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We present a distributed framework for predicting whether a planned reconfiguration step of a modular robot will mechanically overload the structure, causing it to break or lose stability under its own weight. The algorithm is executed by the modular robot itself and based on a distributed iterative solution of mechanical equilibrium equations derived from a simplified model of the robot. The model treats intermodular connections as beams and assumes no-sliding contact between the modules and the ground. We also provide a procedure for simplified instability detection. The algorithm is verified in the Programmable Matter simulator VisibleSim, and in real-life experiments on the modular robotic system Blinky Blocks.
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Distributed Prediction of Unsafe Reconguration
Scenarios of Modular Robotic Programmable Matter
Benoit Piranda, Pawel Chodkiewicz, Pawel Holobut, Stéphane Bordas, Julien
Bourgeois, Jakub Lengiewicz
To cite this version:
Benoit Piranda, Pawel Chodkiewicz, Pawel Holobut, Stéphane Bordas, Julien Bourgeois, et al.. Dis-
tributed Prediction of Unsafe Reconguration Scenarios of Modular Robotic Programmable Matter.
IEEE Transactions on Robotics, 2021, 37 (6), pp.2226 - 2233. �hal-03549347�
1
Distributed prediction of unsafe reconfiguration
scenarios of modular-robotic Programmable Matter
Benoˆ
ıt Piranda1, Paweł Chodkiewicz2, Paweł Hołobut3, St´
ephane P.A. Bordas4, Julien Bourgeois1, and Jakub
Lengiewicz3,4
1University of Bourgogne Franche-Comt´
e, FEMTO-ST Institute, CNRS, France
2Faculty of Automotive and Construction Machinery Engineering, Warsaw University of Technology, Poland
3Institute of Fundamental Technological Research, Polish Academy of Sciences, Poland
4Department of Engineering, Faculty of Science, Technology and Medicine, University of Luxembourg
Abstract—We present a distributed framework for predicting
whether a planned reconfiguration step of a modular robot will
mechanically overload the structure, causing it to break or lose
stability under its own weight. The algorithm is executed by
the modular robot itself and based on a distributed iterative
solution of mechanical equilibrium equations derived from a
simplified model of the robot. The model treats inter-modular
connections as beams and assumes no-sliding contact between
the modules and the ground. We also provide a procedure
for simplified instability detection. The algorithm is verified in
the Programmable Matter simulator VisibleSim, and in real-life
experiments on the modular robotic system Blinky Blocks.
Keywords: Self-reconfiguration, Modular robots, Distributed al-
gorithms, Mechanical constraints, Programmable Matter.
I. INTRODUCTION
Materials able to autonomously assume any shape are
the dream of engineers. Currently, the most advanced arti-
ficial systems possessing elements of this functionality are
modular self-reconfigurable robots—machines composed of
robotic units (modules) which can bond together, move over
one another and communicate, as well as store and process
information [1]. They may be compared to swarms of fire ants
forming bio-mechanical structures from their own bodies [2].
Cooperation of millions of tiny, densely-packed modules is
expected to produce the desired emergent shape change of the
entire ensemble. A system of this kind would be a realization
of the futuristic concept of Programmable Matter [3].
The operation of densely-packed self-reconfigurable robots
is based on the movement of modules from one part of
the robot to another. This poses not only the challenging
hardware problem of designing miniaturized modules able to
operate in large 3D ensembles, but also the software problem
of controlling this motion. If the robot is autonomous, its
operation must be collectively planned and controlled by the
modules, taking into account geometric and mechanical con-
straints on reconfiguration at each stage of motion. Although
offline approaches could be used to check these constraints
efficiently for a number of selected configurations, the less
efficient online approaches must be used in general to handle
any shape or any possible interaction with obstacles.
Geometric constraints result from the fact that modules
need sufficient space to make a planned move but also must
constantly remain in physical contact with other modules [4].
Mechanical constraints, in turn, result from the requirements
of integrity and stability of the entire robot—the structure
cannot break at inter-modular junctions or lose balance during
reconfiguration. The mechanical constraints can be neglected
in some special cases, like reconfiguration under no external
loading (weightlessness) or 2D reconfiguration on flat ground
with perpendicular gravity as the only loading. Otherwise, they
usually need to be considered.
Currently, almost all algorithms for planning and control-
ling self-reconfiguration of densely-packed systems take only
geometric constraints into account, like in [5], [6], [7], [8],
[9]. An up-to-date survey of self-reconfiguration algorithms for
modular robots can be found in [10]. By contrast, works on the
mechanical behavior of densely-packed modular structures are
few, present mostly centralized procedures, and rarely discuss
reconfiguration. Examples can be found in [11], focused on
optimizing the compliance of modular tools, and in [12], aimed
at predicting the mechanical behavior of structures produced
by additive manufacturing. As a separate research direction,
special types of modular structures were investigated in [13],
[14] and [15] to check the possibility of using modular robots
as collective actuators. Autonomous reconfiguration planning
that takes into account both geometric and mechanical con-
straints remains a challenging open problem, some aspects of
which are investigated in this work.
In the present paper, we develop the approach introduced
in [16] much further into a more realistic framework. Arbitrary
3D structures are investigated, for which a linear-elastic FE
model is again adopted, with the addition of unilateral contact
conditions which represent interaction with the surroundings.
Two failure modes are considered: overloading of inter-
modular connections and loss of balance, both checked in
a distributed manner. Two methods of checking the loss of
balance are proposed: (1) a simplified one, valid for structures
standing on a flat surface, and (2) a model-based one, which is
more general but requires solution of the mechanical balance
equations with contact conditions. Verification is performed
in a dedicated simulator VisibleSim [17], as well as exper-
imentally on the real robotic modules Blinky Blocks [18].
The computational cost and several possible extensions of the
applied weighted Jacobi iterative solver are also discussed.
2
II. DI ST RI BUTED PREDICTION OF THE MECHANICAL STAT E
OF A RO BOT
A. Basic characteristics of a modular robot
For the purposes of algorithmic mechanical analysis, a
modular robot will be represented only by its connection
topology and inter-modular connection strengths. We will
focus on structures built of cubic Blinky Blocks [18], which
are arranged on the Cartesian grid. However, in principle, the
proposed approach can handle other connection topologies.
From the information-theoretic point of view, a modular
robot has a distributed and asynchronous computing archi-
tecture, with mesh connection topology corresponding to the
communication network of modules [19]. In Blinky Blocks,
information can only be directly transferred between adjacent
modules, so the communication network has the same con-
nection topology as the modular robot itself. Such neighbor-
to-neighbor-only communication imposes strict limits on the
information-passing abilities of the system and requires special
algorithms for effective prediction of mechanical failures.
B. Problem to be solved
Reconfiguration of a modular robot can be broken down
into simple steps. A single step consists of (a) releasing some
inter-modular connections, (b) shifting modules which have
been freed into nearby positions and, finally, (c) creating new
connections at the new locations. As an alternative way of
restructuring one may consider attaching new modules to a
structure and discarding some old ones. Each reconfiguration
step can potentially cause failure of the structure, which
is in general irreversible. To avoid it, a prior mechanical
analysis can be performed to ascertain that the planned new
configuration is mechanically safe.
We restrict further presentation only to the case of attaching
additional modules to an existing structure. Nevertheless, the
whole idea can also be applied to a complete reconfiguration
step. In such a case, the mechanical analysis would need to be
performed for each of the above stages (a), (b) and (c) of the
planned reconfiguration step. The step would be considered
permissible if the structure was predicted to be safe after each
of the three stages. Since this kind of reconfiguration cannot be
easily performed on Blinky Blocks, we do not further develop
this topic in the present paper.
Algorithmic prediction of two failure modes, shown in
Fig. 1, will be analyzed, expanding on the ideas proposed
in [16]. The first one is when addition of new blocks increases
stresses at some inter-modular junctions beyond the holding
capacity of connectors, as a result of which the structure
breaks, Fig. 1a. The second one is when the structure loses
balance when the new modules are added, Fig. 1b.
C. Overview of model-based failure prediction
We propose a distributed procedure which can predict both
types of failure simultaneously. It approximately solves a
special mechanical problem for the robot with new modules
attached and can be customized to handle different module de-
signs. The procedure has several distinctive features, described
in detail in the succeeding sections:
(a) (b)
Fig. 1. Two types of failure after an additional module (gray dashed line) is
attached: (a) breakage of a connection; (b) loss of overall stability. The red
lines designate failing connections.
The modular robot is represented by a Finite Element
(FE) model; see Sec. II-D.
Two types of connections are assumed: the inter-modular
connection, modeled as a linear-elastic beam, and the
connection between a module and an external support
(e.g., the ground), modeled as a linear-elastic beam with
unilateral no-sliding contact conditions at the support end.
The mechanical state of a planned configuration with
new modules attached (see Sec. II-E) is obtained by
solving a non-linear problem (non-linearities result from
the contact conditions; see Sec. II-F).
The problem is solved in a distributed fashion using the
weighted Jacobi iterative scheme (see Sec. II-G).
After the iterations converge, two failure criteria are
checked: for the loss of balance (see Sec. III-C) and
overloading of connections (see Sec. III-D).
D. Standard 3D frame model
In the adopted FE model, each module pis represented
by a node with 6 degrees of freedom up(3 displacements,
ux,uy,uz, and 3 rotations, τx,τy,τz) and each pair of connected
modules is represented by a beam joining their nodes. A
module in contact with the ground is represented by a special
beam between the module’s node and a “ground” node g, with
ug=0, as it is explained in Sec. II-F. In Fig. 1, the beams are
presented as lines joining the centers of adjacent modules.
In a coordinate system CS whose zaxis points upwards, the
stiffness matrices for a beam joining module pand module q
lying below it read K11
pq =K11 and K12
pq =K12, where:
K11 =E
L3
12Ix0 0 0 6IxL0
0 12Iy0 6IyL0 0
0 0 AL20 0 0
0 6IyL0 4IyL20 0
6IxL0 0 0 4IxL20
0 0 0 0 0 JνL2
,(1)
K12 =E
L3
12Ix0 0 0 6IxL0
012Iy0 6IyL0 0
0 0 AL20 0 0
06IyL0 2IyL20 0
6IxL0 0 0 2IxL20
0 0 0 0 0 JνL2
,(2)
while E,L,A,Ix,Iyand Jνare the elastic modulus, length,
cross-sectional area, area moments of inertia in the xand y
directions, and scaled torsion constant in the zdirection of the
3
symbol value description
E100 MPa elastic modulus
L,A=L240 mm, 40 ×40 mm2length & cross-sectional area
Ix,IyL4/12 mm4xand ymoments of inertia
Jν=J
2(1+ν)2.25 ·L4/41.6 mm4scaled ztorsion constant (J:ztor-
sion constant, ν: Poisson’s ratio)
M0.06106 kg mass of a block
Fmax
V11.98 N strength of a vertical connection
Fmax
L14.97 N strength of a lateral connection
TABLE I
BLINKY BLOCKSGEOMETRIC AND MATERIAL PARAMETERS
beam, respectively (see Tab. I). If the two neighbors pand q
are aligned with the zaxis of another coordinate system CS’,
then K11
pq and K12
pq take the form:
K11
pq =ˆ
RpqK11 ˆ
RT
pq ,K12
pq =ˆ
RpqK12 ˆ
RT
pq ,(3)
where
ˆ
Rpq =Rpq 0
0 Rpq ,(4)
Rpq is the 3 ×3 rotation matrix from CS’ to CS, Tdenotes
a transpose, and block matrix notation is used.
The ensemble is in static equilibrium if, for every module
p, the sums of reaction forces and torques between pand all
its neighbors qare equal to the gravitational force and null
external torque acting on p, respectively, given by the vector
Fext
p= [0,0,99.81 ·M,0,0,0]T. The equilibrium equations are:
p"
q
K11
pqup+K12
pquq#=Fext
p,(5)
with unknown vectors upand uq. After assembling the global
vectors u,Fext and stiffness matrix K, incorporating all degrees
of freedom of the structure, Eq. (5) takes the form Ku =Fext. It
still needs to be extended to account for the modules planned
to be added (Sec. II-E) and contact conditions (Sec. II-F).
The respective quantities for the extended system, called the
perturbed state, will be denoted by symbols with bars.
E. Adding virtual modules
To predict the state of the system one reconfiguration step
ahead, the algorithm must take into account virtual modules
the modules which are planned to be attached. This is done in
a simple way: a system analogous to Eq. (5) is built assuming
that virtual modules are present. During computation, virtual
modules are emulated by their existing neighbors, which store
and process variables and messages related to virtual modules.
F. Contact with external supports
We use a simplified contact model of a cubic module with
an external support, in which the support must be flat and co-
planar with one of the module’s facets. Also, we only analyze
initially existing contact interfaces and assume that no new
ones appear under load. We distinguish two conditions: (i)
a unilateral contact-separation condition (coaxial mode) com-
bined with no-sliding/no-twisting (shear and torsional modes),
(ii) a tilting condition (bending mode). We say that a module
Fig. 2. Regularized contact conditions: (a) a module in contact with the
ground, (b) contact-separation condition for the fzcomponent, and (c) tilting
(stable/unstable) condition for the mycomponent. The dashed lines indicate
the respective “exact” (non-regularized) relationships for the case of a rigid
module in contact with rigid ground.
is in contact if the axial force in the beam representing the
contact is compressive. Otherwise, the module is in separation.
When in contact, the module can only tilt if the bending torque
in the beam exceeds a limit torque which is proportional to
the compressive force (see the explanation below).
Without loss of generality, let us consider a module sup-
ported from below. The module is loaded with the forces and
torques F= [ fx,fy,fz,mx,my,mz]T, which correspond to the
kinematic variables u= [ux,uy,uz,τx,τy,τz]T. The contact con-
dition (see Fig. 2b) takes the form of the Signorini problem:
fz0 & uz0 & fzuz=0.(6)
Additionally, we require that when the contact is active ( fz<
0), there is no tangential slip (ux=0 & uy=0) or twisting
(τz=0), and the module can only tilt if at least one of the
bending torques exceeds a limit torque. The tilting conditions
(see Fig. 2c) can be conveniently written in the following form:
Φx0 & τx·sign(ˆmx)0 & Φx·τx=0,
where ˆmx=mxfy·L/2,Φx=|ˆmx|+fz·L/2; (7)
Φy0 & τy·sign(ˆmy)0 & Φy·τy=0,
where ˆmy=my+fx·L/2,Φy=|ˆmy|+fz·L/2; (8)
where Lis the module’s size. The tilting (bending) condition
expresses the fact that the torques |ˆmx|or |ˆmy|cannot exceed
the torque 9fz·L/2 produced by the compressive force 9fz
about any of the facet’s edges; see also Fig. 2a.
A supported module is modeled as a beam with the same
elastic constants as two connected modules. This gives the
force-displacement relationship for the case of stable contact
(when the module adheres to the support with an entire facet).
This relationship is then used by a special predictor-corrector
scheme, described below, which provides regularization to the
contact conditions given by Eqs. (6–8).
In the regularized contact scheme for module p, a trial state
is computed first, assuming a linear-beam-type connection
with each support q(here, the contact direction is z):
Ftr
pq = [ ftr
x,ftr
y,ftr
z,mtr
x,mtr
y,mtr
z]T=K11
pq ¯
up,(9)
where the term K12
pq ¯
uqis absent because the support is assumed
to be immobile, ¯
uq=0. Then, a corrected vector Fpq is
determined, taking into account two conditions:
4
(i) the unilateral (normal) contact-separation condition com-
bined with no-sliding and no-twisting conditions:
(Fpq :=γ·Ftr
pq for ftr
z0 (separation)
(fx,fy,fz,mz) = ( ftr
x,ftr
y,ftr
z,mtr
z)otherwise (contact)
(10)
(ii) the tilting (bending) condition, used only when f tr
z<0, i.e.
when the module is in contact:
mx:=(mtr
xfor Φtr
x<0 (stable)
γ·mtr
x+¯
γ·L
2ftr
ysign(ˆmtr
x)ftr
zotherwise (tilting)
(11)
my:=(mtr
yfor Φtr
y<0 (stable)
γ·mtr
y¯
γ·L
2ftr
x+signˆmtr
yftr
zotherwise (tilting)
(12)
where ˆmtr
x, ˆmtr
y,Φtr
xand Φtr
yare computed as in Eqs. (7) and
(8), but using the components of Ftr
pq;γ=104and ¯
γ=1γ.
The corrected contact tangent matrix ¯
K11
pq is obtained as a
derivative of Fpq with respect to ¯
up. Again, the matrix ¯
K12
pq is
disregarded because supports are assumed to be immobile.
Remark: Ill-posedness of the problem is avoided by intro-
ducing a very weak spring, characterized by γ, that prevents
free rigid-body motion of the structure. The drawback of
this approach is poor conditioning of the resulting system of
equations when the robot is unstable, which can deteriorate
the convergence rate of the iterative solver; see Sec. II-H.
However, as it is shown in Sec. III-C, the knowledge of which
supports are active suffices for assessing stability and those are
usually identified long before convergence is achieved.
G. Weighted Jacobi solution scheme
The global system of equations for the perturbed system, in
which virtual modules are included in the model (Sec. II-E)
and contact conditions are accounted for by the predictor-
corrector scheme (Sec. II-F), reads:
¯
K¯
u=¯
Fext.(13)
Eq. (13) is solved iteratively using the weighted Jacobi
scheme [16]. A single iteration ii+1 for module preads:
¯
ui+1
p=β¯
Dp
1 ¯
Fext
p¯
Rp¯
ui
p
q
¯
K12
pq ¯
ui
q!+ (1β)¯
ui
p,(14)
where ¯
Dp=diagq¯
K11
pqand ¯
Rp=q¯
K11
pq¯
Dpare the
diagonal and the remainder parts of the respective stiffness
sub-matrices, while β=2/3. Initially, we set ¯
u0=0(alter-
natively, the solution for the non-perturbed state, if available,
could be used instead of 0, which would reduce the necessary
number of iterations). Note that in the iteration i+1 only the
values of ¯
ufrom the iteration iare used, and only those from
pand its direct neighbors q. Thus only local communication
is involved and the memory complexity is constant.
In the present implementation, the weighted Jacobi proce-
dure is initiated by the centroid module which sends an Init
message down the spanning tree, broadcasting the number of
iterations to be done (see Sec. III-B for the centroid module
and the spanning tree). Having received the Init message,
each Blinky Block Bpsends its initial vector ¯
u0
p=0to all
CCCT,CM,CSST,CMST CWJ
Execution time O(d)O(˜
d)O(n2)
No. of CPU operat./module O(1)O(1)O(n2)
Total no. of messages sent O(n)O(n)O(n3)
Memory usage per module O(1)O(1)O(1)
TABLE II
COMPLEXITY ASSESSMENTS FOR SUBROUTINES OF THE ALGORITHM.CC
REFERS TO CENTROID SELECTION,CTTO TRE E CO NST RUC TI ON,
CMTO FIN DI NG TH E CE NTE R OF M ASS ,CSSTT O THE S IM PLI FIE D
STABILITY CHECK,CMSTTO TH E MO DEL -BASED STABILITY CHECK,
AN D CWJ TO T HE WE IG HTE D JACO BI PR OCE DU RE.
its neighbors and initializes its iteration counter, iterp=0.
In a given iteration iterp=i,Bpcan receive from any of its
neighbors Bqa message containing ¯
ui
q(displacements of Bq
calculated in iteration i), which is then stored in Bp’s buffer.
When Bphas received ¯
ui
qfrom all its neighbors, it computes
¯
ui+1
p(see Eq. 14), increments its counter, iterpiterp+1, and
sends ¯
ui+1
pto all its neighbors. The process continues until the
prescribed number of iterations is reached.
Remark. The weighted Jacobi procedure behaves like the
Alpha local synchronizer [20].
H. Convergence properties and possible improvements
The Weighted Jacobi scheme converges if the spectral radius
of the iteration matrix Cβ=Iβ¯
D1¯
Kis less than 1:
ρ(Cβ) = max(|λ1|,...,|λN|)<1,(15)
where λiare the eigenvalues of Cβ. Although in the cases an-
alyzed here convergence is achieved, the number of iterations
is very high, which is a well-known drawback of the method
(the convergence rate tends to 1 when the system grows [21]).
For the one-dimensional spring-in-series system of size
n, we have analytically assessed the number of iterations
necessary to attain an arbitrary relative error to be O(n2). This
is also confirmed numerically in Sec. IV-B for more complex
structures. The assessment shows the low efficiency of the
scheme and underlies the complexities provided in Table II.
The framework presented in this work is not restricted to the
weighted Jacobi scheme though. Its efficiency can potentially
be significantly improved by adapting another method to
solving the considered contact problem in a distributed way.
We will briefly outline the three most promising directions.
Direction 1: The Krylov subspace methods [22] guarantee
that the maximal number of iterations is at most equal to the
number of degrees of freedom of the system, if the problem to
be solved is linear. This can be further improved by appropriate
preconditioning (see our preliminary study [23]). However,
the need for global data aggregation and the non-linearities
introduced by the contact problem require special treatment,
which can deteriorate the time and memory efficiency.
Direction 2: Multigrid techniques [24] can more easily
capture long-wave modes of the solution, which should im-
prove the convergence rate. A special version must be devised,
however, taking into account the contact conditions (like
the one recently proposed [25]) and the specific computing
architecture of the modular robot.
5
create the spanning tree (Sec. III.A)
simplied
stability check?
solve mechanical problem (Sec. II.D-G)
model-based stability check (Sec. III.C)
simplied stability
check (Sec. III.B)
is stable? is stable?
solve mechanical problem (Sec. II.D-G)
overloaded?
(Sec. III.D)
send loss of
balance message
send overload
message
no yes
yes yes
yes
no no
no
Fig. 3. Flowchart of the algorithm.
(a) (b)
Fig. 4. (a) A modular robot (unstable) with a selected centroid (thick blue
line), the center of mass (star) and a spanning tree (arrows). (b) Detection of
whether a given point (star) is inside the convex hull of the support points.
The condition is fulfilled if and only if all the straight angles (shaded) sum
up to the full angle (they do not in this example).
Direction 3: The number of degrees of freedom of the
system can be reduced by applying multi-scale methods or
model order reduction techniques [26], [27]. However, it may
be hard to find a suitable reduced space online efficiently.
III. MECHANICAL STABILITY AND OVERLOAD CHECK
Below we describe computational methods using which
a robot can autonomously predict the two types of failure
shown in Fig. 1, one reconfiguration step ahead. The methods
utilize a spanning tree, which is discussed first in Sec. III-A.
Sec. III-B describes a simple method of checking stability—
without iterations, but restricted to robots standing on flat
ground. Sec. III-C discusses stability verification in the gen-
eral case, using the iterative scheme of Sec. II. Finally, in
Sec. III-D, conditions for inter-modular connection breakage
are presented, utilizing the results of the iterative scheme. The
flowchart of the procedure is shown in Fig. 3.
Assessments of complexities of the subroutines of the al-
gorithm are presented in Table II, where nis the number of
modules, dis the radius of the connection graph of the robot,
and ˜
dis the depth of the spanning tree (usually, ˜
dand dare
of the same order; see Sec. III-A for more details).
A. Spanning tree
A spanning tree allows efficient communication inside the
robot. Its construction begins with a choice of the centroid
module, serving as the root, which is selected near the robot’s
topological center; see e.g. Fig. 4a. This can be done automati-
cally [28], but we chose the centroid manually in all examples.
The tree is extended to all modules, starting at the cen-
troid which sends a Tree message to its neighbours. When a
module receives the Tree message for the first time, it becomes
a next-level node and sends the Tree message further. This
usually leads to the construction of BFS-like trees of quasi-
optimal depth, without the need for synchronization. However,
in rare cases the resulting tree may be far from optimal, with a
Hamiltonian path over the structure as the worst-case scenario.
The algorithms relying on the tree are then adversely affected.
B. Loss of balance in a simplified case
It is assumed that the robot is rigid and stands on flat ground
under vertical gravity (Fig. 4a). The modules know their
own masses and positions in a common Cartesian coordinate
system, with z=0 being the ground level. They also simulate
the behavior of their virtual neighbors (Sec. II-E).
The stability check reduces to verifying that the center of
mass of the robot lies over the convex hull of the points of
support. If it does, the robot is stable, otherwise, it is not. The
algorithm proceeds as follows.
(a) The center of mass of the robot is computed. Starting at the
leaves of the spanning tree, each node sums up the masses
of all its subbranches and its own, mi, and likewise the
weighted centers of mass of all its subbranches and its own,
miXi. The two sums are propagated to the parent node and
the process continues. At the centroid, the center of mass
of the robot is retrieved as [X,Y,Z] = (ΣmiXi)/(Σmi).
(b) Xand Yare broadcast over the tree.
(c) For any supporting module i, its safe angle range [αi,βi]is
determined as the sum of safe angle ranges of its corners.
The 180safe angle range of a corner pj= [Xj,Yj,0]is
swept by the planar vector [XjX,YjY]when turning
90left or right. It covers those directions in which the
structure cannot tilt; see Fig. 4b. The safe angle range of
any corner pj= [Xj,Yj,Zj6=0]is assumed to be empty.
(d) The safe angle ranges are summed up over the tree, just
like masses were in step (a). The summation always gives
a single interval, because all considered ranges are either
empty or not less than the straight angle.
(e) The structure is stable if and only if the aggregated angle
range at the centroid equals the full angle.
C. Loss of balance in the model-based approach
In the general case with arbitrarily placed supports, there is
no simple method to predict stability. Sometimes, if a model
of the robot is simple, like under the rigid-body or elasto-static
assumptions used here, there may even be no unique answer.
Since more accurate modeling goes beyond the scope of the
present paper, we will show how to utilize the proposed elasto-
static model with contact and the iterative solution scheme to
check stability in more general cases. The method, however,
does not assure finding the correct solution in difficult cases
(e.g., when the solution is non-unique by definition).
The method is based on the observation that, when the
iterative solution scheme has converged, the local state of the
contact conditions is fully determined. It is only necessary to
check whether active supports prevent rigid-body rotation of
the structure (at least three noncollinear support points must
be active), which is achieved by the following procedure:
6
(a) Solve the mechanical problem (see Sec. II) and initiate the
stability check (the spanning tree is used again).
(b) For every module, determine the set of its active corner
points by checking the contact conditions (Sec. II-F):
No contact return the empty set.
Tilting in two directions return a single corner—the
common point of the two edges of rotation.
Tilting in one direction return two corners—the end
points of the edge of rotation.
Contact & no tilting return a special ‘stable’ state.
(c) Aggregate information starting at the leaves and moving
up the spanning tree towards the centroid:
At each module, sum the sets of active points of its
subbranches and its own, obtaining set S. By convention,
adding any set to ‘stable’ gives ‘stable’.
If the points in Sare noncollinear then set S=‘stable’.
If multiple points in Sare collinear then leave only two.
Pass Sup the spanning tree.
(d) The stability check ends at the centroid, with the result
being either ‘stable’ or not.
Excluding the expensive phase of determining active sup-
ports (weighted Jacobi iterations), the complexity of the ap-
proach is the same as that of the simplified case; see Table II.
The memory complexity per module is constant because each
returned set of points has at most two elements.
D. Overloading of inter-modular connections
Connection overloading is checked when the iterative
scheme has sufficiently converged and after checking that the
structure is stable. The forces and torques which act between
module pand its neighbor qare predicted as follows:
[fx,fy,fz,mx,my,mz]T=1
2
ˆ
RT
pq(Fpq Fq p) =
=1
2
ˆ
RT
pq(¯
K11
pq ¯
up+¯
K12
pq ¯
uq¯
K11
qp ¯
uq¯
K12
qp ¯
up),(16)
where ˆ
RT
pq rotates the resulting vector into a coordinate system
in which axial forces are aligned with the zaxis.
To avoid connection breakage, the tensile force fzand
torques mxand my, computed in Eq. (16) in the middle of
the connection, must not overpower the magnetic forces Fmax
binding the modules. (Shear and torsion are omitted, because
Blinky Blocks’ connectors are assumed to be resistant to those
modes of breakage.) The vertical and lateral connections of
Blinky Blocks differ, so that Fmax can take two values; see
Table I. The safety condition for both tension and bending is:
Fmax >2·max(|mx|,|my|)/L+fz.(17)
The check is performed for all connections and the results are
aggregated by the centroid over the spanning tree.
IV. IMPLEMENTATION,SIMULATIONS AND EXPERIMENTS
A. Implementation details
The procedures have been implemented and tested in the
integrated environment developed at FEMTO-ST1. It consists
1Programmable Matter project at FEMTO-ST: https://projects.femto-st.fr/
programmable-matter/.
Fig. 5. Blinky Blocks: functional ones (left and right), and two pieces of a
dismantled one with a top view of the motherboard (in the middle).
of the virtual test bed VisibleSim [17] and the reconfigurable
modular robot Blinky Blocks [18], so the same implementation
could be executed on both platforms. The software was
adjusted to be compatible with the real Blinky Blocks Version 1
hardware: reduced floating-point precision of 4 Bytes was used
and the maximum message size was set to 17 Bytes (messages
containing 6 ×4 Byte-long vectors were split in half).
The program’s flowchart is shown in Fig. 3, and the con-
secutive steps of the algorithm are discussed in the previous
sections. The choice between the simplified (Sec. III-B) and
the full (Sec. III-C) stability check to be performed is preset
by the user. In the case of the Blinky Blocks hardware,
a preliminary step is additionally performed, in which the
same main program is loaded into each Blinky Block, and a
common coordinate system for a given configuration of Blinky
Blocks is propagated, starting from a special block with preset
coordinates.
The material parameters of the Blinky Block model de-
scribed in Sec. II-D are provided in Table I. All have been
assessed experimentally, except Young’s modulus which was
chosen arbitrarily (its exact value is not essential for assessing
overload and stability). The dimensions and mass have been
measured directly. Connection strengths have been obtained
in a simplified manner, by finding the maximum number
of Blinky Blocks that their magnets could hold hanging in
a vertical alignment. The top/bottom and lateral connection
strengths differ because the former is produced by a Lego-
like system reinforced with a central magnet, and the latter by
4 magnets placed in the corners of each face; see also Fig. 5.
B. Simulations and experiments
Six different modular configurations and failure scenarios
were investigated (see Fig. 6), both in VisibleSim and on
Blinky Blocks2. The sizes of structures ranged from 8 modules
in test #1(a) to 29 modules in test #6(c). Experiments on
substantially larger structures would be difficult due to the high
computational complexity of the algorithm combined with the
limited processing and communication speed of the Version 1
of Blinky Blocks. In the physical experiments, reconfiguration
of Blinky Blocks was done by manually attaching new modules
to an existing structure. In all the presented cases, the model-
based analysis was involved (addressing both overloading and
loss of stability), which required execution of the weighted
Jacobi iterative scheme. Additionally, in the loss-of-stability
scenarios (Fig. 6 #1-#3), the results obtained with the simpli-
fied and the model-based stability checks were successfully
cross-validated.
2Videos of selected experiments: https://youtu.be/d3aE8GjbYd8
7
Test#1
Test#2
Test#3
Virtual blocks
Global instability detected
7 min / 4000 it.
a) b) c)
22 min / 12000 it.
a) b) c)
Overstressed
Safely stressed
Instability detected
16 min / 9000 it.
a) b) c)
Test#4
Test#5
Test#6
Virtual blocks
Bond breakage detected
Overstressed
Safely stressed
Fixed
a) b) c)
a) b) c)
a) b) c)
9 min / 5000 it.
28 min / 15000 it.
28 min / 15000 it.
Fig. 6. Experiments in VisibleSim and on Blinky Blocks. Computation times and iteration numbers are shown in insets.
Because it is in general difficult to automatically assess the
necessary number of weighted Jacobi iterations, this number
was adjusted manually case by case. The criterion was to
make the number of iterations possibly low while obtaining
correct predictions at the same time. The problem of how the
necessary number of iterations scales with the system size in
a stable case is briefly discussed later. In an unstable case,
this number is expected to be much higher. Following the
conclusion in the final Remark of Sec. II-F, in unstable cases
we stopped computations just after all contact states stabilized,
but before numerical convergence was achieved.
Each of the six tests in Fig. 6 shows the results of execution
of the program for two consecutive construction steps of a
particular Blinky Blocks structure. In every figure, the first
construction step (ab) is designed to be mechanically safe,
while the second one (bc) to result in failure, which is then
demonstrated in the third part of the figure (c). Additionally,
in the top row, VisibleSim results are shown for the tests #1
and #4. The tests are split into loss-of-stability (left column)
and overloading (right column) scenarios. From top to bottom,
the scenarios are ordered by complexity, i.e., 2D cases in
tests #1 & #4, 3D cases in tests #2 & #5, and 3D cases with
more complex connection topologies in tests #3 & #6.
The results of calculations are displayed using colors: the
color of a block corresponds to the highest tensile/bending
stress level in any of its connections, as given by the right
hand side of Eq. (17). Green to orange colors represent the
safe stress range, while red indicates potentially overstressed
connections. Blinky Blocks were programmed to blink in red
when tensile stresses in some of their connections exceeded
the critical level, while global imbalance of a structure was
signaled by the centroid module blinking in purple. Blue
Blinky Blocks are fixed—they are attached to the floor.
In all tests except #6, the predictions are confirmed by phys-
ical experiments. In test #6, breakage is correctly predicted
but ill-localized. This can be observed in Fig. 6-#6(b) which
(a)
size =9
size =10
size =11
size =
12
size =13
size =14
0 20 40 60 80
0.001
0.005
0.010
0.050
0.100
0.500
1
Iteration number 10-3
Relative error
(b)
N(err<1%)
Fit(N(err<1%))
N(err<0.1%)
Fit(N(err<0.1%))
9 10 11 12 13 14
0
10
20
30
40
50
60
70
Number of modules
N. of iterations 10-3
Fig. 7. Number of weighted Jacobi iterations necessary to attain a given
accuracy for fixed-arms of different sizes. Size 10 refers to the example in
Fig. 6-#4(a); other sizes have shorter/longer arms. (a) Convergence character-
istics. (b) A quadratic polynomial fit for two accuracy levels.
indicates breakage of the pillars, while the actual breakage
occurs as it is shown in Fig. 6-#6(c). There are two possible
reasons for the observed discrepancy. The first one is that the
assumed mechanical model of the modular robot is too simple.
The second one is the omission of twisting torques from the
adopted criterion of breakage. It was also very difficult to keep
the structures #6(a) and #6(b) operational—an effect which
was not expected. In both cases, weight-induced deforma-
tions caused separation of electrical connectors, despite the
structures did not break. This necessitated using additional
supports just to perform computations. We view test #6 as
one of benchmark cases for future research on more accurate
models and failure criteria.
CPU time and convergence properties. Computing ¯
ui
pand
exchanging messages with neighbours takes a Blinky Block
a nearly constant time T110.5 ms (9.05 runs per second).
Because communication is local, Tis also the global time
of a single weighted Jacobi iteration, independent of the
configuration. Since the time cost of the other steps of the
algorithm is negligible (see Tab. II), the overall execution time
can be assessed by multiplying the number of iterations by T.
The number of iterations needed to attain a given accuracy
8
greatly depends on the system’s configuration and generally
grows with the number of modules n. Assessment of the num-
ber of iterations is generally not straightforward, even without
considering unilateral contact conditions; see also Sec. II-H. In
Fig. 7 we demonstrate the expected trends for a given family
of configurations. Fig. 7a shows linear convergence of the
relative error k¯
ui¯
uk/k¯
ukas the number of iterations i
grows, where ¯
uis the numerically exact solution. Fig. 7b
presents the necessary numbers of iterations from Fig. 7a for
two example relative errors, displaying quadratic growth with
nand confirming the assessments in Tab. II.
V. C ONCLUSIONS AND FUTURE RESEARCH
We presented a distributed algorithm for checking if a
modular robot will retain its mechanical integrity and stability
after new modules are attached to it at prescribed positions.
The algorithm can be used to assess the mechanical safety
of a reconfiguration step planned by a self-reconfigurable
robot. The procedure is designed to run on the modular robot
itself, and we have verified its predictions through tests in
the dedicated simulator VisibleSim and on the real modular
system Blinky Blocks. To our knowledge, this is the first time
three-dimensional modular-robotic structures compute their
mechanical state in a fully distributed manner.
The algorithm can be improved towards: adopting faster
iterative schemes, as discussed in Sec. II-H and tried in
[23]; extending the application range to soft modular robots;
checking the construction/reconfiguration several steps ahead;
as well as addressing other module geometries and broader
module-support contact scenarios. Future experimental vali-
dation will use the currently produced new version of Blinky
Blocks with faster CPUs and communication, and possibly
quasi-spherical catoms [29] having up to 12 neighbors per
module and electrostatic connectors.
Acknowledgement. This work was partially supported by
the EU Horizon 2020 Marie Sklodowska Curie Individual
Fellowship MOrPhEM (grant No. 800150), by the NCN
Project “Micromechanics of Programmable Matter” (grant No.
2011/03/D/ST8/04089), by the ANR (ANR-16-CE33-0022-
02), the French Investissements d’Avenir program, ISITE-BFC
project (ANR-15-IDEX-03), EIPHI Graduate School (contract
ANR-17-EURE-0002), Mobilitech project and EU Horizon
2020 research and innovation programme (grant No. 811099
TWINNING Project DRIVEN for the Univ. of Luxembourg).
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