The Weed Weasel Prototype: Electric Walking Tractor

     The Weed Weasel Project was divided in to 3 phases; phase 1, construction; phase 2, improvement; and phase 3, documentation.  So I report on each phase in sequence in this report.  This report will be under the heading “project outcomes”.  The final conclusions of this report will be under “assessment of project approach/areas of further study needed”. 

       Phase 1 of the Weed Weasel project, the design and construction of 3 prototypes, was on time and on budget.  Here are some of the design details that were incorporated in to the three new prototypes:

*Wheelbase:  The three participating farms finally agreed on a single wheelbase, based around a 30” bed.  The small wheelbase made the weed weasel more maneuverable and better for something like a greenhouse operation, and worse for a bed system based on, say, the wheelbase of an Allis Chalmers G series tractor.  For keeping shop costs down, a single smaller wheelbase was important.  However, we found during the season that because the Weasel is stronger and more powerful than I anticipated, a larger wheelbase (large enough, for example, to carry a standard Budding Basket Weeder) is possible.

*Wheels:  I originally proposed to use “fat-bike” wheels rather than rivet two standard 26”x1.5” wheels together for the extra traction and flotation desired on farms.  While this seems like the obvious future choice, the price of fat bike equipment is still a premium, so I returned to the homemade “double-tire” as a lower cost solution.  For an individual farmer/builder the acquisition of used or discarded fat-bike wheels is still an option.  Also, later in the season, during the Improvement phase of the project, I experimented with making a “double-tire” wheel from all steel rims and hubs.  This drastically reduced the price of materials and labor for the wheels, and the weight penalty of using steel rather than aluminum was not significant (the whole machine is more powerful than I thought it would be).

*Thumb throttles:  The first prototype used a single thumb throttle driving both motors, with no reverse option.  On the three prototypes for this project I used dual thumb throttles and dual motor controllers, with reverse.  This dual throttle system allows the operator to use the throttles to steer (“skid steer”), to select reverse for backing, and to use one motor in reverse and one in forward when turning in tight spaces.  The dual throttles and controllers is a tad more expensive but gives much better performance overall.  Two details bother me about the throttles, one major problem and one minor one.  The major problem is that the quality of the speed control that the throttles/controllers produce is just not great.  The motors respond too slowly to changes in throttle position, and there is not enough sensitivity through the range of motion of the throttle to give the operator a good feeling of control.  One possible solution to this problem is for the builder to just program an arduino and buy their own relays and transistors.  Code for high performance motor controllers is already available on the web.  For the less technology-minded, the solution is just to wait for the scooter industry to take this better, cheaper route to designing their low-cost controllers.  The minor problem is that the inexpensive scooter throttles are right handlebar only, so for this project the left side is mounted backwards.  That didn't cause a major problem.  One other detail about the throttles; the new throttles come with a led-based battery charge monitor, potentially useful for the operator.

*Dual Controllers:  To make the skid-steer and reverse gear possible required using two (smaller) controllers rather than one larger one.  This makes the cost slightly higher but the improvement in performance is worth it.  It also makes the wiring take a little longer (more connections), but again, with a good wiring plan the extra wiring is not a significant labor nor materials cost.

*Drive train.  The original prototype used a bicycle chain drive chain from the electric motor output to the wheel.  In order to achieve the desired gear reduction (from scooter speeds, 15mph, to walking speeds, 3mph), a large (65T) chainring was used at the bike wheel.  While bike chain is ubiquitous and therefore a great choice, 65T chainrings are difficult to find.  So for the drive train of the three prototypes for this project I chose 8mm chain and more commonly available cogs and chainrings.  The motor output needed to be adapted to the 8mm standard cog, and the chainrings needed to be adapted to the bicycle wheel.  An additional detail of difficulty in the drivetrain is the fact that since a reverse gear was incorporated in to the motor controller, the drivetrain has to be capable of operating in either direction.  Since all standard bicycle hubs are “right-hand-drive”, some care has to be taken attaching the chainrings to the bike wheel or they will loosen in use.  In the case of the aluminum rims and hubs, twisted wire was used to latch the chainrings to the spokes.  In the case of the steel bike wheels, there is  a much quicker and easier solution:  tack-weld the chainrings to the hubs!

*Batteries:  Two of the new prototypes use a 20 A-h AGM type 36 volt battery bank, and one of them uses a 26 A-h AGM type 36volt battery bank.  The difference in weight and cost are not large.  In practice the smaller battery banks were never exhausted in use, so the benefit of the larger bank is probably not worth the extra cost.  What is astonishing is that such good performance was accomplished using a battery technology that is obsolete!  The promise of cheaper better lithium-ion batteries means this Weed Weasel design can use more power for longer durations in the near future.

*Frame:  As noted above, a single smaller wheelbase was adapted for these three Weasels.  In addition, redundant structural parts in the wheel  wells were eliminated, saving cost and weight.  The frame was made a little taller, so the wheels clear under the top of the frame, creating the opportunity to tune the weight balance more by shifting the battery box forward or aft.  At the farmer's request the design was made adaptable to left or right walking (the Weasel can be set up so the operator is on the right or left side).  The tool bar can be raised up or down with indexed holes and pins.  In the future, the design should be modified so the tool bar is raised and lowered by some kind of mechanical lever or pulley.  Finally, the motor mount design was changed to make the motor mounts more adjustable, so the alignment of the drive train can be tuned. 

*Battery/Electrical box:  The box got smaller and more efficient, and was made entirely from pine boards.  Wood is inexpensive, renewable, recyclable, sunlight resistant, safe and easy to work with, and does not conduct electricity, making it an almost ideal material for housing batteries and electronics.

*Electronics:  The electric scooter motor controller used is inexpensive but not ideal.  It has many control features that are unnecessary (turn signals, brake lights), but the one important feature, the throttle, doesn't perform well (as described above). It is easy to imagine that cost/performance improvements could be realized by a custom controller, but engineering electronics is outside the scope of this project.  It is inevitable that the electric scooter industry will adopt newer, cheaper, better controllers in the near future.  In the meantime it is possible someone with electronic acumen will make a custom controller and share the plans.  Because the Weasel works well, the tactic of putting a mass-produced item to novel use on the farm is a good strategy today.

     Although the farms did not report using the 12 volt converter much, I still think its cost is justified.  Imagine the frustration of needing to charge a cell phone in the field and losing an important call despite sitting next to a fully charged battery bank that's the wrong voltage!

     One electronic feature from the original prototype which was adapted and improved for the new models was the hi-low switch on the throttle.  The hi-low switch is a switch that routes the signal from the throttle either straight to the controller (hi power) or through an adjustable potentiometer to the controller (low power).  This allows an adjustable low power setting to be engaged by the operator during precision cultivating or training.  The hi power setting is then normally used during transport to and from the field.  With the dual throttle/controller set-up, there are two independently adjustable potentiometers for the low power settings, allowing the low power settings of the two motors to be different from each other, such as when cultivating on an incline.  After some trial and error, it was determined that audio taper 10K-ohm pots work best.  When an operator is properly trained in the use of the hi-lo switch, they can tune the motor speed of the Weasel to match their own working speed and comfort level.  This is helpful for training.  It also helps during operation, as three-row precision cultivating usually requires a slower speed than blind cultivation.  I'm grateful to Nat York for consulting on this feature of the design. 

     Phase Two of the Weed Weasel Project, the Improvement Phase, was not as smooth as Phase One, but the difficulties brought in to sharp focus the details that make adoption of mechanical cultivation possible.  The purpose of the Improvement phase as it was conceived was to allow the three nearby participating farms to test the Weasels and recommend changes; and then to make those changes and support further testing and feedback of the machines.  Unfortunately, all three farms were unable to extensively test the machines and/or had problems early on that caused them to stop using the machines.  The problems were varied, from trouble steering to problems with ground-contact tools.  Finding time during a hectic growing season was also clearly a factor.  But underlying these proximal problems was a fundamental observation that became gradually clear; mechanical cultivation on a small vegetable farm is not simply the deployment of a machine that scratches weeds.  Rather, it is the adoption of, and investment in, a whole-system strategy of preparing fields and growing crops.  The weeding machine is just a part of this larger plan and strategy.  In order to use a mechanical weeding system effectively, a farm must plan to use uniform bed and row spacing for minimizing tool set-up time.  The individual beds must be groomed smooth, cleared of rocks and residual crop fibers, and somehow planted with more uniform spacing and plant depth than would  normally be used for hand weeded crops.  And weeding must be timely—crop size and weed size are important factors.  The extent to which planning, preparation, and investment matter in mechanical weeding was made clear when one of the Weasels was moved to Waltham Fields Community Farm in July.  Because Waltham already extensively uses cultivating tractors, they were able to set up and use the Weed Weasel with relative ease.  In late July 48 200' rows of two-row brassicas were weeded in 2 hours with the Weed Weasel, representing a time saving over hand hoeing of approximately 8 person-hours.  The operator's comment was “as fast as the G” [the Allis Chalmers G series cultivating tractor].    When I visited Waltham to see the Weasel in action, it was obvious that I was observing the dividends of much past investment in soil structure, bed layout, crop planning, and precision planting.  For example, Waltham direct seeds a three-row crop with a three-row, tractor mounted seeder.  Considerably more expensive than a single-row hand pushed seeder, this set-up sows correctly spaced crops in extremely straight lines.  By contrast, Skinnydip farm uses a single-row pushed seeder, making it difficult if not impossible to achieve the uniform spacing necessary for a machine mounted weeder carrying 4 tools. 

     “Driver's Education” :  As an experiment in the “human factors” to be considered in the adoption of the Weed Weasel, we used an unplanted bed at Waltham to lay out a 100' three row crop of popsicle sticks.  Using popsicle sticks instead of live crops allows people training on the Weasel to experiment with going fast and steering without having to worry about damaging actual crops.  A volunteer with no experience using tractors was sought to try the Weed Weasel.  In just four 100' passes the volunteer improved her skill with the Weasel by a factor of three or four, moving quickly from knocking over many popsicle sticks to only knocking over a dozen or fewer.  It can't be stressed enough that during a busy growing season even taking the time to pull a string and “plant” popsicle sticks in 100' rows is more “messing around” than any of the participating farms felt inspired to undertake.  However, it is also clear that training time on the Weed Weasel (or any cultivating machine) is absolutely necessary for its implementation.  For a hard-pressed farm, this is a catch-22; off season training and planning may end up being the key to the adoption of any mechanical cultivating strategy. 

     One relatively minor yet important improvement made was the recognition that the original specification (by the electric scooter company's engineer) of  a linear taper potentiometer was wrong; an audio taper potentiometer gives a better range of control over the low-power setting of the throttle.

     Finally, it is hard not to reconsider hand weeding:  The hand-eye coordination of even a poorly trained human allows them to make such fine adjustments in tool angle, depth, and position.  Combine this motor-control with the decision making and processing abilities of the rest of the brain and you have a marvelous (though expensive and slow) weeding machine!  There are probably some farmers who simply don't want to impose on their farm the uniformity and details necessary to adopt mechanical cultivation.  For these people single-row (and half-row) pushed implements are a good choice if they can be made fast and precise enough to cultivate effectively.  Ultimately, mechanization should be considered for eliminating drudgery and thus making farmers and farm workers happier, but avoided if it adds stress and complications.  There is no one-solution for every farm.

     Even though Phase 2, the Improvement Phase, didn't turn out as I expected, several improvements were made, others were confirmed, and a lot was learned that is valuable not only to the design of the Weed Weasel, but also to the way in which its benefits are communicated to farmers.  In the end, the two farms that had success using the Weed Weasel prototypes, Waltham Community Fields and Vanguarden CSA, were both farms that already use cultivating tractors.  Despite this, both farms want a Weed Weasel and saw opportunities for the Weed Weasel to augment their current weed control regime.  I am especially grateful to Tim Cooke, at Waltham Fields Community Farm, for taking the time to get comfortable driving the Weasel, testing it, and providing valuable feedback.   Chris Yoder (my brother) of Vanguarden CSA, also helped with the testing and provided an example of a farm that uses mechanical cultivation but where the beds are not as perfectly smooth or free of crop residues as at Waltham.

     Phase 3 of the Weed Weasel Project, creating the plans and video, was fine.  Emily Vogler (with whom I'm married) agreed to take time from her demanding schedule to make the 3-D model and render the 2-D plans.  Once again, Emily's computer drafting skills and expertise in visual communication led to a set of plans that will at once be understood by farmers, engineers, and metal fabricators.  The step-by-step progression is so much better for the builder than a mere blueprint, as it organizes the work in to simple, sequential parts.  And, with one of the prototyped machines in front of us, dimensions and details were double and triple checked for accuracy.

     Meanwhile, Olaf Bertram-Nothnagel produced an excellent and mercifully short video of the Weed Weasel, describing its potential benefits but also showing the limitations; not an advertisement but an introduction.  Although intended for farmers, the video covers just enough of the essence of weed-control on organic farms so that farm workers and farm customers can appreciate it as well as farmers.                    

     The Weed Weasel video can be viewed at:

https://www.youtube.com/watch?v=l_s7aTKpiQs&t=9s

The free, complete, step-by-step plans for making a Weed Weasel are at Farmhack.net:  

http://farmhack.org/tools/weed-weasel-electric-walking-tractor

      We were lucky to have good weather every day we shot footage,  but the weather is the only detail Olaf leaves to luck. 

     In addition to the plans and youtube video, the Weed Weasel was also featured at the Eastern MA Craft Workshop on Cultivation, July 18, at Waltham Community Fields Farm, and made an appearance at the Umass Ag Extension Twilight Meeting on reduced tillage and cover cropping August 28.