Sunday, May 22, 2016

Biological Agriculture for Roboticists, Part 4

So how can robotics contribute to agriculture, or, more generally, to land management? Let's start with a relatively simple example, where the robot need not concern itself with differentiating between crops and weeds, and the only required manipulations are of nonliving materials. I'm talking about erosion control. In the video linked below there is no mention of robots, but the speaker does describe one of the techniques he employs — close placement of stones such that they aren't vulnerable to being washed away in the next flood — as time consuming and tedious. This is almost certainly a task that could be automated.

One step up from this is weed elimination in a recently tilled field which has not yet been seeded for the next crop, sometimes referred to as 'fallow' although that word is also used to mean a field that is simply being left alone for a time and has not been tilled. In that recently tilled field, any green growing thing can be assumed to be a weed — well, not exactly, but for the present purpose yes; I'll get back to this in a future installment — so all the robot has to be able to do is differentiate between the color of tilled soil and the color of any green growing thing within a geofence. Spot application of herbicide directly onto the plant, scaled to the size of the plant (less on smaller plants), is a huge improvement over area-wide application of the same herbicide, because it results in much less being used, and there are already machines available that do this.

Still using herbicides, further improvement is possible if the robot can model the plant in some detail, instead of only detecting a green blob, and determine where various parts of the plant are in space and relative to each other, how they are attached. Given such a model, more precise application of still smaller amounts of herbicide becomes possible, dropwise onto the point or points where cell division takes place (the meristem or meristems) or, in the case of an established plant, by injection into the main stem, just above where it emerges from the soil. Drop application can be tricky in a breeze, with the plant moving around, but if that model includes the structure of the plant and how it moves in response to air currents — something that is extractable from video (see below) — then those motions can be predicted and compensated for in the positioning of the drop dispenser. Other methods, not involving herbicides, are also possible, and made a great deal easier by this sort of modeling.

Pastures are a bit more complicated than recently tilled fields, since the contrast between weeds and everything else is more subtle, but help is available. Herbivores can be picky about what they eat and don't eat, and what they don't eat can produce seed or spread vegetatively, reducing the value of the pasture. In combination with holistic management of the animals themselves, keeping unpalatable plant species in check can help protect and improve pasture land. This is a task that can be performed by robots, at its simplest by clipping off at ground level anything left standing more than a few centimeters high immediately after a paddock has been grazed, after the animals have already decided for themselves what's good to eat and what isn't. This approach won't control low-growing plants, like goatheads (photo courtesy of Forest & Kim Starr), but it will control thistles and other erect plants. (Again, more in a later installment.)

Weed control becomes more challenging when the machine must be able to distinguish between crops and weeds, and research into accomplishing this is ongoing, but given that perceptual ability, the ways in which a robot might deal with a weed are numerous, and generally fall into three categories, control of seedlings, control of established plants that can easily be uprooted, and control of established plants that break off if you attempt to uproot them, leaving the root behind to produce a new shoot.

Seedlings are difficult to distinguish, but easy to kill. Most are easily uprooted, but moving a mechanism into position to do the uprooting takes time, especially if there are many weed seedlings to be dealt with in this manner. Using a high-pressure water jet to expose the root or sever the stem should take less time, since no movement reversal (pulling) is required. Using projectiles (ice, dry compressed compost, ...) might accomplish much the same thing from a slightly greater distance, given accurate targeting. With even more precise targeting, a laser might heat just the meristem enough to stop a seedling's development, from an even greater distance, requiring even smaller movements for retargeting, enabling faster operation. Laser heating might be combined with LIDAR sensing as a precisely timed high-energy pulse. A downside for both projectile and laser methods is that they require a clear path from a remote position to the target, something that becomes more problematic as the growing season progresses and leaf canopies become more dense, but seedlings that emerge later in the season are less of a concern, both because they will develop more slowly due to the shade created by established plants and because, by the time they have developed enough to represent significant competition for resources, annual crops will already be maturing or have already been harvested.

Plants with established root systems can sometimes also be uprooted, and any established plant can be clipped off mechanically near the soil surface, or cut off with a jet of high-pressure water. All of these techniques require positioning a mechanism at or near the base of the plant, and uprooting also requires support sufficient not only for the weight of that mechanism but also to offset the force required to accomplish uprooting, which can be considerable. Clipping or cutting a plant off near the soil line may not kill it, but it will set back its growth, and doing so repeatedly can eventually exhaust the resources it draws upon to regenerate above-ground growth, provided that the root system isn't being fed by foliage elsewhere. Machines relying upon such methods should be programmed to revisit those locations periodically, checking for regrowth. Plants with very tenacious root systems may require more aggressive treatment, which could mean herbicides but might also mean coring (cutting a deep cylindrical plug from the soil around the plant's stem) or steam injection, to cut the node from which that stem emerged off from the rest of the root system or locally kill the root system. All of this is a little like the botanical equivalent of Whac-A-Mole, which means tedium, something robots excel at coping with.

Next I'll go into perceptual systems (sensing and sensory processing) and plant differentiation in greater depth.

Previous installments:

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