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Inductosyn Position-dependent mutual inductance (and thus signal

transfer) between two meander-like flat coils

Excitation:

current into the fixed scale, OR

current into the moving slider

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Inductosyn driven slider

* One rotor (slide) driven by current

tKUtutU == coscos)(cos 2

* Two rotors driven

)cos()cossincos(cos)(u)(u)(u 22212 =+=+= tKUttKUttt

== cos)2cos(2 KUpxKUU

Coarse scale: incremental, fine scale : measurement of phase

Mutual inductance

M ~cos

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Inductosyn - driven stator (scale)

sincos)(usinsin)(u 2221 tKUttKUt ==

tKUtKUtu == sin)sin(sin)sincoscos(sin)(3

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stator

rotor

stator

rotor

a) b)

Resolver Selsyn

Scott transformer (3 phase / 2 phase)

Conversion of selsyn signal to resolver form

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Sensors based on eddy currents

the depth of field penetration (attenuation to 1/e)

G

Zm

y

i~

a) b)

2

=

Difficult field penetration to conductors

(low resistivity => high eddy currents)

Used for detecting presence of conductive targets

(proximity switch)

!!!

Eddy currents in

the material

compensate the

external field

(Lenz law)

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Sensors based on eddy currents -construction

field concentration (focusing) :ferrite core, ev. magnetic shielding

The sensor in typical

threaded-cylinder shape

Metallic target

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Sensors based on eddy currents :

signal conditioning circuits

Bridge and transformer circuits (compensating sensor) resonant circuits LC-oscillator: f, Q

pulse driven - defectoscopy

low f: change of Re(Z)

high f: change of L

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Sensors based on eddy currents : applications

sensors of translational motion

binary sensors of position (proximity switch)

detection of vehicles (or any conducting objects - mines, cable,pipelines)

diagnostics

cracks

material composition

noncontacting

operation in presence of dirtiness

target conductive

for d > independent on target parameters

u2

um

u1

us

is

iw

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~

~

1

2

e)

~

d)

~ 1

2

b)

~

k)

~

h)

~

a)

~

g)

~

j)

~

l

c)

~

f)

~

i)

l

~

l)

Sensors based on eddy currents : applications

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Magnetostictive sensors of position

max. length up to 4 m (attenuation)

hysteresis 0.4 m linearity 0.02 %

elastic wave in ferromagnetic material .... v = 3000 m/s = 3m / ns (approx. 10x speed of sound in air)

Interaction of magnetic fields (current pulse + permanent magnet) creates pulse of mechanical strain

(Wiedemann effect ) propagating along the wire. Time of flight => position of permanent magnet

S N S N S NSN SN SN

Inductionpickup coil

Magnetostrictivewire

Magnet inmovablefloat

InnerTube

Outerguide tube Reflectionterminator

Strain pulse

Strain pulse Strainpulse

reflectedoff bottom

Inductionpickupsensesinitial andreflectedstrainpulses

A B C

Patriot

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Capacitive sensorsd

SC

=

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Capacitive sensors contdd

SC

=

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Capacitive sensor with variable

area of electrodes overlapping

-

1323

1323

CC

CC

+

dSC =

a)

b)

13

2

x

+xx

C C13 23+C13

C23

ratiometric measurement: influence ofd, eliminated

C i i i h i bl

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Capacitive sensor with variable

area of electrodes overlapping

2313

2313

V23213

21

232131

232131232131

u)()(

U,if

)()()(U)(U0)(U)(U

CC

CC

UCUuCUu

UUU

CUuCUuCjCjCjjCjj

VV

VV

+

=+=

==

===+

resolution: 1 m, uncertainty 5 m

a)

b)

13

2

x

x

x

x

+xx

C C13 23+C13

C13

C23

C23

1

2

u1

u ; u1 2

U1

U1

U1

u2

U2U2

U2

u3uv

uv

uv

3

P1

P2

Reg.

c)

td)

1 2

3

3

1 2

e)

f)

S

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Modern signal conditioning circuits for

capacitive sensors

Main problem -

influence of capacitance of leads (cable)

(driven from voltage source, current measured by ideal

ammeter )

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charge pump

realisation by CMOS technology and inductive dividers

coils and transformers are not necessary

C/f converter

ADC not necessary

converter C/U capacitor in feedback eliminates dependence on frequency

transformer bridges

expensive, noncompatible with IC

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+

Cp2

C1

Cp1 Cs

G

U (j )1

U (j )2 U(j )

-A

Linearity even for variable air gap sensor (vibration measurement),U2 ~ d

Amplifier for capacitive sensors:

Parasitic capacitances of the cable to Cs will not apply:

Cp1 is on virtual zero, Cp2 is on low output impedance of the OpAmp

Applications of capacitive sensors

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Applications of capacitive sensors

Honeywell

Omeg

Sensing humidity of

material in dryer

Sensing level of liquid dye in

printworks

Bottle

Conveyor

belt

Control of filling

drums

reservoir

Checking presence of parts

in product completionControl of filling

Typical applications:- sensing level in tanks

- checking filling of products

inside packages- sensing level of powder /

granules in storage

- sensing non-metalic objects

on conveyor belts

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Hone well

rubber gasket

sensor

Checking presence of products in mass production:

sensor

metal object