Okay. So what duty cycle percentage represents what speed? Forget about LabVIEW and the PID for the moment. If you want 0 RPM, is duty cycle 0%? If you wanted full speed, what is that speed and is that 100% duty cycle?
I'm worried that you are using a software time loop writing an on or off to a digital output to define the duty cycle. That might work just fine, but Windows timing can be unreliable. If the OS decides to take some cycles away from your process and go do something else like a virus scan, your timing of the duty cycle will get thrown off. The pulse getting stuck on for a period of time will look like a sudden change to 100% duty cycle. Ideally, you would use a DAQ device that is smarter than a USB-6008 that has a hardware timed output so you can create a digital output task that is a duty cycle, or PWM - pulse width modulated, so that the hardware controls the timing of the ON/OFF and not the software and the Windows operating system.
Let's assume that your USB-6008 will work for you, which I doubt. You do have a problem with your lower loop as it isn't a duty cycle. The loop toggles a digital output on and off, waiting a given number of milliseconds. But that math is wrong and the connection to the 20 constant is illogical.
If your duty cycle control is 0, then the time off (false will be) 20 milliseconds, and the time on (true) will be 0 milliseconds. Okay, that sounds like 0%.
If your duty cycle control is 20, then the time off will be 0 milliseconds, and the time on will be 20 milliseconds, That is actually 100% duty cycle, not 20.
If the duty cycle control is 10, then off for 10, on for 10, that is 50% duty cycle, not 10.
If your speed scales from 0 to x RPM for 0 to 100% duty cycle, (please give us the numbers), then the RPM output from the PID will need to be divided by Max RPM so you get a number from 0 to 100. Feed that through an In Range and coerce so you don't get negative numbers nor something bigger than 100.
Now take your total duty cycle frequency. Do you want 20 milliseconds, so 50 Hz pulses? Duty cycle divided by 100 times 20 milliseconds to get On time. Off time would be (1 - duty cycle/100) times 20 milliseconds .
