High speed cameras measure motions faster

The University of Pretoria (UP) has obtained four new high-speed IDT Vision NX8-S2 cameras that, along with the TEMA motion analysis software, can measure vibrations on moving structures at high speeds or events that are too fast for normal cameras to record.

UP optical and noncontact measurements senior project engineer Dr Abrie Oberholster says the cameras will typically be used at the Centre for Asset Integrity Management (C-AIM) to capture turbine blade vibrations for research by students and the testing of equipment for the power, mining and water industries.

He points out that UP acquired the cameras through the National Research Foundation’s National Equipment Programme. The programme aims to build infrastructure throughout South Africa that supports research in all fields that also makes new technology available for academic and industrial use.

Previously – the university borrowed cameras, which were significantly larger – from other local research institutions, Oberholster highlights.

“The traditional high-speed cameras are bulky and heavy, typically weighing around 6 kg each, which is fine if you just want to capture high-speed footage or if you are going to process images only from one camera to do two-dimensional-type analyses. It is difficult to accurately align these heavy cameras as required for processes, such as stereo digital image correlation analysis.”

Oberholster explains that the new cameras enable the university to do three-dimensional analyses on structures. The IDT Vision NX8-S2 cameras are also smaller than other high-speed cameras, weighing only 0.5 kg each, which significantly simplifies alignment of the cameras.

Further, the IDT Vision NX8-S2 cameras can capture 1.9 megapixel images at 4 000 frames a second. Oberholster says the new cameras are also monochrome cameras, which are “more suitable for science applications”.

Calibration
Mechanical engineering PhD student Benji Gwashavanhu explains that calibrating the cameras before measurements are taken is important when measuring structures that move in more than one dimension. The university uses calibration plates to ensure that the parts of the structure that is being examined are within the camera’s field of vision.

“Instead of equipment with markers on them, we can use a calibration panel with a geometrically calibrated pattern . . . we take a sequence of images while we rotate our calibration panel in the field of view. By so doing, the software can recognise the images to define the volume in space, where the structure is positioned, based on how the markers on the calibration panel have moved,” he explains.

Gwashavanhu highlights that, when the calibration has been done properly using two or more cameras, the vibration on a structure can be measured in the X, Y and Z dimensions with extreme accuracy. If the cameras are not set up properly and the calibration tests on the cameras are wrong, the end-result measurements given are incorrect.

“We can tell when something is wrong before we take our measurements – there are measurements that come out when we calibrate the system. It gives us a calibration file and just by looking at the values in that file, we can tell whether it’s usable . . . ” he explains.

Oberholster points out that calibration is important because it determines the relative position of the structure to the cameras and it converts the pixels of the images into engineering units. It is also important to compensate for any optical distortions inadvertently caused by the camera lenses.

The calibration of the cameras has to be done each time a new structure is going to be tested, and the cameras will be used in several areas of focus of the C-AIM, he adds.

“Using techniques such as digital image correlation as well as motion, shape and optical flow analysis, we are interested in applications such as explosions, crash tests and falling object protection system tests,” he explains.

Oberholster concludes that the cameras have multidisciplinary applications and their being noncontact equipment allows for measurements on small and light structures, with their field of vision also allowing for measurements ranging from very large to extremely small motions depending on the choice of lenses.