Industrial sensors and control – The basics – Part V
CRACK DETECTION SENSORS FOR COMMERCIAL, MILITARY, AND SPACE INDUSTRY USE
Accurate and precise detection of crack propagation in aircraft components is of vital interest for commercial and military aviation and the space industry. A system has been recently developed to detect cracks and crack propagation in aircraft components. This system uses optical fibers of small diameter (20 to 100 ?m), which can be etched to increase their sensitivity. The fibers are placed on perforated adhesive foil to facilitate attachment to the desired component for testing. The fiber is in direct contact with the component (Fig. 6.26). The foil is removed after curing of the adhesive. Alternatively, in glass-fiber-reinforced plastic (GFRP) or carbon-fiber-reinforced plastic (CFRP), materials used more and more in aircraft design, the fiber can be easily inserted in the laminate without disturbing the nor- mal fabrication process. For these applications, bare single fiber or prefabricated tape with integrated bundles of fibers is used. The system initially was developed for fatigue testing of aircraft components such as frames, stringers, and rivets. In monitoring mode, the system is configured to automatically interrupt the fatigue test. The system has also been applied to the inspection of the steel rotor blades of a 2-MW wind turbine.
FIGURE 6.26
A surveillance system has been developed for the centralized inspection of all critical components of the Airbus commercial jetliner during its lifetime. This fiber nervous system is designed for in-flight monitoring and currently is accessible to flight and maintenance personnel.
An optical-fiber mesh has been tested for a damage assessment system for a GFRP sub- marine sonar dome. Two sets of orthogonally oriented fibers are nested in the laminate during the fabrication process. When the fibers of the mesh are properly connected to LEDs and the detectors, the system can be configured to visualize the location of a damaged area.
As an alternative, a video camera and image processing are applied to determine the position of the damaged area. The fiber end faces at the detection side of the mesh are bundled and imaged into the camera tube. Two images are subtracted: the initial image before the occurrence of damage and the subsequent image. If fibers are broken, their location is highlighted as a result of this image subtraction.
CONTROL OF INPUT/OUTPUT SPEED OF CONTINUOUS WEB FABRICATION USING LASER DOPPLER VELOCITY SENSOR
A laser Doppler velocimeter (LDV) can be configured to measure any desired component velocity, perpendicular or parallel to the direction of the optical axis. An LDV system has been constructed with a semiconductor laser and optical fibers and couplers to conduct the optical power. Frequency modulation of the semiconductor laser (or, alternatively, an external fiber-optic frequency modulator) is used to introduce an offset frequency. Some commercial laser Doppler velocimeters are available with optical-fiber leads and small sensing heads. However, these commercial systems still use bulk optical components such as acoustooptic modulators or rotating gratings to introduce the offset frequency.
With an LDV system, the velocity can be measured with high precision in a short period of time. This means that the method can be applied for real-time measurements to monitor and control the velocity of objects as well as measure their vibration. Because the laser light can be focused to a very small spot, the velocity of very small objects can be measured, or if scanning techniques are applied, high spatial resolution can be achieved. This method is used for various applications in manufacturing, medicine, and research. The demands on system performance with respect to sensitivity, measuring range, and temporal resolution are different for each of these applications.
In manufacturing processes, for example, LDV systems are used to control continuous roll milling of metal (Fig. 6.27), to control the rolling speed of paper and films, and to monitor fluid velocity and turbulence in mixing processes. Another industrial application is vibration.
FIGURE 6.27
FIGURE 6.28
With a noncontact vibrometer, vibration of machines, machine tools, and other structures can be analyzed without disturbing the vibrational behavior of the structure. Interestingly, the LDV system proved useful in the measurement of arterial blood velocity (Fig. 6.28), thereby providing valuable medical information. Another application in medical research is the study of motion of the tympanic membrane in the ear.
ULTRASONIC/LASER NONDESTRUCTIVE EVALUATION SENSOR
Ultrasonic/laser optical inspection is a relatively new noncontact technique. A laser sys- tem for generating ultrasound pulses without distortion of the object surface is shown in Fig. 6.29. A laser pulse incident on a surface will be partly absorbed by the material and will thus generate a sudden rise in temperature in the surface layer of the material. This thermal shock causes expansion of a small volume at the surface, which generates thermoelastic strains. Bulk optical systems have been used previously to generate the laser pulse energy. However, the omnidirectionality of bulk sources is completely different from other well- known sources, and is regarded as a serious handicap to laser generation.
Figure 6.29
To control the beam width and beam direction of the optically generated ultrasonic waves, a fiber phased array has been developed. In this way, the generated ultrasonic beam can be focused and directed to a particular inspection point below the surface of an object (Fig. 6.29). This system has been optimized for the detection of fatigue cracks at rivet holes in aircraft structures.
The combination of laser-generated ultrasound and an optical-fiber interferometer for the detection of the resultant surface displacement has led to a technique that is useful for a wide variety of inspection tasks in manufacturing, including areas difficult to access and objects at high temperature, as well as more routine inspection and quality control in various industrial environments. Such a system can be applied to the measurement of thickness, velocity, flaws, defects, and grain size in a production process.
NEXT: PROCESS CONTROL SENSOR FOR ACCELERATION
About the Author
Sabrie Soloman, Ph.D, is the Founder, Chairman and CEO of American SensoRx, Inc.
Excerpted from Sensors Handbook, 2nd Edition by Sabrie Soloman (McGraw-Hill; 2010) with permission by McGraw-Hill.
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