A system has been developed to replace manual measurement of small jet engine blades during safety and performance inspections. The high-speed, automated system consists of a non-contact laser sensor, signal processing hardware, and standard off-the shelf designs, fixturing, and sensor software. Inspection is more efficient, repeatable, and reliable than alternative methods, such as solutions that involve fastening the engine on an inoperable grinder to manually index the rotary intake.
The inspection system has been installed at a southeastern aircraft overhaul facility. To automate the inspection process in this application, the laser sensor was positioned at a diagonal position below the rotor. As the rotor spins, the sensor measures the length of each blade tip from the centerline of each wheel. Measurements are made within 200 milliseconds with a tolerance of ±0.005 in. and resolution of ±0.0001 in. Signals from the sensor are sent to Oryx's PC-based controller. The controller software provides a report of the average readings for all the blades in each rotary intake along with high and low readings. The system automatically monitors the position of each blade within the intake to assure a very precise fit. Also, a report gives the average measurement for each stage in order to select the blades that need to be replaced or repositioned. After replacement, the system also assures that new blades are positioned properly within each intake and ground down to meet specifications.
A second installation is planned at a Canadian aircraft facility that overhauls CF34 jet engines. Additional applications include measurement of commutator segments in electric motors and grinding calendared rolls.
The system's laser sensor, LMI's Laser Twin Sensor (LTS), performs the measurement function. It integrates twin viewing axes and emits a single laser beam that strikes the object's surface, reflecting a spot (30 µm in diameter) 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 also compensates for intensity variations within the laser spot and lateral sensitivity of the sensor is greatly improved. Analog signal processing electronics translate the PSD output currents into a distance.
Differences in the color and surface (i.e. frosted and refurbished surfaces) of an engine blade, or a variety of objects in other applications, can affect measurement data. The laser sensor's controller software automatically compensates for this occurrence with variable power to the laser diode. When averaged at 100 Hz, the sensor can resolve thickness variations as fine as 0.007 um. It also has a large observation angle of 43 degrees at stand-off and the ability to optimize contour tracking with sharp increases in object height.