Magnetostrictive Transducer : Schematic Diagram, Types, Advantages & Its Applications

The electromechanical transducer is a device used to convert either an electrical signal into sound waves or a sound wave into an electrical signal. These transducers are more versatile & contain magnetostrictive & piezoelectric devices. At present for power ultrasonic applications, there are two basic transducer designs used magnetostrictive & piezoelectric. A piezoelectric transducer uses the property of a piezoelectric material for converting energy from electrical to mechanical. A magnetostrictive transducer uses the property of a magnetostrictive material for converting energy into mechanical energy within a magnetic field. Here, the magnetic field is provided through a coil of wire which is covered around the magnetostrictive material. So this article discusses an overview of a magnetostrictive transducer – working & its applications.

What is Magnetostrictive Transducer?

A device that is used to change the energy from mechanical to magnetic energy is known as a magnetostrictive transducer. The magnetostrictive transducer working principle uses a type of magnetic material where an applied oscillating magnetic field will squeeze the atoms of the material, creates a periodic change within the material length & produces a mechanical vibration with high frequency. These types of transducers are mainly used in the lower frequency ranges & these are very common in ultrasonic machining & ultrasonic cleaner applications.

Magnetostrictive Transducer Schematic Diagram

The working of a magnetostrictive transducer can be described by using the following schematic diagram. This diagram explains the strain amount produced from null to complete magnetization. This is divided into discrete mechanical & magnetic attributes that are set in their effect on the magnetic induction & magnetostrictive core strain.

In the first case, figure c shows when the magnetic field is not applied to the material, then the change within length also be null with the magnetic induction produced. The magnetic field amount (H) is increased to its saturation limits (±Hsat). This increases the axial strain to “esat”. In addition, the magnetization value will be increased to the +Bsat value shown in Figure-e or reduces to –Bsat shown in the figure.

When the ‘Hs’ value is at its maximum point, then the magnetic induction & highest strain saturation can be attained. So at this point, if we attempt to increase the field value, then it will not change the magnetization value or field of the device. So, when the field value strikes saturation, then strain & magnetic induction values will increase and move from the outward of the central figure.

In the second case, when the ‘Hs’ value is kept fixed and if we raise the quantity of force on the magnetostrictive material, then compressive pressure within the material will rise onto the reverse side with a decrease in the axial strain & axial magnetization values. In figure-c, there are no flux lines available because of null magnetization whereas in Figure. b & figure. d has magnetic flux lines of a much lesser magnitude based on the magnetic domain alignment in the magnetostrictive driver. Figure-a has flux lines but their flow will be in the reverse direction.

Figure. f shows the flux lines based on the applied ‘Hs’ field & the magnetic domain arrangement. Here the produced flux lines are measured with the Hall Effect principle. So this value will be proportional to the force or input strain.

Types of Magnetostrictive Transducer

There are two types of magnetostrictive transducers; spontaneous magnetostriction and field-induced magnetostriction.

Spontaneous Magnetostriction

Spontaneous magnetostriction occurs from the magnetic ordering of atomic moments under the Curie temperature. This type of magnetostriction is utilized in the NiFe-based alloy called invar and it shows zero thermal increase up to its curie temperature.

The material’s saturation magnetization decreases on heating to the Curie temperature because of a decrease within the amount of arrangement of the atomic magnetic moments. When this arrangement and the saturation magnetization reduce, the expansion of volume also decreases through the spontaneous magnetostriction & the material contracts.

In the invar case, this contraction because of spontaneous magnetostriction loss is equivalent to the expansion caused through usual thermal vibration methods & hence the material will show there is no change within dimensions. But over the Curie temperature, normally thermal expansion occurs & there is no longer any magnetic ordering.

Field Induced Magnetostriction

Field-induced magnetostriction mainly occurs mainly from the magnetic domain arrangement on an applied field application. The Terfenol material shows the largest useful magnetostriction, which is the mix of Tb, Fe, and Dy. Terfenol material is utilized for position sensors, field sensors, mechanical actuators & speakers.

Magnetostrictive arrangement (or) load sensors simply work through the fact that whenever a magnetostrictive material experiences a strain, the material’s magnetization will change. Usually, Terfenol actuators include a Terfenol rod that is arranged under compression to arrange the magnetic domains to the rod length in perpendicular. A coil is used around the Terfenol rod, a field is applied to the rod to line up the domains through its length.

Difference between Magnetostrictive and Piezoelectric Transducer

The difference between a magnetostrictive and piezoelectric transducer includes the following.

How to Choose a Magnetostrictive Transducer?

The selection of a magnetostrictive transducer can be done based on the specifications below.

• This transducer must use a type of magnetic material so that it can interact and can map distances very exactly.
• Transducer must allow contact-free & wear-free measurements.
• Its range must be from 50 to 2500 mm.
• Its maximum resolution should be approximately 2 µm.
• Maximum linearity must be ±0.01 %.
• Displacement speed should be less than 10 m/s.
• Analog output is 0 to 10 V, 4 to 20 mA.
• 24 VDC ±20 % Voltage supply
• IP67 Protection class
• The operating temperature must range from -30..+75 °C.

Advantages and Disadvantages

The advantages of a magnetostrictive transducer include the following.

• These transducers are reliable, maintenance-free, significantly reduce the potential for operational errors & machine downtime
• Magnetostrictive transducers don’t have contact parts, so they have a longer life.
• These are more accurate as compared to fixed contact transducers.
• They have good sensitivity, long-range inspection, durability, easy implementation, etc.

The disadvantages of a magnetostrictive transducer include the following.

• Magnetostrictive transducers are expensive.
• The magnetostrictive transducer has physical size limitations, so it is restricted to operate at below 30 kHz frequencies approximately.

Applications

The applications of a magnetostrictive transducer include the following.

• The magnetostrictive transducer is used for position measurement.
• This transducer plays a key role in converting mechanical energy into magnetic energy.
  Previously, this device was utilized in different applications which include torque meters, hydrophones, sonar scanning devices, telephone receivers, etc.
• At present, it is used to make different devices like high-force linear motors, noise control systems or active vibration, medical and industrial ultrasonic, positioners for adaptive optics, pumps, etc.
• These transducers are mainly developed to make surgical tools, chemical processing, material processing & underwater sonar.
• The magnetostrictive transducers are used for measuring torque developed by rotary shafts within the moving parts of machines.
• This transducer application is divided into two modes; implying Joule Effect & the other one is Villari Effect. When the energy from magnetic to mechanical is converted then it is used to produce force in the case of actuators & can be used to detect a magnetic field in the case of sensors. If the energy from mechanical to magnetic is changed then it is used to detect motion or force.

Thus, this is an overview of the magnetostrictive transducer. This transducer is also called a magneto-elastic transducer. These transducers possess extremely high mechanical input impedance & are appropriate for the measurement of large static & dynamic forces, acceleration & pressure. They are strong in constructional features and when these transducers are used as active transducers, the output impedance will be low.