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SCARA triumphs over 'pick-and-place'

11 February 2013

A leading automotive parts manufacturer recently asked the experts at machine builder, ALPHR Technology to automate the assembly of a moulded plastic spigot. Their requirements were simple – they wanted a system that would be cost-effective, compact, reliable and easy to access. However, the necessary processes to achieve these criteria were not so simple.

After careful consideration and a thorough analysis by the ALPHR team, the decision was taken to base the new assembly module on SCARA (selective compliant articulated robot arm) technology. An Omron SCARA robot, used in conjunction with a vision system and other Omron automation products were the products of choice for this project.

The automotive supplier, whose identity we are not at liberty to reveal for the purposes of this article, largely specialises in designing and producing precision injection-moulded parts and it has been supplying injection-moulded spigots to a leading car manufacturer for some time. In the past, the assembly of these components had been a largely manual process, but when an increase in demand for the spigots meant the need for an additional production line, the company decided to investigate the possibility of automated assembly.

In essence, 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.

Following detailed discussions, the ALPHR team proposed a machine with a PLC-controlled rotary assembly station, which would produce one completed part every 20 seconds. It would only require one operator, who would need to load the parts into the machine, and everything else would be conducted automatically, including inspection, testing and the delivery of the finished part, either to a 'pass' or 'fail' bin.

ALPHR concluded that a machine using conventional multi-axis pick-and-place technology would not only be complicated to design and build, but also inflexible, and ultimately difficult to maintain. A system based on a SCARA robot, on the other hand, would solve all of these problems as well as addressing the client’s request for a machine with the smallest possible footprint. Not least, when taking the cost of the machine into consideration, including the price of all equipment and materials as well as the entire design and construction, the Omron robot proved to be the most financially attractive option.

But there was one problem: while the client had a wealth of experience using large, multi-axis robots, it had no previous involvement with SCARA robots, and it needed assurance that the new machine would perform as predicted. Application engineers from Omron provided this assurance by using software to simulate the assembly machine operation. This not only confirmed the machine’s viability, but also that the required cycle times and efficiencies could be achieved.

The machine produces the assembled spigots in the following way: an operator first loads them manually into one of the ‘nests’ in the machine’s rotary assembly table, and a pneumatic cylinder then pushes the part fully home where 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 the table indexes again, moving the part to a station where the limiters are inserted by pneumatic cylinders. A force push out test is then 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, the latter being kept behind a locked door within the machine to ensure that defective parts can never be accidentally mixed with good products. Passed and failed parts are automatically counted, and the machine notifies the operator when the bins are ready to be emptied. ALPHR Technology’s Paul Bridgwater takes up the story:

“Understandably, as this was the company’s first machine with a SCARA robot, they kept a close eye on this project, so I’m delighted to say that it went very smoothly. Programming the robot was straightforward and it worked ‘straight out of the box’, as did the vision system and all the other key automation components supplied by Omron.”

The new spigot assembly machine has now been working on site for several months, and is fully 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 percent inspection by the vision sensors make it virtually impossible for incorrectly assembled parts to go forward to the final testing stage.

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