Machine Vision Guides Robot Motion

Machine vision can quickly and accurately determine the location of parts so they can be inspected, measured, or manipulated by a robot. An example is using machine vision to guide a robot unpacking one-gallon cans from a large pallet of cans. Machine vision components — cameras, vision processors, and software — were provided by DALSA, and Faber Industrial Technologies developed the can-picking robot and integrated the robot with the machine vision.

The cans arrive on a large wooden pallet stacked in six layers, each with 56 cans. A brown paper “slip sheet” separates each layer and the stack is topped with a wood rectangle called a “picture frame.” Three workers de-palletized the cans. One worker moved the pallet into the factory and cut the retaining straps, while two unpacked the cans. The unpacked cans are placed on a conveyer belt for filling and sealing. It was difficult for workers to keep up with the demand from the can fill and sealing line — about 1.5 cans per second — so Faber was asked to develop a visionguided robot system to take over can depalletizing and placement on the fill line conveyer belt.

Viewing the Work Area

A machine vision system works best with an undistorted view of properly lighted and well-controlled parts. The first tasks in developing a machine vision application are to fix the view geometry, control the lighting, and limit the variation of the presented parts. That’s the theory, but the practice was quite different in this application.

There was no practical way of controlling the lighting in this situation, as the area had to be open to allow pallets of cans to be presented to the unpacking robot and to allow the robot to place cans on the fill line conveyer belt. Bright, high-frequency fluorescent lights were used so that changes in ambient illumination would have a relatively small effect on images of the parts. The best view geometry would be to put the camera directly above the pallet, looking down on the layers of cans. A layer of 6"-diameter cans is about 48 by 40" and, to allow for variation in pallet position, a field of view (FOV) of 5.3 by 4' was used. To get this large field of view with low optical distortion, a camera would be far above the top layer of cans, perhaps 16'. The factory ceilings were 12' high and it was not feasible to knock a hole in the roof.

To get the required FOV, a camera with a short focal length lens was mounted above and at an angle to the can layers. Due to optical distortion from the short focal length lens, the can openings don’t look circular. The vision system corrects the image for some of this distortion. Due to perspective, the FOV decreases as each layer of cans is removed. The vision system compensates for changes in FOV and plane of focus by adjusting the lens’ zoom and focus for each layer of cans.