Stable stacking for the distributor's pallet packing problem

Abstract

We present a novel algorithm that solves the distributor's pallet packing problem. In contrast to existing algorithms, our method optimizes stack stability in addition to stack volume. Furthermore, our algorithm explicitly handles cases where the construction of homogeneous layers of packages with equal height is impossible due to differences in package heights and quantities. The algorithm is a nested beam search that separately optimizes local and global evaluation criteria. We show successful results on both real world and synthetic data sets, compare our performance to an existing algorithm and demonstrate experimental applications in simulation and on a real palletizing robot.

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Stable Stacking for the Distributor’s Pallet Packing Problem
Martin SchusterRichard Bormann Daniela SteidlSaul Reynolds-HaertleMike Stilman
Abstract—We present a novel algorithm that solves the
distributor’s pallet packing problem. In contrast to existing
algorithms, our method optimizes stack stability in addition
to stack volume. Furthermore, our algorithm explicitly handles
cases where the construction of homogeneous layers of packages
with equal height is impossible due to differences in package
heights and quantities. The algorithm is a nested beam search
that separately optimizes local and global evaluation criteria.
We show successful results on both real world and synthetic
data sets, compare our performance to an existing algorithm
and demonstrate experimental applications in simulation and
on a real palletizing robot.
I. INTRODUCTION
The problem of mixed palletizing plays an important role
in the distribution industry. In grocery, beverage distribution
centers and parcel services, boxes of different shapes are
packed onto pallets. In order to minimize delivery costs, each
pallet must be packed efficiently with respect to maximal
volume utilization and inter-layer support, making it stable
for safe transport. This paper addresses the distributor’s pallet
packing problem, where an order contains n different types
of rectangularly shaped boxes of known dimensions li×wi×
hifor i = 1..n. The task is to pack a given number niof
each type of box on a pallet of a given size L × W × H.
Finding an optimal solution to this problem with respect to
maximal volume utilization was proven by [1] to be NP-hard.
The main objective of previous research was the max-
imization of volume utilization on the pallet. Instead, our
algorithm focuses on maintaining stability in addition to
volume utilization. We accomplish this by using a nested
beam search. An outer search optimizes the global density
and stability of the pallet, while an inner search finds the
best local position for the next box. In contrast to previous
work, our algorithm does not create the solution layer-wise.
This allows us to construct a pallet using a large number of
different box types even when building layers is not possible.
Figure 1 shows a plan generated by our algorithm on real
world data from a beverage distribution center. Notice that
layers differ in height over the area of the panel.
In section III, we present the assumptions made by our
algorithm and describe our method in detail. In section IV,
results from running the algorithm on both synthetic and real
data are shown and compared to the planner presented in
[2]. To prove that the algorithm is applicable to problems in
the real world, we used data from an American wholesaler
and a beverage distributor. In section V, the algorithm is
TheauthorsareaffiliatedwithRobotics andIntelligent
Machines (RIM) at the Georgia Institute of Technology, Atlanta,
Georgia30332, USA.Email:
martin.schuster@gatech.edu,
richard.bormann@gatech.edu,
saulrh@gatech.edu, mstilman@cc.gatech.edu
steidl@gatech.edu,
Fig. 1. Simulated result generated by our algorithm for a package list from
a beverage distribution center. The pallet is shown from two points of view.
Fig. 2.
GT/KUKA palletizing cell used for the ICRA-2010 VMAC competition.
Algorithm execution in the URSARSim simulator and on the
evaluated and section VII discusses strength and weaknesses
of our approach. Furthermore, we demonstrate an application
of our method in simulation and on a real palletizing robot
as shown in figure 2.
II. RELATED WORK
The problem of palletizing is divided into several categories.
The manufacturer’s pallet loading problem is the simplest
form of palletizing. The task is to find a loading pattern for
identical boxes for each layer of the pallet [3]. More complex
formulations are known as the distributor’s pallet packing
problem and the multi-pallet loading problem, which both
pack non-homogeneous items onto one or several pallets,
respectively [4], [5], [6]. This paper addresses the distribu-
tor’s pallet packing problem. In contrast to previous work,
we focus on the stability of the stack in addition to density.
Previous work considers stability mainly for the manufac-
turer’s pallet loading problem. However, the manufacturer’s
pallet loading problem only contains identical boxes as
shown in [3] and [7]. Previous solutions to the distributor’s
pallet packing problem as [4] take non-identical parcels into
account, but pack them only considering the volume utiliza-
tion and weight constraints [8], not considering stability as
a main optimization criterion. In contrast to previous work,
our algorithm also focuses on the stability of the pallet while
packing non-identical boxes.
IEEE/RSJ International Conference on Intelligent Robots and Systems
(IROS'10) Oct. 2010

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In [3] and [7] solutions to the manufacturer’s pallet loading
problem are presented which create layers of homogeneous
items. To maximize the stability of the pallet, adjacent layers
are constructed with different loading patterns. [7] defines
two criteria for stability, which we also take into account:
each box must be supported by two boxes below and a certain
percentage of the base must be in contact with boxes below.
[6] gives a solution to the multi-pallet loading problem.
For each pallet, it calculates a sub-contingent of the total
package list and builds the solution layer-wise. It therefore
prefers building homogeneous layers. If this is not possi-
ble, heuristics for creating layers with uneven surfaces are
provided. Although a stability criterion is mentioned in this
paper, the results do not show stable pallets where each box
is supported at least by two boxes below.
The distributor’s pallet packing problem is addressed in
[4], [5] and [2]. They solve the problem for an arbitrary
number of box types with the objective to maximize volume
utilization of the pallet. The algorithms may coincidentally
create interlocks between layers but do not explicitly enforce
stability for the entire pallet. [2] constructs layers and sub-
layers using a heuristic algorithm, that aims to imitate human
packing strategies. They solely optimize pallet density, [5]
taking additional weight considerations into account.
More advanced solutions are discussed in [9] and [10].
These describe a robot picking one out of six available
boxes, packing two pallets at the same time. Unfortunately,
no details about these algorithms are released.
III. METHODS
In this section we describe the details of our algorithm. The
main objective is to maximize the stability and density of
the entire stack. Therefore we designed a nested beam search
with an outer, global search presented in section III-G and
an inner, local search explained in section III-E. The stack
is constructed sequentially; adding a new box creates a new
search state. The global search criteria aim for stack density
and global stability. The local search guarantees additional
constraints and local stability. After presenting the underlying
assumptions, the state representation, and the constraints on
valid box placements, we will explain the search in detail.
A. Assumptions and Specifications
We define the distributor’s three-dimensional pallet-packing
problem with the following assumptions:
• Packages are right cuboids with real valued width,
depth, height and weight.
• Each package has two distinct orientations: 0° and 90°
in the horizontal plane.
• Both the pallet and the packages have constraints on the
total weight that each can support.
• Weight is uniformly distributed over the area supporting
a package.
• There is a finite, initially known number of packages of
each type.
• The algorithm can choose the order of the packages.
We do not orient each box in all six different rotations as
done in [2] since the height is usually the shortest dimension
Fig. 3. Feature maps for two different states: Height, weight and dropindex
map from state representation and drop map for dropping a red box. Colors
range from white to black, indicating the maximum value in the map in
white and the lowest value in black.
in real applications and most palletizing environments are
designed for pick and place operations where robots grasp
each package on the top surface and place it on its bottom
surface, e.g. using vacuum grippers. We do not consider un-
certainty in the final box placements due to robot inaccuracy.
However, in section VI we use a post processing algorithm
to calculate approach paths for a robot arm that do not rely
on complete accuracy of previous placements.
B. State Representation
In order to facilitate local analysis of the continuous space of
object placements we discretize the problem in the horizontal
plane. Each search state consists of three feature maps
represented by 2D matrices. Each matrix assigns a single
number to every cell in a top-down view of the pallet.
The values stored in the feature maps represent:
• height map: height of the stack
• weight map: maximum weight that can be added
• drop index map: index of the topmost box
Examples of maps for two distinct intermediate states can
be seen in column 2-4 of figure 3.
We generate a new state by adding a box to the last state
and incrementally updating the feature maps for the state.
C. Constraints on Placement
In this section we define hard constraints on the valid
placement of a package, referred to as a drop. A package
can be placed at a specific position on the pallet if all of the
following conditions hold true:
• Box placement does not exceed the pallet dimensions.
• Weight of the box does not exceed the maximum weight
which can be supported by the boxes underneath.
• Adding the weight of the box does not exceed the
maximum weight of the pallet.
• Resulting height of the pallet, after placing the box, does
not exceed the maximum height of the pallet.
• At least two opposite edges of the base are supported
from below and the box is placed horizontally, i.e. such
that the base is parallel to the pallet.

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Fig. 4.
local search
Planning algorithm overview: combination of global and nested
Our weight constraints are similar to the hard constraints
defined in [8]. Additional application-specific constraints like
the availability of a valid approach path for a robot arm can
be enforced as hard constraints for a drop when necessary.
D. Search
Having explained the underlying assumptions about the
problem, the state representation, and the definition of valid
placements, we will now show how the nested beam search
optimizes the stability of the stack. The search starts with an
empty pallet and then drops packages from above. In order
to generate children for each selected state in this search tree,
the feature maps for that state are used to search locally for
a fixed number of drops. These drops are evaluated only in
the context of their local neighborhood, using the criteria
explained in section III-F.
We employ a beam search as the outer search over the state
space which maintains a constant number of possible stacks.
For these stacks we optimize the global evaluation criteria
presented in section III-H. Those criteria take the entire pallet
into account. The process of selecting and expanding local
nodes, referred to as one iteration of the global search, is
repeated until the algorithm has either placed all available
boxes or is unable to place any more packages due to
placement constraints. Therefore the maximum search depth
of the algorithm corresponds to the maximum number of
packages in a search path.
An overview of this algorithm is visualized in figure 4.
E. Local Search
We now present our local search and show that it satisfies the
constraints described in section III-C and optimizes for local
stability. In order to find suitable candidate drops according
to these criteria we evaluate a dropmap for each package
type and orientation in each state. A dropmap is a two-
dimensional grid which contains a score value in each cell
indicating the utility of dropping the center of the box at that
position. This score is calculated as given in section III-F. It
also indicates when a drop at this location is impossible due
to height, weight or stability constraints. Improper drops are
discarded by our algorithm, so that the imposed constraints
are enforced. A constant number klocal of the best drops
of each of these maps are then returned as candidate drops
to create a pool of child states for the beam search. These
drops are characterized as maxima in the dropmaps. In order
to explore the search space in different directions we want to
create significantly different candidate drops in one iteration
of search. We ensure that by decreasing the values in the
dropmaps in the vicinity of a chosen drop. This guarantees
that the same or slightly different placements are not selected
again in the same iteration due to their low score value.
F. Local Evaluation Criteria
We evaluate each possible drop position for each package
according to the following criteria:
• base supports: # of boxes that support the base of a box
• sides supported: # of sides supported by other boxes
• surface support area: percentage of the area of side
and base surfaces touching other boxes or the pallet
bounding box
• base height:
pallet height
• volume: the volume of the box divided by the volume
of the largest box still available for placement
All of these values are normalized to an interval between
0 and 1. For base supports we only consider supporting
boxes as stable support if they touch the dropped parcel for
at least 25% of its base area. Consequently, base supports is
normalized by division of 4, which is the maximum amount
of supporting boxes. A weighted sum of all criteria defines
the score in the dropmap. We empirically determined that
setting all weights to 1 was the most successful configuration.
The base supports criterion leads to interlocks between
packages which increases the stack stability, while the other
criteria are designed to ensure a dense stack. The volume
criterion prefers the placement of large boxes at the bottom
of the stack. The base height criterion is intended to prefer
stacking at low heights first and filling gaps.
The fifth column in figure 3 shows two examples of drop
maps. For each cell, they visualize the value (white: best
score, black: no drop possible) of dropping a red package
centered on that cell. The lower image indicates that it can
be placed on top of the middle of the other red packages.
pallet height−box stacking height
G. Global Search
In this section we present the method by which global search
optimizes for a stable and dense stack. The outer search
keeps a list of a constant number of states in memory,
later referred to as kglobal. In each iteration, we generate
children for each state from the klocalcandidate drops that
are calculated by the local search as explained in section III-
E. From these kglobal× klocalchild states, we select kglobal
new states to explore in the next iteration. Figure 5 depicts

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Fig. 5.Beam search schematic
this process. We additionally check for duplicate states using
a hashmap and remove them prior to selection. The selection
takes the kglobalbest states according to the global evaluation
criteria defined in section III-H. The global search iterates
until either all available packages are placed or the algorithm
cannot find a valid placement for any remaining package
according to section III-C.
H. Global Evaluation Criteria
We calculate the scores used in the beam search selection of
section III-G as a weighted sum of the following two values,
which cause the algorithm to prefer dense and stable stacks:
• Stack density:
Volume of bounding box of stack
• Average box support: Average number of packages that
each package is placed upon.
The latter criterion favors plans with high interlocks. Taking
the average might lead to a high variance, however our local
criteria prevent this by choosing plans with high values for
box supports in each optimization step.
In our experiments, we used the following weights:
Volume of stack
scoreglobal= stack density + avg box support/2.5 (1)
The division helps to normalize the average box support, as
high values lie around 2.5 and both factors are intended to be
weighted equally. The chosen criteria ensure that the most
stable solutions proposed by the local search are selected
globally with the goal of yielding a stable stack which is
densely packed.
I. Iterations
As shown in the previous sections, our algorithm relies on
the choice of klocaland kglobal. Empirical evaluation on our
data set Set-S1, which we will introduce in section IV-C,
revealed that results were best when k = klocal= kglobal.
However, we could not determine a clearly preferable value
for k in the range between 1 and 25. Therefore we run the
algorithm for k = 1 to kmaxin increments of 5 and select
the best result.
IV. EXPERIMENTS
We evaluated our algorithm on two real world and two
synthetically created data sets. In this section, we present
the experimental setup, describe our evaluation criteria and
give details about the data sets. We then discuss our findings
in section V. To our best knowledge, we are the first to
evaluate an algorithm in this domain both on real wold and
synthetically created data sets.
A. Setup
The input to our algorithm is a description of the pallet
and a list of package types. The pallet description contains
its width, depth, maximum height and maximum weight.
Each entry in the package list consists of width, depth,
height, weight, maximum weight on top and the number
of packages of that type. The output is an ordered list of
packages and their positions, (X,Y,Z)-coordinates relative
to the pallet.[11]
For all experiments we used a planning resolution of 1cm
in the horizontal plane and a height tolerance of 1cm because
we expect this to be in the range of inaccuracies in robot
movements and box dimensions. A higher resolution might
reduce gaps between boxes but requires a longer runtime
for the algorithm. Unless noted otherwise, experiments were
performed on the widely used Europool pallet [12] with a
base area of 120cm×80cm and a maximum height of 150cm.
The actual size of the pallet is not important. Our algorithm
only depends on the ratio of pallet to the package dimensions,
which is diverse over the test cases.
B. Evaluation Criteria
We consider five criteria to evaluate the results of our
experiments: number of boxes stacked, the stack density
and average box support as defined in section III-H, the
average face contacts and the total algorithm runtime over all
iterations as described in section III-I. Average face contacts
is defined as the average number of packages each package
is touching on all faces.
The stack density criterion indicates the volume utilization
of the pallet. We use the average box support and average
face contacts criteria to evaluate stack stability.
C. Synthetic Data Sets
We tested our algorithm on two different synthetically gen-
erated data sets. The first, Set-S1, consists of 10 synthetically
created package lists. We defined packages with random
dimensions from 10cm to 55cm. We ran our algorithm for
six iterations with kmax= 25.
The second artificial data set, Set-S2, consists of five
randomly created package lists. It was presented in [2] as
“Set#1-5” for benchmarking their layer-wise planner. We
used the same pallet size of 104cm × 96cm × 84cm, the
package dimensions range from 1cm to 67cm. As one list
contains more than 1000 packages, we ran our algorithm
only with kmax= 5.
Visualization for stacks generated from package lists from
Set-S1 and Set-S2 are shown in figure 6.
D. Industrial Data Sets
Next, we performed tests on real world data sets from two
major American companies. Set-R1 was obtained from a
beverage distributor and contains 278 package lists. The
package dimensions range from 2cm to 49cm, the number
of package types from 2 to 20. Set-R2 consists of 8 package
lists from a large wholesale distribution center with 10 to
38 package types having dimensions from 3cm to 51cm. A

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TABLE I
AVERAGE RESULTS AND STANDARD DEVIATION (STD) FOR SYNTHETIC
DATASETS Set-S1, Set-S2 AND REAL WORLD DATASETS Set-R1, Set-R2
FOR OUR ALGORITHM AND BALTACIOGLU [2]
package lists
Set-S1
std
Set-S1 [2]
std
Set-S2
std
Set-S2 [2]
std
Set-R1
std
Set-R1 [2]
std
Set-R2
std
Set-R2 [2]
std
# package types
5.30
1.34
5.30
1.34
14.60
11.26
14.60
11.26
9.17
5.12
9.17
5.12
26.13
10.82
26.13
10.82
# packages stacked
42.1
13.52
59.6
20.49
584
503
839
741
44.10
17.87
59.72
38.40
35.38
16.14
35.38
16.14
stack density
77%
7.36%
90%
7.29%
89%
4.53%
89%
11.74%
70%
7.94%
85%
13.40%
68%
4.01%
56%
9.10%
avg. box support
1.82
0.31
1.04
0.04
1.92
0.54
1.14
0.22
1.54
0.25
1.07
0.08
1.44
0.17
1.03
0.08
avg. face contacts
6.56
0.44
6.27
0.84
8.88
2.32
6.30
2.21
5.84
0.93
5.94
1.90
6.34
0.98
5.18
1.15
runtime in s
681s
189s
2.3s
6.2s
71s
56s
14s
17s
196s
125s
0.4s
0.6s
460s
283s
0.1s
0.4s
max. runtime in s
940s
20s
139s
41s
537s
6s
854s
1s
Fig. 6.Plans for list from Set-S1 (left) and Set-S2 (right)
high percentage of the package types, 57% for Set-R1 and
98% for Set-R2, have at most four boxes per pallet.
For Set-R1 we ran our algorithm for three iterations up
to a maximum number of kmax = 10 search-nodes per
level. For the smaller Set-R2 we used four iterations up to
kmax = 15. We present the average results in table I and
show visualizations for our plans in figure 7 and figure 8.
V. DISCUSSION
A. Quality
1) Synthetic Data Sets: On the generated data set Set-S1
our algorithm achieved an average density of 77%, having a
mean of 1.82 packages supporting each one from underneath.
These high values were reached because the package lists
contained only 4 to 9 different package types. This simplified
the generation of sub-stacks with similar height onto which
further boxes can be placed.
For the second synthetic data set, Set-S2, our algorithm
was able to achieve a volume utilization of 89% on average.
Fig. 7.Plans for two package lists from Set-R1
Fig. 8.Plan for one package list from Set-R2, shown from both sides
Such high density values can be achieved due to the avail-
ability of unrealistic small packages that allow the planners
to fill gaps.
2) Industrial Data Sets: The greater variation in real
world package dimensions, especially heights, makes the task
more difficult than handling synthetic data. This results in
lower average values for density and box support, presented
in table I. Our algorithm is still able to build dense and stable
stacks by combining a large number of different package
types as shown in figures 1, 7 and 8.
3) Comparison to Baltacioglu [2]: We performed a direct
comparison with the planner presented in [2], using all of our
data sets and evaluation criteria. The results are presented in
table I. Our higher values for average box support indicate
the stability of our stacks. Values greater than 1.5 indicate
that at least every other package is stabilized by two packages
below. The resulting interlocks ensure that packages do not
fall off the stack when the pallet is moved or lifted. In
contrast to that, the values for [2] are below 1.15 for all
test cases. This results in pallets that are not as stable or
safe delivery because most packages are only supported by
one other package from below, as can be seen in the visual
comparison in figure 9. Although there is a visible tradeoff of
density vs. stability, our density values are equal to or higher
than [2] for two out of four test sets. The lower variance in
stack density also shows that we constantly produce a dense
stack while maintaining stability.
4) Challenges: During development, we observed two
significant challenges. First we aim to maximize the stability
of the stack by placing boxes directly next to each other
without any gaps. However this may result in gaps near the
opposite corner of the first package placement. If those gaps

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Fig. 9. Comparison: One package list from set Set-R1: Our algorithm (left)
and Baltacioglu [2] (right)
are created in the lower part of the stack, they steadily reduce
the available area as the height of stack increases.
In some cases, our algorithm produces problematic high
stacks of packages of a single type. For instance, consider
the right image in figure 7. We plan to introduce additional
constraints on placement to directly address this problem.
B. Runtime
The maximum runtime of our algorithm depends on the dis-
cretization resolution, the pallet size, the number of package
types and kmax, which gives the number of iterations. By
updating the feature and drop maps only in the vicinity of
the last drop, we gained significant performance increases
compared to a recalculation of the entire map.
Table I shows the runtimes for all test sets. Every plan
presented took less than 10min to generate on a standard
laptop with an Intel Core 2 Duo 2.5 Ghz. Although our
algorithm is significantly slower than [2], it is fast enough
for real world applications. The algorithm is also well suited
for future parallelization because the generation of each state
is independent of other states at the same level.
VI. APPLICATION
We performed preliminary tests in simulation and on a real
robot. Both tests show that the output of our algorithm can
be used to build a stable pallet with a real robot arm, as
shown in figure 2.
A. Execution in Simulation
For simulation we used the USARSim simulator (Unified
System for Automation and Robot Simulation) which is a
high-fidelity dynamic simulation of robots and environments
based on the Unreal Tournament game engine. In order to
handle uncertainty of robot placements, we used a post-
processing algorithm to calculate approach paths for placing
a package onto the pallet. Based on these, the robot arm
approached each package placement position with a small
lateral velocity in both x and y. This ensured that small
errors in previous package placements are corrected by the
current package.
B. Execution on Palletizing Robot
In addition to simulation in USARSim, the algorithm was
successfully executed on the GT/KUKA palletizing cell used
for the ICRA-2010 VMAC competition. The cell included a
conveyor, a vacuum-gripper and a pallet. The execution on
a robot arm shows that our algorithm is applicable to real
palletizing systems.
VII. CONCLUSIONS AND FUTURE WORK
Our new method is a promising approach to solving the dis-
tributor’s pallet packing problem while maintaining stability
of the stack. We showed how the inner search guarantees
constraints and ensures local stability. By evaluating global
criteria, the outer search aims for density and stability of
the entire stack. We showed the success of our algorithm
by evaluating it on real-world data, comparing it to another
planner and executing plans in simulation and on a real robot.
The performance of our algorithm depends heavily on the
definition of appropriate local and global evaluation criteria
for the searches. A thorough analysis of the effect of single
criteria as well as a search for additional criteria to cope with
particular problematic cases is subject to future research. In
real applications, our planner could be used in combination
with a layer-wise algorithm, handling only the part of the
pallet the layer-wise planner cannot generate layers for.
All the data sets used in this work are available at [11].
We encourage future researchers to compete with our results
and share the details of their algorithms to further improve
the performance and efficiency of solutions to the practical
challenge of mixed palletizing.
VIII. ACKNOWLEDGMENTS
We thank Henrik Christensen for his support in defining the
goals and gathering data for this research. This work was
initiated in the course Robot Intelligence: Planning at the
Georgia Institute of Technology.
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