Friday, August 12, 2011

Agricultural Robots: Locomotion, Walker, Crawler or Wheels?

Locomotion:  How an agricultural robot is going to move itself around will depend on
1). the nature of the crop it has to move through,
2). the specific tasks it has to perform, and
3). the condition of the field’s surface.

Tasks such as plowing and harrowing are done using heavy-pull tractors.  Tilling is done using a lighter-duty tractor.  It’s doubtful that the tractor operators responsible for these tasks will ever be replaceable with robotics. 

The tasks that lend themselves to robotics are those that are currently done by hand, tasks such as thinning, weeding, pre-harvest prep, harvesting, packing and loading.  Also an interesting new option for robotics, that humans currently can’t do, is the task of pest and disease control. 

Tasks, Harvesters:  Of the hand-harvested crops, you'll see several different field/crop configurations. 

Strawberries are the only crop that I am aware of that is laid out in wheel-accessible rows that are also maintained that way throughout the planting/growing/harvesting life cycle of the plants.

Strawberries, Corralitos Road, South Santa Cruz County
But in general, if a farmer can still see dirt between their plants come harvest time, then they probably aren't getting the yield they could have out of that piece of land.
Brussels' Sprouts, Jensen Road, North Monterey County
So, while crops may be planted in rows, by harvest those plants have generally grown to fill the space between the rows.

For example, some crops, like lettuce or celery, while grown close together, are harvested in one pass.  The harvester platform follows behind the workers picking the crop, so the crop is already up by the time the harvester might roll over the plants.

Broccoli, Beach Road, Watsonville
In the photo above, you can see the furrow left by the harvester platform's tires up the center of the image.  Since the part of the broccoli plant that gets harvested is the center, the fact that some of the plant's non-saleable outer stalks/leaves get damaged during the harvesting operation isn't an issue.  A wheeled or tracked harvester would be a workable choice for crops like these.  

Green Beans, Green Valley Road, South Santa Cruz County
But there are other crops like green beans or squash that are re-harvested every few days over the two-month (give or take) productive life cycle of the plant.  For crops such as these, "walking" would be the only workable choice for a harvester's method of locomotion.

Since robot harvesters of this last category will need to be able to "walk" down a crop's rows without damaging the plants they are harvesting, several conclusions follow.

1). Tethering, that is, dragging a power cord behind, is not going to be a workable design option.

2). Compromising a walker-robot's agility of movement with a battery pack weighing several hundred pounds will also not be a workable design option.

3). Computationally, the act of "walking" down a crop's row will consume as much machine vision and motion control resources as any of its harvesting/tending duties.  Not only will these robots have to be able to watch what they are doing, but they are going to have to watch where they are going too!

Tasks, Tenders:  Activities such as thinning, weeding, pre-harvest prep, packing, loading and pest/disease control fall under this heading. 

One example of pre-harvest prep is the de-leafing of Brussels’ sprouts prior to harvest.  In this case the field's surface is not only broken earth, but is now covered with a thick carpet of leaves.

De-Leafing Brussels' Sprouts, location uncertain, North Santa Cruz County?

A Field’s Surface:  One of the functions of tilling is to get rid of weeds.  Since organic farms cannot use herbicides to get rid of weeds, tilling turns out to be the only effective large-scale way to do this.  But after tilling, a field’s surface will be broken earth, and after irrigation that broken surface turns into soft mud.  This will be the most common surface condition a robot harvester/tender is going to experience.     
Tilled Field, Pioneer Road, South Santa Cruz County

Next Post:  The motion of an agro-bot's "upper body" is not independent of the motion of its undercarriage.  Undercarriages that have wheels or tracks are going to sway, tip side-to-side and bump as the robot travels down the broken surface of the field it's working in.  This is going to result in a rather shaky/jerky platform for the robot's arm/hand actuators and vision system to have to work from.  One way to deal with this problem is to give a robot an active suspension system of some kind. 

Another way to deal with this problem is "legs".  Having "legs" gives a robot the means to compensate for the uneven surface it has to travel while also simultaneously being able to adjust the position of its "upper body" to keep its actuator arm/hand/vision platform moving in a controlled fashion.  

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