Senzory elektrického proudu
ANMM Iasi 2003
Current sensors using
magnetic materials
Pavel Ripka
Czech Technical University, Prague
Current Sensors: Overview
• Resistive Shunt
• Contactless sensors – use B
– current transformers
– current comparators
– Rogowski coils
– magnetic field sensors
»
»
»
»
in gapped core
compact
remote
multisensor configuration
GMI current sensor
Current clamps
Engineer’s wishlist
Resistive shunts
no galvanic insulation
dissipated heat
measured current should be interrupted
DC current sensor with gapped core
Magnetic yoke
Measured current
n2
n1=1
I1
I2
R2
V2
IH Hall sensor in
narrow airgap
Problems: bulky,
sensitive to external fields
offset drift – only Hall
Large currents = Large yokes
LEM
Magnetooptical current sensor
•
•
•
•
•
optical fibre x bulk glass
1% accuracy even after temp. compensation
expensive
> 1000 A
good for high voltages
Current transformer
Magnetic
core
lS
I1
I2
s
Secondary
Primary
N1
r1
N2
ui
Z2
r2
h
I1
Y’11
L1, Rv1
Y11
C11
M
L2, Rv2
I2
C21
Y21
Y’21
Cm1
Ym1
Y12
C12
Cm2
C22
C’21
Y22
Rv1
Lr1,
L’r2,
I01
Ic
Z2
Ym2
C’11
I1
U1
Ui1
Cp
IR
Rz
R’v2
I’2
IL
Lh
U’2
Z2
AC Current clamps
Leakage
Shielding
I1
Secundary
winding
Measudary
winding
Core
Flux Φ2 generated by
secondary winding
Φ1 generated by
measured
conductor
DC current sensor
using oscilloscopic clamps
AC Current Comparator
Magnetic
shielding
Detection
ring core
I2
I1
Primary
winding
1
2
Secondary
winding
N2
N1
Detection
winding
DET
AC Current Comparator
DC Current Comparator
N1
Nb
N2
Magnetic
shielding
Detection
ring cores
I1
I2
NS
f
G
ref
2f
I
PSD
R
Out
DC Current Comparator
cover
Secondary
winding
magnetic
shielding
Detection
winding
Detection cores
Modulation winding
Electrostatic
shielding
Novel AC/DC Comparator
Detekčn
í toroidy
Magneti
Kryt
Sekundární
vinutí
Primární
vinutí
Vitrokov 8116 –as cast
B (A/m)
0.8
10Hz
1kHz
10kHz
20kHz
40kHz
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
H (A/m)
-0.8
-80
-60
-40
-20
0
20
40
60
80
Vitrokov 8116 – annealed
B (A/m)
0.8
10Hz
1kHz
10kHz
20kHz
40kHz
0.6
0.4
0.2
0
-0.2
-0.4
-0.6
-0.8
-80
H (A/m)
-60
-40
-20
0
20
40
60
80
Excitation current
Nexc = 10
N1
N2
16 A p-p
I1
I2
N
S
f
G
I
PSD
R
Out
Testing core homogeneity
Testing coil
V
out
~
α
Generator
Ratio error
I1
I2
R2s
N1=
10
RN1=0,
1Ω
A
’
A
V
N2=1
00
B
’
B
RN2=
1Ω
εI [%]
0.02
0.01
0
-0.01
-0.02
-0.03
-0.04
-200
-150
-100
-50
0
I1 [A]
50
100
150
200
Current transformer mode
Voltmeter
Gen
(sin)
I1
Shunt
1mV/A
I2
N2=100
N1=1
0
In
Lock-in
A’
Ref
B’
RN1=0,1
Ω
RN2=1
Ω
A
B
40
-0.1
burden 1 
εI
[%]
amplitudová chyba
fázová chyba
30
-0.2
20
-0.3
10
-0.4
0
-0.5
10
100
1000
f [Hz]
C
10
4
δI
[mi
n.]
Current transformer mode
δI [min.]
εI [%]
40
Bm  ls '
I  
sin 
0 z N 2 I 2
-0.1
amplitude error
phase error
30
-0.2
I 
Bm  ls '
cos 
0 z N 2 I 2
20
-0.3
10
-0.4
0
-0.5
10
burden 1 
100
1000
f [Hz]
10
4
AC/DC current comparator mode
Voltmeter
Gen
(sin)
I1
Shunt
1 mV/A
I2
N2=100
N1=10
Lock-in
In
A’
Ref
B’
RN1=0,1
Ω
Electro
nics
RN2=1 Ω
A
B
Závislost chyb komparátoru s
elektronikou na frekvenci pro I1=56,6 A
0.05
25
amplitudová chyba
fázová chyba
0
εI
[%
]
20
-0.05
15
δI
[mi
10
n.]
-0.1
-0.15
5
-0.2
0
-0.25
-5
100
f [Hz]
1000
C
10
4
AC/DC current comparator mode
Voltm
etr
Generá
tor
(sin)
I1
Boční
k
1 mV/
A
Lockin
I
n
R
e
f
I2
N2 =
100
N1
=1
0
RN1=0
,1 Ω
Závislost chyb komparátoru s elektronikou na frekvenci pro I1=56,6 A
εI [%]
A
’
B
’
A
B
Ele
ktr.
čás
t
RN2=
1Ω
25
0.05
amplitude error
amplitudová
chyba
phase chyba
error
fázová
0
εI [%]
20
15δI
-0.05
[min.]
10
-0.1
-0.15
5
-0.2
0
-5
-0.25
100
f [Hz]
1000
C
f [Hz]
10
4
Rogowski coil measures di/dt
Digital integrator for Rogowski coil
IC for power meter
Digital power meter with di/dt sensor
Magnetic sensors
for current sensing
• Magnetic field sensors
– semiconductor
– ferromagnetic magnetoresistors
– other (GMI, optical, resonant, SQUID…)
Magnetic field sensors
Scalar
Vector
Measure the size of B
(“total field B”)
Measure the projection
of B into the sensitive axis
• single-axis
• tri-axial
B  Bx2  B y2  Bz2
only resonant sensors
most
magnetic sensors
Magnetic field sensors: DC and AC
AC
DC
Measure only changing field:
induction coils
Measure DC and AC fields
d
d
Vi  
   NAB 
dt
dt
Vi .. Induced voltage
 .. Magnetic flux
A .. Coil area
N .. Number of turns
most
magnetic sensors
Current sensor specifications
•
•
•
•
•
•
FS range, linearity, hysteresis
TC (“tempco”) of sensitivity
Offset, offset tempco and long-term stability
Perming (= null change after magnetic shock)
Geometrical selectivity
Noise
– PSD , rms or p-p value
• Resistance against environment
– temperature, humidity, vibrations
Types of magnetic field sensors
•
•
•
•
•
•
•
•
Semiconductor sensors (Hall, …)
Ferromagnetic magnetoresistors (AMR, GMR, …)
Resonant magnetometers (Proton, Cesium, ...)
SQUIDs (LTS + HTS)
Induction coils
Optical (Fibre optic, bulk )
Fluxgate
Other principles (GMI, magnetoelastic, …)
Basic rules
Dipole field (from small objects)
B  1/r3
Long iron pipe
B  1/r2
Long straight current conductor
B  1/r
Linear Hall sensor
Asahi Kasei Electronics: InSb Hall element (HW series)
Hall integrated circuit
Analog electronics:
• Delivers constant current
• Amplifies VH
• Flips contacts
• Performs compensations
• May compare with threshold
Honeywell Hall Sensor Using Four CrossConnected Hall Elements
Vertical Hall sensor
B is parallel to the substrate
B
I
V
J
V2
V3
Expected Advantages:
long-term stability
robustness
Active zone is buried into a
mono-crystal, far away from
the chip surface.
V1
currently not used:
expensive
no so good
Permalloy Flux concentrators
Used for Hall and MR
Increase sensitivity
Possible problems:
• TC of sensitivity
• perming
• linearity
Cylindrical Hall device with integrated magnetic
flux concentrators
(Sentron AG: developed, but not in production)
Feedback Hall current sensors
1 A .. 1 kA sensors
LTS 25-NP
25 A, 200 kHz
error 0.02 %
sensitivity TC: 50 ppm/K
LEM
PCB - integrated current sensor
1 – the current lead
2 – the ferromagnetic yoke
3 – Vertical Hall sensor or MR
Sentron
Hall current sensors
– compact design
vnější magnetický
obvod
pouzdro
pouzdro
vodič s měřeným
proudem
vodič s měřeným
proudem
vývody Hallova
generátoru
vývody Hallova
generátoru
magnetokoncentrátory
směr citlivosti
vývody Hallova
generátoru
zapouzdření
1mm
Classical Hall with Flux concentrators
Inside structure of C-MOS Hall
AMR bridge sensor
Philips KMZ
Full bridge made of
meandered resistors
with barber-pole
strips
AMR current sensor (F.W.Bell)
Vout
range
5 ... 50 A
linearity
0.1 %
sens. tempco 50..100 ppm/K
typ. offset (250C) 15 mA
max. offset
Imeas
(-25 .. 850C)
30 .. 50mA
developed by:
GMR bridge sensor
GMR resistors configured as a
Wheatstone bridge sensor
(NVE)
•
•
R2, R3 are shielded
R1, R4: field is
concentrated by approx.
D1/D2
250
Still has nonlinear
(NVE)
40
150
30
100
20
50
0
-2.5
10
0
-1.5
-0.5
0.5
Applied Field (mTesla)
1.5
2.5
Output (mV/V)
unlike AMR bridge
Voltage (mV)
response
50
200
GMR Contactless current sensor
Long straight current conductor
B  1/r
NVE GMR sensor measures
current in close wire
Advantages of magnetoresistors
compared to Hall sensors:
• higher sensitivity
• no piezo effect
• higher operational temperatures
AMR very good
GMR, SDT ... too much nonlinear
Fluxgate sensors
Most sensitive room-temperature
magnetic sensors
Based on non-linear
magnetization characteristics of
ferromagnetic core.
Measure up to 1 mT
with 100 pT resolution
Classical fluxgates:
precise, but expensive
(CTU Prague)
Fluxgate principle
• Ferromagnetic core
- non-linear B-H
• Excitation and sensing
coil
• Core is periodically
saturated by Iexc,
 drops to 1
twice each period
• Measured B0 causes
2nd harmonics in Vind
Vind
Iexc(t)
(t)
B(t)
N
Bo
B()

Fluxgate principle
H
Hexc
a)
Vi
Hm
t

t
Vi
B()
•
In absence of external
field, magnetisation is
symmetrical
External measured
field causes assymetry
– detected in induced
voltage
H
Hexc+H0
t
b)
Vi
t
H0
t
•
t
Hm
Micro-fluxgate sensors
(in development)
•
•
•
•
•
Shizuoka University
flat coils
electrodeposited core
or amorphous strips
electronics on chip
cheap
resolution still higher
than MR
Magnetic amplifier
= current sensor based on fluxgate principle
Magnetic amplifier
100mA/div
DC current2 =80mA
compensates I 1=40A
= current sensor based on fluxgate principle
Fluxgate current clamps
unsymmetrical core
Фext
3
3
Фext2
z
y
Фext1
1
Фext2
z
Фext
y
x
2
x
2
Фext
Фext1
1
Фext
Fluxgate current clamps
symmetrical core
Φext
2
1
z
y
2
Фext
Фext
x
2
Φext
2
1
Leakage flux
Leakege
Shielding
Sekundární vinutí
I1
Measured
current
Core
Flux Φ2 generated by
compensation
winding
Φ1 generated by
measured
conductor
Fluxgate current clamps
jádro senzoru
core
stínění
shielding
z
y
x
no shielding
symmetrically shielded core
assymetrical
shielding
Suppression of external currents
Error caused by external 40 A current
100
Iv [mA]
unshielded
non-symmetric shielding
symmetrical shielding
10
1
0.1
10
100
1000
distance [mm]
40 A current clamp
Relative error( % FS)
0.6
10mm
unshielded
non-symmetric shielding
symmmetric
0.4
0.2
δ
[%
]
0
-0.2
-0.4
-0.6
-0.8
-40
-30
-20
-10
0
I1 [A]
10
20
30
40
Frequency characteristics
B (dB)
2
0
-2
CSLA1CD
CSNE151
KZB464/501
Currclamp
-4
-6
-8
-10
-12
10
100
1000
10
4
10
5
10
6
f (Hz)
Error of Hall sensors
δ2 (%)
1
CSLA1CD
CSNE151
0.5
0
-0.5
-1
-40
-30
-20
-10
0
10
20
30
I (A)
40
Error of fluxgate-based sensors
0.4
KZB464/501
Currclamp
0.2
%)
0
-0.2
-0.4
-40
-30
-20
-10
0
f (Hz)
10
20
30
40
Resistance against external currents
Sensor type
FS
error response
to 40A
CSLA1CD
(Hall)
57 A
2160 mA
CSNE151
(Hall)
35 A
180 mA
KZB464/501
(Siemens)
40 A
120 mA
Currclamp
(our design)
40 A
clamps
14 mA
problems: low-impedance networks
injected interference
Fluxgate-based DC/AC current sensor (PEI Ireland)
Magnetic circuit:
 7 mm/10mm ring
 material: electrodeposited permalloy
 sandwiched into PCB.
excitation winding:
 integrated in the PCB
 40 turns, R= 700 m.
Toroid with magnetic core
embedded in PCB
Wire carrying current to be
measured.
Current sensing: Sensor Array
Array of six sensors:
• increased sensitivity
• resistant against external
currents and fields
Sentron Hall sensors with field concentrators
measure current flowing through the hole
Remote current sensing
Hall with field
AMR
5 mT
6 mm
0.1 < 0.2 %
200 ppm/K
offset TC
resolution
perming, hyst.
BW
power cons.
600 nT/K
6 mm
6 mm
50 T
1 T
100 kHz
55 mW
fluxgate
flipped
concentarors
linear range
size
linearity
sensitivity TC
[email protected]
AMR
10 nT
10 nT
0. 5 mT
30 mm
1 ppm
30
5 nT
0.1 nT/K
100 pT
< 1 nT
1 kHz
150 mW
GMI current sensor
Ibias=
Idet~
~
Imeas =
GMI current sensor
Ibias=
demonstrator:
2 turns of 150 % GMI tape
Idet~
~
Imeas =
GMI current sensor - parameters
2 turns
200 T + feedback
FS range
2A
100 A
sensitivity
0.24 Ω/A
24 Ω/A in open loop
197 ppm/ºC
30 ppm/ºC
Z (0 A)
23 Ω
2.3 k Ω
∆Z/∆T
0.24 Ω/A
???
105 mA/ºC
???
sensitivity TC
offset drift
Future trends
AMR
compact current sensors
circular sensor fields
di/dt coreless coils with digital integrator
embedded pcb sensors
Optical sensors for large currents
Engineer’s wishlist
•
•
•
•
constant high , high Bsat materials
constant high , low Bsat materials
low TC GMI materials
trick how to linearize GMR, SDT
Resources
•
•
www.nve.com (GMR)
www.Sentron.ch (vertical Hall)
•
www.ssec.honeywell.com/magnetic/ (AMR)
•
•
www. Micronas.com (Hall)
www.Infineon.com (Siemens: Hall, GMR)
•
www.semiconductors.Philips.com/automotive/sensors_discretes (AMR)
•
•
•
www.Geometrics.com (resonant magnetometers)
measure.feld.cvut.cz/groups/maglab (fluxgate)
Magnetic sensors and Magnetometers (book)
Artech, 2001,www.artechouse.com
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