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  1. #1
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    Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    Greetings -
    After finishing up my Arduino keypad radio, I decided to reward myself with an eBay binge. I picked up a Cessna ARC IN-514R nav indicator (pic attached) for the hefty sum of $1. I'm successfully controlling the CDI & to/from flag, but the OBS knob is a brain scratcher.
    $(KGrHqUOKp!E5(4bOqlBBOcPvTDnGw~~60_57.JPG

    The OBS knob is not a synchro as I assumed it would be, but a continuous wire-wound potentiometer with 4 taps. I made a schematic of the device:
    OBS Encoder schematic.jpg
    I've never seen anything like it.

    I went ahead and hooked it up to an arduino, using 4 analog inputs. Feeding the wiper with 5v, wiring the 4 poles to ground with a 1k resistor, & hooking up the analog between the pole & the resistor. I wrote a Max/MSP patch to dump the values into a table as I incremented the OBS knob by 10. Take a look at the attached graph made in Excel from the values.
    Its a bit lumpy; I'm sure the wiper could use a cleaning, but lumps aside, I just can't figure out how to take these values and turn them into a rotational angle. I'm hoping there's a simple analog circuit or equation that can turn this into something meaningful. I suppose I could do it with a big, convoluted look-up table, but that just seems lame.
    Any ideas?
    OBS Encoder readings.jpg
    Thanks!!

  2. #2
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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    Hello, forty-2
    I have the same instrument and I'm in the same experiment.

    VOR EN PROTOBOARD.jpg
    The original system works as follows:
    Between the inputs 0 and 180, is injected with a sine signal, and between the inputs 90 and 270, cosine signal is injected.
    The phase difference between both is sensed by the wiper. (The signals are generated by the ground station, the phase difference is calculated by the receiver in the plane, and show in the instrument)


    Your approach is the same as the one I try to use, but with 10k resistors instead of 1 k (no matter).
    The values in the following picture, are the internal values of the wire, between the taps. (in my instrument, of course).
    Im working with opencockpits cards, with 4 analog inputs of 8 bits each.
    puentevor.jpg

    vor 2 destripado.jpg


    If you can, attached excel sheet, maybe we can do something.
    Excel can infer or approximate the formula automatically. I've seen some tutorial how to do it, but I forgot where ...
    Luck.
    Sorry if my english is no good.
    Horacio

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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    Hi Horacio,
    Your English is great, and a heck of a lot better then my Spanish!
    I've posted the Excel spreadsheet. Yeah, I was wondering if there was a way Excel could do some of the work. I'll ask our Excel expert at the office, but that curve looks awfully daunting in a calculus kind of way.

    Can you post the attachments again? I'm getting an error on the site when I click on them.

    I was wondering about the phase difference approach, but thought it was only applicable to Synchros / resolvers. With this 'multi-tap potentiometer', I would think you'd just see a varying mix between the two signals, but no changing phase shift. Couldn't hurt to try, I suppose. I can generate the waves & read the result on the wiper in Max/MSP (Cycling 74) using my laptop's audio jacks. I'm gonna go hack apart some 1/8" cables!

    Cheers,
    Paul
    Attached Files Attached Files

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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    Paul:
    The attachment are only pictures of my instrument and internal meditions of the resistor between taps. I dont know why arent showed.
    Anyway, here again
    puentevor.jpg

    Let me see the excel sheet tonight in calm.
    Regards.
    Horacio.

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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    I couldn't find any 1/8" cables to sacrifice, so sketched up a quick simulation / schematic.

    I'm assuming 2k between adjacent poles (though I'm reading closer to 1.75k), and the wiper is shown as a potentiometer between 270 & 360

    My first thought didn't work, because of a glaring error - the grounds; one adjacent pair will always be at the same potential:
    OBS.jpg

    However this approach shows promise:
    OBS2.jpg

    The scope confirms that the phase on the wiper shifts as it's moved.

    Scope.jpg

    YES! I can mock this up in Max/MSP using 4 audio outputs towards the end of the week. Then the question is: how do we do this in hardware?

    Cheers,
    Paul

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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    Ok, the images seems to be fixed right now. (not my fault).
    Maybe you not understood my reply.
    The original operating of the instrument is based on signals (sinus and cosines). Its just for your information.
    I have a simple description (in spanish, sorry) of the diagram of operation. (wikipedia can give you an idea but very faint)
    (Just search "VHF omnidirectional range")
    In real life, the equipment to check the operation, its a signal generator with differents types of outputs. (serves several instruments, like adf, ils, etc)
    Anyway, your approach with audio signals is like the original operating of the instrument, but not practical for doing in hardware.
    (Four sinus generators, to much, and another circuit for phase-shift measure).
    The approach to using four analogs inputs is like an idea I had long ago.
    But.... (why ever there is a but....)
    If you look the internal potenciometer (photo and diagram), you can view a very strange connections.
    The internal resistance between taps are diferents. (1.82k and 2.43 k).
    Why the order of the taps is ilogical? I dont know.
    I expected
    0 (yellow), 90 (brown) 180(red) and 270 (orange),
    but the photo and the diagram shows you
    0 (yellow) - 270 (orange) - 90 (brown) and 180 (red)
    expectedvor.jpgvor 2 destripado.jpg
    The orange wire in the photo is missing (photo is not of my instrument, but when i open mine, is the same, with all the wires in the same order, like the original squematics).
    The wipe (green cable) go from 0 to 270 its ok, but why from 270 to 90 or from 0 to 180? The logical dictates 270 to 180, or 0 to 90.
    Anyway, deducting a function of the excel its complicated (ive tried using polinomics, but without luck). Maybe a fourier series.....but is beyond my knowledge.
    Sorry for the long and late reply, (a big thunderstorm in my city, let me without electricity, internet, water...)
    I hope I've helped...I'll keep experimenting in others alternatives (like 2 opposite common linked pots with limit removed and slave in sync with the OBS card, like the simkits approach)
    Keep in touch.
    Horacio.

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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    I can tell you what I did,I split it into 2 pot inputs to my oc card. One counts up 0 to 180 and the other down from 180 to 0. By using SIOC I was able to take the inputs, add them together,then subtract 360 and I got it to work very well. The only minor issue is if you turn it to fast while passing 180 or 0 it looses track. I just make sure I turn it slow when at the the poles.

    Rob

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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    Quote Originally Posted by hyamesto View Post
    Why the order of the taps is ilogical? I dont know.
    I expected
    0 (yellow), 90 (brown) 180(red) and 270 (orange),
    but the photo and the diagram shows you
    0 (yellow) - 270 (orange) - 90 (brown) and 180 (red)
    expectedvor.jpgvor 2 destripado.jpg

    Horacio.
    Why? I answer myself. Ohms law. Parallel resistance between 0 & 90 < 0 & 270 or 0 & 180.

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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    Rob:
    Have you used some external resistor between oc card inputs and the instruments?
    Maybe can you make a squematics?
    Thanks in advance.
    Horacio.

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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    Horacio -
    woah, sorry you were without water & electricity! I was just traveling for work, and happy to be back home!
    Yeah, I'm familiar with the real-world operational details, and yes, you're right that doing what I did in the schematic simulation would be a pain to do in hardware (but a fun challenge!).

    Rob - Thanks for chiming in! Sounds like you've got a simpler approach, and like Horacio, I'd love it if you could share a schematic. I can almost visualize it, but it escapes me.
    thanks!

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