| Utilizing photogrammetry and theodolite systems
has brought many advantages to industry, such as mobility, flexible handling and shortened
measuring time. However, the absence of an acceptable "Calibration Artifact" has
been a problem in the application of these systems. Today, using the "KOBA-optima" system, the demands of monitoring
the instruments per VDI / VDE guideline 2634 (Acceptance and monitoring of optical
3D-sensor system) are met. KOBA-optima utilizes the principle of a scale that can be
probed optically. A suitable calibration procedure and the necessary calibration structure
for the scale were also developed.
The KOBA-optima scale system was tested for applicability to
the VDI / VDE guidelines in year 2000 utilizing a comprehensive study that involved the
PTB (Physikalish-Technische Bundesanstalt), and various manufacturers and users of
photogrammetry and theodolite systems. Results of the study verified the Optima scale
system’s applicability to the VDI / VDE requirements, and also that the system
fulfills the demand for an optical physical body traceable to the physical value
"length."
The 2700 mm cylindrical scale body is made of carbon
composite material (CFK), and may be separated at the center for transport. The joining
ends of the scale body are lapped for consistent, accurate set-up. There are six targets
located on the neutral fiber of the body. Each target has a viewing entry access angle of
130° . The clear glass area of each target has a diameter of 10mm. In the center of each
of these clear areas is a site ring, used for sighting the theodolite system. Each site
ring has an internal diameter of 1.25mm, surrounded by a hairline of 0.25mm thickness.
Targets that use retro foil as a background guarantee that
sufficient light is reflected, even if the angle is not favorable for picture taking.
Targets that use a white background or background illumination are optionally available.
The KOBA-optima optical scale system provides 15 possible
measuring distances per scale with six targets. The combination of measuring distances and
scale positioning guarantees a statistically safe measurement. Scales are calibrated on a
measuring machine specially modified with a CCD camera to register the targets, and with
two distance-measuring laser systems for length measurement. The center position of the
measuring mark is determined by using an elliptical trace-out program.
Certification is provided by the DKD (German calibration
service), or the PTB can carry out calibration of the scale. The Measurement Uncertainty
of calibrated values is 5 µm, which is significantly below that provided by
photogrammetry and theodolite systems. As a result, the certifications performed by the
KOBA-optima system are suitable as the SI basic unit "length" according to the
demands of the VDI / VDE standard.
The patented KOBA-optima optical scale system has been
designed to be a standardized artifact, traceable to the national standard. It is suitable
as a calibration standard for optical measuring systems, e.g. acceptance and monitoring
inspections. If can also be used as a reference standard for traceability of the
particular measuring task to the "SI basic unit length" specification, as
defined by the KOBA-optima optical scale system as a physical body.
The scales can be easily fixed to tripods, or on carrying
handles on the object to be measured, or on the object itself by means of fastening clips.
The KOBA-optima® scale is available exclusively from Paul W. Marino Gages,
Inc. |
