I ma curious as to how well their current 4-prong weeder is working. I use a stirrup hoe (see: http://www.johnnyseeds.com/p-5967-3-stirrup-hoe.aspx?utm_source=Nextag&utm_medium=CSE&utm_campaign=MerchantAdv&zmam=80483139&zmas=1&zmac=5&zmap=9489.0) like what you are talking about, and find it insanely fast and easy.
The issue that I see with trying to use one on FarmBot is the lateral stresses on the gantry. I am wondering if the current weeded could be modified with a stirrup hoe add-on. (you may need to make 3 weeders, which can be mounted so that stirrup hoe part is 120 degrees apart. The weeding function would then be:
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weed is detected. Farmbot identifies a “safe” place nearby (2" away) to :land" the weeder. If the orientation requires a different weeder orientation, it swaps to the right one.
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By descending on the Z access, Farmbot “lands” the weeded head, which goes (perhaps 1") into the soil, before further penetration is stopped by a plate on the prongs. This secures the bottom end of the Z arm, and allows for some minor torque to be applied.
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As the Z arm descends a bit further, the downward moment is converted by a cam to a lateral movement, which would pull a small stirrup hoe through the stem of the weed. Since this cuts off the entire plant above ground level, it is VERY effective. The stirrup hoe only needs to move 1/2" or so.
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As FarmBot raises the Z arm, the stirrup hoe is reset (by a spring) for the next weeding action.
This has the added advantage of allowing weeding to happen closer to desired plants than the current weeder.