Magnetostrictive Sensor

1. OVERVIEW
• Magnetostrictive Sensor

2. Magnetostriction Principle
What is Magnetostriction??
• Magnetostriction is a property of ferromagnetic materials such as iron, nickel, cobalt and
their alloys to expand or contract when placed in magnetic field.
• Initially when these ferromagnetic materials are not magnetised the magnetic domains of
the ferromagnetic material are randomly distributed.
• But when they are placed in magnetic field the magnetic domains undergo changes and are
arranged in parallel.

3. • A magnetostrictive sensor is used to measure linear position.
• It basically senses the position of the permanent magnet(position magnet) to determine
the distance between the permanent magnet and the sensor head.
• It is governed by two effects: i. Wiedmann Effect. ii. Villari Effect.

4. Wiedmann Effect:
• 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 centre of the wire and the maximum at the wire surface.
Reverse of the Wiedmann Effect is
Villari Effect.
Applying stress to a magnetostrictive material
changes its magnetic properties
(e.g., magnetic permeability).
This is called the
Villari effect.

5. •The main components of the magnetostrictive sensor are:
1. Waveguide
2. Position magnet
3. Electronics
4. Strain pulse detection system
5. Damping module
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.

6. Working
• 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 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.

7. • 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, 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.

9. Advantages of Magnetostrictive sensor:
Since it is non contact( the position magnet does not touch waveguide) there is no wear and
friction. So there is no limitation on the number of operating cycles and is not affected by
vibrations. Linear measurement