Laser sensor vision system improves surface quality inspection

Jet engines typically contain hundreds of small stainless steel and Inconel fan blades to generate thrust. These blades usually wear and bend due to erosion and foreign object damage during normal flight operations. Overhaul facilities regrind or replace these blades after manual inspection. This slow, labor-intensive process checks the precise length and position of each blade in the engine's ...

Jim Montague, news editor

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Jet engines typically contain hundreds of small stainless steel and Inconel fan blades to generate thrust. These blades usually wear and bend due to erosion and foreign object damage during normal flight operations. Overhaul facilities regrind or replace these blades after manual inspection. This slow, labor-intensive process checks the precise length and position of each blade in the engine's rotary intake.

To help replace manual inspection and measurement, LMI Technologies and system integrator Oryx Systems have jointly developed a vision system that can accurately inspect the blades on the fly at high speeds. LMI provided its Laser Twin Sensor (LTS) non-contact laser sensor, while Oryx contributed software, signal processing hardware, off-the-shelf designs, fixturing and a sensor software package. LMI reports that its LTS has twin-viewing axes and projects a small laser spot, about 30 micrometers in diameter, which allows it to meet extreme accuracy requirements.

LMI and Oryx recently implemented their first blade inspection system at an aircraft overhaul facility in the Southeastern U.S. Their second system will be installed soon at a Canadian aircraft facility that overhauls CF34 jet engines.

David Lacey, Oryx's president, reports that because the two firms' blade inspection system can measure very small items moving at high speeds, it can also be used in similar applications. These include commutator segments in electric motors and grinding calendared rolls.

Automating measurement

To automate the formerly manual blade inspection process, Oryx positioned the LTS sensor at a diagonal position below the rotor. As this rotor spins, the sensor measures the length of each blade tip from the center line of each wheel. These measurements are made within 200 milliseconds with a tolerance of 60.005 in. and at a resolution of 60.0001 in. LMI and Oryx's system likewise monitors the position of each blade in the intake to assure a precise fit.

LTS' measurement process begins when its sensor emits a single laser beam that strikes the blade's surface, and then reflects a spot through twin imaging optics onto two position-sensitive detectors (PSDs). If the reflected spot on one PSD is obscured by a change in the blade's surface, the other PSD will see it. Using the two PSDs compensates for intensity variations in the laser spot, which improves lateral sensitivity of the sensor.

Analog signal processing electronics then translate the PSD output current into a distance. Because differences in the blade or other object's color and surface can affect measurement data, LTS' software automatically compensates by varying power to the laser's diode.

Consequently, LTS can resolve thickness variations as fine as 0.007 mm when averaged at 100 Hz. It also has a 43-degree observation angle at stand off; ability to optimize contour tracking with sharp increases in object height; and can measure on frosted and refurbished surfaces.

Signals from the sensor are sent to Oryx's PC-based controller, and its software generates a report of the average readings for all the blades in each rotary intake, along with high and low readings. LMI and Oryx's system also reports the average measurement for each stage, which allows selection of blades that need to be replaced or repositioned. Following replacement, the system also makes sure that new blades are positioned properly in the intake, and that they're properly ground down to meet specifications.

"This system eliminates the need for by-hand measurements, which drastically reduces set-up time," says David Lacey, Oryx's president. "Repeatability is also much more consistent with our LMI/Oryx system because blades can be loaded uniformly outward. Manual measurement makes it difficult to load each blade in exactly the same manner."

For more information, visit LMI Technologies at www.lmint.com or Oryx at www.oryxsystems.com .

jmontague@reedbusiness.com