1.0% f.s. reading.
➢ This means that when the instrument is reading pressure, the possible the
obtained reading is expected to be ±0.1 bar of the true reading.
➢ The term measurement uncertainty is frequently used in place of inaccuracy.

Precision/repeatability/reproducibility
➢ Precision is defined as the measure of closeness of values for a large number
of readings that are taken of the same quantity.
➢ 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.
➢ The degree of repeatability or reproducibility in measurements from an
instrument is an alternative way of expressing its precision.

Tolerance is a term that is closely related to accuracy and defines the maximum
error that is to be expected in some value.
Error
➢ Error is the difference between the result of the measurement and the true value
of the quantity being measured:
error = measured value − true value
➢ Thus if a measurement system gives a temperature reading of 25◦C when the
actual temperature is 14◦C , then the error is 11◦C .
➢ If the actual temperature had been 36◦C then the error would have been 21◦C .
➢ A sensor might give a resistance change of 10.2 V when the true change should
have been 10.5 V. The error is -0.3 V.

We can divide errors in measurement systems into those that arise
 during the measurement process
 due to later corruption of the measurement signal by induced noise during
transfer of the signal
➢ Systematic errors describe errors in the output readings of a measurement system
that are consistently on one side of the correct reading,
 that is, either all errors are positive or are all negative.
➢ The main sources of systematic error in the output of measuring instruments are
 effect of environmental disturbances, often called modifying inputs
 disturbance of the measured system by the act of measurement
 changes in characteristics due to wear in instrument components over a period of
time
 resistance of connecting leads

Systematic errors can be minimized by:
 careful instrument design
 calibration
 signal filtering etc.
➢ Random errors are perturbations of the measurement either side of the true
value caused by random and unpredictable effects.
➢ Random errors in measurements are caused by unpredictable variations in
the measurement system. Typical sources of random error are
 measurements taken by human observation of an analogue meter,
especially where this involves interpolation between scale points.
 electrical noise.
 random environmental changes, for example, sudden draught of air.

The static characteristics of an element is experimentally determined by
measuring the procedure known as calibration.
➢ Calibration consists of comparing the output of the instrument or sensor
under test against the output of an instrument of known accuracy when
the same input (the measured quantity) is applied to both instruments.
➢ This procedure is carried out for a range of inputs covering the whole
measurement range of the instrument or sensor.
➢ For use of instruments and sensors under different environmental conditions,
appropriate correction has to be made for the ensuing modifying inputs.
➢ The instruments and techniques used to quantify these variables are referred to as
standards.

Transducers
In measurement system, the measurand (quantity under measurement) makes its
first contact with system through a detector
The measurand is converted into an analogous form by the detector.
The mesurand or the input signal is called information for the measurement
system.
The function of the detector is to sense the information and convert it into a
convenient form for acceptance by the later stages of the system.
The process of detection and conversion of input signal or information from one
form to another does require energy.
If energy is extracted from the signal, it will not be faithfully reproduced after
conversion which will lead to errors.

Distinction Between The Sensor And Transducer
A sensible distinction is to use sensor for sensing element itself and transducer for
the sensing element plus any associated circuitry.

Data transmission

Data Display & Storage
The data may be analog or digital form. The display device may be any of the
following types:
1. Analog indicators – motion of needle on meter scale
2. Pen trace/light trace
3. Screen display – CRO, TV
4. Digital counter – counter wheel
5. Digital printer – data in printed form
6. Punches – punched cards/tapes
7. Electronic displays – LED, LCD, etc
8. Storage may be on cards, magnetic tapes, disks.
9. Permanent record of data from a computer

Example 1:
➢ A particular micrometer is designed to measure dimensions between 50 and 75 mm.
What is its measurement span?
Solution:
➢ The measurement span is simply the difference between the maximum and minimum
measurements. Thus, in this case the span is 75 − 50 = 25 mm.

Example 6:
➢ The width of a room is measured 10 times by an ultrasonic rule and the following
measurements are obtained (units of meters): 5.381 5.379 5.378 5.382 5.380 5.383
5.379 5.377 5.380 5.381. The width of the same room is then measured by a
calibrated steel tape that gives a reading of 5.374 m, which can be taken as the
correct value for the width of the room.
a. What is the measurement precision of the ultrasonic rule?
b. What is the maximum measurement inaccuracy of the ultrasonic rule?
Solution:
a. The mean (average) value of the 10 measurements made with the ultrasonic rule
is 5.380 m.
The maximum deviation below this mean value is -0.003 m and the
maximum deviation above the mean value is +0.003 m. Thus the
precision of the ultrasonic rule can be expressed as ±0.003 m.
b. The correct value of the room width has been measured as 5.374 m by
the calibrated steel rule.

The maximum measurement error can be calculated as:
5.383 - 5.374 = 0.009 m (9 mm). Thus the maximum measurement inaccuracy can
be expressed as +9 mm.
Example 8:
➢ A packet of resistors bought in an electronics component shop gives the nominal
resistance value as 1000 Ω and the manufacturing tolerance as ±5%. If one resistor
is chosen at random from the packet, what is the minimum and maximum resistance
value that this particular resistor is likely to have?
Solution
➢ The minimum likely value is 1000Ω − 0.05 × 1000Ω = 950Ω.
➢ The maximum likely value is 1000Ω + 0.05 × 1000Ω = 1050Ω.

chapter one.pptx

Empfohlen

Empfohlen

Weitere ähnliche Inhalte

Was ist angesagt?

Was ist angesagt? (20)

Ähnlich wie chapter one.pptx

Ähnlich wie chapter one.pptx (20)

Kürzlich hochgeladen

Kürzlich hochgeladen (20)

chapter one.pptx