Timing Settings in LabView

[Eric Ellison]

We purchased a 9174, 9237, and 9401 units to capture the pressure sensor data and have a control to turn on the motor. The NI salesperson recommended these things when I told him about the application and what we needed to do.  The computer is connected to the 9174 via the USB cable.

We have an AST pressure transducer (Model AST4000A00025PAB1000).  Plus a solid state relay which was already approved by another solution center posting so that the 9401 can send a signal to the relay to turn the motor on and off.

Anything else?

[Dennis_Knutson]

Should have asked about the hardware (or you should have provided it) in the first place. If you read the specs, you will see that the minimum sample rate is 1.613 kS/s.

[Eric Ellison]

Great so what I asked for I was sold something that I don't need.  We are not experienced in Data Acquisition as this is our first time.  Thus, the many conversations with the salesperson and what we wanted to do.

Options:

1.) Please specify what an item then that can have a min sample rate that would fit our needs, that would work with our program and we could plug into the 9174 right away.

2.) Also the informaiton on the website for 9237 specification only shows "Sample Rate" of 50 kS/s.  I did find what you are talking about in the 9237 manual.  How do we quickly and easily change to an external time base to bring down the rate to the 391S/s?

Thanks for your help but this is a learning experience I would have preferred to avoid.

Eric

[Dennis_Knutson]

Personally, I think you are better off having hardware that can take more samples than you think you need than the opposite. You can reduce the number of samples in software. I don't use Express VIs but give a look at the Sample Compression function.

[IanW]

Looks like the hardware was chosen because of the pressure
transducer you had. If you had one with a 1v to 5v output you could have used
simpler and cheaper voltage measuring DAQ hardware, I have used 9215 and 9205
for measuring pressure transducers.

As Dennis says, you can over sample and reduce your data to
the rate you’re looking for. If you set your task to measure at 2 ks/s then
collect together 20 samples and take the average of these you will arrive at a
sample rate of 100 s/s. I use this technique all the time it has the advantage
of reducing any random noise on the signals.

I think there’s an express vi to do this data reduction for
you, if I have time I may do a quick example to show you.

Ian

[Jake_H]

I'm guessing the 9237 was chosen because it's ideal for pressure, force, strain, and other bridge based measurements. The module can provide variable excitation voltages and be configured in quarter, half, and full-bridge applications. It uses a delta-sigma ADC, which oversamples and filters the data to provide a higher resolution and prevent aliasing. The oversampling and minimum sample rate of 1.613 kS/s is what's preventing you from getting exactly 500 data points. If you need 500 points in 5 seconds, I would recommend sampling at the minimum rate and decimating some of your data. You can't import a different clock to the module. 

You definitely have the right hardware for your application - you'll just have to condense your data to fit your needs. Definitely let us know if you have any further questions.

[Eric Ellison]

We have used the sample compression function as suggested before and that is working fine.

P.S. in regards to the hardware comments earlier we talked to NI first and then purchased the pressure transducer based on the NI equipment.  If I would have known of the "cheaper" option I would haev gone with that one and then purchased a 1V-5V pressure transducer.

Either way it is working now and providing good data.