Optical inspection systems such as automated bulk processing systems are used for determining optical characteristics of articles. In such a system a conveyor moves the articles through an inspection region, where they are illuminated with light produced by an illumination source. An inspection station including cameras or other optical detection devices views the illuminated articles and determines characteristics of the articles based on that view. The inspection system ordinarily also performs operations on the articles according to instructions from a logic unit; the instructions are based on the determined characteristic of the articles. The inspection station sends signals to a sorting or treatment station where the articles are sorted or treated by category. For example, defective or foreign articles may be removed from the flow of articles carried by the conveyor. The most con, non method of removal is by directing an accurately timed blast of a fluid, such as water or compressed air, at an article to be sorted so as to direct it out of the flow of acceptable articles.
Automated bulk processing systems can perform a variety of tasks such as inspecting or sorting bulk articles including raw or processed fruit, vegetables, wood chips, recycled plastics, and other similar products. The articles may be inspected for optically-detectable characteristics such as size, color, or shape. The systems can rapidly separate very large quantities of articles into two or more categories.
The effectiveness of such bulk processing equipment is greatly affected by the level and consistency of the illumination provided to the articles as they move through the inspection region. One of the factors determining the level and consistency of the illumination is whether the sources of the illumination have clean surfaces. Illumination sources used in optical inspection systems are typically fluorescent light tubes separated from the inspection region by a protective tube transmissive of the illumination. The protective tube is most commonly of the polycarbonate plastic sold by General Electric under the trademark LEXAN.RTM. or by Rohm & Haas.
Keeping the protective tube clean is especially important when the inspection region is not located on a conveyor belt but rather is a region of space through which articles pass by inertia after leaving a rapidly moving conveyor belt. In such a system contaminants such as dust become airborne and are especially likely to settle on exposed surfaces of the protective tubes. Moreover, when the articles are treated in a region of space through which they move, the processes of treatment (such as directing a blast of water or compressed air at an unwanted wood chip) can generate further contaminants and air currents, which increases the possibility that contaminants will settle on exposed surfaces of protective shields. To keep the protective tube clean, it is commonly rotated during operation of the inspection system, and a wiper is held against a surface of the protective tube during the rotation.
A wiper presently in use resembles a squeegee and is a plastic blade attached to a metal bar by spaced apart bolts. This type of wiper is effective in cleaning the tube where the plastic blade has good contact with the surface of the protective tube, particularly in applications where the contaminants or the surface of the protective tube are wet. However, such contact depends on the straightness of the metal bar that holds the plastic blade and on the extent to which, during use, the plastic of the blade expands and thus pulls away from the protective tube between the bolts with which the plastic blade is attached to the metal bar or loses uniform contact with the tube by wearing. Where the plastic blade contacts the surface of the protective tube, it causes scratching of that surface, in part by forcing contaminants into the surface; where it does not contact the surface of the protective tube, it is less effective in dislodging contaminants from that surface. When its contact with the surface of the tube is variable, it causes a variable degree of scratching. These problems lead to an uneven pattern of removal of contaminants from, and an uneven degree of scratching of, the surface of the protective tube. Both of those factors cause an uneven level of light transmission along the length of the tube, which interferes with proper illumination of articles being inspected.
There is thus a need to provide for optical inspection systems such as bulk optical processing systems a protective shield from which contaminants are removed throughout the illumination portion (the portion through which light reaches the part of the inspection region at which articles are optically inspected) of the protective shield in a reliable and consistent manner and without scratching the protective shield.