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AIM.2017.8014252

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2017 IEEE International Conference on Advanced Intelligent Mechatronics (AIM)
Sheraton Arabella Park Hotel, Munich, Germany, July 3-7, 2017
Fast robot task and path planning based on CAD and vision data*
Christian Friedrich, Akos Csiszar, Armin Lechler, and Alexander Verl

Abstract— Real manufacturing environments are cluttered
spaces, where the environment matches its digital model (the
CAD model) only to a certain degree. Robot systems, which
autonomously execute tasks in these environments should
combine CAD and vision data in order to successfully carry out
their tasks. This way the model data (coming from the CAD
model) can be corrected by the sensor data (coming from
computer vision) to properly reflect the real environment. In this
paper, a novel method for autonomous task-space exploration is
presented, which is based on space exploration and object
recognition. During the exploration of the environment, the
CAD data is used to determine the initial belief of the newly
gained information. After constructing the environment model,
a task planner based on a geometric contact analysis and
symbolic inference computes the necessary manipulation
sequence. To compute the disassembly space, a novel timeefficient algorithm is proposed. The theoretical aspects are
applied to disassembly sequences. A variety of heuristics for
computing a local optimal sequence of the required
manipulation steps and the corresponding paths is also
presented in the paper. To plan the paths well-known global path
planners are applied with a step-size control which reduce the
planning effort. The approaches are experimentally validated
and the results are discussed.
I. INTRODUCTION
Robot systems should use all available information to
autonomously plan and execute their tasks (incl. their paths).
Manufacturing processes like product assembly, disassembly
for recycling tasks or maintenance automation would benefit
from this higher degree of autonomy. Especially in
disassembly or maintenance automation, where CAD data is
available but can differ from the real environment, additional
information improves the planning process. Also, it is
important that the planning time is short enough for the robot
to adapt it to different task settings within an acceptable time
span. This paper proposes a solution for robot systems to plan
in environments with prior uncertainties by combining CAD
and vision data via an autonomous task-exploration approach.
We will also present novel methods, which allow a fast task
and path planning. Through the combination of these
methods, robots will be able to solve complex tasks in future
manufacturing with the focus on maintenance automation.
II. STATE OF THE ART
Today, robot systems for maintenance automation are
mostly used for specific tasks and non-human-friendly
environments like transmission networks [1] or sewer systems
[2]. Also, there are systems known for the use in material flow
*Christian Friedrich, Akos Csiszar, Armin Lechler, and Alexander Verl
are with the Institute of Control Engineering of Tool Machines and
Manufacturing Units (ISW), University Stuttgart, 70174 Stuttgart, Germany.
(phone: 0049-711-685-82402; fax: 0049-711-685-72402; e-mail:
978-1-5090-6000-9/17/$31.00 ©2017 IEEE
systems [3] or process plants [4]. The key focus of this work
is on navigation and tele-operation tasks. Because the
automation of maintenance tasks offers an enormous
economic potential by comparing the life cycle costs of
machine tools [5] (maintenance is the largest, susceptible cost
factor), there has been research recently in regard to novel
manipulation planning approaches [6]. These allow planning
of the different manipulations for solving a task, for example
to exchange a broken component. However, this work
provides no concept to acquire task-based information from
vision, but shows only a solution of how to plan in a symbolic
manner. To execute the symbolic task planning it is also
necessary to find a task-corresponding path and a method for
interfacing the symbolic layer with the robot controller.
A related task like maintenance is (dis-)assembly
automation. Current systems for assembly automation use
CAD data to generate a robot program automatically [7]. The
approach is focused on planning nearly optimal assembly
sequences, which makes it very time-consuming. Due to the
requirements, the system uses no vision data for task planning
and makes the task execution impossible if the CAD differs
from the real environment. In disassembly automation, the
combination of CAD and vision data is very common [8], [9]
because the digital model matches the real product only to a
certain degree. But this work does not present a general
approach to plan the different manipulations nor a method to
explore the task-space autonomously. However, these are
central skills for increasing the autonomy of existing robot
systems.
To extend the autonomy of present systems, it is necessary
to provide novel methods, which allow fast task and path
planning based on CAD and vision data and to combine them
in a general scheme. By solving these problems, future robots
will be able to support humans and to increase the economic
attractiveness of manufacturing in high-wage countries.
III. SYSTEM OVERVIEW
This section explains the robot system architecture in order
to give a general overview of the different functionalities.
Figure 1 summarizes the complete system, which is integrated
into the Robot Operating System (ROS) [10]. The control
architecture interfaces the robot manipulator via position and
velocity set points, which allows a high reusability. For
planning the manipulations, the hybrid manipulation planner
(HMP) uses CAD and vision data. The actual belief about the
environment is handled in the environment model, which is
also responsible for task-space exploration. This way, the
{christian.friedrich, akos.csiszar, armin.lechler, alexander.verl}@isw.unistuttgart.de). Research supported by Graduate School of Excellence advanced
Manufacturing Engineering by German Research Foundation (DFG).
1633
RGBD-data
CAD-data
robot manipulator
hybrid manipulation
planner
set of manipulations
recognized
objects;
voxel map
task-exploration and
environment model
next task point
manipulation sequence
parser
periphery commands
actual
component
state
peripheral devices
vision system
path
query
path planner
sensors
path
task program
tools
controller
control architecture
set points
Figure 1. Overview of the robot system architecture with the main elements. The robot uses a RGBD sensor for visual perception and a 6D-forcetorque sensor for admittance control. To execute different tasks, the robot can use different tools like a gripper or a screwdriver.
robot plans autonomously the best view pose for object
detection and space exploration and merges the results with
the data from CAD. If the degree of belief about the
environment is sufficient, the symbolic plan from the HMP is
sequenced and parsed into a task program. This contains all
required information for task execution using the established
formalism of skill primitives [11] as interface between
planning and execution.
IV. FAST TASK AND PATH PLANNING
between the components must be determined. To solve this
problem, a novel sampling-based approach is considered.
For this, a uniform sampled sphere = { ∈ ℝ3 | ‖‖2 = 1}
is used for the representation of  (all normal vectors on 
represent possible translational disassembly directions).
Further, for every defined ℛ for a component  the
disassembly space , =  (ℱℛ, ), ∀ is computed.
Thus, the resulting disassembly space for  is given by

In this section, different methods for task and path planning
are introduced. At first, the hybrid task planner is described,
which allows the computation of symbolic robot skills further
referred as manipulation primitives. To solve this problem we
introduce a new method to compute a possible disassembly
space. Also, we introduce a novel task-space exploration
approach, which allows the acquisition of information from
the vision system depending on the actual uncertainty. Based
on this, a local optimal sequence can be determined. The last
paragraph of this section will deal with a novel solution for
global path planning. Through the use of an exploration step
size control the path planning time can be drastically reduced
without significantly affecting the quality of the path.
A. Hybrid manipulation planning
The term "hybrid manipulation planning” refers to the
nature of the proposed planning paradigm, which uses
semantic, topological and geometric information. For this, we
use an extended description of the relational assembly model
[12]. This contains, among others, the information about the
symbolic spatial relations ℛ ∈ {, ,
} between two components , which are defined on
a feature geometry ℱℛ ∈ {, , , } with
the feature vector ℛ = [ℛ ℛ ]. Here, ℛ =
[   ] represents the origin and ℛ = [   ]
the normal vector at ℛ . The relations are stored in a
graph ℛ (, ℛ). This information can be derived from a
common CAD model and partly from vision data. For
manipulation planning the available disassembly space 
, = (⋂
 (ℱℛ, )) ∩  .
(1)
=1
The evaluation of (1) is now straightforward. Because all
subspaces are represented by a discrete set, the intersection
set can be computed using sort and compare operations. For
sorting a subset with -elements and a further subset with elements the complexity is ( ∙ ()), with  ≥ . The
compare operation can done in ( + ), confirms the
computational efficiency. Figure 2 explains the method. The
results are stored in a relational graph ℴ , where the
components  represent the vertices, and the edges ℴ ∈
{, , , , , } the degree of freedom in a
symbolic manner, compare [12].
Figure 2. Sampling-based pre-processor: Exemplary computation of the
relative disassembly space for the component 1.
1634
The relational graph can be further used for planning the
different manipulations. For this purpose, so-called
manipulation primitives ℳ are used to describe the skills of
the robot in a symbolic manner. To solve a task, the state
transitions are described by :  ×  → , for a set of
states  ≔ ℴ ( ), ∀ ∈ ℴ , in dependency with an
applied action (here the manipulation primitive) Σ ≔
{ℳ} ∈ {, , , }. So we can write the state
transition with
(2)
ℴ ( ) ≔ (ℴ ( ), ℳ ).
+1




This means that the application of ℳ to a component with
the state ℴ ( ) results in a new state ℴ+1 ( ). The
derivation of the required manipulation primitive can mostly
be done using a rule-based inference approach. For a more
detailed view, please refer to the previous work [6]. The
results are a set of {ℳ}, which is not necessarily executable
due to considering only the relative disassembly space.
Figure 3 shows the planning using an easy example. In the
first step the disassembly space of the base (B1 ) and the block
component (B2 ) is determined (Figure 3, left). The solution of
the disassembly space allows to build the relational graph
after [12] and to create a plan to solve a task. Here specified
with the disassembly of B2 (Figure 3, right).
Figure 3.
voxel  , free voxel  , and unknown voxel  ). Thus, an
∗
optimal view pose for exploration 
provides the
maximum number of unknown voxels, and an optimal view
∗
pose for object recognition 
the maximum number of
objects, which can be seen in one view. Let us define  as an
evaluation function, which returns the amount of unknown
voxels | | and , which returns the number of objects || in
the view frustum of the sensor for a pose . Then, the
combined task-exploration function, which considers space
exploration and object recognition can be written with
 =
∗
∗
(1 ∙ 
+ 2 ∙ 
)
1 + 2
,
(3)
∗
∗
with 
=    () and 
=    (). To
∈
∈
consider dynamically the next-best view, the two metrics
−1
1 = 1 − (| | + | |) ∙ (| | + | | + | |)
for space
exploration
and
2 = 1 − (∑
(
, )) ∙
=1
−1
(∑
=1 , ) , 1 , 2 ∈ ]0, … ,1] for object recognition are
updated in every view pose. As a result, always the view pose
is more considered, for which the task is less fulfilled. The
evaluation is done using a ray-tracing algorithm. In Figure 4
the validation of the algorithm is presented by a simple
example. The robot knows from the prior CAD data that there
are two screws in the assembly group.
Example of the hybrid manipulation planner.
Considering only the relative disassembly space results in
a very good complexity, but has the problem that it is not
ensured that the component is actually manipulable. To solve
this problem a sequencing approach is required, which
considers only visible and accessible components, compare
[6]. In section C we will discuss several sequencing methods.
The next section will explain a new method to explore all
visible components through an autonomous task-space
exploration approach.
B. Autonomous task-space exploration
To gain autonomously visual information from the
environment we propose an approach, which combines space
exploration and object recognition in one suited view pose.
This allows creating an online map, which can also be used
for path planning as well as merging the recognized
components with the CAD data. For this purpose, the initial
belief that a known component from CAD exists is described
by (,0 ). For every view the belief is updated by applying
a probabilistic model ( | ) for an object recognition  .
For map representation, we consider a probabilistic voxel
map ℳ = ⋃
  , with the voxel , which can have the
following three states with  ∈ { ,  ,  } (occupied
Figure 4. Results of task-space exploration with four views: First view
(left) and last view (right) with a plate-screw example (first row);
Corresponding point cloud data (second row); recognized screws (third row);
belief for object recognition (last row, left) and metric for space exploration
(last row, right).
1635
In the first view pose the robot recognizes only screw 2,
because screw 1 is occluded. Through equation (3), the robot
plans a view pose, which allows also the recognition of screw
1. For optimizing equation (3) the twiddle algorithm is used.
The method allows the confirmation of the CAD model and
also the integration of previously unknown components in the
environment model.
C. Task sequencing
Knowing the exact environment model allows the
computation of transition costs between the manipulation
primitives. Having multiple choices for sequencing and being
able to compute the costs of these choices yields an
optimization problem similar to a travelling salesman
problem (TSP). The costs for transitioning between
manipulation primitives are computed as a sum of travelling
time and tool change time (if applicable). Interdependency of
the manipulation primitives is solved by reducing the set of
primitives in a sequence to the independent ones. In the
scientific literature many methods can be found to solve TSPlike problems. Approaches vary from nearest neighbor to A *
with different heuristics. Solving the sequencing problem
using a well suited optimization method guarantees that the
optimal sequence is found.
For this we compare Dijkstra, A* with a nearest neighbor
heuristic (A*-NN), A* with a minimum spanning tree heuristic
(A*-MST) and a classic nearest neighbor approach (NN). All
experiments are repeated 100 times with a randomly created
cost matrix for four, five and six manipulation primitives.
Figure 5 shows the planning time (left) and the
corresponding time costs for the optimized sequence (right).
Using A* with different heuristics guarantees an optimal
solution. The nearest-neighbor approach satisfies a good
planning time through the complexity which is in ( 2 ),
with  = |{ℳ}|, but does not guarantee an optimal
sequence.
However, for a task sequencing problem with eight
manipulation primitives the (time) advantage of finding the
optimal sequence in comparison to the non-optimal solution
from NN does not exist.
Figure 5.
Results task sequencing.
To execute the optimized sequence also path information
must be determined. For this, the next section introduces a
novel method to reduce path planning time, while respecting
also the optimality of the path.
algorithms divided into search-based (Greedy, A*, e.g.) and
sampling-based planning (RRT [13], RRT* [14], e.g.). But
still a big issue is that the planning time depends on the
occupancy of the map. To solve this problem, we propose a
pre-processing step, which allows the adaption of the
exploration step size  , so that less occupied regions can be
explored with a larger step size Δ↑ and more occupied
regions with a smaller step size Δ↓ .
Using the representation from ℳ, compare section B,
the global occupancy of the map can be described by
| |
 =
.
(4)
| | + | | + | |
To consider not only  , the method divides ℳ
iterative in subregions ℛ with  = 1,2, … 8. So ℳ
represents the parent node and ℛ , the corresponding leaf
nodes in an octree . Every leaf node is also described by a
cuboid  =  ×  ×  = [ ,  ] × [ ,  ] ×
[ ,  ]. The iteration is continued until the local
occupancy  meet  (ℛ ) <  . The computation of the
exploration step size can then be basically chosen depending
on the local depth of the subregion in the tree , , so
(5)
 = ( ) = (( )).
,
,

The experimental results in Figure 6 explain the method.
The exploration step size is here computed linearly
with , = 0.5 ∙ , −1 respecting the actual depth of the
tree  ∈ ℕ+ and ,0 =  . As path planning algorithms
we use Greedy, A* and also two sampling-based methods with
RRT and bi-RRT.
The experiments are repeated five times for the searchbased and thirty times for the sampling-based planners. To
compare the adaptive exploration step size  ∈
[ , … ,  ] in contrast to the state of the art, the
experiments are also done with a constant minimal  and
maximal step size  . The performance is evaluated due
different metrics, similar to [15] with:
 path planning time  in [s],
 collision detection time  in [s],
 path length || in [m],
 deviation optimal path | ∗ |,
 path smoothness  in [rad],
 number of explored vertices ||,
 success rate .
Table I stated out the results for the scenario in Figure 6.
Also the number of explored nodes, the path planning time
and the deviation from the optimal path is shown in Figure 6,
using a Box-Plot representation to show the statistic
properties for the sampling-based planners.
TABLE I.
D. Global path planning with exploration step size
control
The problem of path planning in robotics has been discussed
in detail, and there exist many well-known planning
1636
Results path planning for the example in Figure 6.
Greedy
()
Greedy
( )
Greedy
()
A*
()
A*
( )
A*
()
RRT
()
RRT
( )
RRT
()
bi-RRT
()
bi RRT
( )
bi-RRT
()
  []
0.067
0.082
0.011
3.267
0.077
0.029
3.801
3.632
3.903
0.135
0.075
0.026
  []
0.241
0.657
0.054
13.34
0.538
0.277
11.49
9.941
9.669
1.667
0.819
0.288
||  []
0.872
1.140
0.829
0.896
0.844
0.829
1.515
1.639
1.569
1.228
1.269
1.196
|∗| ∈ ℝ+\1
1.209
1.582
1.150
1.243
1.171
1.150
2.101
2.273
2.176
1.703
1.761
1.658
  []
0.347
2.009
0.468
0.299
0.253
0.468
0.839
1.049
1.452
0.943
1.142
1.440
|| ∈ ℕ+
336
330
106
3985
359
143
991
747
742
117
62
23
 ∈ [0 … 1]
1
1
1
1
1
1
1
1
1
1
1
1
skill primitives [11]. This formalism includes all required
information for describing elementary hybrid robot
movements (motions and forces). But, there is no interpreter,
which could convey the information described this way to a
computational system. Defining a domain-specific language
(DSL) based on the established formalism closes the gap
between symbolic planning and execution. The programming
language obtained this way reflects the mathematical
formalism already used, but in a form, which is interpretable
by robot controllers. An application program for the
disassembly tasks will be generated by the planner in this
language. It closely reflects the mathematical formalism and
it also allows the definition of symbols as variables, which
can be instantiated with values coming from the environment
model. Current robot application programming languages
(e.g. Kuka KRL, ABB Rapid or C++ for ROS) are of lower
level than required and instead of describing skills, they
describe robot actions (logic and motion). The new DSL is
capable of describing skills used for solving a task including
the required controller (e.g. position, force, and visual
servoing) for the skill via the combination of manipulation
and skill primitives. For example, instead of specifying pointto-point or linear motions, it specifies the (∙) primitive
used for loosening a screw. DSLs are defined using a syntax
description language like the extended Backus-Naur Form
(EBNF). To interpret the DSL an interpreter generator (like
ANTLR) is used for the introduced language. This way, based
on mathematical formalism an interpreter is obtained, which
is compatible with the operating system used in the robot
(ROS).
VI. CONCLUSION AND FUTURE RESEARCH
Figure 6. Results of path planning using the exploration step size control:
Adaptive space division (first row, left); search space using A* (first row,
right); sampling space using RRT (second row, left); pre-processed paths
using A* with different step sizes (second row, right); pre-processed paths
using RRT with different step sizes (third row, left); amount of explored
nodes (third row, right); path planning time (last row, left); deviation from
optimal path (last row, right).
For search-based planning it can be seen clearly that the
number of explored nodes can be reduced massively, resulting
in a small planning time. Also for sampling-based planners,
the control of the exploration step size achieves
computational advantages. The deviation from the optimal
path (here the shortest path) is also negligible in contrast to
the smallest step size. By controlling the exploration step size,
the planner can automatically adapt to the local conditions of
the map, whereby a smaller search space is reached.
V. INTERFACING TASK AND PATH PLANNING
In the last contributing section, we propose an interface
between the task and the path planning unit. In the state of the
art an elementary formalism has been established to describe
This work presents an overall concept and novel methods,
which allows fast task and path planning for robots, also in
environments with uncertainty in the a priori data. Through
task-space exploration, the robot can detect autonomously the
environment with a purely information gain-controlled
approach. The hybrid manipulation planner provides a
powerful concept for solving different tasks in the domain of
disassembly or maintenance automation. With the concept of
the exploration step size control, a general method is
available, which massively reduces the planning time. In
future, we will focus on task execution and hybrid control of
the different manipulations.
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