burning question - HSI?

[cscotthendry]

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

[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

[P1IC]

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

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!

[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.

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.

[cscotthendry]

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.

[P1IC]

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

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.

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.

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.

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

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.

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.

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

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!

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.

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 .

[P1IC]

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

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

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.

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.

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 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.

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.

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.

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.

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 ) 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?

[ruprecht]

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

Genius! Where do you get them from?

[P1IC]

Genius!

Thanks you!

Where do you get them from?

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.

[cscotthendry]

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...