Featherweight Tilt checker/timer

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The size benefit is a big thing for me. As I understand it, it would work with the Raven?

Do you think it would also work with the Wren if and when you come out with it?

Yes, this product will work with the Raven, including the continuity checking and beeping that the Raven does. The tilt check switch should also work with any other altimeter or timer. It will act as if it were a mechanical switch on the + line to the deployment charge, that enables current to flow until the rocket tilts over too far. That way you can set up ignition conditions based on velocity, altitude, time, acceleration, burnout detection, or some combination using a Raven, and then this device would provide an additional safety check for the rocket tilt.

This product will also have a simple timer switch that can be hooked up in series with the tilt-checking switch, so that it will work in a stand-alone way as a safe airstart timer. This is how you would need to use it with a simple altimeter like the Wren would be, which is only set up to fire at apogee and main deployment. The altimeter and this airstart device could share a battery, but would otherwise be independent.
 
The gyro already does that on-chip, but even after that correction there is still a residual sensitivity that's random.

Here's an app note on this:

https://www.parallax.com/portals/0/downloads/docs/prod/sens/27911-GyroscopeAppNote1.pdf
Folks

Please read this app note Adrian posted so you don't have to assume what data these sensors collect, how it's analyzed and what you get out. The app notes goes into the details of how Adrian's tilt-sensing instument will work. While there's a lot of math involved, the app note has wonderfully illustrative diagrams of the process.

Thanks.

Bob
 
Adrian- you're way smarter than me-so just build the darn thing! If I crash my rocket-then I'm the idiot and not you! Here's something we really need and everybody wants it to make coffee and predict the stock market-jeesh! If you read anything about the pull pins being outlawed-you'll see where I'm coming from. JUST BUILD IT!!!! (pretty please????) Put a notch in your gun for one already sold- I'll put a deposit if required! While it's good everybody has input and concerns, you are a known flier and won't put something out there that's doesn't meet your pretty high standards and I trust that. Gopher it dude! :D
 
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I'm getting farther in the layout. The electronics parts are pretty much all on one side, and the terminal block and the 2 rotary DIP switches for the setpoints are on what will be the top side. The smallest I can make the board is about 3/4" x 1", but that won't leave a lot of room for labels for the DIP switch settings. I'm thinking about making the board a little bigger just for that, maybe 1" x 1.25 or so. Any opinions on size vs. legibility? Or instead, maybe I'll keep the board small and provide a larger label or card that goes over the top?

Is it possible to provide color coded dots instead of labels? Put the color schematic on the back of the unit or on a field card ?

Personally, the lighter the better, although.. in this case, if it weighed a pound it would be so totally worth it..

On another note, You and a few other guys are DAM SMART !!! Keep up the awesome work.. Im learning so so much from you guys its better than school.

Tom
 
This thread prompted me to drag out the code I had played with a few years ago to compute attitude from gyro data. I thought I might be getting reasonable results but I didn't really like the code so I set it aside.

This time around I found some references I hadn't seen before and now I think I have something that works. (I am probably wrong. :) It is a quaternion implementation and requires no trig functions. Each update requires 15 multiplications so it can run pretty fast.

Even checking for the angle requires no trig functions. You could of course use an acosine to get a human friendly form but for this application it isn't required.

Attached is a sample plot produced from this data.

quaternion.png
 
One final feature I just realized would be really nice for failure analysis, especially as systems get more complex would be some sort of indicator that the maximum angle had been exceed and it had locked out, so in case the sustainer doesn't fire, it would be easy to check and see whether the tilt meter did it's job (or got a misreading) or if the altimeter never tried to fire the igniter.
Something as simple as a different series of beeps or flashes would work.

Oh, and one more thing. The way I see it, you have two options for the unpowered switch. Either have it always open, and only closed while the tiltmeter says it is safe, but if power were to fail for some reason, the sustainer would not light. Or, you could have it always closed, and just opened if the tiltmeter things it isn't safe. This would lead to less accidental failures, but also lower chances of catching a catastrophic event. My leaning would be towards the former.
 
That would be tougher than you would think, especially for people who do multiple-airstart clusters. Disabling the airstarts after one ignition would need to either rely on the accels (a bit iffy what with vibration and all), or have a current measurement. That would also prevent putting the common ground or common high shared among airstarts through the angle-check switch so that as long as the rocket goes straight, all of airstarts are enabled, and once the rocket tilts, they are all disabled.

Debugging is a concern, though I don't know how you could do it without a third device measuring voltages.

Just a note: probably the best way to maximize the safety and success rate for a mega airstart project is to use four of these tilt checkers: two series pairs in parallel. That way it would be immune to both failures to open the circuit and to close the circuit.
 
For the newer members who have joined TRF recently and may not know, Adrian is a professional electronic design engineer who has worked on a number of projects for NASA. We're lucky to have him as an active and responsive TRF member.

Bob
 
...While it's good everybody has input and concerns, you are a known flier and won't put something out there that's doesn't meet your pretty high standards and I trust that. Gopher it dude! :D

Thanks, Fyrwrxz. I've already gotten some good feedback I'll incorporate into the design. Rest assured you guys aren't slowing me down.

Is it possible to provide color coded dots instead of labels? Put the color schematic on the back of the unit or on a field card ?
Im learning so so much from you guys its better than school.

Tom

Color coding would be a good way to mark each position in a small space, but unfortunately anything but white on green increases the board cost several times over.
I'm learning more than I did in school too, so that makes 2 of us.

This thread prompted me to drag out the code I had played with a few years ago to compute attitude from gyro data. I thought I might be getting reasonable results but I didn't really like the code so I set it aside.

This time around I found some references I hadn't seen before and now I think I have something that works. (I am probably wrong. :) It is a quaternion implementation and requires no trig functions. Each update requires 15 multiplications so it can run pretty fast.

Even checking for the angle requires no trig functions. You could of course use an acosine to get a human friendly form but for this application it isn't required.

Attached is a sample plot produced from this data.

Cool. At work we use quaternions for spacecraft attitude. I should look into that more before I make the final call between doing it with a direction cosine matrix or the quaternions. Bill Premerlani and Frank Hermes have a very easy-to-follow white paper on the DCM implementation that I know will work. But I think 15 multiplies is less than the DCM method, but once you have the DCM it takes less to calculate a control output. If I remember correctly, the quaternions also don't need to be re-normalized as often, or perhaps at all.

Your graph looks pretty reasonable, though it looks like the apogee charge went off when the estimated flight angle was 50 degrees. Was the apogee charge early, or is that just accumulated error in the attitude propagation?

One final feature I just realized would be really nice for failure analysis, especially as systems get more complex would be some sort of indicator that the maximum angle had been exceed and it had locked out, so in case the sustainer doesn't fire, it would be easy to check and see whether the tilt meter did it's job (or got a misreading) or if the altimeter never tried to fire the igniter.
Something as simple as a different series of beeps or flashes would work.

You would have to know when the flight angle limit was exceeded relative to when the firing circuit triggered, since you would expect the flight angle limit to be exceeded sometime on any flight that doesn't tail-slide. If I incorporate a check for an open firing switch, then it would also be able to tell when the timer or altimeter tried to fire the charge, if it uses low-side switching. It might be cool to flash out the flight angle that was estimated at the time ignition was detected.

Oh, and one more thing. The way I see it, you have two options for the unpowered switch. Either have it always open, and only closed while the tiltmeter says it is safe, but if power were to fail for some reason, the sustainer would not light. Or, you could have it always closed, and just opened if the tiltmeter things it isn't safe. This would lead to less accidental failures, but also lower chances of catching a catastrophic event. My leaning would be towards the former.

The former is both more natural to implement, and more in keeping with the purpose of the device, IMO. When the power is off, the switch is naturally open, and I would keep it that way until the microcontroller is powered on, performs a self-test (using the built-in self test capabilty for the gyro), measures a near-vertical orientation on the accelerometer and constant, small rates on the gyro. Then zero out the gyro rates and verify that they're small. Also check to make sure the output isn't shorted. Then (still pre-launch) set the state of the switch according to whether the accelerometer says the rocket is within the angle setpoint. Then as soon as liftoff is detected, transfer control of the switch to the propagated gyro attitude. The switch will stay closed until the flight angle threshold is exceeded, and it will stay open from then on until the next time the device is powered on.
 
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I bought a Tiltometer2 on the strength of the information on the rocket electronics website. Specifically, the section of the User Manual "Theory of Operation - Tilt Monitoring in High Powered Model Rockets" and the 3 part video series that demonstrates the features and operation. I had the same concerns over size and cost that others have expressed here. Here's what I found after installing the unit (have not flown it yet)

size - It's not the size of the board itself that creates the issue. The problem is that it necessarily adds an additional board (which should be centered in airframe) and 2 batteries. This could be minimized some by going with the horizontally mounted version and installing on a bulkplate.

cost - I wouldn't trade the functions of the RTOM2 for a cost savings as most relate to safety or the reliable operation of the unit as intended. A less expensive unit that fails more often by incorrectly inhibiting ignition after a solid boost would disappoint most who will only fly one airstarted/ staged flight over a launch weekend.
 
That would be tougher than you would think, especially for people who do multiple-airstart clusters. Disabling the airstarts after one ignition would need to either rely on the accels (a bit iffy what with vibration and all), or have a current measurement. That would also prevent putting the common ground or common high shared among airstarts through the angle-check switch so that as long as the rocket goes straight, all of airstarts are enabled, and once the rocket tilts, they are all disabled.

As long as the tilt is o.k. I would keep the switch closed. You could use one of these to tilt-check switches for multiple air starts by branching the output of this switch to multiple igniters that are connected to their own altimeter or timer channels.
Just a note: probably the best way to maximize the safety and success rate for a mega airstart project is to use four of these tilt checkers: two series pairs in parallel. That way it would be immune to both failures to open the circuit and to close the circuit.

That would make the tilt check function tolerant of any single failure mode that doesn't screw up all 4. Sometimes this kind of series-parallel arrangement can be found in NASA designs. It gets a little ridiculous to use this approach for functions designed for 2-fault tolerance because you need 9! Dis-similar redundancy, like an altitude/time check, is more effective at that point.

For the newer members who have joined TRF recently and may not know, Adrian is a professional electronic design engineer who has worked on a number of projects for NASA. We're lucky to have him as an active and responsive TRF member.

Bob
Thanks Bob, the same can be said for you too. Ironically, my formal education is in mechanical engineering. I don't do any detailed hardware design at work, but I have picked up a thing or two working with other people's designs. The contrast with my regular job is part of what makes hobby rocketry electronics appealing for me. The Parrot altimeter was my second or third design project when I first started designing my own electronics in 2006 or so.
 
What I was saying is that fault-checking would be difficult if you have multiple airstarts going through one tilt-checker.
 
Some design updates:

The board layout is pretty much done now, except for a couple of issues. I like to have a robust power system, so that using a single battery, there is enough on-board capacitance to keep the board up and running for a couple of consecutive 1-second firings. Just as I found out with the Raven, there is only one capacitor that's of reasonable (though not tiny) size and moderate cost that can hold up the board when it's drawing 10 mA for a few seconds, and that's the one the Raven is already using. Up until now I have been able to maintain my 1.5" x 0.75" initial estimate. Now with this extra cap it's 1.575" x 0.75" This will also add to the cost, but my original goal of keeping it under $70 was a little conservative in case I ran into things like this.

In previous posts, I have talked about accommodating a remote status LED. I decided to use 2 terminals for altimeter power instead. This way, the board can be inserted into a design without any splicing or multiple wires per terminal. I may still put a couple of solder pads where they could be used by an external LED, though.
 
How many channels will it be able to handle; 1-4?


JD


Some design updates:

The board layout is pretty much done now, except for a couple of issues. I like to have a robust power system, so that using a single battery, there is enough on-board capacitance to keep the board up and running for a couple of consecutive 1-second firings. Just as I found out with the Raven, there is only one capacitor that's of reasonable (though not tiny) size and moderate cost that can hold up the board when it's drawing 10 mA for a few seconds, and that's the one the Raven is already using. Up until now I have been able to maintain my 1.5" x 0.75" initial estimate. Now with this extra cap it's 1.575" x 0.75" This will also add to the cost, but my original goal of keeping it under $70 was a little conservative in case I ran into things like this.

In previous posts, I have talked about accommodating a remote status LED. I decided to use 2 terminals for altimeter power instead. This way, the board can be inserted into a design without any splicing or multiple wires per terminal. I may still put a couple of solder pads where they could be used by an external LED, though.
 
It could handle 1 channel, though that could be shared among many airstarts (including its own timer). That totals up to use 6 terminal blocks, just like a Raven, I believe.
 
How many channels will it be able to handle; 1-4?
JD

By using the large hold-up capacitor, it will be able to stay up with the battery shorted for probably 4-8 seconds. I'll know better when I get the prototype done. So it should handle at least 4 consecutive full-short outputs, or as many as you want if you space them out. The tilt-checking switch can be connected to as many different external altimeter or timer outputs as you want, and there is one on-board timer channel.
 
ARE YOU STILL AT THE WATER COOLER? Jeesh- the guys in R&D are waiting for your design. Would you just please go back to your desk and BUILD IT????

(sorry if that sounded a little impatient...)
 
By using the large hold-up capacitor, it will be able to stay up with the battery shorted for probably 4-8 seconds. I'll know better when I get the prototype done. So it should handle at least 4 consecutive full-short outputs, or as many as you want if you space them out. The tilt-checking switch can be connected to as many different external altimeter or timer outputs as you want, and there is one on-board timer channel.

I think his question was whether you'd need one per channel, or if you could use one unit for multiple channels.

Let me use our big Delta III as an example -- if I didn't want to light the 3 airstarts if things weren't going right, would I need one unit, or would I need 3 (one for each airstart).

-Kevin
 
I think his question was whether you'd need one per channel, or if you could use one unit for multiple channels.

Let me use our big Delta III as an example -- if I didn't want to light the 3 airstarts if things weren't going right, would I need one unit, or would I need 3 (one for each airstart).

-Kevin

The tilt checker will provide a + output that gets switched off when the tilt goes too far. You can split that out to any number of channels, if all the channels are sharing the same battery, you splice the + sides of the ignitors together off-board, and the (-) side of each channel is given its own altimeter/timer channel.
 
Or a common ground, for altimeters that have that configuration instead of a common high.
 
Not to throw cold water on all of this, but how litigation-prone is all of this? Without an airtight disclaimer, you're one unfortunate prang away from a lawsuit.

-Larry (Vectors were bad enough. Now you throw quaternions at me!) C.
 
Not to throw cold water on all of this, but how litigation-prone is all of this? Without an airtight disclaimer, you're one unfortunate prang away from a lawsuit.

-Larry (Vectors were bad enough. Now you throw quaternions at me!) C.

You can say the same thing for every rocket motor or piece of electronics in the industry.
 
You can say the same thing for every rocket motor or piece of electronics in the industry.

To an extent, but this device is complex and untried (even the device in production now hasn't been around long) and prangs are spectacular.

Indeed, any device that ignites an upper stage is at particular risk. Virtually all of the fire and smoke injuries you see are associated with prepping of those things.
 
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To an extent, but this device is complex and untried (even the device in production now hasn't been around long) and prangs are spectacular.

Are you referring to the Tilt-checker or the hypothetical stabilization system?

The tilt-checker device is going to inhibit the ignition of staged motors if the rocket isn't vertical. If it fails, all that's going to happen is that the rocket won't stage. Once it reaches apogee, the altimeter will deploy the parachute. A failure of the tilt-checker is not going to cause a prang.

The stabilization system could be designed to not kick in until the rocket is above the altitude (or other threshold) where the altimeter would fire if the rocket started going down instead of up. It could also disable itself if the rocket goes too far away from vertical.

-- Roger
 
The tilt-checker device is going to inhibit the ignition of staged motors if the rocket isn't vertical. If it fails, all that's going to happen is that the rocket won't stage.
No, it could allow ignition when the rocket was too far from vertical.

IANAL, but I presume that vendors would be well-served to put a disclaimer on their documentation trying to deny liability from accidents. It might not work but I don't see how it could hurt.
 
It could allow ignition when the rocket is too far from vertical, but that's no change from the status quo for staging and airstarts, which is "ignore orientation entirely".
 
Product liability is definitely a concern. I could be one accident away from losing the business, so I definitely try to go the extra mile for safety, product testing, etc. The LDRS 29 accident really made me think about how I would feel if one of my products were involved in an accident. That's definitely part of the motivation for developing this tilt-checking product. If I ever have a tilt-checking failure combined with someone else's rocket failure that would result in a rocket igniting in a bad direction, I would hope I could make the case that the tilt checker is better than nothing.
 
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I have a project that has been sitting in the corner for years that has 6 outboards. If I were to air-start them in pairs of 2 or 3; I need at least 2-3 events.
Add-in: Any change of it continuing a set of sequences?
Say: tilt check then a series of air-starts at given intervals?

I was wondering if this would be possible?

JD


By using the large hold-up capacitor, it will be able to stay up with the battery shorted for probably 4-8 seconds. I'll know better when I get the prototype done. So it should handle at least 4 consecutive full-short outputs, or as many as you want if you space them out. The tilt-checking switch can be connected to as many different external altimeter or timer outputs as you want, and there is one on-board timer channel.
 
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