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“The automotive industry continues to promote more efficient fuel consumption and reduce carbon dioxide emissions. Compared with existing Hall solutions, GMR (Giant Magnetoresistance) technology improves performance and expands operational capabilities.

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The automotive industry continues to promote more efficient fuel consumption and reduce carbon dioxide emissions. Compared with existing Hall solutions, GMR (Giant Magnetoresistance) technology improves performance and expands operational capabilities.

GMR sensors (magnetoresistance effect magnetic field sensors) are characterized by their response to magnetic fields, which is independent of frequency. Therefore, it is possible to obtain good sensitivity and maximum output signal at the frequency of kHz.

GMR technology

GMR technology utilizes the quantum magnetoresistance mechanism observed in various ferromagnetic multilayers. This research won the Nobel Prize, thanks to the research of Albert Fert and Peter Grunderb.

This effect significantly changes the resistance according to the magnetization of adjacent ferromagnetic elements. The magnetization direction is organized by applying an external magnetic field. The result is the scattering dependence of electrons on rotational orientation.

The following formula describes magnetoresistance:

Where R(H) is the resistance of the sample in the magnetic field H, and R(0) corresponds to H=0. An alternative form of this expression can use resistivity instead of resistance.

The basic principle depends on the rotation of electrons. In a magnetoresistive resistor, the electron scattering speed varies with the interaction of the electron spin and the magnetic orientation of the medium, and the electron travels in the medium-the resistance of the GMR transducer changes under the action of a magnetic field.

GMR sensors are particularly promising for the development of innovative hybrid sensor models that can detect and characterize underground discontinuities in conductive multilayer boards by electromagnetic means. These hybrid sensors integrate a conventional coil and a GMR sensor. The coil generates an alternating magnetic field in the material to be tested. The GMR sensor is a detection element that generates magnetic field interference due to interaction.

Like Hall effect technology, GMR is closely connected with signal conditioning circuits. The output signal is larger than the Hall signal, which has a higher signal-to-noise ratio and lower output signal jitter. These qualities allow the GMR sensor to detect objects at greater distances.

The main characteristics of evaluating GMR sensor selection include several key elements. First of all, the first thing to consider is ease of design, “The solution must be a fully integrated module, including magnets and protection components, to achieve the best performance in the user’s sensor design. Including magnets and EMC protection, which greatly simplifies GMR The design of the sensor.” Allegro spokesperson said. The second consideration is external magnetic field interference and its ability to interfere with the output signal. The common mode field can be eliminated by using differential sensing technology. However, stray fields other than the common mode field will destroy the output signal of the magnetic sensor, so it needs to be considered in the design process. “

The main design consideration is to optimize the magnetic circuit to obtain excellent performance using GMR technology, so this is the reason. This is necessary and is done by the GMR IC supplier. Those familiar with GMR design are best to design GMR IC and magnets as a system.

The gearbox design needs to be smaller and lighter to improve efficiency and fuel economy. This creates a lot of space constraints on the mounting position and tolerances around the speed sensor. The solution is to provide greater flexibility by operating at a further air gap, without compromising the expected performance of the previous part that functions at the closer air gap.

Figure 1: Functional block diagram of ATS19580

Allegro MicroSystems announced the launch of its ATS19580, the industry’s first fully integrated and reintegrated giant magnetoresistive transmission direction and speed sensor (GMR). Due to its high degree of integration, ATS19580 has high vibration resistance, and reduces the system size, complexity and cost, thereby saving fuel. Allegro’s proprietary technology and leading digital processing technology set a new standard for transmission speed sensing. The functionality of the ATS19580 simplifies the integration of customers’ speed sensors and enables them to design safe and fuel-efficient systems (Figures 1 and 2).

“During the design of a GMR sensor, the most critical part is the sensor’s working range and signal accuracy. It must operate at a far air gap, but it cannot be left within the total effective air gap for our customers and car manufacturers. Enough tolerance, the car manufacturer will not be considered as a solution, because it will increase the cost of designing such a precise mechanical system. Similarly, signal accuracy must be weighed, so dynamic air gap capability, vibration resistance and heat resistance must be maintained Gradient compensation. The development of these algorithms for end system compensation has been going on for two decades,” Christine said.

Figure 2: Typical application circuit of ATS19580 GMR sensor

“Transmission is the target market for the ATS19580LSN. However, identifying any gear used for speed, direction or pulse counting can definitely benefit from the functionality of the sensor. Examples include recreational vehicles (ie UTV, snowmobiles, forklifts, etc.),” ​​Chris Ting said.

Hall and GMR technology

Magnetic field detection uses GMR and Hall effect sensors. Both technologies are compatible with integrated circuit processing.

Magnetoresistive sensors provide higher sensitivity than Hall sensors. The sensitivity of the GMR sensor can be adjusted by selecting the film thickness and line width. On the contrary, the Hall effect is conducive to highly linear measurements without saturation effects up to extremely high field strengths (Figure 3).

Figure 3: Hall (a) and GMR (b) sensor layout

Hall-effect sensors can detect vertical magnetic fields, while magnetoresistive sensors can handle parallel magnetic fields. Therefore, GMR sensors consist of unipolar sensing for precision, non-contact displacement applications such as medical analyzers and magnetic field encoders. However, Hall-effect sensors can determine the gear pitch of CNC machine tools and measure the transmission speed.

Driver assistance systems (ADAS) require higher accuracy and reliable systems in terms of sensitivity. Giant magnetoresistance (GMR) can well meet these advanced requirements and can replace the Hall effect as a sensor transducer. In principle, both GMR and Hall are magnetic sensors, but their basic operations and functions are different.

Unlike GMR technology, the minimum difference value of the Hall-based magnetic induction field is less than 30 Gauss, and the difference value can be as low as 5 Gauss. GMR requires stricter design conditions and provides a defined linear range, and the ideal peak-to-peak magnetic signal amplitude should be within 100 Gauss.

Generally, all equipment can operate outside the magnetic design range without permanent damage. However, depending on the signal processing algorithm, performance may decrease. In GMR sensors, the cost of this degradation may be higher.