See and sort see-through plastics

A systems integrator combined a special lighting technique, a specific camera-acquisition mode, image processing in color space, and a well-synchronized rejection device to create a new machine vision solution.

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Vision SystemBy Charles Magnan, Averna Vision & Robotics Inc.


RECYCLED PLASTIC is only as pure and profitable as its sorting and presorting processes can make it. So, plastic recyclers and their operations must precisely separate plastic by category because the purity of each group (PETE, PVC, PP, etc.) correlates directly to revenues. Because these processes and their products often aren’t consistent, however, added personnel usually are needed to do manual sorting.

Though this means automation has an opportunity in many recycling applications, technological limits and costs have restricted its adoption. “Existing automation techniques for plastic sorting include using X-rays and infrared spectrometers,” says Sebastien Parent, principal engneer at Averna Vision & Robotics Inc., a system integrator in Montreal. “The advantages of these techniques are that they can determine the nature of a plastic by its signature in the wavelengths at a molecular level. However, these technologies are complex and expensive.”

A Visual Opportunity
The benefit of using machine vision in the visible light domain is lower cost and that it can use standard machine-vision algorithms. To develop an inexpensive, automated sorting system for clear/transparent polyethylene terephthalate (PETE) plastic with machine vision, our company recently combined a special lighting technique, a specific camera-acquisition mode, image processing in color space, and a well-synchronized rejection device to create a solution, which would operate at 40 images per second.

Averna's Automated Sorting System
Averna’s automated sorting system identifies transparent plastic, and coordinates with 20 air nozzles to blow identified bottles into a bin.

Sorting Out Sorting 
This request for Averna’s vision system came from one of our existing clients, a large plastics recycling facility in eastern Canada, which collects material from all over the nation’s eastern provinces. This recycler takes in plastic that’s already been partially compacted and sorted, and had used three people to further presort it, mostly to separate transparent from opaque plastics. Thorough inspection and sorting is vital because PETE plastic can be recycled many times only if it maintains high purity. The team works on a line that moves at about 40 ft/min, and processes about 5,000 lbs/hr of plastic bottles with a 5-10% error rate. This facility required a more automated presorting system to improve accuracy, and also needed to smooth some peaks in its operating schedule.

“We developed a Bottle Sorter and Inspection system that could go as fast as the recycler’s other equipment, and do a good enough job that the facility’s staff could be redeployed in other areas,” says Parent. The vision-based system was developed nine months ago, and installed six months ago.To achieve low-cost automated plastic sorting, Averna selected National Instruments’ (NI) hardware and software tools to ensure quick testing, efficiency, and robustness. “The new system uses a high-resolution camera and NI’s vision builder software, computer, and I/O cards,” adds Parent. “These tools helped us develop and fine tune our new inspection system in about half the time it takes to build a traditional vision system, and it only cost about one-third as much as a typical $200,000 vision system.”

Coordinating Vision and Actuation
The plastic parts to be sorted arrive via an in-feed conveyor, which drops bottles in front of a camera looking at a specially designed backlight. As long as no plastic, labels, or opaque plastic blocks the lighting, the camera acquires no information. However, as soon as a translucent plastic comes between the camera and the lighting, it’s detected by the inspection system, which activates the sorting process. When the system detects a piece of colorless PETE plastic, a series of 20 coordinated air valve/jet nozzles fire to push that item into a different recycling bin. Other material types, such as colored PETE, opaque plastic, labels, and non-PETE, fall onto an output conveyor without disturbing the drop path (see sidebar).

The challenge for the new vision-based sorting system was to perform its required tasks in the time available to ensure 100% plastic inspection. The conveyor’s speed is 4.5 ft/sec, but the speed of the plastic parts is around 9 ft/sec due to added acceleration caused by gravity. “To accurately handle these falling items, the system must process 40 images (64 x 640 pixels) per second in a 2.8 in. high x 28 in. long field of view with a resolution of 0.044 in. per pixel,” says Francois Vachon, Averna’s vision system developer. “At the end of each process, the system applies rules to decide which air nozzle to open.”

To achieve the needed image acquisition rate with the selected field of view, we selected a CMOS color camera from PixeLink that increases frame rate by “windowing” the sensor. This method focuses on a small area in the view field to increase the acquisition rate, and get a more optimum per second image flow. “Synchronization was a challenge, but we now can trigger more than one nozzle if a bottle shows up in two images if it isn’t falling as fast as the others,” says Parent. “To help distinguish transparent plastics from others, we altered our system’s lighting to use the differences that occur when light passes through transparent plastic.”

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