The system chosen is based on liquid crystal shutter technology. A high
quality graphics monitor, with a colour resolution of 1280 x 1024 x 8 bits, has
been fitted with a liquid crystal screen which can be circularly polarised in
synchronisation to the rate at which the image processing system displays the
alternate left and right hand images on the screen, usually at a frequency of
100 or 120 Hz to avoid user fatigue. The user has only to wear circularly
polarised glasses or clip-ons to view high resolution stereo images.
For the measurement process, the Non-Contact Measurement System reduces
the display to monochrome 1024 x 512 pixel x 8 bit resolution.
The benefits of the viewing system can be summarised as follows:
* The viewing system can be set-up and operated without a high level of
skill or experience.
* The viewing system allows multi-user access.
2.5 Photogrammetric Software
The system is used in a user selectable mode. This may be as a monocompara-
tor, stereocomparator or full digital plotter.
The first two of these modes exist mainly for the benefit of users with
photogrammetric experience, and allow the acquisition of data for input into
external software such as bundle adjustment or lens calibration programs. In
most cases however the system will operate in digital plotter model, as this
allows on-line stereo viewing and the ability to take measurements at will.
The standard phases of inner and relative orientation are performed (inner
orientation being trivial with digital images). There is then the choice of
scaling the model with a known distance or performing an absolute orientation
with control points. As the setup phase is potentially the most difficult
operation for inexperienced users great effort has been taken to hide as much
of the process as possible from the user.
The system operates on a single screen which displays both the user interface
and the stereo model simultaneously. Commands are input using a three
button trackball or mouse.
2.6 User Interface Software
One of the most critical development objectives has been to design the user
interface of the Non-Contact Measuring System such that it could be used by
the engineer who required the data, rather than by a specialised technician.
This has resulted in:
Software that has been written as a Microsoft Windows application. The
graphic user interface will be familiar to all who have used the Microsoft
Windows environment and the complete system is controllable through a
"mouse driven cursor” by pointing and clicking. Typical menus and dialogue
boxes are shown in figure 6.
The de-skilling of the photogrammetric set-up and operating commands such
that the choice of commands is logical and easy to interpret without extensive
training or detailed photogrammetric knowledge.
The dimensional data extraction process has been designed from the require-
ments perspective of an engineer rather than a map maker. In general the
engineer is concerned with specific parts of the image rather than the whole.
Marketing has shown that measurement requirements are likely to be points,
dimensions, angles, profiles and diameters, rather than surface models. For
the construction of CAD drawings a polygon function, which can be used to
describe all the features of one part of the subject and keep that data as a
discrete entity when transferred into the CAD system, is essential. However,
the features required by the map maker are still to be found within the user
interface system.
To allow the engineer to build-up the measurements selectively from the
image, a three-dimensional graphics overlay is integral to the user interface
system. This allows all the measurements taken, be they discrete measure-
ments, surface models or any other option, to be overlaid in three-dimensions
on the stereo image. Editing of the overlay allows individual measurement
"nodes" to be deleted or repositioned; additional nodes to be inserted; entities
deleted; and groups of entities toggled on and off.
The CAD transfer utility saves the graphics overlay as a three- dimensional
CAD file in the DXF format, such that it can be transferred to such CAD
packages as AutoCAD and Intergraph Microstation for further processing or
presentation.
2.7 Performance
In operation the system has been able to meet its design objectives and is used
by engineers for the extraction of the data that they require. With video input
the system can achieve typical pro-rata measurement accuracies of +/- 2mm,
and the digital system +/- 0.5mm, within a subject field of view of one metre
square. The better calibration and ease of interpretation of the images from
the digital camera makes sub-pixel measurements possible.
3 APPLICATIONS
The applications of the system can be categorised as follows:
* Metrology Applications;
* Robotics Applications.
3.1 Metrology Applications
The metrology applications of the system largely follow the applications of
the conventional photographic based system.
However, the range of applications has been increased to include those tasks
in which the dimensional data is required immediately. This extends the
applications to construction related tasks.
One side effect of the availability of the system is that tasks that had been
carried out by other techniques, such as the sizing of the marine growth fouling
on the underwater structures, can now been carried out more conveniently and
cost effectively with the Non-Contact Measurement System.
The metrology applications can be summarised as including practically any
dimensional measurement requirement underwater or in air.
3.2 Robotics Applications
One of the most interesting applications for the system is in the supervisory
control of robotic manipulators that are used on ROVS to substitute for the
manipulative capability of divers.
In most cases an underwater manipulator is under the direct control of an
operator on the surface. As he moves a master control into the desired
configurations, this movement is mimicked by the underwater manipulator.
The positioning of the manipulator is normally carried out solely with mono
video monitoring. Unless the ROV can be positioned such that it is stationary,
which it cannot commonly be, the operation has to be carried out with
uncompensated relative movement between the manipulator and the subject.
This process is termed tele- manipulation.
The construction of these manipulators means that they can be positioned
under software control reproducibly, but not accurately. Their in-built optical
and electronic sensors are still unable to compensate for the tolerances in the
mechanical construction of the joints and hydraulic actuators.
In order for the control of these manipulators to be automated, a closed-loop
supervisory control system is required to inform the manipulator of where it
actually is, rather than where it thinks it is.
Anologue Video Signol
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Figure 3. The data stream from a video image.
