Magnetic field detection

Kermit2 said:

I don't think the sensor(hall) needs to be outside of the tube. If the drawing you show is what you will be making, the sensor can be inside the tube and not 'rotate' at all. putting the magnets in the blades will work for more than a one off, plastic can be extrusion molded around anything and there are also wonderfully strong epoxy glues in these modern times. a smaller support rod inside the 'rotating' tube would not have to move at all, and could place the device(hall) extremely close to the edge of the blades shroud. Hope you can picture what I'm trying to say, it would not be impossible to do, but actually very easy.

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Thanks Kermit2 but as I replied to Ken, it is not possible to get anything inside the tube.

Thanks for your help though it all goes into the melting pot I call my brain.....who knows what will come out?

if nothing can go inside the tube and you don't want anything outside the tube because of weathering, I'm not sure what is left to consider except another design.

Wonder if you could place a tube attached to the top of the box, concentric with the rotating tube. A ring pickup coil on top of the tube, also concentric with the rotating tube. The coil very near the fan. Not sure if you could induce a signal like that.

ken

Thanks Guys for all your suggestions. However, the original question still remains, I have the mechanical arrangement I have and cannot change that. I have seen wind speed monitors that have this arrangement and they work fine. What I need is a circuit that will detect the rotating magnetic field produced as the fan rotates. I will try a tape head to see what happens but again this may not be practical for more than 1 off. I would like to use an off the shelf inductor or something similar to act as the detector. The box will not shield the magnetic forces so anything inside should be subject to this field. I feel that what I need is a simple - or maybe not so simple, amplifier circuit. Maybe I need to build in a low pass filter so that the local radio station is not picked up but the rotating field will be. I am not an electronic designer I am afraid, although I can build whatever is designed with surface mount or whatever. Any circuit suggestions would be most appreciated.

Thanks again for everyone’s help with this project.

Is your magnet of the rare-earth variety?

hgmjr

I'm certain you won't use this idea since you have set on a different course and don't wish to deviate from it, but perhaps others reading this thread in the future can benefit from this idea.

Kermit

hgmjr said:

Is your magnet of the rare-earth variety?

hgmjr

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Magnet is Neodymium Rare Earth Magnet - diametrically magnetised.

Kermit2 said:

I'm certain you won't use this idea since you have set on a different course and don't wish to deviate from it, but perhaps others reading this thread in the future can benefit from this idea.

Kermit

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You are quite correct, this would have been a way to do it but I am afraid I am stuck with my set-up. It is too difficult to implement this idea for my device. One thing I have not mentioned, the vane and propeller part of the tube lift off for transportation. The electronics box just has a short stub shaft sticking out for the propeller and wind vane to drop on to. The bottom of that shaft has the wind direction magnet fitted to it and this almost rests on top of a special magnetic direction detecting IC.

Nothing is ever simple..............still we persevere......thanks Kermit2

Can you post the part number of the inductor featured prominently in your photo?

hgmjr

edmo117 said:

In the final unit, the prop will be less than 20mm diameter so the struts will need to be VERY small so as not to influence the wind speed measurement.

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Looks to me as if you're using an ordinary fan blade in reverse, I do hope you realize these were designed to BLOW air instead of the reverse so unless you've got some fantastic bearings it's going to take a bit of wind movment to overcome the initial friction from a dead stop.

OK, here is an idea to consider.

Place a piece of reflective tape along the edge of each of the fan blades. Then you could use a reflective photo emitter/detector sensor similar to the one that I have attached.

You may need to make a cutout in the cowling that encircles the fan and then use a transparent thin plastic patch to seal it back. The sensor could then be mounted so that it looks through the transparent window and sees the reflective tape on each of the blades. This approach yields more pulses per revolution which should help your anemometer be more accurate at lower wind speeds.

This approach if successful would overcome the problems associated with trying to accomplish your goal with magnetics.

hgmjr

OK I am assuming the prop/turbine is at the point on your instrument, facing the wind with the tail/vane behind; Right?
A couple of thoughts ...

If the prop was magnetically permeable (say steel) it could actually be a pole piece for a central magnet.

You could mount a magnet on a blade tip, better still all the tips, with a standard hall sensor reading them from an embedded position in the housing.

Have you considered using an off the shelf brushless fan (CPU cooler or similar)
If you take out all the electronics you will be left with a fan on a bearing that sweeps magnets over several coils ....
The cheeper ones run sintered bushes on stainless or chromed shafts and you could pot the coils so they unit would be essentally waterproof and corrosion resistant.

If all else fails put the sensor in a fixed shaft and rotate the magnet around it attached to the turbine.

Whatever you do you will have to balance it and the closer your sensor is to its actuator the easer it will get.

Just musing ... its late
Al

Sorry just reread the whole thing ... trying to understand why this should be so hard, there are literally thousands of these things out there and most are really simple.

Are you saying that you cant arrange an electrical connection between the vertical shaft and the head because this is where the direction sensor sits with it activating magnet?

If so I have a possible solution that keeps all active conponants in the vertical shaft.
I will sketch it for you but will need a little more info first.
Al

hgmjr said:

Can you post the part number of the inductor featured prominently in your photo?

hgmjr

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The inductor detailed in the photo is just a standard 1mH inductor. It happens to be made by Wurth Elektronik see:-
http://uk.rs-online.com/web/search/searchBrowseAction.html?method=searchProducts&searchTerm=4846462

But I will use whatever is best for the job.

marshallf3 said:

Looks to me as if you're using an ordinary fan blade in reverse, I do hope you realize these were designed to BLOW air instead of the reverse so unless you've got some fantastic bearings it's going to take a bit of wind movment to overcome the initial friction from a dead stop.

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I realise that thanks - the bearings are needle point - like a watch has.

Thanks for the update though

Dyslexicbloke said:

Sorry just reread the whole thing ... trying to understand why this should be so hard, there are literally thousands of these things out there and most are really simple.

Are you saying that you cant arrange an electrical connection between the vertical shaft and the head because this is where the direction sensor sits with it activating magnet?

If so I have a possible solution that keeps all active conponants in the vertical shaft.
I will sketch it for you but will need a little more info first.
Al

Click to expand...

Al - thanks for looking - I am stuck with my mechanical arrangement so cannot get to the bottom end of the shaft. The shaft is aluminium - solid as it happens but could be a tube if required. The bottom end is drilled 3mm Dia and a diametrically magnetised magnet glued in. This almost, but not quite, touches the surface of a detection IC - Austria microsystems AS5030 to determine wind direction.

You are correct about there being lots of anemometers out there and I have seen one in particular where the fan has a magnet, the same principle as mine, and works fine. I know they too use an inductor for detection but it goes into a custom IC so no joy there.

This MUST be possible but unfortunately, I am not an electronics guru so don't know where to start. I can build anything from a 'fag packet' circuit diagram but cannot draw the circuit myself.

thanks agin for lookin at my problem

hgmjr said:

OK, here is an idea to consider.

Place a piece of reflective tape along the edge of each of the fan blades. Then you could use a reflective photo emitter/detector sensor similar to the one that I have attached.

You may need to make a cutout in the cowling that encircles the fan and then use a transparent thin plastic patch to seal it back. The sensor could then be mounted so that it looks through the transparent window and sees the reflective tape on each of the blades. This approach yields more pulses per revolution which should help your anemometer be more accurate at lower wind speeds.

This approach if successful would overcome the problems associated with trying to accomplish your goal with magnetics.

hgmjr

Click to expand...

Thanks for the thought but, as I stated earlier, I cannot get any electrical connection from the tube or housing. Also, optical methods are subject to dirt, rain etc and take too much power.

thanks again for looking.

Have you considered using a tape head from those old cassette tape recorder?

Problem is that they are hard to obtain new.

There are big difference between a tape head and an current inductor. The tape head has a fine gap in the otherwise closed magnetic path and so is very sensitive to small magnetic flux change. It can produce signal hundred of times larger than a plain ferrite inductor.

eblc1388 said:

Have you considered using a tape head from those old cassette tape recorder?

Problem is that they are hard to obtain new.

There are big difference between a tape head and an current inductor. The tape head has a fine gap in the otherwise closed magnetic path and so is very sensitive to small magnetic flux change. It can produce signal hundred of times larger than a plain ferrite inductor.

Click to expand...

Thanks you. This was suggested early on in the tread but, as you point out, the problem is obtaining these in any sort of quantity.

Good idea though

100 mm? Should be a cinch with a fluxgate magnetometer. Just Google "fluxgate". You can make one yourself if you are good with analog circuits. Otherwise, skip the magnet and try an optical pickup as Hgmgr suggested.

http://forum.allaboutcircuits.com/member.php?u=2236