June 24, 2024

Vegebot: Agricultural Robot Design

by robotlux in

“You know, farming looks mighty easy when your plow is a pencil, and you’re a thousand miles from the corn field.” – Dwight D. Eisenhower

The day before, the Vegebot’s end effector had exploded on first contact with a lettuce, scattering acrylic shards across the lettuce lanes and rendering the crop in that corner of the field unsaleable. Now, under a dark grey sky and light rain, the wind was blowing hard, rocking the overhead camera back and forth. Confused, the mechanical arm lunged repeatedly, missing its targets. It had not been a good week. Then, with a peal of thunder, a bolt of lightning struck a nearby tree. The author and collaborators were stranded in the middle of a flat, desolate field, next to a large, malfunctioning metal robot.

The Vegebot project began in 2016 with the goal of building the world’s first lettuce harvesting robot. It was a collaboration between the Bio-Inspired Robotics Lab (BIRL) at the University of Cambridge and G Growers, a major UK farming group. The accompanying video gives an overview of the robot’s development, which had a particular methodology:

Development in the field. Sometimes there’s a reluctance on the part of researchers to get their hands dirty, and a preference to stay as long as possible in the nice comfortable lab. In the case of agricultural robotics, this is unwise. Rain, wind, bumpy ground, variable lighting conditions profoundly condition the robot’s design. The lab is nothing, repeat nothing, like life in the field. We worked in the field as often as possible, sometimes multiple times per week.

Build and test, rather than design. As the video shows, a core problem was how to effectively cut the lettuce stalk. It’s tough! Rather than making elaborate plans or designs, the team built quick and dirty prototypes of a stalk cutter, trying out different form factors and actuator systems: linear electric, rotary electric and linear pneumatic. The lettuce was initially held in place by a plastic bucket from Poundland. The linear pneumatic actuator had a good force-velocity profile, and was the only solution that reliably cut; it became the basis of all future versions.

One problem at a time. Once the core cutting mechanism was identified, the rest of the end effector development was a series of rapid, field-tested iterations, each one solving the next remaining major problem. Early versions had the actuators in line with the blade and next to the ground; these would get stuck on neighbouring lettuces. So we raised the actuators off the ground and tried various systems of transmission (struts, timing belt) to drive the blade. This created a robustness problem, which led to a solid, stainless steel end effector. This solved robustness, but created a weight problem. And so on, and so on. The point is that iterations were fast, immediately tested in the field and solved one problem each time. A similar process was followed with the software.

Behaviour separation. The classic way of thinking about robot architecture is as a perception-planning-action pipeline. We divided the work up in slightly different chunks, which corresponded to behaviours: detection (find the lettuces and output their screen coordinates), approach (move the end effector to touch the target lettuce), manipulation (grasp the lettuce and cut the stalk). Each of these behaviours potentially has its own perception-plan-action loop. For some crops, detection might involve pushing aside leaves that obstruct the view; manipulation likely involves real-time sensing. Importantly, each behaviour can be dealt with by a different team or engineer: detection (Julia), approach (Simon), manipulation (Josie), and can be tested pretty much independently. The interface between the behaviours is key. By having the output of detection be a set of 2D viewport coordinates (not inferred 3D positions), it meant that the approach behaviour could be tested independently by simply clicking on one of the lettuces on the screen. Manipulation could be tested by manually placing the end effector over the lettuce.

The curse of the seasons. Having made a big deal above of doing as much as possible in the field, agritech’s dirty little secret is that the field isn’t available for much of the year. What do you do in the winter? That’s when you focus on software, simulation (maybe) and dataset processing. And get ready to move as quickly as possible come the spring.

Robotics is astonishingly hard, even for the technically sophisticated who encounter it for the first time. There’s something about the real physical nature of the world that defies the most sophisticated algorithm. Agricultural robotics makes normal robotics look like a walk in the park.