Measuring Underwater Radiated Noise With PXI and LabVIEW

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"By using LabVIEW Real-Time, the pod client takes advantage of the additional power from running under a fully deterministic, embedded real-time operating system without compromising programming power."

- Mark Trotman, ICON Technologies Pty Ltd, Australia

The Challenge:
Developing a cost-effective system for measuring underwater radiated noise.

The Solution:
Building a custom data acquisition system based on NI PXI hardware and NI LabVIEW software that optimally combines input signal resolution and integrity; compact, high-performance hardware for seabed installation; and full PC programmability with network support.

Author(s):
Mark Trotman - ICON Technologies Pty Ltd, Australia
Alex Le Dain - ICON Technologies Pty Ltd, Australia
Kurt Friday - ICON Technologies Pty Ltd, Australia
James Stewart - ICON Technologies Pty Ltd, Australia
Paul Cornes - ICON Technologies Pty Ltd, Australia

Introduction to Underwater Radiated Noise

Naval operations and the oil and gas industry monitor underwater radiated noise to characterize and track underwater vehicles such as submarines and remotely operated vehicles (ROVs). Underwater radiated noise monitoring is a subset of a much larger range of marine applications that need high-resolution multichannel data acquisition systems capable of continuous logging at sample rates up to 200 kHz. Dynamic range is particularly important in the marine environment because signals are acquired in a complex background that ranges from high-amplitude, low-frequency swell events to low-amplitude, high-frequency transients – both natural and man-made.

No standard off-the-shelf systems met all the requirements of our system. Almost all marine-noise monitoring applications require some degree of system design and custom physical installation. With tight coupling between PXI and LabVIEW, PXI hardware offers an ideal combination of technical characteristics that ensures timely, cost-effective custom integration.

Submerging the PXI System

The underwater radiated noise measurement system consists of a hydrophone array floating above a data acquisition pod that sits up to 150 m deep on the ocean floor. The pod is a watertight steel cylinder that contains a real-time PXI data acquisition unit housing an NI PXI-1000B chassis, an NI PXI-8176 real-time 1.26 GHz embedded controller, an NI PXI-4472 dynamic signal acquisition (DSA) module(s), and an NI PXI-6052E multifunction I/O module. Up to 2 km of cable incorporating Ethernet link tethers the pod to a monitoring vessel on the surface.

In a typical operation, we expose the data acquisition system to the noise source for a few minutes to an hour, depending on the nature of the source. During the exposure, the local PXI disk continuously logs and samples all hydrophone inputs in the pod at 24-bit resolution and a 96 kS/s sample rate. You can select any of the inputs from the monitoring vessel and retransmit them as audio over the Ethernet link to get direct feedback. We use a link from LabVIEW to Microsoft DirectX audio-streaming technology to implement the retransmission. After the exposure period, we use FTP to transfer the binary data file from the pod to the master server on the monitoring vessel for long-term archiving and analysis. The binary data file is typically about 100 MB. We then repeat the cycle as required.

LabVIEW is a client-server architecture so the pod client and remote control station log onto the data acquisition system server automatically. In addition to its main task of acquiring and logging the hydrophone data, the pod client monitors the pod environment and controls the hydrophone transducer interface. During an acquisition run, the pod client streams audio packets to the DAS server and environment and streams status packets to the remote control station. The remote control station provides the operator interface to the DAS server and pod client. It sends control packets to the pod and initiates the FTP data transfer from the pod to the DAS server once the run is complete.

A Great Solution to a Generic Problem

Measuring underwater radiated noise is part of a wide range of applications that need high-resolution, multichannel data acquisition in the marine environment. Achieving a particular resolution is important because signals are acquired within a complex background that ranges from high-amplitude swell effects at frequencies of several hertz, to a multitude of low-amplitude natural and man-made signals at kilohertz and higher frequencies.

Because PXI is rugged and compact and has modest power requirements, it was suitable for encapsulation within the submerged pod without compromising data acquisition speed and computer processing power. The 24-bit PXI-4472 DSA module offers a combination of dynamic range, channel density, and sample rate that makes it ideal for many underwater data acquisition tasks. Each PXI-4472 module includes eight simultaneously sampled channels, so the existing system could be upgraded to 56 input channels without significantly affecting the pod architecture.

By using the LabVIEW Real-Time Module, the pod client takes advantage of the additional power by running under a fully deterministic, embedded real-time OS without compromising programming power. This is important because retrieving the pod from the seabed to address any computer-related issues is a major logistical task. The client and server applications running in the less demanding environment of the monitoring vessel are Windows-based. They take maximum advantage of the many industry-standard productivity tools available for the Windows desktop.

Individual hardware and software component functionality was critical in completing the task, but the tight integration between NI hardware and LabVIEW Real-Time was equally important in our platform architecture choice. The close coupling of software and hardware gave the client a high degree of confidence that the custom system could be designed, built, and deployed in a cost-effective and timely manner.

Author Information:
Mark Trotman
ICON Technologies Pty Ltd, Australia
Tel: +618 9470 4275
mt@icon-tech.com.au

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