University of Technology,
Department of Production Eng. & Metallurgy
M.Sc. course
Case Studies:
A temperature measurement
Requirement: Determination of temperature of a liquid in the range 0ºC to
100 ºC . Determination of the temperature of the cooling water for an engine
and its display as a pointer moving across a scale marked to indicate safe
and unsafe operating temperatures.
Sensor: use a thermistor as a sensor.
Signal processing: The resistance change of the thermistor has to be
converted into a voltage which can then be applied across a meter and so
converted to a current through it and hence a reading on the meter related to
the temperature.
Figure below shows a possible solution involving a potential divider circuit
to convert the resistance change into a voltage change. Suppose we use a 4.7
k ohm, bead thermistor. This has a resistance of 4.7 k ohm at 25°C, 15.28 k
ohm at OºC and 0.33 k ohm at lOOºC. The variable resistor might be 0 to 10
k ohm. It enables the sensitivity of the arrangement to be altered.
However, if the variable resistor was set to zero resistance then, without a
protective resistor, we could possibly have a large current passed through the
thermistor. The protective resistor is there to prevent this occurring. The
maximum power that the thermistor can withstand is specified as 250 mW.
Thus, with a 6 V supply, the variable resistor set to zero resistance, the
protective resistance R, and the thermistor at 100°C, the current I through
the thermistor is given by V = IR as 6 = I(0 + R + 330), and so:
I= 6 / (R+330)
The power dissipated by the thermistor I ^2 x 330 and so if we want this to
be significantly below the maximum possible, say 100 mW, then we have:
0.100 = ( ( 6 / ( R+ 330) )^2 ) x 330
Hence R needs to be about 15 Q.
M.Sc. course, Measurement & Control Instrumentations, Dr. Laith A. Mohammed
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Display: When the temperature of the thermistor is OºC its resistance is
15.28 k ohm. If we set the variable resistor as, say, 5 k ohm and the
protective resistor as 15 Kk ohm then the voltage output when the supply is
6 V is:
Output voltage = ( 5.015 / (15.28 + 5.015)) x 6 = 1.48 V
When the temperature rises to 100°C the output voltage becomes:
Output voltage = ( 5.015 / (0.33 + 5.015)) x 6 = 5.63 V
Thus, over the required temperature range, the voltage output varies from
1.48 V to 5.63 V. A voltmeter to cover this range could be used to display
the output.
An absolute pressure measurement
Requirement: Measurement of the manifold absolute pressure in a car
engine as part of the electronic control of engine power.
Sensor: A sensor that is used for such a purpose is a diaphragm pressure
gauge fig.a . The diaphragm is made of silicon with the strain gauges
diffused directly into its surface. Four strain gauges are used and so arranged
that when two are in tension the other two are in compression fig.b.
M.Sc. course, Measurement & Control Instrumentations, Dr. Laith A. Mohammed
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Signal processing: The four gauges are so connected as to form the arms of
a Wheatstone bridge (Figure (c)). This gives temperature compensation
since a change in temperature affects all the gauges equally. Thus the output
from the sensor with its signal processing is a voltage which is a measure of
the pressure.
Display: If required, the output voltage could be displayed on a meter,
possibly following some amplification.
M.Sc. course, Measurement & Control Instrumentations, Dr. Laith A. Mohammed
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Detection of the angular position of a shaft
Requirement: Detection of the angular position of the throttle shaft of a car
to give an indication of the throttle opening, and hence the driver's power
demand on the engine, as part of a car engine management system.
Sensor: A rotary potentiometer (Figure below) is generally used with the
potentiometer wiper being rotated over the potentiometer track.
Signal processing: For a 5 V d.c. voltage connected across the
potentiometer, with the throttle closed and the engine idling the wiper can be
at a position close to the 0 V terminal and so give a small voltage output,
typically about 0.5 V. As the throttle is opened, the shaft rotates and the
wiper moves over the track so that at wide-open throttle the wiper is nearly
at the end of its track and the output voltage has risen to about 4.3 V. The
engine management system uses an operational amplifier to compare the
output from the potentiometer with a fixed voltage of 0.5 V so that the opamp gives a high output when the potentiometer output is 0.5 V or lower and
a low output when higher. This high-low signal, together with signals from
other sensors, is fed to a microprocessor which then can give an output to
control the engine idle speed.
M.Sc. course, Measurement & Control Instrumentations, Dr. Laith A. Mohammed
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Fluid level monitoring
Requirement: Monitoring the level of a liquid to indicate when the level
falls below some critical value.
Sensor: One method would be to use a magnetic float (Figure below) which
rises with the liquid level and opens a reed switch when the level falls too
low.
Signal conditioning: The reed switch is in series with a 39 ohm resistor so
that this is switched in parallel with a 1 k ohm resistor by the action of the
reed switch. Opening the reed switch thus increases the resistance from
about 37 ohm to 1 k ohm. Such a resistance change can be further
transformed by signal conditioning to give suitable light on-off signals.
Dimension checking
Requirement: A method by which the dimensions of components can be
checked.
Sensor: LVDT displacement sensors can be used.
Signal processing: The e.m.f induced in a secondary coil by a changing
current I in the primary coil is given by e = M di/dt, where M is the mutual
inductance, its value depending on the number of turns on the coils and the
M.Sc. course, Measurement & Control Instrumentations, Dr. Laith A. Mohammed
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magnetic linkage between the coils, thus the material in the core of the coils.
Thus, for a sinusoidal input current to the primary coil of an LVDT,
alternating e.m.f s are induced in the two secondary coils A and B. The two
outputs are in series so that their difference is the output. Figure below
shows how the size and phase of the alternating output changes with the
displacement of the core.
The same amplitude output voltage is produced for two different
displacements. To give an output voltage which distinguishes between these
two situations, a phase sensitive demodulator, with a low pass filter, is used
to convert the output into a d.c. voltage which gives a unique value for each
displacement, fig. below.
M.Sc. course, Measurement & Control Instrumentations, Dr. Laith A. Mohammed
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The coils with simple LVDT sensors and parallel-sided coils exhibit nonlinearities as the ferrite core approaches the ends of the coils. This can be
corrected for by using stepped windings or, more cheaply, by using a
microprocessor system and programming it to compensate for such nonlinearities. Such a microprocessor system could also be used to receive the
inputs from a number of such LVDT sensors and compare the outputs with
the required dimensions of the components and give outputs indicating
divergence.
Measurement of relative humidity
Requirement: Direct measurement of relative humidity without the need for
using the operator to use tables to convert temperature values to relative
humidity. The traditional method of measuring relative humidity involves
two thermometers, one with its bulb directly exposed to the air and giving
the 'dry temperature' and the other with its bulb covered with muslin which
dips into water. The rate of evaporation from the wet muslin depends on the
amount of water vapour present in the air; when the air is far from being
saturated then the water evaporates quickly, when saturated there is not net
evaporation. This rate of evaporation affects the temperature indicated by the
thermometer, so giving the 'wet temperature'. Tables are then used to convert
these readings into the humidity.
Sensor: Rather than use a 'wet' thermometer element, a capacitive humidity
sensor can be used. The sensor (Figure (a)) consists of an aluminum
substrate with its top surface oxidized to form a porous layer of aluminum
oxide. On top of the oxide a very thin Gold layer is deposited, this being
permeable to water vapour. Electrical connections are made to the gold layer
and the aluminum substrate, the arrangement being a capacitor with an
aluminum oxide dielectric. Water vapour enters the pores of the aluminium
oxide and changes its dielectric constant and hence the capacitance of the
capacitor. The capacitance thus gives a measure of the amount of water
vapour present in the air.
M.Sc. course, Measurement & Control Instrumentations, Dr. Laith A. Mohammed
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Signal processing: Figure (b) shows the type of system that might be used
with such a sensor. For the capacitive sensor, signal conditioning is used to
transform the change in capacitance to a suitable size voltage signal. A
temperature sensor is also required since the maximum amount of water
vapour that air can hold depends on the temperature and thus to compute the
humidity the microprocessor needs to know the temperature, this also
requiring signal conditioning to get a signal of the right size. An ADC is
then used to convert the signals to digital for processing by a microprocessor
system; a microcontroller is likely to be used with an integrated ADC,
microprocessor and memory on a single chip, there then being a number of
input connections for analogue signals to the system. The microprocessor
takes the values for the two inputs and can use a 'look-up' table in its
memory to determine the value of the relative humidity. This is then
outputted to a digital meter.
M.Sc. course, Measurement & Control Instrumentations, Dr. Laith A. Mohammed
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