503E- MKM Week3
Nonconventional Sensors
prepared by Assist. Prof. Pinar Boyraz, PhD
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Outline
Piezoelectric sensing
Close look at an analog accelerometer
LIDAR and IMU
Non-diagonal
terms
represent
smart
materials
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Piezoelectricity-1
An effect in which energy is converted
between mechanical and electrical
forms
Discovered by 1880s by Curies
a pressure (piezo >> pressure in
Greek) is applied to a polarized crystal,
the resulting mechanical deformation
results in an electrical charge.
Example: piezoelectric microphones
Alternatively, when an electrical
charge is applied to a polarized
crystal, the crystal undergoes a
mechanical deformation which can in
turn create an acoustical pressure
Example: piezoelectric speakers
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Piezoelectricity-2
Electrets are solids which have a permanent electrical polarization.
Electrets are electrical analogs of magnets.
The alignment of the internal electric dipoles would result in a
charge which would be observable on the surface of the solid.
In these structures, each cell of the crystal has an electric dipole,
and the cells are oriented such that the electric dipoles are aligned.
Again, this results in excess surface charge which attracts free
charges from the surrounding atmosphere making the crystal
electrically neutral.
If a sufficient force is applied to the piezoelectric crystal, a
deformation will take place. This deformation disrupts the orientation
of the electrical dipoles and creates a situation in which the charge is
not completely canceled.
This results in a temporary excess of surface charge, which
subsequently is manifested as a voltage which is developed across
the crystal.
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Piezoelectricity-3
Piezoelectric linear transducer
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Piezoelectricity-4
In order to utilize this physical principle to make a sensor to measure force,
we must be able to measure the surface charge on the crystal.
This charge results in a voltage ,
where
is the charge resulting from a force f, and C is the capacitance
of the device.
K^2: Permittance
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Analog accelerometer-1
Springs (within their linear region) are governed by a physical principle
known as Hooke's law.
Specifically, F=kx, where k is the constant of proportionality between
displacement (x) and force (F).
The other important physical principle is that of Newton's second law of
motion which states that a force operating on a mass which is accelerated
will exhibit a force with a magnitude F=ma.
Note that this system only responds to accelerations along the length of
the spring.
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Analog accelerometer-2
Micro-machined stand-alone accelerometer which consists of a mass
spring system as well as a system to measure displacement and the
appropriate signal conditioning circuitry.
The mass is a bar of silicon, and the spring system is implemented by the
4 tethers which attach to each corner of the mass. It responds to
accelerations that occur in line with the length of the mass. When an
acceleration occurs, the mass moves with respect to the anchored ends of
the tethers.
Roughly speaking, the amount of acceleration is proportional to the
amount of displacement of the mass. This is not quite true in this case
since the spring system is not an ideal spring as presented earlier. This
fact is compensated for by some sophisticated signal conditioning circuitry
present in the device.
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Analog accelerometer-3
Next problem which needs to be solved is that of measuring the
displacement of the bar. The principle upon which this is based is that of
the property electrical of capacitance. Capacitors are electrical
components which store charge.
The amount of capacitance that a device such as this would exhibit is
exhibit is given by
, where k is a property of the material between the
two plates. Using this, if one knew k and could measure capacitance, they
would be able to determine , the spacing between the plates.
If the device is at rest, and the spacing
between each of the plates is x0 , then
each of the capacitors exhibits a
capacitance of C=k/x0 . If the middle
plate is moved by a distance x, then
this results in:
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Accelerometer-4
A negative feedback control loop to make sure that the movement of the
mass is kept small so that the below expression remains correct
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LIDAR- 1
LIDAR (Light Detection And Ranging) is an optical remote
sensing technology that measures properties of scattered light to
find range and/or other information of a distant target.
It uses laser pulses in the current technology.
LIDAR uses much shorter wavelengths in the electromagnetic
spectrum: ultraviolet, visible and near infrared.
A laser typically has a very narrow beam which allows the
mapping of physical features with very high resolution compared
with radar.
In general there are two kinds of lidar detection schema:
"incoherent" or direct energy detection (which is principally an
amplitude measurement) and Coherent detection (which is best for
doppler, or phase sensitive measurements).
Coherent systems generally use Optical heterodyne detection
which being more sensitive than direct detection allows them to
operate a much lower power but at the expense of more complex
transceiver requirements.
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LIDAR- 2
In both coherent and incoherent LIDAR, there are two types of
pulse models: micropulse lidar systems and high energy
systems. Micropulse systems have developed as a result of the
ever increasing amount of computer power available combined
with advances in laser technology. They are eye-safe.
High-power systems are common in atmospheric research, where
they are widely used for measuring many atmospheric parameters:
the height, layering and densities of clouds, cloud particle
properties .
There are several major components to a LIDAR system:
Laser, scanner, photo-detector and receiver electronics and finally
position and navigation systems (IMU and GPS).
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LIDAR- 3
In both coherent and incoherent LIDAR, there are two types of
pulse models: micropulse lidar systems and high energy
systems. Micropulse systems have developed as a result of the
ever increasing amount of computer power available combined
with advances in laser technology. They are eye-safe.
High-power systems are common in atmospheric research, where
they are widely used for measuring many atmospheric parameters:
the height, layering and densities of clouds, cloud particle
properties .
There are several major components to a LIDAR system:
Laser, scanner, photo-detector and receiver electronics and finally
position and navigation systems (IMU and GPS).
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IMU
An inertial measurement unit, or IMU, is an electronic device
that measures and reports on a craft's velocity, orientation, and
gravitational forces, using a combination of accelerometers and
gyroscopes.
IMUs are typically used to maneuver aircraft, including UAVs,
among many others, and spacecraft, including shuttles, satellites
and landers.
A major disadvantage of using IMUs for navigation is that they
typically suffer from accumulated error. Therefore, it has to be
corrected by GPS.
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