We have rugby poles. I have attached a picture below. We need to know the best sensor(s) to get to be able to pick up when a ball has been kicked between these poles and then do something funky like flash big red lights. I have thought of an ultrasonic sensor but not sure if it will cover the whole area or how many can cover the entire area.
You need to figure out what the shortest time the ball will spend between the posts will be.
I think that's going to rule-out ultrasound.
Fan beam laser/IR?
How about hanging a net between the posts? 
You have 5.6m between the two poles, that's pretty wide for any low cost sensor to stay "focused"
You score points from a kick even if the ball is "virtually" between these poles but above the height of the poles (the horizontal bar is at 3m from the ground and the rules state that total height is 3.4m minimum, so poles could stick out only by 0.4m) - so having a system that only tracks what's happening in between the poles is not enough...
I don't think it's an easy problem to solve in a tinkerer's budget (I'm not even aware of a commercial system that works)
How about two referees as sensors, one below each post ?
Give them a remote that they press if they saw the ball passing between their position and the corner across the pole. If both referees pressed the button then the ball was in between the bars and you can do your sound and light animation
Give them a remote that they press if they saw the ball passing between their position and the corner across the pole. If both referees pressed the button then the ball was in between the bars and you can do your sound and light animation
+1.
Very hard !! And what if a bird flys between the posts ?
There are commercial systems: light-beam curtains
which means once every inch a photoelectric barrier
Still has the bird problem.
With 0,5 mW lasers that beam vertical up in the air and on the top-end cat-eye-reflectors to reflect the laserbeam down again might work. The photosensors have to match the lasers lightcolor = wavelength of the light.
You should do testings with a prototype. I estimat there are quite same problems until it works realiaby
Still you need multiple lasers to catch a ball passing with the smaller diamater
Some kind of net that guides the ball to fall down a certain path will make detection much easier
best regards Stefan
Yes typical Light Curtain have a short (< 1m50) focus range as they are meant to protect a perimeter around machinery.
You can find some that go to 6/7m (this company for example has some that can go up to 35m) but they are not usually very tall and kinda expensive.
Typical size for good rugby poles is 13m above the horizontal bar
So with the right budget you could equip the poles with such a safety Light Curtain but you would still miss possibly the shots above the poles. (of course if they are 12m high you will get most of the kicks)
The fact that these dont exist as such commercially is notable ( they would be useful) and points to difficulty /cost -there are other issues too; power, damage from the ball hitting the posts , difficulty with access etc which will be reflected in high costs
Might be worth looking at alarm type sensors , radar speed , and other technologies used in similar detection scenarios , camera technology,
“ hawk eye” is a camera based technology used in a number of sports and offers a number of other useful bits
A camera with an attached AI should work in a fixed installation.
A Jetson Nano with a Pi camera would be a cheap try.
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