High-channel count audio and vibration applications - Advanced Concepts and Applications

This tutorial is part of the NI Analog Resource Center. Each tutorial will teach you a specific topic by explaining the theory and giving practical examples. This document presents three applications that have different system requirements, discusses their challenges, and offers a proposed solution to each.

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For more information, return to the NI Analog Resource Center.

Application A - High Speed Data Logger

System Requirements:

• Acquire from 24 channels simultaneously at a rate of 1 MS/s
• Stream all of the data to disk for 10 minutes
• Data will be post processed

Discussion:
First, we need to decide which data acquisition board will best fit our requirements. The PXI-6133 is an 8 channel 2.5 MS/s simultaneously sampled 16-bit device. In order to get 24 total channels, we need 3 of these devices and an 8-slot PXI chassis to hold them.

Next, we need to decide on what kind of a controller we need for the system. Since this is a streaming to disk application, we should calculate how fast of a hard drive we need to be able to keep up with the acquisition. The formula for how much data you will be saving to disk is:

In this application, we have 24 channels * 1 MS/s * 2 bytes/sample = 48 MB/s. This means that we must have a controller with the capability to write to the hard drive at least this fast. PXI controllers have laptop hard drives which limits them to about 20 MB/s. Most new desktop PCs come with SATA hard drives, but even these are strained at around 45 MB/s. In order to achieve very high streaming to disk rates, a hardware RAID system is the best solution. With a RAID system configured for striping (RAID 0), you can stream to disk at rates upwards of 100 MB/s!

A MXI-4 link is used to connect the PXI chassis to the external PC. We need to be careful in choosing a PC to use for the application since the data rates are very high. In this particular case, we would need a PC which has multiple independent PCI bus segments. The PCI MXI-4 card would be placed in one bus while the PCI RAID card would be placed in a separate bus. If these devices were placed in the same PCI bus, they would have to share the same bus bandwidth and likely not achieve the desired performance. This is because the combined required bandwidth of the devices is 96 MB/s (48 MB/s each) which is approaching the theoretical maximum of the PCI bus (132 MB/s).

Proposed Solution:
An external PC with multiple independent PCI buses will be used as the controller. The controller will be connected to a PXI chassis with 3 PXI-6133s through a MXI-4 link. In a separate PCI bus from the PCI-MXI-4 board will be a RAID controller which will be used to stream the data to disk.

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Application B - Online Processing

System Requirements:

• Acquire data from 32 microphones simultaneously
• Perform online power spectrum to monitor spikes
• 20 kHz bandwidth
• monitor continuously

Discussion:
The PXI-4472 is a good choice of a data acquisition device for this application. It has 8 simultaneously sampled channels and can provide IEPE excitation to microphones. We will need 4 PXI-4472s in an 8-slot PXI chassis to get a total of 32 channels.

In order to achieve a 20 kHz bandwidth, we will use a sampling rate of 51.2 kS/s (2.56 x bandwidth). If we use some of the benchmarking numbers as guidelines, we’ll see that the PXI-8187, PXI-8350 or and external PC are all capable of performing a power spectrum fast enough for our application. The decision on which one to choose may be determined by other preferences for the system that are not necessarily requirements. Examples of these may include available space, environment, additional display or processing needs, and cost.

Proposed Solution:
A PXI-8187 will be used as the controller with 4 PXI-4472s in an 8-slot PXI chassis.

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Application C - Very High Channel Count System

System Requirements:

• Acquire data from 600 channels simultaneously
• Sampling rate of 51.2 kS/s
• Stream data to disk for 1 hour
• Data will be post processed

Discussion:
Immediately looking at these system requirements, we know the system will be more complex than the previous ones and it will require several controllers and chassis. The question is how many and how do we configure them.

In this system, we have two main challenges, the number of channels and the streaming to disk rate.

For this application, we will use PXI-4472s as our acquisition device. These devices have 8 channels, are 24-bits, and can sample at up to 102.4 kS/s. Seventy-five PXI-4472s will be required altogether.

A modular architecture is the best choice for this system. By breaking the system into smaller blocks, we will be able to more easily tackle the overall challenges. At the same time, we add more complexity to the implementation of the system since we now need to guarantee synchronization between all of the blocks in software as well as hardware. A dedicated master chassis and controller will be used to control the synchronization. In the master chassis, there will be several PXI-6653 boards which will distribute precise timing signals to all of the slave chassis. One PXI-6651 will also reside in each of the slave chassis to receive these timing signals.

The largest slot chassis we can use to synchronize PXI-4472s is the 14-slot PXI-1044. The first slot is reserved for the controller and one slot is needed by the PXI-6651 for synchronization. This means that we can place 12 PXI-4472s in each slave chassis and we will need a total of seven slave chassis to hold 75 devices. If we evenly distribute the modules between the chassis, there will be 10 or 11 devices in each slave.

Now, we must decide on a controller to use for our system. Since we have chosen a modular architecture, we have divided the workload of the controllers between all of the blocks. Let’s evaluate our stream to disk rate on the slave level:

We also need to be able to have space for an hour’s worth of data which is about 65 GB per slave. The required streaming rate is very close to the limit of what the PXI-8186/8187 can handle. An external PC will work and so will the PXI-8350, both can also have multiple large hard drives if extra storage capacity is needed. The PXI-8350 1U server is probably the best solution in this case since it geared to being rack mounted and headless, two characteristics that are useful in slaves.

Each slave consists of a PXI-8350 1U server, PXI-1044 14-slot chassis, one PXI-6651 for synchronization and 10 or 11 PXI-4472s. Every slave is capable of acquiring data and streaming it to disk for one hour.

The master controller does not need to be the same as the controller used by the slaves and its only requirement is to guarantee synchronization which is not processor or hard drive intensive. Any of the previously mentioned controllers would work great as a master.

Proposed Solution:
In this application, there is one master system as well as several slave systems. The master system will consist of a PXI-8350, 4-slot PXI chassis, and 3 PXI-6653s. The slave system consists of a PXI-8350, 14-slot PXI chassis, PXI-6651, and either 10 or 11 PXI-4472s.

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Relevant NI products

Customers interested in this topic were also interested in the following NI products:

• LabVIEW Graphical Programming Environment
• Digitizers/Oscilloscopes
• Dynamic Signal Acquisition (DSA)
• Data Acquisition (DAQ)

For the complete list of tutorials, return to the NI Analog Resource Center.

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