One article to understand the magnet design of giant magnetoresistance multi-turn position sensor

Multiturn sensors essentially combine magnetic write and electronic read memory with a traditional magnetic angle sensor to provide high-precision absolute position. If the magnetic field is too high or too low, magnetic writing errors may occur. Care must be taken when designing system magnets, taking into account any stray magnetic fields that may interfere with the sensor as well as mechanical tolerances over the life of the product. Smaller stray fields may cause errors in measured angles, while larger stray fields may cause errors in magnetic writing and therefore total turns.

Designing the ideal magnets and shields requires careful understanding of system requirements. Generally speaking, the more relaxed the system requirements, the larger and more expensive the magnet solution required to achieve the target specifications. ADI is developing a series of magnetic reference designs that meet various mechanical, stray field and temperature requirements and will be available to customers of the ADMT4000 true powered multiturn sensor. The first design developed by ADI covers a system with relatively loose tolerances: sensor-to-magnet distance of 2.45 mm ± 1 mm, total sensor-to-rotation axis displacement of ±0.6 mm, and operating temperature range of –40˚C to +150 ˚C, the stray magnetic field shielding attenuation is greater than 90%.

There are some key considerations to consider when designing a magnet, and the next section outlines the main aspects to consider when designing for a GMR sensor.

The GMR sensor operates within a defined magnetic window (16 mT to 31 mT)1; in addition, the maximum and minimum operating ranges have a thermal coefficient (TC), as shown by the red trace in Figure 1. Select magnet materials that match the TC and GMR sensors to maximize the allowable variation range of the operating magnetic field. This helps increase variations in magnet strength and/or tolerance variations in the distance of the magnet relative to the sensor. Low-cost magnetic materials such as ferrite have a much higher TC than GMR sensors and have a limited operating temperature range compared to materials such as samarium cobalt (SmCo) or neodymium iron boron (NeFeB).

Knowing the TC of the chosen magnetic material and the variation in field strength due to manufacturing differences, the required field strength at room temperature (25°C) can be determined. The design can then be simulated at room temperature with high confidence that the system will operate as expected over the entire temperature range. In Figure 1, the solid green line represents the window of magnetic field strength that the magnet is designed to produce within the active area of ​​the GMR sensor. Due to differences in the magnetic material manufacturing process, this window is smaller than the maximum and minimum operating windows of GMR sensors. The green dashed lines represent the maximum and minimum expected magnetic fields due to typical manufacturing variations of >5%.

The simulation of magnets in a mechanical operating environment can take different forms. There are two types of simulation commonly used to design magnets: analytical simulation or finite element analysis (FEA). Analytical simulations solve for the magnetic field using the overall parameters (size, material) of the magnet being simulated, without considering the surrounding environment except for the assumption that the magnet is operating in air. This is a quick calculation that is useful when there is no adjacent ferromagnetic material. FEA can model the effects of ferrous materials in larger magnetic systems, which is critical when combining magnets with stray field shielding or ferromagnetic materials in close proximity to magnets or sensors. FEA is a time-consuming process, so it usually starts with a basic magnet design in an analytical analysis. FEA is used to simulate reference designs of magnets and stray field shielding.

The reference design magnet produced by the simulation consists of a SmCo magnet with integrated steel stray field shielding, as shown in Figure 2. The magnet is an injection molded design so it can be mass produced. Injection molding of SmCo magnets is common due to its ability to produce complex shapes and is widely used in automotive and industrial applications. This assembly is designed to form an interference fit with a 9 mm diameter shaft; however, the bushing can be modified to connect to a different size shaft.

We performed careful characterization of the magnet assembly to demonstrate robust magnetic solutions for GMR sensors. Key to characterization is the ability to draw detailed maps of magnetic field strength in a controlled environment over an extended magnet-to-sensor distance window. The key to successful characterization is to fully understand and calibrate the magnetic field probe used. Figure 3 shows an example of magnetic field strength measured at two different air gaps. Repeating these measurements over the entire operating temperature range and air gap range is very time consuming, but this operation is useful for understanding the magnet's performance to ensure it is functioning properly under the required conditions. Running is crucial.

In summary, the reference design magnet has been proven to meet the requirements for operation at temperatures from –40°C to +150°C with an air gap of 2.45 mm ±1 mm and an axial distance tolerance of ±0.6 mm from the sensor. Details of stray field shielding will be covered in a subsequent article.

ADMT4000 is the first integrated true power-on multi-turn position sensor, which will significantly reduce system design complexity and workload, ultimately achieving smaller, lighter and lower-cost solutions. The reference design will be made available to ADI's customers, allowing designers with or without magnetic design capabilities to add or improve existing functionality for current applications and open the door to many new applications.

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