Motion Control and Motor Drives

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PID - USB 6008


Hi, everyone!

I'm Student. I found some example about PID. Reset Counter and make a PWM on Internet, I don't remember those Links. Now I Use them to control motion of DC motor. I use USB - 6008 of my teacher (I borrowed). When I select time to reset counter 20ms but elapse higher 20ms. The purpose of my project only to study, sorry when I changed those Program. Would you please explain for me and help me solve this project . Thank you.


This my VI and Picture my test.

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Message 1 of 3

I'm sorry that I need to disappoint you, but your approach doesn't sound very promising. Low cost USB DAQ devices are the worst choice for closed loop control systems due to the lack of hardware timing for the digital outputs or a counter output. With this approach you will run into multiple problems:

  1. Lack of hardware timed output results in a PWM signal with a high amount of jitter.
  2. Compared to other bus technologies like PCI or PXI, USB adds even more jitter in single point output mode.
  3. Running your control loop under Windows (non-deterministic operating system) adds even more jitter and makes tuning very hard.

This is really a worst case scenario for a control system and you will probably end up with a poor solution. Here and here are some other threads about the same topic.

Kind regards,

Jochen Klier

National Instruments

Message 2 of 3

Thanks you, Jochen!Smiley HappySmiley Wink


Your explain very useful for me. You can't imagine it help me so much.


One again, Thanks you so much.Smiley Happy Smiley Wink

Message Edited by aludlak on 07-20-2009 06:47 AM
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