Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)
16Simulation for Collision Prediction Based on“Visionplus” Non-contact Measurement System

HIROSHI OISHI*1

KENJI KURA*2

HIDEAKI YAMAMOTO*3

HIDETAKE KIRYU*1

Mitsubishi Heavy Industries, Ltd. (MHI) developed an on-machine shape measurement
system with non-contact sensors and modeling technology. These technologies can accomplish
various objectives, and the entire system is named “Visionplus.” The Standard Triangulated
Language data (STL data)(Note 1) constructed with this system produce on-machine, real-timesimulation for collision prediction. Furthermore, this simulation has been developed to a practicallevel. This paper introduces a method of three-dimensional modeling and examples of simulation
for collision prediction based on “Visionplus.”
Note 1. Standard Triangulated Language is the industry-standard file format for 3D CAD systems
developed by 3D Systems, Inc.

|1. Introduction
In machining, various factors often cause unexpected issues for machine tools that cannot be
predicted from the machining condition. Consequently, to maintain efficient and stable machining
precision, machine conditions must be determined precisely at all times. To minimize the negative
effects of poor workpieces, mistakes in machining, variations of machine characteristics and otherproblems, a shape recognition system that can model the shapes of workpieces and tools, and thenquantitatively evaluate and control them is necessary. Therefore an on-machine measurement
system with non-contact sensors has been developed.
This paper introduces the principles and characteristics of a three dimensional shapemeasurement system that can be used to accomplish various objects, the method to construct athree-dimensional model and on-machine and real-time simulation for collision prediction based on
this system as practical examples.

|2. Outline of Visionplus System
2.1

Principles of Visionplus“Visionplus” is a collective on-machine measurement system with non-contact sensors thatrecognizes the state of the workpiece, jig, tool and other components and models their shapes on amachine. Visionplus includes a hole position measurement system with non-contact sensors used
specifically for the construction of machine parts (automatic compensation of hole position,
determination of workpiece fixing, quality of workpiece, etc.) and a vision-based tool shapemeasurement system. Figure 1 shows examples thereof.
This paper introduces the technology of a three-dimensional shape measurement system witha laser scanner. This measurement system with a laser scanner enables the measurement ofworkpieces and jigs at any stage of the machining process, and makes three-dimensional models
thereof.

*1

Engineering Department, Machine Tool

*3

Engineering Manager, Engineering Department, Machine Tool

*2

Manager, Engineering Department, Machine Tool

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)
17Figure 1 Concept of Visionplus

2.2

Figure 2 Equipment configuration

Mechanism of measurement system

Figure 2 shows the configuration of the system. The system only requires a sensor unit, adedicated personal computer, and a wireless communication device. Consequently, it is a very
simple configuration. To make the measurement system automatically connect to and disconnect
from the machine tool, the sensor controller, wireless communication device and battery areintegral to the housing, so that the measurement device is made wireless. Specialized software must
be installed on the personal computer for this system. Figure 3 shows the configuration of thesoftware.

Figure 3System configuration

The measurement procedure is as follows:(1) Set workpiece on machine(2) Input specification of workpiece(3) Initiate machine operation according to automatically calculated measurement path
(4) Scan the workpiece from four sides and the top surface and measure displacement(5) Calculate position dates (group of point dates) with displacement data
(6) Construct three-dimensional model
Figure 4 shows an example of the constructed model.

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)
18Figure 4

Operation flow

2.3 Setting for measurement
The developed software, which enables shape measurement by simple operation using aGUI, can automatically construct the measurement path by simply inputting the estimatedsize and position of the workpiece. The measurement path is constructed while considering thesensor characteristics (measurement range and direction). Unnecessary measurement processes
can be eliminated by the operator choosing the appropriate surface and measurement path in theGUI based on the measurement conditions. Figure 5 shows the GUI.
(Note 2)

Note 2. “GUI” refers to “graphical user interface,” a type of interface that represents the display andmotion graphically on a screen in order to improve usability for the operator.

Figure 5

GUI screen for measurement

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)19|3. Characteristics of 3D Model Generation System
3.1

Measurement sensor unit

In general, measurement with a laser scan sensor is effected by the material characteristics
and the surface properties of the subject. Laser light tends to be diffusely reflected on glossy
surfaces such as metal, and be absorbed on black painted surfaces. Thus, it is difficult to ensurestable precision in the measurement of workpieces with various surface properties
The system prevents the effect of the surface properties of a subject through the adoption of
an automatic light modulating function to optimally adjust the laser light in real time during
measurement. The system has an automatic laser light adjustment system. The system can preventthe effect of the surface properties. Even if the subject is large or significantly irregular(unevenness) in shape, it is possible to measure large workpieces on a large machine tool. The
system has a wide dynamic measurement range, so that it can measure large or significantly
irregular workpieces (unevenness) on a large machine tool. Furthermore, no special rotation
mechanism is required to measure the subject from four sides and the top surface because thescanning direction can be freely changed by mounting the sensor unit on a standard attachment.

3.2

Model generation

To apply the measurement data to various objectives, surface and solid data in STL format
are often constructed. In general, if solid data is to be generated from point group data, it is difficult
to generate data satisfying the input restrictions of the application software that performs thesimulation of machine operation and geometric operation. In general, solid data generated with thedata of points is inconvenient. The solid data doesn’t satisfy the input restrictions for the simulationof machine operation and geometric operation must be corrected.
The operator must correct the data. This system can generate model data that satisfies theinput restrictions for application software, thereby eliminate the manual correction.

|4. STL Data of 3D Solid Model Generation Technique
4.1 Problem and task of solid modeling(1)

Purpose of development
Depending on the use of measurement data, conventional surface data(Note 3) issometimes enough. However, in order to be used for the application software for
performing various types of geometric operations, it is no enough. Therefore the developed
system is required. In the restrictions of geometric operation, the most important one is thatthe input data must be a closed polyhedron. And it provides easy implementation in theapplication software and to improved robustness. The precise characteristics of input shapedata are “a closed polyhedron(Note 4),” “a manifold polyhedron(Note 5)” and “no selfintersections(Note 6).”Note 3. “Surface data” refers to a group of shape data represented only by planar elements.
Note 4. A “closed polyhedron” refers to the state in which vertices, sides and planes constituting thepolyhedron are precisely combined and closed.
Note 5. “Manifold” refers to the state in which regarding the connection between the sides and planesof a polyhedron, one side is not shared by more than two planes.
Note 6. A “self-intersection” refers to the state in which triangles constituting a polyhedron overlap
each other.

Figure 6

Restrictions on input shapes

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)20(2)Problems
In general solid model generating process, cannot provide a completely closed shapemodel if there are any missing parts in combining point group data. In addition, there is amethod that divides three-dimensional space into discrete voxel modeling (Figure 7) forattaining closed polyhedrons. However, with this technique, the result of data modeling isrepresented as discrete shape data. If high definition data is required, the amount of memory
usage and the number of processes increases in proportion to the cube of the original values.

Figure 7 Voxel modeling

Figure 8

3D Delaunay division

4.2 Explanation of solution
The shape measurement technique of the system is divided into following three processes:(1) The measurement subject is scanned from multiple measurement directions with themeasurement sensor, thereby obtaining point group data for each direction.
(2) Implicit functions are generated from the point group data, which represent the shape of thesubject.
(3) Based on the implicit function, polyhedron data is generated with a 3D Delaunay diagram.
“3D Delaunay diagram” refers to an aggregation of tetrahedrons elements (cells). Each of
the cells satisfies the restriction that the circumscription-balls do not include vertexes of other cells(Figure 8).
The polyhedron generation processing, listed above as item (3), is the most importantprocess. The details are discussed below. Figure 9 shows the flow of polyhedron generation
processing.
i. The entire space of the measurement range is filled by a group of cells.
ii. Inside/outside determination is made as to each of the vertices of the cells.
iii. Boundary cells are extracted.
iv. Intersection positions are calculated from the boundary cells and surfaces.
v. Slice surfaces are generated from the boundary cells.
vi. STL data is generated from all the slice surfaces.

Figure 9

Flow of polyhedron generation processing

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)214.3

Effects of techniqueThis technique based on a 3D Delaunay diagram divides the entire space of the measurement
range into tetrahedron cells without gaps or overlaps.
Closed and manifold polyhedrons can be generated by combining surfaces (cutting planes),even in boundaries. Furthermore, it can be assured that each of the cells is free fromself-intersections, according to the definition of a 3D Delaunay diagram (Effect 1).
Since MHI has succeeded in providing a technique that can easily and reliably generate dataon manifold and closed polyhedrons without self-intersections, it is no longer necessary to
manually correct the shapes, which is otherwise required when using other application software(Effect 2). Compared with voxel modeling and other discrete modeling methods, this technique hasa higher modeling precision to reduce the deterioration of shape accuracy because it allows the useof coordinates of consecutive intersections on boundaries (Effect 3).

4.4

Results of measurement

The results of workpiece measurement using the system are illustrated in Figures 10 and 11.
Figure 10 (workpiece dimensions: L 950 × W 900 × H 650 mm) shows that the system is evencapable of accurately modeling fixtures. Figure 11 (workpiece dimensions: L 1,300 × W 950 × H350 mm) indicates that even the details of the letters engraved on glossy and casting surfaces of themetal can be measured. Note that the time required to measure a subject of approximately 1 m3
was about 15 minutes, including the time for arithmetic operation. The measurement can beperformed at an accuracy of ±1 mm.

Figure 10

Measurement result (fixtures)

Figure 11

Measurement result (mold)|5. Example Application of Collision Prediction Simulation
5.1(1)

Outline and characteristics of collision prediction simulation

Background
When machining a single workpiece on a large machine tool, debugging is often carriedout simultaneously with the processing, frequently causing collisions between the tool and theworkpiece.
There have been a number of interference simulators, but in reality they are not suitablefor practical use because they require CAD data of workpiece materials and fixtures.
Therefore, MHI has contrived to integrate interference simulation and the Visionplus
system so as to provide a practical simulation technology to predict collisions by performing
on-machine workpiece measurement and modeling, and by utilizing STL data obtainedaccordingly.
(2) Features
The simulation can predict collisions irrespective of the auto/manual operation mode andcan accurately determine collisions on portions other than the cutting part of a tool. In addition,

Mitsubishi Heavy Industries Technical Review Vol. 49 No. 3 (September 2012)22

the function can even recognize the constantly-changing cutting state, and it allows continuous
monitoring until the final machining process. The generated model data is compatible with thesimulation and thus correction is unnecessary. The function calls for a simple operation before
measurement. During the period from measurement to debugging, it allows the automation ofprocessing without operations.

5.2

Effects and advantages

Unnecessary machining sometimes occurs due to the inferior shapes of materials, because
CAD models of materials are normally not available. Thus, measuring the material shapes inadvance is helpful in deriving the optimum machining conditions. In addition, since the function
accurately simulates the shapes and mounting positions of fixtures after setup, it is capable ofpredicting collisions with workpieces as well as fixtures. As a consequence, the use of the system isexpected to reduce machine repair expenses that are likely to be incurred as a result of collisions, aswell as the possible risk of operation shutdowns and the discarding of inferior workpieces. By
incorporating the 3D solid models generated by the Visionplus system into a commerciallyavailable machine operation simulation, the efficiency of debugging work for machining programs
can be improved.

|6. Conclusions
We have discussed the measurement performance of the Visionplus system, which canestablish an essential technology of a 3D model generation system for performing measurement on
a machine tool, and which can be applied to various fields. We have also discussed the feasibilityof the collision prediction simulation function of the system. We will provide a comprehensive
measurement system by promoting the further development of application software.