Synchro Interface for 737 Flap Indicator

[RadarBob]

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

[RadarBob]

Would also be interested to know what the 26VAC 400 Hz power requirement is likely to be for this sort of instrument.

I have seen some small inverters at around 6VA which I'm guessing should be ok for a flap indicator, but probably need a bit more for something like an HSI?

If anyone has done any work on this I would be grateful for your thoughts.

Cheers,

Rob

[737NUT]

I can tell you what i did, i found some synchro transmitters and used a small servo driven from servo card to turn the synchro which in turn made the instrument follow that i had it wired to. I have several 28Vdc to 115Vac 400hz inverters if you need a bigger one than what you have.

[RadarBob]

Hi 737NUT,

Many thanks for your input on this one.

I had thought about doing it the way you have, and may still go that way in the end, but at the moment I am trying the more direct approach.

I guess you must arrange the gear ratios to overcome the 180 degree servo travel limitation?

I did look around for some synchro transmitters, but there didn't seem to be many around.

What synchro based instruments do you have in your setup?

I had a discussion with Mike Powell about this who very kindly took the time to explain how synchro works and point me in the right direction.

In fact, in his latest book Mike has some projects for synchro interfacing which we think will do the job.

The plan is to use Mikes approach as the basis for the project, but I will probably implement it on a USB PIC. I also thinking of making my design modular so that if I can get it working ok, then it can be easily expanded in the future to provide more synchro channels as required using the same controller.

I picked up a little 10VA 26V/400Hz inverter off eBay which should drive the Rotors ok - I measured the windings with an LCR bridge and calculated that the Rotors would require in the order of 3.3VA each. Stators should require about 3.9VA each - But these will be driven by an audio amp rather than from the inverter supply.

I took the instrument apart and found that one of the Stator windings was internally commoned to Ground (400Hz RTN) which explains why only two phase inputs (Right/Left Pointer X/Y) were shown for each pointer synchro on the diagram.

Interested to hear about your 28V-115V inverters - Do you use transformers to provide the 26VAC for your instruments?


Best Regards,

Rob

[737NUT]

Hi,
Yes, i used transformers to get my 24-26Vac 400Hz. The gearing was fairly simple, full servo travel gave me anywhere from 280 to 360deg travel depending on which gauge and gearing i used. Usually like 35-38 tooth on servo and 7-12on syntrans. I made oil pressure, flaps, duct pressure, hyd. press just to name a few. Wasn't to hard using OC servo boards and SIOC software. The inverters i have will handle up to 10amp load easy or more with cooling air. I have one on Ebay under user name saraccc

[737NUT]

Let me know if you come accross a 0-5Vdc PWM input to 0-5Vdc analog output card that is reasonable. LOL I found one that is very easy to use but only has 2 inputs and outputs for 85.00. yikes! I need to drive 15 or more gauges.

[RadarBob]

Interesting - Thanks for that.

Are you using real instruments for all of your gauges then?

I had a look at your inverter, and it looks a nice piece of kit.
What are the appx physical dimensions? - Looks massive on the listing photo, but it's probably not.

I guess one would need a pretty hefty 28V power supply to drive this at full load - ~50A?

Any idea what sort of current it draws with no load?

I will keep an eye out for the converter you are looking for - Always on the look out for useful bits at the right price, so if I see one I'll let you know.

How were you thinking of using it?

I suppose the ideal thing for the job would be an off-the-shelf digital to synchro converter, but I haven't seen any surplus, and the new price is high
- I suppose because demand is relatively low.

Cheers,

Rob

[737NUT]

Rob,
The Inverter is only 3" thick by 8" tall by 12" long, pic is deceiving. lol

I used a 24Vdc 8.5amp power supply to run mine. Not sure on the current, i think was only 3-6 amp load on the power supply if i remember right.

As for the DAC i need, i found digital/mechanical gauges for an efis 727 that use an analog voltage input to function.

My goal is to one day have all real instruments but so far all have been testing purposes only. To me that has become half the fun, finding ways to make things work. I will post a picture of the servo to synchro i threw together.

[RadarBob]

Thanks for the pictures - Nicely done!

Thanks for the info on the inverter - Size is not a problem then.

I had a quick look around the web, and there seem to be plenty of transformers around www.surplussales.com have one at 28V/11A (model# T-1846-1) which should suit it nicely.

I suppose the thing with inverters is that as long as you have enough current to supply the no load condition, then you only need to size your DC supply above that to suit the load current - So your 8.5A supply will probably drive a good few synchros.

ok on the DAC application. Interesting one - Like you, I enjoy trying to find solutions to these little problems - Probably why I've got lots of little projects on the go, but nothing that resembles a cockpit!

If you fill me in on the details, I will apply some thought to it too.

Does the converter have to be driven with a servo (pwm) input or could it be something else? -I assume you want to build on your O/C SIOC Servo solution, but if the variables are available to FSUIPC, then I guess any means of converting a gauge variable to 0-5VDC would be acceptable?

What sort of current output would you require on the 5V analogue output?

How many channels do you need to drive each function?

I guess that you have already considered using your servo to drive a potentiometer from a 5V supply in the same way as you drive your synchro?

Are you into building your own circuits, or does it need to be an off-the-shelf
solution?

Cheers,

Rob

[737NUT]

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

[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



Posted via Mobile Device

[Mike.Powell]

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

I was thinking about breadboarding a simple 30 Hz square wave oscillator and connecting it to the tach input. You could scope the signal to see if the tach is loading the oscillator too heavily. Based on what we see we revise and retest.

We can build an oscillator with parts like a LM555 or almost any general purpose opamps. Have any parts floating around?

We probably should start a separate thread. We've completely hijacked this one. :oops:

[737NUT]

Thread moved to here,
http://www.mycockpit.org/forums/showthread.php?p=95117#post95117

Sorry Bob.. Moderators can move the last several post to new thread if they like.

[RadarBob]

No problem to me Rob - It's all good interesting stuff related to interfacing real aircraft instruments as far as I'm concerned!

However, I suppose it's not synchro, so good idea to split the topics - It keeps things tidy anyway.

I'll continue this one when I have more info.

Cheers,

Rob