FLIGHT SYSTEM ARCHITECTURE OF THE SORATO LUNAR ROVER 1 *John Walker 1ispace, 3-1-6 Azabudai, Minato-ku, Tokyo, Japan, E-mail: [email protected] XPRIZE (GLXP) as “Team Hakuto.” Although ABSTRACT GLXP and Team Hakuto have ended without a ispace, a Japan-based private lunar exploration winner and without a lunar mission, Sorato is the company has developed lunar rovers over the baseline rover technology for ispace. course of several iterations, originally for its Google Lunar XPRIZE mission called “Team This paper presents the final architecture of the Hakuto”. A flight model rover called “Sorato” was 3.8kg Sorato rover, results of qualification and developed and qualified for the mission. briefly shows the conceptual design of the next ispace rover, carried to the lunar surface by its Sorato is a 4-wheel, skid steer rover with passive own lunar landers. ispace’s first orbiter mission is suspension and a minimum of actuators. The planned for 2019/2020 and the first landing/roving rover has two redundant controllers, inertial mission is planned for 2020/2021. measurement units, four cameras, and one time-of- flight camera for detecting obstacles and making 2 HISTORY 3D images. The rover also carries a deployable The conceptual design and prototyping for a antenna and has payload capacity. GLXP rover stretches back to 2010 at the Space Robotics Lab (SRL) of Tohoku University1 as Sorato is smaller than any lunar or martian rover, shown in Figure 1. at 3.8 kilograms. This mass was achieved by reducing the power consumption as much as possible, thereby reducing the surface area required for both radiators and solar cells, minimizing mass and by a novel thermal design. The rover makes use of magnesium alloy, carbon fibre structure and low-thermal conductivity plastics, to thermally isolate the solar panels and lunar surface from the electronics. The top of the rover is concave which acts as a heater in the lunar morning and a radiator at lunar noontime. Many parts in Sorato are Commercial-Off-The- Shelf (COTS), qualified through extensive Figure 1: The “Moonraker” 10kg four-wheel screening, especially radiation screening. The prototype rover, with the “Tetris” two-wheel entire system was designed, manufactured, and rover (inset) qualified at the system level. COTS components allow rapid development and higher performance ispace has since developed both two- and four- compared to traditional space products. wheel rovers, for independent operation as well as for cooperative operation to explore a lunar skylight. The rovers are described in Table 1. The 1 INTRODUCTION power consumptions shown are for 10% “duty ispace is a lunar exploration company based in cycle” driving unless otherwise stated. That is, Tokyo, Japan. It intends to travel to the lunar 10% of time is spent driving, and 90% of time surface, find and utilize resources, particularly making decisions. This number is chosen based on water-ice which is thought to exist at the lunar ispace’s field test experience for operation at sub- polar regions. ispace plans to do this in a cost- 100kbps bandwidth. effective and profitable way through its expertise in miniaturization of spacecraft and lunar rovers, All rovers include ispace’s qualified motor beyond the lower bounds established by space controllers and Inertial Measurement Units agencies. (IMUs). All of the four-wheel rovers use the same basic principle of 4-wheel skid steering, with a ispace created a four-wheeled lunar rover called simple passive differential gear system to keep all Sorato as part of its entry into the Google Lunar four wheels on the ground. Table 1: History of ispace rovers since 2013. Name Description “Pre- Description: 4-wheel rover Flight Camera: 5MP omni-directional camera + mirror Model” Radio: 900 Mhz proprietary + 2.4 Ghz wi-fi (1 (PFM) antenna for each) Controller: FPGA space-ready main controller + ARM imaging controller Mobility: 4 in-wheel motor Solar Power: none Battery: space ready Li-ion, 77Wh Mass: 7.0kg Power: 18W Description: 2-wheel rover (tethered) Figure 3: The “FM1” Sorato four-wheel rover Camera: 5MP Radio: 900 Mhz proprietary + 2.4 Ghz wi-fi (1 In PFM, the omni-directional camera system was antenna for each) Controller: ARM controller usable but its height required a tall support Mobility: 2 in-wheel motor structure to support the rover during launch, which Solar Power: none added significant mass. It was replaced by 4 Battery: space ready Li-ion, 38Wh cameras with overlapping fields of view in PFM2. Mass: 1.5kg Power: 10W PFM had a heterogeneous architecture, with a “Pre- Description: 4-wheel rover high-reliability FPGA-based controller plus a Flight Camera: 4x 5MP COTS ARM controller to meet the imaging Model Radio: 900 Mhz proprietary + 2.4 Ghz wi-fi (1 2” antenna for each) requirements. The ARM controllers passed (PFM2) Controller: 2x ARM controller (redundant) radiation screening so PFM2 was updated to a Mobility: 4 in-wheel motor redundant ARM controller architecture. This Solar Power: None general concept was kept until FM2. Battery: space ready Li-ion, 77Wh Mass: 7.0kg Power: 15W The FM1 rover was designed for the thermal “Sorato Description: 4-wheel rover environment at 45N and was qualified at the Flight Camera: 4x 5MP component and system level2, especially by Model Radio: 2.4 Ghz proprietary 1” Controller: 2x ARM controller (redundant) mobility, thermal vacuum, and vibration testing. (FM1) Mobility: 4 in-wheel motor Testing was used to validate analysis models used Solar Power: 18W @ 45N for FM1, and allow three major, related changes to Battery: space ready Li-ion, 77Wh Mass: 5.0kg be made for the FM2 rover: power reduction, Power: 13W thermal design, and mass reduction. “Sorato Description: 4-wheel rover Flight Camera: 4x 5MP Partway through the FM2 design, the landing Model Radio: 2.4 Ghz proprietary 2” Controller: 2x ARM controller (redundant) partner was changed and the landing site was (FM2) Mobility: 4 in-wheel motor changed from 45N to 28N. This hotter location Solar Power: 11 W @ 28N required improved thermal design by less heating Battery: space ready Li-ion, 38Wh Mass: 3.8kg and more cooling. Less heating was accomplished Power: 13W (at 20% duty cycle) by lower power consumption, primarily by using more efficient motors. Less power consumption allowed smaller solar panels and batteries. Smaller solar panels allowed a shorter rover. More efficient motors, smaller batteries, and a shorter rover all contributed to a lower mass, along with change to a monocoque structure. 3 SORATO FM2 ROVER 3.1 Requirements The requirements for the rover came from four main inputs: 1) The expertise and business Figure 2: The “PFM” two- and four-wheel rovers planning of SRL and ispace, 2) The GLXP requirements, 3) The specific environmental Since the PFM rover, all components have been requirements of the mission and landing site and qualified through radiation (total dose, single 4) Constraints of the landing partner. The landing event), thermal vacuum and vibration testing, and site at 28N was selected by the landing partner as system environmental testing. Progress was made a relatively safe site with few rocky outcrops and by reducing the power consumption with each with relatively good available terrain data. Table 2 iteration, thereby decreasing required power, solar gives a summary of the requirements. All of these panel size, and total rover mass. In each phase, were met through design or testing. screening and qualification testing was performed. Table 2: Top level requirements for Sorato Source Category Requirement Total mass of the rover is 3.8kg (without the GLXP Imaging NRT Video “interface” to the lander structure. The maximum HD Video High res panorama speed is approximately 0.1m/s and the power PR GLXP logo shown consumption is 4.5W at idle (including Looks cool communication) and an average of 21.5W while Mobility Travel 500m total moving across varied terrain. Travel 250m from lander ispace Mobility 22 degree slope climb The rover is designed to be dropped from the deck Overcome 10cm rocks Autonomous stop of the lander to the lunar surface. The component 0.1m/s speed which holds and drops the rover is called the Budget Mass < 4kg interface, shown below. It has 3 main functions: PR Sponsor logos deployment using a COTS actuator, thermal Power 10% driving time isolation of the lander to the rover, electrical Landing Thermal Surface temperature to 100C site Power Sun angle to 28 degree interface for heaters inside the rover (used during Radiation Up to 80MeV cruise). Landing Radiation Up to 4krad total dose Partner Structure > 60 Hz eigenfrequency 3.3 System Architecture 20G quasi-static Mission time (proprietary) Aside from connecting the subsystems, the Thermal Isolate from lander defining feature of the architecture is the two Comm 2.4 Ghz, 1W radio redundant ARM controllers located on one ispace- 125 bps uplink (33% of designed board. time) 118kbps downlink (50% of time) CCSDS, Proprietary ground station The GLXP imaging requirements are detailed but essentially require a narrow field of view, with 1- 2fps “Near-Realtime” (NRT) 640x480 video for operation and 10fps 1280x720p HD video sent to Earth (non-realtime)3. Based on the above requirements, the mobility, thermal, and structure detailed designs were created. 3.2 System Figure 6: Sorato Rover Architecture 3.4 Mobility The four-wheel skid steer concept was kept as it has been successful for ispace. The mobility system comprises an ispace motor controller, 4 COTS (but customized) brushless DC gearmotors inside each of four wheels, and ground clearance and wheel size maximized for the available space on the lander. The wheels are mass-optimized based on previous research at SRL which concludes approximately 15 grousers with 15mm length is an optimum point for travel efficiency vs performance4.