Unloading empty paint buckets from a pallet and placing them on a conveyer line for filling is fast-paced, stressful and tedious work. One worker moves pallets from a truck, clips retaining straps, and rolls pallets into reach of the other worker. The other worker sweeps or grabs buckets and puts them onto the conveyer to the fill stations.

This de-palletizing process was a “pinch point” in a paint manufacturer’s production line—if the workers could not keep pace the line stopped. So the paint manufacturer asked Faber Industrial Technologies to design and build an automated de-palletizing system using Dalsa’s machine vision components.

As before, pallets are prepared and rolled into a safety cage by one worker, but the eyes and hands of the other worker are replaced by a machine vision system and a robot with custom “end effectors”—the robot’s “hands”—to unpack and pass buckets to production. In comparison to our human abilities, this task seems easy to do, but is really quite challenging.

A pallet of cans is a stack of materials. The top layer is a “picture frame” of wood that, when bound with retaining straps, holds the layers of cans in place without damaging them. Each layer of cans starts with a protective sheet of cardboard. There are 56 cans in each layer and up to 6 layers of cans on a pallet. The bottom of the stack is the pallet itself. Each of these layers needs to be “seen” by the vision system in order to be picked up by the robot’s end effectors. To do this, the camera is mounted so it is looking down on the stack. The robot puts cans on the production line conveyer, and packing material in piles to be recycled (Figure 1).

When a new pallet is brought in, the vision system finds the position of the “picture frame” and directs the robot to remove it. Then the robot removes a layer of protective cardboard and the vision system finds the position of each can. Individual cans that are more than a few inches out of packed, hexagonal grid position are a problem, as we shall see. Once all the cans in a layer are found, the robot arm’s end effectors pick up half the cans in that layer at a time and place them on the conveyer for filling.

The stack of cans is about 6 feet high when first seen, but shrinks to about 8 inches high when all the cans are removed. Thus the vision system camera must adjust to focus on the current layer of cans. When the pallet itself is finally exposed, the robot uses another of its end effectors to pick up and remove the pallet.

The first step in any machine vision solution is to select lighting that emphasizes important part features and suppresses unwanted details. In this application the vision system is in an open-mesh safety cage and is therefore subject to uncontrolled, ambient illumination. Bright lights were positioned at an angle to the can tops to “wash out” most of the influence of ambient illumination. This results in images with bright ovals where the can rims reflect the light and dark centers for the insides of the cans.

The lens is specified by the field of view (FOV), the working distances and the camera specifications. The pallet stack is 48 x 40 inches (width x height), but a slightly larger field of view, 58 x 44 inches, is used to allow for skew in a layer of cans and variations in the location of the pallet. Optical magnification, M, is the camera’s sensor size (SS) divided by the FOV, so M = SS / FOV = 0.00429. The lens must reduce the FOV by about 233 times to fit into the sensor size.

The working distance (WD) is the distance from the camera to the top of the pallet stack. If the camera looks straight down on the pallet stack, the WD is about 48 inches. The lens focal length = WD * M / (M + 1), where M is the magnification computed above. This gives a lens focal length of 5.2 mm.