1. Introduction
Even now, the Moon is a one of interesting celestial
bodies for exploration or investigation in space
development. The Moon has a lot of unclear questions
as the origin, the chemical components, and
the internal structure. Thus, according to these remarks,
it is required to obtain the global information
about the Moon for future lunar exploration missions.
Therefore the authors propose a robotic system
for burying a long-period seismometer under the
lunar surface. In general, the lunar regolith has high
adiabatic property, and the temperature is constantly
-20 degree even if at night. This consequently enables
the seismometer to measure for a long term.
Also, the buried seismometer can realize better contact
with surrounding regolith. This paper firstly describes
the strategies of subsurface propulsion of the
burrowing robotic system and required mechanisms.
Then, this paper especially discusses an excavation
mechanism which is very important for the system,
and proposes a novel excavating mechanism.
Fig. 1 Schematic of lunar subsurface exploration
Table 1 Planetary excavation systems
Mechanism Size Weight Robustness Purpose
Bucket Wheel simple large heavy ~ sampling
Penetrator simple
middle
long
middle ~ investigation
Drill simple
narrow
long
light sampling
Burrowing Robot complex compact light investigation
Fig. 2 Overview of planetary excavation robots
2. Burrowing Robot System
2.1. Conventional Robots
Planetary exploration with excavation of soil has
received a lot of attention in the world. The comparison
on planetary excavation systems is shown
in Table 1, where the robustness means a capability
to have some excavating points. So far, there have
been proposed burrowing robots for planetary
subsurface exploration [1]-[5]. However, those robots
do not meet the scientific requirements. The
overview of some proposed ideas which are classified
in the length and the diameter is shown in Fig.2.
ICEBREAKER and the Lunar-A penetrators of
JAXA are not actually burrowing robot, but these
are robotic systems for subsurface investigation or
the similar objective. Therefore these two different
methods are shown in Fig.2 for a comparison.
2.2. Strategies for subsurface propulsion
Based on the past researches, the authors set the
following assumptions:
EThe burrowing robot is carried by the rover
EPower is supplied by a cable from the ground station
ETarget depth is several meters from the surface
ETarget soil-layer is lunar regolith
EThe robot diameter is about 0.1 m
Next, the authors define the strategies for subsurface
propulsion by the burrowing robot, and consider
the following two phases. Several concrete
methods are shown in Fig.3.
(1) Make space
ECompress fore-regolith
ERemove and back transport fore-regolith
(2) Advance forward
EUtilize a contact with surrounding regolith
EUtilize an excavated regolith
ESelf generation unrelated to regolith
Fig. 3 Strategies of subsurface propulsion
3. Propulsion Limit Depth
Before considering the active generation of propulsion
force as shown in Fig.3, this paper discusses
the propulsion limit depth by only its weight. The
main resistances for propulsion are lateral friction
and excavating resistance by applying the simplified
dynamics model as shown in Fig.4. For subsurface
propulsion, the lateral friction is universal resistance.
Firstly the soil pressure underground is calculated by
the following equation.
( p gh)K 2c K 0 = + } i1j
where, the notations used in this paper are as followings.
: lateral soil pressure [kPa],
p0 : external stress on the surface [kPa],
: soil bulk density [kg/m3],
g : gravity acceleration [m/s2],
h : depth from surface [m]
c : cohesion of soil [kPa],
K : Rankinefs soil pressure coefficient,
H : robot length [m],
D : robot diameter [m],
A : robot cross-section [m2] ( := (D/2)
2
),
: friction coefficient (soil ? robot ),
L F : soil friction resistance [N],
M : robot mass [kg],
Also, the robot shape assumes to be cylindrical.
When p0 and c are zero, and the total lateral friction
resistance L F is calculated by the integrating
through total lateral surface as following.
( )
2
2 2 A K g hH H
F A K gh dh
h
h H
L
?
=
= ?
?
@@
i2j
Thus the propulsion limit depth is estimated for the
burrowing robot which mass is M from the following
equation (3).
F ? Mg = 0 i3j
As the above consideration, the propulsion limit
depth respect to the robot mass is shown in Fig.5
where is 1600 [kg/m3], D is 0.1 [m] and H is
0.2 [m]. Thus, given the strict weight limits of
space mission, the burrowing robot needs to generate
the propulsive forces subsurface.
Fig. 4 Simplified dynamics model of soil friction
Fig. 5 Advancing limit depth by its weight
4. Compression Limit
This paper considers the capability of compression
fore-soil method as an excavation mechanism. The
relationship at an extreme state between vertical
compressing stress and shear stress by applying
Mohr-Coulombfs failure criterion theory is
shown in following equations (4) and (5).
Ӂ@
sin
2 2
1 3 1 3 ?
?
+
= i4j
= + c
?
=
cos tan
2
3 1 i5j
Here the following new notations are defined.
: shear stress [kPa],
0 : initial stress [kPa],
1 : maximum soil pressure [kPa],
3 : minimum soil pressure [kPa],
: internal friction angle [rad],
e : void ratio,
e0 : initial void ratio,
CC : compression index,
B : generated space distance [m],
x : effect compressing range [m],
The compression index C C is introduced, which
denotes the relationship between void ratio and
compressing pressure as described in the following
equation (6).
10 10 log d log
e de
CC = ?
= ? i6j
Therefore, by integrating the above equation (6),
the following equation is obtained.
@0
0
10 e C log e C + ? ?
?
?
? ??
?
= ?
i7j
Then, the effect range is assumed by the following
equation (8). This means that compression force
affects to distance X from the bodyfs center line as
the same force, and this can indicate the best condition
about the compression to make space.
? ??
?
? ??
?
+
?
+
= ?
e
e
e
e
B xD
1 1 0
0 i8j
The simulation result is shown in Fig.7. In the
simulation, the parameters are 1 ? x ? 10 and
e0=1.2, and D is 0.1 [m]. The simulation result
shows that it can be too hard to make space. Thus it
is concluded that because the lunar soil layer has
high-filling state under the surface, a method to
compress fore-soil is an unrealistic to make space
and the robot has to remove and transport fore-soil
to backward.
Fig. 6 State model of compression method
Fig. 7 Compression limit by Mohr-Coulombfs
failure criterion with CC
5. Non-Reaction Mechanism
5.1 Proposal of Double Rotation System
From the above considerations, the required properties
for robotic excavation mechanism are following:
EFore- soil removal and transportation backward
EContribution of propulsion force
EDust prevention mechanism
Therefore, a screw drill is one of candidates, which
has a series of spiral wing. However, there is a
problem that a single spin drill has reaction to the
body. This reaction can be friction as reducing the
efficiency as well as leading to a wobbling of propulsion
axis. So this paper proposes a new type spiral
screw drill unit as a kind of novel excavation
mechanisms to improve the problem.
The double rotation screw is developed with
N-RDM (Non-Reaction Drilling Mechanism). The
double rotation methods are classified into three
types as shown in Fig.8. The contra-rotor type (a)
has one drilling unit which has the contra-rotation
axis at the same of another rotation axis. The
twin-rotor type (b) consists of two drilling units
which have regular and contra rotation axis each
other at binary positions. The dual-spin type (c) has
one drilling unit which has dual different spin axis.
Each type has two rotation axes, so it is possible to
cancel the reaction. On the other hand, firstly the
contra-rotor type is compact size, and it can be estimated
that it has an equivalent excavating performance
with the single screw drilling. Secondly the
twin-rotor type has unexcavating space compared
with the contra-rotor type because the shape of an
excavated hole is anomalous. It is impossible to
make one circular hole, and there is constraint for
the body shape. Thus it can be estimated that it can
be lower efficiency. Thirdly the dual-spin type has
some external driving parts, and the efficiency is
getting lower. According to these considerations, the
authors firstly adopt contra-rotor type and will do
some experiments in the near future.
5.2 SSD
The schematic of the basic model, where it is call
SSD (Single Screw Drilling Unit) unit, is shown in
Fig.9(a), and also the prototype model is shown in
Fig.9(b). The developed SSD is a conceptual model
of the contra-rotor type screw. And it is a basic dynamics
model for the theoretical analysis.
The SSD unit consists of a body-part and an excavation-
part. Furthermore, the excavation-part has an
inner cone, called CONE, and a helical screw wing
which wind around the CONE, called SCREW.
From Fig.9(a), a parameter denotes the volubile
angle from that central axis along the SCREW upper
surface through from the top to end of the SSD.
Then SC [deg] denotes the angle of inclination of
the SCREWfs center position and p denotes the
SCREW pitch, where these parameters are a function
which is defined by .
Fig.8 Schematic of N-RDM systems
(a) Drawing
(b) Picture
Fig. 9 Model of SSD unit
6. Conclusion
In this paper, the authors discuss a novel robotic
burying seismometer system. And firstly this paper
describes the limit of compression method for making
space in subsurface and need of generation of a
propulsive force for advancement. Then the authors
propose a new excavating screw mechanism with
N-RDM. Some experiments are under going for the
validation of the proposed excavating method as future
works.
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