Magnetic Position and Angle Sensors: Common Types, Key Components, Parameters
Introduction
Magnetic position and angle sensors are widely used in a variety of sectors because of its non-contact, wear-free, and sensing capabilities. These devices enable the measurement of linear and angular displacement in a variety of applications ranging from automotive systems and industrial automation to consumer electronics and aircraft. This page discusses the working principles of various kinds, performance metrics, critical components, design considerations, and applications of magnetic sensors.
Common Types of Magnetic Sensors
Magnetic position sensors and angle encoders work by interacting with a magnetic field and a sensing device. A magnetic field is created by either a permanent magnet or an electromagnet, and its intensity changes with the target's location or angular displacement. Sensing components within the devices detect changes in the magnetic field and transform them into electrical signals according to the target's location or angle. Magnetic position sensors and angle encoders are built with numerous sensor technologies, each with its own set of benefits and drawbacks. We will now go through the many types of magnetic sensors and their fundamental operating principles.
Induction sensors, which are based on Faraday's law of induction, are the most fundamental type of magnetic sensing. It can only detect changes in the magnetic field. Figure 1 depicts the operation of an inductive magnetic sensor coil. When a magnet is brought closer to a coil, the magnetic flux density in the coil rises, resulting in opposing forces in the form of induced electromotive force (emf) and induced current. When the coil stops rotating, the variations in the magnetic flux density stop as well, as do the induced electromotive force and current. The coil's output voltage is proportional to the rate of change of the magnetic field. Only the rate of change of magnetic flux density and its direction may be calculated using the induced electromotive force. Integration is necessary to calculate the magnetic field density. Coils have limited usefulness; but, when combined with other sensors, they can give important functionality.
These sensors are based on the Hall effect, which produces a voltage differential across a conductor when a magnetic field is applied across the cross-section of a current carrying conductor (see Figure 2). The Fleming left-hand rule allows us to calculate the direction of the electromotive force (emf) that is created orthogonally to the current and magnetic field. The magnitude and relative angle of the current and magnetic field assesses the resultant vector voltage proportional to the Hall's constant.
Hall effect sensors are very inexpensive, sturdy, and dust and dirt resistant, although they have poorer sensitivity and resolution than other varieties. These sensors are used in applications such as proximity switching, location, speed detection, and current sensing. The majority of Hall sensors are utilized in automotive applications for position sensors (linear or angular) and contactless DC motors.
Magneto-Resistive (MR) Element Sensors
Magneto-resistive (MR) elements are another prominent form of magnetic sensor. When a magnetic field is applied across an MR element, its resistance varies. Anisotropic magnetoresistive element (AMR), giant magnetoresistive element (GMR), and tunnel magnetoresistive element (TMR) are three popular MR sensor types that use a ferromagnetic thin film material. Because of their reduced noise levels, these sensors often provide superior resolution and accuracy than Hall effect sensors.
Anisotropic Magneto-Resistive (AMR) Sensors
The change in resistance of ferromagnetic materials when exposed to a magnetic field is used by AMR sensors. Figure 3 shows the scattering degree of electro changes when the ferromagnetic film's magnetization direction is parallel to the current direction and when the magnetization direction is vertical to the current direction. As a result, the resistance value changes as well. The magnetoresistive ratio (MR ratio), which represents the rate of change in resistance induced by the state of the magnetic field, is an important metric in MR sensors. An AMR sensor element has an MR ratio of roughly 5%. Because of its simple construction, AMR is frequently utilized in magnetic switches and rotation sensors.
Giant Magneto-Resistive (GMR) Sensors
GMR sensors are based on a thin-film construction made of alternating ferromagnetic and non-magnetic layers (see Figure 4). The degree of electron scattering varies depending on whether the magnetization directions of the Ferromagnetic film pinned layer and the free layer are antiparallel or parallel. The resistance value varies as a result of this process. The MR ratio of GMR sensor elements is roughly 20%. GMR sensor elements are used for magnetic heads, rotary sensors, and other devices due to their increased sensitivity.
Tunnel Magneto-Resistive (TMR) Sensors
TMR sensors detect magnetic fields by utilizing the quantum mechanical phenomena of tunneling magneto-resistance. Figure 5 depicts the construction utilized in TMR sensors, which consists of a laminated ferromagnetic film (pinned layer), an insulator film, and a ferromagnetic film (free layer). The fraction of electrons traveling through the insulator fluctuates due to the tunneling effect. As a result, the resistance changes depending on whether the pinned layer and free layer magnetization directions are antiparallel or parallel. TMRs have an MR ratio that can exceed 100%. Because of its high sensitivity, TMR has become the ideal choice for hard disk magnetic heads or high sensitivity rotation angle sensors. MR technology is used in a wide range of applications including magnetic storage, position and angle sensing, navigation, current sensing, non-destructive monitoring, and biomedical sensing systems.
Fluxgate Sensors
Fluxgate sensors are a conventional innovation with several advantages. These feature a high level of sensitivity and resolution. These sensors, however, can be rather massive. It has been demonstrated that fluxgate sensors can be shrunk, although the magnetic resolution suffers as a result. In these sensors, the generated alternating signal, which is modulated by a low-frequency signal, is proportionate to the measured magnetic field, making noise and offset reduction simple. These have been used effectively in geophysics and astronomy, wearable electronic devices, and non-destructive testing.
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