A friend is looking to retrofit an old blueberry harvester that doesn't have pickerhead rpm display or electronic controls.  I've attached a pic of the current model's magnetic sensor and the overlarge nuts that it senses.  16 nuts per revolution give quick speed change results when a typical pickerhead speed is 26-30rpm.  His model uses a simple hydraulic flow control with no way to get an easily repeatable speed.  I was thinking that he could use something like a calc-an-acre hooked to the pickerhead to get a repeatable speed, but something that would display exact rpm would be preferable.  He might be able to fudge the numbers so that, say, 26mph is 26rpm, and that would be good enough but would require some experimentation to get the rpms right.

 Is this the best way to go, or is there something out there that can cheaply duplicate the factory method of sensing nuts and dividing by 16 to get actual rpm?  Maybe something that'll sense stick-on magnets and yield rpm instead of fudging it with a mph readout?

(pickerhead_mod.jpg)

pickerhead_mod.jpg (90KB - 125 downloads)