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TMAG5231: Interference with metal frame

Gaurav Gothoskar
Prodigy 80 points
Part Number: TMAG5231

Hello,

I have multiple sensors in the design on the bottom side of the PCB which are 6.25mm apart. There is a metal frame mounted on top of the sensors such that there is a square cutout 4mm X 4mm in the metal frame.

the magnet is inserted from the top side of the PCB. The full part number I am using is TMAG5231C1DQDMRR. I have 3 questions.

1. Can the sensor's magnetic field interfere with each other? can 1 sensor false trip when a magnet is in proximity of the sensor next to it?. How apart do they have to be ideally? 

2. Does the type of metal used in the frame make any difference? we are using : 0.048” CRS, ASTM A1008. 

3. is there a type of magnet TI recommends for optimal performance with these hall sensors?

Thank you.

  • 0
    TI__Genius 16320 points

    Guarav,

    Welcome to E2E, and thanks for reaching out!

    The sensors themselves do not generate any magnetic field, but rather are sensitive to changes in field caused by the moving magnet.  Each sensor will vary based on the absolute field that they are exposed to, so if two sensors are close enough to the same magnet, they both may be triggered.  The magnetic field diminishes with the square of the distance from the magnet, so the sensor further from the magnet will experience a weaker field.  The spacing needed to keep the second sensor from detecting the magnet will depend on the size and strength of the magnet.

    ASTM A1008 is a type of steel which is ferromagnetic.  As a result, the steel will interact with and channel the magnetic field from each magnet in the system.  The field will channel through the metal to travel from the N pole to S pole in as efficient of a path as possible.  This will likely reduce the peak field which would be experienced by each sensor, and will likely reduce the effect from one magnet to the neighboring sensors.  The challenge will be to ensure that the magnet does not stick to the steel and that there is enough field to trip each sensor with the metal present.

    There are a variety of magnet materials, shapes, and sizes to choose from.  All of these are acceptable but often the details of the system will govern what options might be selected.  Neodymium Magnets are commonly used because they are strong, but they are brittle and cannot withstand large impacts.  Samarium Cobalt magnets have better stability over temperature, and are commonly used in high heat applications.  Ferrite magnets are often the least expensive, but are also the weakest option typically.  There are also bonded (rubberized) ferrite and neodymium magnets which are more durable, but are weaker than their core magnet material. 

    As long as the field the sensor is exposed to exceeds BOP Max when placed in the closest position, and is less than BRP Min in the furthest operating position, the sensor should be able to switch normally.  It would probably be worthwhile to create a small prototype to test how strong of a magnet is suitable in your case.  If it is too strong, the magnet may become stuck.  If it is too weak, the sensor might not toggle. However, the closer the surface of the magnet gets to the sensor, the more likely to always provide a strong enough field, even with a weak magnet.  At a 6 mm spacing, I would expect that your magnet size should be quite small.  I would certainly try to use something smaller than the 4mm x 4mm opening in the steel.

    Thanks,

    Scott