LUNAR ROVER

ELECTRONICS

The moon, without an atmosphere, is an environment every bit as extreme as the depths of space. Extreme temperature swings and radiation are the norm on the lunar surface, making operation difficult for traditional electronics. Although recent progress in alternative materials such as silicon-germanium hold promise for the future, current electronic components require a warm electronics box (WEB) for space applications. The WEB provides a controlled temperature system, allowing electronic components to operate at temperatures comparable to those on Earth (typically ±40ºC). In addition to maintaining a stable temperature, the WEB provides minor shielding from total dose radiation, but does little for single event upsets. Since silicon-germanium components are still experimental and extremely expensive, the rover will make use of a standard WEB and conventional silicon electronics. The WEB will be mounted in between the rover chassis, just below the solar panel to shield the WEB from direct solar irradiation and maintain a balanced center of mass. As previously stated, temperature control is the primary function of the WEB. During the lunar day, surface temperatures are in excess of 100ºC, and the lack of an atmosphere precludes traditional cooling methods. By shielding the box with the solar panel and using silica aerogel insulation, the temperatures should be kept to an acceptable level. On the other hand, temperatures can fall below -200ºC during the lunar night. Eight radioisotope heater units (RHUs), combined with the silica aerogel insulation, should maintain a stable temperature while the rover is in its nighttime hibernation mode. Each RHU produces 1 W of thermal energy, weighs 40 g, and is smaller than a C-cell battery. Unfortunately, RHUs contain radioactive material; therefore, they will require government approval to obtain.

Inside the WEB, the heart of the rover’s electronics system is the central processing unit (CPU), which controls all of the major systems including navigation, power management, motor control, and digital signal processing. Given the critical nature of the CPU, radiation hardening is imperative because the WEB only provides limited protection. Radiation hardening drives the cost up considerably, and few options exist in the market. BAE and Space Micro both manufacture complete radiation-hardened CPU boards that include necessary periphery elements such as RAM, EEPROM, etc. The RAD750 by BAE is the ideal choice but carries a price tag of approximately $200,000. For increased performance, Space Micro manufactures two models, the Proton 100k and Proton 200k, with presumably an even higher cost. When the computing needs of this mission are considered the RAD750 is the most logical choice between cost and performance. A cheaper alternative that was considered was to use BAE’s older model, the RAD6000, but the processing power was deemed to be insufficient for the needs of the rover’s multiple systems.

RAD750 CPU and Board

Clock Frequency

133-200 MHz

Memory

128 MB SDRAM

Power Consumption

10 W

Mass

549 g

Cost

$200,000

Table 1. First Choice CPU and Board Summary

Proton 100k CPU and Board

Clock Frequency

400 MHz

Memory

128-256 MB SDRAM

Power Consumption

7-9 W

Mass

Unknown

Cost

Unknown

Table 2. Second Choice CPU and Board Summary

In addition to the main CPU and board, the WEB will also contain the telecommunication electronics, which are described separately HERE. Also included in the WEB is the MPEG-4 encoder chip, the Texas Instruments DM355. This chip compresses and encodes the raw data from the high definition image sensors before passing it along to the CPU. Finally, the WEB will contain six lithium ion batteries to provide sufficient power to disengage the rover from the lander and sustain the electronics during the hibernation mode through the course of a lunar night. During hibernation mode, the rover will also draw upon the batteries periodically to transmit short bursts relaying the continued functionality of the rover. The Tadiran TL-5930 battery was chosen for its blend of size, cost, and output.

DM355 MPEG-4 Encoder

Power Consumption

< 400 mW

Cost

$9.75

Table 3. MPEG-4 Encoding Chip Summary

TL-5930 Lithium Ion Battery

Output Voltage

3.6 V

Output Current (cont)

230 mA

Output Current (peak)

500 mA

Capacity

19 Ah

Mass

92.5 g

Cost

$16.48