Data acquisition system design for 20 LVDT sensors to laptop via USB

[SRGardner]

Hi, I'm very new to data acquisiton and am working on constructing a 20 station/sample creep tester (very smal displacements) for testing which will span 40+ days. On arecent project, I worked with SignalExpress and was able to configure two tilt-sensors (one axis accelerometer type) onto a human body-form for some field testing for relative positions of different body parts. This creep tester I'm working on will need to be hooked up to a laptop, since my projects vary and often times the projects/PC need to be movable, too.

I intended on using 20 Penny & Giles LVDT's (http://www.romdevices.ro/deppdf/SLS190.pdf ) (one per station) to monitor linear displacement at a  very slow sampling rate. In my last project, I used a USB 6009 DAQ card, but I will need more analog input channels for this next test. Ideally the next DAQ system would be USB friendly for ease of use/portability. I'd like to be able to collect data very slowly, due to the longevity of the project. Ideally, I would be able to collect just one data point every 15 minutes or so, but had trouble doing so with the 6009 (limitation of the DAQ or SignalExpress?)

Any help would be appreciated. I'm a little LOST!

Thanks!

[AdnanZ]

There is no limitation of the 6009 where it wouldn't be able to collect a sample point every 15 minutes. What happened when you tried doing that? How did you program the application?

[SRGardner]

Using SignalExpress, my program is structured in the following way. (This is how it was set using my two tilt sensors, I just figure if I can't acquire data at slow enough of a rate with them, the LVDT will be no different):

Steps:

1) DAWmx Acquire -> Voltage (Two desired channels, Ai0 and Ai1)

2) Scaling and Converting Step (Scale the voltage for the first channel)

3) Scaling and Converting Step (Scale the voltage for the second channel)

4) Formula (make voltage output into angle value for first channel)

5) Formula (make voltage output into angle value for second channel)

6) Save to ASCI/LVM (data saved to ASCI .txt file)

The program is extremely simple, and works when acquiring data at higher speeds. I get a DAQ Assistant error when I try to put in the sampling rate as 0.001111 Hz (1.11m Hz). If 15 min = 900 sec, 1/900sec = 0.001111...etc. The error reads that it is restricted to a lower limit of 5.58x10^-3 and upper limit of 48x10^3.

I thought that by extending the error "time out" in the advanced timing settings tab, I'd be able to override something like this... But 1) it doesn't and 2) I don't know if overriding errors is really what I want to be doing, even if it would collect data - might it be possible it is inherently doing something wrong anyway (hense the error in the first place?)

Thanks for your help!

[SRGardner]

Looking through some old purchase orders, I've found that we own [just have to find it 🙂 ] some of the following. Could it be possible that some of what we have be a better fit for the project - or be utilized with a minimal purchase (New DAQ cards with more space?) :

C-DAQ-9172 8-Slot USB 2.0 Chassis for CompaqDAQ (120 VAC)

NI9263 4-Channel +/- 10V, 100KS Per Channel, 16-Bit, Analog Output Module

779564-01 cRIO-9014 Real-Time PowerPC Embedded Controller for CompactRIO, 2GB Disc on Chip Non-Volatile Storage, 128 MB DRAM

779521-01 NI 9237 4-Ch 50KS/s per channel, 24-Bit Bridge Analog Input Module

779052-01 cRIO-9101 4 Slot, 1 M Gate Reconfigurable Chassis for CompactRIO

***Sorry for the late addition to my post, thought it might be helpful to know.

[Henrik_Volkers]

For one measurement every 15min on 20(+) channels ... how about using a relay muxer  to scan your channels?