Connectors Enable Precise Movement of Lunar Robotic Tool Changer
- Saturday, 01 December 2012
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 Corporation — 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.
Carlson described how it works: “We actually used a total of 6 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 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 the reason they chose these particular guides was that they were some of the smallest slides he could find. His one caveat was that he wanted to work with one of the slide suppliers that Honeybee had worked with before, and not 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 without any plastics. 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.
Honeybee was able to choose from the widest selection of miniature linear slide guides on the market. 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 manufacturers do not claim that their preload eliminates all clearance. Their standards are plus to minus, which allows gaps, i.e. clearance, to exist. Minus means there is some preload so there’s no gap. NB’s are from zero to minus as a standard, making for greater accuracy because 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.
There can be instances where no preload 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. But the space mission was not such a case.
This article was contributed by NB Corporation, Hanover Park, IL. For more information, visit http://info.hotims.com/40440-321.