Synchro Interface for 737 Flap Indicator

[Mike.Powell]

Hi Rob,
I found the docs on a set of gauges i want to get working. The input signal is in Hz and it states 100% rpm = 4200rpm at the sensor which should output 70Hz signal to = 100% on the gauge. No idea on voltage though. The gauge itself outputs a 0-5Vdc analog signal proportional to eng the input. I have the factory docs if you would like to see them. I was//am trying to figure out a way to interface them as i can get them fairly cheap and with a 727 i need a few as you know. In theory, the PWM signal coming from the servo cards is also a frequency based signal but i need more/exspensive equipment to monitor and test.

As for DAC, you are correct, i could just use a servo turning a pot but i am trying to cut down on parts count and such.

There is always an option, problem is most i can't afford.

Rob

Can you post the docs?

[RadarBob]

ok, so the engine sensor outputs a signal in the range 0Hz to 70Hz which drives the gauge and the gauge in turn produces a dc output signal which varies in the range 0%RPM = 0V and 100% RPM = 5V

The unknowns then are the waveform of the output of the engine sensor, the amplitude and how much current the gauge input requires (or what the input impedance is).

I would guess at sinusoidal and 26V, but no idea on impedance - We could do with finding these out.

I don't think a servo output is any use in this case because, as I understand it, the servo signal is a train of width modulated pulses at a fixed frequency.
Pulses are varied in width typically between 0.9-2.1mS to set the position of the servo between 0 degrees and 180 degrees respectively.

There must be a chip somewhere that can be bolted on the output of a PIC to do the job.

But as you say, keeping the parts down in number and cost is likely to be an issue.

I doubt that there is anything low cost out there that will do the job - Probably find that by the time you add up the cost of buying the gauge and the parts to build a driver you will be somewhere in the region of the cost of a replica.

On the other hand, replicas may not be available for all your instruments, and they are pretty pricey so if you can crack it, then I think that the solution using real instruments will be superior.

Not really possible to estimate the cost until we know what bolt on chips would be required, and whether the output needs amplifying or not.

Rob

[737NUT]

Here are the docs on the gauge. It uses MIL spec mil-g-26611 which is a 22-26Vac 70Hz@4200rpm signal. The gauge docs state the needed specs. So i need to convert the signal from FSX to an output that matches the MIL Spec. Or convert a PWM signal to Hz which i know is possible. You'll see reading the docs hook-up is simple other than replicating the input signal needed by the gauge. Just needs 28Vdc and 5V for lighting.

Thanks,
Rob

[Mike.Powell]

MIL-G-26611 defines the standard tachometer generator. This is actually a three-phase alternator that was originally designed to drive an old style electromechanical tachometer. The old style tach contains a three-phase permanent-magnet motor which spins a magnetic drag cup indicator very much like the automotive speedometers of the 50s and 60s.

A MIL-G-26611 tachometer generator produces a three-phase AC voltage that varies with RPM. The frequency also varies with RPM. The waveform is generally sinusoidal. The generator can supply enough power to drive three 40 Ohm resistors in a wye configuration.

So, the “standard” signal to drive your N1 tachometers is a three-phase AC voltage varying linearly from 0 to 21 volts and 0 to 70Hz as rotational speed varies from 0 to 4200 RPM. The signal has enough power to drive a wye arrangement of three 40 Ohm resistors.

However, you do not need the standard signal. The doc for the tachometer indicates that the tach uses only a signal-phase input, and is capable of responding to a 0.5 volt input level. You probably don’t need a sinusoidal waveform. The tach doc mentions input hysteresis and filtering. These are means of rejecting noise on the input. I suspect a square wave would work well.

We don’t know how much power the input will demand. There’s no requirement that it use as much as the old style electromechanical tachs, and probably it does not, possibly it requires much less.

So, there is a possibility that the tach can be driven by a 5 volt logic signal passed through a capacitor to remove the DC voltage component.

Running a few tests before pursuing the interface could pay good dividends. If the tach does respond to logic level signals, the interfacing becomes much easier.

What sort of test gear do you have? Can you breadboard some simple (1 or 2 chips, etc.) circuits?

[RadarBob]

Hi Rob,

I've had a look at the PDF, and it doesn't say much about the input signal - Other than that it needs to conform to MIL-G-26611 - Which I can't find any info on.

It does say though that the instrument should detect the tacho signal down to amplitude +/- 0.5V pk, so I think it probably is sinusoidal.

Maybe Mike will know the answer to this?

If it were me, I would measure the resistance between Input and 28V DC RTN (Pins C/D) and see what it looks like - If it is a couple of hundred ohms or more I would stick an audio generator into it and see what happens.

I had a bit of a look round on the Web last night and found a nifty design for a (function) sine wave generator that will run on a cheap PIC (few dollars) and uses a few external components (op amp filter/resistor network) - Might be handy as the basis of a solution.

http://mondo-technology.com/signal

I was thinking maybe a few of something like these to generate the signals controlled by one USB PIC to handle the comms with the PC and pass the values to the generators. Can always stick an amp on the end if you need more drive.

What do you think Mike?

Cheers,

Rob

[RadarBob]

Sorry, I crossed with your reply there Mike as I was writing my post off-line.

Thought you would know the answer!

Would certainly be loads easier if input doesn't need to be sinusoidal and will work with logic levels.


Rob

[737NUT]

MIL-G-26611 defines the standard tachometer generator. This is actually a three-phase alternator that was originally designed to drive an old style electromechanical tachometer. The old style tach contains a three-phase permanent-magnet motor which spins a magnetic drag cup indicator very much like the automotive speedometers of the 50s and 60s.

A MIL-G-26611 tachometer generator produces a three-phase AC voltage that varies with RPM. The frequency also varies with RPM. The waveform is generally sinusoidal. The generator can supply enough power to drive three 40 Ohm resistors in a wye configuration.

So, the “standard” signal to drive your N1 tachometers is a three-phase AC voltage varying linearly from 0 to 21 volts and 0 to 70Hz as rotational speed varies from 0 to 4200 RPM. The signal has enough power to drive a wye arrangement of three 40 Ohm resistors.

However, you do not need the standard signal. The doc for the tachometer indicates that the tach uses only a signal-phase input, and is capable of responding to a 0.5 volt input level. You probably don’t need a sinusoidal waveform. The tach doc mentions input hysteresis and filtering. These are means of rejecting noise on the input. I suspect a square wave would work well.

We don’t know how much power the input will demand. There’s no requirement that it use as much as the old style electromechanical tachs, and probably it does not, possibly it requires much less.

So, there is a possibility that the tach can be driven by a 5 volt logic signal passed through a capacitor to remove the DC voltage component.

Running a few tests before pursuing the interface could pay good dividends. If the tach does respond to logic level signals, the interfacing becomes much easier.

What sort of test gear do you have? Can you breadboard some simple (1 or 2 chips, etc.) circuits?

Hi Mike,
Thanks for the input on this. Yes i can breadboard up a project. I have a good fluke and o-scope. What did you have in mind? I am new to the PIC use and programming but eager to learn.
Rob

[Mike.Powell]

Before I go off into a breadboarding frenzy...

What other test instruments do you have? As RadarBob says, using a audio signal generator is a good test. No point building something if you've got suitable gear on hand.

[RadarBob]

Sorry if you tried that link to the function generator project I was talking about and it didn't work - I've fixed it now.

Cheers,

Rob

[737NUT]

I'm trying to devise an interface to drive an original 737 Flap Position Indicator instrument.

I have a drawing showing the connections used to drive the instrument in the simulator environment as follows:

+5VDC LIGHT
-5VDC LIGHT
Left Pointer (X)
Left Pointer (Y)
Right Pointer (X)
Right Pointer (Y)
26VAC 400Hz (H)
26VAC 400Hz (H)
26VAC 400Hz (R)

The two 26VAC 400 Hz (H) pins are linked.

As the instrument is a synchro, My original plan was to drive it with a servo/synchro transmitter combination or build a digital to synchro converter

However, I expected to see three phase wire connections for Stator S1-S3 and another pair for the Rotor?

I don't know much about synchro instruments, and this has thrown me a bit.

I would be grateful for any advice on the signals/phase relationships I would need to apply to get this working.


Best Regards

Rob

Mike I don't have any kind of signal generator. Being looking for one on eBay



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