Inductive versus Magnetic Position Sensors

Inductive versus Magnetic Position Sensors
Technical Article
(Ref: ZET14_V1)
21 July 2011
Inductive Versus Magnetic Position Sensors
Both inductive and magnetic sensors are the design engineer’s preferred choice
for measuring position in harsh environments. Both offer the advantages of noncontact sensing over the traditional potentiometer. This paper describes the
fundamental physics behind each technique and outlines the consequent
strengths and weaknesses of each approach.
Operating Principles – Magnetic Sensors
The term ‘magnetic sensor’ can be somewhat confusing since this term covers a range
of techniques including Hall effect, magnetoresistive and magnetostrictive.
Hall effect sensors are, by far, the most common magnetic sensor and are widely used
in high volume applications in automotive and domestic appliance sectors. A typical
example is in brushless DC motors for power commutation.
The Hall effect is the production of a voltage difference (the Hall voltage) across an
electrical conductor, transverse to an electric current in the conductor and a magnetic
field perpendicular to the current. It was discovered by Edwin Hall in 1879. In simple
terms, a Hall effect sensor measures the strength of a magnetic field. This effect can be
used to measure position since field strength is proportional to the distance between the
Hall sensor and the magnet.
y
If we consider a simple bar magnet – as
shown here on the right – we can see that
the lines of magnetic flux extend from and to
its poles. The density of the flux lines
decreases with distance away from the
magnet. You might recall seeing this effect
from the experiment with the iron filings and
a salt shaker in your school days. As the
magnet moves closer to the Hall device the
Hall effect increases – thus providing a
basis for position measurement.
x
N
Hall device
S
Tens of millions of Hall effect devices are made each year and for a high volume
application in domestic appliances or automotive, a Hall chip for $1 is achievable. With
around 10UScents for a magnet, it is possible to engineer a really low cost sensor.
So far, so good. Snag is, the magnetic field experienced by the Hall device also varies
in proportion to a bunch of other factors too. These factors include variation in the
position of the magnet in y & z axes; magnetic hysteresis; extraneous DC/AC fields;
distorting effects of nearby magnetically permeable materials (e.g. steel); variations in
temperature and differences in field strength between one magnet and the next.
All these ‘other factors’ means that Hall sensors are only suitable for those position
sensing applications where:- only modest measurement performance is required (typically >1% linearity)
- the mechanical tolerances for the relative motion of magnet & sensor are tightly
controlled
- extraneous AC fields, DC fields or nearby metal objects (notably swarf or
ferromagnetic particles in oil which may build up on the magnet over time) is
either well controlled or non-existent
- temperature is well controlled or the sizeable temperature effects are not
significant to the accuracy of measurement.
Hall devices which are capable of operation at >120Celsius are rare because
measurement performance drops off rapidly at elevated temperatures.
In summary, Hall effect offers a fairly robust, cheap but low precision technique which
can work well – but only if mechanical tolerancing and EMC is well controlled.
Magnetoresistive devices are similar to Hall effect devices but rather than detecting field
strength, they detect field direction. In terms of position measurement, the strengths
and weaknesses of the technique are also similar.
Magnetostriction is completely different to Hall effect or magnetoresistive techniques.
Magnetostriction refers to an unusual property of some ferromagnetic materials which
causes them to change their shape or dimensions during the process of magnetization.
The variation of material's magnetization due to the applied magnetic field changes the
magnetostrictive strain until reaching its saturation value. The effect was first identified
in 1842 by James Joule. This phenomenon can be used to measure position by
measuring the time of flight of a sound wave along a length of magnetostrictive material
such as nickel.
In a magnetostrictive position sensor a length of magnetostrictive material extends
between two fixed points as shown below:-
2
Sensor
N
y
S
x
A pulse of energy sent from one end bounces back from the other end in a time t. If a
magnet is brought in to close proximity with the strip, the time taken for the pulse to
bounce back reduces in proportion to the distance between magnet and sensor.
Magnetostrictive sensors are most suited to measuring linear position over long lengths
– most notably position sensors for hydraulic rams. This is where >90% of all
magnetostrictive sensors are used.
Over the last decade or so, magnetostrictive sensors have gained market share over
the more traditional linear transformers (LVDTs) because they are more compact. In
the last few years, the trend is reversing, with linear transformers winning back market
share back because problems are now become apparent from actual practical
application:
- large temperature effects (speed of
travel of energy thorough a solid is
highly dependent on temperature)
- clamping of the Magnetostrictive
materials must be tightly controlled
and any damage to the clamping
arrangements or wave guide through
shock or harsh vibration causes
catastrophic failure
- low measurement performance at
lengths of <150mm. The longer the
measurement scale the better the
accuracy (in % terms) of a
Magnetostrictive device.
Magnetostrictive technique is generally not applicable to rotary position measurement.
Its technical features make it particularly well suited to in-cylinder applications on long
stroke lengths of >250mm, where the delicate magnetostrictive strip can be protected
against shock from a substantial mechanical housing.
3
More generally, magnetostrictive devices are not more widely used because of their
high cost. Whilst the sensor itself is relatively inexpensive, by the time the cost of
mechanical parts for the wave guide, strip clamps etc. are counted, the basic sensor
price multiples by >10 and prices are typically measured in 100s of US$.
Operating Principles – Inductive
Michael Faraday became the father of electrical induction at the
Royal Society in 1835 when he found that an alternating current
in one conductor could ‘induce’ a current to flow in an opposite
direction in a second conductor.
Since then, induction
principles have been widely used as a basis for position &
speed measurement with devices such as resolvers, synchros
and linearly variable differential transformers (LVDTs). The
basic theory can be seen by considering two coils - a transmit
coil (Tx) and a receive (Rx) coil. The following equation applies:VRX = - K dITX
dt
Rx
♦ VRX is the voltage induced in the Receive coil
♦ K is the mutual inductance coupling factor
depending on the coils’ relative areas, geometry,
distance, and relative number of turns.
♦ dITX /dt is the rate of change of current in the
Transmit coil.
Tx
The receive signal is therefore proportional to the relative areas, geometry and
displacement of the coils. But as with magnetic techniques other factors can come in to
play such as temperature which changes the resistance of the coils, causing a
disturbance to any position measurement. This effect is negated by the use of multiple
receive coils and calculating position from the ratio of the received signals.
Accordingly, if temperature changes, the effect is cancelled out since the ratio of the
signals is unaltered for any given position. Since the coils can be a relatively large
distance apart, the mechanical installation is
much less onerous. Again, this is assisted by
the basic ratiometric technique.
This robust, reliable and stable approach has
meant that inductive sensors are the preferred
choice in areas where harsh conditions are
common – such as defence, aerospace,
industrial, oil & gas sectors.
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So why aren’t inductive sensors used more widely if they are so robust and
reliable?
The answer is simple. Traditional inductive sensors use a series of wound conductors
or spools. The spools must be wound accurately to achieve accurate position
measurement. Further, in order to achieve strong electrical signals, lots of wires are
needed. This makes traditional inductive position sensors bulky, heavy and expensive.
Zettlex technology uses the same inductive principles but printed, laminar constructions
are used rather than wound spools. This means the coils can be produced from etched
copper or printed on substrates such as polyester film, paper, epoxy laminates or
ceramic. Such printed constructions can be made more accurately than windings.
Hence a far greater measurement performance is attainable at less cost, bulk and
weight - whilst maintaining the inherent stability and robustness
Since inductive techniques work at greater separation distances than capacitive
techniques, this allows the principle components of inductive position sensors to be
installed with relatively relaxed tolerances.
Not only does this help to minimize costs of
both sensor and host equipment, it also
enables the principle components to be
encapsulated. This enables the sensors to
withstand very harsh local environments such
as long term immersion, extreme shock,
vibration or the effects of explosive gaseous
or dust laden environments.
Electromagnetic noise susceptibility is often
cited as a concern by engineers considering
inductive position sensors. The concern is
misplaced given that resolvers have been
used for many years within the harsh
electromagnetic environments of motor
enclosures for commutation, speed and
position control.
5
High resolution
Resilience to foreign matter
Robust EMC operation
tiv
e
ind
uc
le x
Ze
tt
Low thermal drift
Easy to install
na
l in
High repeatability
High accuracy
Tra
dit
io
Ma
gn
e to
str
Ha
ll
eff
ec
t
ict
iv e
du
cti
v
(>2
e
50
mm
)
A summary of the benefits of each of the techniques is shown below:-
Compact
Lightweight
Mark Howard
Zettlex General Manager
MACCON GmbH, Aschauer Str. 21, D-81549 Munich; www.maccon.de
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