motional feedback (MFB) with inductive sensor - diyAudio
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Old 9th November 2005, 08:52 AM   #1
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Default motional feedback (MFB) with inductive sensor

This picture from a 25 cm Backes & Müller sub was discussed in a German forum. An inductive sensor will monitor the velocity of the voice coil.

Apparently, there are four rectangular (ferrite?) magnets, setting up two gaps of the same or opposing B direction.

There is one trace in the center of the PCB that sits at the upper end of the gaps. The voltage induced in this trace would essentially be a half wave with a strong peak every time it crosses the rest position. This might actually be a position detector used for a servo that maintains the rest position of the coil constant.

The velocity sensor is probably a coil that runs around the circumference of the PCB, so the trace at the upper end might make up 1/4 of that sensor. As the VC and hence the PCB moves, the area that is inside the gaps changes and if the field in those gaps is homogeneous, a voltage proportional to the velocity will be induced. If it really works this way, the two gaps would have to have the same orientation.

The questions we could not get a satisfactory explanation for are:
- why the use of two gaps rather than one large one?
- what is the function of those fingers on the center trace that we presume is a position detector? The are parallel to the motion of the VC, so no voltage gets induced.
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Old 9th November 2005, 09:47 AM   #2
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Intriguing. There is a different finger pattern on the left with something that looks like a type number shown. I wonder whether those fingers have maybe a mechanical or dissipation purpose. Another possibility would be a capacitive sensor. Would it be possible that the inductive part senses the mechanical mid point position and that the fingers are part of a capacitive pick-up to sense the velocity and/or position?
Do we have an electrical diagram of the correction circuit?

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Old 9th November 2005, 12:08 PM   #3
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well, this is the page the pic came from:

http://users.skynet.be/accupulse/sub_woofer.html


I am going to write him an email to ask if he has more pics.


For a capacitive pick-up, we'd need
a) a (high) voltage source
b) a motion in the normal direction of the PCB

-> not likely, I think


Back to induction:
Those textbook pics of loop area A and dphi have made me blind. There is absolutely no need that the trace run around the circumference of the PCB, the loop can be closed anywhere as long as it is not inside the gap.

So a single trace centered in the gap is enough as a velocity sensor.

Any trace that does not sit in the center when the driver is in rest position is more like an overexcursion sensor. This might be the function of the trace at the top of the pcb.

Still does not explain the function of the trace that sits exactly at the top of the gap and has the fingers.
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Old 9th November 2005, 12:20 PM   #4
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Well, if the fingers have a mechanical purpose, they would keep the center trace from moving relative to the PCB, but this seems overkill...

Also, one would want to do this only for the velocity pickup trace where excellent linearity is needed, not on something like a position or overexcursion sensor.


Coming to think of it, maybe the picture was taken with the cone pushed up, so the fingered trace really IS the velocity pick-up?
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Old 9th November 2005, 03:01 PM   #5
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Already got a reply. Seems he did push up the cone while taking the picture.

I have sent him a sketch of what I understood from his reply. If he confirms, I will post it here.
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Old 9th November 2005, 04:28 PM   #6
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Not really related, but he has some very interesting and impressive building equipment.
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Old 11th November 2005, 09:30 AM   #7
moamps is offline moamps  Croatia
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Default Re: motional feedback (MFB) with inductive sensor

Quote:
Originally posted by capslock
....This picture from a 25 cm Backes & Müller sub was discussed in a German forum.....
Any link?

Thanks.
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Old 11th November 2005, 10:38 AM   #8
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Second page of this thread:
http://www.audioavid.com/wbb2/thread...adid=2085&sid=

Warning: signal to noise on that forum is - 10 dB!
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Old 11th November 2005, 11:54 AM   #9
moamps is offline moamps  Croatia
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Quote:
.....Second page of this thread...
Thanks.

My opinion is that the sensor's linearity is the biggest issue here. The Xmax is long and I don't see how it is possible to get homogenous magnetic flux with those four little ferrits. Fingers should be used IMHO for the linearization of received voltage. The upper trace looks broken if I am not mistaken. Also, the PCB looks like it has been hand-dremmelled (fine adjustments?).

Position sensors are usually independent of velocity and acceleration, which isn't the case here. Also inductive and capacitive sensors are out of the question because hard ferromagnetic materials are used.


Quote:
Warning: signal to noise on that forum is - 10 dB!


Regards,
Milan
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Old 11th November 2005, 01:33 PM   #10
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Quote:
Originally posted by moamps


Also inductive and capacitive sensors are out of the question because hard ferromagnetic materials are used.


Milan, this is an inductive sensor for velocity. There are four ferrite magnets altogether, and there is a static magnetic field between each of the two magnets you see and its respective counterpart that you cannot see in the pic. The fingered trace is essentially vertical and sits in the vertical center of the magnetic gap in rest position. As it moves up and down, a voltage proportional to velocity is generated.

The magnetic gap should be linear inside the height of the magnets, but not outside.

I still have no explanation for the vertical fingers on the sensor trace. They certainly have no inductive function
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