Worldwide there are about 400,000 species of plants. Seeds from those plants can be scattered by the wind, deposited by birds, and carried by animals. Given the exuberance with which many plants produce seeds and the ease of dispersion, it’s inevitable that when you sow a field with just one type of plant many different types will grow. And wherever these uninvited guests emerge they’ll do everything they can to snatch sunlight, nutrients, and water away from the crops you want.
Farmers wage a perpetual battle against weeds, and for decades chemistry has supplied them with a potent weapon. Modern crop protection chemicals largely enable farmers to prevail over their tenacious foe. But not without cost. Herbicides can cause collateral damage—poisoning beneficial organisms, running off into rivers and streams, and persisting in the environment long after the job is done. Nor have weeds surrendered. They counter attack by developing immunity to our best chemical weapons.
The worrisome aftereffects of herbicide use partly explain the growing interest in organic farming. Organic farming eschews external inputs in favor of managing the farm ecosystem using local tools. Rather than input high-tech herbicides, many organic growers rely instead on an ancient and venerable tool, the hoe.
In many ways the humble hoe beats the pants off herbicides. Removing weeds mechanically leads to few unintended consequences. Nothing is poisoned, nothing noxious escapes, and the decomposing weeds can even return nutrients to the food crop. Furthermore, no weed yet has developed immunity to being plucked from the ground. With these clear advantages why don’t we see armies of diligent hoers saving our crops from weeds? Two reasons: it costs too much and no one wants to enlist—the hoeing army recruitment office is a lonely post.
Hoeing weeds: a hot, dusty, low-paying yet important job that no one wants to do—sounds ideal for a robot, no? Indeed yes and many scientists, engineers, and robot enthusiasts have pondered this tantalizing challenge. But how realistic is it? My purpose here is to address this question. Can we, today design weeding robots that work as well and cost less than chemical alternatives?
It’s easy to envision the ideal solution. We see teams of robot weeders swarming through fields. Day and night they work uprooting weeds of every size and description but never disturbing any food plant or causing mischief beyond the boundaries of the farm. Plus the robots cost far less than the billions of dollars farmers now spend annually on herbicides.
This notion seems very attractive—why aren’t such robots already in widespread use? It turns out that the bar for robots is set very high. In spite of their drawbacks herbicides mostly work. To conventional farmers they are familiar and predictable, each use costs relatively little (typically ones to tens of dollars per acre), and as weeds become resistant farmers use different chemicals and larger quantities. Society’s reasons for wanting to limit herbicide use appear a little less compelling from the farmer’s point of view. And it’s farmers not society, who must be convinced to trade sprayers for robots.
Technologists seek a solution that satisfies both sides: minimize herbicide use, eliminate weeds, and reduce cost. But to do so we must ask the right question. It is not, “Can we build robots that remove weeds?” The answer to that question is an easy but academic “yes.” Rather, the important question is, “Can we build weeding robots that make economic sense to farmers?”
As with most proposed jobs for robots, the really tricky part is neither technology nor economics but the confluence of the two. First, the macroeconomic case is convincing—there is clearly a billion dollar plus potential market for weeding robots. Second, relevant technology appears to be available. (See here for example.) In the video a commercial vision system developed by Aris of Eindhoven, The Netherlands grades and sorts plants in a greenhouse. Why can’t we just mount this or a similar device on a mobile base and achieve a decisive victory over weeds?
Existing solutions like Aris’ work indoors, but indoor systems have a big advantage over outdoor systems—they give engineers control. In the grading/sorting example the lighting is controlled, the backdrop behind the plants is carefully chosen, the plants can be observed from various angles, electrical power is abundant, and heat, cold, rain, dust, condensation, and bugs crawling on lenses need not be considered. Another subtle but significant benefit is that indoor operations can be conducted year round while outdoor equipment is used only during a limited growing season. Return on investment calculations make it more difficult to justify purchasing equipment that must sit idle in the barn part of the year rather than earn its keep every day. This means that minimizing cost is more important for outdoor robots than for indoor systems even though ruggedness and other outdoor requirements make low cost harder to achieve.
As is the case here, it’s common in mobile robotics to find that a reliable solution available in a structured environment does not function or is impractical in an unstructured setting. Instead, we must reimagine problems from the ground up. We look for alternative approaches that benefit from the unique strengths of robots and suffer only minimally from their weaknesses.
Weeds aren’t giving up and neither should we. So let’s look at the essence of what the robot needs to do. Our desired weeding robot must satisfy these functional requirements:
- Remain confined to the assigned field
- Visit every accessible point in the field
- Classify each point appropriately
- Apply an eradicating mechanism to every point where a weed is present
- Refrain from applying an eradicating mechanism to points where a food crop is present.
Various engineering solutions at a range of costs are available to address most of these items. But requirement number three turns out to be the make or break challenge for our weeding robot.
A standard suggestion for deciding between weed and not-weed is to give the robot a vision system. That system would follow one of two strategies. The first solution involves providing the robot access to a database of relevant weed types. The vision system points toward the ground and, moment by moment, decides whether any observed visual feature matches a known weed type. The robot then attacks the features it identifies as weeds. The second approach is to train the vision system to recognize the type of plant we want to grow in the field. The robot then applies its eradication mechanism to all ground surfaces that do not contain the desired plant.
At the current state of the art both these strategies are problematic in uncontrolled environments. A wavelength analysis of the spectrum of light reflected from the ground can usually discriminate between plant and non-plant. (For example see the WeedSeeker system.) But to date, I am aware of no vision-based outdoor system that reliably and economically discriminates between weed and desired plant. No such system is in widespread commercial use in outdoor fields.
The critical element of the weeding robot we wish to design can’t be found in any catalog. That is, the vision part of our ideal robot remains a research project. Research projects often merit support but putting one on the critical path to a commercial robot has scuttled many a promising product.
Manufacturers of agricultural equipment use a couple of strategies to deal with weeds without actually having to recognize them. Crops are usually planted in rows while weeds just sprout anywhere. Thus it is possible to build purely mechanical weeding implements. Dragged behind tractors these devices use passive mechanical fingers to disrupt weeds that grow in the space between crop rows. Such devices are available in the marketplace.
A second approach relies on giving plants a head start over weeds. If we begin with a freshly plowed field then transplant seedlings rather than plant seeds, we ensure that the desired plants start out taller than competing weeds. There are systems (see the Robovator) that rely on this strategy to discriminate between big crop plants and small weeds. This enables attacking weeds anywhere, even those that emerge between desired plants in the same row.
I often think that designing a successful robot is a lot like constructing a magic trick. A magician performing a trick could never actually do what the audience thinks he or she is doing. Rather, the trick only works because the magician wears a blue shirt rather than a brown one and because the apparatus is back lit rather than front lit and because the magician moves the hand that does not conceal the ball rather than the one that does, and so on. A bunch of factors exquisitely specific to the trick being performed must be arranged just so to make the trick work.
The situation is similar in the robotics domain. The robots I have built are never as general-purpose as most observers suppose them to be, and each robot exploits every possible advantage in its application space just to do one useful task. Lay observer, “If your robot can do A, then it must also be able to do B and C and D!” Me, “No, it’s really only cost effective doing A. But I could design a robot that does B or C or D.”
There’s one more similarity between robots and magic—like the magician, the roboticist must cheat at every opportunity. In magic the only thing that counts is mystifying and delighting the audience. To make it seem that your lovely assistant has magically transported from one location to another, use twin lovely assistants. Rank cheating! But it accomplishes the goal. In commercial robotics the only thing that counts is performing a desirable task at a competitive price. It’s not necessary that the solution be elegant, or be done the way conventional wisdom expects, or be something a researcher could write a paper about.
I see a couple of opportunities for cheating in the design of the weeding robot. First, although it seems natural and obvious, we’re not required to use a vision system at all. And second, although we started with the problem of weeds in farmers’ fields, we may achieve initial success by focusing on a related but different target—executing a sneak attack rather than a frontal assault.
The function we would like a vision system to perform is to decide whether a point within its field of view is or is not a weed. Rather than insist that the robot figure this out, why don’t we cheat and just tell it? One way we might do this is to employ centimeter-resolution Real Time Kinematic (RTK) GPS. Record the position of each seed when planted and then later avoid applying the weed eradication mechanism to those spots. Unfortunately this calls for high precision and high precision’s favorite companion is high cost. Too much cost will render our robot as desirable as a weed.
An alternative strategy is to indicate the location of each seed or seedling with a marker the robot can identify reliably and inexpensively. For example, we could place an RFID tag or biodegradable barcode near every seed or surround each seed with a short physical barrier. Tagging every seed with a this-is-not-a-weed marker allows us to dodge the vision research project and to keep robot cost low. This means that we can build our weeding robot today with inexpensive, off the shelf technology. But admittedly, this solution poses another problem.
An acre of corn typically contains tens of thousands of plants. If we assign our robots to this weeding task, we must commit to buying, installing, and later maybe removing tens of thousands of tags per acre—and corn is grown on a very large number of acres. Commercial growers care about total cost, not whether the cost is due to the robots or tags. They also care about worker availability not whether the workers are assigned to hoe or tag plants. Our strategy doesn’t seem appealing these growers.
Robot-friendly markers change the problem but have not yet solved it, so let’s cheat again. Farmer’s fields are not the only reluctant venue for weeds. Every home gardener battles the same scourge only on a smaller scale. Marking tens of thousands of plants per acre on the massive scale of commercial agriculture is currently impractical, however performing a similar task for tens of plants in a typical home garden is entirely reasonable.
Home gardeners plant their crops by hand, are often reluctant to use herbicides, and (if they’re like me) intend to weed regularly but are frequently diverted by other priorities. But for a small increment of work at the beginning—tagging plants as they are put in the ground—gardeners can have a better-looking, weed-free garden all season long. Furthermore, time freed up from weeding can be spent tending plants, pruning plants, and generally realizing healthier, higher yielding crops. Priced and executed correctly, the home garden weeding robot should have a strong appeal.
Let’s take stock. We set out to see if robots could change the weeding paradigm—eliminate hand hoeing for organic farmers and maybe give conventional farmers a compelling alternative to herbicides. So far we’ve discovered a strategy that enables cost-effective robots on a home gardening scale. However, our approach accomplishes something more significant than may be apparent at first blush. We now have a well founded path toward greater functionality and larger scale.
The strategy that enables the weeding robot to be simple and inexpensive makes planting more complicated and costly. But in situations where crops are planted and weeded by hand, this tradeoff makes sense. Thus our robot is appropriate for home gardeners and likely also applies to small-scale agricultural production, e.g. market gardens and maybe market farms. In these instances the cost of robots and tags is more than offset by a reduction in the hassle and cost of hand weeding.
Looking forward, the cost and inconvenience of tagging plants is not fixed. Like any technology, once a market is established, competitive forces will work to reduce the tag cost and automated methods will be found to simplify tagging. As cost decreases scope increases. That is, given further development the weeding robot becomes attractive to commercial scale growers.
Our exercise has proven fruitful. Rather than waiting for an unpredictable breakthrough in computer vision we have found a “shovel-ready” strategy. One that offers a limited but useful robot today and through predictable, incremental development promises to fulfill our vision of a generally applicable, cost effective weeding robot for large growers in due time.
Watch out weeds, the robots are coming.