Feature Sensors & Sensing Systems
CARRYING OUT ACCURATE PISTON RING INSPECTIONS
PSA carries out accurate automatic piston ring inspections using Keyence laser displacement sensors
ocated 20km from Metz, the Trémery factory has a staff of 3,740 and specialises in assem- bly of four cylinder diesel engines and has a daily production of 6,500 units, which makes it the biggest diesel engine assembly factory in the world, and the fourth largest taking all power sources together. The piston rings are fitted by a robot. The rings feature a gap and when they are expanded, they slide into grooves which have been cre- ated into the piston to house them. The pistons are critical components whose main purpose is to prevent combustion gases from escaping into the engine crankcase, so as to avoid loss of gas pressure and engine power. They also help to keep the engine’s lubricating oil out of the combustion chamber, while leaving a lubricating film of a predeter- mined thickness over the full sur- face area of the cylinder walls. Correct fitting of these moving seal- ing parts is essential to keep the engine running efficiently.
L Three rings
Pistons are fitted with three rings - the scraper ring (which is the most critical in regards to inspection because it is very thin and has a small spring that breaks easily), the com- pression ring, and lastly the top or ‘fire’ ring that is 3mm high and is the easiest to inspect. The piston rings are checked automatically at the Trémery PSA factory. A robot places the piston on a turntable, and four sensors are located on each side of the piston, opposite the first two rings. Each sensor inspects half of a ring. An initial rotation through 180° is made to check the first two rings with the four sensors. The turntable then rises along the Z-axis. A second rotation is
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made in the opposite direction to inspect the third ring, using two sen- sors. If a ring is found to be non-con- forming, it is inspected again, and then a third time. If the defect is con- firmed, the piston is sent to an opera- tor for further analysis. Claude Tritz, an assembly mainte- nance technician at the Trémery PSA factory, commented, “The displace- ment sensors measure all the dimen- sions of each of the half perimeters that they are analysing. About 2,500 mea- surements are made per full turn, giving a pitch of less than one tenth of a millimetre. An inspection cycle of the three rings takes about five seconds. “We have determined thresholds to check the presence and the position of the three rings in the middle of the groove, together with the size and position of the gap. We also check the spring on the scraper ring via its gaps. To do so, we have selected the Keyence LK-G32 high precision model with a small, 30 micron spot, as otherwise the spot was too big to enter the gaps.” Two LK-G models are used - two heads with small spots, 30 microns in diameter (LK-G32), to inspect the scraper and compression rings, and two LK-G82 heads to detect the top ring. The LKG sensors are laser displace- ment sensors. They provide a sam- pling speed of 50kHz with a repeatability level of 0.05µm for the most accurate models. Piston rings are reflective parts, and this affects the measurements. The LK-G range enables stable detection of transpar- ent plastic and metal objects, thanks to its correction functions.
The ABLE (Active Balanced Laser control Engine) technology detects the surface of a target and adjusts the laser light intensity to the optimum level. ABLE intelligently controls
Below: the Trémery factory, set up in 1979, specialises in assembly of four cylinder diesel engines and has a daily production of 6,500 units
three parameters - laser emission time, laser power level, and gain (CCD amplification factor), thus pro- viding a wide range of light intensity adjustment. The MRC (Multiple Reflection Cancel) algorithm enables elimination of multiple reflections from a metal surface. When at least two peaks are generated by multiple reflections, the algorithm compares the wave forms with the last light wave form received and determines which one shows the greatest similar- ity with the ‘correct wave form’.
Summary Tritz concluded, “We have been using this solution for years now - in com- parison to the previous solution, we have considerably reduced the num- bers of false defects. It is to be noted that 3,000 engines are assembled on a continuous basis every day on the lines inspected by the LK-G sensors, in a very difficult environment. Nonetheless, the LK-G sensors stand up to the conditions very well.”
Keyence
www.keyence.co.uk T: 01908-696 900
Enter 213 NOVEMBER/DECEMBER 2013 Automation
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