PowerPointPrsentation

1
6 Magnetic Field, Displacement, Rotation,
Proximity
Presence and proximity Photoelectric proximity
switch Inductive and capacitive proximity
sensor Angle Resolvers Encoders Date
4.10.2007
Magnetic field Dimension Applications Hall
effect Magnetoresistive devices Fluxgate
sensors Position Differential transformer
(LVDT) Encoders
2
The magenitic field B is defined by the Lorentz
force
Fig. by Gerthsen
In the air gap of a permanent magnet (E.g. in
loudspeaker) there is ca. 1T Earth magnetic field
is around 50 µT 0,5 G (at equator) Classical
measurement Oscillation frequency of a compass
needle
3
Magnetic sensors Many measurement problems can be
measured indirectly by magnetic sensors
Position µm ... m Movement Proximity Rotation Curr
ent Orientation (compass)
Hall sensor Magnetoresistive sensor Fluxgate Diffe
rential transformer Induction coil Reed switch
dB
4
Hall effect
Lorentz force
n concentration of charge carriers Semiconductor
s show small n ( large mobility) and large Hall
Voltage
Fig. by Sinclair
5
Hall sensors and switches Technology similar to
Si-electronics monolithic integration Not very
sensitive Range 0,1 mT ... 10 T Resolution down
to 100 nT possible Sell in large numbers for lt50
Cent Hall switches have a typical setpoint at
2...10 T Movement detection Proximity switches
(with permanent magnet) Rotation Open/close of
doors, valves... Current sensing
6
Magnetoresistive sensors R R(B) A number of
magnetoresistive materials is used. One effect is
the Hall resistance effect In a Hall probe, the
electrons do not move parallel to the field
gradient. Increase of path length causes
increase of resistivity
Conductive needles are immersed in a
semiconductor matrix Sensitive Linear only for
small B T-sensitive
Fig. by Schaumburg Sensoren
7
Fluxgate sensor principle Nonlinearity of
magnetic response of a saturable core causes
change of the efficiency of a transformer, if an
external magnetic field Bext is applied. Bext
0 high slope Bext gt 0 smaller slope
B Magentic field H Magentic intensity Figs.
by Kovacs
8
Fluxgate sensor Two coils with saturable cores
are placed within one pickup coil. Bext 0
Fields cancel totally, zero pickup voltage Bext gt
0 Fields do not cancel totally, net voltage at
pickup coil Extremely sensitive 100 nT ... 200
µT Resolution down to 1 nT Electronic compass,
earth science (finding ores)
Figs. by Blackburn Modern Instrumentation
9
Fluxgate sensor measurement
Current switches from negative to positive
saturation No external field Signal
cancel External field One core switches earlier,
the other one later. At the beginning and at the
end of the switching the two fields do not cancel
Figs. by Blackburn Modern Instrumentation
10
Position, proximity and angle Control of
relative movement (linear and rotation) with
contact (not inertially) on large scale
(mm...cm) Angle of a hinge, Speed of rotation,
Tachometer, Control of machines, status of the
arm of a robot... Proximity and touching
Detection of objects, counting objects,
Security These measurements are the most
important ones for automation.
11
Position Differential transformer Linear
variable differential transformer (LVDT) Most
common position sensor Advantage Small friction,
Linear, High resolution, Overrange ok, Robust
Fig. by Sinclair
12
Proximity, optical measurement Photoelectric
proximity switch Normally IR light is
used Modulation around 1000 Hz Distance 0 ... 3m
(with reflector) Optical distance measurement
analyses the position or the size of the focus of
a light beam
Fig. by Sinclair
13

• Proximity, inductive measurement
• Coil in an oscillating LC-circuit. Metal close to
  core
• High loss of the circuit (all materials)
• Higher self induction (metals)
• Good for detection of metals. Setpoint typical
  some mm.
• Frequency typical 5 kHz
• The coil may have an iron (increases sensitivity,
  generates cross-sensitivity to magnetic fields)
  core or be driven without core
• Sensor Loss is measured
• Switch Setpoint determined by breakdown of the
  oscillator

14
Proximity, capacitive measurement
Also for nonmetals Very sensitive to
contamination, surface must be kept clean Good
temperature stability Range 25µm-50mm Planar
capacitor needs large area Guard electrode
(Schirmelektrode) on same potential as
measurement electrode C is part of a LC or a RC
oscillator
Fig. by E. Obermeier Sensoren
15
Angle resolver Inductive resolver transformer
with 2 secondary coils placed at 90 angle The
primary coil is rotated, the transmitted
amplitude and phase of the secondary voltages is
a function of the rotation angle Resolves 0,1
for rotations up to 12.000 rpm
Fig. by Sinclair
16
Angle optical encoders
An optical grid on a rotating plate is detected
by a photoelectric switch in reflection or
transmission Mode 1 Differential Two sensors
needed to discriminate up-down No true zero after
start, errors add up Typical 2000
pulses/rotation, up to 5000 rpm
Fig. by Sinclair
17
Angle optical encoders
Mode 2 fully coded on several tracks True zero
any time 8 tracks 256 bit 1,4
Fig. by Sinclair