360 Degree View Ensures Efficient Part Inspection
- Fig. 1a: A camera with a pericentric lens images both a 360 degree view of the top and sides of an object
- Fig. 1b: A camera with a pericentric lens images both a 360 degree view of the top and sides of an object
- Fig. 2a: Multiple mirror systems with telecentric lenses allow different side views of an object
- Fig. 2b: Multiple mirror systems with telecentric lenses allow different side views of an object
- Fig. 3a: Using multi-mirror optical systems, Polyview optics provide the inner and outer surfaces view of objects such as washers or grommets
- Fig. 3b: Using multi-mirror optical systems, Polyview optics provide the inner and outer surfaces view of objects such as washers or grommets
In machine vision, many applications require a total view of all of the object's sides. Such inspections are performed using multiple cameras placed at strategic locations. But they increase the cost of vision systems. However, there are more economic possibilities to solve such tasks: intelligent high-tech lenses which ensure a 360 degree inspection.
Beer, wine or water - before filling liquids into bottles, it is necessary to inspect the bottles making sure they are free of slivers and contamination. The circumference of the container is checked for any defects in the neck. Also, the barcode has to be read out. In order to inspect the inner and outer surfaces of an object, a system of multiple cameras is normally used: Three or four cameras control the object from different angles of view. A further camera is required to also get the top view. With a computer and machine vision software, the images taken from all object surfaces have to be combined to form one single image. Although multiple camera systems work reliably, their usage goes along with high costs. Moreover, little available space often complicates the multiple cameras' integration. For these reasons, system developers often look to optics vendors to provide single-camera solutions that alleviate the need for image stitching or part and camera rotation.
Churn Out Outer Surfaces Inspection
When the outer surfaces of parts moving on a conveyor must be inspected, multiple camera systems often cannot be used since neighboring parts may obstruct the side views of the object to be inspected. By using pericentric lenses with a single camera, however, both a 360 degree image of the top and sides of the object can be captured. In such systems, an image of the lateral walls of the object is displayed around the image of the top surface of the part (fig. 1). The user gets one image that contains all information needed for the defect detection for this part. Such pericentric lenses simplify system integration, since the parts to be inspected can pass freely in the space below the lens. Captured images are then unwrapped using software to obtain a linear view of the sides of the object and allow image analysis to be performed.
Inspecting the other surfaces of objects can also be accomplished by using multiple mirror systems. By this approach, an object is viewed through a mirror assembly using a telecentric lens. This allows four or more different side views of the object to be obtained (fig. 2). Because a telecentric lens is used, all the images are magnified equally allowing them to be correctly matched and measured using software. Even metrology tasks can be solved effectively this way.
Lenses for the Inspection of Inner and Outer Surface
One variation of this technique that provides multiple side views of the same object can be achieved using polyview optics. These multi-mirror optical systems provide four, six, eight or more side views of the same object. This makes such systems suitable for inspecting the inner and outer surfaces of objects such as washers or grommets. Since the inner and outer side surfaces are displayed in the same image, all the relevant information needed to inspect such products is simultaneously displayed (fig. 3). PolyView optics provide a good image resolution and allow for high-speed image acquisition due to their short focal length.
PolyView optics can also be used for the inspection of cavities. In some cases, however, other optical solutions can be more efficient and compact. When bores need to be inspected, specialized optics can be used to generate images similar to those provided by pericentric lenses. Like pericentric lenses, these optics are designed to image curved surfaces. The wide perspective angle obtained provides a detailed view of these surfaces without the need to place an optical probe inside the part.
While such optics are useful, they cannot be used to image the interior of threaded objects. In such cases, it is necessary to insert a fiberscope or borescope inside the part. Such products use bundles of coherent optical fibers to transfer images from the cavity to a camera. Unfortunately, the maximum number of fibers a bundle can contain limits the achievable resolution and when high-resolution images are required, a direct optical coupling with the camera must be used. This can be achieved by using optical probes that consist of a macro lens that images the cavity walls through a spherical or aspherical mirror. Such optical probes alleviate the need to scan the entire thickness of the cavity if the inspection can be limited to a specific cavity height.
When defects inside a cavity become very small, however, a much higher optical magnification is needed and a smaller field of view is required. Since only parts of the interior surface can be inspected in a single image, the probe must both scan the height of the cavity and rotate to provide a full 360 degree view. Besides requiring an accurate positioning system for probe movement, this also requires specific software for image matching, since the image of the cavity will consist of numerous images obtained by scanning the interior surface and depth of the cavity.
Intelligent optics systems enable effective solutions for the complete image acquisition of a three-dimensional object. The necessary investment in cycle time, material and thus cost is significantly lower than that of a multi-camera system. In addition, the 360° high-tech optics solution can be used when space restriction prohibits the integration of multi-camera systems.