lecture 1
TRANSCRIPT
Exam:
• Oral examn with known questions, but no preparation at the examn. • Grades are given in the 7 scale
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Example of a process control system
The skecth shows the transistion from a process to our processor and the
transistion from the processor to processor to a process.
Example: sound
The sound is sensed by a microphone diaphgram(1), it is converted to an electric
signal (2), which is transmitted through a cable (3). The signal is amplified (4) and
afterwards converted to a digital signal (5) that is lead into a signal processor.
The same goes the other way from processor to speaker (10)
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Example: Capacitiv humidity sensor (Philips H1)
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Example: Capacitive proximity sensor
http://www3.sea.siemens.com/step/pdfs/sensors.pdf
The sensing surface of a capacitive sensor is formed by two concentrically
shaped metal electrodes of an unwound capacitor. When an object nears
the sensing surface it enters the electrostatic field of the electrodes and
changes the capacitance in an oscillator circuit. As a result, the oscillator
begins oscillating. The trigger circuit reads the oscillator’s amplitude and
when it reaches a specific level the output state of the sensor changes. As
the target moves away from the sensor the oscillator’s amplitude
decreases, switching the sensor output back to its original state.
Standard targets are specified
for each capacitive sensor. The
standard target is usually
defined as metal and/or water.
Capacitive sensors depend on
the dielectric constant of the
target. The larger the dielectric
number of a material the easier
it is to detect. The following
graph shows the relationship of
the dielectric constant of a target
and the sensor’s ability to detect
the material based on the rated
sensing distance (Sr).
The following table shows the dielectric constants of some
materials. If, for example, a capacitive sensor has a rated
sensing distance of 10 mm and the target is alcohol, the
effective sensing distance (Sr) is approximately 85% of the
rated distance, or 8.5 mm.
One application for capacitive proximity sensors is level
detection through a barrier. For example, water has a much
higher dielectric than plastic. This gives the sensor the ability to
“see through” the plastic and detect the water.
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Example: Resistance thermometers
Platinum resistance thermometers (PRTs) offer excellent accuracy over a wide temperature range (from -200 to +850 °C).
Standard Sensors are available from many manufacturers with various accuracy specifications and numerous packaging
options to suit most applications. Unlike thermocouples, it is not necessary to use special cables to connect to the sensor.
The principle of operation is to measure the resistance of a platinum element. The most common type (PT100) has a
resistance of 100 ohms at 0 °C and 138.4 ohms at 100 °C. There are also PT1000 sensors that have a resistance of 1000
ohms at 0 °C.
The relationship between temperature and resistance is approximately linear over a small temperature range: for example, if
you assume that it is linear over the 0 to 100 °C range, the error at 50 °C is 0.4 °C. For precision measurement, it is necessary
to linearise the resistance to give an accurate temperature. The most recent definition of the relationship between resistance
and temperature is International Temperature Standard 90 (ITS-90).
http://www.picotech.com/applications/pt100.html
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Example: NTC Thermistor NTC=negative temperature coefficient
Source: EPCOS
Temperature dependence of resistance: The dependence of the resistance on temperature can be approximated by the following equation:
Where:
RT NTC resistance in Ω at temperature T in K
RR NTC resistance in Ω at temperature TR in K
T, TR Temperature in K
B B value, material-specific constant of the NTC thermistor
e Eulers number (~2,7183)
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Example: Strain gauge
The strain gauge is connected into a Wheatstone Bridge circuit
with a combination of four active gauges (full bridge), two gauges
(half bridge), or, less commonly, a single gauge (quarter bridge).
In the half and quarter circuits, the bridge is completed with
precision resistors.
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Principals of different magnetic sensors
When V and ω are constant, it means that the formula is
changed to:
Each time a tooth is approaching and moving away
from the N-pole there is a change in the magnetic flux
in the windings around the magnet, thereby inducing
a voltage across the winding. The voltage amplitude
is proportional to the change in the flux and the
frequency of the voltage signal is proportional to the
rotational speed.
V
The power loss in the metal plate
changes the inductance of the
sensor which can be converted into
a voltage change.
Source: Measurement and Instrumentation principles, Allan S. Morris
Where K is constant. Larger distance means smaller L and thereby larger I
V = voltage, ω = angular frequency, L = inductance , I = current.
Magnetic body
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Microphone
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Dynamic Microphones
A dynamic microphone takes advantage of electromagnet effects.
When a magnet moves past a wire (or coil of wire), the magnet
induces current to flow in the wire. In a dynamic microphone, the
diaphragm moves either a magnet or a coil when sound waves hit the
diaphragm, and the movement creates a small current.
Electret Microphone
Electret microphones are a special form of the
cendenser microphone, which eliminates the need
for a polarizing power supply by using a
permanently charged material´. The preamp circuit
uses an FET in a common source configuration. The
two-terminal electret capsule contains an FET which
must be externally powered by a a bias voltage
(~1.5-9V). The resistor sets the gain and output
impedance.
Condenser Microphones
A condenser microphone is essentially a capacitor,
with one plate of the capacitor moving in response
to sound waves. The movement changes the
capacitance of the capacitor, and these changes are
amplified to create a measurable signal. Condenser
microphones usually need a battery to provide a
(phantom) voltage(~11V-48V) across the capacitor.
MEMS microphones Over the last few years the MEMS(Micro Electro-Mechanical Systems) microphone
has grown to be a big competitor to the electret microphone. This is due to its very small size, compareable audio quality and often build-in ADC, which makes them very useful in devices like cell phones.
The mems microphones are often variants of the condenser microphone design.
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Magnetic proximity sensor
http://www3.sea.siemens.com/step/pdfs/sensors.pdf
Electromagnetic Coil and Metal Target
The sensor incorporates an electromagnetic coil which is used to detect the
presence of a conductive metal object. The sensor will ignore the presence
of an object if it is not metal.
Siemens BERO inductive proximity sensors are operated using
an Eddy Current Killed Oscillator (ECKO) principle. This type of
sensor consists of four elements: coil, oscillator, trigger circuit,
and an output. The oscillator is an inductive capacitive tuned
circuit that creates a radio frequency. The electromagnetic field
produced by the oscillator is emitted from the coil away from
the face of the sensor. The circuit has just enough feedback
from the field to keep the oscillator going.
When a metal target enters the field, eddy currents circulate
within the target. This causes a load on the sensor, decreasing
the amplitude of the electromagnetic field. As the target
approaches the sensor the eddy currents increase, increasing
the load on the oscillator and further decreasing the amplitude
of the field. The trigger circuit monitors the oscillator’s amplitude
and at a predetermined level switches the output state of the
sensor from its normal condition (on or off). As the target moves
away from the sensor, the oscillator’s amplitude increases. At a
predetermined level the trigger switches the output state of the
sensor back to its normal condition (on or off).
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Hall effect
Where K is the Hall constant.
The voltage V is in the horisontal
direction.
Where:
I is the current across the plate length,
B is the magnetic field
n is the density of charge carriers,
e is the electron charge (approx. 1.6 × 10−19 C)
d is the depth (thickness) of the plate
If an electric current flows through a conductor in a
magnetic field, the magnetic field exerts a transverse
force on the moving charge carriers which tends to
push them to one side of the conductor. This is most
evident in a thin flat conductor as illustrated. A buildup
of charge at the sides of the conductors will balance
this magnetic influence, producing a measurable
voltage between the two sides of the conductor.
Note that the direction of the current I in the diagram is
that of conventional current, so that the motion of
electrons is in the opposite direction.
The voltage across the plate can be calculated as:
As the plate doesn’t change, the formula can be
simplified to:
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I3GFV: Optisk fiber
The larger the pressure, the more light
dissaperears in the cladding which means
that less light is detedted.
no is the refractive index
In the air outside the fiber, n1 is
the refractive index in for the
kernal, n2 is the refractive index
for the cladding.
n1 ˃ n2 ˃ no
The refraction at change of the
mediums can be determined by
Snells law:
As long as 2 90, All the light
stay in the kernal.
Air to kernal:
Kernal to cladding:
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Smoke detector
Ionization Detectors: Ionization Chamber
An ionization chamber is very simple. It consists of two
plates with a voltage across them, along with a
radioactive source of ionizing radiation, like this:
The alpha particles generated by the americium have
the following property: They ionize the oxygen and
nitrogen atoms of the air in the chamber. To "ionize"
means to "knock an electron off of." When you knock
an electron off of an atom, you end up with a free
electron (with a negative charge) and an atom missing
one electron (with a positive charge).
The negative electron is attracted to the plate
with a positive voltage, and the positive atom is
attracted to the plate with a negative voltage
(opposites attract, just like with magnets). The
electronics in the smoke detector sense the
small amount of electrical current that these
electrons and ions moving toward the plates
represent.
When smoke enters the ionization chamber, it
disrupts this current -- the smoke particles attach
to the ions and neutralize them. The smoke
detector senses the drop in current between the
plates and sets off the horn.
http://home.howstuffworks.com/smoke3.htm
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