CAD Model-Based Planning and Vision guide Accurate Geometric Measurement
by Heikki Ailisto
The geometric accuracy of manufactured
objects affects both manufacturing costs and quality. Control over the
geometric accuracy of products is seen as an important issue in the construction
of machinery, cars and ships, for example. This control can be achieved
by measurements of dimensions and the coordinates of vital points, for
instance, but measurement planning is needed to ensure that the goals are
reached in an efficient manner.
Laser rangefinding or LIDAR is an excellent technique for measurement
of dimensions and shapes, since it is accurate, fast and has a large measuring
range. Typically, the laser range measurement is combined with very accurate
angle measurement, servo motors for panning and tilting the measurement
beam and sophisticated computer control. The x,y,z coordinates of the measured
point can be calculated from the measured range, pan and tilt values. Repeatability
of 1 mm can be achieved in good conditions within a measuring range up
to 20 meters. Commercial systems specially designed for wear monitoring
of converters and ladles used in steel making and for ship building industry
have been developed in Oulu, Finland in cooperation between University
of Oulu, VTT Electronics and hi-tech companies.
Measurement Planning based on CAD Data
The widespread use of computer aided design and manufacturing has led
to a situation in which product geometries are available in electronic
form and machines and measurement devices are controlled by computers.
At the same time, the design and manufacturing staff are becoming familiar
with the use of computer aided techniques in their everyday work. This
is opening up a path for developing CAD-based measurement planning systems
serving the purposes of geometric control.
In this work a concept of CAD model-based automated 3D measurement was
developed (see Figure 1) and its feasibility demonstrated by implementing
and testing experimental systems for two cases drawn from industrial needs.
Cases from shipbuilding and steel making were considered.
Figure 1: Concept of CAD model-based automated 3D measurement.
A CAD compatible tool for measurement planning and analysis was implemented.
The tool imports CAD files of the objects or products to be inspected and
offers the user various graphical means for measurement planning. The result
of the measurement planning is a special command file for controlling the
optical coordinate measurement system.
Vision Sensor finds Vital Points to be measured
The LIDAR based optical coordinate measurement system is excellent in
measuring 3D coordinates of single points. However, it lacks the ability
to find the point of interest in a changing environment where, for example
the position of the measured object may vary. This ability would be valuable
in automating the measurement function so that points marked with special
targets or points with visible features, like corners, could be measured
without operator intervention.
We implemented an auxiliary vision sensor for controlling LIDAR based
optical coordinate meters. A CCD camera is placed either on the same optical
axis with the laser range finder or paraxial with the measurement head.
A vision system analyzes the view of the camera and finds predefined target
points or visible features, and then guides the pan and tilt motors so
that laser range finding can be focused on these points. Pointing accuracy
better than 1 : 20000 was achieved with the prototype system within a measurement
range of 16 meters. A LIDAR based coordinate measuring system equipped
with vision is shown in Figure 2.
Figure 2: A LIDAR based coordinate measuring system equipped with vision.
CAD Model-based Automated Measurement Feasible
The feasibility of the concept of automated 3D measurement was demonstrated
by following the whole operation chain (as seen in Figure 1) of the system
from measurement planning to comparison between as-measured and as-designed
values in examples involving planning the measurement of targeted points
in a large hall, measurement of a curved shell plate used in shipbuilding
and view point planning for refractory lining monitoring measurements.
The operating principle was found to be sound, but certain limitations
emerged, some of which were due to features of the CAD system. Further
work with measurement planning tools is required, eg in order to improve
their user interface and to incorporate tolerancing in measurement planning.
When considering further development of the vision system, it would
be beneficial to integrate it with the 3D measurement system, since this
would lead to a less costly, more compact and more reliable solution. Interesting
possibilities are offered by integrated vision systems, which have the
capabilities of a video camera, frame grabber and processor in the same
package.
Some of the ideas and solutions developed in this work are already being
utilized by industrial companies. Vision is being used for coordinate system
set-up in a laser profiler-based measurement application in steel manufacturing
and development work in this field is continuing. Work on the measurement
planning tool is also going on, and the tool is being adapted for photogrammetry
applications.
Please contact:
Heikki Ailisto - VTT Electronics
Tel: +358 8 551 2111
E-mail: heikki.ailisto@vtt.fi
