PXI Labview-RT Read Speed

[Floriiii]

Hey guys,

As some of u might rmb, I wanted to measure a voltage (pressure) with an 80 kHz A/D converter using FPGA. My point was that I wanted to "read" the single value every few microseconds (most current value). As u guys recommended I called the sales man and asked for the according setup. So he told me that I should use a PXI (NI PXIe-8880) , the following A/D converter (NI PXI-6259 ) and the LabView RT system. He also told me that this setup can read the current value every few microseconds. My question is: Is that true? Is a PXI in combination with LabView RT able to read the according value that fast? I always thought, that only LabView FPGA was able to do that.

Regards,

Flo

[mikeporter]

With the right daq card a pxi system can acquire data that fast running Windows. The point you miss is that NI's daq cards are intelligent devices with their own clocking independent of the computer they are installed in.

The real question is what are you going to do with the data once you have it?

Mike...

[Norbert_B]

Flo,

as Mike says, it very much depends on your use case. There is only one use case which requires a system to acquire single values for computation: controller.

So if your application is NO controller, you should use a simple M- or X-series device running on a Windows machine and acquire the data using the onboard clock (as Mike says).

If your application is a controller software (e.g. PID controller), your system has to be deterministic. That rules out Windows leaving RT or FPGA.

Going for 80kHz control loop cycle time is very much depending on the power of the system compared to the complexity of the controller and the performance of the IO hardware. My experience tells me that this is not easy to achieve 80kHz (but possible).

Please note that all items very much depend on the algorithm, the power of the CPU, the IO hardware and the skills you have implementing the algorithm.

There are (scholastic) benchmarks for RT going to close 200kHz, but in reality, you probably fail to get more than 10kHz.....

My hint for you:

If you have experience in LV RT and LV FPGA, try to loan both systems to compare the result.

Norbert

[Floriiii]

Thank you for ur answers!

The program is gonna control a pressure valve. The analog input is the voltage of a pressure sensor. I attached a screenshot of the program structure.  I am using the sampling points for a controller (PWM output signal).  A Sampling point consists of 7 values (7 values in a row, that's why I need single values). The controller updates the duty cycle of the PWM Signal. It only updates the duty cycle after certain measurements took place. I hope u guys know what I am talking about. Can't I read the most recent value without a timed loop using the Single-Point Analog Input. 

So does it work better if I take a controller which doesn't work with windows (for example NI PXIe-8135 RT)

I know, that the program doesn't seem logic, but I can't change it. Other ppls want me to program a code which works exactly like that.

//Edit: The wait doesn't have to be exactly 40 µs or s.th. like that. If there is a jitter of 4 µs the code s still gonna work