240+ Channel 10KHz/channel Pressure Data Acquisition

[RThodal]

Hello, I am building a data acquisition system for measuring pressure on the hull of an experimental boat.  The system will consist of half a dozen pressure sensor array strips mounted transversely on the hull. There will be a minimum of 240 channels and each channel must be at least 10 KHz for my application. The pressure sensors are resistive type. I'm hoping for suggestions on what NI DAQ system would be suitable for this application. I appreciate your help.

Thanks,

-Robert 

[mallorim]

Hello RThodal,

Could you provide more information about the type of pressure sensor you will be using. I would be more than will to make suggestions as to Data Acquisition (DAQ) hardware, but I was unclear about what you meant when you said that the pressure sensors were of the resistive type. Does this mean that you will need to measure resistance or will you be measuring voltage? Also, will your pressure sensors need a voltage excitation or a current excitation? This affects what hardware you will want to purchase.

The following are great documents on getting started measuring pressure:

Measuring Pressure with Pressure Sensors

Pressure and Load Measurements: How-To Guide

Regards,

Mallori M.

Message Edited by mallorim on 12-02-2008 10:11 PM

[RThodal]

Mallori,

Sorry I was unclear on the sensor type. I am still trying to decide on exactly what sensors to use, maybe you could advise me on this as well. What I need is a pressure sensing array like those made by tekscan, sensor products, or verg/vista medical. In my case the array must be a strip ~900mm in length with at least 40 sensing elements. These companies all make DAQ equipment to go with their sensors but none of the equipment can do 10,000 Hz per sensing element and this is critical. Right now I have a sample sensor array from tekscan. My understanding is that the resistance of the ink in the array reduces as pressures increase. I've tested the sample sensor and I'm seeing resistance in the Megaohm range, but that may not be representative of the final sensor choice. I don't know whether I should be measuring resistance directly with a product like the NI DMM or if I should build a 2, 3 or 4 wire circuit to pass a current and measure the voltage?

Thanks,

-Robert

[AnalogKid2DigitalMan]

Robert:

Interesting project, this sensing technology is new to me. Just some random thoughts-

Will the sensors be exposed to water? That may be a problem.

What is the response time of the sensor? That will dictate how fast you will need to sample. No need in sampling at 10Khz if the response bandwidth is 500Hz.

Tekscan does have a high speed system: http://www.tekscan.com/technology.html#2

Good Luck!

-AK2DM

[RThodal]

The rise time on the sensors is 5 microseconds according to tekscan, so 10 KHz should be possible. The high speed I-Scan system is capable of a maximum of 10 KHz with some of their sensor arrays. At 10 KHz it is limited to about 40 sensors, so I would need one DAQ system per sensor array. I plan on having 6-12 arrays and I would really prefer the flexibility of using national instruments. I plan on testing some different ideas for mounting the sensors and protecting them from the water.

[RThodal]

Any other thoughts on this?

[Seth_B.]

There are a couple of possibilities, depending on how much of an issue that space/size/weight will be.  Are you looking for a standalone system or can we connect this to a PC?  If we can connect it to a PC, does it need to be a laptop, or can we use a desktop?

One major consideration is the amount of data that you are collecting: At 10 kHz, 240 channels, and 8 bit data you are collecting about 19 MB per second.  That's a lot of information to move over a bus.  Another is that voltage measurements will be MUCH easier than resistance measurements.

You have several options if you would like to use NI hardware:

CompactDAQ/RIO: Use either several CompactDAQ chassis (one chassis can't transfer 19 MBps over USB) or a CompactRIO chassis.

PCI: Use an NI switch and an NI DMM to scan through your 240 channels and take resistance measurements.

PXI: Use a PXI chassis in your boat to do that same as the PCI listing above.  Can act as a standalone computer with the proper equipment.

Gathering that much data will not be simple, but it can definitely be done and we will be happy to help get you set up.  To start, we'll need to determine what form factor (PC, PXI, or CompactDAQ/RIO) would fit best with your boat.  Then, we'll need to know whether we will be taking resistance or voltage measurements.  From there, we can help you configure a system that will work best for you.