Launch Detection (almost)

Discussion on ground control/GSE and recovery equipment. This includes launch pads, triggers, chutes, streamers etc. Includes other items such as simulation and other computer software, etc.

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lorstin
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Launch Detection (almost)

Postby lorstin » Mon Nov 09, 2015 8:11 am

I recently butted into someone elses thread and asked a question about launch detection, I received a couple of helpful replies. However both methods suffer from a delay, I suppose what I really want is ignition detection. So my next question is - is it permissable to attach a sensor (mounted on the rocket) across the wires from the launch controller? Question 2. Most launch contollers ( I believe ) send a signal to do a continuity test - so what does my sensor have to filter out so that it does not false trigger?
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Re: Launch Detection (almost)

Postby SpaceManMat » Mon Nov 09, 2015 9:04 am

Have you considered using a break wire, delay should be very sort. Also the issue with detecting ignition is this happens just before launch. A break wire if set right should I would think be able to detect when the rocket has moved about a centimetre or so.
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Re: Launch Detection (almost)

Postby OverTheTop » Mon Nov 09, 2015 10:49 am

Make sure you don't have a breakwire set too sensitive. You don't want a "chuff" on the rail to trip it.
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Re: Launch Detection (almost)

Postby vance2loud » Mon Nov 09, 2015 11:37 am

Are you after something mounted to/inside the rocket or something mounted to the launchpad?
For photography I found this link a while back it might help.
http://rocketsetc.com/rocket-photography/lift-off-sensor/
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Re: Launch Detection (almost)

Postby air.command » Mon Nov 09, 2015 1:39 pm

It all depends on what you are trying to achieve with the signal on the rocket. What happens if the igniter fires, but not the motor? Do you just want to know when the igniter had current pass through it, or when the motor actually lights and the rocket starts moving?
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Re: Launch Detection (almost)

Postby lorstin » Tue Nov 10, 2015 7:16 am

Thanks for all the replies. :D
In a nutshell
*I would like to detect ignition rather than lift-off
*The device I want to trigger is mounted inside the rocket and basically starts the clock
* the device is passive (doesn't affect flight or deploy chute etc.)so it doesn't matter if lift-off doesn't occur

The following bit of video shows the delay which can occur. The video has been slowed by 50% just to make the delay a bit more obvious.



OTT - what's a "chuff"?
Vance - Liked the link you posted, but not suitable for my purpose, however it did give me an idea for something similar that is rocket mounted - Photo Interrupter from Jaycar (part no. escapes me)
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Re: Launch Detection (almost)

Postby OverTheTop » Tue Nov 10, 2015 9:23 am

Sometimes the motors can "chuff" when ignited, producing just a little burst of thrust, then going back to a gentle smoulder. Sometimes enough to move it a little up the rail, and possibly fall back. The might or might not go on to light and launch fully.

If you are looking at measuring ignition delays, some of the commercial altimeters measure that, looking at the initial thrust burst and then the time to some higher pressure. Not sure what algorithm they use for the calcs.
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Re: Launch Detection (almost)

Postby rocket_troy » Tue Nov 10, 2015 10:16 am

OverTheTop wrote:Sometimes the motors can "chuff" when ignited, producing just a little burst of thrust, then going back to a gentle smoulder. Sometimes enough to move it a little up the rail, and possibly fall back. The might or might not go on to light and launch fully.

If you are looking at measuring ignition delays, some of the commercial altimeters measure that, looking at the initial thrust burst and then the time to some higher pressure. Not sure what algorithm they use for the calcs.

If I was to hazard a guess, I'd be inclined to think he was using it to initiate a video recording or some kind of data recording (more likely video though) ie. so you can capture the entire ignition and take off process.
There's probably a thousand different ways you can do it, but my personal preference would be a wireless RF link - can be done pretty cheaply with some low power 433Mhz modules.
You could also do it via an LED that's loosely attached to the outside of the airframe via some kind of loose umbilical wire that's easily pulled off during take off. That illuminates some kind of (shielded from sunlight) LDR inside the rocket. You'd need to run it parallel with a resistor to restrict the current running through it, but you can safely run it in parallel with an igniter without any concern about it affecting the reliability of the igniter or false triggering from continuity testing.
Continuity testing is always done with extremely high impedance so the only thing likely to be triggered by such is either a FET or a direct digital input to a microprocessor. Nothing that requires current or power should be affected by the high impedance voltage from a continuity test.
Again though, there's a thousand different options here.

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Re: Launch Detection (almost)

Postby rocket_troy » Tue Nov 10, 2015 10:27 am

rocket_troy wrote: Continuity testing is always done with extremely high impedance so the only thing likely to be triggered by such is either a FET or a direct digital input to a microprocessor.

I should add: false (continuity) triggering of either can be very easily biased out via a suitable resistor to the opposing rail anyway (pretty obvious and simple to electronics geeks)

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Re: Launch Detection (almost)

Postby strud » Tue Nov 10, 2015 10:51 am

You could use a UV sensitive photodiode or similar since the products of combustion typically emit significant levels of UV compared to ambient.
This is how some flame detectors work.

These ones are particularly nice (but a bit expensive) since they are inherently blind to the visible and IR spectrum :

http://www.farnell.com/datasheets/1836028.pdf


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Re: Launch Detection (almost)

Postby joeman » Tue Nov 10, 2015 1:48 pm

Perhaps somehow measuring the current flow? The igniter resistance will change...increase I assume as it heats up...before it finally breaks.
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Re: Launch Detection (almost)

Postby rocket_troy » Tue Nov 10, 2015 2:22 pm

joeman wrote:Perhaps somehow measuring the current flow? The igniter resistance will change...increase I assume as it heats up...before it finally breaks.

You only need to measure current (the presence of a current) or voltage (provided you bias the presence of high impedance voltage to zero). You're only after an indication of low impedance voltage or current flow. No need to detect the profiling of such - it's pretty much a digital on/off condition.

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Re: Launch Detection (almost)

Postby lorstin » Tue Nov 10, 2015 2:49 pm

rocket_troy wrote:If I was to hazard a guess, I'd be inclined to think he was using it to initiate a video recording or some kind of data recording (more likely video though) ie. so you can capture the entire ignition and take off process. ................. You could also do it via an LED that's loosely attached to the outside of the airframe via some kind of loose umbilical wire that's easily pulled off during take off. That illuminates some kind of (shielded from sunlight) LDR inside the rocket............ Again though, there's a thousand different options here.

TP


You hazard a pretty good guess Troy, very close, I won't elaborate any more at the moment in case it all falls in a heap. I gather from what you say about the umbilical that such things attached to the ignition wires are allowable. LED & LDR sounds nice and simple, or maybe an :idea: opto-coupler.
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Re: Launch Detection (almost)

Postby rocket_troy » Tue Nov 10, 2015 3:04 pm

I think the RF solution is the more elegant one though.

Ok, I'm just assuming you want to avoid microprocessors and intricate electronics here:

one of these connected to (switching) your on board electronics:
https://www.adafruit.com/products/1097

and one of these connected via relay to the igniter leads:
https://www.adafruit.com/categories/259

You basically power the relay coil with the igniter leads (***parallel*** to the igniter) and the relay switch contacts with a hacked button contacts of the transmitter (remote).

I've based this solution on pure simplicity for an RF solution.

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Re: Launch Detection (almost)

Postby lorstin » Thu Nov 12, 2015 6:51 am

You're right Troy, I'd prefer to keep away from microcontrollers & complex electronics for this part of the project as I'm just looking for a signal to start the processes rolling in a microcontroller & the bits & pieces hanging off it.
I quite like the RF solution you suggested, not quite what I imagined when you first mentioned it. Plus it's got three spare channels, hmmm (sound of brain :roll: ticking over). I'll soon need to design a bigger rocket to lift it all off the ground, good thing I like slow take-offs.

P.S. Strud, your avatar sounds like a conversation I frequently have with myself.
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