Pivot sprinklers

 

Do you ever think about those giant pivot sprinklers out in agricultural fields?  Great invention.  You can look at the crops underneath and see what a difference they make.  But think about the structure of them.  A long tube filled with water, about a quarter of a mile long.  A bunch of wheels set on trusses at intervals underneath.  Power the outside wheels, the ones farthest from the pivot, to make it go around.  But what about all the other wheels?  Does the water pressure in the pipe keep the rest of the mechanism rigid enough that all the other wheels can just roll to keep up?  I don’t think so.  Not for a quarter mile.  I think there would be a big bend in the middle.

 

So you have to power all the wheels to keep everything in line, but that creates its own problem.  This thing travels in an arc.  A different radius for every set of wheels.  Every set of wheels travels a different distance in the same amount of time.  They travel at different speeds.  They would all have to be calibrated perfectly or this thing would end up a mess.  And besides, some of the wheels must slip in the mud they’re making sometimes.  And sometimes the land isn’t even flat.  No, that can’t be it.

 

Watch a pivot sprinkler in action for a while.  Most of the time the wheels aren’t moving.  The outside wheels set the pace.  All the inside wheels only move when it’s time to catch up.  How do the wheels know when it’s time?  The central water pipe only snakes a tiny bit.  Overall, it stays pretty straight, but with each truss seemingly moving randomly.  One might move, the next one moves, then the next one might move again before the first repeats.  There are no obvious triggers.  I don’t see any lateral guy-wires that would support this function; something that would pull a switch when it was time to move, then release and stop the motion when it was time.  No obvious light-beams triggering alignment adjustments.

 

Bill Grenemeyer should be my source of information for all things agricultural, and he is.  He explained it to me a couple years ago, but I didn’t understand.  I had to look it up.  It’s all about angle sensors.  The long pipe bends a little as each set of wheels moves.  There are angle sensors within each truss that tell it when it is falling behind and it’s time to catch up.  They all operate independently.  The ones on the inside of the arc, closest to the pivot, hardly have to move at all.

 

That takes care of the motion, but we still have to deal with the amount of water the sprinklers put out.  You can’t just pick one size.  The ones on the outside of the arc cover a lot of ground around the circle, while the ones on the inside hardly move at all.  If every sprinkler put out the same amount of water, the inside part of the arc would be a lake if the ground on the outside of the circle got an adequate watering.  So not only do we have to make the wheels cover the right amount of ground, we have to scale back each set of emitters so they’re delivering the appropriate amount of water at each point on the radius.

 

Well, I was just thinking about pivot sprinklers.