An improved alignment mechanism and mating procedure have been devised for a robotic drilling system in which there is a need to assemble drill stem rods for sampling soils and rocks on a distant planet or asteroid. This mechanism is applicable to systems requiring positive axial alignment between segments. Similar mechanisms could be used on Earth, not only for assembling long drills but also for any system where a series of rods must be robotically assembled, such as in truss construction.

Figure 1. Conventional Mating Schemes incorporating polygonal configuration are susceptible to jamming.

Conventional robotically actuated drill-stem segmenting systems use alignment keys that often feature polygonal protrusions. Figure 1 shows a conceptual example. These mechanisms, while ensuring correct rotational alignment, are susceptible to jamming in a misaligned position. To prevent such jamming, complex control systems must be implemented that can provide fairly accurate alignment between the stem segments prior to any mechanical engagement. Additional control must also be implemented to deal with the possibility that jamming occurs.

Figure 2. An Improved Alignment Mechanism does not require rotational alignment prior to engagement.

The improved alignment mechanism and mating procedure preclude any chance of jamming during assembly. The mechanism also does not require rotational alignment prior to engagement. The interface is a “half dog clutch,” a mating scheme where two half cylinders are mated together (see Figure 2). However, the alignment keys are cut so as to produce a helical face. These keys are used for rotational alignment as well as torque transfer between segments. In the current implementation, an additional pilot is used to ensure axial alignment, but this feature may not always be necessary. The following steps are used to mate the segments:

  1. The segments to be aligned are moved toward each other until contact between them is sensed. It may be beneficial to slowly rotate one of the rods in the direction opposite that of a flat cut that is part of the alignment key to help ensure that the helical faces of the dogs contact first.
  2. The rods are moved apart to a distance that is fraction of the height of the alignment key.
  3. The rods are re-engaged by simultaneous rotation and translation in such a way that the tip of the alignment key travels parallel to the helical alignment-key surface. This motion ensures that next contact will be between driving faces, at which time positive axial alignment is achieved.
  4. The axial-rotational movement can continue until full engagement of the segments is achieved and detected, assuming that one segment is held freely so that the other may drive it.

Note that for this entire process only two simple sensory inputs are required. At no point must the control system know the rotational position of either segment.

This work was done by Benjamin Dolgin and Stephen Askins of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Mechanics category. NPO-21164


This Brief includes a Technical Support Package (TSP).
Improved Alignment Mechanism for Robotic Drilling

(reference NPO-21164) is currently available for download from the TSP library.

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