SCARA robot assembles spigots cost effectively

12 December 2012

To automate the assembly of a moulded plastic spigot, an automotive parts manufacturer asked industrial automation company ALPHR Technology to devise a cost-effective solution that would be compact, reliable and easy to maintain.

After analysing the application, the team concluded that a solution based on an Omron SCARA robot, used in conjunction with a vision system and other automation products from Omron, would be the solution.

The customer supplies injection-moulded spigots to an automobile manufacturer. In the past, assembly of the spigots had been a largely manual process, but an increase in demand for the spigots required an additional production line, so the company decided to investigate the possibility of automated assembly.

The assembly process involves fitting four compression limiters of two different sizes to the spigot. The limiters have to be accurately positioned and pressed into place to a precisely defined depth. Once fitted, they have to be retained with a specified force. The final stage of the process is to carry out a leak test on the completed spigot assembly to verify not only that the limiters have been fitted correctly, but also to check the integrity of the part itself.

The team proposed a machine with a PLC-controlled rotary assembly station, which would produce one finished part every 20 seconds. Just one operator is needed to load parts into the machine, with all other operations performed automatically, including inspection, testing and the delivery of the finished parts to pass and fail bins.

It was decided that a machine using conventional multi-axis pick-and-place technology would be complicated to design and build, inflexible, and ultimately difficult to maintain. A solution based on an Omron SCARA robot would, however, solve all of these problems as well as addressing a requirement for the machine to have a small footprint.

A careful costing of the machine, taking into account not only the cost of equipment and materials, but also the cost of design and construction, revealed the robotic solution to be significantly more financially attractive. These factors made the solution very interesting but the customer had no previous experience of SCARA robots.

Application engineers from Omron used sophisticated software to emulate the operation of the proposed assembly machine which confirmed that the principle of operation of the machine was sound, but also that the required cycle times and efficiencies could be easily achieved. With its questions answered and concerns dispelled by this demonstration, the customer gave the go ahead for the project.

As built, the spigot assembly machine works on a part that the operator has manually loaded into one of the ‘nests’ in the machine’s rotary assembly table. A pneumatic cylinder pushes the part fully home and then the table indexes. Next, the robot takes the four compression limiters, one at a time, from vibratory feeders and places them into the part.

Two Omron FQ vision sensors confirm that the limiters are all present and correctly positioned, and then the table indexes again, moving the part to a station where the limiters are inserted with pneumatic cylinders and then a force push out test is carried out by applying a precisely controlled force.

The table indexes once more to bring the part to the final station where it is pressurised and tested for leakage. Depending on the results of the test, the part is transferred either to the pass bin or the fail bin. Passed and failed parts are automatically counted, and the machine notifies the operator when the bins are ready to be emptied.

The new spigot assembly machine has now been working on site for several months, and is living up to expectations. While, as planned, the rate of production of the new machine is not substantially different from that of the manual assembly line, the percentage of reject parts is greatly reduced, as the dependable robotic placement of the compression limiters and the 100% inspection by the vision sensor’s make it virtually impossible for incorrectly assembled parts to go forward to the final testing stage

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