Sensors Slide 07B - Systems and Computer Engineering - · PDF file · 2017-11-20Sensors Slide 07B.8 Strain Gauge Examples Bonded: (more typical) bonded to device with adhesive (superglue)

Copyright © by A. Adler, 2009-2017 (including Material from J.G. Webster)

Sensors

Slide 07B.1Sensors

Sensor: device which detects changes in quantities of interest and provides a

readable output

ExamplesThermocouple converts temperature to voltage.Mercury thermometer converts temperature to a reading on a calibrated glass tube.


Sensors

Slide 07B.2

Instrument Characteristics:Resolution and Sensitivity

Resolution Smallest change

measurable

Sensitivity

= (∆signal)/(∆measurand)

Low sensitivity High sensitivity

measurand

sig

nal

∆measurand

∆si

gn

al


Sensors

Slide 07B.3Linearity and operating range

Many sensors have a linear operating range.

Outside this range we have the maximum operating range (that doesn’t damage the instrument)

50

-20

Thermometer

50

-20

-20 50

Melting

Temperature(input)

Reading on thermometer

(output)

Maximum Range

LinearRange


Sensors

Slide 07B.4Sensors

Sensor Types: Displacement Sensors: measure movement

Resistive Inductive Capacitive Piezoelectric

Temperature Measurement Thermistors Thermocouples


Sensors

Slide 07B.5Potentiometers

Resistive ElementWiper

Fixed ContactsFixed Contacts

Image source:WikipediaConstruction

Wire wound Carbon film Ceramic Conducting plastic


Sensors

Slide 07B.6Question:

If we apply 10V across a single turn potentiometer with 50 wire turns covering 360º.

What is sensitivity (in volts/degree)?

What is resolution?


Sensors

Slide 07B.7Strain Gauges

Strain gauge measures strain (deformation) by a change in resistance.

Measurement circuits typically use “Wheatstone bridge” – describe next section

Connecting Pads

Se

ns

itiv

e D

ire

ctio

n Gauge Factor: measure of gauge sensitivity

GF = ( ∆R/R ) / strain

R: undeformed resistance∆R: change in R due to strainstrain: fractional change in length (∆L/L)


Sensors

Slide 07B.8Strain Gauge Examples

Bonded: (more typical) bonded to device with adhesive (superglue)

Resistance Wire Foil Helical Wire

Unbonded:mounted freely (ends bounded to device)

Armature

Strain-gage wires

A

DC

B

Diaphragm

Unbonded strain-gage pressure sensor. With increasing pressure, the strain on gage pair B and C is increased, while that on gage pair A and D is decreased


Sensors

Slide 07B.9Strain Gauge Examples

Integrated pressure sensor. Diaphragm deforms if pressures on each side are unequal.

Deforming strain gauges

References gauges

Pressure(measurand)

P. reference(atmosphere)


Sensors

Slide 07B.10Strain Gauge: analysis

Analysis of SGArea

(A)

Length (l)

resistivity (ρ)

Poisson’s Ratio (µ)

For incompressible media µ=0.5. Calculate from Vol = D2L is const

∂ A2A

=−∂ LL

R =LA

∂R =LA

∂

A∂L−

L

A2 ∂ A

∂RR

=∂

∂LL

−∂ AA

∂RR

=∂

12

∂LL

G =∂ R/R∂L /L

=12∂/

∂ L/L


Sensors

Slide 07B.11Strain Gauge: analysis

Gage Factor

DimensionalEffect

Piezoresistive Effect- Metals ≈ 0- Ceramics / Semiconductors

have large effect Examples:- Metals G = 1+2(0.3) = 1.6- n-Si G ≈ 100- p-Si G ≈ -100

(large temperature drift in semis)

G=∂ R/R∂L /L

= 12 ∂/

∂ L/L


Sensors

Slide 07B.12Question

Mercury plethysmograph measures change in leg blood volume after pressure cuff applied (venous occlusion)

µ for Hg is 0.5 Calculate ∆R/R

if blood makes10% increase inleg cross section


Sensors

Slide 07B.13Inductive Sensors

Inductance:

Inductance sensor measures displacement by changes in geometry.

Tend to be non-linear, since geometry to inductance relationship is non-linear

Many applications: metal detectors, proximity detector, traffic light car presence detector

L=n2Gn=number of turnsG=geometry factor=permeability of medium


Sensors

Slide 07B.14Inductive sensor types

c

d

c

c

d

c

d

aa

b

b

d

Self-inductance Mutual inductance


Sensors

Slide 07B.15Inductive sensor: LVDT

Linear Variable Differential Transformer (LVDT)

AC current in primary, causes a voltage induced in each secondary proportional to its mutual inductance with the primary.

As the core moves, these mutual inductances change, causing the voltages induced in the secondaries to change. The coils are connected in reverse series,

When the core at the central position, equal but opposite voltages are induced, so the output voltage is zero.

When the core is displaced in one direction, the voltage in one coil increases as the other decreases. When the core moves in the other direction, its phase is opposite.

Output voltage is proportional to distance (up to its limit of travel)

Because the sliding core does not touch the inside of the tube, it can move with low friction, sealed against the environment

a

be

d

c

Source: Wikipedia


Sensors

Slide 07B.16Capacitive sensors

Capacitive sensors Low cost, small, mechanically strong Quite non-linear, better to indicate contact

Source: Salpavaara, et al,, 2008.


Sensors

Slide 07B.17Capacitive sensors

Electromagnetic analysis

Electronic Circuit

Area (A)

Permittivity of free space

Relative Permittivity

Non-linearSensitivity

xC=0r

Ax

, 0=8.86×10−12 F

m

K=dCdx

=−0 r

A

x2


Sensors

Slide 07B.18Piezo-electric sensors

Piezoelectricity is the ability of some materials to generate an electric potential to applied mechanical stress.

Apply stress → Generate voltage Apply voltage → Generate stress

Piezoelectric materials Crystals (most common is quartz) certain ceramics, including bone Films (polyvinylidenechloride: PVDC)

Disadvantage Because of very high impedance, charge leaks away. Can’t be used to measure static forces


Sensors

Slide 07B.19Piezoelectric sensors

Analysis of sensitivity

Note that most piezosensors are very stiff, so we can assume that x is constant

Example: Calculate V for 10g weight on a 1cm2 area 1mm thick quartz. k= 2.3pC/N, ε

r = 10

Relative Permittivity

q=kf q : charge

V =qC

=kfx

ϵ0 ϵr A

K=dVdf

=kx

ϵ0 ϵr A


Sensors

Slide 07B.20Time constant of Piezosensors

Piezosensors act as high pass filter.

Response:

Time const: τ

+

Equivalent circuit ofPiezo-electric sensor

Force creates a current, which is held on an capacitance

Charge leaks via internal and amplifier leaks

RinternalRamplifierC

=C Rinternal∥Ramplifier


Sensors

Slide 07B.21

High freq. behaviour piezo-electric sensors

High-frequency circuit model for piezoelectric senor.

Rs: sensor leakage resistance

Cs the capacitance.

Lm, Cm, and Rm: mechanical system.

Output voltageInput force

Resonance

Log Frequency

RtCsCm

Lm

Rm

f c

Usablerange

Frequency Response


Sensors

Slide 07B.22Questions

What is the Gauge factor? What kinds of materials have large G? When is this useful?

Why is temperature coefficient (tempco) in a strain gauge a problem? What strategies can be used to help deal with it?

Name some applications for inductive sensors?

What makes the LVDT linear, when most inductive sensors aren't?

Since capacitive sensors are highly non-linear, what kinds of applications are they useful for?

Are piezo-electric sensors linear? What are some advantages and disadvantages?

Sensors

Slide 07B.23Temperature Measurement

Why measure temperature Body is a heat engine. We burn food + oxygen to get

energy for life. Temperature monitors the functioningof the engine

Temperature increase – hyperthermia– typical cause: infection

Temperature decrease – hypothermia – typical cause: shock

Instruments Thermistors Thermocouples Radiation (hot objects emit IR radiation – not included)

Sensors

Slide 07B.24Thermistors

thermistor is a type of resistor with resistance varying according to its temperature.

thermal and resistor = thermistor Many applications for thermistors: current-

sensing, thermal protectors, self-regulating heaters.

Biomedical applications: thermometers, flow sensing, breathing (nasal thermistor)

All resistors have some temperature variation. Thermisors have large tempco (%change/ºC)

material is generally a ceramic or polymer

Sensors

Slide 07B.25Thermistor: circuit

As temperature increases, the thermistor resistance decreases, yielding more current that flows through Rf, thus Vo increases.

Many different sizes: Small Thermistors are more fragile,faster (2s) Larger Thermistors respond slowly (10s)

Offset

Thermistor

+V

–VV

0

Rf

+

–

Sensors

Slide 07B.26Thermistors

1B. (5 points) A thermistor, RT is used in the circuit below. At 35◦C, RT = 100Ω and at 36 ◦C,RT = 101Ω. What is VO for at 35◦C and 36◦C?

1C. (5 points) What is the sensitivity of at the output of the sensor, VO , in V/◦C over the rangefrom 35◦C to 36◦C?

−

+

100+10V

RT

−10V

10 kVO

+10V

−10V

The circuit is a summing inverting amplifier, whose output we can therefore write as

VO = −10 k10V

100 −10VRT

At 35◦C, RT = 100Ω and so VO = 0V, while at 36◦C, RT = 101Ω and so

VO = −10 kΩ100 Ω

1−1

1.01(10V) =−9.90V

from which the sensitivity is seen to be −9.90 V/◦C.

Midterm#2 2016

Sensors

Slide 07B.27

Typical thermistor zero-power resistance ratio-temperature characteristics for various materials.

Linear model:

ΔR = kΔT

where ΔR = change in resistance ΔT = change in temperature k = first-order temperature coefficient of resistance

Linear model only works over small range

Thermistor: sensitivity model

Sensors

Slide 07B.28Thermocouples

Based on Seebeck effect: when a conductor (such as a metal) is subjected to a thermal gradient, it will generate a voltage.

Thermocouples measure the temperature difference, not absolute temperature.

Traditionally, one of the junctions—the cold junction—was maintained at a known (reference) temperature, while the other end was attached to a probe.

Thermocouples are faster, smaller, more robust, more linear than thermistors

Source: Omega.com

Sensors

Slide 07B.29Thermocouples: Usage

−

+

The hot junction is at the thermocouple. The LT1025 electronic cold junction compensates for ambient temperature changes. The noninverting amplifier provides a high input impedance and high gain.

Type K (chromel–alumel) commonly used general purpose thermocouple. Inexpensive. Available in the −200°C to +1350°C range. Sensitivity ≈ 41 µV/°C.

Ri

Rf

COffset C

LT1025

Thermocouple

– +

V0

±V

+V

Sensors


Thermocouple or thermistor? Cheap Mechanically strong Simplest electrical circuit Capable of high temperatures Fastest response

Which has the highest sensitivity (V/ºC) at 50ºC Type K thermocouple Thermistor with constant current of 1mA. R0= 1kΩ. Β=

4000K.

Sensors


How does a thermistor differ from a thermocouple? Which is more linear? Which is less brittle? Which can have the fastest response?

What would you build the temperature cut-off switch in a computer from?

Why does a thermocouple need a reference circuit?

What strategies are used to help reduce drift in radiation thermal detectors?

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Sensors Slide 07B - Systems and Computer Engineering - · PDF file · 2017-11-20Sensors Slide 07B.8 Strain Gauge Examples Bonded: (more typical) bonded to device with adhesive (superglue)