Research and Application of General Collision Detection Simulation Platform
L. Zeng1, T. Ning1, P. Xi1
1
School of Mechanical Engineering and Automation, Beihang University, Beijing 100191
(lzeng2012@sina.cn)
Abstract - System architecture of simulation platform
based on assembly model with constrained motions is
presented. By using accurate collision detecting algorithm
library, collision detection function is implemented in the
simulation platform. Control mechanism of the simulation
platform is illustrated. To verify the practicality of the
system and validity of the algorithm, a three dimensional
numerical control tube bender simulation system is built
based on this platform.
Keywords - assembly model, collision detection,
constrained motion, pipe bending machine, simulation
I. INTRODUCTION
Collision detection simulation system has a wide
range of practical applications, for example, there are
great demands for it in engineering fields, such as
assembly process simulation, motion simulation of CNC
machining, scheduling of transport equipment in
warehouse, port constructing simulation, cargo handling
management and so on [1-4]. The main research of this
paper is about the collision detection simulation system
for mechanical equipment. The related softwares include
Dassault Systèmes’s VirtualNC, CGTech’s VeriCut,
etc. Existing simulation systems generally use OpenGL as
internal graphics platform, models that can be imported
include 3DS [5], VRML and so on. Chen [6] uses Solid
works to establish simulation system directly.
SimMechanics[7] realizes the export of assembly model of
commonly used CAD software by plug-in method, it can
also be exported through the API function provided by
SimMechanics. Yang [8] uses XML technology to store
assembly model information, which facilitates the followup data retrieval and application. Literature [5] uses
sample-point method to implement collision detection.
Literature [4] adopts bounding box subdivision to deal
with collision detection. Ali[9] uses linear programming to
implement real-time collision detection of convex
polyhedral in translation and rotation conditions. Zheng[10]
implements collision detection of assembly model by fast
detection method based on hierarchical subdivision.
Miguel[11] uses double-layer multi-resolution to improve
detection efficiency, and OBB-tree[12] is a common
method. However, existing collision detection simulation
systems have the following problems:
 VirtualNC and other software are basically for
CNC machine tool, but can’t implement the simulation of
other types of mechanical equipment (such as CNC pipe
bending machine).
 In assembly modeling, some simulation systems
depend on the specific CAD software, or lack the
necessary assembly relations, assembly constraints and
some other information, which would hinder the
subsequent simulation application.
 In the collision detection algorithm, the current
literatures generally adopt discrete method based on
bounding box, which is hard to ensure the detection
precision and speed.
To improve the practicality and usability of the
simulation platform, this paper derived assembly
relations, constraint relations and parts geometric data
based on the assembly model of CAD software. Related
assembly information is recorded in text file, and parts
geometric information is stored in file similar to STL.
Objects defined in simulation platform correspond to
these defined in CAD system’s assembly models, which
would facilitate data transformation and motion constraint
relations building. The G-code is used to define the
system’s trajectory and control equipments’ running
status. To improve the precision and efficiency of
collision detection, we developed our own exact
algorithm library, based on which we implemented
collision detection in simulation platform.
The rest of this paper is organized as follows. In the
methodology, Section A introduces the relations and
definitions of three kinds of objects in assembly model.
Section B discusses kinematics constraint object based on
the degrees of freedom of assembly model and the driven
approach. Section C introduces the interface design of
collision detection algorithm library. Section D gives the
process analysis of model simulation. The results analyze
the collision detection algorithm library and the test
results of assembly model simulation. The last section
summarizes the research contents of this paper.
II. METHODOLOGY
A. Representation of simulation model
Product assembly mathematical model mainly
describes hierarchical relations and assembly relations
between product parts and components, as well as the
constraint relations among assembly design parameters in
different levels of assembly body:
1) Hierarchical relations. Product parts and
components have hierarchical relations. A product can be
decomposed into several components and parts, a
component can be decomposed into several parts. These
hierarchical relations can be visually represented as the
assembly tree. The root node of the assembly tree
corresponds to the product, every leaf node stands for
each part, and the mid-side nodes stand for components.
Assembly tree visually expresses the affiliate relations
among products, components and parts.
2) Assembly relations. Assembly design in product
parts and components is expressed by assembly relations
between each other. The basic assembly relations include
positional relation, connection relation and fit relation,
etc.
3) Kinematic relations. Kinematic relations describe
the relative motion and transmission relations between
components.
The core data of assembly mathematical model in this
paper is component and fit relations. Component consists
of parts and subassemblies, while fit relations consist of
fitting, aligning, coaxial, parallel, vertical and tangent, etc.
All components in assembly model constitute a
hierarchical model according to the nested relations
between each other. Components in the same level belong
to the same assembly, and they have fit relations but no
nested relations. Components in different level do not
belong to the same level, and they have no fit relations but
nested relations. Degrees of freedom (DOF) of a part is
the number of possible movement patterns of the part
while it is assembled with the connected parts under the
constraints relations, and the connected parts are assumed
to be fixed. Degrees of freedom can be divided into
translational degrees of freedom (TDOF) and rotational
degrees of freedom (RDOF), DOF=TDOF+RDOF. If a
part is connected with no any other parts, then its DOF=6,
that means it has three translational degrees of freedom
and three rotational degrees of freedom. When the six
degrees of freedom have all been constrained, then
DOF=0, it is full constraint state. When 6>DOF>0, it is
under constrained state.
Related model objects are briefly described as
follows. The data members of assembly model object
CAsm mainly include name, bounding box and
component set. The data members of Component object
CComp mainly include local coordinate system, part
model (or assembly model) and degrees of freedom
attribute. The data members of part model object CPart
mainly include name, material property, triangle set and
bounding box. The degrees of freedom attribute of
component contains specific geometric information. For
example, rotation axis information is recorded for the
rotational degrees of freedom. The geometric information
will be used to define kinematics constraints.
B. Realization of the constrained motion
First we should note that each component of
assembly model has a local coordinate system and a
number of assembly constraints (such as fitting, aligning
or coaxial, etc). Moving parts have certain degrees of
freedom, such as for 3D CNC pipe bending machine, the
bending arm (C axis) has a rotational degrees of freedom,
and the car (Y axis) has a degrees of freedom of straightline motion. To associate the moving parts in assembly
model with the straight-line motion and rotational motion
of mechanism, we proposed axis object (CAxis)
simulation servo motor. Since this paper is only for
collision detection simulation, we deal with simplified
motion control model, and the change of acceleration and
deceleration is not in our consideration. The main
attribute of CAxis include motion type (straight-line or
rotational), current coordinate, movement direction signs,
total displacement, speed, movement starting time and so
on.
The main interfaces of CAxis:
 Initialize. The interface is used to initialize a
movement, and to specify the movement direction,
coordinate increment, movement speed and starting
time.
 Get the current coordinate (YBC data).
 Status update. The interface function is constantly
updating CAxis coordinates, and inputting the
current time, positive and negative limit position,
outputting coordinate increment and limits.
 Reset. This interface is used to reset all state
parameters of CAxis.
Every motion axis of machine tool corresponds to a
CAxis object. When a movement starts, if it is a singleaxis movement, then a corresponding axis CAxis is
initialized; if it is interpolation, then several
corresponding axes CAxis are initialized at the same time.
The input parameters are movement direction, coordinate
increment and starting time. Then, current axes CAxis
coordinates would be updated every time slice and the
local coordinate system of the corresponding components
in assembly model are updated according to axes CAxis
coordinates, then the whole assembly model starts to
move.
C. Collision detection algorithm library
Six types of geometry were defined in collision
detection library to enhance the generality of the system.
These geometries are half-space, cube, cylinder, sphere,
torus and polyhedron. The surface of polyhedron is
triangular mesh, as shown in Fig. 1.
Fig. 1 basic geometries and general polyhedron
To ensure the collision detection precision, the
algorithm library doesn’t adopt discrete algorithm but
dozens of geometrical and algebra accurate algorithm.
There is no complete collision detection algorithm based
on accurate calculation in existing literatures, for
example, no accurate algorithm for cutting torus has been
provided at present. Results from collision detection
contain two collision geometries, collision marks,
interference depth, collision point and some other
information. To detect the collision of assembly model,
first we use the bounding box of each component in
assembly model to judge the collision roughly, then for
the parts whose bounding box intersected, we use
algorithm in collision detection library to detect the
collision precisely. The algorithm is developed by our
own research group and is in patent application, and will
be introduced in another paper. This paper only gives the
test results.
To realize the openness of the general simulation
platform, the collision detection algorithm is designed to
be independent algorithm library, and is associated with
simulation platform by standard interface.
D. Control mechanism of simulation platform
The motion trajectory and running status of model in
simulation platform is defined by G-code in ISO standard.
G-code consists of word and block. Each word has a key
word, key word G stands for movement, M stands for
ready to action, S stands for speed of main axis and F
stands for feed rate. A number of words form a block. For
example, G00 X0 Y100 Z50 is a block formed by three
words, G00 stands for fast straight-line motion, XYZ
stands for coordinate values.
collision detection library in Windows XP system. Based
on the simulation platform we implemented 3D CNC pipe
bending machine simulation system. The whole
mechanical model of pipe bending machine consists of 7
assemblies and 18 parts, and occupies 5.3MB space. The
YBC three axes of pipe bending machine can be manually
controlled to rotate by single step. Pipe can be bent
continuously by executing G-code, and it can also be bent
step by step manually. The simulation system displays the
current coordinates and G-codes real-time. Wall and
ground can be added to the system as collision
constraints. The system status (that is collision detection)
updates every 0.01 second. The pipe bending simulation
system implements collision detection between pipe
fitting and machine tool, pipe fitting and pipe fitting, pipe
fitting and wall or ground.
Parts in collision section are shown in red during the
simulation, as shown in fig. 5. From the test results we
can find that the collision detection algorithm library can
detect collision accurately. Our test hardware platform is
DELL Vostro1400, CPU is Intel(R)Core™2Duo and 1GB
of physical memory. The collision detection algorithm is
relatively the most time-consuming when pipe fitting or
machine tool is about to collide or has already collided.
The specific test results of case c in Fig.3 are as follows:
Pipe
bending
time
duration
12.3s
Fig. 2 system status update mechanism
Table I. Test results of collision detection algorithm
Number of
Total time
Single
Maximum
detection
consuming
detection
single
of algorithm
time
detection
time
300
0.492s
1.6ms
16ms
(a) pipe bending process 1
G-code compiler handles G instruction set inputted
into it and output specific motion control commands to
drive each related axis to produce movement, and then
mechanism starts to move. A timer is defined in
simulation system, and every time slice (such as 0.0010.01s) it would perform a subroutine to update model
status, as shown in Fig. 2.
III. RESULTS
We built general collision detection simulation
platform with VC6.0, OpenGL graphics package and
(b) pipe bending process 2
(c) collision of pipe itself
(d) collision between pipe and machine tool
Fig. 3 collision detection
IV. CONCLUSION
This paper presents the general simulation platform
architecture with kinematic constraints based on assembly
model, and implements CAD model import, G-code
parsing and system simulation control mechanism.
Collision detection of the system based on accurate
collision detection algorithm has also been implemented.
Take 3D CNC pipe bending machine as example, we
verified the practicality of the simulation system.
To further improve the general simulation system and
apply it to assembly and simulation field, the follow-up
work is mainly in three aspects. The first one is to support
more model definition file format, such as the wildly used
VRML file format, as well as lightweight CAD model JT
and 3DXML file format offered by UG and CATIA. The
second one is to integrate the platform with simulation
software, and realize Working Model 3D or ADAMS data
interface. The third one is to optimize the collision
detection algorithm library, and implement accurate
calculation of interference amount (interference depth).
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