The seemingly mysterious "magnetism" is actually not far from us, constantly surrounding us. This is because the Earth itself can be considered a giant natural magnet called the geomagnetic field. If we simplify the geomagnetic field to a bar magnet, then the geomagnetic south pole is actually near the geographic north pole, and the geomagnetic north pole is actually near the geographic south pole. The hypothetical magnetic axes of these two poles are not aligned with the Earth's axis of rotation, but rather tilted at an angle of approximately 11.5 degrees. The geomagnetic field strength ranges from 0.4 Gauss to 0.6 Gauss, and its intensity and direction vary with location.
A magnetic sensor is a device that converts changes in the magnetic properties of a sensitive element caused by factors such as magnetic fields, radiation, pressure, temperature, and light into electrical signals. In modern industry and electronic products, the most widespread application of magnetic sensors is measuring physical parameters such as current, position, and direction by sensing magnetic field strength. Currently, there are many different types of magnetic sensors, the most common being those using Hall elements, anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR), and tunnel magnetoresistance (TMR) sensors as their core components.
What is an AMR magnetoresistive sensor?
When certain metals or semiconductors are exposed to an external magnetic field, their resistance changes with the magnitude of the applied magnetic field. This phenomenon is called the magnetoresistive effect, and the change in resistance is called magnetoresistive resistance.
In 1857, Thomson discovered the anisotropic magnetoresistance effect in permalloy. For strongly magnetic metals with anisotropic properties, the change in magnetoresistance is related to the angle between the magnetic field and the current. Common metals of this type include iron, cobalt, nickel, and their alloys. When the external magnetic field makes a zero-degree angle with the magnet's built-in magnetic field, the resistance does not change with the applied magnetic field; however, when the external magnetic field makes a certain angle with the magnet's built-in magnetic field, the magnetization vector inside the magnet shifts, and the thin-film resistance decreases. This characteristic is called the anisotropic magnetoresistance effect (AMR). The effect of a magnetic field is shown in the figure below.
AMR effect of permalloy
Relationship between change in magnetic reluctance and change in angle
The resistance R of the thin-film alloy changes with the angle. The resistance and magnetic field characteristics are non-linear, and each resistance does not correspond to a unique external magnetic field value. From the diagram above, we can see that the sensor is most sensitive when the current direction is parallel to the magnetization direction. When the current direction and the magnetization direction form a 45-degree angle, the magnetoresistive sensor generally operates near the linear region shown in the diagram, thus achieving linear output characteristics.
The basic structure of an AMR magnetic sensor consists of a Wheatstone bridge composed of four magnetoresistive resistors. The power supply is Vb, and current flows through the resistors. When a bias magnetic field H is applied to the bridge, the magnetization of two oppositely placed resistors rotates in the direction of the current, increasing their resistance; while the magnetization of the other two oppositely placed resistors rotates in the opposite direction, decreasing their resistance. The external magnetic field value can be obtained by measuring the difference voltage signal output from the two terminals of the bridge.
AMR magnetoresistive sensor equivalent circuit
How can AMR magnetoresistive sensors "show their capabilities"?
Anisotropic magnetoresistive sensors have a measurement range centered on the Earth's magnetic field distribution, making them ideal for magnetic sensors operating in Earth's magnetic field environment. They utilize common permalloy alloys, offering advantages such as high accuracy, small size, and good stability. Furthermore, the manufacturing process requires only a single magnetic thin film, resulting in a simple and low-cost process that eliminates the need for expensive manufacturing equipment, making them suitable for mass production and better suited to the demands of the consumer electronics market.
AMR magnetoresistive sensors are well-suited for detecting magnetic fields below 1 Gauss within the Earth's magnetic field range. The sensors can be used to detect ferromagnetic objects such as airplanes, trains, and automobiles. Other applications include magnetic compasses, rotary position sensing, current sensing, drilling orientation, linear position measurement, yaw rate sensors, and head tracking in virtual reality.
Navigation function
Magnetic sensors were originally used as nautical compasses, utilizing their magnetism. Magnetic sensors and accelerometers can be combined to form electronic compasses, a relatively economical electronic instrument for measuring azimuth. Today, electronic compasses are widely used in automobiles and handheld electronic compasses, watches, mobile phones, walkie-talkies, radar detectors, telescopes, star detectors, Muslim clocks, handheld GPS systems, wayfinders, weapon/missile navigation (dead reckoning), position/azimuth systems, security/positioning devices, high-performance navigation equipment in automobiles, marine and aviation, and video game consoles—any device requiring direction or attitude display.
- Vehicle control function
Because the Earth has a massive magnetic field, the passage of large magnetic objects like vehicles causes localized disturbances in the geomagnetic field. Magnetic sensors can detect the presence of vehicles, their direction of travel, speed, and size. Therefore, underground parking garages can calculate the number of available parking spaces and monitor road traffic flow, providing road and parking information for people's travel. This will play a crucial role in the current vigorous development of "smart cities"!
Geomagnetic Detection
Anisotropic magnetoresistive sensors can be used to identify buried materials. Magnetoresistive sensors utilize the property that thin-film alloys change their magnetoresistance when exposed to a magnetic field. When a bridge circuit encounters magnetic fields of varying strengths, it produces different voltage outputs, converting the magnetic signal into an electrical signal. Given the complex network of underground pipelines scattered throughout cities, and the frequent occurrence of leaks during construction that can lead to water leaks, gas leaks, and power outages, this application has practical significance.
Applications of ARM in the field of robotics
Collaborative robots are robots that interact with or work safely alongside humans in shared spaces to produce goods. In 1978, a cutting robot in a Hiroshima factory in Japan mistook a worker for a steel plate, leading to a tragedy. Since then, the safety of industrial robots has been a persistent problem for all robot developers. Since the world's first commercially available collaborative robot, the WAM robotic arm, was developed in the United States in 1996, another core issue has consistently plagued developers—how to achieve safety while simultaneously improving production efficiency and reducing costs.
As an external manifestation of electromechanical systems, the robotic arms of industrial collaborative robots are typically connected to motors via gearboxes. High-resolution current and rotational position information are crucial for achieving precise motor control and efficient commutation. Generally, resolver-based systems can offer very high resolution and accuracy, but end-solutions can be expensive and bulky. Sensorless solutions can also be used to detect back EMF current and reduce sensor weight and cost, but motor starting performance may be an issue. Using three Hall effect sensors to detect the position of the motor magnets is typically used in cost-sensitive applications, but monitoring three signals often presents space and installation challenges. Analog Devices (ADI), a professional manufacturer of semiconductor solutions for industrial automation, offers angle sensors based on anisotropic magnetoresistive (AMR) technology. With ARM sensors, not only can high angular accuracy be achieved, but also a very small sensor subsystem can be obtained, and the sensor can be positioned within the motor assembly.
What is AMR technology?
The resistivity of a sensor based on the AMR (Aspect-Modulated Magnetic) principle depends on the magnetization direction relative to the current direction. This sensor is typically deposited as a thin-film magnetically permeable alloy (magnetic iron-nickel alloy). AMR sensors operate in saturation, therefore the external magnetic field plays a decisive role in the resistance change. The resistance is highest when the external magnetic field is parallel to the current direction and lowest when the applied magnetic field is perpendicular to the plane of the current-carrying magnetically permeable alloy. An angle sensor can be realized by arranging two independent Wheatstone bridges at 45° to each other, with its sine and cosine outputs depending on the direction of the external magnetic field. This configuration provides a sensor with an absolute measurement range of 180°.
AMR working principle
Anisotropic magnetoresistive (AMR) thin-film materials are becoming increasingly important in current position detection technologies. Compared to traditional techniques, magnetoresistive (MR) position measurement offers several advantages. Reliability, accuracy, and overall robustness are the main factors driving the rapid development of magnetoresistive detection technology. Low cost, relatively small size, non-contact operation, wide temperature range, insensitivity to dust and light, and wide magnetic field range—these characteristics collectively contribute to a robust sensor design.
Factors determining robot accuracy and repeatability specifications
When a robot is in operation, the motors rotate at very high speeds, while the robotic arm moves much more slowly. Position sensors are typically mounted on the motors themselves. The joint angle interpretation results sent back to the robot controller show the joint position, which is then interpreted directly from the motor angle sensors or from the robotic arm side. In the case of the robotic arm side, the motor angle sensors are used to control the motor speed. There is also a safety brake that holds the robotic arm in place in the event of a power outage to prevent it from collapsing.
Industrial robot architecture
Repeatability is one of the most frequently used terms in robot design. Once a robotic arm consistently returns to the same position, the user can calibrate it at the start of a task and then know that it will remain consistent. The term accuracy is often used interchangeably with repeatability; it is one of the most important specifications and key parameters in repetitive tasks such as robotics. Systems with high repeatability can achieve very high absolute accuracy after calibration.
After understanding the repeatability specifications and the robot's range of motion, we can deduce the specifications of the rotary encoder. Encoders are typically found on servo motors and robot joints. The gearbox converts the high-speed rotation of the motor into controlled, low-inertia arm motion by increasing torque. Because robots have multiple joints, and mechanical tolerances must be considered, when combining multiple joints to achieve the robot's overall range of motion under sensors, the joint performance should exceed the target angle repeatability specifications.
Encoders in robot architecture
AMR sensor with high reliability and accurate location information
Analog Devices (ADI) offers the ADA4571 AMR angle sensor as a position sensor for use in mature magnetic encoder solutions. Leveraging high-precision position feedback, the ADA4571 improves motor commutation efficiency and enhances torque and speed control, particularly in low-speed applications. This sensor supports motor speeds up to 50,000 RPM, while its direct angle measurement principle reduces vibration and noise effects, eliminating their impact on motor performance. Excellent torque control capabilities contribute to improved motor efficiency, reduced emissions and heat dissipation, and extended motor life. Furthermore, its low phase delay (2µs) enables fast closed-loop control, improving responsiveness in high-dynamic applications such as industrial servo motors, robots, and electric steering.
What is AMR?
AMR is an abbreviation for Anisotropic Magneto Resistance. It is a component that reduces resistance when a magnetic field is applied, and its function depends on the orientation of the magnetic field lines relative to the component (anisotropy).
The AMR element is made of a strongly magnetic metal.
AMR elements are made of strongly magnetic metals such as Ni and Fe alloys.
Because of its symmetrical nature with positive and negative sides, the same action can be performed even if the north and south poles of the magnet are reversed. This characteristic allows for the acquisition of high-precision, reliable data in a non-contact manner. Furthermore, this non-contact detection capability allows for flexible applications such as opening/closing detection, rotation detection, and position detection.
Output type: Digital output
The sensor is formed on the same substrate as the IC and AMR components. The analog signals from the AMR components are digitally processed by the IC, achieving Hi and Lo level digital output, thus eliminating the need for signal processing by the customer.
What is the sensitivity of an AMR (magnetic) sensor?
The sensitivity of an AMR sensor is the magnetic field strength when the AMR sensor is turned on (or off). The magnetic field strength when an AMR sensor is turned on in a zero or weak magnetic field is called the ON sensitivity (Hon), while the magnetic field strength when an AMR sensor is turned off in a strong magnetic field is called the OFF sensitivity (Hoff). Sensitivity varies between different AMR sensors; the Hon and Hoff values on the datasheet only indicate the range for that product.
What are the advantages of AMR sensors?
A wide variety of sensitivity and sizes are available.
AMR sensors come in a wide variety of sensitivities, responsiveness, and current consumption, allowing you to choose the most suitable product for your application.
Suitable for flexible configurations and designs.
Even if the N and S poles of the magnet in an AMR (magnetic magnetic field) sensor are reversed, the sensor's operation will not change.
Compared to Hall ICs, this small, high-precision AMR sensor has a wider sensitivity range, allowing for more flexible configuration of the magnet and sensor, and reducing installation errors during housing and mounting.
Furthermore, since AMR sensors are not structural components similar to reed switches, their small size and reliability can be ensured.
Advantages of using magnetic sensors
Magnetic field lines are invisible to the naked eye. They are not absorbed by non-magnetic materials such as plastics, but rather penetrate to the other side. Magnetic sensors utilize these characteristics for detection.
For example, its ease of application in the following scenarios is a major advantage.
Small and with the switch location not visible: Laptop power-on/off detection and security device action settings.
Most suitable for structures where sealing is important.
Rotation detection of gases and water, etc.
Easy to achieve waterproof structure
Power switch for wearable device
The principle of AMR sensors: magnetoresistive elements
The resistance of a magnetoresistive element changes in a magnetic field perpendicular to the direction of the current. After being configured as shown in the diagram, the AMR sensor can combine elements with changing resistance (R1, R4) with elements with unchanged resistance (R2, R3).
Action of AMR sensor
When a magnetic field is applied, the resistance values of magnetoresistive elements R1 and R4 will decrease, and a potential difference will occur between midpoint A and midpoint B.
When the potential difference exceeds the specified setting value (threshold), the sensor's ON/OFF output will be switched.
Sensor internal histogram
3-terminal structure
The magnetic block is a single IC package used to convert AMR components and their output signals into digital signals.
The 3-terminal structure includes an input terminal (VCC), a GND terminal (GND), and an output terminal (OUT), and adopts a sampling circuit structure that suppresses current consumption.
Vibration damping
Vibration refers to the phenomenon where the power signal is repeatedly interrupted due to minute and extremely fast mechanical vibration when switching relay or switch contacts, and it is one of the causes of electronic circuit failures.
To prevent vibration, the working magnetic field has been made to have hysteresis.
If the magnet is close to the sensor and the magnetic field exceeds MOP, OUT changes from H to L.
If the magnet is far from the sensor and the magnetic field is lower than MRP, OUT changes from L to H.
Difference from Hall IC
The magnet features a compact and lightweight design.
AMR sensors can utilize a wider range of magnetic fields, thus enabling a broader detection range.
Because of its wide detection range, it can reduce housing tolerances and mounting errors. Compared with Hall effect ICs, it has advantages such as allowing for smaller and thinner magnet designs.
Different directions of the magnet
When the magnet is directly above the sensor, the Hall IC's magnet is configured to be placed vertically, while the AMR's is placed horizontally. The two have different magnetic field detection directions.
Because the magnetic force is stronger near the poles of a magnet, placing a magnet vertically can potentially affect the data on a card if there are magnetic data such as credit cards nearby. However, for electronic devices like smartphones and laptops where magnets cannot be placed vertically, AMR (Aspect-Modulated Resonance) shows an advantage. (Horizontal placement does not mean that magnetic data will not disappear.)
Design freedom
To detect horizontal magnetic fields, various placement methods for AMR sensors can be considered, thereby increasing design freedom.
Hall effect ICs typically detect vertical magnetic fields at a fixed point, so it is recommended that you install the magnet directly above the Hall effect IC, which limits design flexibility.
Comparison table of AMR sensors and Hall ICs
Detection principle
Sensor materials
Detection direction
Detection range
| AMR sensor | Hall IC | |
| magnetoresistive effect | Hall effect | |
| NiFe alloy | Si (low price, low sensitivity) InSb (high sensitivity, poor temperature characteristics) | |
| Horizontal magnetic field | Vertical magnetic field | |
| Width | narrow |
Difference from reed switches
Smaller sensitivity fluctuations, smaller size and shock resistance
Compared to reed switches, AMR sensors offer greater flexibility in magnet and sensor configuration design due to their smaller sensitivity fluctuations.
Furthermore, the small size of magnetic sensors allows for smaller circuit boards and surface mounting, thus reducing surface mount costs. Additionally, magnetic sensors are easier to operate than reed switches due to their greater shock and impact resistance, and their contactless nature increases lifespan.
The magnetic field detection direction is the same
Since the magnetic field detection direction is the same, the magnet used in the reed switch may be directly applicable to the AMR sensor.
However, it should be noted that since the switch of the AMR sensor itself has a circuit and requires a power source to operate, the number of wires will vary, or the sensor may generate current consumption inside.
