Inside Machines: Fly Me to the Moon
Tiny motion control technology helps with a prototype lunar mechanical tool changer for a robot that could unload moon landers from a spacecraft and then do various science experiments.
NASA Langley Research Center hired Honeybee Robotics Spacecraft Mechanisms Corporation to develop a mechanical tool changer, for the end of what Langley’s Lunar Surface Manipulation System (LSMS) team describes as a robot that could unload landers. After moon landers are unloaded, the robot could do base assembly, then mate with tools to take science experiments.
The dexterous, autonomous robot incorporates elements of a crane and tool changer for use in multiple applications on Mars or in outer space for a robotic arm with end effectors.
Making it work
The assignment included a footprint to stay within, along with requirements for load ratings and misalignment allowances.
Lee Carlson, a systems engineer who was part of the Honeybee team, described the project: “The crane might be sitting on the lander deck or on the lunar surface and would be driven from quite a distance away from the tool to be mated to; this required designing for large misalignment allowances … this was our first design challenge. The end of the crane and target tool could be misaligned by as much as a couple inches in any direction with up to 20 degrees angular misalignment when attempting a mate.”
Among other considerations, Carlson said, “The tool changer had to be capable of carrying around 1000 lb so (the tool changer) had to be very robust. Also, since this was a lunar project, it has to be tolerant to moon dust. These two design criteria required special seals to protect large roller bearings. If this design was for space, it becomes considerably simpler. All of the loads would be reduced and dust is no longer an issue. But the moon is a very harsh environment, and lunar dust is a major concern when designing for missions there.”
The original assignment called for mechanical tools without power. The crane would do all the work. Tools would include a forklift attachment, a shovel or scoop for acquiring surface samples or digging, or even a bucket for lifting human passengers.
NASA added capability to attach an electronic or electromechanical tool to the end of the crane, adding an electrical connection to the original mechanical link, so the tool changer might accommodate “cameras or tools with cameras on them or even a light jackhammer,” Carlson said. There wasn’t space for an electrical connector because it was not a part of the original contract, and the budget did not allow for starting from scratch.
10 square inches
NASA did not want to redesign the tool changer, which gave Carlson 2.5 x 4 in. of free space to incorporate the male side of the new autonomous connector. The connector has to mate itself to a female connector mounted on the tool, calling for the smallest design Carlson had ever done.
Honeybee designed the male side on the crane and female side, on each tool, which had to be inexpensive and easy to create.
The male connector has all the moving parts. It is cylindrical and populated with eleven 1/16-in. dia. aluminum pins, plated with gold over nickel, and configured in a standard MIL/Spec pattern. The connector rides on compact slides—miniature guides. The top faces of the two glides are facing each other, and Honeybee’s components are in between the two glides, supporting this connector and reducing the moment loads on the slides.
Six slides were used within the space, three on each side. Carlson said, “The slides ride on each other in the manner of drawer slides that are stacked to extend the distance they can open a drawer. Our configuration achieves an extension of the movement equal, approximately, to the length of three slides. So instead of a half-inch stroke, we could get an inch and a half stroke—within a very, very small footprint. Low mass, low load, and very low profile were all required for this application.”
Carlson said that the guides were some of the smallest slides he could find, made of stainless steel, and were from a supplier he had worked with previously. To increase lunar dust tolerance, the electrical connector assembly will be sealed in a bellows to protect it from the harsh lunar environment. The guides chosen have a standard radial clearance that is twice as accurate as other standard miniature guides.
Had Honeybee taken a different design route, there might have been a deformation of the guide block to consider. Mounting the connector on one rail on an arm that extended to the side would have caused block deformation, reducing accuracy. The version used is stiffer with optimized machining on the guide’s top-mounting surface that attaches to the table. This withstands the extra moment load that could have caused some clearance due to deformation.
Waiting for the moon
With moon exploration on hold, Honeybee is waiting to complete LSMS assembly and testing.
Miniature guide technology notes
The connector rides on compact slides—miniature guides made by NB Corporation, offering the widest selection of miniature linear slides. They are called SEBS.
Most manufacturers do not claim that their preload eliminates all clearance. Their standards are plus to minus, which allows gaps, or clearance, to exist. Minus means there is some preload so there’s no gap. NB designs are from zero to minus as a standard; removing clearance adds accuracy. A negative clearance means the ball is larger than the space, adding more pressure and greater rigidity. This increased rigidity is desirable in high-precision applications. NB fabrication requires more control in the assembly and manufacturing process.
There are instances where no preload is desired, where one might want to remove all friction and trade off accuracy and rigidity for minimal friction, but not for this application.
NB SEB-AD version is stiffer with optimized machining on the top-mounting surface of the guide block that attaches to the table, which withstands the extra moment load that could have caused some clearance due to deformation.
For the smallest applications with lesser loads, an extra compact block, SEBS-BS (size 2), is shorter than the standard length block and has two holes instead of four. Either retained-ball (whose elements allow for easier handling since the guide block may be removed from rail without ball loss) or low-cost non-retained-ball lines are available.
SER is a miniature guide with crossed-roller bearings (with more contact areas than ball bearings) providing the greatest rigidity. SER comes in stainless steel, with nonretained rollers, and in the same block sizes and configurations as the SEBS ball bearing miniature guides.
In recent tests executed by Newmark Systems Inc. of Rancho Santa Margarita, Calif., due to their friction-free travel, NB’s miniature guides have proven to not wear after 1.5 years of constant travel.
Honeybee developed harsh-environment, mission-critical end-effectors for more than 25 years, including equipment for other space missions.
– Larry Hansen is general sales manager, NB Corporation of America. Edited by Mark T. Hoske, CFE Media, Control Engineering, www.controleng.com.