“Just in time” and “total quality” in current industrial production represent requirements for subcontractors, non-compliance with which leads to penalties or termination of cooperation. Delivery of materials and parts just before the start of the production operation allows the final manufacturer to reduce stocks and reduce logistics. The condition is total quality, ie zero scrap in the delivery, which will allow the entry control to be omitted. For subcontractors, however, this means moving from statistical control of production quality to control of each piece produced, ie total control. For its implementation, machine vision is an effective means, the construction of which can be relatively simple.
An example of a inspection machine is a device for inspecting a simple product: 6 mm diameter steel bars. The bars are manufactured in several lengths from 132 to 176 mm and are provided with a recess on one side. The length of the rod should be measured with an error up to 0.25 mm, the dimensions of the recess with an error up to 0.1 mm. It is necessary to check about 10,000 rods per hour, so the inspection time is about 0.3 s. It is a separate inspection machine with a manually filled hopper and sorting the rods into satisfactory and unsatisfactory
The design of the machine vision depends on the required testing speed. The picture shows a pair of drive wheels that pick up the bars from the magazine and move them under the cameras and telecentric lenses in a smooth motion. Once the stick is in the appropriate position to take the picture, the light barrier is activated, which triggers the cameras.
Inexpensive cameras with a relatively low resolution of 640 × 480 pixels are used, which occupy an image measuring approximately 14 × 10 mm. Thus, an area of about 0.03 × 0.03 mm is projected from the plane of the image per pixel in the camera. Using subpixel resolution algorithms, it is possible to determine the position of the object edge in the image with an error of up to 0.01 mm. This value guarantees the fulfillment of the specified error for grooving measurement as well as the rod length measurement error, which is calculated from the values measured by two cameras.
Because the difference between the longest and shortest part design is 44 mm, the ends of the rods would not fit in the camera’s field of view in all cases. For this reason, there are a total of four cameras in the machine, which guarantees that the end of the rod is in some field of view in any case.
Images from the cameras and telecentric lenses are processed in a panel PC. According to the inspection result, the electromagnetic damper located under the drive wheels is activated and synchronized with their movement so as to sort the falling bars into good and unsatisfactory ones. The panel PC display contains machine operation information, inspection statistics and other data.
Machine vision control is therefore fast, non-contact and non-invasive, fully automated and easy to adapt to changes in production.