Optical Patternator -- 4D Imaging for Spray Characterization Using LabVIEW, IMAQ Vision, and DAQ

Author(s):
Wing T.. Lai - Aerometrics
Jeff Su - Aerometrics

Industry:
Consumer Goods

Products:
Vision, LabVIEW,

The Challenge:
Studying the spray patterns of nozzles and injectors quantitatively.

The Solution:
Developing a PC-based automated system that acquires laser-illuminated images in four dimensions using National Instruments LabVIEW, IMAQ Vision, and HIQ software and DAQ and GPIB boards.

Introduction
Sprays play a key role in many diverse applications, including fuel injection, painting, coating, inkjet printing, agriculture, safety, and medicine. Because they involve complicated two-phase processes, sprays can be highly turbulent, highly transient, and not necessarily repeatable. We used LabVIEW and other software and hardware from National Instruments to develop a new four-dimensional (three dimensions in space and one in time) system to characterize spray devices. The Aerometrics Optical Patternator (AOP) is a laser-based system that uses planar imaging techniques to rapidly measure and study the temporal and spatial characteristics of steady and unsteady sprays. These characteristics make the AOP a valuable tool for studying spray formation and behavior and for verifying the quality of different nozzles and injectors during both development and production phases.

The Aerometrics Optical Patternator
The AOP employs both elastic light scattering and fluorescent emissions from an ensemble of spherical droplets to compute geometric characteristics (such as spray cone angle and spray symmetry), fuel mass distribution, and the Sauter mean diameter (SMD).

The figure depicts a typical configuration for the AOP. It consists of a laser, light-sheet generator, camera, frame-grabber board, timing controller, motion controller, computer, and data acquisition, analysis, and visualization software. We use the light-sheet projector to illuminate a two-dimensional plane within the spray field of interest. With fuel injectors, the fuel droplets lying within the measurement plane scatter the incident light and the CCD camera captures the image of the illuminated plane. We sequentially record both the elastic light scattering pattern and the laser-induced fluorescence pattern by positioning appropriate optical filters in front of the camera. Capturing the image and processing it in the PC yields a representation of fuel surface area distribution (Mie scatter), mass distribution (fluorescence), and SMD (ratio of the Mie to fluorescence scatter) distributions within the measurement plane.

By incorporating a high-speed shuttering system, such as a pulsed laser and/or a gated-intensified CCD camera, it is possible to freeze the flow field at an instant in time. Also, by properly synchronizing the laser/ camera system to the electronic fuel injector, it is possible to study the spray characteristics at a specified instant after the injection has begun. We can repeat the synchronized data acquisition over several cycles and digitally average the images for improved statistical accuracy. Therefore, we can study the transient evolution of the global spray with all the necessary information resolved in time.

By mounting the laser-sheet/camera system (or the spray, depending on the specifics of the installation) on a computer-controlled traverse system, we can obtain several two-dimensional, averaged image slices within the spray - we can study the full spray volume. By combining synchronization and traverse positioning, we can generate a complete, four-dimensional data set representative of the combined spatial and temporal behavior of the spray.

System Hardware
The AOP employs a Pentium II PC running Windows NT as the host computer platform. The system uses a National Instruments PC-TIO-10 counter/timer and digital I/O board to achieve the synchronization between the laser, the injector, and the camera. By changing the triggering time of the injector (or other atomization device), we can make a sequence of measurements of the spray. The counter/timer board also controls the injection duration when we test a pulsed injector. To capture the spray pattern in different planes, the system uses a traverse mechanism to move the nozzle to the desired locations. The traverse has three translation stages (X, Y, and Z axes) and one rotation stage (q) so the user can easily position the laser light sheet with respect to the spray axis. We control the traverse over the IEEE 488 bus using the AT-GPIB/TNT board.

Software Design
The powerful, flexible graphical programming language, the graphical user interface (GUI), and the extensive hardware interface options (VI libraries) made the LabVIEW and IMAQ Vision combination a natural choice for the AOP. There are two main functions in the AOP software -- acquisition and analysis.
Acquisition controls all the hardware devices and stores the images in specified directories. The user can acquire images using four different modes:
• 2D data - fixed time and fixed location
• 3D data - fixed location but variable time
• 3D data - fixed time but variable location
• 4D data - variable time and variable location

We use different laser/camera configurations for different applications. For example, the automotive fuel injector industry uses intermittent sprays and thus requires pulsed lasers operating in the ultraviolet range; however, the gas turbine industry deals mostly with steady sprays and thus requires continuous- wave (CW) lasers.
The analysis portion of the software incorporates National Instruments IMAQ Vision for the image processing and analysis operations, as well as HiQ for report generation and presentation of results. Using IMAQ Vision, we can display the images in real time and easily handle several different camera formats. HiQ displays the final results so the user can manipulate, annotate, and print them.

Conclusion
By combining the long-established Aerometrics product, the Phase Doppler Particle Analyzer, with National Instruments products -- DAQ and GPIB hardware and LabVIEW, IMAQ Vision, and HiQ software --we quickly developed the AOP, a new product with extensive features. The AOP gives our customers a complete tool for spray research and development. We are currently planning additional features, such as QC-oriented protocols and 4D visualization. With the flexibility of LabVIEW, we can produce a simple, powerful, and cost-effective system useful in many industries.

For more information, contact:

Jeff Su

Aerometrics/TSI, Inc.

1288 Reanwood Avenue

Sunnyvale, CA 94089

Tel: (408) 738-6688

Fax: (408) 743-9664

E-mail: jsu@aerometrics.com

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