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Magnetic Strength Sensor

I took my friend, Rick (Paintball Mini-gun) Galinson, to dinner for his birthday the other evening. He asked me if I new of a sensor with an analog type output that corresponds to the strength of a magnetic field.

I don't know of such a device.

I'm hoping some of you might and willing to share your thoughts.
Jon McPhalen
Hollywood, CA
It's Jon or JonnyMac -- please do not call me Jonny.
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Comments

  • 34 Comments sorted by Date Added Votes
  • jmgjmg Posts: 11,657
    JonnyMac wrote: »
    .... He asked me if I new of a sensor with an analog type output that corresponds to the strength of a magnetic field.
    ..
    If you mean packaged, that's harder, but search for Hall Effect Linear, or Magnetoresistive gives sensor elements that could be used.

    Google also finds Analog/PWM/SENT output ones, like SiLabs Si721x series, that look easy to talk to.

  • What kind of range are you looking for? Magnetic compass, or MRI chamber stuff?

    -Phil
    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
  • Not sure of your application but what about a linear velocity transducer. Trans-telk
  • Rick wants to use it to detect an actor's jaw movement, condition the value, and then drive servos.
    Jon McPhalen
    Hollywood, CA
    It's Jon or JonnyMac -- please do not call me Jonny.
  • Will the actor wear some type of appliance on his jaw??
  • Yes.
    Jon McPhalen
    Hollywood, CA
    It's Jon or JonnyMac -- please do not call me Jonny.
  • YanomaniYanomani Posts: 563
    edited July 22 Vote Up0Vote Down
    Hi JonnyMac

    There is a great range of choice, when it comes to linear hall-effect sensors, from AllegroMicro,TI and other companies.

    A bit of experiment will be needed, in selecting the proper (neodimium?) magnet/sensor pair that best matches the sensitivity range of mechanical measurement your friend wants.

    In the same line of thinking, but not fully OT; since actresses and actors often use ear-phones, have ever you heard/thinked about using something like this:

    iplab.cs.tsukuba.ac.jp/paper/international/ando_uist2017.pdf

    They are talking about tiny barometers (Bosch BMP280) and commercialy available canal-type ear-phones (Panasonic RP-HJE260).

    At least they seem to be easier to wear, different from having to glue something to the actor's face, near the muscles or teeth, like magnets or flexible piezoelectric sensors.

    The main problem I can foresee with air pressure-based sensing, is that they will also catch the swallowing movements that happen, for example, when we compensate for changes in atmospheris pressure, so as to avoid feeling pain in the ears, though the authors of the study seems to didn't tested that possibility.

    Henrique
  • I have used a sensor like this one as a weather vane to determine wind direction in the 360 range.

    AS5145

    Not exactly what you are looking for but could be used to determine what angle the jaw is at.

    Mike

  • iseries wrote: »
    I have used a sensor like this one as a weather vane to determine wind direction in the 360 range.

    AS5145

    Not exactly what you are looking for but could be used to determine what angle the jaw is at.

    Mike

    This is the type of sensor that Diver Bob uses to sense the position of the legs on his hexipod bot. He and Duane Deign have developed Prop drivers for this type of chip that uses a magnet with a magnetic field orentation across the magnet rather than lengthwise to the magnet. Angular position results can be quite accurate.

    Jim
  • I'm thinking if the actor can wear some type of chin strip harness you could use potentiometers in the hinge points. Another more complex approach would use a 3D scanner with small glitter sized reflectors glued on various face, chin, and jaw points.
  • JonnyMacJonnyMac Posts: 6,060
    edited July 22 Vote Up0Vote Down
    DigitalBob wrote: »
    I'm thinking if the actor can wear some type of chin strip harness you could use potentiometers in the hinge points. Another more complex approach would use a 3D scanner with small glitter sized reflectors glued on various face, chin, and jaw points.
    The costume requires more flexibility that than -- I'm certain multiple sensors will be required to get to what Rick wants. The scanner approach will not work as this has to work live on set.

    I appreciate everyone's input. Keep in coming!
    Jon McPhalen
    Hollywood, CA
    It's Jon or JonnyMac -- please do not call me Jonny.
  • What about attaching gyro IC's to some type of chin strip to get positions. I'm thinking magnets could be tough solution because you need some type of receiving antenna or coil.
  • The jaw movement has to be independent of the head position. I have a couple leads on sensors that we're going to try.
    Jon McPhalen
    Hollywood, CA
    It's Jon or JonnyMac -- please do not call me Jonny.
  • Are you aware of magnetic paint? Might be handy to paint the areas you want to capture, without adding much bulk.

  • How about accelerometer? You can place then independently and then process them from a reference zero over any range of motion. The limiting factor would be the processing power required to create the absolute points from the relative data. Link the with differential twisted pair or CAN to a Bluetooth transponder to keep as much of the electronics as possible out of the mask area. Just my 2€ worth. (Yeah, inflation and exchange rate arbitrage)
    Ordnung ist das halbe Leben
    I gave up on that half long ago.........
  • I was thinking accelerometer as well. Perhaps one at the chin and one at the upper jaw to get comparative data. Maybe even use a flex PCB to mount both and bond to the face with cosmetic adhesive so it is barely noticeable if at all.
  • Tracy AllenTracy Allen Posts: 6,262
    edited July 24 Vote Up0Vote Down
    Mutual inductance might work. That is, magnetic coupling between two coils that move wrt one another. Usually one is excited with a sine while the other is the pickup to detect amplitude as a function of angle or distance. There are variations on the principle, for example, one coil can be a passive resonant circuit while the other detects the reflected impedance. Or as a metal foil affects inductance.

    Maybe Beau will chime in.

    TI makes an inductance to digital converter chip LDC1000, which would be any easy path for this but probably overkill.

  • jmgjmg Posts: 11,657
    edited July 24 Vote Up0Vote Down
    A variometer might work. That is, magnetic coupling between two coils that move wrt one another. Usually one is excited with a sine while the other is the pickup to detect amplitude as a function of angle or distance. There are variations on the principle, for example, one coil can be a passive resonant circuit while the other detects the reflected impedance.

    That sounds useful, like a LVDT approach where 2 or 3 coils are coupled.
    Some use A and !A sine drive with a null at centre balance, and others use COS and SIN, and give 45' phase shift at balance.
    That would be contactless, and could be quite light, and should have good noise immunity and be easy to set up, with a range related to coil size.

    I wonder if a hinged PCB with Printed coils could be made good enough here ?


    or, maybe a small stepper motor can be connected as a sensor/generator, and give useful enough velocity related feedback to sync mouth movement ?
  • Tracy AllenTracy Allen Posts: 6,262
    edited July 24 Vote Up0Vote Down
    I originally used the term variometer, referring to an antique way of using two coils to tune a radio. Well, I figured it might not be understood and changed it to mutual inductance. Still, it is instructive to search up how a radio variometer works. These days variometer more often refers to a fast response aural altimeter.

    There are arrangements of coils that bring them into good null where the fields cancel out in the receiving coil so that small disturbances can be easily detected. Similarly for a single excitation coil and a passive second element.

    Here is a nice article from Digikey on the differences and applications of Hall effect vs magnetoresistive sensors.



  • There's this:



    -Phil
    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
  • jmgjmg Posts: 11,657
    edited July 24 Vote Up0Vote Down
    TI makes an inductance to digital converter chip LDC1000, which would be any easy path for this but probably overkill.

    Perhaps, but this sort of application is less price sensitive. Something that is reliable is worth a lot.


    There is also this device LDC0851 - somewhat lower cost.
    -- that has a compare out, but seems to run 2 coils, alternating for ~ 4318 (4096?) counts each. A comparator on each coil could feed to a Prop Freq Ctr, to use this for analog compare use.
    the simple compare out could drive a LED to confirm operation.

    addit: another approach, could be to use the nifty small SOT353 74AHC1G42xx series
    - use that to make a LC Oscillator, in the MHz region, and feed the Osc/2^N into a prop pin, to measure frequency (eg 15M/2^14 ~ 1kHz divided) - pretty much the same idea as the TI parts, but much cheaper.
    Two could make a differential design. An 80MHz Prop would resolve to > 16 bits on that ~ 1KHz/1ms update rate. Measure over 16 cycles/16ms, and it's ~ 20 bits.
  • tonyp12tonyp12 Posts: 1,914
    edited July 24 Vote Up0Vote Down
    http://www.ti.com/lit/ds/symlink/drv5056.pdf
    EXAMPLE:
    Magnet 10mm × 6mm ferrite cylinder
    Distance from magnet to sensor From 20 mm to 3 mm linear.
  • tonyp12 wrote: »

    ...that is one interesting device.

    Thanks.
    Well-written documentation requires no explanation.
  • jmgjmg Posts: 11,657
    tonyp12 wrote: »
    http://www.ti.com/lit/ds/symlink/drv5056.pdf
    EXAMPLE:
    Magnet 10mm × 6mm ferrite cylinder
    Distance from magnet to sensor From 20 mm to 3 mm linear.

    Nice looking part, but does need a bulky magnet. I'd guess any magnet will 'work', but their linearity would vary - for this, that may not matter too much.
  • tonyp12tonyp12 Posts: 1,914
    edited July 25 Vote Up0Vote Down
    That example use the DRV5056A3 on weaker ferrite magnets:
    Neodymium is 8x stronger at same volume (though FE is 40% lighter in weight):

    That this tiny at 40cent each should work, it got a step-in to better secure it to a holder:
    https://www.kjmagnetics.com/proddetail.asp?prod=GD05H1&cat=185

    There is a calculator on the right side http://www.ti.com/product/drv5013,
    and using the above magnet 0.15" & 0.1" grade 42:
    at 20mm = 0.62 mT
    at 5.3mm = 19.93 mT (the max for A1 version)
    at 3.9mm = 39.25 mT (the max for A2 version)
    at 2.7mm = 79.77 mT (the max for A3 version)
    at 1.7mm = 160.12 mT (the max for A4 version)

    Ferrite or neodymium magnet pro/cons:
    https://www.supermagnete.de/eng/faq/Should-I-buy-a-ferrite-or-neodymium-magnet
  • JonnyMac wrote: »
    He asked me if I new of a sensor with an analog type output that corresponds to the strength of a magnetic field.

    Though I haven't toyed with one for years, an analog Hall effect sensor indeed falls into this category. The threshold digital output typical of Hall devices is added to make them useful in more applications; internally they are all analog.

    I'd worry some about where to put the magnet(s). I imagine Rick has talked to the stunt coordinator for his/her input on possible use of magnets placed inside the actor's mouth. Maybe sensors for use with a much lower Gauss magnetic strip (therefore not swallowable) could be used. Anyway, an interesting project.
  • A friend/client uses this vendor for Neodymium magnets:
    https://www.magcraft.com/

    He uses them in a specialty scale designed for shooting enthusiasts who reload ammo. Funny thing is that the scale is controlled by a BASIC Stamp 1 (it's still useful).
    Jon McPhalen
    Hollywood, CA
    It's Jon or JonnyMac -- please do not call me Jonny.
  • I would definitely not put super magnets inside anyone's mouth. If you accidentally swallow more than one of them, they can cause huge intestinal problems:

    https://www.webmd.com/parenting/baby/news/20041026/swallowed-magnets-are-dangerous

    -Phil
    “Perfection is achieved not when there is nothing more to add, but when there is nothing left to take away. -Antoine de Saint-Exupery
  • Even one could cause significant injury in case it did not pass. Usually an MRI requires a metal screening survey, and an x-ray of the orbitals if there was any possible metal fragments in the eyes. But that's about it. Google for MRI and tables, chairs, and tanks. Then imagine that same force pulling on a small super-magnet. Our newer MRI suites have metal detection systems at the access, but older ones may not.

    Unless the magnet was firmly embedded in a dental appliance, I would not recommend that route.
    Ordnung ist das halbe Leben
    I gave up on that half long ago.........
  • Is the actor actually on set in front of the camera or off to the side with the servo controlled device on camera? What range of motion is required? Probably would have been a better question before suggesting a solution.
    Ordnung ist das halbe Leben
    I gave up on that half long ago.........
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