Usually, you want to find a way, how to transfer some physical
quantity into voltage, to display it on the digital device (or just gain
digital data).
Take measuring resistivity. You can easily
transfer information about resistivity to voltage and there is lots of
ways how to impact resistivity. If you put a conductor in to the place
with high temperature, its resistivity will change, so you can measure
the temperature.
If you put pressure on the conductor, it will change resistivity, so you can measure weight.
This
is just simple look of it. Now what we actually use to measure
temperature. There is an effect called Seebeck effect. If there is a
conductor heated only on the one side, the differences of temperature
along the conductor will cause a voltage between ends of the conductor.
What value the voltage will have depends not only of the temperature,
but also on the material of the conductor. Now take two conductors made
from different materials, connect them on one side and heat them there.
On the non-connected side you get voltage depended on the chosen
materials and temperature.
Now take another effect. For example
piezoelectric. By applying some kind of force, that will cause a
deformation of the Krystal, the voltage will be produced. So this effect
is used for example for measuring acceleration or pressures.
What
about metal detectors? They contains two coils. One of them (coil 1) is
under the AC voltage and generate magnetic field. If there is no metal
object, nothing happens. But if there is, the magnetic field from the
coil 1 will cause indication of the "Foucault currents" and the metal
object will produce weak magnetic field on its own, which will sense the
other coil in the detector.
This is just what came to my mind
right now. There is lots and lots more. It's always using one or more
physical quantities to transform them to something readable.
Sensors
(a device) work in a way by responding to stimuli or an input quality
(generally physical parameters) by generating processable outputs.
After
receiving signals from a sensor, these signals need to be processed.
The conversion of energy from one form to another is an essential
characteristic that governs the sensing process. For instance, if the
sensor output is voltage, we utilize analogue-to-digital and sample and
hold circuits, as well as a circuit that transfers the digits into the
micro-controller or computer. If a sensor produces a time varying signal
where the information is embedded in its frequency signatures, then a
frequency counter and possibly a frequency analyzer are needed. If
output of the sensor is a change in colour then a visible spectrometer
is necessary.
Sample Capacitive Sensor example: Capacitive devices
are extensively used as fluid gauge, humidity, pressure, and
non-contact sensors.
Q=CV and C = k.q/V* = kC*
If we want to
design a capacitive sensor, we need to make a “bad” capacitor, whose
value varies with temperature, or humidity, or pressure, or whatever we
need to sense. By allowing a capacitor’s parameter to vary selectively
with a specific stimulus, we can build a useful sensor.
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