Study on Small Body Exploration Rover
with Intelligent Behavior Control
Takuma Honda(Graduate School of Engineering, The University of Tokyo),
Takashi Kubota(Japan Aerospace Exploration Agency)
ABSTRACT
In recent years, small body exploration missions have been proposed and implemented. Small body exploration gives us a lot of hints about the origin of the solar system, the evolution of Earth and the beginning of
life. In the future, rover mission is expected. There are a lot of researches that consider moving mechanisms
in microgravity. However there are few researches to consider exploration planning. The purpose of this
study is to propose an intelligent behavior control, which means an autonomous exploration with variety of
controls. In this paper, the proposed rover has been studied by simulations from the view points of thermal
control and mobility under microgravity.
知的行動制御を行う小天体探査ロボットの提案
本田拓馬 (東大院)， 久保田孝 (JAXA)
摘 要
近年，小天体探査ミッションが盛んに計画されている．小天体は誕生してから熱的な変性をあまり経
験していないことから，探査することで太陽系の起源や地球の進化，生命の起源を知るヒントが得ら
れると考えられている．小天体探査をより詳細に行いたいという要求から，探査ローバによる小天体
表面を移動しながらのその場探査への期待が高まって いる．微小重力環境での移動性の検討が多くな
されているが，探査戦略についての検討をしている研究は少ない．そこで本研究では，知的行動制御
と呼ぶ様々な行動制御を行いながら自律探査するローバを提案する．本稿では，知的行動制御の１つ
である熱制御性と，提案ローバの微小重力環境下での移動性の 2 つの検討をシミュレーションにて行
なったものを紹介する．
Key Words: Space Robot, Mechanism, Thermal control, Small body exploration, Mobility
1. Introduction
In recent years, small body exploration missions have
received a lot of attention. Asteroids may have better clues
about the origin of the solar system because they were
formed without thermal deformation[1]. A wide range of
direct surface exploration by small rovers is important to
investigate asteroid in detail. Asteroid exploration rovers
are required to adapt to the asteroid environment. Required main functions are shown as below.
• Robust mobility under microgravity environment
The gravity of asteroid surfaces is very small, 10−7
∼10−3 [G]. In microgravity environment, it is difficult for rovers with wheeled mechanism to move.
So, hopping mobility by pushing the surface is considered to be effective[2, 3, 4, 5, 6, 7]. In addition, moving mechanisms other than hopping are
proposed.[8, 9]
• Countermeasures against heat environment
Asteroid surface is very hot during the daytime. The
daytime may be much longer due to the inclination
of the rotation axis. Therefore rovers are required to
have aggressive thermal control[10].
• Simple mechanism
Small and light mechanism is desirable because payloads are limited.
The conventional studies of the asteroid rovers focused
only on mobility mechanism under microgravity. These
rovers have not considered exploration planning except
mobility.
In this paper, a new type of rover to move under microgravity as well as perform thermal control and landing
control by deploying the body of the rover is proposed.
This paper shows the effectiveness of two functions of the
proposed rover by some simulations. Those functions are
thermal control and mobility under microgravity.
Figure 2. Exploration senario
part to reduce rebound.(Figure 2 (3))
4. The rover explores the surface of the asteroid widely
and long with repeating 1∼3 sequences.
Figure 1. Model of the proposed rover
2.
Proposed Rover and Exploration
Strategy
This section shows a model and a concept of our proposed rover.
2.1
Proposed Rover
Figure 1 shows the model of the proposed rover. The
rover has a deployment part and two internal torquers in
its body. The rover can hop by using torques from these
three actuators and also can control its attitude by using
them. The outside of its body is covered by insulated sides
and the inside is heat release side. So, the rover can control
its temperature by opening its deployment part to expose
internal heat released side to the space and closing if the
rover faces to the sun or surface of the asteroid.
2.2
The proposed rover performs some behaviors as above sequences automatically during exploration.
3. Study on Thermal Control of Proposed
Rover
This section shows simulation studies on thermal control of the proposed rover. A thermal mathematical model
is built and conventional thermal control methods and a
proposed method are compared.
3.1 Heat Environment of The Small Body
Figure 3 shows the relation of heat radiations between
the small body and a rover. Equations of thermal input factors and heat balance equation of rover’s body are shown
by eq.(1)∼(5).
Exploration Strategy
(1)
Qa = aFe AI s
(2)
Qi =
Exploration strategy of the proposed rover is shown as
follows.
1. When actuators(deployment part and internal torquers) are operated, the rover will be pushed against
the surface and will hop by counter force from the
surface. Hopping direction and velocity can be
changed according to a ratio of input torques into
three actuator.(Figure 2 (1))
2. While the rover is hopping, it controls its attitude to
perform scientific investigation for longer time. In
case the rover temperature increases, the rover perform thermal control by using attitude control and
opening and closing its deployment part to decrease
its temperature under operable temperature. Sun direction and body attitude are detected by sun sensor, gyro sensor and pictures from equipped camera.(Figure 2 (2))
3. When the rover is going to land, it perform softlanding control by attitude control and deployment
Q s = α(A − A1 )I s cos θ s
ϵg σFe A(T g4
−
ϵR σFesp A(T R4
T R4 )
Qe =
− 4)
dT
CR
= Q s + Qa + Qi + QR − Q e
dt
α
A
A1
θs
Is
a
Fe
ϵg
σ
Tg
TR
ϵR
Fesp
CR
:
:
:
:
:
:
:
:
:
:
:
:
:
:
(3)
(4)
(5)
Sunlight absorptance
Area of heat release side of rover[m2 ]
Area of shade on heat release side of rover[m2 ]
Angle between sunlight vector and heat release side[deg]
Intensity of solar radiation[A/m2 ]
albedo coefficient
Angle factor between heat release side and surface of asteroid[12]
Infrared radiation ratio
Bortzmann’s constant[W/(m2 · K 4 )]
Temperature of asteroid[K]
Rover temperature[K]
Emissivity of rover
Angle factor between heat release side and the space[12]
Heat capacity of rover[W · s/K]
The rover’s body temperature can be observed according to these equations. During daytime, each power of
thermal input factors is strong. Therefore the rover would
be greatly effected by thermal inputs from the environment
Figure 3. Heat environment
and the rover’s temperature would become to be over operable temperature in a short time if the rover faces to sun
direction or the surface of the small body. The rover has
to reduce thermal inputs into own body by thermal control
so that it can explore for longer time.
3.2 Strategy of Thermal Control
There are some thermal control methods for space exploration rover. One conventional method for a lunar exploration rover is passive type method. This method is to
equip the rover with heat release side on one surface of its
body and insulated sides on other surfaces. This method
is effective for rovers which can keep making its heat release side face to the space like lunar rovers. However, it
would not work for hopping rovers because they are rotating during exploration. Therefore, the proposed rover
perform thermal control with its deployment part. When
the rover faces to sun direction or the surface of the small
body which give the rover large heat input, the rover close
its deployment part to protect its body against heat input.
Meanwhile, when the rover faces to the space, it deploy
its deployment part to radiate its heat to the space. This
proposed method would work well for hopping rovers and
they could explore for longer time. Figure 4 shows images
of these two thermal control methods.
Heat Input and Output Balance According to
Rover Attitudes and Deployment Angles
This section shows simulation study on calculation of
values of heat input and output(I/O) balance of the rover
during hopping according to rover attitudes against the
surface of the small body and deployment angles of the
deployment part. The rover attitude θR and the deployment angle θAB are defined as Figure 5. The rover is assumed to hop over the surface with rotating around only Y
axis because other rotations are canceled by internal torquer. Other simulation parameters are shown as table 1. In
this simulation, initial body temperature is 60[℃] because
60[℃] is assumed to be limiting temperature to explore
for the rover in this paper. Simulation result is shown as
Figure 6. Lateral axis shows attitude angles and vertical
axis shows deployment angles. Red zone shows values of
heat I/O balance are positive. It means that rover temperature will increase if it keep its attitude and deployment
angle. Other color zones show that the values are negative. Therefore, the rover can perform thermal control ef-
Figure 4. Images of two thermal control methods
Figure 5. Definitions of angles
3.3
fectively if the rover’s attitude and deployment part are in
yellow or blue zones.
Table 1. Parameters for Thermal Simulation
parameters
Intensity of solar radiation
Albedo coefficient
Infrared emissivity
Absorptivity of OSR
Emissivity of OSR
Sun direction
Temperature of asteroid
Heat capacity of rover
Rover mass
Area of a heat release side
Range of θR
Range of θAB
value
1515 [W/m2 ]
0.1
0.9
0.1
0.8
λ=60[deg], ψ=85[deg]
140[degC]
1000[W·s/K]
1.2[kg]
0.2 × 0.2 [m2 ]
0 ≤ θR ≤ 360
0 ≤ θA B ≤ 180
Figure 6. Heat I/O balance(body temperature:60[℃])
3.4
Simulation Study on Thermal Control During
Hopping
1. Conventional passive model
The rover has a heat release side on a surface of its
body and no deployment part, and not control its attitude during hopping.
2. Deployment model
The rover has a deployment part and deploy it when
θR < θth1 or θR > θth2 to release its heat to the space.
In other attitudes, the rover closes its deployment part
to be insulated.
3. Attitude control model
The rover has a deployment part and perform attitude
control by using it to release its heat as a long time
as possible. When θR < θth1 , the rover deploy its deployment part. When θR reaches 330[deg], the rover
Temperature [℃]
100
According to the simulation result of section 3.3, thermal control methods for the proposed rover are proposed.
One proposed method is to make the rover deploy its deployment part if θR is under 60[deg] or over 300[deg]
(these degrees are threshold degrees θth1,th2 ), and close if
θR is in other ranges. This method is named ”Deployment
model”. The other method is also make the rover deploy
and close its deployment part like Deployment model, but
additionally try to hold θR in the range the rover can release
its heat to the space effectively by closing its deployment
part gradually. This method is named ”Attitude control
model”. It is considered to be more effective than Deployment model because the rover can keep good attitudes to
release its heat during hopping with rotating. A thermal
transient response of the rover that performs three thermal
control methods during hopping is observed by a simulation to confirm effectivenesses of proposed thermal control
methods. Three methods are shown as bellow.
80
60
40
passive
deploy
attitude control
20
0
0
0.5
1
Time [s]
1.5
2
4
x 10
Figure 7. Thermal transient responses of three methods
begins to close its deployment part gradually to keep
θR . Finally, when θAB reaches 30[deg], the rover close
its deployment part.
Initial state is that the deployment part is closed, initial body temperature is 30[℃] and the rover is rotating
above the surface of the small body with angular velocity
-10[deg/sec] around Y axis. Other simulation parameters
are same as the simulation in section 3.3.
Figure 7 shows the simulation result. Lateral axis shows
elapsed time and vertical axis shows body temperature of
the rover. Table.2 shows operable times of three cases.
According to these results, a thermal control method with
deployment system is more effective for hopping rovers
than conventional passive model. Moreover, attitude control is helpful for hopping rovers with deployment system
to survive for a long time.
Table 2. Time to reach 60[℃]
Model
Case1(Passive)
Case2(Deployment)
Case3(Attitude control)
Time
1,500[s]
12,000[s]
14,500[s]
4. Study on Mobility of Proposed Rover
This section shows a simulation study on a mobility of
the proposed rover to verify the mobility under microgravity. A mobility of a rover with only deployment system
under microgravity is verified by Miyata et. al[11]. In
this chapter, three dimensional dynamics model is built
and omni-directional hopping of the proposed rover with
deployment system and internal torquers is observed in a
three dimensional simulation.
Figure 8 shows a simulation model. An initial states
of the rover in the simulation is that global coordinates
exactly overlap local coordinates in Figure 8 and the deployment angle θAB is 0[deg]. A spring damper model is
applied to the model of the small body surface. Table 3
shows simulation parameters. The movement of the rover
is described by multi-body dynamics with two rigid bodies. The equation of motion is shown as eq.6. M means a
mass of the body, J means a moment of inertia, F means
an external force into the body, N means a torque into the
body, Φ, γ mean constraint equations and Λ means a Lagrange multiplier. Eq.(6) is calculated by 4th Runge-Kutta
method.

 M

 0

ΦV
0
J
Φω
ΦV
Φω T
0
T

 

  V̇   F 

 
 
  ω̇  =  N 

 
 
γ
Λ
(6)
In this simulation, the mobility of the rover under microgravity according to input torques of the deployment
system and internal torquers is observed. Simulation results are shown as table 4. This table shows hop velocities
of the rover according to input torques. The hop velocity
means a velocity of the rover which has just risen off the
surface of the small body. T 1 , T 2 , T 3 mean input torques
as Figure 8. As results of the simulation, it is verified that
velocities for X axis and Y axis can be obtained by using the deployment system and internal torquers simultaneously and a direction of the velocity can be changed by
changing a ratio of input torques. According to the above
result, it is verified that the proposed rover can change its
hop velocity by changing the ratio of input torques into the
deployment system and internal torquers. Therefore, this
rover is considered to be able to perform omni-directional
hopping which directions the rover want to go under microgravity.
5. Conclusion
In this research, the small body exploration rover which
can hop under microgravity and perform thermal control
and landing control while hopping is proposed. In this
Figure 8. 3D simulation model of the rover
Table 3. Simulation parameter
Parameters
Values
Width of rover(L x )，Depth(Ly )
0.2 [m]
Height of body A(LzA )
Height of body B(LzB )
Mass of body A(MA )
0.12[m]
0.03[m]
0.8[kg]
Mass of body B(MB )
Gravitational acceleration of asteroid
0.2[kg]
9.8×10−5 [m/s2 ]
Spring constant of asteroid surface (K)
Damping constant of asteroid surface(C)
Coefficient of friction(µ)
104 [N/m]
102 [Ns/m]
0.4
Table 4. Hop velocities under conditions
Input torques(×10−3 [N])
（T 1 ,T 2 ,T 3 ）
Hop velocity(×10−3 [m/s])
(V x ,Vy ,Vz )
(1.0 , 1.0 , 0.0)
(-1.0 , 1.0 , 0.0)
(3.0 , 1.0 , 0.0)
(-1.0 , -1.5 , 30)
(-1.0 , 1.5 , 30)
(-1.8 , -21 , 42)
(-3.0 , 1.0 , 0.0)
(1.0 , 3.0 , 0.0)
(-1.8 , 21 , 42)
(-4.7 , -0.3 , 48)
(-1.0 , 3.0 , 0.0)
(1.0 , 1.0 , 3.0)
(-1.0 , 1.0 , 3.0)
(-4.7 , 0.3 , 48)
(7.0 , 1.2 , 39)
(1.7 , 5.3 , 49)
(1.0 , 1.0 , -3.0)
(-1.0 , 1.0 , -3.0)
(1.7 , -5.3 , 49)
(7.0 , -1.2 , 39)
paper, effectivenesses of two function of the rover are verified by simulation, the thermal control by using deployment system and hopping mobility by using the deployment system and internal torquers under microgravity. In
the future, microgravity experiment will be performed to
verify the mobility and a strategy of the attitude control of
the rover while hopping.
REFERENCE
[1] T. Kubota, The first challenge in the world, Hayabusa
project, Transaction of the Japan Society of Mechanical Engineers, Vol.114, No.1107, 2011.
[2] T. Kubota, B. Wilcox, H. Saito, J. Kawaguchi, R. Jones,
A.Fujiwara, J. Reverke, A Collaborative Micro-Rover Exploration Plan on the Asteroid Nereus in MUSES-C Mission, In: 48th International Astronautical Congress, 1997.
[3] T. Yoshimitsu, T. Kubota and I. Nakatani, New Mobility
System of Exploration Rover for Small Planetary Bodies,
Journal of the Robotics Society of Japan, Vol.18, No.2,
pp.292-299, 2000.
[4] Y. Nakamura, S. Shimoda, S. Shoji, Mobility of a Microgravity Rover using Internanl Electro-Magnetic Levitation,
Proceedings of the 2000 IEEE/RSJ Internationanl Conference on Intelligent Robots and Systems, 2000.
[5] K. Yoshida, The Jumping Tortoise: A Robot Dsign for Locomotion on Micro Gravity Surface, Proc of 5th International Symposium on Artificial Intelligence Robotics, Automation in Space, pp.705-707, 1999.
[6] S. Shimoda, T. Kubota and I. Nakatani, Proposal of New
Mobility and Landing Experiment in Microgravity Experiment, Journal of the Japan Society for Aeronautical and
Space Sciences, Vol.53, No.614, pp.108-115, 2005.
[7] T. Yoshimitsu, T. Kubota and A. Adachi, MINERVA- surface exploration system in Hayabusa-2 asteroid explorer,
The Japan Society for Aeronautical and Space Sciences,
4509, 2013.
[8] K. Yoshida, T. Maruki and H. Yano, A Novel Strategy for
Asteroid Exploration with a Surface Robot, In proc. of the
3rd International Conference on Field andService Robotics,
Finland, pp.281-286, 2002.
[9] M. Chacin, K. Yoshida, MULTI-LIMBED ROVER FOR
ASTEROID SURFACE EXPLORATION USING STATIC
LOCOMOTION, Journal of the Robotics Society of Japan,
Vol.21, No.5, pp.498-502, 2003.
[10] T. Yoshimitsu, Planetary Rovers for Small Bodies, Journal
of the Robotics Society of Japan, Vol.21, No.5, pp.498-502,
2003.
[11] Y. Miyata, T. Yoshimitsu and T. Kubota, Progress of Research on A New Asteroid Exploration Rover Considering Thermal Control, The 2nd International Conference
on Robot Intelligence Technology and Applications, F1A,
2013.
[12] A. Feingold, Radiant-interchange configuration factors between various selected plane surfaces, Proc. Roy Soc. London, ser. A, Vol.292, No.1428, pp.51-60, 1966.