Robot Sensing System-4

OVERVIEW

• Magnetostrictive Sensor

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.

• 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.

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 theVillari effect.

•The main components of the magnetostrictive sensor are:1. Waveguide2. Position magnet

3. Electronics4. Strain pulse detection system5. 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.

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 sonicwave 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 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.

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