NGCP - Robotic Arm / End Effector

Introduction

In 2016, Northrop Grumman imposed a new mission objective: In addition to autonomously locating the payload, the UGV would now autonomously pick up and retrieve said payload.

As a result, the NGCP mechanical team, created a 5-DoF, articulated robotic arm equipped for autonomous operation.

Mechanical Constraints

Due to the weight, power, and costs limitation of the UGV, special care was given to the mechanical systems of the robotic arm. We undertook the challenge of picking up a 1.5lb payload from 3 feet away, using only small servos.

Mechanical Constraints: Weight

Our first approach was to reduce any unnecessary weight on the robotic arm.

I explored various materials for their high specific strength. Although I considered carbon fiber and fiberglass, these materials were time consuming and labor intensive for us to produce, which did not fit our rapid prototyping environment.

Ultimately I found common 6061 aluminum and 3D printed PLA/PETG to be the most suitable materials for our project. Due to the lack of studies into 3D printed parts, we verified the strength of our prints through testing, eventually refining our printing processing to eliminate mechanical failures.

Mechanical Constraints: Design

Our 2nd approach was eliminate strain on our servos through strong mechanical design. Although many articulated robotic arms ultitize a direct drive, the forces expected in operating the UGV in rough terrain would quickly overload the servos. Stepper motors could better performance due to their ability to “hold” torque, but are heavy, power intensive, and required an additional encoder.

As a result, the majority of the robotic arm is designed to minimize backdrive while providing high mechanical advantage. The two joints of the robotic arm feature a leadscrew, while the end effector is driven through a custom worm gearbox. This configuration allows the robotic arm to carry payloads without drawing any power.

Autonomy

Computer Vision

The robotic arm was built to be capable of fully autonomous operation. A combination of servo torque feedback, along with a modified webcam for OpenCV provided the necessary data to the UGV NUC.

Future versions of the robotic arm may incorporate a break beam sensor in order to aid in “aiming” the end effector.