3D-Printed Robot Arm for Crop Inspection
By Ai-Pinh Hu, PH.D
3D-printed, low-cost robot arm mounted onto the UGA autonomous rover for bell pepper
Traditional manufacturing techniques are subtractive in nature: you start with a block of material (wood or aluminum, for example) and proceed to carve away until you get your desired widget. Additive manufacturing, also known as 3D printing, is the process of making a 3-dimensional object of practically any shape and size from a digital model. Unlike traditional manufacturing, 3D printing builds an object from the ground up by successively adding thin layers of material that are extruded from a nozzle.
Plastic is currently the most common material used, though 3D printers that “print” with metals, ceramics, and even biomaterials are also available. The technology allows rapid prototyping, permitting a low cost of failure in developing and trying out new ideas. The fabricated parts may also become permanent components in their own right.
Although 3D printers have been in existence since the 1980s, only recently have they begun to make their way out of the engineering workplace and into the consumer market. Within a decade (or less), 3D printers are anticipated to become common household appliances. Today, early adopters can choose from desktop models costing on the order of $2,000 that are capable of manufacturing acrylonitrile butadiene styrene (ABS) or polylactic acid (PLA) plastic parts to within a tolerance of 0.1 mm. These desktop units are about the size of a microwave oven.
Researchers at the Georgia Tech Research Institute’s Food Processing Technology Division and the University of Georgia’s Tifton campus are collaborating on the design, fabrication, and deployment of a low-cost robot arm that is more than 80 percent 3D-printed from plastic. The targeted first application is inspecting bell pepper leaves for the early detection of pests, for which the robot arm will be mounted onto one of UGA’s autonomous rovers (see photo) that is capable of navigating up and down the rows of a crop field.
One intent of the project is to demonstrate the efficacy of using consumer-level 3D printing technology for an application that requires both precision and repeatability. Due to the particular geometry of the rover and the layout of the bell pepper plantings, the reach of the arm is about 5 feet measured from its anchor point on the rover. The payload, consisting of a suite of sensors, will be approximately 2 pounds.
Two challenges result from the design parameters of this application: (1) the long reach of the robot arm, made mostly from relatively flexible plastic, can lead to imprecision and vibrations during motion control, and (2) a 2-pound payload cantilevered several feet represents a significant weight to support.
To address the first challenge, the researchers are applying visual servoing, which is a method to control a robot arm using a camera as feedback. The control method closely resembles the way a human senses and manipulates objects. In the same way that people can manipulate (approach and grasp) everyday things without having to know the measurements of their arms and hands, robots implementing visual servoing control do not require precise or even known hardware geometries. This is a benefit in the case of the GTRI-UGA 3D-printed robot, where the relative lack in structural rigidity leads to difficult-to-characterize, or even changing, hardware geometries of the arm.
Regarding the second challenge, researchers are learning how to build with plastic and have devised a novel cam counterbalance mechanism that can support weight passively (with springs), such that actuation (applied by motors) is not needed to overcome gravity, but only during actual arm movement. The ability provided by the 3D printer to fabricate endlessly inventive cam shapes made it feasible to implement the counterbalance in a cost-effective way.
The research team plans to deploy the robot arm/autonomous rover for inspection tasks in the bell pepper fields in Tifton, Ga., during next year’s spring planting.
Ai-Ping Hu, Ph.D., is a senior research engineer in the Georgia Tech Research Institute’s Food Processing Technology Division. His areas of research expertise are controls, robotics, and mechantronics. He can be contacted by email at email@example.com.