ST08 – Resistive based sensors and interfacing 1 Resistive based sensors and interfacing Lecturer: Smilen Dimitrov Sensors Technology – MED4

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ST08 – Resistive based sensors and interfacing

Resistive based sensors and interfacing

Lecturer:Smilen Dimitrov

Sensors Technology – MED4

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Introduction

• The model that we introduced for ST

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Introduction

• 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

• Now, ready to look into actual sensor circuits

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Basic sensing principles

• A brief overview of basic sensing principles from a microscopic perspective, before we start with circuit-theory level– Not an all-inclusive list…

• Sensing light

• Sensing temperature

• 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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Sensing light

• 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:

• 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

• 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.

tNq

m

2

2S

LR ),( LL fIfN ),( LL fIfR

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Sensing temperature

• Basis – phonons – increased motion (vibration) of ions in a crystal lattice

• Increases effective area of ions, and probability of collision witha free moving electronsinfluences the average speed of electrons – which again influences resistivity; for all resistors

• Thermistors: PTC/NTC

• 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

• 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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Sensing pressure / force

• 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.

• 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).

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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Basic sensing principles

• 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).

• … 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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Resistive based sensors

• Resistive based sensors have resistance as the electric parameter functionally dependent on the measured physical parameter (light, temperature, pressure..)

• 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:

• And thus a voltage, that changes ultimately because of the change of the measured physical parameter:

• … 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.

PfRsens

PfU sens

senssens RfU

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Interfacing: Voltage divider

• For a wide class of sensors – photoresistors, force sensing resistors (FSRs)..

• What resistance to choose for the fixed resistor? First guess:

21

22 RR

REU

PfPfR

PfV

RR

RVV uiio

121

2

2max2min2

1

RRR

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Interfacing: Wheatstone bridge

• Some sensors have too small of a change in resistance – for them Wheatstone bridge must be used:

• Can’t be used directly with a DAQ – a differential amplifier is needed first..

I1

I2

I3

2

1

2

2

RR

REU o

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How do we view the data acquisition system

• Any sensing circuit can be seen as a variable voltage source,closing a circuit with an “analog in”

• Any analog input can be seen as a very big equivalent resistance

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Switch (push button [SPST] & toggle [SPDT])

• 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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Switch (push button [SPST] & toggle [SPDT])

• Availability: micro-switches (SPST), and regular ones (SPST and SPDT):

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ST08 – Resistive based sensors and interfacingSwitch (push button [SPST] & toggle [SPDT]) - interfacing

• If switch is OFF: If ON: Vo = Vcc

• Beware of possible short circuits when using (for example: switch in parallel with resistor):

0011 RIRVo

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Resistive switch ladder

• One switch – one analog input; with this method, can interface more – however, can only detect one switch at a time

• Analysis: calculate voltage divider for each state of each switch: – All off:

– S1 on:

– S2 on:

ERRR

RVo

321

1

EVo

ERR

RVo

21

1

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Potentiometer (slider/fader & rotary knob)

• Three-terminal element, made of single chunk of resistive material• Linear displacement – slider/fader, rotary displacement – rotary

knob (trimmer or potentiometer)

• Resistance seen between an end terminal and wiper: • Thus, a voltage divider is formed between these two resistances

S

lR 1

1 S

lR 2

2

ERR

RVo

21

221 lll 21 RRR

ElfERS

l

ER

RVo 1

1

2

Can be used as two-terminal “variable” resistors

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• Availability: trimmers, potentiometers (rotary), faders (linear)

Potentiometer (slider/fader & rotary knob)

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Photosensitive (light dependent) resistor [LDR]

• A two terminal electronic element, which reacts to changes of light intensity I, falling on it, by changing its resistance

• Interfacing – through voltage divider – to obtain voltage dependent on measured light intensity:

)(IfRLDR

ERR

RV

LDR

LDRo

E

IfR

IfVo

)(

)(

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Force sensitive resistor [FSR]

• A two terminal electronic element, which reacts to changes of force (pressure) on its surface by changing its resistance

• Interfacing – through voltage divider – to obtain voltage dependent on measured light intensity:

)(FfRFSR

ERR

RV

FSR

FSRo

E

FfR

FfVo

)(

)(

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Force sensitive resistor [FSR]

• Availability: FSR and bend sensor

• Also strain gauge (Wheatstone !)

Documents

ST08 – Resistive based sensors and interfacing 1 Resistive based sensors and interfacing Lecturer: Smilen Dimitrov Sensors Technology – MED4