Magnetostrictive Linear Position Sensors


Magnetostriction is a property of ferromagnetic materials
such as iron, nickel, and cobalt. When placed in a magnetic
field, these materials change size and/or shape (see Figure 1).


Figure 1
A magnetising force, H,
causes a dimensional
change due to the
alignment of magnetic domains.


The physical response of a ferromagnetic material is due to
the presence of magnetic moments, and can be understood
by considering the material as a collection of tiny permanent
magnets, or domains. Each domain consists of many atoms.

When a material is not magnetized, the domains are randomly
arranged. When the material is magnetized, the domains are
oriented with their axes approximately parallel to one another.

Interaction of an external magnetic field with the domains
causes the magnetostrictive effect.

This effect can be optimized by controlling the ordering of
the domains through alloy selection, thermal annealing, cold
working, and magnetic field strength.

The ferromagnetic materials used in magnetostrictive position
sensors are transition metals such as iron, nickel, and cobalt.
In these metals, the 3d electron shell is not completely filled,
which allows the formation of a magnetic moment. (i.e., the
shells closer to the nucleus than the 3d shell are complete,
and they do not contribute to the magnetic moment). As
electron spins are rotated by a magnetic field, coupling
between the electron spin and electron orbit causes electron
energies to change. The crystal then strains so that electrons
at the surface can relax to states of lower energy. When a
material has positive magnetostriction, it enlarges when
placed in a magnetic field; with negative magnetostriction,
the material shrinks. The amount of magnetostriction in base elements and simple alloys is small, on the order of 10-6 m/m.

Since applying a magnetic field causes stress that changes
the physical properties of a magnetostrictive material, it is
interesting to note that the reverse is also true: applying
stress to a magnetostrictive material changes its magnetic
properties (e.g., magnetic permeability). This is called the
Villari effect.

Normal magnetostriction and the Villari effect are both used
in producing a magnetostrictive position sensor.

Figure 2
The Wiedemann effect describes
the twisting due to an axial
magnetic field applied to a
ferromagnetic wire or tube
that is carrying an electric
current.


An important characteristic of a wire made of a
magnetostrictive material is the Wiedemann effect (see
Figure 2). When an axial magnetic field is applied to a
magnetostrictive wire, and a current is passed through
the wire, a twisting occurs at the location of the axial
magnetic field. The twisting is caused by interaction of
the axial magnetic field, usually from a permanent
magnet, with the magnetic field along the magnetostrictive
wire, which is present due to the current in the wire.
The current is applied as a short-duration pulse, -1 or
2 µs; the minimum current density is along the center
of the wire and the maximum at the wire surface. This
is due to the skin effect.

The magnetic field intensity is also greatest at the wire
surface. This aids in developing the waveguide twist.
Since the current is applied as a pulse, the mechanical
twisting travels in the wire as an ultrasonic wave. The
magnetostrictive wire is therefore called the waveguide.
The wave travels at the speed of sound in the waveguide
material, ~ 3O00 m/s.

The operation of a magnetostrictive position sensor is shown
in Figure 3.


Figure 3.
The interaction of a current pulse with the position magnet
generates a strain pulse that travels down the waveguide
and is detected by the pickup element.

The axial magnetic field is provided by a position magnet.
The position magnet is attached to the machine tool,
hydraulic cylinder, or whatever is being measured. The
waveguide wire is enclosed within a protective cover and
is attached to the stationary part of the machine, hydraulic
cylinder, etc.

The location of the position magnet is determined by first
applying a current pulse to the waveguide. At the same
time, a timer is started. The current pulse causes a sonic
wave to be generated at the location of the position
magnet Wiedemann effect. The sonic wave travels along
the waveguide until it is detected by the pickup.

This stops the timer. The elapsed time indicated by the
timer then represents the distance between the position
magnet and the pickup.

The sonic wave also travels in the direction away from
the pickup. In order to avoid an interfering signal from
waves travelling in this direction, their energy is absorbed
by a damping device (called the damp).

The pickup makes use of the Villari effect. A small piece of
magnetostrictive material, called the tape, is welded to
the waveguide near one end of the waveguide. This tape
passes through a coil and is magnetized by a small permanent
magnet called the bias magnet. When a sonic wave
propagates down the waveguide and then down the tape,
the stress induced by the wave causes a wave of changed
permeability (Villari effect) in the tape. This in turn causes
a change in the tape magnetic flux density, and thus a
voltage output pulse is produced from the coil (Faraday
effect). The voltage pulse is detected by the electronic
circuitry and conditioned into the desired output.

MTS magnetostrictive sensors are available with many
outputs, including DC voltage, current, pulse width
modulation, start-stop digital pulses, CANbus, Profibus,
serial synchronous interface, HART, and others.
____________________________________________________

For more information, please contact :-

MTS Systems Corp. USA.
Tel: 919-677-0100
Email:
info@mtssensors.com



  
 
 

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