Odd myRIO PWM problem

[Thorfinn]

Hi. I have an odd problem with my PWM duty cycle. At 1 = 100% duty cycle I have no power and full power at 0 = 0% duty cycle. How can that be???

Kind regards

Lasse

[GerdW]

Hi Lasse,

I guess the problem is within your VI. You just need to find and remove it!

Btw. which answer do you expect when providing so little information?

[Thorfinn]

I've already tried the basic PWM express VI. Same thing??? Also on another myRIO. It makes no sense to me??

[GerdW]

Hi Thorfinn,

unfortunately I'm stuck at LV2017 right now, so cannot open your VI.

At 1 = 100% duty cycle I have no power and full power at 0 = 0% duty cycle.

It makes no sense to me??

Well, it makes sense to me - for a certain kind of DAQ hardware!

I'm using PWM generating devices in my testbenches (like this). Those PWM modules use "low-side" switches ("open-collector" outputs, switching to COMMON/GND). Because of that I need to use pull-up resistors and handle an inversion of the output signal: 100%PWM signal leads to 100% switched to GND level leads to 0% power at the connected device…

So your observation can make sense - it all depends on the used hardware!

[Thorfinn]

Hmmm... Makes sense if that's the case. But I'm using the Digilent motor adaptor for NI myRIO and a LEGO DC motor which actually states that the A4973 driver used has been configured for a 1.5A max current at 100% duty cycle. Then again I must be missing something here because your explanation makes perfect sense. Thanks!

 

[RavensFan]

What does the signal look like at 25% and 75%.  Does .25 look like 75% power, and .75 look like 25% power?

[Thorfinn]

Sorry for the late reply.

Yes .25 gives a 75% high signal...

[GerdW]

Hi Thorfinn,

some DAQmx devices also allow for logical negation of digital output signals - maybe you incidently enabled a similar feature on your myRIO?

[Thorfinn]

I can't find that option anywhere and I'v also tried on two separate myRIOs. The funny thing is that I didn't had the same problem with Labview 2016 a couple of years ago... For now I'll just negate and add one to correct it. But thanks