Controlling air core gauges with a hacked servo-jittering problem

[rjfon1]

Good morning/evening all.
Building an analogue gauge cockpit in an Aerocommander shell. Want to use the original gauges that I have including various VORs, and engine gauges. I'm using a hacked servo to control the gauge and have initial success, but need to fine tune it a bit. I took out the servo board, removed the potentiometer and hooked up a VOR where the DC motor should be. The good part about this is that the VOR requires both +ve and -ve voltages and the servo does this well. I have two problems:

1)There is a lot of jittering of the gauge. Tried to sort it out by using a 100uf capacitor, but no joy (Capacitor used in one direction only for testing)

2)The range of movement of the needle is extreme. Full deflection is about 1.8V, and the servo puts out 5V at the limit. Any thoughts on this? Do i need a transformer to reduce the scale?

Yes I can use the open cockpits DC motor card, but at 41Euro ($70au) for 6 motors, VS 30USD($33au) for the SimIO card which can do 18 servos, plus I like to tinker, it's better than converting everything to actual servo mvt.

Thanks in advance,
Rob

[rjfon1]

G'Day all.
I seemed to have found a solution. I thought that perhaps they were a bit like LEDs and don't like the current, so I tried putting a resistor in series with the feed line. Hey Presto, it worked, sort of. Takes a few substitutions to get the exact value for precise full deflection, but it works a treat. For some reason it also stopped the jittering. I think that may be due to the full scale being used and it attenuated even the slightest fluctuations. Now I'll be able to connect up most of my gauges without even having to modify them at all.

[xplanematt]

Aero Commander shell? Awesome! We want pics.

You do not have an air core motor, by the way. Sounds like it's a simple D'Arsonval meter. If this is a real VOR gauge, you are just applying a small voltage to either of the two inputs to deflect the needle. The gauge is wired so that polarity is reversed based on which terminal you energize. And yes, they require very little voltage, and a resistor is the way to go (as you have discovered). Pick a resistor that results in your full 100% duty cycle just barely maxing out the needle deflection, so you can take advantage of the full PWM range provided by your controller card.

On an air core motor, you apply two discrete voltages to the two coils of the motor, representing the sine and cosine of the desired angle. It doesn't matter what voltage you run (as long as you don't fry a coil), because the needle simply moves until the magnetic fields of the two coils are in balance.

I would just add that the use of a servo is totally overkill, and may actually be costing you some precision. D'Arsonval meters can be run directly from the PWM channels of an Arduino (or the output terminals of a DC motor controller, as I'm guessing you are using). You can run air core motors with just an Arduino and an H-bridge chip (like the L-293D). Actually, you *should* be able to run them straight from the Arduino without an H-bridge, but I haven't tested this yet (the Darlington transistor arrays on the L-293D provide some current protection to the Arduino, and also allow you to run higher voltages to your air cores).

I spent many happy hours in the right seat of a Turbo Commander 680 as a kid, traveling with my dad. Now I have a Sabreliner cockpit, so I am once again looking at that familiar old Rockwell logo. Actually, I have recently discovered a partially gutted Aero Commander nose section collecting dust in the sim/flight school building of an airport not far away.....I may try to get my hands on it once some of the dust settles on my current projects........

Please share pics, I'd love to see the Commander! There's nothing like a real cockpit for a sim!

Matt

[rjfon1]

Thanks for the reply Matt.
Unfortunately renovating a house is slowing down my project big time.
As far as using the DC motor controller alone, I tried that and there was no precision at all. There seemed to be constant fluctuations (not jittering).

Just out of curiosity, you wouldn't know if it's possible to use the sin/cosine windings in the VOR to detect the bearing and feed it into the Arduino and back to the computer would you? Would make life simpler. At this stage I was going to attach stepper motor and gearing, with an optical interrupter to detect 0. Changing bearing would be done with a rotary encoder. This is how most people do it. There is a book from mike powell on 'Building Simulated Aircraft Instruments' that creates a plug a play version in USB, but I've never been able to get my hands on one. Tried talking him into a reprint and/or an E-version but he wouldn't be in it.
Thanks for the reply.
Rob Fonhof
Melbourne Australia.

[xplanematt]

Ah yes, my wife and I did the remodel thing when we purchased our house about 3 years ago....there was no sim work for a while!

If you are referring to the goodies inside the OBS gauge that are tied to the compass ring, I'm not entirely sure how those are made up, but the assembly as a whole is essentially a phase shifter (not unlike an audio/music phase shifter). There are two discrete signals that make up a composite signal coming into the instrument. One is a fixed reference signal. The phase of the other signal is offset from the phase of the first signal by however much your course is from 360*. The phase of this signal coming into the instrument is shifted by turning the OBS knob. The offset generates a DC voltage that is applied to the D'Arsonval meter that the CDI needle is attached to. Hence, when the two signals are in-phase, there is zero voltage: needle centered. I'm a little fuzzy on some of the other specifics, as it's been months since I dug into this, but that is more or less the basics.

I'm actually using a sound card to drive my OBS gauge. I got help from a ham radio operator who had, for his own amusement, written a piece of software that generates audio tones that simulate the actual radio signal a real VOR ground station emits. Now, PC sound cards introduce phase shift of their own unfortunately, but the good news is that the error is constant for a given PC and a given sound card, which means, you can do a one-time calibration and forget about it. It's very cool to have a real OBS working exactly as it does in the real plane, with zero modification.

My experience with stepper motors is that they are not suitable for very many things on our sims. I can see how they would be good for very discrete, abrupt, and precise positioning of things (doubtless why they are used so much in printers, scanners, CD-ROMs, etc), but they do not look very smooth while they are getting there. If you gear them down like crazy, then you can make much finer movements, but this presents more demanding space and engineering requirements inside your instruments. And then you still have to figure out how to close the loop and determine your gauge's position. Using a "home" location is fine, but if you are doing something like the OBS gauge, you may have HOURS of OBS movement, but never actually hit your home position. Hence, your calibration will drift. Multiple sampling points or a shaft encoder would alleviate this somewhat, and I suppose if you figure out the right stepper assembly and the right shaft encoder for your specific application, you could work out a good solution.

I prefer a more "standardized" and simpler approach. Some ideas would be using a continuous-rotation potentiometer (like a smaller version of those found in real servo altimeters). Another possible solution would be Hall encoders. I am currently experimenting with Hall encoders, and it appears they could be the "magic bullet" to angular position sensing. My current approach piggy-backs onto a synchro or resolver, effectively providing a low voltage, DC-based solution for reading synchro-based gauges, without modifying their internal mechanics. Some OBS gauges are actually synchro/resolver based, but even a traditional gauge like yours should be doable with Hall encoders.

If you are not sharing pics, could you at least tell us what model of Commander you have?

Matt

[rjfon1]

G'Day Matt.
Thanks for the reply. Sorry it took so long. Have been on shift. Work as a flight paramedic, so plenty of time in aircraft. We use Kingair B200C aircraft and N3 dauphin or Bell 412 helicopter. New contract for the helicopters so we're getting 6x brand new AW139s. Next to our hangar is a company that uses only aerocommanders for freight and charter work, plus a dornier for heavy duty stuff. They have about 25-30 Aerocommanders around the country, pistons and turbines.

Mine was a 685. Used to have registration VH-FVJ.

http://www.google.com.au/imgres?imgu...tart=0&ndsp=55

It had the geared engines, turbos and pressurised. It was due for a new spar, but too expensive to do, so they parted out the expensive stuff to use as spares, engines, control surfaces, landing gear etc. I got the leftovers. Here are some pics attached.

Read an article on the net about a chap who bought an early aerocommander for 21k, flew it home, spent 20 on it and has a great aircraft. Love to do that. I'll see if I can find it again.

As far as the ons and other gauges, it looks interesting and I now understand the concept as you described it. Do you still have the link to the information?

When I was typing my last message, couldn't remember the word resolver, but I know what it does.

If I could get my HSI going, it would be fantastic.

Cheers,
Rob.

[xplanematt]

That is AWESOME! Do I see a full instrument panel in there? Did you get to keep all the goodies? Looks like someone got a little crazy with the saw on the right corner.... Easy to fix though. I was lucky, my friend who cut up my Sabreliner is not a professional demolition or metalworking guy by trade, but he did a fabulous job on the cutting (he has done a few cockpits before).

I LOVE real cockpits...and that one is a beauty!!! I could not find many good pics of the 685 cockpit online, but from what I can see it looks like it would make a very nice sim. Man, keep us posted, I would love to follow your project!

The VOR stuff was a culmination of several weeks of research and digging and experimenting, there wasn't really one specific resource (other than my ham guy) to refer you to. Here are a couple links to get you started:

http://www.pprune.org/archive/index.php/t-471950.html

The poster "ft" explains it pretty concisely. There is also knowledge to be gained from the following patent:

http://www.google.com/patents/US3729683

Or, if you just want the pile of WAVs I generated (all 360 of them), I can ask my ham guy if it's OK to redistribute. I'm sure he's fine with it, I would just have to check. I made 360 wavs...so it's precise to only one degree, but that has worked out just fine even for IFR approaches. You would still have to code up the software to grab radial info from you sim software (FSX, I presume?) then play the corresponding WAV out to your sound card of choice...but that's easy stuff when you already have the composite signal in the form of WAVs. You would also have to apply the needed course offset to allow for whatever phase shift your PC introduces (again, easy stuff).

If you are using X-Plane, I can point you to the appropriate dataref to pull indicated VOR radial. FSX is a different story....I can work with FSX, but I HATE it.....and don't even get me started on SIOC, bleh!!!!

Btw here's my Sabreliner project, if you haven't seen it by now: http://sabrelinersim.com/ I hope to make a lot more progress this winter....just got my building application submitted to the city today. The plan is to get my shop at least enclosed before cold weather hits, then I can move the Sabreliner in and start restoring it.

Matt

[rjfon1]

G'Day Matt.
Found your site a few days ago. Looks great. Also saw the weather radar video. Again, fantastic. My cockpits came with lots of bits and pieces, and also had access to a treasure trove of other stuff, which included a couple of weather radars. I was not going to use them as it was starting too look too complicated, but you've shown it can be done.
Regarding the system, I love FSX scenery, but I hate the flight dynamics. I got Xplane10 and the difference was very noticeable. So, I'm changing over to XPlane. Have a look at a site http://www.v1avionics.com/projects/sim42911/ it is virtually a plug and play for what we are doing. tried to contact him, but no reply yet. It would be great with my HSI. You've given me a new energy to get moving (After the renovation), and yes I will be following your project as well. Cheers,
Rob.