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  1. #11
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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    Rob,
    Are you using the same gauge as we are? I tried the approach I think you described and I finally realized something: The distances between the numbers on the gauge isn't linear.
    30 to 60 is less then 0 to 30. In fact, it appears that 30 to 45 is the same as 0 to 30. This pattern repeats around the dial, you can see it in my first photo.
    Is this why our purely DC resistive approach isn't working? probably?

  2. #12
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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    Paul:
    I just finished a test with new wiring, the results are quite promising, but it's late here in Argentina. Tomorrow I send the new wiring diagram and excel sheet with the results. Apparently, the answer is almost linear, and excel can calculate the formula.
    Be patient.
    Greetings.
    Horacio.

  3. #13
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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    New wiring.


    CONECCTION VOR 2.jpg

    Resistance connected to two analog inputs of 8 bits (256 steps each one).
    CONNECTED VOR RESPONSE.jpg
    Try this with your instrument. If we share the same or similar curve, we can continue ......A
    Excel file:
    Attached Files Attached Files

  4. #14
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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    Horacio -
    sorry for the delay. Nice work! I wired up as described and got the same curve:
    (will it show up inline, who knows!)

    Looks like we might be wired slightly differently, since mine is essentially upside down from yours, but the curve is the same, so that's what's important.

    I never took mine apart like you did (well I did, but I put it back together), so I'm wired up directly to the terminals:
    0° N/C
    90° Input #1
    180° Ground
    270° Input #2

    Now I see what Rob was talking about: Looking at one input for 1-180, and the other for 181-360.
    Spreadsheet attached as well. I'm going to poke around in Max/MSP a bit more (its just the fastest dev environment for me) and see if I can come up with some math that makes everything line up.

    Cheers,
    Paul
    Attached Images Attached Images
    Attached Files Attached Files

  5. #15
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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    I just finish a new alternate wiring. The same curve here, but by mistake, in another test, I put direct 4.5 volts between 90 and 180 inputs.....
    Burn the resistance, and now the inputs are unbalanced. I cannot get the original ohms between taps. The curves are erratic, and I think the instrument is now destroyed.... .
    I will try to open and check, but i don´t hope happy results...
    Be careful with your´s instrument.
    Learned lessons:
    The curves are really difficult to calculated by a simple function, (i tried lineal, quadratic, polymodals.... and ever give me an error of 2 to 5 degrees) Too much for the OBS. And the analogs inputs are very sensitive (never a fixed number, ever jumping from one to another in proximity). I go back to use an encoder, and a stepper, like the first design made a long time ago. (and works fine!!)
    Let me know yours advances. maybe in the future,i can find another Vor like this. Good Luck. and remember, i´m always to help you.
    Horacio.

  6. #16
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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    OH NO! So sorry to hear it got toasted! At ~2mA, damn...
    after subtracting the two, I was able to get within 3 - 5 degrees from 0° - 90°. then it flips back @ 90° & again @ 270°. The 'lump' in the curve coincides with the varying spacing around the dial (surely by some electrical math beyond me).
    graph.jpg
    It is jumpy, I haven't tried cleaning the contacts, that will help, but it would still need some smoothing in software anyway. Honestly, I'm kind of OK with 3 -5 degree accuracy for this piece: It'll be 'close enough' for NAV2, and it IS almost 50 years old, so I'll give it some respect

    Were you able to find an equation that turned this into a continuous line? I can't figure out how to straighten this out to a 0 - 360 without using a series of IF THEN functions.

    All the best,
    Paul

  7. #17
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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    I´m working with your excel file. Give me some time. And good news. I find the broken connection in my VOR, an maybe i can fix it.
    Keep in touch.
    Horacio.

  8. #18
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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    Paul:

    Here is your´s last excel file, with my notes.
    The function calculated for your instrument is:

    A=VALUE OF 90 TAP
    B=VALUE OF 270 TAP
    C= VALUE OF HEADING (OBS)

    IF A=B AND A= 200
    THEN C=0 (for exact value)
    GOTO END
    ELSE
    IF A=B AND A= 100 THEN C=180 (for exact value)
    GOTO END
    ELSE
    IF A>B
    THEN C=-1,841B+368,25 (with 200<B>100)
    GOTO END
    ELSE
    IF A<B
    THEN C=1,8505A-8,7421 (with 200<A>100)
    END

    The max error is about 4,5 or 6,28 degrees. (check excel).
    Regards:
    Horacio.
    Attached Files Attached Files

  9. #19
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    Re: Interfacing an old Cessna nav indicator w/ resistive OBS encoder

    Paul:

    Any advance with the instrument and the function posted?
    Regards.
    Horacio.

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