LabVIEW PID controlled PWM output

[GerdW]

Hi nad,

please ask your questions here in the forum. PMs are not meant to discuss LabVIEW problems…

why you use tick count and when you multiplie by 0.001 why 0.001 and not 1000.

Do you understand the math taking place in this snippet?

It's the conversion from ms to s…

[vpereira]

Hello everyone, 

I have a similar problem as described in this thread and the solution was very helpful, I kindly thank you. 

However, I have a question regarding the effective minimum and maximum PID output. 

Imagine that each iteration of the loop  takes 100 ms and the PWM cycle is 3 seconds. This makes the effective minimum and maximum PID output to be 100/3000= 3,33% and 96,66% respectively. When the PID output is outside of this band, there is no action on the system. In my particular case, since I want the code for the PID to run in parallel with the rest of the VI for my application, the band is even smaller (around 5-95%). My system is sensitive to this PID output, so my question is if there is any way to increase the "resolution" of the program or if it is a limitation of the code. 

I tried to increase the PWM cycle but since the electric resistance I use has an high power output, it causes a little fluctuation in the system that I wanted to avoid by using small cycles.

Thank you very much.

[GerdW]

Hi vpereira,

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@vpereira wrote:

However, I have a question regarding the effective minimum and maximum PID output. 

Imagine that each iteration of the loop  takes 100 ms and the PWM cycle is 3 seconds. This makes the effective minimum and maximum PID output to be 100/3000= 3,33% and 96,66% respectively.

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Your limit calculation is wrong: your PWM signal can be "all time low" (0%) or "all time high" (100%). The steps for duty are 100/3000*100% = 3.33%…

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@vpereira wrote:

my question is if there is any way to increase the "resolution" of the program or if it is a limitation of the code. 

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You can either decrease the "100 ms" loop time or increase the "3000 ms full cycle time" to increase the resolution (aka duty step size) of your PWM output…

[vpereira]

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@GerdW  wrote:

Your limit calculation is wrong: your PWM signal can be "all time low" (0%) or "all time high" (100%). The steps for duty are 100/3000*100% = 3.33%…

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Yes, I omitted the 100% because I thought it was obvious, I am sorry.

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@GerdW  wrote: You can either decrease the "100 ms" loop time or increase the "3000 ms full cycle time" to increase the resolution (aka duty step size) of your PWM output…

Increasing the duty step size causes a lot more fluctuation in my system and I need to avoid that.  I already knew I had to decrease the loop time, my question was how can I do it. I was not clear, I am sorry. 

Please find attached a image of my VI. Basically I read 3 temperatures and when the user clicks "Reinitialize" it creates a ramp to the SetPoint with the required ramp. 
Thanks for the help.

[GerdW]

Hi vpereira,

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@vpereira wrote:

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I already knew I had to decrease the loop time, my question was how can I do it. I was not clear, I am sorry.

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In this image there is no timing function visible.

The only place where a timing might occur is the DAQmxRead function: did you set any sample timing in the DAQmx task?

(It always helps to attach the VI instead of a cropped image of the block diagram!)

[vpereira]

I am sorry for the mistake, here it goes.

I did not put any timing DAQmxRead function, I assumed that by being on demand it would be as fast as possible. 

[GerdW]

Hi vpereira,

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@vpereira wrote:

I assumed that by being on demand it would be as fast as possible. 

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Your assumption is correct.

You wrote you are reading some temperatures: which kind of module do you use in your cRIO?

Did you read its specs, especially about the supported sample rates?

[vpereira]

Dear GerdW,

After inspecting the specs of the cRIO, I can tell that the loop time is indeed caused by the sample rate. 

So it is fair to assume that the problems I am having are limitations of the equipment I am using and I need to start thinking in acquiring the correct one. 

Thank you very much for your time and help