Hi bigfarmerdave,
One simple but not perfect way would be to hang something like a length (or a very narrow "V"-shape) of lightweight plastic pipe from near the end of each boom. The pieces would be able to rotate freely from front-to-back, but not side-to-side. They might be clamped into their rotating parts so that their heights above the ground could be adjusted initially, depending on crop conditions, etc. A switch (or you could possibly use a potentiometer or rotary encoder if you needed to get fancier) would be activated whenever the bottom of the piece of plastic pipe (or whatever) was pulled backward by contact with something below, by a certain (probably small) angular displacement, which would tell the system to raise the boom. When the pipe or whatever was no longer contacting anything below, and fell back to being vertical, the switch would open and the boom would stop rising.
That would enable maintaining a (rough) minimum clearance, but would not lower the booms if they ended up too high. Actually, if nothing else were done, it would almost guarantee that they'd end up too high, eventually, since they would follow upward contours but not downward ones. So, for "rigid, hanging ground-contact sensors" in general, it seems like you'd either a) have to have the rest of the system constantly try to lower the booms, to "feel for the ground" with the sensors, or, b) have them always be in contact with the ground and have two switches, to keep them within some preset range of "drag angle", so that the boom would raise if the angle of the sensor got too shallow and the boom would lower if the angle got too steep.
Also, you'd probably have to experiment with the weight of the hanging "sensors", to avoid crop damage. And it would probably be nice to find a way for their whole assembly to be pushed very easily from side to side by a row-width or so, so they might be able to "automatically" slide into the middles of rows of things like 3 foot tall corn, so that they and the corn wouldn't bother each other as much. You might be able to attach a "V" shape (or something) to them, near the ground, to help guide them. (Or go crazy and put video cameras and motors on them so you could guide them from side to side.)
You'd probably want an easy way to disengage the sensors' switch system, for manual control, and for when starting at the ends of the rows (until they got centered between rows, for example), and would probably also want to make sure that the system couldn't lower the booms unless you were moving, etc etc. And you'd want to make sure to have over-riding limit switches for the total height-adjustment limits, if you don't already have them.
- Tom Gootee
**broken link removed**