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In previous articles, we have mentioned soft magnetic and hard magnetic materials.
In fact, all of these materials belong to the category of ferromagnetic materials.
Ferromagnetic materials share a common characteristic: they can be magnetized.
Based on the degree to which they can be magnetized, i.e. the magnitude of their permeability, they can be divided into materials with high permeability, such as permalloy and silicon steel, and materials with ordinary permeability, such as cobalt steel and cast steel.
Magnetic permeability of several ferromagnetic materials: Silicon steel sheets are used for transformer cores because of their high saturation magnetic flux density and low coercivity and remanence, meaning low hysteresis loss. Permalloy, due to its high permeability, is an ideal magnetic shielding material. Image source: JO Aibangbee1 ands.O. Onohaebi2, Ferromagnetic Materials Characteristics: Their Application in Magnetic Cores Design Using Hysteresis Loop Measurements.
01
magnetic permeability
In daily life, we often use magnets to attract objects such as nails, screws, and needles.
These things can be attracted by magnets because the magnets induce (magnetize) a magnetic field inside them, and the magnetic field in the magnet and the induced magnetic field attract each other.
Therefore, in layman's terms, magnetic permeability can be considered as the ease with which a material can be magnetized; the higher the magnetic permeability of a material, the easier it is to be magnetized.
Numerically speaking, magnetic permeability is the ratio of the strength of the induced field to the strength of the applied magnetic field. The stronger the induced field, the higher the magnetic permeability, and the stronger the attraction.
Air, wood, plastics, brass, and titanium alloys are non-magnetic materials; external magnetic fields do not induce magnetism within them, therefore they are not attracted by magnets. Iron, nickel, cobalt, gadolinium, and silicon steel are magnetic materials; when exposed to external magnetic fields, they can induce magnetic fields within themselves, thus attracting magnets.
02
Magnetic domains
Why are some materials easily magnetic while others are not?
This has to start with the microstructure.
From a microscopic perspective, ferromagnetic materials contain many magnetic domains.
A schematic diagram of magnetic domains and their alignment changes under the influence of an external magnetic field.
Magnetic domains can be understood as numerous tiny elliptical permanent magnets within a ferromagnetic material.
When a material is not magnetized, the magnetic domains are arranged randomly. If the material is magnetized, the domains are oriented approximately parallel to the external magnetic field, thus changing the overall size of the material.
The basic principle of magnetostriction: In the middle image, without an external magnetic field, the magnetic domains are randomly arranged, and the material is at its natural length. In the top image, an external magnetic field is applied in a vertical direction, causing the magnetic domains to align vertically as well, resulting in the material shortening. In the bottom image, an external magnetic field is applied in a horizontal direction, causing the magnetic domains to align horizontally as well, resulting in the material lengthening.
03
Magnetostriction effect
Based on magnetic domains, we can easily understand magnetostrictive effects.
A. Magnetostriction effect (Joule effect):
Almost all ferromagnetic materials, such as iron, nickel, cobalt and their alloys, will change in size and shape due to changes in magnetization. This effect is called magnetostriction.
Because this effect was discovered by Joule, it is also called the Joule effect.
All ferromagnetic materials undergo magnetostriction; for example, when a magnetostrictive rod is placed in a magnetic field parallel to its length, the rod will change length.
The length variation of materials used in magnetostrictive sensors is very small, typically on the order of 10⁻⁶ m/m.
Since magnetostrictive sensors utilize not only the magnetostrictive effect but also several other related effects, I'll briefly mention them here as well.
B. The Villari effect:
Conversely, applying stress to a magnetostrictive material alters its magnetism (permeability). For example, twisting a magnetostrictive element or a magnetized wire causes a change in magnetization, a phenomenon known as the Villari effect.
C. Wiedemann effect:
An important characteristic of wires made of magnetostrictive materials is the Wiedmann effect: when an axial magnetic field is applied to a magnetostrictive wire and current flows through the wire, the wire will twist at the position of the axial magnetic field.
Torsion is caused by the interaction between the axial magnetic field from the permanent magnet and the circumferential magnetic field around the magnetostrictive wire. (The current-carrying conductor generates a circumferential magnetic field around the conductor, which we also mentioned in our previous article, "What is the principle of an induction motor? What are its advantages and disadvantages? What are its typical applications?")
04
Working principle of magnetostrictive sensors
Magnetostrictive sensors operate based on the Joule, Villari, and Widman effects.
On the one hand, the applied force or torque generates stress, which changes the magnetic permeability of the magnetostrictive material, causing its magnetization intensity to change.
On the other hand, the applied magnetic field will change the magnetic state of the magnetostrictive material, thereby causing its elastic constant to change.
Working mechanism of a magnetostrictive position sensor: The image shows the core components of a magnetostrictive position sensor. A current pulse is applied to a ferromagnetic wire, generating a circular magnetic field around the wire. A permanent magnet (rectangular or ring-shaped) is attached to the moving object whose position is to be measured. The magnetic field of the permanent magnet interacts with the magnetic field of the current pulse, causing the waveguide to elastically deform in the form of a mechanical pulse. This wave propagates bidirectionally along the waveguide at the speed of sound and is eventually picked up by a pickup coil. Image courtesy of MTS.
Let's look at a more detailed picture.
The working principle of a magnetostrictive position sensor, image courtesy of Sensorland. At one end of the waveguide, a small piece of magnetostrictive material called tape is soldered to the waveguide. This tape passes through a coil and is magnetized by a small permanent magnet called a bias magnet. When sound waves propagate along the waveguide to the tape, the stress caused by the sound waves induces a change in the permeability of the tape (Villarley effect). This, in turn, causes a change in the magnetic flux density of the tape, thus inducing an output pulse from the coil (this utilizes Faraday's law of electromagnetic induction, which we discussed when talking about induction motors; interested readers can click here to view).
At the other end of the waveguide, unused pulses are attenuated by a damping module to prevent further interference with the returned/reflected pulses.
Therefore, the position of an object can be accurately determined by using the time t that elapses from the transmission of the current pulse to the reception of the strain pulse: the position of the object = Vs * t / 2, where VS is the waveguide propagation speed.
To ensure the accuracy of the output, a laser interferometer is typically used in the final stage of production to determine or calibrate the actual propagation speed in the waveguide.
Magnetostrictive sensors provide absolute position information and, unlike incremental encoders, do not require repositioning when power is off.
A real magnetostrictive position sensor, image courtesy of Temposonics.
The most distinctive feature of magnetostrictive sensors is that they can also use multiple position magnets with a waveguide, making them ideal for detecting the position information of multiple components along the same axis of motion, such as the blades on a slitting machine, and can measure a travel range of more than 5m with a resolution of up to 1 mm.
05 Applications of Magnetostrictive Sensors
Magnetostrictive sensors are used to measure liquid levels: a permanent magnet is hidden in a float. When the liquid level changes, the magnet moves, and the waveguide is excited and propagates at different positions, eventually being detected by the detector in the sensor.
Magnetostrictive sensors are used to measure: (a) Pressure: The structure mainly consists of upper and lower rigid plates, and an excitation coil and a detection coil surrounding the iron core. When an external force is applied to the rigid plates, it changes the strain of the magnetostrictive iron core, thereby changing the permeability and magnetic flux density in the detection coil. According to the electromagnetic induction effect, a changing current will be induced in the detection coil. (b) Vibration: Vibration force acts on the magnetostrictive strip, causing strain. The magnetic susceptibility of the magnetic core in the strip changes, and a fluctuating current is induced in the coil.
Magnetostrictive sensors are used to measure drill bit torque: Three coils are located outside the drill bit. The excitation coil surrounds the inside of the drill bit, enclosing the shank (cylindrical without cutting grooves) and the notch. Two induction coils, one above the notch and one above the shank, are connected in series in opposite directions to measure permeability. Compared to the notch, the permeability of the shank is less sensitive to torque changes, and the output voltage difference between the two coils varies proportionally to the applied torque. Image from: Overview of Magnetostrictive Sensor Technology. Frederick T. Calkins, Alison B. Flatau and Marcelo J. Dapino.
