Group Members Alia Khalid

CIIT/FA12-BS(EE)-003/Atk

Baqar Naseer

CIIT/FA12-BS(EE)-013/Atk

COMSATS Institute of Information Technology Attock Campus

MAGNETOSTRICTIVE SENSOR

Magnetostriction is a transduction process in which electrical energy is converted to mechanical energy.

This 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.

As a magnetostrictive material is magnetized, it strains; that is it exhibits a change in length per unit length. Conversely, if an external force produces a strain in a magnetostrictive material, the material's magnetic state will change.

Magnetostriction

Magnetostriction Transducers A magnetostriction transducer is a device that is used to

convert mechanical energy into magnetic energy and vice versa.

This bi-directional coupling between the magnetic and mechanical states of a magnetostrictive material provides a transduction capability that is used for both actuation and sensing devices.

This device can also be called as an electro-magneto mechanical device as the electrical conversion to its appropriate mechanical energy is done by the device itself. In other devices, this operation is carried out by passing a current into a wire conductor so as to produce a magnetic field or measuring current induced by a magnetic field to sense the magnetic field strength.

Theory of Magnetostrictivesensors 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.

Normally following effects are used in producing magnetostrictive position sensor.

i. Magnetostrictive effect

ii. Villari effect

iii. Wiedemann effect

Magnetostrictive effect

The physical response of a ferromagneticmaterial is due to the presence of magneticmoments, and can be understood by consideringthe material as a collection of tiny permanentmagnets, or domains. Each domain consistsof 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 magnetostrictiveeffect.

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

Magnetostrictive effectCont…. 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., theshells 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, couplingbetween the electron spin and electron orbit causes electronenergies 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.

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.

Reverse of Magnetoestrictive:

applying stress to a changes its magnetic properties magnetostrictivematerial

Villari effect:

Wiedemann effect 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 magnetostrictivematerial 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. Thisis due to the skin effect.

Wiedemann effectcont…. 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.

Operation of a magnetostrictiveposition sensor The main components of the magnetostrictive sensor are

Waveguide

Position magnet

Electronics

Strain pulse detection system

Damping module

The operation of a magnetostrictive position sensor is shownin Figure 3.

Fig.3:The operation of a magnetostrictive position sensor

Operation of a magnetostrictiveposition sensor cont…. 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. Thewaveguide wire is enclosed within a protective cover andis attached to the stationary part of the machine, hydrauliccylinder, etc.

The location of the position magnet is determined by firstapplying a current pulse to the waveguide. At the sametime, a timer is started. The current pulse causes a sonicwave to be generated at the location of the positionmagnet Wiedemann effect. The sonic wave travels alongthe waveguide until it is detected by the pickup.

Operation of a magnetostrictive position sensor cont…. This stops the timer. The elapsed time indicated by the

timer then represents the distance between the positionmagnet and the pickup.

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

The pickup makes use of the Villari effect. A small piece ofmagnetostrictive material, called the tape, is welded tothe waveguide near one end of the waveguide. This tapepasses through a coil and is magnetized by a small permanentmagnet called the bias magnet.

Operation of a magnetostrictiveposition sensor cont…. 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 manyoutputs, including DC voltage, current, pulse widthmodulation, start-stop digital pulses, CANbus, Profibus,serial synchronous interface, HART, and others.

Advantages & Disadvantages 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

Disadvantages of Magnetostrictive sensor: Dead band on both side of the sensor.( Some manufacturers can reduce the dead bands based on your requirement but cannot make it to zero)

Applicationsof magnetostrictive sensor The applications of this device can be divided into two modes. That is,

one implying Joule Effect and the other are Villari Effect. In the case where magnetic energy is converted to mechanical energy it

can be used for producing force in the case of actuators and can be used for detecting magnetic field in the case of sensors.

If mechanical energy is converted to magnetic energy it can be used for detecting force or motion.

In early days, this device was used in applications like torque meters, sonar scanning devices, hydrophones, telephone receivers, and so on. Nowadays, with the invent of “giant” magnetostrictive alloys, it is being used in making devices like high force linear motors, positioners for adaptive optics, active vibration or noise control systems, medical and industrial ultrasonic, pumps, and so on. Ultrasonic magnetostrictivetransducers have also been developed for making surgical tools, underwater sonar, and chemical and material processing.