Quality Digest  |  06/24/2008

Cognex Checker 202 vision sensors

Error-Proofing Oil-Cap Assembly

Two vision sensors are used at the station where caps are pressed onto the O-ring loaded on the assembly dial.

Supplying parts to the automotive industry 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, Wisconsin, facility. 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.

“Machine vision has been a key component of our automation strategy for the last seven years,” says Shane Harsha, MPC manufacturing engineering manager.

An automated oil-cap assembly system is a case in point. MPC engineer Brian Champion recently augmented traditional tooling and sensor technology with Checker vision sensors from Cognex Corp. of Natick, Massachusetts.

The MPC oil-cap assembly system installs O-rings into thermoplastic caps, and then prints on top of the caps. After an O-ring is loaded onto the dial fixture, it indexes to the second station. There, another in-line conveyor feeds blank caps to a second pick-and-place arm that presses the caps onto loaded seals.

Tight control of O-ring and cap orientation is critical to ensure that the oil cap functions correctly. The cap must also be in the right orientation prior to printing. MPC selected three Checker 202 vision sensors to ensure proper O-ring and cap orientation. The first detects inverted O-rings. A second checks that the O-ring is positioned properly on the dial fixture before the cap is pressed onto the ring. A third ensures cap orientation is correct prior to assembly and printing.

Tooling on the feeder bowl was designed to prevent inverted O-rings from entering the process, but the tooling was unreliable. O-rings that were slightly flawed occasionally made it past the tooling and were loaded upside down, causing the machine to shut down.

“Having the operator flip these seals and restart the machine was eating into our efficiency,” says Harsha. “If the production rate dropped from 360 to 200 caps per hour, it cost us about $20,000 a year in downtime. At full production volume, that cost could increase to as much as $120,000 per year.”

Harsha and Champion decided that Cognex Checker 202 vision sensors were suited to this application. They chose the sensors because they include a powerful graphical form of ladder logic that allows tying the Checker’s individual inspection sensors directly to outputs that easily solve more complex applications.

“The small size, built-in lighting, variable working distance, ladder logic, and free-running capability make these devices very simple to install,” says Champion.”

The checker device includes three types of sensors:

Brightness sensors look for light and dark areas.

Contrast sensors check features such as date codes, threads, and barcodes.

Pattern sensors are trained to recognize a feature and signal when it’s spotted.


Champion first trained the part-finding sensor to detect the O -ring. Then he positioned a pattern sensor to verify the presence of the sealing bead. The pattern sensor finds the sealing bead and signals when it is detected, remaining in a fixed position relative to the part-finding sensor so that it is always in the correct location to detect the sealing bead. If the sealing bead is inverted, the vision sensor sends a signal to a pneumatic solenoid that blows the O-ring back into the feeder bowl to be recirculated.

Because it was such an effective solution, Champion decided to error-proof the oil-cap assembly process by adding two more sensors. Champion set up one vision sensor on the moving arm, first training the part-finding sensor to look for the O-ring in the image, then a pattern sensor to verify the presence or absence of the sealing bead. This ensures that the O-ring is properly loaded before the cap is pressed onto the ring.

The final vision sensor mounts just upstream from the pick-and-place arm to check cap orientation. Its part-finding sensor first recognizes a corner radius of the oil cap, then two pattern sensors recognize the graphics on the cap. By training on two patterns, the vision sensor determines cap orientation. If it is not correctly oriented, the sensor signals the pick-and-place arm controller to rotate the cap 180˚ before placing it on the assembly dial.

“Checker vision sensors have helped us achieve zero-defect rates in the manufacturing process, while lowering scrap,” says Harsha. “They are the perfect solution for many of our inspection and error-proofing applications.”


Cognex Checker 202 Vision Sensors


• Inspects unlimited multiple-part features simultaneously with a single sensor.

• Parts on a line typically vary in position, and Checker tracks all of them without requiring precise part handling.

• Sensor detects part by finding an actual part feature, a capability unattainable with photoelectric sensors.

• Employs ladder logic for complex custom inspections.



About The Author

Quality Digest’s picture

Quality Digest

For 40 years Quality Digest has been the go-to source for all things quality. Our newsletter, Quality Digest, shares expert commentary and relevant industry resources to assist our readers in their quest for continuous improvement. Our website includes every column and article from the newsletter since May 2009 as well as back issues of Quality Digest magazine to August 1995. We are committed to promoting a view wherein quality is not a niche, but an integral part of every phase of manufacturing and services.