burning question - HSI?

P1IC, I'd love to see pix of your design if you are willing to share them. 3D is better of course. It sounds like a novel solution but I have questions the Pix will answer.

No problem if you don't have the book, its not a requirement. But it is very close to the 'standard' way instruments are built and when done, you have a very high quality instrument that perfectly imitates the original. (Well, almost perfectly)

Once this design is finalised, it will be fairly easy to duplicate for nearly anyone since we are using off the shelf parts, with the possible exception of the gears which can be sourced fairly easily.

Thanks for posting and adding to our progress!

Ruprecht:
It would be better if the optos were all in line, then they could all be mounted on one piece of circuit board. Like Andy, I would prefer to keep the home sensors as optos. With the reed switches, I've found that you have to home them from either clockwise or anticlockwise. Also they're a bit inaccurate so there are some calibration issues as well as accurately placing the magnets on the gears. Not to mention them falling off (possibly).

Re: The RMI pointer?? I'm not familiar with the F16 gauge, but on the BK gauge the course pointer is just an extension of the CDI needle. It doesn't rotate independently of the CDI. Additionally, the CDI card rotates in sync with the compass card except when the course is being changed. Ditto for the heading bug.

P1IC: welcome to the thread. I didn't quite work out what you were suggesting, but I'd sure like to see some drawings of your ideas. Like Ruprecht, I'm a visual person. However I agree with the sentiment of making software do as much as possible. Having said that though, the whole thing could be done with a small color LCD but that's not really what we're trying to achieve.

Ruprecht: When we get down to cutting gears and bits of metal, we gotta get together. I'm sure there will be a thousand details to sort when we start making metal shavings. At that point your skill with the CAD package is really going to be crucial.

*EDIT*Ruprecht with respect to clearances in the optos, no problem, just make the gears thinner. 3mm at the sizes we're working is WAY overkill anyway. You could take them down to 1.4mm no problem.


Cheers

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Originally Posted by cscotthendry

Re: The RMI pointer?? I'm not familiar with the F16 gauge, but on the BK gauge the course pointer is just an extension of the CDI needle. It doesn't rotate independently of the CDI. Additionally, the CDI card rotates in sync with the compass card except when the course is being changed. Ditto for the heading bug.

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The CDI is on NAV1/ILS and the the CRS pointer is, as you say, affixed to the CDI card coaxial with the CDI indicator in its centre position.

The RMI flag is exactly that - an RMI. an independent pointer that points to the VOR tuned on NAV2 (with no course information other than direct bearing-to/from).

I suppose you could make it work as an ADF instead in software by using the NDB bearing tuned on ADF1 instead of the VOR bearing on NAV2. Could even include a VOR/ADF toggle on the instrument I guess! Maybe for version 2 :)

Ruprecht:
OK, I've got an RMI gauge that has a pointer that does that. But it only has the card and the pointer not all the other stuff as well. I guess then that modding your design to the BK will mean reducing some of the functionality (which is just fine by me :D)
Given what you say, it was inevitable then to have four gears involved. No matter though as you've done the hard stuff fitting it all in.
Just thinking on it though, I would rather have the GS bars than the extra VOR indication. VOR 2 could always be done on a "conventional" RMI (another project???)

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Originally Posted by cscotthendry

Ruprecht: When we get down to cutting gears and bits of metal, we gotta get together.

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No worries mate, that would be great. PM me if you're going to be in the city during the week, I can meet up for lunch and show you over the model.

I reckon we'll be able to make shavings pretty soon - if you wanted to get started cutting some of the 120-tooth gears I reckon they're pretty much set in stone now once we've settled on a width.

Re the gear width, the reason I went 3mm is to give it a little bit of wiggle room with hobbyist construction so even if they're off a bit they will line up with mating gears pretty well. Let's make a call on the width so a) I can finalise the CAD on the gears and b) you can get cutting. I vote 2.5mm if the material is available.

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Originally Posted by cscotthendry

Ruprecht:
OK, I've got an RMI gauge that has a pointer that does that. But it only has the card and the pointer not all the other stuff as well.

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Yup the only place I've seen a combined HSI/RMI is in military aircraft (or on civvy glass). One's initial aviation experiences tend to drive one's preferences for life - I still have trouble with the "pendulum" style VOR gauges and fixed-card ADF, but give me an HSI/RMI and I'll navigate head-down all day :) For the same reason I resist GPS, as I learned IFR doing NDB approaches! Old school!

On the BK, my intention is to proceed with the current design to finished stage to "prove the concept" and then branch it for the BK. I will build the F-16 style, if you guys build BK style (minus the RMI ring) it will be a good proof that the design can be adapted to most any HSI. If the firmware and circuit supports an RMI, it's a simple thing to just not connect that stepper.

Ruprecht:
Yep I agree. If we do the hard one first then it's easy to take stuff out. Also I agree with your sentiments about the D'Arsonval movement based VOR gauges. Never liked the look of them. When I first started working on airplanes they were the mainstay and I always thought they looked icky.

What's this "you guys" build the BK;) You're in too deep now. BTW are you interested in having a go on the mill and lathe? I could probably use some help when we start to make the parts. Besides as I mentioned before there will undoubtedly be changes needed for the practical construction and I sure would like to have you on tap to thrash out the design.

Anyway, If we do the military version that we're working on and then convert it to the BK style, we should have the market pretty well covered. We'll make millions ;) Have you picked out your new house yet? I've got my eye on the new Bugatti.

Money? Theres money involved?? Hmmm... I think I'll buy a new Cessna with the G1000 installed... Gotta go low and slow.

lol, the smiles on the faces of all the happy pitbuilders as they power up their HSI will be all the reward I need :D

of course, my own Bell 47 or Spitfire would be nice too. With an HSI.

Actually to be serious for a sec, I think if we pull this off we will be inundated with requests to supply detailed instructions, plans and cut gears for folks. We should start thinking in advance what our response to that is - personally I am happy to make plans in user-friendly format but have no interest in building complete units for sale. eMachineShop, Serv-o-link and the like should do a roaring trade in the gears. It would be nice to find someone with a CNC machine to cut the plates though - the thought of hand-milling them leaves me cold.

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Originally Posted by cscotthendry

What's this "you guys" build the BK;) You're in too deep now.

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OK let me rephrase: WE shall build an F-16 style for me and 2 BK style (or whatever you prefer) for your guys :)

And I'd love to have a crack at the milling if you're prepared for bits of the mill to go flying around the workshop :) I have 2 left thumbs when it comes to metalwork - to be fair I'm lucky to have any thumbs left at all!

As long as I'm dreaming, the Cessna would be for work and this baby http://www.tecnamaircraft.com/BrP2006T.pdf is for pleasure. In the meantime, I'll be happy with a BK style version.

Well if we're dreaming, I see you and raise you this: http://www.avtechgroup.com/

Yep. Saw that one too. They discontinued work on it for lack of money. Too bad. It was scheduled to be only 3.2 mil each.

At any rate, I'd like to see the new 3Ds of what we have so far.

Been flying tonight mate, no 3ds work. Showed my brother how to fly the Kai Tak checkerboard IGS in a Baron :) Love that same-cockpit multiplayer!

Thanks for the welcome! Actually I haven't drawn out my thoughts for the CDI yet - and I can't do it right now, since I'm moving house, and today is the day I de-commission my computers! Sucks.

However, just so that you know I'm not all full of BS, I've attached three images of my altimeter design, hopefully showing how I have arranged for the opto indexing.

Once I'm settled in my new home and paid an arm and a leg and the sale of my firstborn to pay for satellite internet access, I'll get on to the CDI thoughts I have, and try to explain it visually.

But knowing the speed at which you guys work, you'll have finished the project by then!:)

(I haven't uploaded attachments to this forum before - I hope it works!)

P1IC:
Love the idea with the worm gears. I worked this out just a few days ago and realized that you can get really high ratios with very small gears. BTW for 48TPI gears, a 1.75mm pitch thread is your friend. Fits exactly.

Oops looks like I put the cat amongst the pigeons mentioning money, LOL

By the way, for our readers who do not know what 'The Book' is, here is the link to it.

http://www.mikesflightdeck.com/mfdbooks.htm

(Hey Mike, That'll be a dollar for the advert. LOL )

Tasks for modelling this evening:

- reduce gear thickness to 1.4mm
- expand divider plate thicknesses to 1.2mm
- position optos
- adjust shafts, support gears and driver gears to suit
- check plate dimensions are 3.24" not 3" (we might have more room than we think!)
- model the CRS needle affixed to the CDI plate

Anything else, please advise before tonight. Also please advise: what thickness should the divider plates be? I'm working on 1.2mm (18 gauge) as that's commonly available in hobby/electronics stores here.

Don't forget the thickness of the outer casing.
Actually, I would use 1.5 mm gears just because.
18 gauge is fine for the divider plates.

Hi all:
Here's the latest. Ruprecht and I are going to get together over lunch and discuss the developments so far. I have his 3dsmax model in DXF form and will be translating that into a more precise CAD package. 3dsmax seems to be great for visualization, but doesn't do the things needed for manufacturing. So this topic will go quiet from my end for a while, while I translate ruprecht's model (with a few improvements) into Solid Edge.

I'll keep you all posted on the news as I progress this gauge.

And to add to Scott's comment above, I'm trying to keep my head above water with work atm and the HSI is slipping down the list. The situation should be smoother in a week or so. Progress is occurring, never fear.

Scott and I had a great catchup the other day and, amongst many other things related to the HSI, had a think about our influences with this project.
We should really acknowledge the work of Mr Felipe Lopez Gerola from Bilbao Spain. He put together some amazing 2d drawings of a hobbyist HSI which, when I came across them on Viperpit, inspired me to visualise the instrument in 3d from hobbyist materials.
There are many people we would not be at this place in the design without; Felipe, Mike Powell, and all the amazing and talented builders out there. Here's hoping we can get this instrument done and give something back to you all.

Well I've started to transfer the design into a CAM friendly package called Solid Edge. Along the way, I've started to think about the jigs and machining fixtures we'll need to make the parts. As a proof of concept and to help me learn Solid Edge, I designed a machining fixture to make the gear for the heading bug.

I've started modelling standard parts for the construction. One of the things we'll need is servos. I've settled on the Hitec HS-55 as the modelled standard servo and created a model for it.

Nice work guys. I'm working on the driver circuits that go inside the housing. Just block schems for now till I get some more precise measurements from Scott.

Modelling, modelling modelling:D
I needed a reference for the positions of the control knobs etc so I modelled the BK facia

Looks good. I like the 'FlightSim Avionics'. What is DG? Directional Gyro?

Flightsim Avionics is my company name. I've registered a .com with Planet Domain for it too. So that's ready when the time comes.

It's actually DC for Direct Current vs AIR for air ;-) In the pic it also looked a bit like BC to me, but it's not that either. If you look closely at the lettering it is actually extruded from the front surface.

I have an RFQ in with a local plastics company for getting moulds made and a production run of about 100. Just to see what it would cost.

Hello again guys. Probably way too late, as I suspected, but it has taken toooo long to get Internet access here at my new address. Satellite, unfortunately.

Anyway, I said I would post my thoughts on a system which avoids the need for brushes in the CDI section. Four pics. Pic A shows the arrangement for moving the CDI indicator across the back panel (where all the TO, FROM etc stuff is, not shown here for clarity). This is a back view looking behind the CDI backing panel. You can see the white CDI needle strung between the two lines of cord.

When the red cog is spun, the green shaft through it also spins, driving the cords and moving the needle. Here I am assuming the green bits are fixed to the frame, ie. the CDI doesn't rotate (yet).

Pic B shows the drive arrangement in red for the CD needle. The stepper applies torque through the worm gear to the red shaft which turns the original little cog.

Pic C shows the arrangement for turning the CDI, through the green shaft. What you don't see in this pic is that the green shaft is fixed at its front end to the CDI assembly. Running the green stepper turns the whole CDI assembly including the little red cog - and this is where the problem lies.

If the green motor runs, the red cog will spin around the red drive system, making it turn and causing the needle to move. We don't want that, so whenever we turn the green drive we also need to turn the red drive the same amount in the same direction, so that the needle stays in the same position on the CDI face. So no brushes are needed. It's all done in the PIC software.

Pic D shows a bit more clearly how the assembly looks with the CDI drive cage mostly fleshed out. Of course, the rollers for the cord are grooved pulleys (I didn't draw them like that for simplicity) and the wrap of the cord around the driving shaft is more involved than shown here, but not difficult.

Hey that's a really neat idea. I see some issues with the physical construction, but nothing that couldn't be solved.

Definitely an intriguing approach. How would you do TO/FROM and OFF flags? If my understanding is correct, turning the green stepper causes the whole cage to rotate. If you had flags on the cage, they would require some sort of wiring (whether you use solenoids, LEDs or whatever) and wouldn't the wiring wind up?

Just trying to understand, possibly you've already thought of the solution (or indeed, possibly I've totally misunderstood!).

Cheers

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Originally Posted by ruprecht

Definitely an intriguing approach. How would you do TO/FROM and OFF flags? If my understanding is correct, turning the green stepper causes the whole cage to rotate. If you had flags on the cage, they would require some sort of wiring (whether you use solenoids, LEDs or whatever) and wouldn't the wiring wind up?

Just trying to understand, possibly you've already thought of the solution (or indeed, possibly I've totally misunderstood!).

Cheers

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No, you are correct, that is the same problem, and it's solved in exactly in the same way. The flags might be controlled by solenoids or steppers or whatever, but if you drive these coaxially from a shaft concentric with the existing ones, and rotate them exactly as with the CDI, they will work just like the CDI assembly. No coiled wires, no brushes.

Actually, one stepper will serve to display TO, From, Inactive. It's just a matter of angle of rotation in a window to display the required icon. This would save on the number of coaxial shafts involved!

Coiled wires just don't work. What if your aircraft is held for an hour in a hold, circling clockwise every four minutes? Your wires are toast. And brushes are iffy in a digital environment. They might be OK, but I prefer the software solution. Plus all the mechanical issues involved in carrying all the motors around with you as you rotate the various bits. Testing required!

Of course as you have seen, there are still issues to resolve. Mainly, how to display the RMI heading card and the heading bug in front of the whole assembly. I really REALLY don't like the rim-drive approach which has been published here, because of the mechanical complexity - having a lathe and mill, I can cut gears along with the rest of you, but I'd rather not! The answer has to be in software.

I prefer a coaxial drive system, but the CDI cage kinda puts a stopper on that for driving the flags, OBS and bug.

I haven't tried it out yet in 3D, but I think we can provide coaxial drive from behind the cage with stand-offs outboard of the CDI back plate on each display disk (which sit in front of the CDI assembly), allowing the CDI assembly to rotate freely inside with no coax shaft interfering with the CDI needle (or, in the case of an ILS, needles). But I hate it, because it's mechanically complex, with fine tolerances.

The rim drive system was incorporated into one of the designs I had for general instruments like RPM, ASI, and the like, with a simple DC motor driving the rim with a rubber sleeve to provide some traction, and a pot centrally feeding back its position - and, incidentally, the pot shaft supported the disk which made it mechanically very simple, and it worked OK (for angles of rotation less than about 240 degrees), but in the HSI we have no access to the centre forward of the CDI, and the angle of rotation is infinite, so rim drive plus a zeroing strobe driven by a stepper may be the only realistic solution.

But I worry about slip, just like with the cord arrangement in my cage suggestion - but maybe it's not an issue. With pot feedback, slip is irrelevant, but with open-control like a stepper, slip is a real issue. Gears work, but I hate them on the rim! Sorry.

I also worry about going outside the standard instrument cage, and driving a rim would seem to need a stepper out there beyond the edge.

By the way, my pics don't show that each channel has a strobe disk at the back, connected to the appropriate drive shaft, with one single LED/Photocell firing backwards through all strobe disks and monitoring all channels for zeroing, with one zero blanking pad on each disk. Software sorts it out.

But maybe there's a better solution? I'm actually sure there is - it's just a matter of finding it. More questions than answers!

A 90 degree worm gear would get rid of all the string and pulleys which I don't care for. That gives you direct drive for the CDI needle.

I've got the general direction you are heading with this design. Its going to be 12 inches deep into the panel using this layout.

There is nothing wrong with rim drives, quite often they are the best design for the application. How they are implemented is the thing. In our case, the drive is well inside the instrument casing.

Also, we do not have to use 'brushes' as brushes. We should use a slip ring here. They are made for this kind of application.

What happens if your strobe disks get out of synch with each other? The software will fail and the instrument becomes useless.

I seem to get that you are not mechanically inclined and tend to shy away from those type of solutions. Nothing wrong with that, but there are somethings that software should NOT do.

P1IC:
I have some suggestions for your design, hope you don't mind me butting in.

The CDI pointer in your diagram looks like it would scrape on the face of the CDI plate. You could solve this by shaping the attachment to the cable so that the needle sits forward of the cable and protrudes past the face of the CDI plate. Looking side on to the needle, think of a squarish C shape.

For the home position sensor on the needle stepper, if you set the gearing such that it only takes one (or less) turn of the gear driven by the stepper worm, then you could have an optointerrupter straddling that gear and just have a hole in the relevant place in the gear.

The interface between the worm gear and the straight gear will be a small issue to solve as well. There are two options which are a) cut the teeth on the straight gear at the appropriate angle for the worm or b) mount the steppers at an angle to the straight gear. If you mount them straight and with straight cut teeth, there will be a fair bit of wear intil they "break in" then you might get a bit of backlash in the movement.

Apart from the issues that Ruprecht raised, these are the issues I saw with the construction. As I said, they're solvable. Rup's issues may be a bit more difficult though.

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Originally Posted by AndyT

A 90 degree worm gear would get rid of all the string and pulleys which I don't care for. That gives you direct drive for the CDI needle.

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I don't care for it either! I'm not familiar with 90 deg worm drives - could you describe these?

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I've got the general direction you are heading with this design. Its going to be 12 inches deep into the panel using this layout.

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Actually, the dsign as shown in Pic 4 is only 3.2 inches deep from the front of the panel to the end of the longest shaft.

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There is nothing wrong with rim drives, quite often they are the best design for the application. How they are implemented is the thing. In our case, the drive is well inside the instrument casing.

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Excellent. I suppose you place the motors in the corners. I can do that too in my design. Good thought.

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Also, we do not have to use 'brushes' as brushes. We should use a slip ring here. They are made for this kind of application.

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OK. I was avoiding these, not just because I don't like the thought (silly me) but also because it means carrying the drive system around. Maybe that's not an issue.

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What happens if your strobe disks get out of synch with each other? The software will fail and the instrument becomes useless.

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If the drives get out of sync, that's probably because of lost pulses or slip. I said I am concerned about slip, but I haven't commented on lost pulses in a stepper. I don't know how common that is!

But if things go wrong in hardware or software, the instrument is useless no matter what the design is. I agree that this system relies on sync between the channels - and if that gets screwed, then physical damage can result in my design. Not pretty. So maybe some end-stop sensor which shuts down the instrument if it goes AWOL?

Anyway, how do you solve any problem with any mechanical or software glitch? They will happen, and it's not just my design which is the issue!

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I seem to get that you are not mechanically inclined and tend to shy away from those type of solutions. Nothing wrong with that, but there are somethings that software should NOT do.

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Actually, no. I have fairly good mechanical building skills using my mill and lathe and a few other gizmos. I just think that if I get a design wrong in software, the fix is relatively painless. If I get it wrong in hardware, I have to start over :roll: .

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Originally Posted by cscotthendry

P1IC:
I have some suggestions for your design, hope you don't mind me butting in.

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Not at all! I'm here to get ideas, just like everyone.

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The CDI pointer in your diagram looks like it would scrape on the face of the CDI plate. You could solve this by shaping the attachment to the cable so that the needle sits forward of the cable and protrudes past the face of the CDI plate. Looking side on to the needle, think of a squarish C shape.

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I thought that the positioning of the string pulleys (which protrude beyond the front of the backing plate) would place the needle off the face. Maybe the angle of the 3D image doesn't show that.

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For the home position sensor on the needle stepper, if you set the gearing such that it only takes one (or less) turn of the gear driven by the stepper worm, then you could have an optointerrupter straddling that gear and just have a hole in the relevant place in the gear.

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The opto-interrupter is used on the shaft - or rather, on a clear plastic disk strapped to the shaft. And yes, the sizing of the various drive components require less than one rev for full deviation, so that there is no confusion about the zero point. Agreed.

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The interface between the worm gear and the straight gear will be a small issue to solve as well. There are two options which are a) cut the teeth on the straight gear at the appropriate angle for the worm or b) mount the steppers at an angle to the straight gear. If you mount them straight and with straight cut teeth, there will be a fair bit of wear intil they "break in" then you might get a bit of backlash in the movement.

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Actually I have got bags of plastic gears and worm drives really cheap from eBay which mesh perfectly, and I will be using these for my instruments. And backlash will not be an issue because of the high gearing inherent in worm drives. Slop of less than one tooth will not be visible on the CDI needle.

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Apart from the issues that Ruprecht raised, these are the issues I saw with the construction. As I said, they're solvable. Rup's issues may be a bit more difficult though.

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I replied to that post, but maybe you still see issues there? Thinking again about the to / from / inop flag, that's behind the backing plate, so there is no problem about the mechanics. A clear plastic disc (I will use plastic Petri dishes, 90mm diameter, 40 discs for $8, they come sterilised, but that's not really important :p ) with the same drive system as the other indicators will (hopefully) work OK. A set of windows in the backing plate will reveal the icons when they are rotated in to the correct position.

But as AndyT said, any loss of sync is a real problem. Not for the to / from / inop system, which simply fails to display correctly, but it's a problem for the CDI needle, which may go out of range if sync goes belly-up, and wrap itself around the backing plate with extreme prejudice. So my thought is, under what circumstances might that happen?

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Originally Posted by P1IC

I will use plastic Petri dishes, 90mm diameter, 40 discs for $8, they come sterilised, but that's not really important :p )

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Genius! Where do you get them from?

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Originally Posted by ruprecht

Genius!

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Thanks you!

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Where do you get them from?

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I was looking for clear plastic discs for some time when I stumbled upon these.

http://www.indigo.com/science-suppli...etri-dish.html

There is some work required to cut off the rims, which will be required in many (but not all) applications, but if the application suits keeping the rims on, the lids fit over the bases quite nicely so that the two faces can be really very close together while still having both rims available for rim drive. Needles and suchlike can be pasted on quite easily. Getting a good needle cut out of paper is probably harder than cutting off the rim! :(

I picked two sleeves up in Waterloo, but they also mail out.

Regarding cutting off the rims, I've been quite successful with a Dremel tool with a sharp pointed cutter. The plastic (polypropylene) melts a bit, but I think if I make a jig for rotation using my lathe and a sharp cutter I can get a pretty good circumference cut for rim drive if I need that. Testing required! As always.

P1IC:
Re missed pulses on steppers. This only happens (in my experience) for one of two reasons. a) there is a binding on the stepper that is greater than the torque produced by the stepper or b) the pulses are applied faster than the stepper can physically react. The first is a construction/tolerance issue that is a real devil. The second one is easy solved.

Using worm drives, a) may rear it's head from time to time because of the sliding type contact between the worm and the gear. The obvious solution is...