PWM output

[nano_era]

Hello,

I would like to see if someone could make a recommendation for me. I am looking to use an NI device to output PWM to control 2 motors. I have only used usb 6008 before and have concluded that I will need a different hardware for this, but also believe that it does not need to be Compact RIO. I need something that does just what I need it to do. Just output PWM for controlling 2 motors. The hardware to drive the motors is not a concern right now, I will build that later. I just would like to see what are the cheapest solutions for something like this(even though I know it won't be "cheap").

Any recommendations would be greatly appreciated.

Thank you

[John_P1 · Accepted solution]

Hi nano,

If there is feedback involved, then your latency requirements will be an important factor in choosing a hardware setup.  In order of latency:

    USB DAQ < PCI DAQ (Windows ) < PCI DAQ (RT) < FPGA

For the DAQ-based solutions, I suggest looking into X Series (e.g. PCIe 6320, USB 6341).  These have 4 counters which support a wide range of functionaity including generating PWM signals.  If you do not require feedback, these boards do have the option of buffered counter outputs.  If you do require feedback, I'd recommend considering the PCIe variant of the board as the latency is much lower than that of USB.  RT and FPGA solutions would give lower latency but can be quite expensive.

Best Regards,

[nano_era]

Thanks for the reply.

As of right now, I do not expect to use any feedback, just PWM output according to user interface. I believe the results of the PWM would then be measured externally. The two recommendations you gave me seem like pretty good ones. I will look into them a bit more.

[John_P1]

Since you mentioned you don't require feedback I'll go ahead and add in another USB option (USB 6210) which has 2 counters that support generating PWM signals.  However, you cannot buffer the PWM characteristics, and any updates to the PWM would have to be made in software (but can still be done on-the-fly).  The 6210 would be significantly lower cost than the USB X Series, but only has 2 counters (instead of 4) and lacks some of the features found on X Series boards.

Best Regards,

[nano_era]

It looks like that one could work as well, but for controlling two motors accurately and consistently, is it really that much of a difference between this usb device and the x series one?

[John_P1]

I don't know if you need this or not for your application, but the buffered counters on the X Series could be useful if you're outputting a pre-defined array of pulse widths.  On the 6210 any updates would be software-timed and non deterministic.

With buffered counter outputs, you can deterministically output an array of pulses at a specific frequency and duty cycle.  For example, your sequence could be N pulses at a certain duty cycle before switching to another duty cycle.  You could also indefinitely output at a specific duty cycle and use a hardware trigger to switch to the next pre-defined duty cycle.  For most practical purposes there is no limit on how long this sequence can be (other than your computer's memory).

If the duty cycle is going to be manually set by a user in software, then it probably won't matter which board you go with (out of the USB options) as long as you don't plan on having any more than 2 PWM signals at a given time.  The PCIe version of the X Series will have the lowest latency of these boards (due to the inherent latency in USB), but if the input is just coming from a user control it probably won't be noticeable.

If you have specific requirements it might help further narrow down the hardware.

Best Regards,

[nano_era]

That's a pretty good explanation. For now the 6210 looks like it would be a good enough choice, but I will also consider the other ones as well. I am still planning the project and its details.