Cognex prevents assembly defects at MPC

Miniature Precision Components (MPC) is using three vision sensors from Cognex to error proof the automated assembly of oil caps at its Prairie du Chien facility in the US.

With 41 moulding machines ranging in size from 25 to 550 tonnes, this 100,000ft2 facility employs about 450 people.

In addition to oil caps, the facility manufactures a host of other thermoplastic parts and assemblies including PCV valves, thermostat housings, and quick-connect ports for emission control systems.

'We achieve quality through automation and machine vision has been a key component of our automation strategy for the last seven years,' said Shane Harsha, MPC manufacturing engineering manager.

MPC automation and tooling engineer Brian Champion recently augmented traditional tooling and sensor technology with Checker vision sensors from Cognex.

'As Checker vision sensors are simple to set up and easy to install, they offer a cost-effective alternative for inspections where traditional sensors are not reliable and a full-blown vision system is too expensive,' said Harsha.

The MPC oil-cap assembly system installs o-ring seals into moulded thermoplastic caps, and then prints on top of the caps.

The system uses two vibratory bowl feeders, about 4ft 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.

The assembled cap and seal then continue on the rotary dial through the pad printing and final inspection stations to complete the process.

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.

The hard tooling and traditional sensors in the cap assembly system proved unreliable, so 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 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.

'Having the operator flip these seals and restart the machine was really eating into our efficiency,' said Harsha.

MPC corporate headquarters in Walworth had previously introduced Harsha and his team to the Cognex Checker vision sensors.

Harsha and Champion decided these vision sensors were a perfect fit for this application.

After a demo, they chose Checker 202 vision sensors because they include a powerful graphical form of ladder logic that allows tying Checker's individual inspection sensors directly to outputs to 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.

'There was no need to wire them to a PLC, no need to install and wire trigger sensors, and the four-step set up makes it by far the easiest vision sensor that I've ever used,' said Champion.

'Unlike the vision systems that we've used in the past, I am able to set up the entire inspection with Checker in just a few minutes.' The Checker device includes three types of inspection sensors: brightness sensors that look for light and dark areas; contrast sensors that check features containing light and dark areas; and pattern sensors that are trained to know what a feature looks like and then signal when it is spotted.

To detect the inverted O-rings in this application, Champion set up the Checker 202 by first training the part-finding sensor to look for the o-ring in the image.

Then he positioned a pattern sensor in the correct location to verify the presence or absence of the sealing bead.

The pattern sensor looks for the pattern of the sealing bead on the o-ring and then signals when it is detected.

The pattern sensor remains in a fixed position relative to the part-finding sensor so that it is always in the correct location to look for the shape of the sealing bead.

If the sealing bead is missing, the vision sensor sends an output through an optic coupler to a pneumatic solenoid that blows the inverted o-ring off the line and back into the feeder bowl to be recirculated.

As it took less than an hour to set up and install the first vision sensor, Champion decided to completely error-proof the oil-cap assembly process by adding two more.

Both are used at the next station where the cap is pressed onto the loaded O-ring.

One is mounted on the moving pick-and-place arm.

The other is fixed above the inline conveyor that feeds caps to the process.

Champion set up the vision sensor on the moving arm in a similar way to the one looking for inverted O-rings coming out of the feeder bowl, first using 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 allows the vision sensor to ensure the o-ring is properly loaded before the cap is pressed on.

The final vision sensor mounts above the inline conveyor feeding caps to the process, just upstream of the pick-and-place arm that presses caps onto the loaded o-rings on the assembly dial.

This vision sensor checks cap orientation.

However, it was set up in much the same way by first training the part-finding sensor to recognise a corner radius of the oil cap, then training two pattern sensors to recognise the oil-can handle and oil-drop graphics.

By training on two patterns, the vision sensor can determine cap orientation.

If it is not in the correct orientation for installation, the vision sensor signals the pick-and-place arm controller to rotate the cap 180deg before placing it on the assembly dial.

'Checker vision sensors have helped us to achieve zero-defect rates in the manufacturing process, while also lowering scrap,' added Harsha.

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