Mad Scientist and the Hoplite

As part of my daily hunt around various news sites, spotted this one again from Tomshardware :

DIY Sentry Gun Blasts Intruders with Paintballs.

Putting aside the question of how much you'd get sued if you had something like this ... What would a Mad Scientist software/computer/electronic engineer do for putting one of these together ? Let's call the Sleepypete system Hoplite (no trademark infringement intended, no way am I likely to sell this !) after the Phalanx gun that's used on ships.

You've got a few things to include :

Turret and gun
Control mechanics
Targeting system

For something like this, it's always best to follow a thread through logically. Like : Target -> Aim -> FIRE. So let's start with that targeting system. They use a webcam on that site but that's got a few shortcomings (like no night vision). So what does Hoplite need :

Webcam - aye. It's simple, cheap and interfaces well. Hook it up to image processing software to detect differences between reference pic and "what's there now" pic (trees don't move, so that's your reference). You'd want a fixed camera to designate targets (what to shoot at) and I'd look for a camera that moves with the gun to allow for aiming.

But I'd also want to have a thermal imager going for night time. A thermal imager is actually easier to get targeting info from too, just aim for the centre of the Warm Blob. But ... you have to do that motion detect thing again so you don't have false positives from the sun warming stuff or people tricking the system with flares.

There should also be a lockout so that if the turret is pointing at something in the reference, it shouldn't paint your tree.

Hoplite Control mechanics - this is the fun part.

The key reason why I went into Electronic & Electrical Engineering is because I was predicting that the influence on electronics on what we use was increasing daily. Silicon chips are everywhere. Every button you press is likely to have software triggering off behind it. My first car had barely any transistors in it, my latest car is wholly software driven even down to drive-by-wire pedals. And the thing that links software to the bits that move is the electronic engineering.

They use a laptop for control in that link, which is fair enough. You could also use the laptop as a display for CCTV, plus software on it provides the brain. What's the brain need to do ?

Lead the target - this is aiming in the right place to tag 'em when they run
Let you control and monitor stuff
Set the rules for how ruthless it is

The control is the most interesting bit and it's where the geeky people can really come out to play. The trick is to start simple and then add the clever bits. That's called Integration Engineering. Start with something that forms a foundation and make sure that works. If you start at a more ambitious level, then if you have a problem you have to go back to that simple foundation anyway to figure out what the problem is. Engineering is all about taking a concept and taking away everything that doesn't matter until you have something clean, efficient and elegant.

And there's all sorts of really complicated control theory behind that foundation. Let's just say it's dead simple to make something point roughly where you intend it to but it's much tougher to tune a negative feedback control system to make it point precisely where you want while still following things that move.

They do a pretty decent job on the one in the video, the ballistics look good and most of the time it's pointing in the right direction. It does jump off beam occasionally though (a control system issue that should have it stop firing when it jumps) and there seems to be a little stickiness.

Hoplite turret system :

This is where I might need a little help as I'm very rusty on the mechanical side of things. You'd want :
Weatherproofing,
Access to reload the system
Mounting sturdy enough to manage the rifle and recoil, plus cameras
Reliable motors to steer the turret (2 axis rotate + tilt would cover it)

It's one thing to do the geeky thing and cobble something together that does a job. It's another thing entirely to engineer something solid that you can put outside and forget about until you hear that "Rat-Tat-Tat- OUCH" of a gotcha.

PS Just need a suitable location to put a Hoplite, this place is too small.