I have been trying to

Question

use the vi with PID control. Everything is working fine on the labview. However, when I get my ruler to measure really what happened in the experimental apparatus, I do get way off results from the VI results. Can the problem due to the real time issue?To clarify the problem, I have a piston moves through the cylinder. In the simplified attached vi, the left hand side of the block diagram defines the velocity profile of the piston which is basically a trapezoidal. Then, I use the time in seconds coming from the loop delay on the right hand side of the block diagram. Then the output of the formula node goes to PID module to give the desired voltage to the selonoid valve to move the piston. Problems are;
1) When I keep the loop d
elay time at 1ms, the piston moves 2-3 times further than it suppose to move. Then, I change loop delay time to 2ms, 5ms, 10ms, 20ms... I always get DIFFERENT piston movement. Can it be due to the real time card problem? Do we need to buy a board which performs real time applications? Or Is there any other solution I can get away this problem. I only need time in seconds. I used the delay time to use the seconds. But, it is not helping me. Is there any source in labview like clock, I can get only in seconds so that I do not need to worry about delay time?
2) I am planing to use image processing tools in the Labview at the same vi? Do you think that will be a problem in terms of real time issue?
Thanks...
Attachments:
Test15.vi ‏121 KB

Answer 1

use the vi with PID control. Everything is working fine on the labview. However, when I get my ruler to measure really what happened in the experimental apparatus, I do get way off results from the VI results. Can the problem due to the real time issue?Hello,
First of all, in a software-timed application, delay time is the actual control loop rate, and it has a direct effect on the system behavior. According to control theory, a control system must sample a physical
process at a rate about 10 times faster than the fastest time constant in the physical process. Therefore, you first need to determine the fastest time constant in the system ans choose your loop rate accordingly. If the required loop rate is faster then 1 ms (the fastest loop rate you can obtain in a software-timed loop) then you need to use a real-time hardware and apply hardware-timed loop. You can find many on-line example at
http://www.ni.com/devzone/dev_exchange/ex_search.htm
about this method.
Your current loop might take mo
re then 1 ms. To calculate it, you can use Sequence structure as shown in the attached VI. The problem might also be a PID tuning issue. As far as I understand this is a position control application and you are getting a very big steady-state error. You can refer the document linked below to get more information about PID tuning.
http://zone.ni.com/devzone/conceptd.nsf/webmain/4B7775373E4AE64986256B6000691505?opendocument
I hope this helps.
Best Regards,
Remzi A.
Application Engineering
National Instruments
Attachments:
benchmark.vi ‏19 KB