A Study of Experimental Facility for Lunar Rover Development

Transcription

1 A Study of Experimental Facility for Lunar Rover Development S. C. Fan, Y. Q. Feng, Q. J. Zheng Beijing Institute of Spacecraft Environment Engineering, Beijing, China, Abstract. The lunar exploration project of China is underway, and a lunar rover will be developed and launched onto the lunar surface in the next step. In order to verify the reliability and diverse functions (such as landing impact, autonomous navigation and tele-operation) of the rover, an experimental facility for the lunar rover development is needed. Based on the knowledge of the lunar surface environment and the existing simulation technology, this paper presents an idea for realization of an integrated experimental facility for the lunar rover development. It is well known that the lunar environment is very tough and the moon geological feature is complicated too. A kind of knockdown simulator for lunar surface environment is suggested. The main part will include various mobile units, each of which simulate a kind of lunar surface topography in different dimension, such as craters, hills, and ravines. One unit is one grid, which is a sand table in small size, but the sand is a sort of man-made lunar soil simulant, which has similar chemical composition, particle size distribution, and engineering properties to the true lunar sample. According to the request of the one kind of test, corresponding units or grids are selected and assembled to be a test bed, which dynamically achieves the simulation of different terrain on the lunar surface in a limited area. At the same time, a relative fewer units can be chosen to assemble a smaller test bed, which can cooperate with other existing environmental simulators (for example, vacuum container, dropping tower) to fulfill an integrated environmental simulator. INTRODUCTION A number of new explorations to the moon have been announced by several countries, most of which plan to develop unmanned mobile vehicles for the scientific survey. Especially, NASA/JPL have succeeded in Mars Pathfinder mission, two of their rovers have moved on the Martine surface and gathered and transmitted voluminous ata back to the Earth. Many researches have focused on the study of lunar rovers, which can be sent to the moon after lunar orbit surveys the surface topography and resource distribution by remote sensing. However, there are many engineering and technological difficulties in the development of lunar rovers. Rover field tests have been performed for evaluating its performance. N.Yoshiioka described a preliminary testing of a lunar rover by using an indoor field, which has a simulating lighting condition and surface feature (N. Yoshioka, 1997); Takashi Kubotaa has proceeded a series of experiments to verify the new mobility system when they developed their small, light weight Micro5 (Takashi Kubotaa, 2003); T. Adachi represented their experimental facility for lunar development (T. Adachi, 1995). The vehicles used on the lunar surface in the final three Apollo missions(lrv), extended the range of exploration and carried two astronauts over the lunar terrain, but had a limited capacity to withstand elements of the lunar environments. Future lunar rovers will be required to be used repetitively over a prolonged period of time. Therefore, more relative environmental experiments should be carried out to verify and improve the design of the rover. This paper proposes an idea of experimental facility for lunar rover development. At first, a simple review about the tough lunar environment is illustrated, and main factors are focused on the lunar terrain and lunar dust. And then an experiment facility to simulate the lunar terrain and lunar dust is described, and its main part will be a kind of knock-down equipments. The simulating methods for the two environments are discussed. International Lunar Conference

2 LUNAR ENVIRONMENT The lunar environment presents many technical challenges for the design of rovers. Table 1 summarizes the main characteristics of the lunar environment that will influence these designs. Of these characteristics, lunar terrain and lunar dust presented the greatest challenges, which have never been met in the design of the other orbital explorers before. Two fundamental large-scale morphologic types of lunar terrain are clearly evident Mare and Upland regions. Well formed young craters are superimposed on both of these surface types and constitute a widely distributed third basic surface. The Mare surface is characterized by relatively gentle topography with low normal albedo and features, such as craters, ridges, rills, domes, ray systems, and scarps. In Mare region, maximum slope is about 17 o, and mean slope is 0~10 o. In contrast, the Uplands have higher albedo, and are rugged with complex superimposed craters. Its maximum slope is almost 34 o, and mean slope is about 4.5 ~ 6 o. In spite of various slopes, distribution of the blocks and the rocks is complicated too, it is therefore necessary for the rover to have an autonomous navigation function. Secondarily, the lunar dusts once are disturbed and lofted; they travel everywhere, and easily adhere to the surface of the photovoltaic array, optical lens and ribs of the radiator. The accumulating dust will do great harm to life and efficiency of these components. TABLE 1. Lunar environment attributes critical to lunar rover design (Niranjian S. Rao, 1993) Environment Radiation High Vacuum Optical characteristics Magnetic field Gravity field Micro meteoroids Terrain Lunar soil Physical properties and Lunar dust Thermal Moon quake Effects Single particle event, accumulating effect Outgas; no damping vibration; Select proper optical instrument; Almost no effect; Low gravity; A small percentage of impact are dangerous Rover must be designed to be able to climb slops, and avoid obstacles Probably the most adverse factor for rovers: surface accumulating; electrostatic effect; mechanic component wear or failure; Worst case -150~+150oc, Thermal cycling can cause fatigue and over stress Secondary effect, rock cascade from the wall of the crater METHOD AND FACILITY FOR THE SIMULATION OF THE LUNAR SURFACE ENVIRONMENT Due to the limitation of the reality, it is very difficult to simulate the full environment elements on the ground, but some key factors should be integrated according to the requirement of the test. Lunar Surface Terrain Simulation A kind of knock-down units to simulate the lunar surface terrain is suggested. The main part is a series of mobile units, each of which is designed to simulate a sort of lunar terrain, such as craters, ridges, domes, slopes and scarps. International Lunar Conference

3 In fact, each unit is a kind of sand table, but the sand is the lunar soil stimulant, which has the similar chemic composition and engineering properties to some kind of lunar soil sample. Each mobile unit is modularized, and can be assembled through their interfaces. The mobile means that the units are not fixed on the ground and their supporting mechanism are adjustable, so they can be transferred in horizontal and vertical directions by the tracks and rails. When the mobile units are not used, they can be stored vertically, and when they are in a functional mode, they can be selected and assembled at suitable positions to simulate a specified region on the lunar surface. The terrain characteristic of one unit is determined by the corresponding matrix unit. One kind of matrix unit represents one specific topographic feature. Therefore a matrix library should be developed in advance, which includes the representative features, such as craters, ridges, domes, slopes, domes, rugged land and obstacles. Before a test, the required mobile units are picked and the lunar stimulant is laid into the unit bed to a given thickness, then the matrix units are picked to mold the mobile unit. The matrix unit is overlaid on the mobile unit and pushed down by appropriate force, at the same time, the two units are vibrated synchronously to adjust the soil porosity ratio in the mobile unit, which controls the engineering properties of the soil, such as bulk density, bearing capacity. By this method, the same soil initial condition and topographic feature of the different tests are guaranteed in a high precision, which is repeatable and comparable, when two different rovers or the same rover with different parameters, are compared through a series of contrastive tests. Testing Room Transfer Rail Vertical Track Horizontal Rail Preparing Shop Measuring and Control Room Matrix Unit Mobile Unit FIGURE 1. Sketch of knock-down units to simulate the terrain of the lunar surface International Lunar Conference

4 As depicted in the figure 1., the experimental facility should include a preparing shop, a test room and a measuring & control room. Assembly of the rover and preparation of the mobile units are fulfilled in the preparing room, and prepared mobile units are fixed at a proper situation to simulate an area of the lunar terrain in the test room. A rail is installed to transfer the mobile units from the preparing room to the test room. Testing data from the sensors in the testing room are transferred to the measuring & control room for analysis, where the rover and the mobile units can be controlled remotely in the measuring & control room. The experimental facility is purposed to simulate the terrain of the involuted lunar surface, which can be used to study on the performance of the dynamics of the rover and the effect of the soil on the wheel of the rover. The rover can advance forward and backward, it also can turn, climb slopes, cross over ravines, and sample in the area, even autonomously cruise, with obstacles identification and avoiding. The terrain feature parameters are controllable in some high precision, which is useful for the parameter optimization for the structure and mechanism of the rover, such as the span and width of the wheels. Lunar Dust Environment Simulation The lunar surface is covered with a blanket of fine particles, along with occasional random rocks. There are several natural and anthropogenic mechanisms for the suspension of the lunar dust. The natural mechanisms include meteoroid impacts and electrostatic transport phenomena, which are geographically indiscriminate and fall outside human control. However, it is anticipated that the amount of dust suspended and transported by various human activities will be significantly greater than that disturbed by natural processes in a local area, where the spacecraft will land, the rover will cruise, or if the astronaut will walk (Charles L. Johnson, 1991). Under consideration of the unmanned landing and the non- frequently spacecraft landing, the main elements to be integrated for the lunar dust simulation will include high vacuum and low gravity, which are the key factors for the suspending of the lunar dust. Because the dust particles once are disturbed, they will travel under the influence of a low gravity (one sixth that of Earth), and in flight, particles will experience no atmospheric friction in a high vacuum environment. If the two factors are simulated in a container successfully, the rover wheel will be rotated as the disturbing source of the dust to investigate the suspending and depositing of dust on the component surface. Sedimentary cover Sensor Vertical Sedimentary cover level Sedimentary cover Sensor CCD camera Xenon Lamp Horizontal Sedimentary cover level Wheel Windows Mobile Unit Lunar soil stimulant Rotating Table Supporting Mechanism FIGURE 2. Sketch of the lunar dust simulator on the ground International Lunar Conference

5 The high vacuum environment is not difficult to be implemented, as figure 2 shown. The vacuum container will be designed according to the dimensions of the rover. However, the low gravity is a big problem. It is one direct method that the electric field is utilized to counteract part of the gravity of the dust particles. But the polarities of the particles are not uniform, and the electric quantities of the particle are unpredictable. Using magnetic field is another way. But the new problems will be induced, such as, how to charge magnetism to the particles, and if there are other side effects of the magnetic particles. It is shown that lunar dust simulation will only integrate the high vacuum and the lunar soil, without the low gravity at present. The experimental results from the equipment of the simulator of figure 2 are not reliable enough to be used directly, but extrapolation can be employed by the laws of similarity to get reasonable results including the low gravity. CONCLUSION A kind of knockdown simulator for lunar surface environment is proposed. It is shown that it will be functional in lunar terrain simulation, but in the simulation of the lunar dust environment, the low gravity is still an open question. In the future, a lot of research work should be done in order to investigate the feasibility of the proposed facility and methods. REFERENCES N. Yoshioka, and Y. Wakabayashi, Driving technology and preliminary testing of a lunar rover, Control Eng. Practice, Vol.5, No.6, (1997) Takashi Kubotaa, Yoji Kurodab, Yasuharu Kuniic, and Ichiro Nakatania, Small, light-weight rover Micro5 for lunar exploration, Acta Astronautica, Vol. 52, (2003) T. Adachi, T. Iijima, S. Okamoto, and T. Takan, An experimental facility for Lunar Rover development, 46 th International Astronatutical Congress, Norway, Niranjian S. Rao, and Brian E., Wallace, Consideration for the Design of Lunar Rover Structures and Mechanisms for Prolonged Operations in the Lunar Environment, AIAA Charles L. Johnson and Kurtis L. Diez, Effects of the lunar environment on optical telescopes and instruments, Space Astronomical Telescopes and Instruments, Vol. 1494, (1991) International Lunar Conference