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Sensing Systems and Signal Processing
Dr Richard
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Acceleration
Acceleration
Where might accelerometers be used?
Acceleration
Accelerometers are used in many applications from measuring vibration, shock, displacement, to velocity, inclination and tilt.
There are a number of different approach’s to sensor design depending on the application area and the forces experienced by the device.
http://oukas.info/?u=Accelerometers
Acceleration
How to measure Acceleration?
We know this is the rate of change of velocity so we could monitor the velocity and obtain it that way.
Newton’s second law of motion also gives us another method
So if we know the mass and can measure the force acting on it we can obtain the acceleration through
Velocity (m/s)
Gradient is Acceleration
Force = Mass x Acceleration
Acceleration
A simple accelerometer could use:
A known mass, a box and a spring
Let’s attach a pen to the mass, if we move the box the mass lags behind the motion due to the spring stretching (F=-kx) with a force due to the acceleration.
The spring then pulls it back into position.
The pen will trace a line on the floor as it does this. This line is a measure of the acceleration. (This is how a seismometer works)
Acceleration
Rather than using a pen, we could us a capacitive sensor to sense the distance of the mass to the box edge and generate an electrical signal.
You could use a piezoelectric crystal to generate a signal, when the mass moves it ‘squashes’ the crystal and generates a voltage.
These all work well but are large devices
The accelerometer in your phone is much smaller.
Capacitor plate
Piezo electric crystal
Acceleration
Microchip accelerometers often referred to as MEMs devices (micro-electro-mechanical systems) allow very small footprint devices for measuring acceleration.
Main sensing is via capacitive sensing of an electrode that is free to move.
https://www.machinedesign.com/motion-control/nist-develops-way-precisely-characterize-mems-accelerometers
Acceleration
An electrode is suspended by a thin cantilever.
The cantilever allows the mass to move when it experiences a force and the electrodes sense its position in space through changes in capacitance.
Acceleration
Accelerometers have many applications and are found in a vast array of devices.
Obvious examples:
vehicle acceleration,
vibration monitoring in cars, machines,
Structural monitoring in buildings
Acceleration
Medical applications
Measure the depth of CPR chest compressions,
fitness watches for running / walking logging.
https://spectrum.ieee.org/the-human-os/biomedical/devices/smart-watch-beats-smartphone-in-lifesaving-cpr
Acceleration
Electronics
For spinning disk HDD free fall sensor parks the heads of the disk if detects fall so the drive isn’t damaged. (laptops) less important these days with SSD
Orientation sensing – which way up is phone / tablet.
Games consoles – input sensors (Wii etc)
Camera – image stabilisation – detect vibration and cancel by moving sensor (or optics) or wait for no vibration before exposing.
https://www.embedded.com/design/system-integration/4028129/Accelerometers-and-free-fall-detection-protects-data-and-drives
What is piezoelectricity?
Atomic structure has balanced charges
Pulling on the structure displaces the charges
But the average charge location is unchanged
This material doesn’t have an piezo electric effect
Applies across the entire crystal structure.
Different materials have different arrangements and therefore different outcomes are possible..
Average charge location doesn’t change with lattice distortion
For this atomic arrangement distorting the lattice causes a change in the mean location of the charges
This produces a potential difference.
For piezo electric materials this effect can be used for sensing
The opposite effect can also be used – applying a voltage to a piezoelectric material causes the material to change dimensions which can be used to interact with other materials (e.g. in an ultrasonic transducer)
Average charge location changes with lattice distortion
Piezoelectricity
In the large scale the material must be a single crystal – this is not the case for most piezo materials.
The poly crystalline material needs to be processed so that it becomes more like a single crystal.
This is done via ‘poling’ – applying a strong electric field and heating the material so that the domains within the sample become aligned – leaving a strong piezo electric effect
This process is called ‘polarisation’
A material can become depolarised if exposed to extreme electric fields, mechanical stress or temperatures – so the materials have limitations on their use.
Piezoelectricity
Random domains
Strong DC Field + Heat
Aligned domains
D = electric displacement [C/m2]
d = piezoelectric coefficient [C/N]
T = stress [N/m2]
= permittivity [F/m]
E = Electric field [V/m]
Piezoelectricity
If we stress the material we develop charge at the surface proportional to the stress, the size depends on the piezoelectric coefficient of the material (which depends on many things)
The reverse is also true: applying a voltage to the device induces a stress which changes the dimensions of the device so can be used to induce vibrations.
Applications:
Buzzer – ac voltage applied cause it to vibrate at the frequency
Nano positioning – carefully controlling the voltage causes the piezo element to expand or contract giving very precise control over the position – used for nanometric positioning in scientific instruments.
Electric lighter – press the button – get a spark – how electric cooker lighters work
Energy harvesting – when impacted energy generated and harvested.
Ultrasound transducers (NDE) – more advanced version of buzzers above – convert electrical signals into vibrations
Piezoelectricity
http://www3.gehealthcare.com.au/en-au/products/categories/ultrasound/ultrasound_probes
http://www.nanopositioning.net/XYZ_nanopositioning_stage.php
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