Section 5.3: Ruggedization of Imaging Lenses
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Imaging lenses used in industrial machine vision applications have
special requirements beyond those of standard imaging lenses. The
lenses used in factory automation, robotics, and industrial inspection
must work in specific, demanding environments involving vibration,
shock, temperature change, and contaminants. As such, new classes
of ruggedized lenses have been designed to work specifically in a
multitude of different scenarios. There are three distinct types of ruggedization
available:
1. Industrial Ruggedization
2. Ingress Protection Ruggedization
3. Stability Ruggedization
Industrial Ruggedization
Industrial ruggedized lenses are designed to survive vibration and
shock without taking damage or changing focus or f/#. Flexibility is
sacrificed by eliminating moving parts to achieve this.
A standard fixed focal length lens uses a focus mechanism and an
iris that is comprised of thin leaves and ball detents to adjust f/#
which can spring out of place during shock and vibration. The iris
is removed and replaced with a fixed aperture stop in industrial ruggedized
lenses and the focusing mechanism, typically consisting of
a focusing mechanism of a threaded barrel within another threaded
barrel, is replaced by a single thread and rigid lock mechanism.
Stability Ruggedization
Like industrial ruggedized lenses, stability ruggedization protects the
lens from damage, but also ensures optical pointing and positioning is
maintained after shock and vibration. In addition to replacing the iris
and a simplified focus mechanism, individual lens elements are glued
in place to prevent them from moving within the housing. Figure 5.7
shows a stability ruggedized lens in which the lens elements are glued
in place and a clamping lock is used to simplify the focus.
Figure 5.7 Glue
Glue
Glue
Glue
Simple Focus With Clamping Lock
Lens elements sit within the inner bore of the barrel of an imaging
assembly. The space between the outer diameter of the lens and inner
diameter of the barrel is typically less than 50 microns. Despite the
minimal amount of space, decenters, on the order of tens of microns
are enough to significantly affect the pointing of the lens. When using
a stability ruggedized lens, if an object point is in the center of the
FOV and falls on the exact center pixel, it will always fall there even if
the lens has been heavily vibrated (Figure 5.8). Stability ruggedization
is important in applications where the FOV must be calibrated, such
as measurement equipment, 3D stereo vision, lenses used for sensing
in robotics, and lenses used for tracking object locations. These applications
often require the optical pointing to be stabilized to values
much smaller than a single pixel.
Figure 5.5
Double Threaded Focus Ball Detents
Multiple Leaf Iris
Single Threaded Focus
Aperture Stop
Figure 5.5: A standard lens with complex mechanics and an adjustable
iris vs. an industrial ruggedized lens with simplified mechanics.
Industrial ruggedization is ideal for applications where the system
will be set up once and not changed. An added cost advantage is
also present in this type of lens due to the removal of the complex
movements and adjustments, resulting in significant part reduction
and cost savings. There are more applications for industrial ruggedization,
such as high vibration factory environments, situations where
the camera is rapidly accelerated, inspection systems where many
similar camera setups are repeated, and robotic vision.
Ingress Protection Ruggedization
Ingress protection ruggedization ensures a lens assembly is sealed
using O-rings and RTV silicone to prevent moisture and debris from
entering the lens. This protection is typically added to an industrial
ruggedized lens as sealing an adjustable focus and iris would be problematic.
These lenses are used in environments of high humidity/
moisture, sputter, dust, or small particles, and where space to fully
enclose the lens and camera is not available.
Figure 5.7: Stability ruggedized lens with all lens elements glued
in place.
Figure 5.8b
Object Crosshair Image Crosshair
Figure 5.8a: Unperturbed system where object crosshair is mapped
to the image crosshair.
Figure 5.8b
Object Crosshair Image Crosshair
Figure 5.8b: Perturbed system where lenses are decentered within
the barrel and the optical pointing stability changes. The object
crosshair is mapped to a different place (yellow) on the image than
the unperturbed image (red). This example is highly exaggerated
and actual changes tend to be on the order of a pixel or less.
Figure 5.6 O-ring Seal
Figure 5.6: An ingress protection ruggedized lens with an O-ring to
seal out contaminant.