Modular exploratory robotic vehicles that will be able to reconfigure themselves in the field are undergoing development. These vehicles at the initial concept stage were described in “Reconfigurable Exploratory Robotic Vehicles” (NPO-20944), NASA Tech Briefs, Vol. 25, No. 7 (July 2001), page 56. Proposed for use in exploration of the surfaces of Mars and other remote planets, these vehicles and others of similar design could also be useful for exploring hostile terrain on Earth.

Figure 1. This Two-Wheeled Vehicle is a prototype of an Axel2.
To recapitulate from the cited prior article: the modular vehicles are denoted generally by the term Axeln, where n is an even number equal to the number of main wheels. The simplest vehicle of this type is Axel2 — a two-main- wheel module that superficially resembles the rear axle plus rear wheels of an automobile (see Figure 1). In addition to the two main wheels, an Axel2 includes a passive caster wheel attached to the axle by an actuated caster link. The motion of the caster link can be used to control the rotation of the axle in order to tilt, to the desired angle, any sensors mounted on the axle. In addition to the sensors, the axle of an Axel2 houses computer modules and three motors and associated mechanisms for driving the main wheels and the caster link. An Axel2 is powered by rechargeable batteries located inside the wheel hubs.

Figure 2. The Prototype Axel2 and a Prototype Instrument Module are depicted here during a docking maneuver that ends in coupling at the module interface.
One constructs an Axeln (n > 2) as an assembly of multiple Axel2s plus one or more instrument module(s) connected to each other at module interfaces (see Figure 2). The module interfaces contain standardized electrical and mechanical connections, including spring-loaded universal joints that afford some compliance to enable the modules to rotate, relative to each other, to adapt to terrain. Data are communicated between modules via fast serial links in the module interfaces.

An Axeln amounts to a train carrying n/2 – 1 instrument modules. The instrument modules contain additional computational units that, in addition to processing of instrument readings, contribute to coordination of motion. In other words, the “intelligence” of an Axeln, and thus the sophistication of the maneuvers that it can perform, increase with n. The symmetrical design of the modules enables them to operate in any stable orientation, including upsidedown; this feature contributes to robustness of operation in rough terrain. A fully developed Axeln would be able to diagnose itself to detect nonfunctional modules.

Going beyond the de scription in the cited prior article, the following additional major items of the hardware can now be reported.

Also contained within the axle of an Axel2 is a stereoscopic pair of electronic cameras to be used for navigation across terrain, for scientific observations, and for guidance in docking maneuvers.

Each module interface is an electromechanical module located at the mid-length of the axle of an Axel2. The module interface carries female parts of mating mechanisms, while instrument modules carry the male parts. The mating mechanisms include conical mating surfaces that correct for small initial misalignments to facilitate autonomous coupling of an Axel2 with an instrument module.

Information on the Axeln software has become available since the prior article was published. To enable self-diagnosis and automatic reconfiguration of modular hardware, the architecture of the Axeln software provides for autonomous adaptation of the software to the hardware reconfiguration. More specifically, an Axeln uses software that can determine when physical reconfiguration is necessary (e.g., in response to task requirements or hardware failures), controls the hardware reconfiguration, and reconfigures itself to conform to the changed hardware configuration.

The capability for autonomous reconfiguration of the hardware depends heavily on the supporting software. One of the goals of the development of the Axeln system is to simplify and generalize through modularity. The reconfigurable software architecture mirrors the modularity of the hardware by providing that, as hardware modules are connected or disconnected, associated software modules are also put into or taken out of operation.

This work was done by Ayanna Howard, Issa Nesnas, Barry Werger, Daniel Helmick of Caltech; Murray Clark and Raymond Christian of Arkansas Tech; and Raymond Cipra of Purdue University for NASA’s Jet Propulsion Laboratory. For more information, contact This email address is being protected from spambots. You need JavaScript enabled to view it..


This Brief includes a Technical Support Package (TSP).
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(reference NPO-30890) is currently available for download from the TSP library.

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