■ True Power-On Multi-Rotation Sensor Expected to Revolutionize the Sensing Market

True power-on (TPO) multi-turn sensors based on giant magnetoresistance (GMR) sensing technology are expected to revolutionize the position sensing market in both industrial and automotive applications.

Because it can reduce system complexity and maintenance requirements compared to existing solutions.

This article discusses key factors to consider when designing a magnetic system to ensure reliable operation even in extremely demanding applications.

We also present a magnetic reference design that can be utilized by pioneering developers of this technology.

■ For magnetic writing, the incident magnetic field must be maintained over a specific operating range.

A multiturn sensor essentially combines a conventional magnetic angle sensor with a magnetic write and electronic read memory to provide very accurate absolute positioning.

'Multi-turn position sensor providing true power-on (TPO) function even when power is lost&rsquAs explained in a technical article from Analog Devices entitled “o;”, magnetic writing requires that the incident magnetic field be maintained over a certain operating range.

Magnetic write errors can occur if the magnetic field is too high or too low. Therefore, the system magnet must be carefully designed, taking into account any stray magnetic fields that may interfere with the sensor, and considering mechanical tolerances over the life of the product.

Low level stray magnetic fields can cause errors in the measurement angle, while high level stray magnetic fields can cause magnetic write errors, which can lead to total rotation count errors.

■ Magnetic Reference Design

Designing the optimal magnet and shield requires a thorough understanding of the system's requirements.

In general, the looser the system requirements, the larger and more expensive the magnet solution must be to achieve the target specifications.

Analog Devices (ADI) is developing a series of magnetic reference designs that address a variety of mechanical, stray field, and temperature requirements to help customers adopt the ADMT4000 true power-on (TPO) multi-turn sensor.

The first reference design developed by ADI is for systems with relatively loose tolerances, featuring a sensor-to-magnet arrangement of 2.45mm±1mm, total sensor displacement about the rotational axis of 0.6mm, an operating temperature range of -40°C to +150°C, and stray field shielding attenuation of greater than 90%.

■ Magnetic Considerations

When designing magnets for giant magnetoresistance (GMR) sensors, the following are key considerations:

○ Magnet material

GMR sensors operate over a defined magnetic field range (16 mT to 31 mT). Additionally, as can be seen in the red line in Figure 1, a thermal coefficient (TC) is applied to the maximum and minimum operating ranges.

Choosing a magnet material whose TC value matches that of the GMR sensor will maximize the allowable deviation of the operating magnetic field. This will allow for greater deviations in both magnet strength and magnet placement tolerance relative to the sensor.

Low-cost magnetic materials such as ferrites have much higher TC than GMR, which can limit their 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 magnetic field strength due to manufacturing variability, the required magnetic field strength at room temperature (25°C) can be calculated.

The design can then be simulated at room temperature with confidence that the system will perform as expected across the entire temperature range.

The green solid lines in Figure 1 represent the range of magnetic field strengths that the magnet must generate across the active area of the GMR sensor.

Due to manufacturing process variability of magnetic materials, this range is lower than the maximum and minimum operating range of GMR sensors.

The green dashed lines show the minimum and maximum expected magnetic fields assuming typical manufacturing variability of 5% or greater.

○ Magnet Simulation

Magnet simulation in a mechanical operating environment can take different forms. Two types of simulation are mainly used when designing magnets.

These are analytical simulation and finite element analysis (FEA).

Analytical simulations calculate the magnetic fields using the bulk parameters (size, material) of the magnet to be simulated, without considering the surroundings, except that they are assumed to operate in a normal environment.

This simulation is fast to compute and is useful when there is no ferromagnetic material nearby. FEA can model the effects of ferrous materials in larger magnetic systems, which is necessary when combining magnets with stray field shielding or when combining ferromagnetic materials close to the magnet or sensor.

Since FEA is a time-consuming task, the field typically starts with a basic magnet design derived from analytical analysis. For this reference design, FEA was used to simulate the magnet and the stray field shielding.

○ Magnet design features

The reference design magnet obtained through simulation results consists of a SmCo magnet integrated with a steel stray field shield (Figure 2). These magnets are designed to be injection molded, making them suitable for mass production.

Injection molding is widely used for SmCo magnets because it can manufacture complex shapes and is widely used in automotive and industrial applications.

This assembly is designed to fit a 9mm diameter shaft, but can be adapted to other sized shafts by changing the bushings.


○ Magnet characteristics analysis

We thoroughly characterized this magnet assembly to ensure it is a robust magnet solution for GMR sensors.

A key aspect of characterization is to generate detailed magnetic field strength maps over an extended magnet-to-sensor mounting range in a controlled environment.

Successful characterization requires understanding and good calibration of the magnetic field probe being used.

Figure 3 shows the magnetic field strength measured with two different air gaps.

Repeating these measurements over the entire operating temperature and air gap range can be time consuming, but is essential to understanding the magnet's performance and ensuring it is operating under the required conditions.


■ ADMT4000, the first integrated TPO multi-turn position sensor

In summary, this reference design magnet has been verified to meet the operating requirements of -40℃ to +150℃, 2.45㎜±1㎜ air gap, and 0.6㎜ shaft-to-sensor placement tolerance.

The ADMT4000 is the first integrated TPO multi-turn position sensor that will significantly reduce system design complexity and developer effort, ultimately reducing solution size, weight, and cost.

ADI will also provide reference designs to help design engineers, regardless of their design expertise, add new or enhanced features to current applications and realize new possibilities in a variety of applications.

More information about the ADMT4000 and magnet reference designs is available at analog.com, or contact your local ADI sales representative to advise on your customer application.

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※ About the author
- Stephen Bradshaw holds a BSc in Electrical Engineering from the University of Leeds and an MSc and PhD in Optoelectronics from the University of Glasgow. He has extensive experience working on lens design and characterization for first generation mobile phone cameras at STMicroelectronics, Gbps optical transceivers at Maroni, and various optical transceivers at Nanotech Semiconductor. He has been with ADI for over 10 years, working as an applications engineer primarily on Lithium Iron (LiFe) and Lead-Acid battery monitoring product lines and magnetic position sensors.

- Christian Nau is a Product Application Manager at Analog Devices, specializing in automotive electronics and sensors. He joined ADI in 2015 as an FAE and was responsible for magnetic sensor support in the EMEA region. Since 2019, he has been working in ADI’s Magnetic Sensor Technology Group, supporting customers for the products and driving the future direction of the Group.

- Enda Nicholl is a strategic marketing manager for magnetic sensors at Analog Devices, Limerick, Ireland. He joined ADI in 2006 as a mechanical engineer and has nearly 30 years of experience with sensors and sensor interface products across a wide range of applications and markets, including automotive and industrial. He has held a variety of roles including product applications, field applications and sales, strategic business development and marketing.