Today, modern measurement technologies enable high-precision detection of the movement of liquids and gases, providing valuable data for numerous applications. How does air flow around an airplane? How does blood move through our veins? And how can pollutant emissions in combustion processes be minimized? For these and other purposes, speed, direction, pressure, and turbulence within a flow are analyzed to increase efficiency, ensure safety, and drive innovation in numerous areas.

Various methods are available to measure these flows, including visual ones such as particle image velocimetry (PIV). High-resolution cameras are used to track marked particles within a flow and analyze their movement. iLA_5150 GmbH, from Aachen, Germany, also relies on EBIV (event-based particle image velocimetry). It’s a new optical method for the qualitative and quantitative visualization of flows and flow velocities. It combines PIV with event-based cameras, in this case with a uEye EVS from IDS Imaging Development Systems. The innovative sensor technology of this industrial camera enables highly dynamic and energy-efficient detection, especially for fast and turbulent movements.

Application

In the EBIV method, tiny particles are added to a flowing fluid and illuminated in a “light section plane.” They generate individual light pulses as they enter and exit the LED light section. This change in local brightness is recorded independently by the camera pixels and transmitted to the PC as a data stream of “change events.” In contrast to conventional cameras, event-based models only react to the changes in brightness registered in the image field. Stationary scattered light, such as background or a nonchanging illuminated surface, does not generate a measurement signal. This reduces the amount of data considerably.

Experimental setup with flow channel, flow model, illumination source, control unit, and uEye EVS camera. Image: IDS

Frame rates of up to 10,000 frames per second

The data stream essentially contains information about what happens, when, and where. In detail, these are the pixel coordinates on the sensor, microsecond time stamps of the pixel events, and the information about the events: on or off.

This allows a distinction to be made between increasing intensity (on event) and decreasing intensity (off event). Using suitable software, the stream can be converted into an image matrix in which both the spatial information and the linear time base of the stream are available. The result is comparable to the extremely high frame rate of a high-speed camera.

André Brunn, head of development at iLA_5150, says, “The EBIV measurement method differs fundamentally from conventional imaging methods. They usually generate very large amounts of data and require powerful peripherals that can process them. For exceptional frame rates of 1,000 Hz and more, the image-based cameras required are themselves very complex and expensive. With the help of event-based camera technology, comparable frame rates of 10,000 frames per second are possible, whereby only standard PC interfaces such as USB with a few gigabits per second are required. The price of the event-based models themselves is significantly lower than that of corresponding high-speed cameras and is therefore also very interesting for smaller teaching and research institutions.”

Further processing

The data stream is converted into image data and displayed in the front end—the so-called EBIV viewer—for direct online flow visualization. The user can also select the integration time, which corresponds to the exposure time of an image camera, as well as the time increments of the sequence, i.e., the period between two consecutive shots. Classic image filters can also be used. With the help of these settings, the user can optimize the display of the movement paths and adapt them to individual examination objectives. Both fine flow details and large-scale patterns can be made more clearly recognizable. This ability to readjust the measurement results is another advantage of the event-based approach.

In the qualitative visualization of the flow in the EBIV view, particles are continuously illuminated so they’re visible as luminous traces. The method is well suited to visualizing the flow. However, it doesn’t provide exact measurements of its speed or direction. For a precise, numerical analysis of the flow properties, individual particles are illuminated for a moment using short, time-defined light pulses.

Flow visualization in the EBIV viewer. Image: IDS

Thus, their exact position can be recorded and their movement quantified. By comparing the particle positions in successive images, the speed and direction of the flow can be precisely calculated using PIV, for example. The result is a transient 2D vector field that changes over time; in other words, the vectors don’t remain constant. As with classic PIV, the image sequences of the event-based camera can also be statistically analyzed—for example, to determine mean values and fluctuations in the flow velocity.

PIV-view: Unsteady flow field (left) and time-averaged flow field (right). Image: IDS

Camera and software

On the camera side, iLA relies on a uEye XCP-E from IDS. The small, lightweight industrial camera offers event-based sensor technology in a robust die-cast zinc housing (29 × 29 × 17 mm) with screw-on USB Micro-B connection. It’s compatible with all standard C-mount lens sizes. This makes it ideal for both industrial and nonindustrial areas. The integrated, event-based vision sensor (EVS) was developed by Sony and Prophesee. It’s supported by Metavision SDK, a seamlessly integrated suite of software tools and models, APIs, and other training and development resources from Prophesee for efficient analysis, visualization, and customization. Building on this, the EBIV viewer from iLA_5150/PIVTec is specially tailored to flow visualization applications.

Outlook

The optimization of fluidic systems and processes requires detailed knowledge of the flow conditions. A qualitative flow visualization is often sufficient to understand effects and develop suitable control mechanisms. Until now, however, imaging fast flows with high temporal resolution was usually only possible with expensive high-speed cameras. Event-based camera technology offers a cost-effective alternative that requires significantly less technical effort.

These “simple” methods have been lacking in teaching and research in particular. Thanks to the extreme data reduction, this technology also allows the use of several cameras or large camera arrays without the periphery becoming a limitation for data transfer. The compact design of the uEye EVS models also makes them ideal for mobile applications. This means that real application environments can be analyzed directly for the first time without having to rely on artificial flow models or channels.

In short: Event-based cameras enable efficient, cost-effective, and high-resolution visualization and quantification of flows. Due to the small amounts of data generated, many processes can be analyzed almost in real time, which also makes them interesting for use in fully automated systems. Established flow measurement methods such as PIV can be seamlessly integrated and expanded. The technology can be used wherever flow information is captured by changes in scattered light intensity—be it from moving particles or vibrating surfaces.

Published by IDS Imaging Development Systems.

Camera: uEye EVS (industrial camera) with event-based vision sensor; model, uEye XCP-E; camera family, uEye EVS. Image: IDS