topic Re: Real-time plot with changing acquisition rate in LabVIEW

Real-time plot with changing acquisition rate

kjemmett — Fri, 25 Jul 2008 20:05:37 GMT

Hi all,

I have a simple program that collects current and voltage data from a Keithley sourcemeter, plots it in real-time, and saves the data to a text file. A feature of the program is that the frequency of measurement changes during data collection - high frequency measurement for the first (more interesting) period, and low frequency measurement for the duration of the experiment. The problem I'm having is that I can't get the x-axis to adequately scale to reflect this change in frequency. I'm currently using a waveform chart and an x-scale multiplier tied to 1/frequency. It works for the high frequency period, but bonks the scaling when switching to low frequency.

My understanding is that waveform chart is designed for constant rate acquisition. This is probably the problem. Should I be using the XYgraph function? I collect time data, so should be able to build a {time,current} array and just repeatedly redraw the graph. Can I get the XYgraph to behave like a waveform chart?

Basically, I want to keep a fixed scale on the x-axis, but have the chart know that after a certain time each data point represents a smaller delta-t. Any simple approach I'm overlooking?

Cheers,
Kevin

Re: Real-time plot with changing acquisition rate

Kevin_Price — Mon, 28 Jul 2008 19:19:37 GMT

I don't know a simple way to get you exactly where I think you're trying go. There *are* some things you can do, but they each have their own side effects to consider.

1. To make an XY graph behave somewhat like a chart, you'd need to create a software buffer for the XY data, and then periodically graph it all. This may end up requiring you to do careful memory management, and will also consume extra CPU for the display.

2. You could wait until the end of acquisition and plot a single XY graph of the full dataset against actual sample time. But I suspect you chose a chart because you need the live feedback while the data collection is in progress, and this method wouldn't give you any.

3. A combination approach would be to simply chart the live data with an X-axis meaning "sample #". Once the collection is over, *then* draw a single XY graph that includes actual time scale data. This method doesn't give you good X-axis info until the collection is done, but at least you can easily see your Y-axis data in near real time.

-Kevin P.

Re: Real-time plot with changing acquisition rate

sardarsree — Mon, 01 Mar 2010 08:25:03 GMT

Hi,

I amtaking sensor input signals through NI FPGA cRIO-9004. i am analysing this using wave graph. the problem is i am not getting accurate singal.How to decrease the frequency?

Re: Real-time plot with changing acquisition rate

Anna_K. — Mon, 01 Mar 2010 22:03:53 GMT

Do you want to decrease your acquisition frequency or how often you plot to the graph? The acquisition rate depends on how fast your I/O loop runs on the FPGA. Are you using a DMA FIFO to send the data to the RT Host or are you passing the data to your PC? Are you displaying the data in a Chart or Graph?

Re: Real-time plot with changing acquisition rate

Anna_K. — Mon, 01 Mar 2010 22:04:43 GMT

You will need to give alot more information about your application and problem

Re: Real-time plot with changing acquisition rate

sardarsree — Tue, 02 Mar 2010 08:56:17 GMT

Hi,

I am sorry for unsufficient data. Actually I want to capture engine crank singal.

Through tekronix scope i am getting correct signal. But when i use NI cRIO-9221 module I am not getting clear graph like tekronix scope graph. I attached the both pictures.I want to build scope using labview(NI cRIO-9221).

( NOTE: i taken history of wavechart then i attached this into wave graph. bcoz to get cursors.)

Re: Real-time plot with changing acquisition rate

A_Patel — Tue, 02 Mar 2010 22:49:57 GMT

Hi,

At what frequency do you want to collect the data?

FLash

Re: Real-time plot with changing acquisition rate

sardarsree — Wed, 03 Mar 2010 03:29:58 GMT

Hi,

Max frequency is 10khz.

Re: Real-time plot with changing acquisition rate

Anna_K. — Thu, 04 Mar 2010 15:39:23 GMT

We still need more information about your code, how fast is your FPGA loop running?

- are you using a DMA FIFO to pass the data from FPGA to host?

Can you post you code?

Re: Real-time plot with changing acquisition rate

sardarsree — Mon, 08 Mar 2010 04:12:09 GMT

HI,

Sorry For the delay. I attached my project setup image. and also i attached my program.

Re: Real-time plot with changing acquisition rate

sardarsree — Mon, 08 Mar 2010 09:11:17 GMT

Hi,

Sorry. i forgot to attach my project setup. I attached that setup pic here.

Re: Real-time plot with changing acquisition rate

George_P_Burdell — Mon, 08 Mar 2010 22:57:20 GMT

It appears you don’t have any VI under your cRIO controller. You can’t just go directly from your FPGA to the development computer. You need to go through the controller. The FPGA talked to the controller, then the controller talks to the PC. Therefore you will need to setup this communication as well.

I would suggest looking at the FPGA Tutorial linked below for additional help.

NI Developer Zone: FPGA Design, Development and Programming Tutorial
http://zone.ni.com/devzone/cda/tut/p/id/3358

Re: Real-time plot with changing acquisition rate

sardarsree — Tue, 09 Mar 2010 04:57:07 GMT

Hi,

Thank you so much. Please tell what modules i need to add in the fpga VI. For my application I have to take the wave form signal from NI-9221.

Re: Real-time plot with changing acquisition rate

George_P_Burdell — Tue, 09 Mar 2010 21:34:14 GMT

I would suggest looking at some of the shipping examples for an idea of how a cRIO project should be set up. You can access them by going Help>>Find Examples from within the LabVIEW environment. My suggestion for one to get started with is under Hardware Input and Output>>CompactRIO>>Basic IO>>Analog Input – cRIO.lvproj. Take a look at this example and see where everything is. From there you can translate this into your project.

Re: Real-time plot with changing acquisition rate

sardarsree — Tue, 06 Apr 2010 08:41:56 GMT

sir,

Just tell me. Is it possible to take graph for 120khz singnal (the signal is speed of wheel with 6000rpm)with NI-cRI 9221.

thank you

Re: Real-time plot with changing acquisition rate

George_P_Burdell — Tue, 06 Apr 2010 21:16:21 GMT

Is your 120khz signal from an encoder? Most encoders generate a digital PWM signal that can be read with a counter. The NI 9221 is an analog input module that cannot read digital signals or give you access to the counters. I believe you would have more success with a NI 9401 or NI 9402 which are our digital modules.

NI 9221
http://sine.ni.com/nips/cds/view/p/lang/en/nid/14590

NI 9401
http://sine.ni.com/nips/cds/view/p/lang/en/nid/201849

NI 9402
http://sine.ni.com/nips/cds/view/p/lang/en/nid/204678