2009

Using Vision Sensors to Eliminate Manufacturing Defects

Supplying parts to the world’s leading automotive companies leaves no room for error. That’s why Miniature Precision Components Inc. (MPC) uses three vision sensors to error-proof the automated assembly of oil-caps at its Prairie du Chien, WI facility. With 41 molding machines ranging in size from 25 to 550 tons, this 100,000 square-foot facility employs about 450 people. MPC’s four manufacturing plants generate approximately $167 million per year supplying the automotive and commercial industries with high-quality injection-molded parts and assemblies such as PCV valves, thermostat housings, and quick-connect ports for emission control systems.

alt“We achieve quality through automation, and machine vision has been a key component of our automation strategy for the last seven years,” explains Shane Harsha, MPC Manufacturing Engineering Manager.

altAn automated oil-cap assembly system is a case in point. MPC Automation and Tooling Engineer Brian Champion recently augmented traditional tooling and sensor technology with Cognex Checker vision sensors. The MPC oil-cap assembly system installs o-ring seals into molded thermoplastic caps, and then prints information on top of the caps. The system uses two vibratory bowl feeders, about four feet in diameter. One feeds o-rings and the other feeds oil caps into the process. A vibratory bowl feeder consists of a large bowl with a spiral ramp up the side. As the bowl vibrates, the parts work their way singly up the ramp, to an inline conveyor. At the end of the inline conveyor a pick-and-place arm transfers o-rings to the first station on a rotary assembly dial.

After an o-ring is loaded onto the dial fixture, it indexes to the second station. There, another inline conveyor feeds blank caps from the other vibratory bowl feeder to a second pick-and-place arm that presses the caps on top of the loaded seals. Then the assembled cap and seal continue on the rotary dial through the pad printing and final inspection stations to complete the process.

Control Part Orientation

Tight control of o-ring and cap orientation is critical to ensure the seal is properly installed so that the finished oil cap will function as intended. The cap must also be in the right orientation prior to pad printing in order to meet stringent quality requirements.

Because the hard tooling and traditional sensors in the cap assembly system proved unreliable, MPC selected three Checker 202 vision sensors to ensure proper o-ring and cap orientation. The first detects inverted o-rings between the vibratory feeder bowl and inline conveyor. A second vision sensor checks that the o-ring is positioned properly on the dial fixture before the cap is pressed on. A third ensures cap orientation is correct prior to assembly and printing.

Each o-ring has a sealing bead on one side. The bead must be placed face down when the seal is loaded onto the assembly dial. If it’s not, the machine shuts down. The operator must then access and reposition the seal before restarting the machine. Mechanical tooling on the feeder bowl was designed to prevent inverted o-rings from entering the process. However, the tooling was unreliable, according to Champion. O-rings that were very slightly warped or not perfectly flat occasionally made it past the tooling, and were loaded upside down causing the machine to shut down.

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