burning question - HSI?

[AndyT]

By the way, Stagger the sensors slightly. Do not put the one right behind the other. That way they take less room. I meant to add that above.

[ruprecht]

I'm not sure mate, here's my thinking. Overall length is not a worry, the thing that worries me is the distance from gear 3 to the faceplate. That is the length of the RMI flags, which are thin strips of metal. That distance is currently about 30mm.

Mounting the optos between the gears will add, I'm estimating, at least another 10-15mm. that takes the RMI flag length to 40-45mm. if those flags sag, or suffer repetitive bending stresses during stepping, they will eventually conflict with the heading bug flag and tear each other apart, necessitating a quite difficult repair. remember that the steppers don't move the flags smoothly (in an analog way) they move them digitally, one half-degree step at a time. The longer the flag, the longer the intertial moment of the flag head and the more repetitive stress it encounters.

My thinking is that we need to *reduce* the gear 3 - faceplate distance as much as possible, not be thinking about making it bigger. I'm all ears, but I think the above is a valid concern given the tolerances involved?

[Michael Carter]

I can see that there is no room in the housing for this, but many HSI's use a rotating drum with the CDI needle fixed to the drum rather than the needle swing back and forth across the dial face.

That's probably not an option at this point.

[ruprecht]

BSW, I think I know what you mean and yes all I have to do is quickly model it. I've just been focused on the gear train so hadn't done it yet. I really need to do an animation to show people how it all hangs together

[AndyT]

Ok, I'm not sure we are on the same thought here, so let me add in a simple sketch and this:

What is the strength of aluminum versus the weight of the bugs that will have to be held? Inconsequential. The shaft of the flag mounts can be larger than what you have drawn because it will all be hidden behind the faceplate and CDI plate. This is not a problem we need to put that much worry into.

What are the measurements I have question marked in the sketch? These are your sizes for the plate distances.
Sorry, I did not label my sketch very well....

[ruprecht]

ah, you're a genius. Let me measure up the opto for you and see if it will work.

We were definitely not on the same page, I'm a visual person

-- rup

*edit* Ok it appears there IS room to do it like that, depending on distance x in the attached diagram. So long as x < about 3.5mm, we're gold with 3mm gears. the tolerances are quite tight (1/8 mm either side of the gear) which worries me a little, but the optos could probably stand some grinding out without dying.

The H21A1 optos are near as dammit a half-inch wide, so the existing spacers can stay! sweet as. Little bit of rework to position the gears in the longtitudinal axis, but that's no biggie.

[AndyT]

One more scribble for you to address your concerns about the bending of the shafts.
The shafts have the same curve as the inside of the gear. The shape is triangular to increase the strength and durability. With this design, no bending should be possible under normal use.

[AndyT]

We can always move the index hole closer to the edge of the gear.

[P1IC]

Hi, guys. I have just found this thread, and it’s very interesting to me, since I have just started to design an instrument panel for my own (steam gauge of course) IFR procedures training simpit, based on stepper motors and PICs.

I started with the altimeter because it seemed to have all the nasties in there that I would encounter in many other instruments - like multiple 360 degree rotation, a knob for tweaking, a subscale, a couple of high-res needles. No CDI of course, which is an additional complication, but I decided I could get to that later when I came to VOR / ILS design. I hadn’t planned on HSI!

Then I found this thread, and started (prematurely) thinking about the problem.

For what it’s worth, I have taken a different approach to you guys (maybe because I don’t have “The Book” that you all rave about, so I’m on my own there, which I hope you will believe is not necessarily a bad thing, since I’m not constrained or influenced by the ideas in there, whatever they are).

I’m intending to use needles visibly up front, combined with clear plastic disks on to which I’ll paste cards, subscales and the like, and I’ll also use these for zero location behind the visible faces. So the problem recently announced here about having no room for the various LED zero point indicators does not exist in my design. I just use one pair of IR devices, shining through all of the discs, behind the visible instrument face, each with its own mask (a bit of tape, probably) to blank out the IR, and I can program the pics to zero each disk in turn.

There’s plenty of room for that (using a 3" instrument face), and my 3-D design mockup using Cinema 4D XL 7 shows that this will work. For the altimeter I use three small stepper motors at $3 each (Futurlec, whom I hate just like you, and Sayal in Toronto, whom I also hate, but what’s a guy to do?), and some plastic worm gears and gear wheels for the drives, which I obtained at very low cost on eBay, so no machining of gears is required (although I do have a lathe and a mill, just in case).

However, the reason I’m posting this is in response to the problem of having a rotating card on which must be mounted a motor which controls the position of the CDI, and the resultant need for brushes to overcome the problem of twisted wires.

Nonsense, he cried! The design you guys are working on already involves PIC microcontrollers, and so you can solve your issues entirely in software. Remember, write once, use everywhere (to paraphrase the Java launch blurb). If you require a design with complex hardware requirements, everyone suffers and output is reduced to a quality-threatened crawl. Create a design where the software does the work on a (relatively) simple hardware platform, then write the software once, and you can produce hundreds of accurate gauges. And each simpit requires quite a lot of these devices.

If you drive the CDI by means of the angular position of a cog wheel, probably then driving a pulley arrangement (a bit of thread around a shaft ) then of course whenever the carrier card rotates, the CDI will move because its driver cog will rotate too. But not if you program the CDI cog to compensate. After all, you do know what the new position of the carrier card is going to be, right?

So the carrying card moves, and the CDI is commanded to compensate, so that it stays fixed in its commanded position.

Now, what happens if the carrying card rotates through say 100 times 360 degrees? It doesn’t matter, and your software will not crash through the limitations of an 8-bit microcontroller, because the delta values you feed in to the CDI will not actually be part of its control structure. You are just re-adjusting the zero position of the CDI continually.

Sorry for the lengthy post!

[ruprecht]

Hi P1IC, welcome to the thread. I'm very interested in your suggestions, but as stated above I'm a dumb helo pilot and need pictures to understand things Can you sketch how you think you could design an independently rotating CDI assembly without the use of a brush contact rotor? The problem as I see it is not rotating the assembly itself to match rotation of the compass card (easily done in firmware as you say) but mounting a servo on that rotating assembly to give you a localiser bar. That is what requires the brush contacts in my book.