Fly Me to the Moon

As Langley’s Lunar Surface Manipulation System (LSMS) team describes the project, it is a tool changer that is for the end of a robot that could unload landers. Then, after the landers were unloaded, it could mate with tools to take science experiments as well as doing base assembly. This robot could be likened to a crane, but with more dexterity.

The crane was designed to work remotely for use on unmanned missions. It is completely autonomous, which is one of the driving factors, as well as the fact that the tool changer has other applications. It is capable of use on Mars or in outer space as a robotic arm.

Since Honeybee Robotics Spacecraft Mechanisms Corp. has been developing harsh-environment, mission-critical end-effectors for more than 25 years – having worked on the equipment for other space missions – they were well-suited to tackle this challenge.


Making it Work
Basically, Honeybee was given an envelope – a footprint to stay within – and all the requirements for load ratings and misalignment allowances.

“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 and 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° angular misalignment when attempting a mate,” says Lee Carlson, systems engineer, Honeybee.

“The tool changer had to be capable of carrying approximately 1,000 lb, so the tool changer had to be very robust while being tolerant to moon dust,” Carlson explains. “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 would no longer an issue. But, the moon is a very harsh environment and lunar dust is a major concern when designing for missions there.”


Changes
The original assignment called for dummy tools requiring no power, allowing the crane to do all the work. Tools would range from a fork lift attachment to a shovel or a scoop for acquiring surface samples or digging, or the tool could even be a bucket for lifting human passengers.

NASA then decided it wanted the capability of attaching an electronic or electromechanical tool to the end of the crane that would allow the tool changer to provide an electrical connection. However, they had not left space to accommodate an electrical connector because it was not a part of the original contract, and the budget did not allow Honeybee to start from scratch.

Carlson had to work within the constraints of the current design since NASA did not want to redesign the entire tool changer. They wanted to add an electrical connector to it without increasing the current envelope. Carlson only had 2-1/2" x 4" 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.

“We make small stuff all the time and if there was more space, there are many different ways that I could have designed it,” Carlson says.

Honeybee designed both the male and female sides of the connector. The female side had to be inexpensive, and easy to create, since each tool would need to have its own female connector versus a single male connector attached to the crane. The female connector has no moving parts but is slightly compliant.

The male connector has all the moving parts. It is cylindrical and populated with 11-1/16" diameter aluminum pins, that are plated with gold over nickel configured in a standard MIL/Spec pattern. The connector rides on compact slides – miniature guides made by NB Corp. – called SEBS. The top faces of the two glides are facing each other and Honeybee’s components are in between the two glides, supporting this connector. This configuration reduces the moment loads on the slides.


Precise Movements
“We actually used a total of six slides within the space, three on each side. 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 approximately equal to the length of three slides. Instead of a 1/2" stroke, we could get a 1-1/2" stroke – within a very small footprint. Low mass, low load, and very low profile were all required for this application,” Carlson explains.

Carlson says that the reason that they chose these particular guides was that they were some of the smallest slides that he could find. His one caveat was that he wanted to work with one of the slide suppliers that Honeybee had worked with before – not wanting to take chances on a new supplier. It also had to be a guide that – even though this was a prototype – was completely made of stainless steel. Plastics are generally avoided unless they are specially chosen and approved. As for lunar dust tolerance, the whole electrical connector assemblage will be sealed in a bellows to protect it from the harsh lunar regolith.

The standard SEBS guides major advantage is that they have a standard radial clearance that is twice as accurate as other standard miniature guides. Most manufactures do not claim that their pre-load eliminates all clearance. Their standards are plus to minus – meaning there is some preload so there is no gap – which allows gaps or clearance to exist. NB’s guides are from zero to minus as a standard, making for greater accuracy since there is no clearance. In other words, 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’s standard fabrication requires more control in the assembly and manufacturing process in order to adhere to this higher quality standard.

One Rail Solution?
Of course there can be instances where no pre-load is desired, where one might want to remove all friction and trade off accuracy and rigidity for minimal friction. In such a case, one might want clearance. The space mission was not such a case.

Had a different design route been taken by Honeybee, there might have been a deformation of the guide block to consider. For instance, had the connector been mounted on only one rail on an arm that extended to the side, this would have caused deformation of the block, reducing accuracy. The solution to this problem is called the SEB-AD. The AD version is stiffer because NB optimizes the machining of the top-mounting surface of the guide block that attaches to the table. This withstands the extra moment load that could have caused some clearance due to deformation.

For the smallest applications, there is an extra compact block for lesser loads – SEBS-BS (size 2), which is shorter than the standard length block, having two holes instead of four. Either retained-ball (whose elements allow for easier handling since the guide block may be removed from the rail without ball loss) or low-cost non-retained-ball lines are available.

The miniature guide that provides the greatest rigidity is the SER, which uses crossed-roller bearings. These offer greater contact areas than ball bearings, which,  in turn, delivers the required rigidity. SER comes in all stainless steel, it has non-retained rollers, and it is available 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, CA, due to their friction-free travel, NB’s miniature guides were proven to not wear after one and a half years of constant travel.
 

Patience
As Honeybee waits to complete assembly and testing with the LSMS, they look forward to the future when, hopefully, the project will make its way to the moon – the final test of all.


NB Corp. of America
Hanover Park, IL
nbcorporation.com


Newmark Systems Inc.
Rancho Santa Margarita, CA
newmarksystems.com


Honeybee Robotics Spacecraft Mechanisms Corp.
Pasadena, CA
honeybeerobotics.com