How does this magnetism detector work?
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  1. #1
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    Default How does this magnetism detector work?

    This is a bit off the beaten path but I know there are some serious electronics guys here. This little thing is for detecting magnetism in watch components. It runs off a 9v battery, the base is cast bronze. The arm/spring carries some sort of sensor, and a part is placed on the cover over that, and it gets "boinged" and the meter shows if magnetism is present and how much. There are two settings, the second will detect magnetism in a screw 0.30mm diameter 0.60 long that has only been in proximity of a normal magnet, or in the smallest hairspring.

    It's super cool, the company Greiner made all sorts of very innovative equipment for watch work over the years, but I'm dying to know how it works. Does anyone have a clue?

    On a more machinist tack, the control knob is something totally new to me. The switch shaft is smooth. The knob has an aluminum insert, anodized, with a tapered mouth. A second anodized aluminum insert with female threads on the closed end and slits and corresponding taper fits into the knob. A firm but normal tightening of the screw secures it on the shaft extremely well. Seems pretty slick!

    20200220_083515-large-.jpg 20200220_083658-large-.jpg 20200220_091333-large-.jpg

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    The sensor looks like a coil. If you move a conductor in a magnetic field an emf is induced, causing a current to flow. So the instrument is based on magnetic induction.

    Similar knob clamps using collets, or similar, have been around since at least the 1970s. I think the manufacturer was Elma.

    Andrew

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    google hall effect sensors, very common type of sensor used all over the world, from cars (wheel speed sensing, crank/cam shaft position sensing) to home security (door/window open positions), they are also common in manufacturing machinery, milling machine axis end/reference position sensors, motor speed control, spindle orientation sensors etc.

    edit: some contain integrated electronics to make sensor work as on/off switches, but older ABS wheel sensors for instance were just a coil basically, would cost 10-20$ to replace, then they "upgraded" the sensor and placed a 0,01$ IC in the same housing to filter and process the signal inside the sensor, and now the replacement costs 50$, progress!

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    Not a hall sensor, might be a fluxgate:

    Magnetometer - Wikipedia

    Tapered split-collet knobs have been around since the 1920s, at least, I have several radios with
    that feature from that time.

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    Quote Originally Posted by jim rozen View Post
    Not a hall sensor, might be a fluxgate:

    Magnetometer - Wikipedia

    Tapered split-collet knobs have been around since the 1920s, at least, I have several radios with
    that feature from that time.
    Looks like a fluxgate to me as well. It is not a Hall device, as they do not use coils at all.

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    Quote Originally Posted by Joe Gwinn View Post
    Looks like a fluxgate to me as well. It is not a Hall device, as they do not use coils at all.
    Looks to be two (counter-wound?) coils on a ferromagnetic core, one pair of wires visible.
    For it to be a fluxgate there has to be a drive coil pair and a sense coil pair, not visible in that photo.
    Tough to be sure given the pictures

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    Quote Originally Posted by jim rozen View Post
    Not a hall sensor, might be a fluxgate:

    Magnetometer - Wikipedia

    Tapered split-collet knobs have been around since the 1920s, at least, I have several radios with
    that feature from that time.
    you're right, I didn't look carefully enough at it, jumped to conclusion thinking about something I'm familiar with

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    Quote Originally Posted by Screwmachine View Post
    On a more machinist tack, the control knob is something totally new to me. The switch shaft is smooth. The knob has an aluminum insert, anodized, with a tapered mouth. A second anodized aluminum insert with female threads on the closed end and slits and corresponding taper fits into the knob. A firm but normal tightening of the screw secures it on the shaft extremely well. Seems pretty slick!

    20200220_083658-large-.jpg

    How very Swiss

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    There is no real reason for the spring and "boing" for a fluxgate sensor. It does not need any motion of the sensor system, the "motion" is provided by the "gating" or "drive" coil.

    The only reason for the moving coil system that I can think of is to pick up the field by induction into a plain coil. So it appears to me that the device is just picking up the signal from the coil moving through the field from the magnetized item, and displaying the magnitude on the meter. The switch has gain settings, which provide "ranges".

    I suspect there is a filter in the electronics that is tuned to the resonance frequency of the spring and coil mechanical system, which is picked to be different from 60 Hz. The open unshielded PC board could otherwise pick up external signals from the surrounding area and give false readings due to external magnetic interference from motors etc, even with the cover.

    So, when you put the item on the "spot", and "boing" the lever, you are creating a signal at a specific frequency that is amplified, while all other frequencies are rejected. That allows a pretty high gain without running into trouble, meaning a rather sensitive device that is not overly complex.

    Clever, actually.

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    I could not find a US patent for this device, so no helpful answer there. However, Rudolf Greiner of Greiner Electronic of Switzerland did have some US patents based upon his Swiss patents, including one for an electronic watch rate analyzer.

    Patent Images

    Perhaps the magnetism device has a Swiss patent that could be researched.

    Larry

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    I missed the boing motion - it,s not a fluxgate.

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    I think JST has it. You're just wiggling a coil next to something that either is or isn't magnitized. The signal is amplified, filtered, rectified and read out on the meter. Cute little thing.

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    I like JST's guess as to how that device works. Without seeing the circuit, it is probably as good a guess as any.

    Those collet grip knobs have been around for quite a while. I have seen thousands of them used in better grade electronic equipment. They aren't terrible expensive.

    They have advantages and solve several problems with other kinds of knobs.

    They fit on round shafts, not special splines or flats needed.
    They don't bugger up the shaft.
    There are no screw holes that are visible.
    They mount concentrically so they are centered on the control and on the panel label.
    They do have a real tight grip so they don't usually come loose. Not even with repeated use and vibration.
    The top cap usually snaps on and off to hide and access the screw. This gives you a two color option at no additional price if style is important to your project/product.
    And they are easily installed and removed for maintenance.

    They are my preferred style of knob when I am building something. They are widely available at the electronic supply houses and probably on the internet as well. I would avoid ones that do not have brass/metal inserts for the taper.


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