In the IoT market, industrial equipment and consumer applications, sensors are often needed to sense angles so that the equipment can make accurate movements or make corresponding judgments. One of the new TMR(Tunnel Magneto-resistance) sensors has the advantages of high sensitivity, low power consumption, high linearity and low-temperature dependence. This article will show you the features of TMR sensors and the product features of Crocus Technology TMR angular sensors sold by Murata.
Magneto-resistance effect can be applied to storage and sensing fields
TMR (Tunneling Magneto-resistance) refers to a magneto-resistance effect that occurs in a magnetic tunnel junction (MTJ). It is a component composed of two thin insulators and their separated ferromagnets. In the case of a thin insulating layer (usually a few nanometers), electrons can be tunneled from one ferromagnet to the other side. Its tunneling resistance changes with the relative direction of the ferromagnetic materials on both sides. The magneto-resistance effect is the scientific basis of magneto-resistance random access memory (MRAM) and magnetic read-write heads in hard disks, and is also used in the field of sensing.
The TMR magnetic sensor is composed of a pinned layer separated by a dielectric barrier and a free (sense) layer. If the external magnetic field changes, the magnetization of the sensing layer will change its magnitude and orientation, which will lead to a change in MTJ resistance. However, there is an important difference between TMR angular sensors and TMR linear magnetic sensors. TMR magnetic sensors usually work in the linear range of TMR characteristics, while angular sensors are in the saturated region. In other words, the magnetization of the sense layer is always at the maximum possible magnitude and changes its orientation vector as the angle of the external magnetic field changes. The obvious limitation of this method is that the angular sensor cannot operate at a low magnetic field. The operating magnetic field range of typical angular sensors is between 25 mT and 90 mT.
The angle is detected by sensing the change of resistance
The TMR effect is manifested in the resistance change of the material stack (including a ferromagnetic alloys). The resistance of the TMR stack is directly related to the angle between the sense layer and the fixed layer. Therefore, the angle can be detected by detecting the change of resistance. Compared with other technologies, TMR technology has many advantages, such as 360° full rotation discrimination. It is only sensitive to angles, not sensitive to external field strength, and because of the contactless system, it will not produce mechanical wear and tear. It can be far away from dust and contamination, and is not affected by mechanical vibration.
Although sensors based on TMR technology are relatively new, they have entered a variety of industrial and automotive applications. These applications require monitoring of rotating mechanical structures, such as position sensing of the angle, speed and direction of motor shafts and knobs, typical of angle sensors. Due to the principle of magnetic measurement, high sensitivity and low noise, the TMR magnetic sensor is very suitable for steering angle measurement, rotor position measurement of motor communication in brushless direct current (BLDC) motors, and speed sensing (ABS sensor) for wheel speed measurement and crank shaft speed and position sensing with direction information.
High precision sensor suitable for low power consumption, high sensitivity and extreme temperature
The magnetic sensor products introduced by Crocus Technology are based on its innovative patented XtremeSense®TMR technology. They can achieve high sensitivity and low noise in a wide temperature range through its unique CMOS lithographic printing technology - a fully integrated monolithic process. These magnetic sensors not only have high magnetic sensing performance, but also have high temperature and high reliability, small footprint, current and thermal isolation, high frequency operation and low power consumption. They providing excellent robustness and stable performance, making them an ideal detection solution for IoT field and industrial applications.
The Crocus TMR angular sensor (2D sensor) introduced in this paper comprises eight TMR resistors sensitive to magnetic fields. Each resistor is composed of multiple MTJ (TMR elements), which can realize the accurate measurement of uniform magnetic field angle. Its excellent functional characteristics make it very suitable for many different operating environments and applications important for accuracy and rapid response, such as knob, encoder, motor/motion control and many other applications.
High-performance full-bridge TMR angular sensor
CT310, introduced by Crocus Technology, is a full-bridge angular sensor developed by Xtreme Sense™ technology. The angular sensor has a working magnetic field range of 25 mT to 90 mT, and can perform contactless angle measurement from 0° to 360°. After compensation in the full working temperature range, the angular error is as low as 0.30°. At the same time, it can also operate in the working voltage range of 1.0V to 5.5V and provide differential outputs for the sine (SIN) and cosine (COS) axes.
The CT310 sensor adopts a dual full-bridge resistance row and is packaged in an 8-pin TSSOP package, which can be used for application fields where the spatial issue shall be first considered. In addition, there is a small and light 8-pin DFN package with a dimension of 2.00 x 2.00 x 0.45mm.
Unlike linear sensors, the CT310 operates in saturated mode, so the external magnetic field has little effect on its performance, which helps it achieve excellent stability in the entire temperature range. The industrial version can work from -40°C to 85°C. The extended industrial version supports -40°C to 125°C. It can be widely used in angle measurement, rotation angle sensors, and brushless DC motors.
Conclusion
The TMR-based 2D angular sensor introduced by Crocus Technology is the best alternative to the old magnetic 2D sensor. Due to its high CMOS integration capability, this solution can support monolithic IC and has the advantages of lowest power consumption, low angular error and high cost-efficiency. It will be one of the best choices for angle sensing applications in industrial and consumer products.