Understanding How GMR Sensors Enhance Vehicle Performance and Safety

Giant magnetoresistance (GMR) technology offers key advantages for a range of automotive applications. Learn how it works, how it compares to alternative approaches, and the types of GMR sensor solutions available.

Giant magnetoresistance (GMR) elements, a breakthrough in nanotechnology, are an important technology for today’s automotive system designs. Offering high sensitivity and precision, these sensors are not only improving the accuracy and reliability of various automobile systems but also enhancing vehicle safety and efficiency.

In this article, we’ll explore the operating principles of GMR sensors, models, and benefits, and introduce some sensor solutions for automotive applications.

 

GMR Sensors: Basic Operating Principles

GMR sensors operate on the principle of giant magnetoresistance. This is a quantum mechanical phenomenon observed in multi-layered thin-film structures having alternating ferromagnetic and non-magnetic layers (Figure 1). The resistance of these structures varies in response to external magnetic fields due to the alignment of electron spins in the ferromagnetic layers.

 

Figure 1. GMR sensors work on the principle of having alternating ferromagnetic and non-magnetic layers. The resistance value changes in response to external magnetic fields.

 

This alignment, which can be parallel or antiparallel, influences electron scattering and alters the electrical resistance. In parallel alignments, the scattering is reduced, leading to lower resistance, while antiparallel alignments increase scattering and resistance. The ability of these sensors to detect changes in magnetic fields arises from this variable resistance property, allowing them to function as highly accurate and reliable sensor elements.

 

Evolution of Automotive Sensing Technology

Traditionally, automobiles have relied on mechanical or less sensitive electromagnetic sensors for various functions within the vehicle. These older systems, while functional, faced limitations in terms of precision, durability, and efficiency:

Speed and Position Sensing: In the past, vehicles used mostly mechanical systems or basic magnetic sensors for determining speed and position. These systems were not only susceptible to wear and tear, but they also suffered from accuracy issues. 

Engine Timing: Earlier engine timing mechanisms were primarily mechanical, often lacking the precision required for modern engines and leading to suboptimal fuel combustion and higher emissions. The mechanical nature of these systems also made them more prone to degradation over time, compromising the overall efficiency and reliability of the engine.

Safety Systems: Initial versions of critical safety systems, such as anti-lock braking systems (ABS), faced efficiency challenges. This was primarily due to limited accuracy of sensors used in detecting wheel speed and slippage. Consequently, early ABS systems were not as effective in preventing wheel lock-up during emergency braking, potentially compromising vehicle safety.

 

Saturated vs. Linear Models

In the saturated model, the GMR sensor operates in a state where the magnetic field strength is sufficient to align all magnetic moments within the ferromagnetic layers, leading to a maximum reduction in resistance. However, after saturation has been reached, the sensor’s sensitivity to further increases in magnetic field strength diminishes significantly. 

Key characteristics of the saturated model include:

• High Magnetic Field: A strong magnetic field is necessary to achieve saturation.
• Binary Output: Ideal for applications requiring a simple on/off response.
• Stability: Less sensitive to minor fluctuations in the magnetic field at saturation.

The linear model operates in a lower magnetic field range, where the alignment of the magnetic moments is incomplete. This partial alignment offers a relationship between the magnetic field strength and resistance that is generally linear, although with potential deviations depending on material properties and sensor design. 

Characteristics of the linear model include:

• High Sensitivity: Highly responsive to small changes in the magnetic field.
• Proportional Response: Offers a continuous, quasi-linear response to variations in the magnetic field strength.
• Dynamic Range: Suitable for applications requiring sensing over a range of magnetic field strengths.

The choice between saturated and linear models depends on the specific application within the vehicle. Linear models are generally more complex and are used in systems requiring detailed information. In automotive applications, they are suited for precise measurements of component positions, offering a gradient of responses that allows for fine-tuning of engine timing. 

Saturated models are used where binary information is sufficient, e.g., detecting the presence or absence of components at specific locations. In terms of cost and integration, saturated models, being simpler, are cost-effective and easier to integrate into various sections of a vehicle as well as in space-constrained automotive applications.

 

Considerations for Component Selection

The choice between saturated and linear models depends on the specific application within the vehicle. Linear models, used in systems like ABS for wheel speed detection, are generally more complex and suited for precise measurements of component position or orientation. They offer a gradient of responses, allowing for fine-tuning in applications such as engine timing.

Saturated models are ideal for applications where binary information is sufficient, e.g., in simple proximity sensors for detecting the presence or absence of components at specific locations. In terms of cost and integration, their simpler design makes them cost-effective, robust, and easier to integrate into various parts of a vehicle, including space-constrained applications.

 

Benefits in Automotive Applications

The ability to detect minute changes in magnetic fields has enabled the development of highly accurate speed and position sensors based on GMR technology. GMR sensors provide accurate measurements of crankshaft and camshaft positions, which is crucial for engine timing. This also contributes to optimizing fuel injection and ignition timing, leading to more efficient fuel utilization and reduced emissions (Figure 2). 

 

Figure 2. A variety of automotive subsystems can take advantage of GMR.

 

Incorporating GMR sensors into safety systems like ABS and electronic stability control (ESC) can also enhance their effectiveness. These sensors provide faster and more accurate detection of wheel speed and slippage, allowing for more responsive and reliable safety systems. 

As the global automotive industry shifts toward electric vehicles (EVs) to minimize emissions, GMR technology will potentially play a role in battery management systems. Accurate sensing of current and voltage is essential for optimizing battery performance and longevity.

Similarly, GMR sensors will be integral to the development of autonomous vehicles due to their high sensitivity and accuracy, which is crucial for the complex sensing requirements of self-driving cars. 

 

Sensor Solutions for Automotive Applications

For its part, Alps Alpine offers a variety of sensors based on GMR technology suitable for automotive applications. Its GMR-based sensors include angle sensors, encoders, and switches offering a range of features that enhance performance and safety. 

Key benefits include:

• High Stability: Ensures consistent performance in critical automotive systems.
• Low Noise: Minimizes EMI, ensuring highly accurate and reliable readings.
• Low Power Consumption: Suitable for automotive applications where energy efficiency is a priority.
• High Sensitivity: Capable of detecting minute angle changes for precision control.
• High Robustness: Durable and resistant to harsh automotive environments.
• Wide Operating Temperature Range: Suitable for various climatic conditions typical in automotive environments.

 

Now lets examine some GMR-based sensor solutions.

 

HGARPS011A and HGARAN Angle Sensors

The HGARPS011A angle sensor is offered in a TSSOP-8 package, ideal for space-constrained applications (Figure 3). This product offers versatility with two directions and two full bridges, enhancing sensor precision. Moreover, the broad operating temperature range (-40°C to +150°C) ensures reliability in high-temperature environments typical in automotive settings. Key applications include steering angle, gear shifting, and motor control systems, such as EPS.

 

Figure 3. Offered in a TSSOP-8 package, the HGARPS011A is an angle sensor well suited for space-constrained automotive applications. (Click on image to enlarge)

 

The HGARAN series uses a dual-phase output and two full bridges, delivering detailed angle detection. Its small DFN1810 package enables seamless integration into compact systems. With low current consumption and a 5 kΩ bridge resistance, designers can leverage the HGARAN to reduce power consumption. Key applications include position detection and motor control.

 

HGPRDT007A and HGDVST Encoders 

The HGPRDT series encoder (HGPRDT007A) is offered in a compact SOT-23 package with two-phase output for high precision in rotation and motor control systems. This product line offers high output stability and ensures consistent performance in varying operational conditions. Key applications include power windows, slide doors, tailgates, and sunroofs (Figure 4).

 

Figure 4. In contrast to conventional Hall encoders, the HGPRDT007A GMR encoder provides two phases of output with 50% phase shift in one package.

 

The HGDVST is designed as a single-polarity, single-output type switch output sensor in a small SOT-23-5 package. This product offers high robustness against EMI, an essential quality for the electromagnetically complex environment of today’s automobiles.

This device is suitable for non-contact automotive applications such as door latch and tailgate position detection. Its non-contact nature ensures longevity, being less prone to wear and tear compared to mechanical switches. 

 

Technology for More Efficient Vehicle Designs

GMR elements are not only transforming existing automotive technologies but also paving the road for future innovations. The precision, sensitivity, and reliability of GMR technology continue to drive advancements in vehicle performance, safety, and efficiency. As technology advances, potential applications of GMR elements in automotive applications will expand, helping to develop even more efficient vehicle designs. 

For more information regarding Alps Alpine’s automotive sensors or application-specific queries, please visit the company’s website.

Alps Alpine will be showcasing these products this week at CES 2024. Viist them at booth 10677, North Hall, Las Vegas Convention Center.

 

All images used courtesy of Alps Alpine

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