By Joe Feeley, Editor in Chief
The vibratory bowl conveyor used in many parts-picking operations provides an economical and reliable method of presenting small parts in a given orientation. However, a key weakness of the vibratory bowl feeder lies in its dependence on mechanical channels and guides to orient parts, which 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 in an application presented to Rixan Associates (www.rixan.com) of Dayton, Ohio, was that the 415 parts involved would have required dozens of different feeders.
Rixan Associates developed a unique take on the vibratory bowl feeder concept by using machine vision to replace the channels and guides in order to handle a virtually unlimited number of parts within a certain size range.
Traditional, but Inflexible
Traditional vibratory bowl feeders use a helical, shelf-like, inclined ramp winding upward from the bottom of a bowl to a discharge passageway. The reciprocating motion pushes the parts up the ramp. Obstructions, cutouts and other orienting devices arranged along the passageway cause parts that are improperly oriented to topple off and allow only those parts that are properly oriented to pass through.
"Vibratory bowl feeders normally can handle only a single family of very similar parts," says Stephen Harris, president. "Each different part in a family requires changing or adjusting the obstructions and orienting devices, which can take considerable time or require the high expense of multiple bowls. With a typical bowl feeder, a part rides up a ramp, and then the part will fall back into the bowl if it did not get oriented correctly. It's really used to singulate and orient the part so it can be fed."
As a result, despite its high efficiency and relatively low cost, the vibratory bowl concept usually is not seriously considered for applications involving large numbers of different parts.
Harris felt that recent improvements in the speed, accuracy and reliability of machine vision systems would make it possible to use a vision sensor to replace channels and guides in the design of a flexible vibratory bowl feeder. "We got the opportunity to validate this concept when Rixan competed for a potential customer application that processes a wide variety of high-value aerospace fasteners in a forging press," says Harris.
Previously, an operator picked parts out of a bin and loaded them in the proper orientation in the press. This was an entirely manual process—just an operator and bins of different parts that had been made all at once. The manufacturer wanted to automate this operation because of the potential danger involved in having the operator place his hands into the press. Additionally, the repetitive nature of the task required the operator to take frequent breaks, which limited the productivity of the application.
New Vibratory Feeding Concept
Rixan engineers, led by Chief Technical Officer Mark Battisti, developed a unique flexible feeding concept that uses a track on a 24-in. vibratory bowl without the obstructions and cutouts (Figure 1). "Every part that reaches the top of the bowl is presented onto a raised semi-transparent plenum so the vision system mounted above easily can identify the type of each part and its orientation," says Battisti. "The robot picks the correct parts, while incorrect parts are recycled into the bowl."
It can be used two ways. "In one case, all the parts are the same, but it only picks those parts in the proper orientation for the press loading" says Harris. "In the second case, multiple parts could be run in the same bowl but only the wanted part in the proper orientation is selected for load. The beauty of it is there's no need to clean out the feeder during a shift. Simply do the setup for a new die, load those parts into the hopper and select the part you want to feed from the operator menu and go."
Rixan selected a Cognex In-Sight 5401 vision system and a Mitsubishi Electric RV-6SL-S11 robot that were integrated through a joint development effort between the two companies. Mitsubishi Melfa-Vision software incorporates robotic programming software, and Cognex In-Sight Explorer software is used to set up both the robot and the vision system simultaneously as an integrated solution. This new approach makes it easier to create vision-driven robotic applications with capabilities such as locating and inspecting parts that go far beyond what robots alone can accomplish.
You could buy the vision system separate from the robot and robot software and integrate them, but Battisti thinks that's not a good idea. "That would be an option," says Battisti, "but the process of getting a camera and a robot talking to each other sometimes involves special programming and is costly to do on a one-off basis. Cognex and Mitsubishi aren't the only companies to collaborate like this, but the other important piece in our selection is the teach-a-part wizard concept that is a part of the higher-level software package and is the key offering of the Melfa-Vision software."