Vision sensors play key role in lock assembly

One of the world's largest lock manufacturers has begun to use machine vision systems to automate inspection during manufacture.

Compx Fort Lock of Illinois, USA has more than 50 years of experience manufacturing and supplying cam, switch, and special purpose locks to a wide variety of industries which include personal computers, automotive products, security devices, office furniture, mail boxes, lockers, safes and coin-operated devices.

To ensure the proper functionality of their locks, the company has recently implemented new machine vision sensor technology to check for the presence of springs during the assembly process.

Up to six springs per lock are inserted during the automated assembly process, and each one needs to be present and correctly positioned in order to keep lock "tumblers" in contact with an inserted key as it turns.

"In the past, we were mechanically checking for the springs, however the process involved contact between the probe and the part, and occasionally this would cause a spring to bounce out of the assembly", explains Mark Mazanec, Manufacturing Engineer at Fort.

"This would require reworking of the parts, which was ultimately affecting our throughput.

At that point, we knew we needed a noncontact form of checking the parts, and decided to look into adding a vision system".

Deciding that a PC-based vision system would be overkill for the application, Mazanec looked into low-cost vision sensors and chose an In-Sight 2000 from Cognex.

In-Sight is a high-performance vision sensor featuring an industrial-hardened vision processing unit, a separate high-speed digital camera, onboard light control, built-in discrete I/O, and a standard VGA display output.

"The main factor in choosing the sensor was cost", says Mazanec.

"We realised that In-Sight had everything we needed to get the job done at a price we were comfortable with, which meant we didn't have to spend money for a PC and everything that goes along with that.

Whether or not a PC-based vision system was warranted in this case is debatable".

During the manufacturing process, springs are automatically loaded into a lock assembly, which moves station to station on an indexing table.

As the lock moves into the inspection station, it stops under the vision camera, which is placed 3.5in above.

"Establishing the camera field of view was challenging for this inspection, because if the camera was too high up, it would have trouble finding the springs, whereas if we went in too close, you couldn't get all the springs in one picture", says Mazanec.

"We moved the camera at 0.5mm increments until we got the field of view that works best for us".

The vision camera, which is set in continuous acquisition mode, captures an image of the lock assembly and transfers it to the vision processor.

There, the image is analysed by the sensor's PatFind part location software tool, which verifies the presence/absence of each spring.

The entire inspection cycle time, between the initial camera trigger and inspection result, occurs in a fraction of a second.

According to Mazanec, a key attribute of the PatFind tool is its ability to locate springs even when lighting is poor, or when springs vary in their anchored position from part to part.

On verifying the presence and proper position of each spring, the vision sensor sends a "pass" signal to a PLC, and the indexing table moves the part along to the next station.

If the part fails the inspection, it is indexed to a rework station.

Green and red "LEDs" appear on the user interface screen, making it easy for operators to monitor activity.

Having no prior machine vision experience, Mazanec was able to set up the vision application on his own using the sensor's spreadsheet interface.

The process involved selecting vision tools and parameters from drop-down menus using a handheld control pad.

The vision spreadsheet then automatically generated tool results into worksheet cells, which were then linked together to perform the assembly verification.

Since the vision sensor was installed, it has been running for 8h per day without any failures.

According to Mazanec, the company is able to better ensure product quality, and can realise higher throughput rates.

"The In-Sight sensor has greatly improved our production process.

It won't let bad parts through, and unlike the mechanical checking process, it won't cause bad parts.

Our initial goal was having a non-contact system that would allow us to get our maximum throughput, and we have finally reached this point".

Fort Lock currently has plans to add two additional vision sensors to its production line, and may also be considering the use of vision to verify stamped codes on lock keys.

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