Robot Excavator for Off-World Deployment
Details
Retrofit or OEM:
OEM
Industries:
Aerospace & Defense
Solution:
Non-captive lead screw assembly provided smooth vertical motion for the excavator.
Worcester Polytechnic Institute (WPI) is a university that provides an education built around science, engineering, and the humanities to create solutions to real-world problems. Recently, an interdisciplinary team of engineers at WPI were participants in the 2022 Lunabotics Competition sponsored by NASA. This competition looks to educate college students in the NASA Systems Engineering process, while also allowing NASA an opportunity to evaluate designs and operational data that can be gleaned for future robotic excavators and builders.
The Lunabotics competition is part of the ambitious NASA Artemis Program. Its initial mission seeks to land a diverse group of persons on the Moon and use innovative technologies to explore the lunar surface more extensively. From that celestial stepping stone, the program will then use any lessons learned on and around the Moon to propel the first astronauts to Mars.
Competition Goals and Parameters
This year’s project goal for the competition was to utilize the NASA process to design and build a semi-autonomous lunar mining robot. Each participating team was tasked with designing a robot to serve three main functions:
- Autonomous navigation through rough terrain
- Mine icy regolith simulant
- Deposit regolith into a collection sieve
Regolith is a region of loose unconsolidated rock and dust that sits atop a layer of bedrock. For this type of rigorous application, the engineers would need reliable hardware that would perform flawlessly in off-world conditions.
Design and Build with Lead Screw Assembly
The WPI excavator subassembly is made up of two pulleys on either end, connected by a belt, support rails, and a linear drive. Grousers are riveted onto a belt, so as the excavator is lowered into the ground, the belt moves and the grousers scoop the gravel. As the excavator moves into mining position, it rotates 55-degrees from its stored configuration to the mining position. Once it is in place for mining, the excavator can be translated by the lead screw to a total depth of 39 cm. Once the WPI team submitted their specifications through our website, PBC Linear was able to match and deliver the components with the exact requirements, allowing for smooth integration.
For the vertical motion of the excavator, the team considered a rack and pinion system. After considering the need for simplicity, compact structure, and dust protection, they decided on a lead screw with non-captive motor and Constant Force nut from PBC Linear.
The lead screw system was the most critical element of the subassembly. This is what allows the excavator to move in and out of the ground to mine the regolith and gravel. The lead screw had to be cut precisely to the WPI team’s specifications, which included a visible length of 77 cm and a width of 2 cm. In addition, M6-threaded journals on either end of the screw provided limits to the screw’s translational motion within the assembly.
Our automated straightening process minimizes runout which can cause vibration, noise, and premature wear (We offer the highest straightness tolerances available for lead screws). In addition, PBC Liner uses a proprietary PTFE coating process that reduces friction and increases efficiency.
Lead screw motor mount system
Lead screw motor mounting plate
Non-captive lead screw actuators
Anti-backlash nut with Constant Force Technology
The non-captive stepper motor and lead screw were selected to perform the linear translation of the excavator in and out of the ground. The motor is held in place with brackets that are fixed to the robot’s base. As the nut rotates, the screw is linearly driven through the stepper motor, vertically adjusting the excavator pulley system.
PBC Linear offers a patented anti-backlash nut design, referred to as Constant Force Technology (CFT). In addition to being self-lubricating, the nut is equipped with a constant force spring that ensures consistent pre-load and reduced backlash (2 to 4 times the performance vs conventional designs). This proved to be very important for keeping the screw in place during mining activities.
Successful Application
The most important part of testing the excavator was to analyze the loading on the lead screw. The engineers needed to confirm that the force of digging did not deform or deflect the lead screw over multiple applications. Testing this limit also allowed the team to determine the effectiveness of the nuts as a stand-in for a keyed lead screw. Digging tests were done in both a dry-sand and wet-sand environment.
Overall, the lead screw, motor, and anti-backlash nuts performed well, providing the excavator and its nine-member team with a consistent and robust linear motion --enough to win a significant improvement award from the NASA Artemis Challenge.
During testing, the lead screw and motor did experience some binding located within the translational extremes at either end of the lead screw. This was a direct result of the increased moment loads generated by the extra weight of the excavator. This was easliy mitigated by drilling out and enlarging the lead screw motor mounting holes, providing the lead screw with more room to self-correct.
Discover your ideal linear motion solution:
Find more Lead Screw-related content: