Rover Subsystem - Texas Tech

Proposal for
Texas Space Grant Consortium
Project: Lunar Rover

October 17, 1995

Submitted by:
Richard Eddings
Tim Horne
Mike McCafferty

TABLE OF CONTENTS

ABSTRACT                                                  1

INTRODUCTION                                              2
     MARS ROVER "ROCKY IV"                                2

ROVER DESIGN                                              3
     PROPOSED ROVER SCHEMATIC                             3
STRUCTURE/MATERIAL                                        4
     INITIAL FRAME CONCEPT                                4
DRIVE/STEERING                                            5
THERMAL SYSTEM DESIGN                                     6
     THERMAL DESIGN DIAGRAM                               6
COMMUNICATIONS/SENSOR DESIGN                              8
     COMM AND SENSOR BLOCK DIAGRAM                        8
     COLOR VIDEO CAMERA                                   9
     AUTOMATIC CONTROL SYSTEM                             9
     IR VIDEO CAMERA                                     10
POWER SYSTEM                                             11
     POWER SYSTEM BLOCK DIAGRAM                          11
     POWER DISTRIBUTION AND CONVERTION                   12
     PHOTOVOLTAIC ARRAY                                  12
     PATHFINDER ROVER PHOTOVOLTAIC ARRAY                 12
     BATTERIES                                           13
MICROCONTROLLER                                          13
     SMARTCORE MICROPROCESSOR                            13

CONCLUSION                                               14

REFERENCES                                               15

APPENDIX                                                 16
     LUNAR FACTS & FIGURES                               16

ABSTRACT

Oxygen production is very important in establishing a lunar space station. Oxygen atoms can be extracted from lunar soil and formed into breathable oxygen.4 This process has been tested on Earth, and it will be tested on an unmanned mission to the moon. A lunar rover is needed to collect lunar soil samples and return them to the processing station.4 The rover will be assisted by remote control, and will also perform other scientific tasks.

INTRODUCTION

The most important resource for survival on the moon is oxygen. Lunar soil contains an abundance of oxygen in the form of oxides such as FeTiO3. The extraction of oxygen from the lunar soil has been demonstrated on Earth using samples collected by the Apollo missions. However, extraction of oxygen has not been demonstrated on the moon. The purpose of this mission is to perform the oxygen extraction experiment with a small unmanned lunar facility.3 A successful demonstration of oxygen production on the moon would be a major step towards the establishment of a permanent lunar base.

The Texas Space Grant Consortium (TSGC) consists of teams of students and faculty from various academic institutions.3 These teams will develop a preliminary design for a small lander to perform oxygen production as well as various other experiments. In order to perform the oxygen production, lunar soil must be gathered and transported from remote areas to the lander. Therefore, a small rover will be designed to perform such a task.

The rover will be fitted with a device capable of collecting up to 1kg of lunar soil and rocks from remote areas away from the contamination region created by the landerÕs rocket thrusters. The rover will be controlled by an Earth-based station, but due to time delays, the rover will have some autonomous control to prevent damage to the rover.1 The rover will also have the ability to inspect the lander and the surrounding area with a video camera. Other scientific equipment will be considered in order to increase the usefulness of the rover and the overall effectiveness of the mission. Figure 1 shows an example of the rover designed for the Mars mission scheduled for 1997.

Figure 1: Mars Rover "ROCKY IV"1

ROVER DESIGN

The preliminary design of the rover is shown in Figure 2. The rover will be able to dock with the lander in order to store lunar soil and rock samples. The lander will contain an array of rechargeable batteries which will charge throughout the lunar day. The lander will act as a radio relay station in order to transmit a high power radio signal to Earth and communicate with the rover. An arm will be located on the rover for sample collection and various other tasks.

Figure 2: Proposed Rover Schematic

The rover must be able to withstand the thermal conditions of the moon. The temperatures vary from -175oC to 115oC.3 Heating and cooling systems are a major concern in order to ensure proper operation and prevent thermal damage to all equipment.3 The rover will be able to control its own temperature and the temperature of the equipment that it is carrying.

The rover will be controlled from an Earth station. This control signal will be relayed through the lander to the rover. The rover will send video images and sensor data to the lunar relay. The rover will be somewhat autonomous due to the transmission delay to and from Earth. This autonomy will prevent damage due to steering blindly.

Structure / Material

Due to the unpredictable terrain of the moon, a flexible rover would be ideal for this proposed mission. This would allow for freedom of movement over a wide range of obstacles. One of the most flexible designs considered will allow for the rover to navigate most terrain it is likely to encounter and still be relatively small. A schematic of the platform is shown in Figure 3.

Figure 3: Initial Frame Concept

The frame will be divided into three equal sections. This will help in distributing the mass evenly throughout the rover. Torsional springs will be used at the hinges as the main damping system. Each section will also have an independent suspension attaching it to the main body of the rover as a supplemental damping system. This independent suspension will also act as a mechanism to compact the rover for storage on the lander.

Although this design was one of the most flexible, there are several constraints. The main body of the rover will have to be anchored from the center section of the frame to allow for the outer sections to flex. Also, the dimensions of the main body will have to be smaller than the inside dimensions of the frame to allow for the required freedom of movement. An additional constraint has been placed on the design in the allocated storage space of approximately 508 mm x 305 mm x 254 mm for the rover on the lander. This space is significantly smaller than any micro-rover design we have encountered so far. Whether or not this size constraint poses a problem has yet to be determined.

The main body of the rover will be completely sealed from the elements. This will allow for total protection from lunar dust and aid in the storage of heat during the lunar night. This design will also protect the rover from any free moving parts on the lander that may damage itsÕ sensitive components. The main drawback to total isolation is the difficulty in cooling the internal components during the day. Due to this drawback, a cooling system will be implemented to "channel" the heat away from the sensitive elements. This cooling system is discussed in more detail in the thermal section of the report.

Drive/Steering

Each wheel will have its own independent drive motor. The use of six wheel drive will assure that the rover keeps in contact with the ground even over rough terrain. Individual motors were chosen to simplify the drivetrain design and lower the number of mechanical parts. This approach should make the rover less vulnerable to damage. The mass reduction from simplifying the drivetrain will compensate for the addition of extra motors.

Variable speed motors will be used so that they may assist in the steering of the rover. Steering mechanisms will also be placed on the front and rear sections to work in conjunction with the variable speed motors. To further conserve mass, the wheels will be constructed of a ridged metal strip attached to the hub by eight thin spokes. This design should adequately support the rover and its cargo while maintaining traction and conserving mass. The wheels are rigidly attached to the individual sections of the frame to constantly maintain seven centimeters of ground clearance. The frame sections will be attached to the main body of the rover by a damping system to prevent damage to the internal components due to rough terrain.

Thermal System Design

Figure 4: Thermal Design Diagram

The temperature on the moon varies from as low as -175oC to 115oC. Since the moon has almost no atmospher, almost all heat is transferred through radiation and absorption. There is very little heat transferred by conduction, such as through the wheels to the moon.

Heat conduction to the moon through the wheels is minimal compared to the heat that will be absorbed from the Sun. The rover will be mirrored on the exterior in order to reflect radiant energy away. This highly reflective coating will keep the rover from rapidly overheating, preventing permanent damage to the sensitive electronic equipment. Some energy will be absorbed by the mirror, and therefore an insulation barrier made up of Aerogel will be placed between the outside and inside shells.

Aerogel has been developed since the 1930s. It cannot be frozen or destroyed by temperatures under 2000oC. This solid made of silica dioxide has a density less than that or air and is impervious to ultraviolet light.

The inside shell will contain a mirrored surface in order to maintain the internal temperature of the rover during a lunar night. The excess heat created by the electrical components, along with a backup heater will maintain the temperature of the rover during the night. A backup refrigeration unit will prevent rapid heating during the lunar day. This will be done using black-body radiation along with a Peltier device which is very rugged, and reliable. A heat conducting material may be used to surround all components in order to control the temperature efficiently.

A Peltier device is a reliable way to heat or cool the rover. There are no moving parts and it is extremely lightweight compared to other refrigeration systems. A thermoelectric refrigerator is made up of two dissimilar metals forming a junction. As current passes through the junction, the device heats up on one side, and cools on the other. Simply by reversing the polarity, the device can be easily changed from a refrigerator to a heater. Efficient Peltier devices are made of semiconductor materials.

All equipment will be insulated from the chassis. The solar array will also be insulated from the chassis. The Peltier device will have a mirrored shutter in order to help retain or disperse heat through the underside of the rover.

Communication/Sensor Design

Figure 5: Communication and Sensor Block Diagram

Communication is an important aspect of the lunar rover. The rover can be controlled from an Earth station. There is a delay of 3-5 seconds when communicating with the rover. This means that the rover will need to be partially autonomous in order to prevent collision resulting in severe damage. The Earth station will be able to control camera switching, motor direction and speed, full arm movement, and steering. A small forward looking microwave radar system will be used to prevent the rover from colliding into rock, etc.

A 3-dimensional color camera system will be implemented on the rover. This will be accomplished by multiplexing two color cameras. A 3D effect is created by spacing two 1:1 cameras within the distance of the average spacing of the human eyes (2.5"). Each camera generates 30 frames per second, but when multiplexed, the result is 15 frames per second per eye. A 3D visor can be worn by the Earth station controller in order to put the view in perspective, enabling easier control. This stereo effect will be especially helpful when operating the collection arm, as well as increasing PR. Each color camera is approximately 1.625 in3, 2.5 oz, and has 450 lines of resolution. As a comparison, SVHS and Hi-8 video have 400 lines of resolution.

An Alpha Proton X-Ray Spectrometer (APXS) will be located on the rover in order to analyze the lunar soil. This equipment can send its results back to Earth through the data link. Also, an infrared camera with IR LED illuminator array will be able to see in "darkness".

Figure 6: 1.625 in3 Color Video Camera

An automatic control system is available for the color camera. The color camera in Figure 6 easily fits inside the ACS. The dimension of the ACS is 2 in3. It performs many basic picture adjustments. These adjustments include: auto gain control, auto white balance, auto black balance, electronic auto shutter (1/60-1/100,000 second), automatic color processor compensates for various lighting conditions, medium wide angle lens 6" to infinity, 2 lux.

Figure 7: 2 in3 Automatic Control System

An infrared camera is an ideal choice for a third camera. It can function at night with or without its illuminator. It has a flexible design that can be easily placed in any position. The range, in total darkness, with the illuminator, is 8-12 feet.

Figure 8: Infrared Video Camera

Power System

The power system will be responsible for providing power to all rover systems. Figure 9 shows a basic block diagram of the system. The system will consist of two power sources, the photovoltaic array and the batteries. The primary function of the photovoltaic array will be to provide enough power so that the rover can accomplish its mission during the daylight hours. The batteries are to provide the power necessary to keep the internal systems of the rover within their survival temperature ranges for the entire lunar night. The power from these two sources must then be conditioned and distributed throughout system.

Figure 9: Power System Block Diagram

Power Distribution and Conversion

The distribution of power will be controlled through a digital interface by the microcontroller. An electronic switching device will be designed to handle the power that each subsystem will need. The microcontroller will be able to control which subsystems will receive power. The design of this device will include a circuit to protect the microcontroller from being damaged if problems with the power system occur.

Conversion and regulation of the power provided by the sources is needed not only because each subsystem has different requirements but to provide clean power. The design of this system will require specific data on each subsystems power requirements.

Photovoltaic Array

All of the power needed for operation of the rover will be provided by the photovoltaic array. This device will use the energy from photons to "kick" electrons in the valence band into the conduction band to produce electricity. Devices of this type only have efficiencies from 10% to 30% depending on their construction. The amount of power that can be produced by the array depends on the amount of direct sun light or photon energy and the size of the array. At the lunar surface the amount of sun light present is approximately 1300 W / m2. Compare these to the Earth with 1000 W / m2 , on a clear day, and Mars with 590 W / m2 and it can be seen that even with a ten percent (10%) efficient Si array with .2 m2 area it is possible to produce around 26 Watts of power. For this design an array made of GaAs will be considered because they offer efficiencies from 25% to 30% and maintain them at temperatures that are present on the moon.

Figure 10: Pathfinder Rover Photovoltaic Array

Batteries

The power supplied by a rechargeable battery source must able to provide power for an extended period of time. For this reason batteries with high energy densities will be considered. These must also have a low mass, be rechargeable and have a wide range of operating and survival temperatures.

Microcontroller

The microcontroller is the brain of the system. It must be able to receive and follow instructions via remote as well as monitor the well being of the rover both internally and externally. Software will need to be developed so that the microcontroller can make decisions based upon the inputs it receives. Memory storage will need to be available to store images that can not be sent immediately and to remember previous movements so a backtrack can be performed if contact is lost. Many types of microcontrollers are available that can be used for just this purpose. An example of this type of microcontroller would be the Zworld Little Giant. It has four serial communication lines that operate at 57,000 baud and seven 10-bit analog input lines. Figure 11 shows the microprocessor that is part of the Little Giant.

Figure 11: Smartcore microprocessor

CONCLUSION

The lunar rover is very important for the oxygen production experiment. The rover will easily complete scientific data collection and sample collection by remote control. The reusability and the ability to exchange equipment with the lander base station allows for versatility and an efficient means for accomplishing tasks.

REFERENCES

King, J.H., "Mars Pathfinder Project Information," NASA, Aug 1995.
Kline, Michael, "Sensor Specifications," http://www.utexas.edu/tsgc/design.html, Aug 1995.
TSGC, "Interdisciplinary Spacecraft Design Project,"http://www.utexas.edu/tsgc/design.html.
Dubois-Matra, Oliver, "Lunar Oxygen Production Plant:Specification sheet," http://www.utexas.edu/tsgc/design.html ,Aug 1995.

Wednesday, 31-Dec-1969 18:00:00 CST
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