DIC case study: A fishery research institute uses DIC technology to improve fishing net fishing efficiency
Do you know how fishing nets catch fish? What factors affect the efficiency of trawling? How do the shape and trajectory of the trawl change during the process of opening and dragging?
Recently, a fishery research institute took the lead in conducting a study in China, hoping to systematically understand the changes in shape and movement trajectory of the trawl during the towing process, to provide scientific theoretical support for exploring the factors affecting fishing efficiency such as net opening, net shape and fish entry.
What are the difficulties in underwater measurement of fishing nets?
Under the action of hydrodynamic forces, fishing nets will produce large deformations and displacements, and are geometrically nonlinear. How to measure the dynamic displacement of fishing nets?
The opening area and shape of the net mouth will affect the entry of fish into the net during the towing process. If the local net shape cannot be fully extended, it will also increase resistance and affect fishing.
How to measure underwater fishing nets? The traditional method is to take a large number of photos of underwater fishing nets with a camera and analyze the morphological changes of the fishing nets through the photos. It is impossible to measure dynamically and capture the morphological change data in real time.
How to solve the underwater dynamic measurement of fishing nets?
In order to realize the dynamic trajectory measurement of underwater fishing nets, after many exchanges and communications between the two parties, XTOP 3D developed and customized a product: XTDIC-STROBE underwater 3D dynamic deformation system. This system uses digital image correlation (DIC) and dual Using stereoscopic vision technology, by tracking speckles on the surface of the fishing net underwater, it can achieve dynamic measurement of the 3D coordinates and displacement of key points of the fishing net during the deformation process.
So, what’s so special about this DIC underwater dynamic measurement product developed by XTOP 3D? In other words, what underwater measurement problems need to be overcome?
Problem 1: Waterproofing underwater camera equipment
As we all know, this is an underwater fishing net measurement process, and the waterproofing of industrial CCD cameras and DIC equipment is crucial.
In order to solve this problem, after many experiments and failures, we finally came up with a set of waterproof devices that meet the requirements: put the equipment into the waterproof device to ensure that the equipment and measuring head are waterproof and ensure the reliability of the calibration model; industrial CCD camera through the glass window of the waterproof device, data is collected on the fishing net. In addition, the camera, housing, and measuring head have also been waterproofed accordingly, adding double insurance to the system's waterproofing.
3D dynamic deformation measurement system waterproof device
Problem 2: Measurement of underwater dark environment
For underwater DIC measurement, how to overcome the underwater light difference and dark environment data collection is also a key difficulty.
Because of the limitations of underwater light and industrial camera shooting, fill light is particularly critical. This link will directly affect the accuracy of the measurement data.
To this end, XTOP 3D special blue light lighting equipment is specially used to supplement underwater image collection; the industrial CCD camera is fixed in the control room through a specially designed tooling, with a measuring distance of 2m. The light source is placed underwater, facing the fishing net being measured, at a distance of about 3m.
Underwater 3D dynamic deformation measurement system setup and collection
Problem 3: Underwater camera calibration
To perform DIC dynamic measurements underwater, calibration tools must be placed underwater, including calibration plates and crosses. Only by obtaining the internal and external parameters of the camera through camera calibration can we accurately obtain the 3D data of the object point.
In order to ensure the collection effect of fishing net feature points, reflective marking points/speckle spots are fixed on the fishing net, and these feature points change correspondingly with the movement of the fishing net opening;
The reflective marking points/speckle spots on the fishing net all move under the camera coordinate system. From this, the movement patterns of the characteristic marking points on the fishing net are collected and calculated, and the movement displacement parameters of the fishing net are obtained.
Dynamic measurement of underwater trajectories of fishing nets
During the measurement process, two industrial CCD cameras of the XTOP 3D underwater XTDIC-STROBE 3D dynamic measurement system simultaneously captured the deformed image of the fishing net. Through stereo matching and 3D reconstruction, the 3D coordinates, displacement and other data of key points on the surface of the fishing net were obtained.
The newly developed 3D underwater DIC three-dimensional dynamic measurement system realizes research on the hydrodynamic characteristics of underwater fishing nets through the generation, identification and tracking of fishing net key points, and 3D trajectory reconstruction, as well as the calculation of fishing net motion trajectory parameters. It is useful for optimizing fishing gear design and enhancing the quality of fishing gear. Fishing efficiency and improving the safety performance of fishing gear are of great value.
Data collection and calculation
The water tank is opened to tow the fishing net, and the XTDIC-STROBE 3D dynamic measurement system is used to obtain the 3D coordinates of the key points of the fishing net. During the towing process of the fishing net, the underwater 3D dynamic changes of the fishing net during the towing process are measured.
Through the XTDIC-STROBE analysis software, the collected data are processed to obtain the XYZ displacements of each key point of the fishing net. Calculate based on the data and analyze the changes in X displacement, Y displacement, Z displacement, and E displacement.
1) Calculation of X-direction displacement of key points of fishing net
- Y-direction displacement of key points of the fishing net
3) Z-direction displacement of key points of the fishing net
4. Displacement in E (comprehensive) direction of key point of fishing net
The underwater 3D deformation measurement method of fishing nets based on 3D digital image correlation (DIC) technology can visualize the displacement changes of the nets at different positions within the camera's field of view, and through the analysis of the 3D displacement data at key points, the dynamic process of changing the shape of the nets can be clearly observed.
Through this underwater dynamic displacement measurement of fishing nets, the feasibility of Xinto 3D
DIC underwater dynamic measurement system has been verified, and the measurement data and accuracy have met the requirements of the user's technical standards, which has been recognized by the customer, and it provides data support for the precise determination of horizontal and vertical expansion of the net opening, and the measurement of the dynamic change of the net opening and the body of the net, and it also provides an important reference for the design and improvement of the structure and material performance of the fishing nets.
