Design and Testing of TRL5 IPEx Actuators

In-Situ Resource Utilization (ISRU) is an essential part that NASA is addressing by framing specific missions
around placing sustainable infrastructure on both the moon and Mars. Regolith is the most abundant resource available on the moon. The IPEx project is developing a 30 kg-class excavator to demonstrate robotic excavation of large amounts (10,000 kg) of granular lunar regolith on a future technology demonstration mission. IPEx uses novel excavation tools, called bucket drums, which are hollow cylinders with scoops staggered around the outside. Regolith is collected with the scoops and flows into the drum where it is captured by an internal baffle system. The excavator can then transport the regolith in the drum and reverse the direction of the drum rotation to dispense the regolith out. IPEx uses two sets of bucket drums that dig simultaneously in opposing directions and results in counter-acting excavation forces. This combination of bucket drum excavation tools and counter-acting excavation forces enables low mass robotic excavators to effectively dig in reduced gravity environments. This is a significant departure from terrestrial excavators that rely on high mass to produce tractive forces to counteract the forces of excavation.

IPEx is made up of several custom actuators that all need to be verified for their intended application. This paper focuses on the initial design, testing and modifications of three actuators: a mobility actuator, a shoulder (arm) actuator, and a bucket drum (excavation) actuator. The design of each actuator incorporates mass and volume constraints of the rover, while intentionally being oversized for the lunar mission to allow for Earth testing such that the lunar hardware could be verified in a lab environment. This consideration allows for identical motor designs between the actuators tested and those that will be used on mission, effectively reducing the project complexity. The mission’s concept of operations (ConOps) predisposed the motor sizing, torque, and speed requirements. The calculations based on the ConOps resulted in choosing a design for all three actuators that integrate a ThinGap motor, harmonic drive, SKF angular contact bearings and hall effect sensors.

Each actuator was tested under four separate test profiles: ambient motor characterization, hot and cold motor characterization, accelerated life test (ALT), and ConOps. The accelerated life tests and successful ConOps have verified the motors’ ability to survive the mission with a relative factor of safety. The first series of all three unique actuators were seamlessly characterized and completed their respective ALT. However, only the bucket drum actuator performed a complete ConOps within the first series without the need to restart the test. The first mobility actuator needed to be decoupled from the test chamber and reinstalled several times before the ConOps completed, while the first shoulder actuator was unable to complete a ConOps. The first series of testing resulted in various failure modes and minor design alterations of each actuator. These failure modes and design alterations are highlighted within this paper.