The camera mounted on the geotechnical centrifuge at Rensselaer is a Phantom v5.1 HI-G manufactured by Vision Research. It provides 8-bit image depth and records at 1200 frames per second at a full resolution of 1024×1024 pixels. The frame rate may be increased up to 95,000 frames per second by lowering the resolution of the video stream. It contains 8 GB of non-volatile internal memory and can capture 6.67 seconds at maximum resolution and frame rate. The fast recording speed is critical for capturing dynamic events. Typical earthquake simulations contain frequencies ranging from 10-300 Hz when incorporating scaling laws. In order to prevent aliasing of the data, at least ten points per sine cycle must be captured. This equates to 100-3000 frames per second for the aforementioned frequency range. Normal cameras, which record at 24-30 frames per second, are not suitable for capturing dynamic motions.
The camera is permanently fixed in a mounting system that can be installed on the centrifuge boom cross-member. The hinges connecting the basket to the boom isolate the camera from shake table vibrations. For high intensity events, such as explosive testing, the mounting system can be fitted with a high-strength transparent shield. The camera connects to a computer mounted on the centrifuge cabinet via an RJ45 Ethernet connector. It records video to internal non-volatile memory. When the camera is waiting for a trigger, it continuously acquires data which allows for post-event capture if necessary. In many instances the camera is connected to the main data acquisition system through an external trigger channel. It sends a voltage pulse to the camera which begins recording on the falling edge. This synchronizes the camera with the data acquisition system and input event.
High-intensity light emitting diodes (LED) are ideal for high-speed camera applications. The color spectrum is closer to natural sunlight when compared to incandescent and fluorescent light. The lights are installed out of the camera frame and at angles to minimize glare and reflection. The lighting system can be coupled with the camera such that it is only in a powered state during the exposure of a frame. This increases image clarity at higher recording frequencies while maintaining the lighting system at an operational temperature.
High-speed 1000 fps versus Normal 24 fps
The following video is a demonstration of the 2D shaker and laminar container when recorded using a normal 24 frames per second camera and the high-speed camera set to 1000 fps. Each video is analyzed using motion tracking software. The high-speed video produces correct data while the normal camera is too slow and results in aliased data.
High-speed Camera Tracking Demo
The following video demonstrates target tracking using video from the high-speed camera. Targets are installed on the center and blades of a box-fan. The fan is recorded using both a normal 24 fps camera and the high-speed camera at 3000 fps. The targets are tracked and displayed as: x versus time, x versus y, and acceleration (g) versus time. Additional information, not displayed, such as velocity and angle are also obtainable through the process.