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sensors and actuators exam answer
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Chapter 1. Introduction1) Why would you want to make sensors in silicon?(Explain the advantages and
disadvantages of silicon sensors.)(Why would you want to make sensors in
silicon? What are the disadvantages?)(What are the properties of silicon that
make it interesting for sensor applications? Which properties are less
attractive?)(Silicon is an excellent material for sensors. Describe three
advantages. In what environmental circumstances would you not choose silicon,
or silicon is no longer advantageous?)
Advantage: Silicon sensors make it possible to combine sensors and integrated
circuits on one chip. Excellent performance to price ratio. It makes it possible to
apply the highly-developed and sophisticated batch-production methods of
integrated circuits to the transducer field. Many physical effects occur in silicon,
high availability, low costs, high efficiency,thermally stable up to 1100 degrees
celsius.
Disadvantage: silicon is not piezoelectric because of its symmetrical lattice
structure and it is also not magnetic. This eliminates silicon as a self-generating
transducer in the magnetic and mechanical domain. Can be tackled with extra
layer. It is also self-heating, Offset, Non-linearity, cross-
sensitivity, brittle, short life cycle, drift, offset, self heating, improper frequency
behavior, time dependence of offset and sensitivity, no piezoelectric effect, not
magnetic.
2) Except for the magnetic domain people have sensors for all signal domains.
Give for each domain an example of a sensor in our bodies, and also a silicon
sensor (sensors) which in principle take over the task.
1. Radiation: Eyes – photodiode
2. Mechanical: Pressure sensor (ear) – Capacitive pressure sensor
,3. Thermal: Temperature sensor (skin) – Thermocouple
4. Electrical: Nerve cells – pn junction/transistor
5. Magnetic: n.a. – hall plate
6. Chemical: pH sensor in bloodstream / nose/tongue – electronic nose /tinoxide
gas sensor?
1) It is not always practical to make a smart sensor, why? (What is a “smart-
sensor” and what are the advantages and disadvantages?)(What is a smart
sensor? Describe 4 advantages and 4 disadvantages.)
(By combining sensor and part of signal-processing circuits on one single chip
can be called a “smart sensor'')
Pro’s: low costs, proven technology, short development time, single package,
bus-compatible output,they can store multiple profiles and also the data of many
systems. Also, lower maintenance,reduced down time, higher reliability, fault
tolerant systems,less complex system architecture, lower weight, adaptability
for self-calibration and compensation.
Con’s: High cost (small series), close proximity of sensor and circuitry might
lead to unexpected feedback loops. Compatibility problems: during fabrication
complications with processing steps,Lower yield: due to complex fabrication
more chance of things going wrong.Also, no smart sensors can be designed
that contain a piezoelectric element and signal conditioning circuits on the same
chip.
2) Give six signal formats that a sensors can produce. Which formats are most
desirable ?(Which are the most desirable for most users and why?)(Sensors can
convert the measurand in to a range of electrical changes. Give 4 of these, with
examples. What is the most desirable output form for the user and why?)( A
sensor can generate many different forms of output (e.g. resistance change).
,Describe four examples. Which output form is most desirable for the user (not
necessarily one of the formats you describe)?)
Analog Voltage
Analog Current
Capacitance change
Resistance change
Frequency
Digital
Pulse duration
Pulse count
Binary output
Change in resistance: piezoresistive effect
Change in voltage: piezoelectric effect
Change in dielectric constant: photodielectric effect. Dielectric constant is also
known as relative permittivity. Relative permittivity of a material is its absolute
permittivity relative to the vacuum permittivity (expressed in a ratio).
Permittivity: a material with high permittivity polarizes more in response to an
applied electric field than a material with low permittivity, thereby storing more
energy in the electric field.
Capacitance change: photoconductive effect
Frequency output: a voltage controlled oscillator. Using a switch to charge and
discharge a variable capacitor. This gives a triangular type wave, which can be
converted into a square wave using a comparator.
As an example, an electronic thermometer , in the input transducer thermal
energy is converted into electrical energy by means, for instance, of the Seebeck
effect.
Sensors yield a wide range of outputs, whereas the user prefers a standard
output, such as digital or frequency ,binary (in terms of noise). Using signal
processing these output formats can be converted into a digital signal. In an
application such as automotive, all sensor units can have a standard output
which is transmitted to the central unit via a bus.
, 2) What are the advantages of self-generating sensors?
It does not require an external power source because it gets the power from the
measured signal. Good examples of this category are the solar cell based on the
photo-voltaic effect and the thermocouple based on the Seebeck effect. Self-
generating sensors do not require an auxiliary energy source; all the electrical
energy at the output of the sensor is delivered only by the input signal.
Therefore, when the input signal is zero, the output signal also has this value;
the input-output relationship is a line that goes through the origin. A solar cell
does not show an output voltage in the dark, and a thermocouple only generates
an output voltage when there is a temperature difference between the ends.
Therefore, when the signal-to noise ratio is satisfactory, in order.
1) Why is the Hall effect a modulating effect? Give examples for 4 other
domains examples of modulating sensors are in silicon.
If an electric current flows through a conductor in a magnetic
field(perpendicular to current), the magnetic field exerts a transverse force on
the moving charge carriers which tends to push them to one side of the
conductor. 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. The presence of this measurable transverse voltage is called the
Hall effect.
When a current flows through a conductor that is placed in a magnetic field an
electric field will be created. A power supply is needed to create the current thus
modulating.
1. piezo resistivity
2. photoelectric effect
3. magneto resistance
4. electrolytic conduction
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