Lunar pit exploration is limited by the capacity of the rover’s battery. Without access to solar energy or other means of recharging, a typical planetary rover might operate for tens of hours or less on a single battery charge. This single-charge energy capacity is inadequate to accomplish even modest exploration and science goals. The purpose of this project is to address the power issue by designing and demonstrating a robotic recharging docking system that could be used in a lunar skylight. Primary deliverables include a prototype robotic docking mechanism, a proof of concept demonstration, and a confident recommendation as to which power transfer method (conduction or induction) should be implemented in a lunar skylight flight mission based on research and experimentation.
Deliverables and Demonstration
At the conclusion of this project, a simulated autonomy test will demonstrate the efficacy of the system designed to charge a robot from a hanging tether. This tether originates at the lunar lander where electric power is received by solar arrays. For demonstration purposes, the existing robot, Cave Crawler, will employ the prototype system to locate and dock with a hanging tether from a distance of approximately 5 meters. An autonomous approach will be simulated by directing Cave Crawler along a wall-following straight path and exploiting the robot's obstacle avoidance software to funnel its trajectory towards the goal using strategically placed obstacles. An array of varying angles of approach (yaw) and axial rotations (pitch and roll) will test the system's reliability under non-ideal circumstances. The metric for effective design is the proportion of successes to failures of electric power transfer.
In the video below, Cave Crawler autonomously navigates its way between two closely spaced obstacles. Note the hanging crane hook in the foreground, which occupies the approximate position of a hanging recharge tether for robotic docking.
Design of Tether
The functional requirements for our tether recharging dock include: low mass, low part count, dust tolerance, rotational symmetry, precision required, grade tolerance, rough terrain tolerance, total envelope size, predicted reliability, and hard dock. With these requirements five basic designs were generated and evaluated using a design matrix. All of the designs (except Design 4) accommodate both conductive and inductive power transfer. While only one of these two methods would be used in a mission, both are being considered, since each may have potentially unique advantages, such as efficiency (conductive) and precision/dust tolerance (inductive).
Design 1: V-shaped capture device on rover that straddles the tether, which is then raised and passively aligns itself into the docking/charging configuration.
Design 2: Cone-shaped compliant connector on the tether that drops into a funnel-shaped receptor on the rover to establish a connection.
Design 3: Similar to Design 2, a cone-and-funnel compliant connector that mates horizontally.
Design 4: Charging pad that lies on the lunar floor onto which the rover drives and charges via induction only.
Design 5: Charging platform that the rover drives onto with guide rails which accept the rover’s electrical connectors.
After preliminary engineering design analysis, Design 1 was chosen. Its fundamental advantages lie in its potential for low mass, low part count (especially few moving parts), dust and precision tolerance, and ability to accommodate both inductive and conductive charging. The animation below depicts how this design concept operates.
A complete analysis of a lunar recharging system must consider the effect of lunar regolith. Regolith is the layer of loose material covering the surface of the Moon. The composition of its constituent substances varies across the Moon, but it contains fine ferromagnetic and paramagnetic particles throughout. Since this project is investigating electromagnetic charging methods, the magnetic susceptibility of the regolith must be investigated to assess its potential impact on charging devices. Furthermore, a lunar simulant with comparable electromagnetic properties to that of true lunar regolith must be identified so that project systems can be developed, tested, and demonstrated to tolerate the pervasive lunar dust.
This project’s prototype will use a commercially available inductive cell phone charger to test the feasibility of inductive rover recharging. A Duracell Powermat, which transfers power wirelessly to a Galaxy S III cell phone case was purchased for this purpose. As an initial experiment, the charger’s power output was characterized by measuring the voltage and amperage transferred to the cell phone case connected to a load simulating a recharging battery. The input and output power were calculated and used to estimate the efficiency of the induction pad. This process was repeated with four different conditions: zeros gap (as intended by the manufacturer), a plastic baggie separating the two halves of the charger, 2 millimeters of lunar simulant (BP-1) inside a plastic baggie between the two halves, and 2.5 millimeters of air between the two halves. The results are shown in the figure below.
The thin plastic baggie appears to make little or no difference in the induction charger’s efficiency. The gap caused by the lunar simulant, however, resulted in a noticeable decrease in efficiency. Compared to the air gap, it seems that the distance of separation accounts for most but not all of the efficiency loss; i.e. the presence of the simulant does have a negative effect.
Current work is focused on detailing the design of the passive docking mechanism, which will lead to fabrication and assembly of the parts. Initial tests will then begin to demonstrate that an electrical connection can been made between the rover and charger. The effects of regolith will then be tested by applying an appropriate lunar simulant on the charger for both inductive and conductive charging methods.