Addition of machine vision enhances rivet handling

  • 23-Apr-2009 08:33 EDT
Fig. 2 - RFS-1000 Rixan Feeding System.jpg

The Cognex In-Sight vision system is shown at top center of the vibratory feeding system.

The vibratory bowl conveyor provides an economical and reliable method of presenting small parts in a given orientation. However, a key weakness of the vibratory bowl feeder is that its use of mechanical channels and guides to orient parts limits it to handling just one part or a family of similar parts. The task of loading aerospace fasteners into a press would seem to be ideally suited for automation with a vibratory bowl feeder and a pick-and-place robot. The problem is that if hundreds of parts are involved, it would require dozens of different feeders.

Rixan Associates, Inc. developed a unique take on the vibratory bowl feeder concept by using machine vision to replace the channels and guides. With machine vision, a virtually unlimited number of parts within a certain size range can be handled. The feeder uses the vibratory bowl concept to randomly present the parts in the bowl in any possible orientation. When the vision system views the right part in the right orientation, it stops the feeder and sends the part’s location to a robot which loads the part into a press, for instance.

The patent-pending Rixan RFS-1000 flexible vibratory bowl feeder system, which costs about $45,000, provides a 20% improvement in productivity and eliminates the need for an operator. Return on investment for most users is accomplished in less than one year, according to Cognex, which supplies the vision system.

Traditional vibratory bowl feeders use a helical shelf-like inclined ramp winding upwards from the bottom of a bowl to a discharge passageway. The passageway’s reciprocating motion pushes the parts up the ramp. Obstructions, cutouts, and other orienting devices are arranged along the passageway to topple off parts that are improperly oriented and allow passage for only those parts that are properly oriented.

Vibratory bowl feeders are normally able to handle only a single family of very similar parts. Each different part in a family requires changing or adjusting the obstructions and orienting devices, which can take considerable time. As a result, despite its high efficiency and relatively low cost, the vibratory bowl concept is usually not seriously considered for applications involving large numbers of different parts.

Stephen Harris, President of Rixan Associates, felt that recent improvements in the speed, accuracy, and reliability of vision systems would make it possible to use a vision sensor to replace the channels and guides. He got the opportunity to validate this concept when he bid on a contract for processing a wide variety of high-value aerospace fasteners in a stamping press.

Previously, an operator picked parts out of a bin and loaded them in the proper orientation in the press. The manufacturer wanted to automate this operation because of the potential danger involved in having the operator place his or her hands into the press. Additionally, the repetitive nature of the task required the operator to take frequent breaks, limiting productivity.

Rixan engineers, led by Chief Technical Officer Mark Battisti, developed a flexible feeding concept that uses a track on a 24-in vibratory bowl without the obstructions and cutouts. Every part that reaches the top of the bowl is presented onto a raised semitransparent plenum so the vision system mounted above can easily identify the type of each part and its orientation. The correct parts are picked by the robot, the incorrect ones recycled into the bowl.

Used in the system are a Cognex In-Sight 5401 vision system and a Mitsubishi Electric RV-6SL-S11 robot that have been integrated through a joint development effort between those two companies. Mitsubishi Melfa-Vision software incorporates robotic programming software as well as Cognex In-Sight Explorer software to set up both the robot and the vision system simultaneously as an integrated solution. This new approach makes it easy to create vision-driven robotic applications with capabilities such as locating and inspecting parts that go far beyond what robots alone can accomplish.

Melfa-Vision includes standard job programs supporting most common robot applications, enabling novices to easily develop robotic vision applications with up to three robots and seven vision sensors. Rixan engineers picked the Mitsubishi Electric robot because it is fast—moving at up to 6.0 m/s—and because it has a long reach. They selected the Cognex In-Sight 5401 vision system because it provides up to seven times the processing power of other models and acquires up to 60 full 8-bit images per second.

To develop the application, Rixan engineers first ran the Melfa-Vision wizard that calibrates the robot in relation to the feeder. Then, they put a part in the robot gripper in the ideal orientation for pickup. They jogged the robot over the feeder and set the part down on the plenum. This process taught the robot to pick up the part in this orientation at this point on the plenum. Next, they selected a Melfa-Vision wizard that runs the Cognex PatFind object location tool and drew a rectangle around the part on the screen. The PatFind tool instantly recognized the part in the zero angle of orientation. This process, which took only a few minutes, is all that was required to program the feeder to handle a single part.

From this point on, the vision system automatically recognizes the specific part as it moves across the plenum. When it sees the part, the system sends a signal to stop the feeder. It then acquires one last image to determine the exact position of the part and calculates its angle of presentation. The robot moves to the location of the part, twists its wrist to match the part’s orientation, picks up the part, and sends a signal to restart the feeder once all “pickable” parts have been removed.

Most parts in this application are symmetrical top to bottom so they can be identified from their silhouette with the plenum backlit as described above. Some other parts must have a certain side facing upwards before they can be loaded on the press. The Cognex PatMax tool is used to recognize which side is up on these parts. All that is required to train the vision system is placing a part on the plenum in the proper orientation and selecting the PatMax feature from within MELFA-Vision. Top-down lighting is used for parts where pattern recognition is required. White lighting is used for some parts and glare-free polarized lighting for others.

The fastener manufacturer originally ordered a single Rixan feeding system and programmed it for a few parts. The first feeding system was so successful that the manufacturer has already purchased two additional systems and is in the process of programming them to handle its full range of parts. The feeding system is slightly slower than an operator working at full speed but, since the system does not take breaks it provides a productivity increase of 20% compared with a human operator. The vision system also eliminates errors that can occur in manual feeding such as putting the part into the press in the wrong orientation, potentially damaging the tooling and causing substantial downtime.

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