How will "multiple reflectors" work with ultrasound. Could you post a quick drawing of that?

Let's say the largest side of the greenhouse is 15m.
Place two additional reflectors partway along the path besides the two end reflectors.
Now the longest distance to any reflector is 15/3 = 5m.
The ultrasonic range finder will detect the closest reflector. If TS is only interested in movement then you only need to detect change in the TOF (time of flight) signal.

 

So, tell us what is supporting this contraption, and what provides its mobility to transverse the area. Tell us why an encoder on the traversing structure can’t be electronically geared to the spreader.

 

So, tell us what is supporting this contraption, and what provides its mobility to transverse the area. Tell us why an encoder on the traversing structure can’t be electronically geared to the spreader.

Agree. Encoder wheel or encoder track seems like the logical direction here.
also, ultrasonic sensors get more and more expensive after 2 meters. There's a reason the ultrasonic backup sensors in cars don't have a six-meter range.

 

Thanks for all the input guys, I'll just add a electrical motor to pull it over a bar above the plants and measure speed on the bar with a encoder.