Featherweight Tilt checker/timer

raohara said:

I know the cost of RocketTiltometer (RTOM) up there, but are you balancing it with the waste and financial loss of a sustainer motor that ignites when you don’t want it to? For example, in one of Frank’s test flights he used a CTI M2020 for his sustainer. Retail on that motor is $370. The flight went awry, but the COS feature aborted ignition and saved the motor for a subsequent flight.

I may be missing something, but if one is using expensive sustainer motors it seems like RTOM has the potential to save you some bucks.

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IF someone is using expensive sustainer motors...

Most of the staging I've seen is with much smaller motors.

Don't get me wrong -- Frank's product is a BMW-caliber product. But a BMW has a lot of extras, and has a pretty high price tag, and what most folks want and can afford is the no-frills car that works.

-Kevin

I think leaving off the magnetometer would be okay for the purposes of most airstarting. Basically anything that airstarts late enough to require increased drift resistance would fit a Tiltometer, with the possible exception of something silly like staging M or N longburn motors to a 29mm I motor or a 24mm G motor. Which would be pretty cool.

Aside from that, for my school's rocketry class we have a powered turntable for calibrating accelerometers and gyroscopes that can pull a few G's on the edge, if I remember correctly, but it would need compound motions to be able to properly test both sensitivity and bias for whichever sensor is getting its rotation axis aligned with the acceleration direction.

If temperature sensitivity is a problem, and computing power isn't so limited, would it be possible to use a temperature sensor to automatically adjust the incoming sensor data based on known drift?

CarVac said:

If temperature sensitivity is a problem, and computing power isn't so limited, would it be possible to use a temperature sensor to automatically adjust the incoming sensor data based on known drift?

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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

Adrian A said:

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

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Great app note.

More impressive history from page 1. ST has shipped more than 1.4 billion MEMS sensors to date and its dedicated 8-inch MEMS fabrication line, which was among the first in the world, currently produces more than 3 million devices a day.

ST makes the chips in your phones and game controllers. Incredibly good and inexpensive devices, low power, high accuracy and high stability. My smart phone has several 3-axis MEMS devices inside and are fun to monitor when your bored on cross country flights.

Bob

A seemingly trivial issue, but an issue none the less:
If temperature deviations over the course of the flight are an issue, when is t=0. From when you turn the device on, or from when the rocket lifts off? Because depending on how someone were to go about using this device, they could in theory turn it on and then let the rocket sit in the sun for 15 minutes in the desert with a black paint job, and roast the poor thing to bits, throwing off the readings if it were to start calculating t=0 at power on.
Alternatively if one were to use it in a rockoon... (Lets not start this whole discussion, but it would be nice if the device could be left on for long periods before the launch without having to worry about drift from that).

Lentamental said:

A seemingly trivial issue, but an issue none the less:
If temperature deviations over the course of the flight are an issue, when is t=0. From when you turn the device on, or from when the rocket lifts off? Because depending on how someone were to go about using this device, they could in theory turn it on and then let the rocket sit in the sun for 15 minutes in the desert with a black paint job, and roast the poor thing to bits, throwing off the readings if it were to start calculating t=0 at power on.
Alternatively if one were to use it in a rockoon... (Lets not start this whole discussion, but it would be nice if the device could be left on for long periods before the launch without having to worry about drift from that).

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A good question, and not a trivial issue at all.
I plan to update the estimate for the gyro drift continuously while the rocket is stationary on the pad, and use the version that was calculated 2-3 seconds before liftoff is detected. This is the same strategy I use for the accel offset now in the Raven.

That would make it difficult for people to angle a launch rail to counteract weathercocking, wouldn't it? They would have to set the angle tolerance wider than desirable.

Also, depending upon how much drift there is, I would suggest disabling the smaller tolerance angles because otherwise no flight would ever have successful staging ignition.

Another question: would the altimeter fire the airstart motor when the rocket happens to be pointing up, or would it disable it after any oscillations?

Adrian A said:

After some requests on the TRA email list, I decided to try my hand at making a simple, small, inexpensive, gyro-based device that measures the rocket flight angle and inhibits an airstart ignition if the flight angle exceeds a user-selected angle threshold.

This is still in the early design stages, and wouldn't be available for a couple of months, at least.

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Wow .. be nice to have it by MHM in May as we, as you know, have a LARGE 2-stage project planned for that launch. I'll be eagerly awaiting updates!

Steve
www.smashrocketry.com
Proton M - an NCR Group Project

CarVac said:

That would make it difficult for people to angle a launch rail to counteract weathercocking, wouldn't it? They would have to set the angle tolerance wider than desirable.

Also, depending upon how much drift there is, I would suggest disabling the smaller tolerance angles because otherwise no flight would ever have successful staging ignition.

Another question: would the altimeter fire the airstart motor when the rocket happens to be pointing up, or would it disable it after any oscillations?

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I can measure the rail angle by taking the accelerometer difference between the prelaunch gravity direction and the liftoff thrust direction. That would even account for misalignment between the electronics and the rocket. But since the liftoff acceleration can exceed the accel range I'm planning to use, I'll probably just state in the manual that the rocket thrust axis has to be aligned with one of the board dimensions, and assume zero misalignment. In that case, I can still measure direction of gravity relative to the rocket, and use that for a starting rotation.

I think I would have the board lock out the ignition if the measured flight angle ever exceeds the threshold. I wouldn't want it to fire during a tumble, and it would be pretty hard (though maybe not impossible) to ignore a momentary oscillation from weathercocking but not ignore a real attitude problem for a rocket that's temporarily vertical.

I've had rockets spin (roll, I guess is the appropiate term, I don't mean cartwheel as they do sometimes when unstable) coming off the launch rod, have heard of some doing a cone roll. Would your design cut off the air start in that situation?

I'm very interested in your idea, but will have to wait for the final price tag before making any commitment.

gldknght said:

I've had rockets spin (roll, I guess is the appropiate term, I don't mean cartwheel as they do sometimes when unstable) coming off the launch rod, have heard of some doing a cone roll. Would your design cut off the air start in that situation?

I'm very interested in your idea, but will have to wait for the final price tag before making any commitment.

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A roll would not affect the air start ignition. Coning would only inhibit the ignition if the angle thst the rocket was pointing from vertical exceeded the threshold.

You could detect coning by taking the magnitude of the cross product of the roll with the long axis of the rocket. Ideally, a rocket would have almost zero rotation rate except along its axis, even if it's shooting off at a 20 degree angle, so that would allow you to prevent airstarts in nonideal conditions.

If that didn't take too much processing power.

One problem. A LOT of rockets roll - watch on board video.

-Kevin

CarVac said:

You could detect coning by taking the magnitude of the cross product of the roll with the long axis of the rocket. Ideally, a rocket would have almost zero rotation rate except along its axis, even if it's shooting off at a 20 degree angle, so that would allow you to prevent airstarts in nonideal conditions.

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I'll apply a rotation with a direction cosine matrix at each gyro sample to compute the true rotation between the ground reference frame and the rocket as I go along. This will provide the angle between the rocket axis and vertical directly, so I don't think the approximation that I think you're suggesting will be needed for this.

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?

If it were up to me I would keep it as small as possible, with just enough markings to identify which switch is which and say, 3 tick marks around each switch one for the minimum, one for maximum, and a midpoint. A separate card overlay for more precise settings I think would be the way to go.

Adrian A said:

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?

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Since so many folks fly minimum diameter upper stages, I think size matters, so I'd keep the board as small as possible.

-Kevin

What I meant was have two conditions: a pitch condition and a rate of pitch condition. That way you could prevent airstarts when the rocket is rapidly oscillating or coning within the tolerance window, which is not desirable either.
With regards to size, smaller is better, as long as the functionality remains.

Adrian A said:

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?

Click to expand...

I like the overlay idea with cut outs for the switches to gauge position settings.

CarVac said:

What I meant was have two conditions: a pitch condition and a rate of pitch condition. That way you could prevent airstarts when the rocket is rapidly oscillating or coning within the tolerance window, which is not desirable either.
With regards to size, smaller is better, as long as the functionality remains.

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Rate of pitch may be hard to establish reliably. Numericl differentiation is error prone.

Larry Curcio said:

Rate of pitch may be hard to establish reliably. Numericl differentiation is error prone.

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Not true. The gyros used are rate gyros, which intrinsically output rotation rates and must be integrated to give angles.

CarVac said:

What I meant was have two conditions: a pitch condition and a rate of pitch condition. That way you could prevent airstarts when the rocket is rapidly oscillating or coning within the tolerance window, which is not desirable either.
With regards to size, smaller is better, as long as the functionality remains.

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I don't see why I wouldn't want to ignite my stage, even if it's rolling or wobbling at high rate, as long as the rocket is never pointed far from vertical.

Adrian A said:

I don't see why I wouldn't want to ignite my stage, even if it's rolling or wobbling at high rate, as long as the rocket is never pointed far from vertical.

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Especially since there are people who intentionally induce roll via canted fins, to help gain stability.

-Kevin

smashburn said:

Wow .. be nice to have it by MHM in May as we, as you know, have a LARGE 2-stage project planned for that launch. I'll be eagerly awaiting updates!

Steve
www.smashrocketry.com
Proton M - an NCR Group Project

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Steve,

It would be nice to have a fully tested product done by MHM, but knowing the way these things drag out, I think it's 50-50 it will be done by then. NCR is my club, and this is our club's big project, so I'd like to contribute. I would recommend planning for using a Raven for an altitude/time check for safety, and using a Rocket Tiltometer for a finer check of go/no-go on the airstarts. I'm planning to get one myself for flight testing, and perhaps I could loan it for this project.

troj said:

Those two, I think, are huge.

The Tiltometer is a cool device. Its downsides are size and cost, with the latter being significant enough, I think, to prevent most people from considering one.

-Kevin

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these are definitely huge advantages over the tiltometer, and with our plans to get into staging, this could definitely be useful to us!



Braden

Adrian A said:

I don't see why I wouldn't want to ignite my stage, even if it's rolling or wobbling at high rate, as long as the rocket is never pointed far from vertical.

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I think that the question was about the rocket looping or about to loop. If it loops, the altimeter would probably fire ejecting the parachute. I don't think you'd want to light the second stage motor after the 'chute's out, so some way to make sure it doesn't fire in that case would be good.

If the rocket is just starting to go wonky, i.e. the pitch is changing quickly, you might want to inhibit starting the motor because, by the time the motor comes up to thrust, the rocket might be pitched over too far. Of course, the likelyhood of something going wrong at the very instant just before staging is slim, but, if it's something you could easily check ....

-- Roger

CarVac said:

Not true. The gyros used are rate gyros, which intrinsically output rotation rates and must be integrated to give angles.

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Then I stand corrected.

Larry Curcio said:

Then I stand corrected.

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BTW, what do the rate data look like? The concern could still hold if the raw data were noisy, and the integrated data were intrinsically smoothed - as is the case with, say, accelerometer data.

-Larry (Just asking...) C.

The yaw rate limit would ignore the rolling if you used the magnitude of the (angular velocity crossed with the vertical axis). Anyway you could simply set the magnitude limit big enough to ignore most coning, and it could still help protect against a rocket which is verging on instability igniting.

@Larry: The data they output is fairly smooth, especially higher-end digital ones. The integration isn't particularly necessary for smoothing, even for the cheapo analog ones with crappy data loggers we use in my school's rocketry class. The noise was on the order of the quantization error, 3/256 volts.

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?