PHILIP : Powering Rovers by High Intensity Laser Induction on Planets by using High Intensity Laser Power Beaming System

Funding organization: European Space Agency (ESA)

Coordinator: Leonardo S.p.A, Italy

Partner: INOE 2000, Romania

Project manager for partner: Dr. Mariana Braic

Team:

Dr. Viorel Braic - Key person,

Dr. Iulian Pana.

ABSTRACT

The scenarios that have been identified anticipate operations on the south polar surface of the Moon, including areas where solar energy collection presents challenges, such as Permanently Shaded Regions (PSR).

Investigating PSR is the goal. Traditional powering systems require the rover to periodically exit the PSR for batteries replenishment. Using radioactive materials as a power source is an alternative strategy. One example of this is the plutonium Multi Mission Radioisotope Thermoelectric Generator installed on the NASA/JPL MSL mission's Curiosity Rover.

In addition to the aforementioned, large batteries would be required for the rover to travel deep within the PSR, which would make designing the vehicle extremely challenging in terms of mass and volume.

In PHILIP project a novel strategy is investigated: using a high power laser beam that is directed out of the lander and into the rover's line of sight, the rover can be powered continuously, allowing it to enter the PSR without interruption and without the need for radioactive materials.

Designing an optical system with the goal of expanding the laser beam out of the fiber connector to a defined diameter and orienting it toward the rover using steering mechanisms is the task assigned to INOE 2000.

The laser beam diameter must be designed to ensure the minimum divergence, allowing the photovoltaic panel to collect the same amount of power density on the photovoltaic cells along the rover range excursion, in order to illuminate the rover within 4 km of the target distance or 15 km.

RESULTS

Initial plan: After the first beam expander (Bex), the movement of the plane mirror revealed a significant shift in the beam position at large distances, which was deemed unacceptable when taking into account a 40 mm laser beam (wavelength 1070 nm) diameter. In this model, a PV panel with a lateral dimension of 0.54 meters was taken into account. Additionally, since the BEx#2 and the steering mirror should only have one moving platform, the mass of the selected mirror was deemed prohibitive.

Our calculations indicated that for a maximum laser beam deflection of 0.0040, the beam will stay on the panel, taking into account the potential angular movement of the mirror and the requirement for the image to remain on the PV panel. This angular value, however, was deemed unacceptable since it would place severe restrictions on the steering mechanism's ability to move accurately.

A second design with a mix of parabolic mirrors was released. The parabolic mirrors M1 and M2 were the ideal arrangement. The selected reflected focal length (F1) for M1 was also calculated issuing a feasible value from the point of view of manufacturing constraints.

The M1 mirror's diameter was selected due to its excellent fit with the incident collimated beam diameter. Additionally, a small F1 value makes it possible to obtain a compact setup system in terms of mass and size by allowing for a short distance to the second mirror. To enable the optical setup to have a compact structure, an extra plane mirror was added. We made use of mirrors, which reputable manufacturers can provide.

Several image diameters (ID) at 5, 10, and 15 km, were calculated using the design with two parabolic mirrors. Several input beam diameters supplied by a collimated laser source were taken into consideration. The variation of the PV panel's image diameter (ID) at 4 and 15 km, as well as the necessary magnification, were calculated to determine the optimal value of F2.

The calculated image diameter (ID) at 4, 10, and 15 km, as well as the irradiance, are modeled in the figure below, taking into account the two parabolic mirrors with optimized focal points (Fi) and a certain collimated laser beam diameter. The Gaussian beam profile at 15 km distance from the laser is also shown.

4 km	10 km	15 km
ID = X mm	ID = X+7 mm	ID = X+17 mm

The optical setup was viewed as a single module to maintain the components' constant positions. The laser beam collimator, which is attached to the laser exit via an optic fiber (FO), is housed on the base plate of the optical system. A plane mirror deviates the beam that emerges from the collimator at a 90-degree angle. In order to connect to their holders, the two parabolic mirrors should be fitted with appropriate tapered holes and machined to the lowest mass permitted.

Conceptual optical arrangement taking the needs for steering into account: There were two distinct steering requirements: coarse mode and fine mode. In the coarse mode, a gimbal holder rotates the optics base in two planes. After accounting for the scanning process that keeps the laser beam in contact with the PV panel, we have determined that few tens of nm resolution of a piezo actuator causes a shift in position of less than 10^-7 radians. Given that a position change of about 10^-5 rad is required to move the laser beam along half of the PV panel, resulting that the piezo-actuator's resolution is found to be adequate. Additionally, the piezo actuators could be commercially available vacuum-compatible piezo-motors.

To track the rover's movement in the fine mode without losing contact with the PV panel, the beam needs to have an appropriate moving resolution. A solution with the base plate positioned on a platform resembling the base plate emerged, taking into account the rover's speed and distances as well as the requirement for beam steering in two planes. Although the solution is comparable to the currently available tilt and rotate optical holders sold in stores, it needs to be modified to account for the optical setup's weight and dimensions. A setup for precise positioning was also defined.