topic Re: mutiple pulse modulation in LabVIEW

mutiple pulse modulation

James3 — Tue, 20 Dec 2005 18:58:00 GMT

Hello,

I am looking for a way to create multiple PWM signals using a NI-6254 DAQ and labview 7.1. For now I would like to create 8 unique digital signals that I have control over the pulse width. I noticed this card has 2 counter outputs and this works well, however I need 8 signals. Is it possible to use one counter signal and then route that to 8 pfi pins? After I do this would I be able to manipulate this signal independantly. By this I mean each of the eight signals could have a different pulse width. If this is not possible would anybody have an idea on how I could accomplish this task?

James

Re: mutiple pulse modulation

James3 — Tue, 20 Dec 2005 19:20:12 GMT

I forgot to mention that I have MATLAB at my disposal, maybe I could create a script to create a pusle width modulated signal?

Just throwing ideas around. As of riht now I am not too concerned about accuracy.

Re: mutiple pulse modulation

Tom_W_[DE] — Tue, 07 Apr 2026 16:02:49 GMT

Hi James-

It's not possible to route the counters to multiple pins and then control them independently- you only have two sets of counter hardware at your disposal, so you'll only be able to create two PWM signals at one time. Your best bet would be to use a card with eight counters such as the PCI-6602.

You could use the correlated digital output capabilities of your board to generate a PWM signal, but the updates to freq/duty cycle would be non-deterministic.

Hopefully this helps-

Re: mutiple pulse modulation

James3 — Wed, 21 Dec 2005 17:39:30 GMT

Tom,

Thanks for the reply. That's what I thought, I didn't know if there was a clever way around it. I can't get a different card because this is for a school project so I have to think of a way around this. Currently I am thinking about using a LMD1825 motor driver. I think that chip and the DAQ will work well together. According to the data sheet one can use digital signals to control the pulse width. I'll post my results after I get the IC in and play around with it.

Thanks again.

James

Re: mutiple pulse modulation

Tom_W_[DE] — Wed, 21 Dec 2005 17:46:00 GMT

Hi James-

The external IC may be the quickest way to reach your goal- please keep us posted on your progress and if you have any additional questions. Best of luck

Re: mutiple pulse modulation

James3 — Wed, 21 Dec 2005 17:53:40 GMT

I am looking now for a way to read ttl voltage levels and reconize them as boolean values on an analog input. Is this possible? So what I have are digital signals, but I want to read them on a designated analog input on the DAQ and then convert the signal to a boolean value within labview and analyze that data. Has this been done before? Is this something worth trying?

Thanks for the fast responses!

James

Re: mutiple pulse modulation

Tom_W_[DE] — Wed, 21 Dec 2005 17:55:09 GMT

Hi James-

Is there a reason why you don't want to use the digital input lines on your device to accomplish this?

Re: mutiple pulse modulation

James3 — Wed, 21 Dec 2005 17:59:15 GMT

Yes, I would be using the digital signals as outputs.

James

Re: mutiple pulse modulation

Tom_W_[DE] — Wed, 21 Dec 2005 18:08:54 GMT

Hi James-

The quickest method I can think of is to just specify a logic threshold division and then check to see if you are above or below that level. I have attached a VI that shows how this might be done; you may want to add a VIH and VIL (rather than a simple threshold) check in order to define a deadband in your AI signal.

Re: mutiple pulse modulation

James3 — Wed, 21 Dec 2005 18:14:42 GMT

Tom,

Excellent! I donnot have access to a machine with labview until later today so I cannot play around with it until then. But I'll let you know how things go, from what you say it sounds like it would work for what I am trying to do.

Thanks again!

James

Re: mutiple pulse modulation

Tom_W_[DE] — Wed, 21 Dec 2005 18:16:04 GMT

Hi James-

Glad to help- let us know if this doesn't work out for you.

Thanks-

Re: mutiple pulse modulation

James3 — Thu, 22 Dec 2005 04:00:38 GMT

I tried the VI and didn't see what I hoped. I didnt see the digital state turn high until I set the logic threshold to -6 (-5.279 after playing around with it more). Originally I had it set to 0. I am sending +5VDC and the level never changes.

I am using dev1/ai0. I have pin 67 connected to gnd (the AI gnd) and I also have pine AI sense connected to gnd. I also tried it with and without apfi 0 connected to gnd. Do know why I see this?

Thanks again Tom,

James

Re: mutiple pulse modulation

Tom_W_[DE] — Thu, 22 Dec 2005 16:23:55 GMT

Hi James-

Let me make sure we are using the same connections: you should have the positive terminal of your source wired to pin 68 (ai0 input) and the negative terminal of your source wired to pin 67 (AIGND). I noticed that I did not specify the RSE terminal configuration directly in my earlier post, so I have modified the VI to force it to use RSE terminal configuration.

When you wire a DC source I would not expect any change in the indicator; you will need to test with a source that slowly oscillates above and below the logic level to see any change on the indicator. Let me know if you're not able to see better results-

Re: mutiple pulse modulation

James3 — Fri, 23 Dec 2005 15:28:23 GMT

Tom,

Thank you for taking the time toshow me how to do that. I didn't have time to look at it too much, all I did was test it against 5 and 0 VDC and that worked. I'll test it against a clock signal and see how fast it can record the data.

Happy Holidays,

James

Re: mutiple pulse modulation

Tom_W_[DE] — Tue, 27 Dec 2005 17:28:40 GMT

Hi James-

That sounds promising- let us know if you're still running into problems.

Re: mutiple pulse modulation

alex_bcn@fantasymail.de — Wed, 03 May 2006 09:23:17 GMT

Hi James!

Did you acomplish your aims with the programme? Seems like I´m having almost the same problem, although in my case I only have to control 4 digital Outs so that I don´t need the analog INs...

Would you mind sharing your knowledge with me? The problem is that I´m almost a complete Labview-Starter and don´t know that much about digital signal processing.

Thank you in advance,

Alex

Re: mutiple pulse modulation

James3 — Wed, 03 May 2006 19:02:10 GMT

Alex,

I did accomplish what I set out to do. I don't quite understand what you are trying to do though. What DAQ card do you have, as I understand they are all different. I needed 8 timers, my card only had 2 so I used different hardware to achieve my goal. Let me know what exactly you are trying to do and I might have a suggestion. If you only have to control 4 digital outputs that program should be very very easy....

Best of luck

James

Re: mutiple pulse modulation

alex_bcn@fantasymail.de — Mon, 08 May 2006 06:41:17 GMT

Hi James,

I have got an PCI 6254 and the aim is to generate 4 (better: 6) digital rectangular signals with pulse width modulation. That is, it should be possible to adjust frequency, duty-cycle and time seperation between these signals for each one. I think that task may not be that difficult, but I´m new in LabView and DAQ so I need some hints or an example. I would be pleased if you could send me a copy or part of your progamm in order to try it out on my system.

Thank you,

Alex

Re: mutiple pulse modulation

James3 — Fri, 12 May 2006 16:14:44 GMT

Alex,

I looked at the data sheet for your card and it says that you only have two counter timers. If you have the option to purchase a card with more counter timers I would recommend that. If not then you could use software to compute your PWM signal, but this is not a good way to accomplish what you are trying to do if accuracy is an issue. What I did was I found an IC (LMD18245) that created a PWM based on an RC pair and a binary number. What exactly are you using the PWM signal for?? Keep searching the net and using the Labview software. Good luck, hope this helps.

James

Re: mutiple pulse modulation

Travis G. — Tue, 04 Jun 2024 17:31:46 GMT

Hello James,

The best way to generate digital pulse trains for PWM applications is using the counters on your DAQ device. Alex is right when he states that your PCI-6254 device has two counters on the card, so you are limited to generating two simultaneous pulse trains with this device. If you are looking for a device with more counters, National Instruments sells dedicated counter/timer devices that contain up to 8 counters per card. Take a look at this comparison chart to see the different channel counts and specifications for the counter/timer series devices.

You can use the same program to implement PWM using the DAQmx driver with your PCI-6254 M Series device or with one of the counter/timer devices. Here is an example program that shows how to change the pulse specs of your pulse train on the fly in terms of duty cycle and frequency. I hope this helps!

Regards,
Travis G.
Applications Engineering
National Instruments