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28 Nov 2002
A video tracking system for monitoring the dynamics of a swimming fish is developed in China.
Researchers based at China's Tsinghua University have developed a video tracking system that simultaneously monitors the position and shape of a swimming fish. The team says its system will be useful in studying the mechanics of creature movement. (Review of Scientific Instruments 73 4381)
The system works by projecting a fringe pattern on to the fish and then monitoring the distortion in the pattern as the fish moves.
Having developed two previous tracking systems, researcher Hao Wang says the current system has a wider field-of-view than its predecessors without sacrificing accuracy. "The challenge was how to achieve high sensitivity, resolution and reliability," said Wang. "It took us some time to select the tracking servo device and design the algorithms for image processing and system control."
The researchers use a mirror mounted on a stepper motor to reflect the fringes onto a fish. The pattern contains a brighter, reference fringe generated from a semiconductor laser operating at 670 nm and 5 mW. The distorted fringe pattern is collected by a second mirror, also mounted on a stepper motor, and recorded by a high-speed camera.
"The two stepper motors rotate to track the fish," explains Wang. "A high-speed camera and a target trajectory algorithm are used to track and predict the position of the fish and reference fringe." Knowing the angles of the stepper motor, allows Wang and colleagues to determine the absolute position of the object and its shape from the distorted fringes. The relative accuracy of the measurement is said to be better than 1%.
The team has successfully used its system to track a 5-cm-long goldfish through a 44-cm-long tank. According to Wang, the next aim is to track a flying insect, such as a dragonfly or a butterfly. "Flying insects have more manoeuvrability, so the system will need to be developed further," he added.
Author
Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.
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