Inductosyn
•
•
•
Position-dependent mutual inductance (and thus signal transfer)
between two meander-like flat coils
Rotary or linear position
Excitation:
current into the fixed scale, OR
current into the moving slider
Farrand controls
displacement
Two sensing coils (sine and cosine) ->
similar to quadrature output in
incremental optical encoders (see later)
Graph: Analog Devices
Inductosyn - driven stator (scale)
u 21 (t ) = KU sin ϕ sin ωt
u 22 (t ) = KU cos ϕ sin ωt
u 3 (t ) = KU (sin ϕ cos α − cos ϕ sin α) sin ωt = KU sin(ϕ − α) sin ωt
Within one electrical/mechanical period: phase φ ~ fine position,
Larger movement: incremental output
Resolver
Selsyn
stator
stator
α
α
rotor
a)
The same signal format
(the same signal processing electronics)
as from inductosyn: e.g. Analog Devices
AD2S1200
Tamagawa SmartSyn
Pancake Resolver
rotor
b)
Scott T-transformer (3 phase / 2 phase)
Conversion of selsyn signal to resolver form
Selsyn as angular position repeater
http://www.allaboutcircuits.com
Sensors based on eddy currents
the depth of field penetration δ (attenuation to 1/e)
!!!
δ =
2ρ
⇒ Difficult field penetration to conductors
ωµ
(x low resistivity => high eddy currents)
⇒ Used for detecting presence of conductive targets
(proximity switch)
G
i~
Eddy currents in
the material tend
to compensate the
external field
(Lenz law)
Zm
y
δ
a)
b)
Sensors based on eddy currents -construction
field concentration (focusing) :
ferrite core, ev. magnetic shielding
The sensor in typical
threaded-cylinder shape
Metallic target
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
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 >δ independent on target parameters
u2
Φ
is
um
u1
us
iw
Sensors based on eddy currents : applications
∅
1
~
∆
~
~
∆
l
2
a)
~
c)
b)
∆1
~
∆2
~
g)
~
j)
h)
~
ρ
~
ρ
µ
i)
~
∆
f)
~
ρ
ρ
~
~
ϑ
e)
d)
k)
l
l)
Magnetostictive sensors of position
elastic wave in ferromagnetic material .... v = 3000 m/s = 3µm / 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
Induction
pickup
senses
initial and
reflected
strain
pulses
Induction
pickup coil
Magnetostrictive
wire
S N
N S
Inner
Tube
Strain pulse
S N
N S
Magnet in
movable
float
Outer
guide tube
A
S N
N S
Strain pulse
Strain
pulse
reflected
off bottom
Reflection
terminator
B
☺ max. length up to 4 m (attenuation)
☺ hysteresis 0.4 µm
☺ linearity 0.02 %
C
Patriot
Capacitive sensors
C=
εS
d
Capacitive sensors – cont’d
C=
εS
d
Capacitive sensor with variable
area of electrodes overlapping
1
3
εS
C=
d
2
x
a)
C13
C13+C23
−x
C23
+x
b)
ratiometric measurement:
C23 - C13
C23 + C13
influence of d,ε eliminated
Capacitive sensor with variable
area of electrodes overlapping
1
3
u1
2
1
x
P1
C13
3
C13
C13+C23
−x
C23
uv
Reg.
u3
a)
U1
U1
uv
b)
c)
x
1
U2
d)
t
2
x
3
e)
U1
uv
2 u2
+x
2
3
S
C23
1
u1 ; u2
P2
U2
U1 ( jω ) jωC13 + U 2 ( jω ) jωC23 = 0 ⇒ U1 ( jω )C13 = − U 2 ( jω )C23
(uV − U1 )C13 = −(uV − U 2 )C23
U2
f)
x
Similar to
digital caliper
if U1 = U , U 2 = −U
(uV − U )C13 = −(uV + U 2 )C23 ⇒ u V = U
C13 − C23
C13 + C23
resolution: 1 µm, uncertainty 5 µm
Modern signal conditioning circuits for
capacitive sensors
Main problem influence of capacitance of leads (cable)
(driven from voltage source, current measured by „ideal
ammeter“ )
• 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
Amplifier for capacitive sensors:
Cp1
Cs
Cp2
C1
+
U(jω)
−
G
U1(jω)
-A
U2(jω)
Parasitic capacitances of the cable to Cs will not apply:
Cp1 is on virtual zero, Cp2 is on low output impedance of the OpAmp
Linearity even for variable air gap sensor (vibration measurement),
U2 ~ d
Applications of capacitive sensors
Typical applications:
- sensing level in tanks
- checking filling of products
inside packages
- sensing level of powder /
granules in storage
Honeywell
Omega
Bottle
Conveyor
belt
Control of filling
- sensing non-metalic objects
on conveyor belts
Sensing humidity of
material in dryer
Checking presence of parts
in product completion
drums
reservoir
Sensing level of liquid dye in
printworks
Control of filling
Checking presence of products in mass production:
sensor
rubber gasket
sensor
metal object
Honeywell
Turck