ROBOTIC ARM STRUCTURAL DESIGN

September 2018 - May 2019, for MRover. General joint design and primary structure for a 6 degree-of-freedom arm.

Problem and Background

Competition objectives at the University Rover Challenge require fine manipulation ability. In response, MRover has traditionally built a 5 or 6 degree-of-freedom (6 DOF) arm to quickly manipulate objects in the field such as supply canisters up to 5kg in mass and instrument panels with a wide variety of features (knobs, switches, keypads, etc.). Due to the tight mass constraints imposed by the competition, the arm must be lightweight -- the team budgets 8.5kg for the arm out of the 50kg for the whole rover.

Ideally, we would like to spend as much mass as possible on the actuators for the arm in order to maximize power output. Previous designs for the arm have involved heavier than necessary primary structures.

Solution

Design Goals

• Minimize mass
• Maximize rigidity
• Standardize actuator interface

Over the course of September to December 2018, I designed a primary structure for the robotic arm to use carbon fiber to cut down mass usage. Additionally, I created a generalized joint design for the arm with lower friction and higher rigidity than before.

REFLECTION

Successes:

• Very high rigidity
• Much lighter than the previous arm
• More range of motion than the previous arm, thanks to the offset joint design
• Sleek design

Failures:

• Links did not have enough clearance with each other: some links rubbed at acute angles
• Initial output interface design for Joints B-E did not feature dowel pins to explicitly carry the torque between parts, causing screw failure and backlash. Dowel pins were added to fix this issue.
• Few provisions for cable routing, making harnessing very difficult
• Stripping of aluminum female threads

Future Work:

• More mass reduction
• Integral cable routing considerations (connector bulkheads, conduits, etc.)
• Incorporation of threaded inserts (i.e. Helicoils)
• Incorporation of wavesprings to maintain appropriate preload on needle roller thrust bearings
• Investigate use of plastic instead of aluminum