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Proximity/Motion Detection Choices?

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(@lydara)
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Joined: 3 years ago
Posts: 90
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So another "best practice" we might embed into a model rocket launch control system: no personnel at the launch pads while powered.

With a variety of launch rods & rockets at the pads, there may be reflections instead of clear space.

Being in the middle of a sunny field, there may be a lot of IR and other light interference.

Having exited Scouts around, there may be a lot of background audio noise.

But would love to put a sensor in the PadBox which could control a relay to automatically keep power disengaged whenever some comes near the launch pads.  Suggestions?


   
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(@davee)
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Joined: 3 years ago
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Hi @lydara,

My experience of rockets is limited to the 'light blue touch paper and retire immediately' sort, several decades ago, so my immediate thought for anything more powerful was a 5 metre high fence, with remote controlled cameras on watchtowers? 😀

But if you were looking for a more subtle approach ...Clearly these are only armchair suggestions, practical systems are much tougher to implement and please forgive my total ignorance/pointing out the obvious/etc.

Will you have some sort of keep out rectangular zone, marked with at least a nominal wire/tape?

Operationally, I presume tripping the 'detector' will immediately disable the firing system. It can only be re-enabled by manual intervention after checking the area is now clear.

If so, maybe you can emulate the typical 'Mission Impossible' laser beam + detector, ring of light ... albeit with a few modifications to make it actually work.

  1. Any light beam must be eye-safe. This has implications in terms of both power and wavelength. I am assuming it will not exceed limits considered safe for (say) a pointer in a lecture room. I assume you will get some 'professional' advice on this matter.
  2. It might be possible to use an infrared beam which is 'more naturally' 'eye safe' at lower powers, but this may be more difficult to align with the detector.
  3. I am assuming you can decide on a height that will be difficult to evade. Unfortunately, some scouts might be wise to that one!
  4. If the 'physical barrier', outside of the light beam area, consists of several taut wires, then slithering through would be more difficult. A simple 'continuity check' would also help protect against a scout with a penknife...
  5. If it is based on light, background audio should not affect it.
  6. To counter the problems of sunlight, etc. expect to put the detector at the end of a tube or similar, so that it is shaded.
  7. The light source should also be shaded, to reduce risk of it reflecting sunlight towards the detector.
  8. Laser light sources are typically well collimated ... that is they tend to stay as a narrow beam, not spreading out over a wide angle typical of say a torch. This means they must be carefully mounted and aligned on a stand which can withstand wind and accidental 'touching'.
  9. The light source, could include a collimator - essentially two (or more) lenses, arranged like a telescope, but with the light entering the telescope tube at the eyepiece end and travelling 'backwards' to exit at the objective lens. This will increase the diameter of the beam but reduce the extent to which it fans out after leaving the objective. This will reduce the effect of a small object such as fly passing through the beam. (Increasing the diameter of the beam will decrease the peak power/square area, which may reduce the risk of eye damage. However, you need to get professional advice on this point.)
  10. The detector may also have a form of telescope optical system.
  11. Perhaps most importantly, the beam should be amplitude modulated with a recognisable pattern that cannot be replicated by sunlight or other stray light sources. Furthermore, by looking for a particular modulation, it is possible to build a much higher gain detection system.

Of course, this may not be cheap and beyond your budget ... it only a 'thought-experiment'.

As an alternate, assuming you have a 'wire boundary', I wondered if the 'touch pad' inputs could be co-opted to work with a length of wire. I am rather dubious but I thought I would mention it.

  1. The length of wire for a given input would probably have to be kept short, impling many independent sense inputs.
  2. The wire would need to be carefully insulated and clamped at the support points to prevent wind movement producing signals.
  3. I am not sure if it would be triggered if only touched by insulated clothing or say a glove.

In principle a wire operating a mechanical switch could attempt similar detection, but it too would suffer from wind, etc.

The AI crowd might suggest cameras and 'person detection' software. Perhaps if the camera were to pointed at a 'black screen', then anything coming in between would be identified. I am not sure how robust it will be, or how easy to 'train', but I guess it should be in the candidate list. Again you would need say 4 cameras to form a rectangular cordon.

I will be interested to see what other ideas come up ... these all seem too obvious, and maybe there is a really simple one I missed.


   
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(@lydara)
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Joined: 3 years ago
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Topic starter  

So the layout of our model rocket range:

It is a simple open field,, a few acres, on the back side of the Scout camp.  We rope off about a third of it, in the middle (we usually keep things on-range, but there are occasional wind gusts). 

At one end we erect two "car port" tent shelters, one for the RSO & firing line and one for "the peanut gallery" of observers.  Everyone within sight must be under cover before a launch is allowed.  People will inevitably look up, and we all like having two working eyes.

As far as cables will allow (OEM controllers, just 17 feet), cables are stretched from the firing line to the launch pads.  These sit atop a "saw horse" frame, keeping the end of the launch rods above eye-level (caps can get lost, and we all like having two working eyes).   The "snake pit" of cables between firing line and launch pads is roped off from all but the RSO.

The only entry onto the field is by a single gate, immediately adjacent to and controlled by the RSO.  No one is allowed onto the field until the RSO declares it safe, opens the gate, & invites them in.  No launch keys may be given out until the RSO clears everyone from the field, closes the gate, and declares a launch is imminent.

For the new launch control system, the distance between firing line & launch pads will be increased.  Instead of just individual launch controller batteries, there will be a big battery & relays at the launch pads.  When opening the field, the RSO will have all safety keys removed & in his pocket. The RSO is to be the first one to the pads, and immediately turn off the local power disconnect.  Once all rockets are recovered, new ones wired & mounted, the last person at the pads is to be the RSO, who turns on the local power as the last step as they return to the firing line.

But we are all human, forgetfulness happens...  So I'm looking to put a sensor in the PadBox which will override and disconnect the power anytime someone is within say ten feet of the pads, in any direction.  The PadBox will sit on the ground by the "saw horse."  So other than the "saw horse" legs and inter-box cables, should be able to "see" at one to two feet off the ground.

As a donation to the Cub Scouts, it needs to be "thrifty" as well as safe.


   
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MadMisha
(@madmisha)
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@lydara

Are there any visual obstructions that prevent a person standing next to the RSO to say "Hey, there's a person out there!"? It would be the most power efficient(you might need to buy him a beer after). You could even give him a big red button that says "ABORT" on it. Also, adding complication doesn't sound that great. It just adds to possible failures of the system if something goes wrong. Troubleshooting can already be hard enough as it is.


   
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(@lydara)
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Topic starter  

@madmisha  Ha!  You've stumble upon _exactly_ how I run the range today.  The RSO has a line-of-sight to everything (gates, controllers, launch pads, etc).  And it is _always_ the responsibility of the RSO to maintain situational awareness & hyper-vigilence for safety!

For the system I am proposing, when at the firing line, the RSO must press--and hold--a momentary switch for countdown to continue and allow launch.

However I am trying to answer the concerns some have raised about local power still being there, available for the electronics to go errant with, as the RSO approaches the launch pads.  Personally I think it is a highly-unlikely boundary case.  However it seems should be easy enough to answer with a pair of back-to-back $5 sensors and a relay to automate 360 degree monitoring (so even if someone else came from somewhere other than the RSO's gate). 

Seems there are motion detectors _everywhere_....gas station & grocery doors, home alarm systems, etc.  But I've never used them in custom electronics, you hoping to learn from your experiences.  Sounds like make sandwich something between two discs, to shelter from sun & reflections.  Have been thinking of marine-grade plywood and many coats of poly for the "saw horse" legs, but that (and cables) might be the only reflections at a foot or two off the ground.


   
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(@davee)
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Joined: 3 years ago
Posts: 1628
 

Hi @lydara,

  I tried to answer your original question and standby my musing, but on reflection, plus your additional information, I agree with @madmisha, in so far as recommending your first line of defence should be ensuring that not only the RSO, but also a small number of supervisory observers (maybe just 1 or 2, that is your call), should all need to be on active duty to enable the 'firing sequence' to begin. (I'll leave any bribery or persuasion needed to recruit the observers to you.)

That does not stop you considering a second line of defence, providing the quality of the first defence line is not neglected. I am concerned that a secondary defence, which works 99% of the time might give a false sense of confidence to the operators. I think it will be very difficult to make a 100% reliable 'person detector'. Also, so far I have failed to think of a 'cheap' solution.

Obviously, you will aready have a firing sequence procedure, which I am sure you regularly review, and hopefully you actively look for any single failure points, such as if a switch fails to open, two wires short together, a cable is miswired, a (single) person is distracted and fails to do their job, and so on. (The latter is the basis of aviation safety, and sadly the results of occasional neglects of the principle have become all too apparent.)

I don't know what the voltage & current requirements are for rocket firing, but for this discussion I am presuming that at an appropriate point in the power system, they can be handled by standard relays.

----------

Perhaps the firing sequence should start with a loud klaxon or similar, and optionally a flashing light or two, with minimum of (say) 30 seconds 'countdown' warning to clear the firing area, before any actual firing is enabled, eventhough it is hopefully clear before the sequence is initiated. Together with a safety pre-briefing, it would add to the audience anticipation anyway!

Whilst I would try to keep the firing enable mechanism as simple as possible, I presume there will be single power source to all of the individual firing mechanisms, which at a convenient point isolates the entire firing mechanism. This point should consist of more than 1 switch/relay in series, all of which must be closed to supply power. Thus failure or misuse of one component cannot make the system unexpectedly 'live'.

I would go further and suggest that all of these observers + RSO are made part of a 'voting circuit' that must unaminously and actively approve the firing sequence to begin. This voting could be something like each holding a sprung switch closed, so releasing any of the switches would immediately abort the firing sequence.

In addition, the RSO and each observer, would need to provide a 'token', perhaps a plug attached to their wrist for the 'duration of danger', that must be inserted into a socket, thus 'proving' they are 'on station' as well as being the person operating the voting switch. I would probably envisage that any remote switches/plugs operate a separate relay (or two) to that of the RSO, so that they can only prevent the firing circuit becoming live, they cannot override the RSO's ability to disconnect power.

Secondary contacts on the relays can be connected to LEDs on the RSO's master control panel, so they can be monitored and tested. (Assuming firing a rocket requires more power than powering a LED, careful positioning of the resistor in series with the LED could also protect against a crosswired relay.)

The procedure might include the RSO watching the observers 'approve' starting the firing procedure, and only activating his/her switch after seing the observers' LEDs light, plus a final visual check of the area.

Using relays located at a 'central' point means that each observer could be some distance away, perhaps with two observers looking from diferent angles, each connected by their own single 3 or 4 core cable.

An independent 'countdown timer' of say 30 seconds could also drive a further relay, such that a 'unanimous vote' will not enable the firing for that first 30 seconds, reducing the chance of 'twitchy fingers' being an issue.

I reiterate I am only thinking about this from an armchair, so apologies for any 'daft' suggestions, but they are provide in good faith and hopefully they will spur a useful thought or two. (Obviously, I do not accept responsibility for any errors on my part.)

Best wishes, Dave


   
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(@lydara)
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Joined: 3 years ago
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Topic starter  

@davee  Your thoughts mirror many of mine!  A more complete description of the rocket launch control system is here:  https://forum.dronebotworkshop.com/project-help/10n-rocket-launch-control-system/


   
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