Download ST08 – Resistive based sensors and interfacing

Sensors Technology
– MED4
ST08 – Resistive
based sensors and interfacing
Resistive based sensors and interfacing
Lecturer:
Smilen Dimitrov
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ST08 – Resistive based sensors and interfacing
Introduction
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The model that we introduced for ST
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ST08 – Resistive based sensors and interfacing
Introduction
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We have discussed
– The units of voltage, current and resistance, from both a microscopic
and macroscopic (electric circuits) perspective
– The definition of an elementary electric circuit
– Ohm’s law and Kirschoff Laws
– Solving and measurement of voltage divider circuit
– Solving of more complicated circuits
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Now, ready to look into actual sensor circuits
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ST08 – Resistive based sensors and interfacing
Basic sensing principles
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A brief overview of basic sensing principles from a microscopic perspective,
before we start with circuit-theory level
– Not an all-inclusive list…
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Sensing light
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Sensing temperature
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Sensing pressure/force
– Electronic sensing principles – not all sensors are necessarily built on
them; some sensors can be a mix of mechanical and electronic system
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ST08 – Resistive based sensors and interfacing
Sensing light
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Basis in photoelectric effect – a valence electron can gain enough energy
from a light wave/particle to leave the atom, and become a free electron. In
vacuum bulbs:
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In materials (conductive or semconductive) - this increases number of free
electrons per unit volume, which directly influences what we call resistivity ρ
– which influences resistance R
N  f ( IL, fL )
•
•

2m
q 2  N  t
R
L
S
R  f ( IL, fL )
Photoresistivity (or alternatively, photoconductivity) – dependant on
frequency and intensity of light
When we sense light, we can obtain resistance [in photoresistive materials],
as the electric parameter functionally dependent on light.
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ST08 – Resistive based sensors and interfacing
Sensing temperature
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•
Basis – phonons – increased motion (vibration) of ions in a crystal lattice
Increases effective area of ions,
and probability of collision with
a free moving electrons
influences the average speed of electrons – which
again influences resistivity; for all resistors
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Thermistors: PTC/NTC
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Temperature also influences possibility for increasing number of free
electrons
– Thermocouple - a metallic contact of two different metals, where one
metal is heated – thermoelectric effect
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When we sense temperature, we either obtain resistance [in thermistors], or
voltage [in thermocouples], as the electric parameter functionally dependent
on temperature.
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ST08 – Resistive based sensors and interfacing
Sensing pressure / force
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Forces acting on a material, either try to change its position (translation) or
try to change its volume (scaling) – pressure is simply force averaged over
an area.
Piezoresistivity – resistance is
related to volume (length and
cross-section area) of
conductors; but compression can
also lead to metallic behaviour.
Piezoelectricity – when a pressure is
applied to a polarized crystal, the
resulting mechanical deformation
results in an electrical charge.
Deformation disrupts the orientation of the electrical
dipoles and creates a situation in which the charge
is not completely canceled.
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When we sense force (pressure), we either obtain resistance [in
piezoresistive devices], or voltage [in piezoelectric devices], as the electric
parameter functionally dependent on force (pressure).
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ST08 – Resistive based sensors and interfacing
Basic sensing principles
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So, in sensors (electric sensor materials) it is important:
– Which mechanism gives rise to the sensor effect; which determines:
– Which electrical parameter changes in response to the measured
physical paramater
(or in other words – which is the electric parameter functionally
dependent on the measured physical parameter).
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… which is important, as this determines what kind of a circuit do we need,
in order to obtain a usable signal – voltage – that we can interface with (that
is, that we can sample with DAQ hardware)
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ST08 – Resistive based sensors and interfacing
Resistive based sensors
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Resistive based sensors have resistance as the electric parameter
functionally dependent on the measured physical parameter (light,
temperature, pressure..)
Rsens  f P 
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As circuits, we can use either a voltage divider, or a Wheatstone bridge, in
order to obtain voltage dependent on the changing resistance of such a
sensor:
U sens  f Rsens 
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And thus a voltage, that changes ultimately because of the change of the
measured physical parameter:
U sens  f P 
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… which is what we need in interface with the sensor, using a DAQ system –
and obtain the change of the measured parameter, as a change of data in a
software environment.
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ST08 – Resistive based sensors and interfacing
Interfacing: Voltage divider
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For a wide class of sensors – photoresistors, force sensing resistors (FSRs)..
U2  E
R2
R1  R2
Vo  Vi
•
R2
f P 
 Vi
 f u P 
R1  R2
R1  f P 
What resistance to choose for the fixed resistor? First guess:
R1 
R2 min  R2 max
2
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ST08 – Resistive based sensors and interfacing
Interfacing: Wheatstone bridge
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Some sensors have too small of a change in resistance – for them
Wheatstone bridge must be used:
I1
I2
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I3
 R2
1

Uo  E 
 
 R2  R 2 
Can’t be used directly with a DAQ – a differential amplifier is needed first..
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ST08 – Resistive based sensors and interfacing
How do we view the data acquisition system
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Any sensing circuit can be seen as a variable voltage source,
closing a circuit with an “analog in”
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Any analog input can be seen as a very big equivalent resistance
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ST08 – Resistive based sensors and interfacing
Switch (push button [SPST] & toggle [SPDT])
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Simplest (from an electrical perspective) – resistance changes between 0 and ∞
Based on establishing electric contact between conductor, upon application
of force
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ST08 – Resistive based sensors and interfacing
Switch (push button [SPST] & toggle [SPDT])
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Availability: micro-switches (SPST), and regular ones (SPST and SPDT):
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ST08 – Resistive based sensors and interfacing
Switch (push button [SPST] & toggle [SPDT]) - interfacing
Vo  R1  I  R1  0  0
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If switch is OFF:
If ON: Vo = Vcc
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Beware of possible short circuits when using (for example: switch in parallel
with resistor):
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ST08 – Resistive based sensors and interfacing
Resistive switch ladder
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One switch – one analog input; with this method, can interface more –
however, can only detect one switch at a time
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Analysis: calculate voltage divider for each state of each switch:
R
E
– All off: V 
1
o
– S1 on:
– S2 on:
R1  R2  R3
Vo  E
Vo 
R1
E
R1  R2
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ST08 – Resistive based sensors and interfacing
Potentiometer (slider/fader & rotary knob)
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Three-terminal element, made of single chunk of resistive material
Linear displacement – slider/fader, rotary displacement – rotary knob
(trimmer or potentiometer)
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Resistance seen between an end terminal and wiper: R1   1 R2   2
S
S
Thus, a voltage divider is formed between these two resistances
l
Vo 
R2
E
R1  R2
l  l1  l2
R  R1  R2
l1
R
Vo  2  E  S  E  f l1   E
R
R

l
Can be used as twoterminal “variable”
resistors
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ST08 – Resistive based sensors and interfacing
Potentiometer (slider/fader & rotary knob)
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Availability: trimmers, potentiometers (rotary), faders (linear)
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ST08 – Resistive based sensors and interfacing
Photosensitive (light dependent) resistor [LDR]
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A two terminal electronic element, which reacts to changes of light intensity
I, falling on it, by changing its resistance RLDR  f (I )
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Interfacing – through voltage divider – to obtain voltage dependent on
measured light intensity:
Vo 
RLDR
E
R  RLDR
Vo 
f (I )
E
R  f (I )
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ST08 – Resistive based sensors and interfacing
Force sensitive resistor [FSR]
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A two terminal electronic element, which reacts to changes of force
(pressure) on its surface by changing its resistance RFSR  f (F )
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Interfacing – through voltage divider – to obtain voltage dependent on
measured light intensity:
RFSR
Vo 
E
R  RFSR
Vo 
f (F )
E
R  f (F )
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ST08 – Resistive based sensors and interfacing
Force sensitive resistor [FSR]
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Availability: FSR and bend sensor
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Also strain gauge (Wheatstone !)
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