Three-dimensional Point Tracking Research Articles

Research on Non-contact Bolt Preload Monitoring Based on DIC

Bolt preload is a key parameter of the reliability of bolt connection. Traditional bolt preload detection generally uses torque wrench or high-precision strain sensor. In this paper, a new method of non-contact preload monitoring is realized by measuring the digital image of bolt elongation. A bolt tensile test device is designed, and the results of laser triangulation and three-dimensional point tracking are compared with those of optical measurement. The embedded load cell is used to monitor the bolt preload. The test results of M18 bolts with different bolt lengths and grades are evaluated. and it is found that the test values are close to the theoretical calculation, which can be used to predict the pretightening force.

Dynamic Characteristics and Damage Detection of a Metallic Thermal Protection System Panel Using a Three-Dimensional Point Tracking Method and a Modal Assurance Criterion.

A thermal protection system (TPS) is designed and fabricated to protect a hypersonic vehicle from extreme conditions. Good condition of the TPS panels is necessary for the next flight mission. A loose bolted joint is a crucial defect in a metallic TPS panel. This study introduces an experimental method to investigate the dynamic characteristics and state of health of a metallic TPS panel through an operational modal analysis (OMA). Experimental investigations were implemented under free-free supports to account for a healthy state, the insulation effect, and fastener failures. The dynamic deformations resulted from an impulse force were measured using a non-contact three-dimensional point tracking (3DPT) method. Using changes in natural frequencies, the damping ratio, and operational deflection shapes (ODSs) due to the TPS failure, we were able to detect loose bolted joints. Moreover, we also developed an in-house program based on a modal assurance criterion (MAC) to detect the state of damage of test structures. In a damage state, such as a loose bolted joint, the stiffness of the TPS panel was reduced, which resulted in changes in the natural frequency and the damping ratio. The calculated MAC values were less than one, which pointed out possible damage in the test TPS panels. Our results also demonstrated that a combination of the 3DPT-based OMA method and the MAC achieved good robustness and sufficient accuracy in damage identification for complex aerospace structures such as TPS structures.

Feasibility of extracting operating shapes using phase-based motion magnification technique and stereo-photogrammetry

Optically-based vibration measurements can be used to determine the full-field dynamic response of structures without the need of mounted transducers (e.g. accelerometers, strain gages, and LVDTs) that can add cost, have wiring, power signal transmission issues, and may affect the dynamic characteristics by adding mass or stiffness. The three-dimensional digital image correlation (3D DIC) and three-dimensional point tracking (3DPT) methods based on a stereo-vision system can provide the full-field dynamic displacements of a structure with sub-pixel accuracy. However, stereo-photogrammetry systems are limited by camera resolution and intrinsic noise of the acquired images. Moreover, exciting a structure with only a small amount of energy may result in a subtle mechanical response that is not perceptible by observing the raw data of the conventional optical measurement systems (i.e. 3D DIC and 3DPT). Thus, in order to use the optical-sensing techniques to identify dynamic characteristics of a structure at high frequencies, the signal-to-noise ratio (SNR) in the sequence of images captured with a stereo-vision system needs to be improved. Within this paper, phase-based video magnification in conjunction with the stereo-photogrammetry techniques are used to measure the higher-frequency operating shapes of a cantilever beam and a 2.3-m long Skystream 4.7 wind turbine blade. The phase-based motion estimation method uses complex steerable pyramids to decompose the original sequence of images to amplitude and phase at different spatial resolutions (subsampled images). Motion information is conserved within the phase values which can be filtered in the time domain and amplified to detect the subtle motions. The magnified sequence of images using the motion magnification technique are post-processed using 3D DIC or 3DPT to quantify infinitesimal deformations that are not recognizable using only the traditional stereo-photogrammetry methods. The results obtained within this paper reveal the great potential of motion magnification in conjunction with stereo-photogrammetry techniques in detecting small motions of structures and extracting 3D operating shapes from the optically measured data with low SNR. Compared to traditional stereo-photogrammetry, additional modes of the test structures are obtainable by means of combining the phase based motion magnification and conventional image-sensing techniques (i.e. 3D DIC and 3DPT). The proposed methodology could be used to enhance the established procedure to extract more information about the dynamic characteristics of structures (only once effectively calibrated), compared to existing image-sensing techniques and contact measurement sensors.

A Noncontacting Approach for Full-Field Strain Monitoring of Rotating Structures

The three-dimensional point-tracking (3DPT) measurement approach is used in conjunction with finite element (FE) method and modal expansion technique to predict full-field dynamic response on a rotating structure. A rotating three-bladed wind turbine rotor was subjected to different loading scenarios, and the displacement of optical targets located on the blades was measured using 3DPT. The out-of-plane measured displacement of the targets was expanded and applied to the FE model of the turbine to extract full-field strain on the turbine. The sensitivity of the proposed approach to the number of optical targets was also studied in this paper. The results show that the dynamic strain on a structure can be extracted with a very limited set of measurement points (optical targets) placed on appropriate locations on the blades. It was shown that the proposed technique is able to extract dynamic strain all over the entire structure, even inside the structure beyond the line of sight of the measurement system. Because the method is based on a noncontacting measurement approach, it can be readily applied to a variety of structures having different boundary conditions.

Extracting full-field dynamic strain on a wind turbine rotor subjected to arbitrary excitations using 3D point tracking and a modal expansion technique

Health monitoring of rotating structures such as wind turbines and helicopter rotors is generally performed using conventional sensors that provide a limited set of data at discrete locations near or on the hub. These sensors usually provide no data on the blades or inside them where failures might occur. Within this paper, an approach was used to extract the full-field dynamic strain on a wind turbine assembly subject to arbitrary loading conditions. A three-bladed wind turbine having 2.3-m long blades was placed in a semi-built-in boundary condition using a hub, a machining chuck, and a steel block. For three different test cases, the turbine was excited using (1) pluck testing, (2) random impacts on blades with three impact hammers, and (3) random excitation by a mechanical shaker. The response of the structure to the excitations was measured using three-dimensional point tracking. A pair of high-speed cameras was used to measure displacement of optical targets on the structure when the blades were vibrating. The measured displacements at discrete locations were expanded and applied to the finite element model of the structure to extract the full-field dynamic strain. The results of the paper show an excellent correlation between the strain predicted using the proposed approach and the strain measured with strain-gages for each of the three loading conditions. The approach used in this paper to predict the strain showed higher accuracy than the digital image correlation technique. The new expansion approach is able to extract dynamic strain all over the entire structure, even inside the structure beyond the line of sight of the measurement system. Because the method is based on a non-contacting measurement approach, it can be readily applied to a variety of structures having different boundary and operating conditions, including rotating blades.

Monitoring the Dynamics of a Helicopter Main Rotor With High-Speed Stereophotogrammetry

Stereophotogrammetry in conjunction with three-dimensional point tracking (3DPT) algorithms has proven to be a highly robust dynamic measurement technique on small, rotating bladed systems. This measurement technique can be scaled up to much larger systems and has several desirable features for helicopter and wind turbine structural health monitoring (SHM) applications that include: (1) it is noncontact and does not require the use of roll/slip rings for signal transmission; (2) the applied measurement targets negligibly affect the aerodynamics or structural mass/stiffness; and (3) position data can be collected on many hundreds of points over what is capable using conventional multichannel data acquisition (DAQ) systems. A field test was conducted in which operating data were collected on the main rotor of a helicopter in both grounded and hovering operating conditions. Part 1 of this work describes the experimental setup and DAQ process of the test performed and part 2 of this work presents some of the results including blade dynamics and extracted operating deflection shape information for the helicopter. The promising results presented in this work will serve as the foundation for the development of noncontacting SHM techniques that can be applied to large, rotating bladed structures while in operation.