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Customer Solutions

Creating a System to Analyze Air Bubbles in Liquids

Author(s):

Ernst Wilding, Joanneum Research

Industry:

Industrial Controls/ Devices/ Systems

Product:

LabVIEW, Vision

The Challenge:

Analyzing air bubbles in liquid in an industrial setting.

The Solution:

Using a powerful LabVIEW development system to build a statistical analysis tool based on image processing.


Introduction
In industrial processing, the size of air bubbles in liquids respective to their size distribution plays a major role in the effectiveness of the production process.A wide range of bubble-generating equipment exists. Each differs in the way it produces bubbles and releases them into the liquid. Some of these differences include bubble speed, amount, and distribution. Until now, there were only analysis tools that qualified single bubbles, revealing the problem of a missing statistical relevance. Engineers could only achieve this relevance through time-consuming and work-intensive process of measuring a large amount of single bubbles.

Our development system results in a reproducible qualification of generated bubbles anywhere in the observed liquid based on a statistical random sample test. We developed a robust imaging sensor that you can freely place in the observed liquid, image processing software, and application user interface.
To achieve statistical qualification of bubble-generating equipment, we needed to develop a new imaging sensor that would make it possible to acquire images of the emerging bubbles anywhere within the liquid.
Because of the relatively high speed of the bubbles leaving the bubble-generating equipment, the image acquisition task was difficult. The purpose of the demanding image processing system were greatly influenced by the high density of the bubbles in the medium, and with it, the light refraction and reflection.
We created a system based on an industry-standard personal computer running Windows NT and the imaging sensor, which includes the mounted backlight unit with adjustable light intensity.

Developing the Imaging Sensor
Initial image acquisition tests showed that, depending on the type of illumination (direct light or backlight) bubbles and their shapes appear quite differently. These tests also revealed the high density of bubbles, which makes it difficult to distinguish between individual bubbles. Partial or total overlay of bubbles was another problem.

With each of these considerations in mind, we developed the new imaging sensor. It consists of a camera mounted in waterproof steel housing that is oriented in 90° to the viewing direction. The image is redirected to the camera using a prism. This configuration gives us a slim design that makes it possible to use the sensor even in a narrow experimental container.

We can freely adjust the light emitting diode (LED) backlight used in relation to the sensor window so that only a small gap is left where the bubbles can pass between the illumination unit and the imaging sensor. This helps the system deal with varying bubble densities. The intensity of the backlight is also adjustable to overcome light intensity problems in fluids that are not fully transparent.

Two Methods for Image Processing
Several morphological image processing steps, such as morphological gradient, morphological filling of holes, and morphological opening are necessary for image preprocessing and bubble segmentation. To minimize false alarm rate in the detection of bubbles, we can perform preliminary analysis of isolated bubbles in two different ways.

One method to avoid the problem of detecting multiple smaller bubbles with partial overlay as one big bubble, which would clearly lead to wrong statistical data, lies in the use of heuristics. Heuristics increase robustness of single bubble identification. A brighter center spot and a somewhat uniform but darker border region characterize the image of a single bubble. Using this information, you can reject ambiguous bubbles.
The second method is to use characteristic parameters such as the Heywood circularity factor, Waddel disk diameter, center of gravity, ellipse orientation, and a set of given parameter ranges for cross-checking the bubble shape. You can choose both methods from the user interface - their use depends on the nature of the occurring bubbles. A summary of the aforementioned processing steps is depicted in figure above.

Functional User Interface
The user interface consists of the following functional parts:

  • Image acquisition (including image source and image processing parameters)
  • Image display (source image, different stages of image processing)
  • Histogram mode
  • Histogram display (including mean, standard deviation, and bubble count as numeric output)
  • Mode of integration (single image, image sequence)
  • Output of histogram data and statistics on serial port

You can acquire images from the sensor in individual shots or as a sequence of images. It is also possible to load images saved during earlier sessions from hard disk to recalculate statistics. Several different modes of operation determine the statistic output of the program, which you can perform after each processed image or at the end of an image sequence. You also can save histogram data on a disk. You can adjust image processing parameters to optimize the processing result for the given situation of bubble speed, liquid transparency, and bubble size.

Histogram options enable selection between different desired histograms that include:

  • Absolute quantity frequency density of bubble diameter
  • Relative quantity frequency density of bubble diameter
  • Relative surface frequency density
  • Cumulative frequency of quantity
  • Cumulative frequency of surface


Rapidly Implementing a User Interface and Reducing Development Time
This application qualifies individual bubble-generating equipment with respect to generated bubble amount, bubble size, and size distribution. The newly developed innovative imaging sensor provides the statistical analysis anywhere in the liquid. The system is capable of measuring bubbles with a speed up to 1.0 m/s and a spatial resolution of 0.2 mm. With the new sensor, you can measure the bubbles directly above the equipment where the bubbles are generated and during their movement towards the liquid surface.
By using NI LabVIEW, we rapidly implemented a functional and clearly structured user interface. In addition, we significantly shortened the development cycle compared to similar projects.

For more information, contact:

 Ernst Wildling

JOANNEUM Research Forschungs-GmbH, Institut für digitale Bildbearbeitung

Wastiangasse 6 A-8010 Graz

Tel: +43 316 876 x 1740

E-mail: ernst.wildling@joanneum.at

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