Inspecting piston-ring fit

13 May 2014

The Renault site in Cléon, France, employs LJ-G laser displacement sensors from Keyence on its G9 engine production line to inspect the fit of piston-rings.

The powertrain site chose the sensors because of their detection reliability and ease of use. 

Piston rings have open ends that are spread apart to allow them to be slid into grooves specially machined on engine pistons. At Cléon, this is done by an operator, who then places the fitted pistons on pallets. These pallets are moved to an automated inspection platform where four pistons are simultaneously checked by four LJ-G sensor heads. 

A profile is generated for each piston and this is compared to a ringless profile to ensure that the rings are in place. Because piston rings play a crucial role in engine operation, they must be inspected to ensure that they are properly fitted.

"Our priority in designing this inspection was to not be tied down by the measurement conditions and to reduce false defects to a minimum," expalined Patrick Guehennec, industrial resources officer at Renault's methods department."We had thought about a vision system, but there were too many lighting issues and it was hard to get a sharp image."

The Cléon site specified the LJ-G laser displacement sensor for the application."It was very easy to set up and we liked its user-friendliness," said Guehennec. "Overall, the laser profile obtained with the LJ-G allows stable detection regardless of the lighting conditions or surface type."

The LJ-G offers a quality control solution in that detection is stable regardless of reflections and light conditions. It features an E3-CMOS sensor which compensates for light intensity and has a dynamic range that is said to be 300 times wider than that of conventional sensors. 

E3-CMOS (Enhanced Eye Emulation CMOS) technology replicates the accommodation function of the human eye and creates complete digital reproductions of surface profiles despite dark or overly bright areas. This gives the LJ-G good measurement stability, even with complex surfaces such as elastomers and metals. In addition, the MFL function adjusts the CMOS sensitivity, the laser intensity and the exposure time (the darker the target, the longer the sampling time and vice versa).

Although the smallest ring inspected at Cléon is just 1mm thick, the LJ-G's accuracy of a few microns ensures problem-free detection. 

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