Standby Compass...

[SwissCharles]

Hi Folks (a question to the electronics nerds and the tinkerers among us, anyway),
(Inspired by a post in 'Networking' : http://aerowinx.com/board/index.php?topic=3175.0 ...)

As the proud owner of a real B-744 standby compass:

I am wondering, if it would at all be feasible to modify such a compass to reflect the mag heading as sent by PSX (through Qi214 as well?) by manipulating the magnetic field around it. E.g. by placing small coils around it to move the compass rose in unison with PSX's compass heading output.

As far as I see, this would have to comprise a kind of an integer to analog converter which in turn would have to effect a phase shift along these coils around the compass by varying the current sent through the appropriate coils. Of course these shifts would have to be smoothed out to properly reflect a magnetic compass's intricacies as they are so faithfully output by PSX.

This manipulating magnetic field would have to be quite stronger as the one presented by mother nature at a given location....

The alternative to this would be to open up the compass, remove all unnecessary parts and insert a teeny weeny itty bitty small dc motor?

Or to use a dc motor to rotate a rather strong magnet externally around the compass as driven by PSX? But this solution could not be elegantly hidden enough though...

Any other ideas?

Charles

...obviously having too much time on his hands and feeling like a nincompoop - even had to google 'Myrmecology' - you guys are a bunch of intellectuals    

[martin]

Moi,

couldn't the same be achieved more simply by mechanically moving a sufficiently large iron mass (or perhaps even better a small permanent magnet)  around the compass (in order to "pull" the "needle" to the desired heading), without the need to fiddle around with magnetic fields?

Then again, while I'm pretty sure it would work with a needle compass, I am not certain if it applies to the Whiskey compass in the same way. It also depends on the "geometry", of course (one would want to hide the little "puller" magnet).

Cheers,
Martin
...and sorry for the "myrmecology" -- "antidom" just didn't sound right.
Note also that above I only mention "iron", to avoid the whole ferromagnetic vs. ferrimagnetic vs. diamagnetic thing...

[SwissCharles]

Hi Martin

Martin: Note also that above I only mention "iron", to avoid the whole ferromagnetic vs. ferrimagnetic vs. diamagnetic thing

That's why I stick with electro-magnetism 

An [admittedly not too scientific] test by moving a  rather weak magnet around the compass showed a notable deflection when passing to the left or the right of the case and only a small movement when passing in front or in the back of it. This seems plausible, depending on where they placed the magnet inside the [horizontally] moving compass rose - you meant that by 'geometry' right? By looking into the compass there seem to be two magnets hanging in parallel underneath the compass rose left and right along the N <> S line .

Well yes, it would be next to impossible to hide a moving and sufficiently strong magnet inside the given structure there (mainly the rather tight fitting 'box' in the center window post... But, say, coils wound around a permanent magnet placed at 120deg. around the compass case?... And then varying the current through these coils to mimic the magnetic field around the compass?.. hmmm... sounds like a helluva trouble to do this.

Remains the solution to drink the whisky and replace the innards with said itty bitty tiny dc electro motor to turn the compass rose in accordance with PSX's output...

Ahhh... Martin... now you have me worried there! ...... 'antidom'.... hmmm... 
no, no, myrmecology is just fine by me then ...

[Roddez]

Hi Charles,

I think the best way to achieve what you are looking for is with a motor connected to the compass.  There is enough space above the compass to couple a shaft from a motor to the compass card.

You could use a DC stepper motor, but remember a stepper motor needs something to tell it where the zero point is.    Another option is to use an AC motor in the form of a synchro receiver/resolver.  The wiring is a little more complex, but it always knows where up is and mechanically it's simpler.  The OEM trim gauge uses a synchro setup.

Food for thought. 

Rod

[SwissCharles]

Hi Rod,

Thanks for your input!

AC motor in the form of a synchro receiver/resolver

Yes, that would be the best way imho. Ahhh... neat idea to bury the motor inside the center window post just above the compass case. Shouldn't need to be a very strong motor ...

Another application for a certain interfacing solution from down under  ?

[farrokh747]

hi charles -
i had played with this some time ago - a relatively small magnet placed above the compass unit inside the center trim enclosure, will be strong enough to turn the ball - you cannot connect a shaft directly to the floating globe without taking apart the sealed compass enclosure... (i didnt want to risk opening it up to have a look inside)  there's enough room in there for the magnet and the driving unit -
You can then turn the magnet with a stepper/synchro/string-and-pulley - if you're using simstack, a small synchro will do the trick (but you may need 26vac 400hz power if it's an aviation grade synchro)

http://www.ebay.com/itm/BOEING-737-Standby-Compass-As-Removed-As-Is-Unserviceable-/261968852758?hash=item3cfe8f7716&vxp=mtr



fc

[Britjet]

Slightly off topic but you have reminded me of an old flight deck trick - magnetic broccoli.
We (or should that be I) several times succeeded in demonstrating to gullible cabin crew how much iron there was in the broccoli in my crew meal by holding it up on a fork to the standby compass and showing them the deviation. Wizard wheeze!

Peter

[SwissCharles]

Thanks for your input!

@ Farrokh: Hey that's excellent info. I wouldn't have thought that a magnet above the compass'es enclosure would be strong enough to turn the rose!! That indeed would spare me from drinking the whiskey and fiddling around with a very small motor plus the associated parts inside the case. Will try that one out ...

@ Peter: LOL   I'll bet my farm that it would have worked with couliflower as well!       

[Pierre Theillere]

Hi BritJet!

Be careful... nowadays, onboard airliners, forks (for "cattle" class) may be fully plastic-made, possibly with a shiny metal-looking paint! Maybe on 1st class, and for pilots, still enjoy metal forks' pleasures?

[Britjet]

Cattle class? I have been there. But happily I often got "mooved" up :-)

[Roddez]

couldn't the same be achieved more simply by mechanically moving a sufficiently large iron mass (or perhaps even better a small permanent magnet)  around the compass (in order to "pull" the "needle" to the desired heading), without the need to fiddle around with magnetic fields?

When you stick a magnet above or below the compass card, you can end up tilting the compass card.  As you move the magnet around, the compass card looks a little odd.  You also have to work out how you are going to move your magnet around the case...

Another application for a certain interfacing solution from down under  ?

Hopefully - if we can get everything else sorted out.  The Synchro boards are coming along - we are slowly putting all the prototype pieces together.

[SwissCharles]

Hi Rodney,

To address the problem of a tilting compass rose, that you mention, I gather that the magnet (i.e. perhaps in the form of a cylinder, say, 2cm long with a diameter of approx 4mm?) would have to be postitioned exactly on the vertical axis of the compass rose above or below it. And this magnetic cylinder could then be driven by a synchro driven interface.. just thinking aloud here 

I will do some manual trials and see how the compass behaves..

If it's tilting too much, it must be because of the whiskey in it of course    ...   Why - anyway - is it called a whiskey compass in certain parts of the world? I can't somehow imagine that the dampening fluid in it is made up of that stuff, huh ?

[Hessel Oosten]

Why - anyway - is it called a whiskey compass ?

I've read: because of the yellow color of the fluid in it.

Hessel

[Roddez]

Why - anyway - is it called a whiskey compass in certain parts of the world?

I have heard it called a Wet Compass - W for Whiskey?  I have also heard that at some stage the liquid in the compass was an alcohol based liquid so that could also be an other reason.

Rod.

[Blake H]

I have a opencockpit stepper motor card with a stepper motor driving the real compass off the shaft.

Hope this helps.

[SwissCharles]

...a stepper motor driving the real compass off the shaft. ..

Hi Blake,

Could you elaborate on this a little? What 'shaft' do you mean?

TIA

[farrokh747]

hi

did a test with a small magnet and a stepper motor....  works well, the issue is to have the stepper find the "home/zero/North" position on start..  also, some issues with the magnet placement .....  but seems to work overall... doing some more tests...

http://farrokhchothia.com/slists/compass/IMG_2079.MOV

code here :  http://farrokhchothia.com/slists/compass/compass_test.ino

cheers,

fc